Practical Penetrant Inspection (NDT30P) : World Centre For Materials Joining Technology

WORLD CENTRE FOR
MATERIALS JOINING
TECHNOLOGY
PRACTICAL PENETRANT INSPECTION
(NDT30P)
TWI Ltd, Training and Examination Services

NDT30P
DAY 1
TIME SUBJECT
9.00 - 9.15 Introduction to the course and administration
9.15 - 10.30 Introduction to NDT
10.30 - 10.45 Break
The 6 steps of Penetrant Inspection, including practical
10.45 - 12.15
demonstrations
12.15 - 13.00 Lunch
First practical exercises of Water washable, Post
13.00 – 15.00
emulsifiable and Solvent removable techniques
15.00 – 15.15 Break
15.15 – 16.30 The theory of Penetrant inspection Part 1
Issue of Coursework paper No. 1
16.30 – 17.00
Product technology video
DAY 2
TME SUBJECT
8.45 - 9.00 Review Coursework paper No. 1
9.00 - 10.30 The theory of Penetrant inspection Part 2
10.30 - 10.45 Break
The theory of Penetrant inspection Part 2
10.45 - 12.15
Continued
12.15 - 13.00 Lunch
13.00 – 15.00 Practical exercises
15.00 – 15.15 Break
15.15 – 16.30 Practical exercises
16.30 – 17.00
Practical Penetrant Inspection WORLD CENTRE FOR

Rev 0 Dec 2005 MATERIALS JOINING
Copyright © 2005, TWI Ltd TECHNOLOGY
NDT30P
DAY 3
TIME SUBJECT
8.45 - 9.00 Review Coursework paper No. 2
9.00 - 10.30 Product technology theory
10.30 - 10.45 Break
10.45 - 12.15 Instruction and report writing
12.15 - 13.00 Lunch
13.00 – 15.00 Practical exercises including writing an instruction
15.00 – 15.15 Break
15.15 – 16.30 Practical exercises including writing an instruction
16.30 – 17.00
and Product technology paper
DAY 4
TIME SUBJECT
Review Coursework paper No. 3
8.45 - 9.00
and Product technology paper
9.00 - 10.30 Course review
10.30 - 10.45 Break
10.45 - 12.15 Final coursework paper
12.15 - 13.00 Lunch
13.00 – 15.00 Final course practical test
15.00 – 15.15 Break
15.15 – 16.00 Final course practical test continued
16.00 – 16.30 Final coursework review

CONTENTS
Section Subject
1.0 PRINCIPLE
1.1 PENETRABILITY
2.0 FLAW ENTRAPMENT EFFICIENCY

2.1 VOLUME
2.2 LENGTH
2.3 CONTAMINANTS
2.4 PENETRANT DYE
2.5 PROCESSING
3.0 PENETRANT PROPERTIES

3.1 WETTING ABILITY
3.2 SPECIFIC GRAVITY
3.3 FLASH POINT
3.4 VOLATILITY
3.5 CHEMICALLY INHERT
3.6 VISCOSITY
3.7 SOLUBILITY
3.8 SOLVENT ABILITY
3.9 TOLERANCE TO CONTAMINANTS
3.10 HEALTH HAZARD
3.11 AVAILABILITY AND COST
3.12 ELECTRICAL CONDUCTIVITY
4.0 FLUORESCENT PENETRANTS AND THE ELECTROMAGNETIC

SPECTRUM
4.1 TYPES OF UV-A LAMP
4.2 FLUORESCENT DYE
5.0 DEVELOPMENT
5.1 CAPILLARITY
5.2 LIGHT SCATTERING
5.3 SOLVENT ACTION
Section Subject
5.4 DEVELOPER PROPERTIES
6.0 PROCESS SEQUENCE

6.1 PREPARATION AND PRE-CLEANING
6.2 CLEANING METHODS
6.2.1 MECHANICAL METHODS
6.2.1.1 BRUSHING
6.2.1.2 BLASTING
6.2.2 CHEMICAL METHODS
6.2.2.1 HOT SOLVING DE-GREASING
6.2.2.2 VAPOUR DE-GREASING
6.2.2.3 COLD SOLVENT DE-GREASING
6.2.2.4 SOLVENT MATERIALS WITH
EMULSIFIERS AND DETERGENTS
6.2.2.5 ALKALINE CLEANING
6.2.2.6 ACID PICKLING
6.2.2.7 STEAM CLEANING
6.2.2.8 PAINT REMOVAL
6.3 APPLICATION OF PENETRANT
6.3.1 SPRAYING
6.3.2 AEROSOL SPRAY
6.3.3 ELECTRO-STATIC SPRAY
6.3.4 DIP AND DRAIN
6.3.5 DIPPING HEATED PARTS
6.3.6 THIXOTROPIC
6.4 REMOVAL OF EXCESS PENETRANT
6.4.1 WATER AS A REMOVER
6.4.2 LIPOPHILIC REMOVER (EMULSIFIER)
6.4.3 SOLVENT REMOVAL
6.4.4 HYDROPHILIC REMOVER (DETERGENT)
6.4.5 SPRAY-SCRUBBER PENETRANT REMOVAL
6.4.6 WATER AND SOLVENT
6.5 DRYING
6.5.1 HOT AIR RE-CIRCULATING OVEN
6.5.2 FORCED WAM AIR
6.5.3 DRY-CLEANED COMPRESSED AIR
6.6 DEVELOPMENT
6.6.1 DRY DEVELOPER

Section Subject
6.6.2 DRY POWDER DEVELOPER APPLICATION

6.6.3 AQUEOUS LIQUID DEVELOPER
6.6.4 WATER-SUSPENDED DEVELOPER (WET
DEVELOPER)
6.6.5 WATER – SOLUBLE DEVELOPER
6.6.6 SOLVENT BASED DEVELOPER
6.6.7 PLASTIC FILM DEVELOPER
6.7 INSPECTION
6.7.1 FLOURESCENT INSPECTION
6.7.2 COLOR CONTRAST PENETRANTS
6.7.3 INSPECTION AIDS
6.8 RECORDING
6.9 POST CLEANING
6.10 PROTECTION
7.0 SYSTEM CLASSIFICATION

7.1 TYPES OF PENETRANT
7.1.1 COLOUR CONTRAST PENETRANTS
7.1.2 FLOURESCENT PENETRANTS
7.1.3 FLOURESCENT V COLOUR CONTRAST
7.1.4 DUAL PENETRANTS
7.2 EXCESS PENETRANT REMOVAL
7.2.1 WATER WASHABLE
7.2.3 POST EMULSIFIABLE
7.2.4 HYDROPHILIC REMOVER (WATER-DILUTABLE)
7.3 TYPES OF DEVELOPER
7.3.1 DRY POWDER DEVELOPER
7.3.2 AQUEOUS DEVELOPER
7.3.3 SOLVENT BASED
7.3.4 WATER OR SOLVENT BASED FOR SPECIAL
APPLICATION
7.4.1 BS EN 571
7.4.2 MIL-L-25135 C
7.4.3 MIL-I-25135 E

Section Subject
8.0 CHOICE OF PENETRANT SYSTEM

8.1 SIZE AND TYPE OF DEFECT
8.2 GEOMETRY AND INTRICACY
8.3 SURFACE CONDITION
8.4 OTHER FACTORES TO BE CONSIDERED
8.5 EXAMPLES
9.0 EQUIPMENT CHECKS

9.1 OVERALL SYSTEM PERFORMANCE
9.2 CONTROL CHECKS
9.2.1 WATER WASH TEMPERATURE AND PRESSURE
9.2.2 COLOUR INTENSITY
9.2.3 PENETRANT REMOVER CHECK
9.2.4 DEVELOPER CHECK
9.2.5 UV LAMP OUTPUT CHECK
9.2.6 UV MONITOR CHECK
9.2.7 WATER REMOVABLE PENETRANT, WATER
TOLERANCE CHECK
9.3 MAINTENANCE CHECKS
9.3.1 TANK LEVELS
9.3.2 EQUIPMENT CLEANLINESS
9.3.3 AIRLINE CLEANLINESS
9.3.4 PROCESSING UNITS
9.3.5 UV LAMP MAINTENANCE
9.3.6 CLEAN TANKS

1.0 PRINCIPLE
Penetrant testing (PT), alternatively referred to as dye penetrant inspection (DPI),
penetrant flaw detection (PFD) and liquid penetrant inspection is often said to function
through the phenomenon of capillarity. This is the elevation or depression of the
surface of a liquid where it is in contact with a solid, such as the sides of a tube to
form a meniscus.
It can be most clearly seen when using tubes of a very fine diameter, known as
capillary tubes, and is dependant upon the balance between the surface tension of the
liquid (cohesive forces) and the wetting of the sides of the tube (adhesive forces). If the
adhesive forces exceed the cohesive the surface of the liquid will be concave and the
liquid will rise up the tube. In practice the finer the bore of the capillary tube the greater
will be the rise seen. If the cohesive forces exceed the adhesive then the surface will be
convex and the liquid will fall below level of the surrounding liquid. Penetrant inspection
requires the former of these 2 situations.
Capillarity
1.1 PENETRABILITY
Penetrability is however a complicated property and not quite so simple as the
analogy with capillary tubes suggests. What is not disputable is that it is influenced by
variables such as surface condition and type of the test object, the type of penetrant,
the temperature, and the presence or absence of contamination. The formula shown
below has been used to link together these factors.
2S Cos θ
P =
D
P = Capillary pressure
S = Surface tension of the liquid
θ = Contact angle
D = Width of the crack
Practical Penetrant Inspection 1.1 WORLD CENTRE FOR

Principle TECHNOLOGY
Copyright © 2005, TWI Ltd

From the formula it can be seen that high capillary pressure requires high
surface tension but it should be stated however that a material with a high surface
tension is not necessarily a good penetrant an example being water which has a high
surface tension but is a poor penetrant.
θ represents the angle formed between the liquid-air interface and the liquid-
solid interface. The importance of the ability of a liquid to wet the sides of a tube has
been stated previously and the smaller the contact angle is, the higher will be capillary
pressure.
Contact Angle
A good penetrant must have high wetting ability and hence have a low contact
angle typically below 5 degrees. The wetting ability of a penetrant can however vary
from one type of surface to another and dwell time should therefore be varied
accordingly.
Contact Angle Wetting Ability Droplet Shape
Less than 90o High
90o Moderate
Greater than 90o Low
Contact Angle, Wetting Ability and Droplet Shape
The formula also shows that the width of the discontinuity or opening influences
capillary pressure, in fact the narrower the opening, the higher the capillary pressure.
Penetrants have been known to enter a space of 0.3 microns width, that is one third of
one thousandth of a millimetre. Evidence of this can be seen from capillary rise
experiments where liquid will rise higher in narrower tubes although it will take longer for
this rise to occur. In terms of a penetrant inspection finer defects will thus require longer
dwell times.
In the USA a similar formula known as the Static Penetration Parameter (SPP)
is utilised.
SPP = SCos θ


S again represent the surface tension the liquid and Cos θ is the contact angle
between the liquid-air interface and the liquid-solid interface. The importance of the
ability of a liquid to wet the sides of a tube is again shown and the smaller the contact
angle is, the higher will be the SPP value.
A fourth factor worth considering is viscosity, which while not significantly affecting
a liquid's ability to enter a discontinuity does influence the rate at which it does so.
Experience and the formula below for Kinetic Penetration Parameter (KPP) both show
that a highly viscous penetrant takes much longer to enter a defect and thus requires a
longer dwell time.
As viscosity is strongly influenced by temperature, so are penetrant inspections.

A viscous penetrant that has been dipped or sprayed will also drain more slowly from a
specimen and cause excessive loss of penetrant due to drag out into the wash station.
S Cos θ
KPP =
η


2.0 FLAW ENTRAPMENT EFFICIENCY
A term used to describe the efficiency of a penetrant inspection is "flaw
entrapment efficiency" which describes the ability of a penetrant to form an indication
large enough to be detected. Some of the factors influencing Flaw Entrapment
Efficiency are:
• Volume of a defect
• Length of defect
• Contaminants
• Penetrant Dye
• Processing
2.1 VOLUME
The size of an indication is based on the volume of the defect it has entered.
The larger the discontinuity is in terms of its depth and width, the more penetrant it will
hold and the more penetrant there will be present to form the indication in the
developer.
2.2 LENGTH
The length of a defect whilst influencing the volume of the penetrant present
has a strong influence upon the ability of the human eye to detect the indication. Very
fine indications such as those formed by fine fatigue or stress corrosion cracks will
have insufficient width to be detected visually and will only be located when they are
of sufficient length.
2.3 CONTAMIMANTS
Penetrating fluids will generally enter fine, clean discontinuities more readily
than wide and contaminated ones but many in-service inspections will encounter
defects contaminated with oil, water and corrosion products. These can both reduce
the volume available to the penetrant and in the case of water adversely influence the
contact angle of the penetrant.
Highly acidic or alkaline contaminants also cause fading of the dye present
within the penetrant, likewise heat and prolonged exposure to ultraviolet light can also
cause penetrant dyes to lose their brilliance.

Flaw Entrapment Efficiency TECHNOLOGY

2.4 PENETRANT DYE
The efficiency of a penetrant inspection will be influenced by
• Type of dye
• Concentration of dye
The most obvious influence of dye type is seen when one changes between
colour contrast and fluorescent penetrant dyes, the latter giving much higher
sensitivity than the former. Within these 2 categories the brilliance and intensity of the
dye colour will strongly influence sensitivity.
Penetrant manufacturers will within a classification such as water washable

fluorescent penetrants, offer varying sensitivity levels. This can be achieved by
altering the dye concentration and in the case of water washable penetrant the level of
emulsifier.
2.5 PROCESSING
Dye concentration can be affected by utilising the dip and drain method of
processing. This allows more volatile constituents of a penetrant to evaporate during
the dwell time and thus increases the concentration of the dye within the remaining
penetrant.
The degree of penetrant removal must also be considered. Components that

are cleaned until there is no background coloration present will offer a high degree of
contrast for any penetrant indications. This absence of background coloration is
interpreted in some cases as evidence of over-emulsification and over-washing and
that penetrant may have been removed from defects. A small degree of background
shows that over-washing has not occurred. This should not be excessive however as
the brightness of the indication must exceed the background's brightness in order to
be detected.

Flaw Entrapment Efficiency TECHNOLOGY

3.0 PENETRANT PROPERTIES
• Wetting ability
• Specific gravity
• Volatility
• Chemical activity
• Solubility
• Solvent ability
• Tolerance to contaminants
• Health hazard
• Flammability
• Electrical conductivity
• Availability and cost
3.1 WETTING ABILITY
The wetting ability of penetrants is an important physical property that affects

their penetrability and bleed-back characteristics. The contact angle and surface
tension of a penetrant control wetting ability.
3.2 SPECIFIC GRAVITY
Specific gravity is a comparison of the density of a penetrant with the density of

distilled water at 40C. It is normally not a problem area with oil base penetrants.
Penetrants used in a tank system must have a specific gravity less than one in order
to ensure that water will not float on top of the penetrant and prevent the penetrant
from covering the test object.

Penetrant Properties TECHNOLOGY

3.3 FLASH POINT
The flash point of a material is the temperature at which enough vapour is given
off to form a combustible mixture and a minimum value of around 93°C is typical.
Insurance companies and transport regulations are tending to dictate a movement
upward in penetrant flash points and this can be a particular problem with solvent-
based removers and developers that are required to be halogen free. Most
manufacturers can, however, supply non-flammable alternatives using chlorinated
solvents.
3.4 VOLATILITY
Many materials with good penetrant ability are unfortunately volatile, which
means that they would evaporate too quickly to be practical. The penetrant would dry
from the inspected surface, leaving it stained, and from any discontinuity, leaving it
contaminated with precipitated dye.
Volatility is characterised by the vapour pressure or boiling point of a liquid.

Low volatility is desirable from a practical standpoint of the evaporation loss of
penetrant stored in open tanks.
3.5 CHEMICALLY INERT
Penetrant materials must be as inert and non-corrosive as possible. Maximum

sulphur, sodium and halogen levels are often specified by the nuclear and aerospace
industries, to avoid the possibility of embrittlement or cracking as failures can occur
years after, caused by quite small quantities of contaminant.
It is important that the penetrant be chemically compatible with the material

being tested and to this end penetrants containing chlorides, chlorine, or sulphur are
frequently restricted from use on austenitic steels, titanium, and high nickel steels.
When new applications are evaluated, the compatibility of an organic penetrant

material or of certain solvents must be considered. The potential reactions must be
considered with respect to the operating environment of the product.
3.6 VISCOSITY
Viscosity relates to the thickness or body of a fluid and is a result of molecular

or internal friction. Viscosity is an important and easy-to-measure property. Tests to
measure viscosity include the Federal Test Method Standard 781 Method 305 and the
ASTM Standard D-445 using the Cannon-Fenski viscometer.


3.7 SOLUBILITY
All penetrants contain a dye, red or fluorescent, in solution. The penetrant must
hold sufficient dye at ambient or high temperature and the dye must not come out of
solution if the temperature drops. Red contrast penetrants tend to cause most trouble
in this respect.
3.8 SOLVENT ABILITY
Having applied the penetrant, it becomes necessary to remove the surplus from
the test specimen to ensure a clean, clear background. Volatile solvents, some
flammable, some not, are often used. These must not dissolve the penetrant in
defects.
3.9 TOLERANCE TO CONTAMINANTS
Penetrants kept in open tanks will become contaminated after a time, even with
the greatest care. Water is the main enemy, especially for water washable penetrants
and so checks must be done at intervals to ensure all is well. Oil, grease and solvents as
well as many strange objects find their way into tanks.
Even though great care in cleaning is taken, contaminants can still remain in a
defect. Therefore the penetrant materials must be formulated to minimise such
problems. Reduction of fluorescent brilliance by chromate residues on water washable
penetrants can be of particular concern, although less so in recent times.
3.10 HEALTH HAZARD
Chemists developing new penetrant materials must comply with or exceed the
most stringent health and safety requirements. Three of the main problems are of
toxicity, odour and skin contact. Again, it is more likely that the solvent-based
cleaners, removers and developers will come under closest scrutiny. For instance,
halogenated hydrocarbons are extremely dangerous in the presence of heat, so
smoking is absolutely forbidden when they are being used. It is very difficult for an
operator to remain uncontaminated by penetrants, even when wearing gloves and
overalls. Different people are allergic to different materials. Nonetheless, reputable
manufacturers will not knowingly use hazardous materials.


3.11 AVAILABILITY AND COST
Materials that are difficult to obtain are unlikely to be used even though they
might have big advantages in terms of capillarity. Similarly there is no commercial
sense in using components which are very expensive, making the final product
uncompetitive in a very open market.
3.12 ELECTRICAL CONDUCTIVITY

Electrostatic spraying of penetrant is becoming increasingly popular in large
automatic processors, even where electrostatic hand-spray guns are used. The
electrostatic spray provides uniform coverage of parts with complicated shapes,
reduces over spraying, and requires less penetrant overall.
The basic principle of electrostatic spraying is that the spray gun applies a
negative electrical charge to the penetrant as it is sprayed. The test object retains
ground potential. The electrostatic attraction between the two opposite polarities
causes the penetrant to be strongly attracted to the part.
Electrostatic spray systems used manually require penetrants that have high
electrical resistance so that there is not a flashback to the operator.
The penetrant should have two characteristics to be suitable for electrostatic spraying:
1. A low viscosity so that the liquid can be readily divided into very small
components (i.e., atomised) and be easily attracted to the part
2. It must readily accept and hold the electrical charge placed on the liquid
particles. Most commercially available penetrants have suitable characteristics
to be used in electrostatic spray systems.


4.0 FLUORESCENT PENETRANTS AND THE
ELECTROMAGNETIC SPECTRUM
Fluorescent penetrants utilise the ability of certain materials to absorb
electromagnetic energy of one wavelength and in response emit light at a different
wavelength. Molecules within fluorescent penetrants absorb ultraviolet light, become
energetic and then shed their excess energy by emitting yellow green visible light.
Indications are viewed under darkened conditions with the operator thus viewing
bright indications against a dark background. They are mainly used in factories, on
castings, forgings, precision parts, aluminium alloy, stainless steels and so on.
Fluorescent penetrants are more sensitive than colour contrast as the indications
produced are 10 times more "seeable".
Ultraviolet radiation can be divided into three categories based on wavelength,

UV-A, UV-B, and UV-C with the shorter wavelengths of ultraviolet radiation being
more dangerous to living organisms. UV-A has a wavelength from 400nm Å to about
315nm, UV-B from about 315nm to about 280nm and UV-C from around 280nm to
150nm.
Absorbs Emits
10 100 200 300 400 500 600 700

ULTRAVIOLET VISIBLE
LIGHT LIGHT
UV-A and Fluorescence
4.1 TYPES OF UV-A LAMP
By far the most common type of light source used to inspect components tested
with fluorescent penetrant is the mercury vapour arc lamp. In fact the mercury arc lamp
is a street or workshop lamp which has a filter over it to reduce the visible light to a
minimum but allow the UV-A to be transmitted.

Flourescent Penetrants and the TECHNOLOGY
Electromagnetic Spectrum
The filter is called a Woods in UK and a Kopp in the USA. On one type of lamp
the filter is integral with the outer envelope but on another, using either a GE or
Westinghouse lamp, it is separate.
The mercury arc is drawn between electrodes enclosed in a quartz tube. The
resistor limits the amount of current in the starting electrode. The quartz tube is mounted
and enclosed in the outer glass envelope, which serves to protect it and filter out any
possible hazardous radiation.
400 watt mercury vapour arc flood lamps can be used where very large
components are tested or to give a background illumination in an inspection area.
Background lighting in a darkened area can however, be more economically provided by
UV-A strip lights.
4.2 FLUORESCENT DYE
Dye concentration and colour shade influence the overall sensitivity of

fluorescent penetrant systems, as with the visible penetrants. There are many other
variables that can be controlled, as well as capabilities that can be developed. In
general, fluorescent penetrant systems have more potential applications than do the
visible dye penetrants.
Fluorescent materials absorb energy from light waves in the ultraviolet region of
the electromagnetic spectrum. This energy is converted and emits photons of energy
at a different light wavelength. Most commonly used in NDT is ultraviolet light (UV)
which peaks at 365nm wavelength. This is the UV peak commonly known as black
light. Penetrant dyes are selected that absorb energy in the 350 to 400 nm range and
emit light in the 475 to 575 nm range. The emitted light is in the visible spectrum in the
green to yellow range.
The quality of fluorescent dyes is determined by how efficiently they will absorb
UV light and convert it into visible light. This is influenced by:
• The intensity of the UV-A light at the surface of the component
• The ability of the dye to absorb UV-A light
• The concentration of the dye
• The ability of the dye to produce visible light
• Film thickness
Of these factors the inspector has the ability to influence the intensity of the UV-
A light at the surface, the dye concentration and the film thickness. With respect to the
first of these factors, all fluorescent penetrant inspection procedures call for a
minimum level of UV-A measured in μW/cm2. Below the specified level the
fluorescence will be insufficient.
Dye concentration, as has been mentioned earlier, will increase during the
dwell time due to the evaporation of the more volatile penetrant constituents.
The final factor, film thickness, is important because fluorescent dyes have a
minimum layer thickness below which, they will not fluoresce. Emulsification, washing,
and development can influence film thickness

5.0 DEVELOPMENT
As the name implies, images that are invisible are revealed. The function of all
developers, regardless of type is to improve the perceptibility of penetrant indications.
Developers will achieve this by:
• Drawing out a sufficient amount of penetrant from the discontinuity to form an

indication
• Expanding the width of the indication enough to make it visible,
• Increasing the brightness of a fluorescent dye above its bulk brightness
• Increasing the film thickness of the indication to exceed the dye's thin film
threshold in order to make it detectable.
Some procedures do not require the use of a developer; however, a high

sensitivity penetrant is then used for an inspection where a lower sensitivity material
could be used if a developer were applied and requires UV-A levels of 3000 μW/cm2 .
Most industrial procedures do however require the use of developers and if correctly
applied have been shown to enhance a penetrant image by a factor of up to 600.
Thus it is sensible to have knowledge of why and how they work.
The basic mechanism of developer action is based on the following:
• Capillarity
• Light scattering
• Solvent action
5.1 CAPILLARITY
The most important mechanism for a colour contrast developer is capillarity. The
capillary attraction of the developer particles overcomes the opposing attraction of the
discontinuity and therefore increases the surface area of the indication.
This action spreads the penetrant laterally on the surface, thus widening the
indication. Capillary action expands the bulk dye into many thin films around the
developer particles to enhance its brightness. Capillary action also works vertically
through the developer to expand the thickness of the fluorescent dye.
The developer particles must be of a particular size and shape. Too large a size
will result in low capillary pressure and too small will cause the particles to block any
orifice.

Development TECHNOLOGY

5.2 LIGHT SCATTERING
Vitally important when using fluorescent penetrants is the light scattering effect.
Through this mechanism the brightness of an indication is significantly amplified per unit
area. Each particle of developer provides a bright scattering multiple reflector. The
developer particles reflect both the exciting UV-A and fluorescent radiation.
This mechanism plus the improved contrast in darkened conditions, gives the
marked improvement in sensitivity of fluorescent penetrant systems over colour contrast.
I0
If
If
If
If
Light Scattering
5.3 SOLVENT ACTION
This mechanism only applies to non-aqueous (solvent suspendible) developers

and dry and water-suspended developers have no capability for drawing the penetrant
out of the discontinuity except by capillary action. The developer should be sprayed
from a distance such that the developer particles are just damp when they strike the test
surface. The remaining solvent on the particle will bridge the gap between the developer
particles and the penetrant in the discontinuity. This is especially important with fine tight
defects where the developer particles do not necessarily contact the penetrant. The
solvent must not however, dilute the penetrant nor significantly reduce the brightness of
a fluorescent penetrant.
Solvents in the solvent-suspended (non-aqueous wet) developers and in the

plastic film developers are also solvents for the entrapped penetrant. The solvent
dissolves into the penetrant, lowers its viscosity, and expands its volume. This


process aids self-development in that the penetrant will flow back to the surface and
into the developer to form the indication by capillary action.
5.4 DEVELOPER PROPERTIES
The properties of a good developer used in penetrant testing are numerous. The
more important ones are listed below:
a) The material must be absorptive, to perform blotting action.
b) It must have a fine texture but not be too fine, as this may block imperfections
c) For colour contrast penetrants it must mask out background contours and colours
d) It must be easily and evenly applicable
e) It must form a light and even coat
f) There must be no fluorescing of the developer when used with fluorescent

penetrant
g) The penetrant bleeding from a discontinuity must easily wet the material
h) When used with a colour contrast penetrant it must obviously be of a highly

contrasting colour. The best colour seems agreed to be white
i) It must be readily removable after the test is completed
j) It must be non-toxic and non-irritant


6.0 PROCESS SEQUENCE
• Preparation and Pre-Cleaning
• Application of Penetrant
• Excess Penetrant Removal
• Application of Developer
• Inspection
• Recording
• Post Cleaning

Process Sequence TECHNOLOGY

Preparation and
Pre-Cleaning
To assure a valid penetrant inspection it is vital that any discontinuities
are free from contamination and open to the surface. The test specimen
Water Post-
surface should also be clean. Solvent
washable Emulsifiable Removable
Typical contaminants are:
a) Machine oils
Water b) Scale
Waterand slagWater Apply Apply
and c) wash flux spray
Welding lipophilic solvent
Solvent d) rinse
Corrosion preventatives emulsifier remover
e) Paint
f) Oxide films
g) Burnt oil Apply
Water
h) Carbon hydrophilic
wash
emulsifier
i) Corrosion products
j) Acid/Alkali
k) Water
Water
wash
The subject is vast and indeed, on some penetrant processes, it takes longer and
costs more to prepare a part than to test it. However, whichever cleaning
process is used, it must be Penetrant
Check compatible with the material being inspected.
removal
Broadly, cleaning methods can be broken into two, physical and chemical. BS
7773 Cleaning and Preparation of Metal Surfaces should be referred to when in
doubt.
Dry Apply water- Dry Apply solvent-
soluble based
developer developer
Apply water
Apply dry
suspendable
powder
Dry developer
developer
Inspect
Record
Clean and
Protect


6.1 PREPARATION AND PRE-CLEANING
Penetrant testing is capable of locating discontinuities open to the surface but

will fail to locate these discontinuities if:
• the penetrant is not able to wet the surface of the test object
• the penetrant is unable to enter a discontinuity due to a blockage
• the bleed out of the penetrant from a discontinuity is restricted
Shown below are a number of potential causes of such problems:
• Unable to wet the surface of the test object
Lubricating oils
Water and hydrates left after water evaporation
Polishing and buffing lubricants
• Penetrant is prevented from entering a discontinuity
Peening or smearing of the discontinuity

Carbon
Scale, rust, oxides and other corrosion products
Paint and protective coatings
Weld metals and flux residues
Anodising
Penetrant residues
• Penetrant bleed out is restricted
Carbon
Varnish
Scale, rust and other corrosion products
Strong acids and alkalis
Anodising


6.2 CLEANING METHODS
Listed below are some standard cleaning methods. Production rate, volume of
parts, and cost will dictate the cleaning method and equipment to be used.
Whichever method is used, care should be taken to ensure that it will not react with or
attack the base metal.
Mechanical Methods
Brushing
Blasting
Chemical Methods
Hot Solvent Degreasing
Vapour degreasing
Cold solvent degreasing
Alkaline cleaning
Acid pickling
Steam cleaning
Paint strippers
6.2.1 MECHANICAL METHODS
Physical methods can usually only remove contaminants from the surface and are
unable to clean out a flaw. Brushing and blasting are typical methods.
6.2.1.1 BRUSHING
Brushing can be extended to any scrubbing action. Usually a wire brush is used
to remove dry scale, flakes of paint, etc. A brush with soft bristles is recommended but
often such a brush will not lift stubborn dirt, so a harsh bristle brush ends up being used.
Then there is a danger that the scrubbing action will peen the lips of possible defects
and so prevent the penetrant entering.
If the danger of peening is suspected, a chemical etch process should also be

specified to follow the brushing.


6.2.1.2 BLASTING
Surface preparation by blasting is a vast subject in its own right. Grit blasting is
the most common method of physically moving debris from a surface. However, exotic
solid removers such as walnut shells and plum stones are sometimes used and the
remover may also be a liquid. Water under very high pressure is often a most efficient
pre-cleaner.
As with brushing, the danger of peening the mouth of a suspect defect exists, so
chemical etching is often specified as a follow-on procedure.
6.2.2 CHEMICAL METHODS
If possible it is preferable to use one of the chemical processes to remove

contaminants. The following are among the most common.
6.2.2.1 HOT SOLVING DE-GREASING
This is probably the most common way to process parts to be batch tested in
house. The parts are boiled in a solvent, usually trichloroethane 1.1.1 and the solvent is
prevented from evaporating by condensing tubes arranged above the solvent tank.
Often an ultrasonic transducer under the tank is fitted to vibrate the parts being cleaned
and thus aid the removal of solids and liquids. The liquid becomes dirty and corrosive
after a time. Therefore adequate filtration is essential as well as regular checks on water
content and acidity.
6.2.2.2 VAPOUR DE-GREASING
The part to be cleaned is suspended above a tank of boiling solvent, usually

trichloroethane 1.1.1. The solvent condenses on to the part and removes liquid
contaminants. Vapour de-greasers are usually used in conjunction with hot solvent
cleaners, as a second stage. The method works well on liquid contaminants but does not
dislodge solids. The solvent action continues until the temperature of the part reaches
the temperature of the vapour. Since the specimen is hot when removed, drying out
water in defects is not a problem.
6.2.2.3 COLD SOLVENT DE-GREASING
This is the most common method of pre-cleaning when a colour contrast system
is to be used, especially on site. It is usually sprayed or flushed on to the area to be
tested and then swabbed or wiped off. The method is not good at removing


contaminants from potential defects and can only clean surface solids, which are in
suspension in grease or oil.
6.2.2.4 SOLVENT MATERIALS WITH EMULSIFIERS AND

DETERGENTS
"Gunk" is a trade name, which comes to mind when discussing this type of
cleaner. The material is usually a mixture of white spirit or kerosene and emulsifying
agents. When brushed or sprayed on to grease and oil-coated parts, the cleaner thins
the contaminants. After a period of time the dilute residues can be washed off with water.
However, water contamination is a problem therefore drying action must be taken.
Detergent cleaners are becoming more common and these are usually diluted with water
to be used hot or cold. We will be discussing the difference between emulsifiers and
detergents during the penetrant remover section.
6.2.2.5 ALKALINE CLEANING
Alkaline de-greasing may be used hot or cold and has advantages over solvent
methods because it will remove soaps and salts. Very thorough washing is necessary
after cleaning to remove residues and probably drying after that. Alkaline cleaners are
not used on aluminium alloys.
6.2.2.6 ACID PICKLING
Acid pickling or alkali de-rusting solutions may be used to remove rust and scale.
After using these chemicals, it is usually necessary to apply a further cleaning method to
ensure that discontinuities are also cleaned. Neutralising, washing and drying may then
have to be carried out.
6.2.2.7 STEAM CLEANING
This process often works well when large areas have to be cleaned, but as with
any process with any process involving water, care must be taken to ensure the part is
dry before penetrant application.
6.2.2.8 PAINT REMOVAL
There are a number of proprietary paint removers available, all of which are
caustic. Residues will often kill the fluorescent material in a penetrant. Neutralising
procedures are necessary after the paint has been removed. Further cleaning must be
done to remove paint remover residues from discontinuities.

6.3 APPLICATION OF PENETRANT
Dipping, brushing, pouring, spraying, or essentially any other way to get

penetrant onto the surface of a test object is acceptable. The penetrant must be
allowed to remain in place, covering any surface discontinuities long enough for the
penetrant to enter the discontinuity. The penetrant dwell time can involve a period of
submersion and partial draining. This draining period during the penetrant dwell time
will enhance the sensitivity of the penetrant due to the evaporation of volatile
constituents when exposed to air in a thin film. This evaporation effectively increases
the concentration of the penetrant.
Two crucial factors to consider when applying penetrant are not how it is done but
what is the temperature of the component and how long will it remain on the component.
BS EN 571 states that the component temperature should fall between 10 and 50oC. In
special cases the temperatures down to 5oC can be used but for temperatures outside
this range penetrant product families specially approved for this purpose shall be used.
BS EN 571 specifies that penetration time can vary between 5 and 60 minutes.
The actual time depends upon the properties of the penetrant, the application
temperature, the material being tested and the defects being sought. Under no
circumstances shall the penetrant be allowed to dry during the penetration time.
Adequate penetration time can, when seeking transgranular and intergranular

corrosion cracks, be increased to hours and involve only the most sensitive (Group VI)
fluorescent penetrant. Dwelling overnight and processing the next day is a common
procedure when such discontinuities are suspected.
The method of penetrant application varies a great deal depending on

circumstances. The most usual method application for colour contrast penetrants is by
spraying from an aerosol. This is not essential and indeed can be quite messy.
Providing that contamination by foreign materials can be avoided it matters not how the
penetrant is applied when using portable kits on site. Thorough wetting of the surface of
interest is the main consideration.
For production line work, especially when using fluorescent penetrant materials,
application is normally by:
• Spraying
• Dipping and draining
• Thixotropic


6.3.1 SPRAYING
Spraying may be more applicable on large items, which are too large to go into a
tank, or where an uneconomic amount of penetrant would have to be kept in a large
tank. After spraying, drained material is recovered and re-used.
Penetrant is usually applied by spraying when spot or local inspections are

performed using penetrant kits. Brushing is a good method to apply penetrant to a
small local area, especially in hard-to-reach places. A small amount of penetrant can
be applied only on the area where it is needed, thus eliminating the need for cleaning
overspray. Compressed air spraying using paint spray equipment is still a prominent
means used throughout industry for applying penetrant, and particularly when
inspecting large parts.
6.3.2 AEROSOL SPRAY

Spraying an object, using material from an aerosol can is expensive, if the price of
the container and gas is considered. However, as the aerosol is sealed there is no
possibility of contamination of the penetrant while still in the can. It is a total loss system
and all excess penetrant is wasted. Aerosol spray is the most common method of
application, when local area checks are done with colour contrast penetrant
6.3.3 ELECTRO-STATIC SPRAY

Electrostatic spraying is being more widely used to apply penetrants, and
especially in automatic processors. This method of spraying applies very thin coats of
penetrant with minimal run-off. The penetrant is expended, but experience has shown
that this process is economical. The penetrant is used directly out of the shipping
container with no draining losses, contamination, or deterioration from sitting for long
periods in a tank.
Electro-static spray means that the object to be tested is positively electrically

charged with respect to the penetrant coming out of the spray gun. The penetrant is
sprayed in a thin even coating upon the object. The method is very good on large, plain,
simple shapes. It does not work well on intricate shapes nor does it penetrate holes in
such items as castings.
Electro-static spraying has a use on stainless turbine blades that have fine cooling
holes in them, often bored by electron beam methods. Curiously the penetrant does not
readily penetrate the cooling holes, but it does enter adjacent cracks.


6.3.4 DIP AND DRAIN
The traditional way of applying the penetrant in factory use is to immerse a

specimen in a tank of liquid. Once wetting is complete, the item is removed and drained
either over the tank or over a separate drain station. In this way the maximum amount of
material is recovered and used again.
Alternative methods that are useful in reducing mess and waste are thixotropic
penetrants and electrostatic sprays.
6.3.5 DIPPING HEATED PARTS
Dipping hot parts directly out of the vapour phase degreaser or those that have
been heated in an oven was quite common at one time. This procedure increased the
sensitivity by reducing the penetrant viscosity so that it would penetrate faster. The
viscosity of commercially available penetrant has been reduced since the late 1960s
so that it is now 1/2 to 1/3 of the previous viscosities. Thus, dipping of hot parts
provides less of an advantage now that penetrant is readily available with low
viscosity.
6.3.6 THIXOTROPIC
Under stable conditions thixotropic liquids increase in viscosity until they gel.
When brushed they become mobile (the text book definition being that their viscosity
reduces when a shear stress is applied). The usefulness of thixotropic penetrants is that
they can be applied by brush overhead, where spraying is difficult and might create a
mess.
6.4 REMOVAL OF EXCESS PENETRANT

After the penetrant has been in contact with the test surface for an acceptable
period, the excess material must be removed from that surface without affecting the
penetrant that has entered a defect.
This is a vital stage for the surface must not be cleaned excessively or defects will
be leached out. However, without proper removal of excess penetrant a high
background will remain, reducing the contrast between potential defects and the test
surface. The more common methods of penetrant removal with their BS EN 571
classifications are:
Method A Water
Method B Lipophilic emulsifier


Method C Solvent
Method D Hydrophilic remover and water
Method E Water and solvent
6.4.1 WATER AS A REMOVER
A water or air/water spray is used to remove water removable penetrants with

fluorescent penetrants being washed under UV-A light (BS EN 571 specifies that such a
lamp will have a minimum output of 300μW/cm2). The water should be used as a fairly
coarse, high volume, droplet spray with a pressure as low as possible and a temperature
below 50°. If air is used it should be maintained at a low pressure and just sufficient to
break up the flow of water into droplets. The wash time should be just enough to slightly
underwash the part.
The penetrant material contains its own emulsifier and is therefore removable by
water. There is, however, a big disadvantage. Water will cause the penetrant and
emulsifier to be washed out of wide shallow defects. Water washable penetrants are
most commonly used on rough parts such as castings or forgings as a rough surface is a
series of wide shallow crevices.
By lowering the amount of emulsifier in the liquid, finer and tighter defects can be
found. This is at a price, for as the sensitivity goes up the washability goes down. So
super sensitive water removable penetrants will not wash from anything but smooth
surfaces.
6.4.2 LIPOPHILIC REMOVER (EMULSIFIER)
A lipophilic material is an emulsifier, which mixes with oil and makes the whole
miscible with water. The normal method of application is immersion followed by
draining but they can also be applied by dipping, pouring, air spraying and
electrostatic spray. Brushing is not recommended since it leaves an uneven
application and it mixes the emulsifier, the penetrant, making control over the
emulsification time impossible.
This method has the advantage of enabling wide shallow discontinuities to be

detected because only penetrant oil enters the discontinuity and washing after
emulsification should only remove the surplus penetrant from the surface. This
method is used on high stress critical parts, one draw back however is that emulsifier
contact time is absolutely critical.
Most references quote a maximum of three minutes for emulsifier contact time
and if exceeded the emulsifier will begin to attack the penetrant in flaws, especially the

wide shallow defects. If a number of specimens are tested in a batch at the same time,
then all of them must have exactly the same emulsification and wash time or results will
vary. Washing with water takes place after the emulsification phase under the same
conditions detailed in paragraph.
Lipophilic emulsifiers are used neat and are often viscous. This results in high
dragout making it expensive to use. Also when large amounts are being flushed down a
drain, pollution control measures may have to be taken. Hydrophilic removers have
generally therefore superseded lipophilic emulsifiers.

The ideal procedure involves using a dry rag to mop up surplus penetrant
followed by a lint free rag or soft paper towel moistened with liquid solvent and then
wiped over the test area. When removing colour contrast penetrant wiping should
cease when the rag is lightly tinged with dye to ensure that the specimen is not over
cleaned.
When solvent removers are used to clean off excess fluorescent penetrant,
removal should again be conducted under ultra violet light (UV-A) to ensure that the
background has to be reduced to an acceptable level. Irrespective of the type of
penetrant to be removed neat high-pressure solvent must never be sprayed directly at
an area of inspection interest as it could enter the surface discontinuity and wash out
the entrapped penetrant.
6.4.4 HYDROPHILIC REMOVER (DETERGENT)

A hydrophilic remover is a detergent which, when mixed with water in a tank, has
the ability to break down the surface tension of penetrant in contact with a test surface
and lift or scrub the penetrant from that surface.
Concentrations of emulsifier can range from 5% to 33% in water, the proportion

of detergent to water being varied to allow greater control of washing. Experiments have
shown that a 5% concentration produces the greatest sensitivity with a continuing
decrease in sensitivity up to about 30% concentration. Penetrant tolerance of the
emulsifier mix is lower at 5% than at 20% and the tank life is shorter even though the
sensitivity is somewhat less.
As the remover is diluted, the test parts are given a pre-wash with a spray to
remove excess penetrant. This extends the life of the remover and means that there is
only a thin film of remover left. In some cases, where an exclusive drain tank is used the
pre-wash allows the penetrant to be separated, recovered and perhaps re-used.
The hydrophilic remover is usually applied by immersion, although a spray of high

dilution remover is sometimes preferred. Remover time is usually a maximum of three
minutes, although there is much more flexibility than with a lipophilic emulsifier process.

Washing with water takes place after the remover stage under the same conditions
previously.
Even though the penetrant used may be called post emulsifiable, hydrophilic
removers are by far the most commonly used. It is ultimately the most sensitive system,
although it takes longer. It combines flexibility with efficiency, but it is sometimes difficult
to remove the penetrant from crevices and the roots of fine threads.
6.4.5 SPRAY-SCRUBBER PENETRANT REMOVAL

Hydrophilic emulsifiers can be used to remove the penetrant by mixing the
emulsifier with the rinse water at the spray nozzle. The spray used is a mix of
approximately 250 parts of water to 1 part emulsifier. As the emulsifier releases the oil
from the surface by detergent action, the water spray scrubs the released oil away.
The spray remover serves to scrub layer after layer of excess surface penetrant from
the part, continually introducing a fresh detergent/water solution. Spray time depends
on the surface roughness, penetrant viscosity, penetrant sensitivity level, complexity
and size of the part, and water temperature.
The processing procedure is the same as for using the dip tank procedure
except that the emulsifier is applied by spraying the emulsifier/water mixture until the
surplus penetrant is removed. This is an ideal application on large parts or in an area
that does not have large tanks. The pre-rinse, spray emulsification, and final rinse can
be accomplished in a single spray booth equipped with a water spray nozzle and an
emulsifier-mixing nozzle.

There are some very specialised penetrant applications where a solvent dip is
used prior to emulsification. Usually, the solvent dip is used on rough surfaces, such
as turbine blades in an as cast condition or with a diffusion coating, or on aluminium
parts that have been anodised. A tremendous amount of background coloration
occurs in these applications that cannot be removed by emulsification. The procedure
normally involves a quick dip in aliphatic kerosene, then a dip into the emulsifier tank.
The procedures must be worked out experimentally for each part.
In some instances, solvent dip followed by a hydrophilic emulsifier in the spray

scrubber mode produce better results than an emulsifier dip procedure.
6.5 DRYING
Drying the test specimen or test surface is an important intermediate stage after
penetrant removal. If the solvent removal method is used, the part being inspected will
dry quickly as the solvent remover evaporates.


Where water has been used during the removal stage drying can be critical.
The essence of drying is that the parts must be dried quickly so that excess penetrant
bleed out does not take place. At the same time the part must not get too hot, 50°c is
suggested as a maximum temperature, or penetrant will evaporate out of defects.
Drying after removal of the penetrant depends on the removal method and
developer used. Drying after solvent removal is usually by air drying alone whereas
after water removal heat is required to drive off the water. At the same time this will
expand the penetrant in any discontinuities and reduce its viscosity to provide better
self-development. Heat is also essential when water-suspended developer and water-
soluble developers are used since the water must be evaporated from the developer.
Self-development from the heat is essential for water-based developers, as they can
only provide capillary action.
Dryers can be heated by gas, electricity, or steam. It is essential that a fan

circulates the air within the dryer or else heat stagnation can build up in the cabinet
near the heater. Moving air will break up this stagnation condition and reduce the
drying time. Re-circulating the air over the heating system is more economical than
continuously heating cold air. Re-circulating hot air dryers are almost always specified.
6.5.1 HOT AIR RE-CURCULATING OVEN
Specimens to be dried are placed in an oven in which the air is moved by a fan.
This helps the part to dry evenly. The maximum temperature of the air may be as high as
85°c to promote rapid drying, but as has been said, the specimen must not get that hot.
10 minutes is the maximum drying time. If drying cannot be accomplished in that time an
improved oven is recommended.
6.5.2 FORCED WARM AIR
Forced warm air means a large volume of dry heated air enveloping unusually
large specimens. For instance large castings which cannot be put into an oven. Hair
dryers are seldom recommended as they take too long to dry all but the very smallest
specimens.
6.5.3 DRY CLEAN COMPRESSED AIR

Compressed air can often be used to advantage before hot air drying, to blow
water droplets from holes and crannies. If a fine nozzle is used, the pressure should be
less than 1 bar and the distance should be more than 300mm.
If a large volume of compressed air is available, the recommended pressure is

not greater than 2 bar.


6.6 DEVELOPMENT
The five classifications of developer listed in BS EN 571 are:
• Form a Dry
• Form b Water soluble
• Form c Water suspendable
• Form d Solvent based
• Form e Peelable
As the name implies, images that are invisible are revealed and the function of
all developers, regardless of type, is to improve the perceptibility of penetrant
indications. A correctly used developer has been shown to enhance a penetrant
image by a factor of up to 600.
The application of developer in visible penetrant applications should be in a

uniform coat obtained by a series of light passes with the spray with only enough
developer as necessary to provide a thin white background being applied. A heavy
coat will mask fine indications and some practice is needed before uniform coats can
be applied. The can or gun should be held about 12 inches from the surface; the
second coat should be applied to the light areas between the passes of the first coat.
Sometimes, spraying the second coat across the direction of the first coat is good
practice.
Developer sprayed over fluorescent penetrant must be carefully done to

provide only a very light coat as the surface of the component should not be totally
obscured. One coating is usually sufficient, as the developer is not required to provide
a contrasting background, as is the case with colour contrast penetrants. If a second
coat is applied, only a touch-up of the light spots is needed.
6.6.1 DRY DEVELOPER
Dry developers can be applied by dipping the part in a bin of developer, fogging
the part with a gun or in a chamber, or by electrostatic spray. They are only
recommended for use with fluorescent penetrants and are best used on parts that
have been heated.


6.6.2 DRY POWDER DEVELOPER APPLICATION
Dry powder developer is applied in a number of ways:
a) Dust storm cabinet.
This is the most common method where the part or parts are sealed in a cabinet,
which is probably warmed to keep the powder dry. The powder is agitated and the
resulting dust settles on the test areas.
Upon removal, the excess powder is either shaken off gently or blown off with a
very light air stream.
b) Electrostatic or flock gun spray
Both methods rely on electrically charged particles of powder being attracted to

the penetrant exuding from a discontinuity. Spraying is more appropriate to automatic
processes and large plane surfaces. Extraction at the developer station is essential.
c) Insufflator.
This method simply relies on puffing the powder on the surface from a rubber or
plastic container. It is somewhat wasteful if applied carelessly. Extraction is advised and
this method of application is really only suitable for local areas or small parts.
d) Fluidised bed.
Fluidisation is a technique in which a mass of solid particles, i.e. dry powder

developer, is brought into a state of suspension by an upward stream of gas blown
through it. It is usually only used in large automatic systems, where large numbers of
parts are passed through the bed. The station must be sealed from other draughts or the
system will fail.
6.6.3 AQUEOUS LIQUID DEVELOPER
Water suspendible and water-soluble developers can be sprayed or flowed on.

However, in the vast majority of cases the application is by dip and drain. Dip time is
usually no more than 30 seconds. The suspendible type must be thoroughly agitated
before the parts are immersed.
When aqueous developers are used they are applied before drying and
therefore development takes place during the drying phase. Thus there can be a
significant saving of processing time.


6.6.4 WATER-SUSPENDED DEVELOPER (WET DEVELOPER)
The water-suspended developers can be applied by dipping or spraying.

References and previous data given provide considerable detail on this method.
6.6.5 WATER – SOLUBLE DEVELOPER
Water-soluble developers become a solution with water. This type of developer

can be applied by dipping or spraying. Due to its water base, it is not recommended
for use with water-washable penetrants. The appropriate mixtures recommended are
available from the suppliers.
6.6.6 SOLVENT BASED DEVELOPER
Solvent suspendible developers are invariably sprayed either from aerosols or

paint type sprayers. The developer must be thoroughly agitated, as there is a
tendency for the particles to agglomerate. It is wise therefore especially with aerosols,
to test the spray quality before applying to the test surface.
The developer must be sprayed from a distance exceeding 300mm so that it

falls on the area of interest in an almost dry thin even coat. If used with colour contrast
penetrant the developer coat should almost obscure the background surface. When
applied as part of a fluorescent system, the coating should be only just discernible in
daylight conditions.
6.6.7 PLASTIC FILM DEVELOPER
Plastic film developers are supplied in a spray can because of their high
volatility. The developers used with visible penetrants are pigmented to provide a
white background; those used with fluorescent penetrants are clear. Two spray coats
are usually applied for developing. If the developer is to be stripped off for recording,
at least three more coats of clear lacquer should be applied over the area to be
stripped.
6.7 INSPECTION
Parts should be inspected initially as soon as the developer is applied. This is

particularly so when using colour contrast systems and a solvent suspendible developer.
The penetrant can sometimes bleed quickly from a crevice or a hole adjacent to a
discontinuity and thus mask it. It is difficult to inspect specimens that are placed in a dust
storm cabinet very soon after application and impractical on water based developers.
As a principle, development time should vary from 0-30 minutes with some authorities
suggesting that development times in excess of 10 minutes are excessive. A general

view is that a maximum of 20 to 30 minutes satisfies the requirements and that
inspection should be carried out throughout and at the end of the development time.
Inspection is a critical part of the penetrant process but no more so than the
processing, because improper processing will prevent indications being seen at the
proper sensitivity level and thus the inspector cannot detect them.
The personnel are the most critical and important elements of the inspection of
penetrant indications. Acceptance or rejection of the part is based on the inspector's
judgement. The inspector must have very good near vision acuity and be capable of
colour discrimination. Eyesight should be checked periodically, and at least once a
year. Inspection is monotonous and tiresome; physically, visually, and mentally. A
good procedure is for the inspector to alternate between processing and inspection.
Inspectors should be properly motivated to realise the importance of their work.

Their job should have the same stature, respect, and compensation as other
inspection classifications within the same facility.
6.7.1 FLUORESCENT PENETRANT
The room or area where fluorescent penetrant inspection is to take place must
be darkened to below 20-lux visible light. Background UV-A strip lights or low output
amber lights may be used in order that the inspectors can just see their way round.
Actual inspection should take place under UV-A mercury vapour light
conditions. For penetrant inspection the minimum level of UV-A light at the test
surface must be 1.0mW/cm2 (1000μW/cm2).
Before inspecting the person viewing should wait in the viewing area for a
minimum of 5 minutes (10 in USA) to allow their eyes to adapt to the low light levels.
Photochromatic spectacles must not be worn although the sodium lens type may
reduce eyestrain.
From cold, mercury vapour UV-A lamps take a minimum of 15 minutes to reach
full output intensity. Once switched on they should be left on throughout the working
day as constant switching not only wastes time but also reduces lamp life. Inspectors
should not view continuously for more than approximately 30 minutes.
Colour contrast penetrants should be viewed in bright white light conditions. A

minimum of 500 lux is recommended in accordance with BS EN 571-1.


6.7.3 INSPECTION AIDS
Aids to correct inspection should not be spurned but at the same time they
should be treated with caution. Hand magnifiers are useful; especially to decide
between scratches, scores and cracks but they should not be too powerful, x8
magnification being about the maximum.
Illuminated magnifiers, having either miniature UV-A lamps or P bulbs, as

appropriate, are also useful. For restricted access, particularly when using penetrants
as leak detectors, cold light endoscopes might be considered. These can now be
bought with a UV-A light box.
6.8 RECORDING
BS EN 571 lists the following methods for recording indications:
• Written description
• Sketch
• Adhesive tape
• Peelable developer
• Photography
• Photocopy
• Video
6.9 POST CLEANING

It is often unnecessary to clean residues from test material. However, in some
cases, such as when a high quality paint surface is to be applied, it is vital to remove
penetrant and developer residues.
To remove colour contrast residues it is best to first apply a thick wet coating of
non-aqueous liquid developer and when it is dry brush the surface clean with a soft
bristle brush. Finally the part can be dipped in or sluiced over with a solvent
cleaner/remover.
If dealing with intricate parts, it is often necessary to scrub them in a warm

water detergent mixture, to remove developer residues.


Specimens tested by fluorescent method using dry powder developer are often
just subjected to a dry air blast. This may be followed by cleaning with cold solvent or
in a hot liquid/vapour degreaser. It is worth a reminder that solids are difficult to
remove with liquid or vapour degreasers.
Post-cleaning is usually not necessary if dry developer has been used. Wet
developer and solvent- suspended developer need to be cleaned; a water scrub spray
will usually suffice. A degreaser or spray solvent can be used in the field. It is
desirable that the developer be removed as soon as possible following inspection, as
there are some types of developer that become more difficult to remove as time
passes. The most difficult developer can usually be scrubbed off with a brush and
detergent. Post-cleaning for LOX-use parts requires special procedures, usually
specified by the component manufacturer.
6.10 PROTECTION
When a penetrant inspection is completed the test surface is invariably vulnerable

to outside contamination. Indeed, many test items can be of high value. Protection, even
with a light de-watering oil is a wise precaution. The appropriate protective treatment is
not within the scope of these notes. Suffice to say it must be compatible with the
material, which is applied.


Penetrant systems of families are classified according to:
• Type of penetrant
• Method of penetrant removal
• Type of developer
7.1 TYPES OF PENETRANT

According to BS EN 571-1 there are three main types of penetrant:
Type I Fluorescent
Type II Colour contrast
Type III Dual (Combined colour contrast and fluorescent)
Colour contrast penetrants are usually dyed red and are mainly used to look for
defects when there is adequate day or artificial light or where no power source is
available to enable use of an ultra-violet lamp. Colour contrast penetrants are used with
spray developers that leave a white background when dry, so the inspector sees a red
against white contrast.
Colour contrast penetrants are mainly designed for use with either solvent or
water removable systems although Mil-L-25135 C does call for post emulsifiable visible
penetrant. The solvent removable form is generally used on site or where local areas are
to be tested on construction joints and welds. Water washable colour contrast penetrants
are mainly used on rough castings although they are used on undressed welds, where a
rag moistened with is used to remove excess.
7.1.2 FLUORESCENT PENETRANTS
Fluorescent penetrants utilise the ability of certain materials to absorb

electromagnetic energy of one wavelength and in response emit light at a different
wavelength. Molecules within fluorescent penetrants absorb ultraviolet light, become
energetic and then shed their excess energy by emitting yellow green visible light.

System Classification TECHNOLOGY

Indications are viewed under darkened conditions with the operator thus viewing
bright indications against a dark background. They are mainly used in factories, on
castings, forgings, precision parts, aluminium alloy, stainless steels and so on.
Fluorescent penetrants are more sensitive than colour contrast as the indications
produced are 10 times more "seeable".
7.1.3 FLUORESCENT V COLOUR CONTRAST
- Fluorescent more sensitive

- Less operator fatigue with fluorescent
- More difficulty in monitoring fluorescent penetrant removal
7.1.4 DUAL PENETRANTS
Dual penetrants are combined colour contrast and fluorescent penetrants. These
materials are something of a compromise and tend to work best in bright sunlight. They
fluoresce at the red/orange wavelengths (650nm) rather than in the green/yellow band.
Absorbs Dual
10 100 200 300 400 500 600 700

ULTRAVIOLET VISIBLE
LIGHT LIGHT
Dual Penetrant


7.2 EXCESS PENETRANT REMOVAL
The method of penetrant removal influences the contents of the penetrant itself
and although a number of different methods of removal are used, the materials fall
into 2 basic types:
• Penetrant chemicals composed of a hydrocarbon and a dye (red or fluorescent) in

solution. These can be either post-emulsifiable penetrants or solvent removable.
• Penetrant chemicals composed of a hydrocarbon, a dye (red or fluorescent) and

an emulsifier. These are called water washable or self-emulsifying penetrants.
• Despite there being only 2 basic types of penetrant five methods of penetrant
removal are listed in BS EN 571 part 1, these are:
Method A Water
Method C Solvent
7.2.1 WATER WASHABLE
Water washable penetrants contain an emulsifier and are therefore removable by

water. There is, however, a big disadvantage as water can cause the penetrant and
emulsifier to be washed out of wide shallow defects. Water washable penetrants are
most commonly used on rough parts such as castings or forgings as a rough surface is a
series of wide shallow crevices.
By lowering the amount of emulsifier in the liquid, finer and tighter defects can be
found. This is at a price, for as the sensitivity goes up the washability goes down. So
super sensitive water removable penetrants will not wash from anything but smooth
surfaces. The removal of fluorescent penetrants should be carried out under UV-A light.
Advantages Disadvantages
Useable on rough surfaces Susceptible to over

washing
Suitable for batch inspection Least sensitive method
Cheaper than other methods Requirement for a water source


A lint free rag or soft paper towel is moistened with liquid solvent and then wiped
over the test area. Before this stage a dry rag can be used to mop up surplus penetrant.
If a colour contrast penetrant is being removed then wiping should cease when
the rag is lightly tinged with dye. The specimen should not be overcleaned.
When solvent removers are used to clean off excess fluorescent penetrant,
removal should be conducted under ultra violet light (UV-A) to ensure that the
background has to be reduced to an acceptable level. Neat, high-pressure solvent must
never be sprayed directly at an area of inspection interest.
Portability Not suited to batch inspections
No water supply required Requires hand wiping therefore time

consuming
More expensive than water washable
Potentially hazardous chemicals
Solvents used for the purpose of penetrant removal present a number of

potential problems. Firstly the compatibility of the remover and the component under
test must be considered. If in doubt guidance can be found in BS 7773, the Cleaning
and Preparation of Metal Surfaces.
Secondly, hydrocarbon solvents and many paint removers are highly flammable
and should not be used near open flames. Chlorinated solvents have high flashpoints,
but will add fuel to a general fire that exceeds their ignition points.
Thirdly, solvents will remove natural oils from the skin. Rubber or plastic gloves
should be used when the hands are to be exposed to solvents for any length of time.
7.2.3 POST EMULSIFIABLE

Penetrants, apart from the self-emulsifying ones used in water washable
inspections, are generally oil based and not soluble with water. Since water is the
most plentiful and the least expensive type of solvent available the need then is for
some chemical that is soluble both in water and in oil that can render them water
washable. The chemical utilised is referred to as an emulsifier and can be either oil
based or water diluted. The former are referred to as lipophilic and the latter
hydrophilic. Both will mix with oil and makes the whole miscible with water and the
normal method of application is immersion followed by draining (brushing being


excluded as it makes it difficult to control the diffusion rate of the emulsifier into the
penetrant).
This method has the advantage of enabling wide shallow discontinuities to be

detected because only penetrant oil enters the discontinuity and washing after emulsifier
application should only remove the surplus penetrant from the surface. This method is
used on high stress critical parts. However, emulsifier contact time is absolutely critical.
Most references quote a maximum of three minutes for emulsifier contact time. If
the maximum time is exceeded then the emulsifier will attack the penetrant in flaws,
especially the wide shallow defects. If a number of specimens are tested in a batch at
the same time, then all of them must have exactly the same emulsification and wash
time or results will vary
Lipophilic emulsifiers are used neat and are often viscous resulting in high drag-
out and therefore making it expensive to use. Also when large amounts are being
flushed down a drain, pollution control measures may have to be taken. Lipophilic
emulsifiers have therefore, generally been superseded by hydrophilic removers.
In addition to the emulsifying action other desirable properties are a colour

which contrasts with that of the penetrant to enable us to ensure that all the surface of
the test item has been covered by the emulsifier. The dye of the emulsifier is also
fluorescent so that, when washing under black light, the complete removal of the
emulsifier can be verified and water soluble so that the dye will not be left on the
surface after the part has been washed.
7.2.4 HYDROPHILIC REMOVER (WATER-DILUTABLE)
Hydrophilic emulsifiers used in penetrant testing are essentially surface-active

agents (surfactants) or detergents and the word "hydrophilic" means water-loving or
water-soluble. Such emulsifiers are supplied as a concentrate and are mixed with tap
water to the desired dilution.
Hydrophilic removers have the ability to break down the surface tension of
penetrant in contact with a test surface and lift or scrub the penetrant from that surface.
In practice the proportion of detergent to water may be varied to allow greater control of
washing. The change in concentration varies the activity of the emulsifier and the rate at
which it acts. In practice sensitivity tests with specimens of the type to be inspected
should be made to determine the optimum level. Low concentration levels near to 5%
provide the best sensitivity but can be expected to leave more background than a 20%
concentration on rough surfaces.
Hydrophilic removers are infinitely water tolerant a fact which enables parts to be
given a pre-wash with a spray to remove excess penetrant. This extends the life of the
remover and means that there is only a thin film of remover left. In some cases, where
an exclusive drain tank is used the pre-wash allows the penetrant to be separated,
recovered and perhaps re-used.

The hydrophilic remover is usually applied by immersion, although a spray of high
dilution remover is sometimes preferred. Remover time is usually a maximum of three
minutes, although there is much more flexibility than with a lipophilic emulsifier process.
Washing with water takes place after the remover stage under the same conditions
detailed for water washable systems.
Maximum penetrating ability Not suited to rough surface
Greater control over penetrant removal More expensive
Ability to locate wide shallow defects More time consuming
Immersion time not as critical

as for lipophilic
Even though the penetrant used may be called post emulsifiable, hydrophilic
removers are by far the most commonly used. It is ultimately the most sensitive
system, although it takes longer. It combines flexibility with efficiency, but it is
sometimes difficult to remove the penetrant from crevices and the roots of fine
threads.
A further advantage in the use of a hydrophilic emulsifier is that the parts may
remain in the emulsifier tank from 5 to 20 minutes. This wide range of dwell times
provides less dependency on accurate time control.
Initial cost of the hydrophilic concentrate is about the same as that of the
lipophilic emulsifiers but the high dilution with water provides a substantial cost saving.
Experience shows that the tank life of the two types of emulsifier is about the same,
depending on the concentration. Less dilution provides somewhat greater tank life for
hydrophilic. Operating costs can be lower with hydrophilic emulsifiers because of
differences in the processing. The hydrophilic removers are highly water tolerant; this
allows for a pre-rinse of water to remove up to 80% of the surface penetrant before
emulsification. The pre-rinse thus eliminates much of the penetrant contamination of
the emulsifier. The pre-rinse water can be collected and separated to salvage most of
the penetrant. The lower viscosity of the hydrophilic emulsifier will drain more rapidly
off the parts, resulting in less emulsifier drag-out than for the more viscous lipophilic
emulsifiers. Diluted hydrophilic emulsifiers are non-flammable and relatively non-toxic.
Lipophilic emulsifiers have a high flash point, but they will add considerably to a
general fire if ignited.


Excess water washable penetrant can be removed by first washing with water
and then subsequently by wiping with solvent dampened cloths.
7.3 TYPES OF DEVELOPER
Developers may be divided into five types:
Form a Dry powder
Form b Water-soluble
Form c Water suspendable
Form d Solvent based (non-aqueous wet)
Form e Water or solvent based for special application
7.3.1 DRY POWDER DEVELOPER
Dry powder developer is almost exclusively used in fluorescent systems in the

U.K. It is a light, fluffy, hygroscopic, highly absorbent powder that will cling to dry
metallic surfaces. It will cling even better to wet surfaces and therefore will generally
stick best to the bleed out from discontinuities. Dry developer is hardly visible in white
light so leaves a contrast of bright fluorescence against a dark background in UV-A
conditions.
By its nature it absorbs moisture so if inhaled or swallowed in significant

amounts, will dry out nasal passages and the throat. It is inconvenient rather than
harmful but, nonetheless, it is important that care should be exercised in use.
Easy to handle Difficult to see if properly applied
No hazardous vapours Fine powders can be hazardous
Easy to remove Do not offer high degree of colour contrast


7.3.2 AQUEOUS DEVELOPER
Aqueous liquid developers can be sub-divided into suspendable and soluble.

The water suspendable type is bought as a white powder and then mixed in the
correct proportions. A much more dilute mixture is used when it is to be used in a
fluorescent system. It must be constantly agitated to keep it in even suspension.
Water-soluble developer is quite common in the USA but has found little favour in
Europe. It is sold in white fine granular form and is, in fact, water-soluble salts, which
dissolve when mixed with water to make a straw coloured liquid. When the test
specimen is wetted with the developer and dried, the surface hardly appears affected in
normal light. This type of developer is therefore restricted to fluorescent systems.
No vapours or dust Difficult to apply evenly
Cheaper than non-aqueous Require drying after application
7.3.3 SOLVENT BASED
This is often called non-aqueous developer and is a suspension of inert white

powders in a volatile solvent. The most common solvent is trichloroethane (1.1.1.),
although if sulphur or halogen free materials are specified flammable acetone or
naptha based materials are usually used. This is the most common type of developer
used with colour contrast penetrant. It leaves a fine white background through which
red indications are readily visible.
Most sensitive Hazardous solvents
Useable with fluorescent or Need to be correctly

colour contrast applied
Higher cost
7.3.4 WATER OR SOLVENT BASED FOR SPECIAL

APPLICATION
When an indication needs to be recorded a peelable developer can be applied

which can be carefully removed once development has occurred.


Throughout this text the classifications for penetrant chemicals contained in BS

EN 571 have been highlighted. These classifications are repeated below along with
those outlined in some USA.
7.4.1 BS EN 571
Penetrant
Type I Fluorescent
Removal
Method A Water
Method C Solvent
Developer
Form a Dry powder
Form cWater suspendable
Form d Solvent based (non-aqueous wet)
Form e Water or solvent based for special application
7.4.2 MIL-L-25135 C
Group I Solvent removed, visible, dry, wet or non-aqueous developer
Group II Post emulsifiable, visible, dry, wet or non-aqueous developer
Group III Water washable, visible, dry, wet or non-aqueous developer
Group IV Water washable, fluorescent, dry, wet or non-aqueous developer
Group V Medium sensitivity, post-emulsifiable fluorescent, wet or non-

aqueous developer
Group VI A High sensitivity, post emulsifiable (hydrophilic), fluorescent, wet,

dry or non-aqueous developer


Group VI B Ultra-high sensitivity, post emulsifiable, fluorescent, wet, dry or
non-aqueous developer
Group VII High sensitivity, solvent removable, fluorescent, non-aqueous

developer
7.4.3 MIL-I-25135 E
Penetrant
Type I Fluorescent
Removal
Method A Water washable
Method B Post emulsifiable, lipophilic
Method C Solvent-removable
Method D Post emulsifiable, hydrophilic
Developer
Form a Dry powder
Form c Water suspended
Form d Non-aqueous
Form e Special application
Sensitivity
Level 1/2 ultra low
Level 1 low
Level 2 normal
Level 3 high
Level 4 ultra-high


8.0 CHOICE OF PENETRANT SYSTEM
The factors influencing the choice of which system to use are:
• Size and type of defect

• Geometry and intricacy
• Surface condition
8.1 SIZE AND TYPE OF DEFECT

From the previous text it has been mentioned that wide shallow defects
(comparing the depth to the width of the opening to the surface) are most likely to be
detected by the post-emulsifiable method. Likewise it has also be mentioned that fine
defects are best located by fluorescent methods due to the higher sensitivity of the
eye to fluorescent rather than colour contrast indications. When seeking very fine wide
shallow defects the fluorescent post emulsifiable technique would be the
recommended system.
8.2 GEOMETRY AND INTRICACY
Highly intricate components containing a large number of changes in section

and those with threaded areas present problems in removing excess penetrant. Post
emulsifiable methods whilst more sensitive than water washable would in all likelihood
leave behind excessive background coloration on such samples. This would thus
reduce the detection of defects.
8.3 SURFACE CONDITION
Likewise rough surface are difficult to fully clean when tested with post
emulsifiable methods, and components such as sand castings would be best tested
by a water washable method.
Fluorescent methods are also less suited to the testing of rough components
than visible methods (colour contrast) due to problems in adequately monitoring
penetrant removal.
8.4 OTHER FACTORS TO BE CONSIDERED
• Component material
Solvent removable methods may lead to surface damage due to incompatibility

between the penetrant and the material under test. The now withdrawn British
Choice of Penetrant System TECHNOLOGY

Standard 6443 contained a procedure for determining compatibility involving the
partial immersion of components of the same composition for periods of 16 hours
in the case of penetrants and developers and 30mins for cleaners and removers.
• Size and position of the item to be tested
On-site welds are unlikely to be tested by fluorescent and water washable

methods due to the requirement of darkened conditions in the former and a water
source in the latter case.
• Equipment and expertise available
Fluorescent water washable and post emulsifiable test methods generally involve
the use of flow lines and are thus more suited to factory use than on site testing.
• Cost
Water washable penetrant methods are obviously much cheaper than solvent
removable and post emulsifiable methods due to the availability of the main
cleaning fluid.
• Number of components to be tested
Fluorescent methods are recommended for batch inspections due to the higher
sensitivity of the eye to fluorescent indications over visible colour contrast ones.
This offers greater sensitivity and reduces the fatigue suffered by the operator
performing the inspections. Operators are thus able to perform at a higher level
for longer periods.
8.5 EXAMPLES
Based upon the criteria listed shown below are a couple of examples showing
what test system would be selected and the reasoning behind the selection
Example 1: Inspection of a large number of threaded components
Batch inspection of components with complex geometry.
Method: Fluorescent water washable with dry powder developer
• Fluorescent for mass inspections

• Water washable more suited than solvents to batch inspections
• Post-emulsifiable difficult to remove from threads


Example 2: Inspection of turbine blades for fatigue cracks
High sensitivity required for a low number of components.
Method: Fluorescent post-emulsifiable with non-aqueous developer
• Fluorescent more sensitive than colour contrast systems

• Post emulsifiable more sensitive than water washable
• Non-aqueous developer most sensitive form of developer


9.0 EQUIPMENT CHECKS
There are few checks on the penetrant materials usually used on site, where the
colour contrast solvent remover system is being used, dispensed from aerosols.
However, where line processes are in use, controls must be undertaken to ensure that
overall system efficiency is maintained. The checks described in these notes are
considered to be the minimum needed and are in their simplest form. This is so that they
can be easily and economically applied in the average workplace.
When material containers are re-filled a sample of each of the materials should be
taken and stored away from heat and light in clean glass containers. The containers
should be labelled with their identity, batch number and date of filling. These are utilised
as controls to check the chemicals in use. These checks can be divided into overall
system performance checks which, as the name suggests monitor the entire penetrant
system and control checks monitoring individual parts of the process such as UV-A light
levels.
9.1 OVERALL SYSTEM PERFORMANCE

This is a check normally done daily or at the start of each shift involving a
specially prepared test piece processed through each system and compared with
previous checks. Any change in results signifies a change in the chemicals being
utilised.
These checks typically employ either a quenched aluminium alloy block or

chrome plated stainless steel panels. The former of these, while quite useful for
comparing the performance of two penetrants, tend to deteriorate quite rapidly after a
number of checks. Typically their use will involve processing one half of the block with
fresh chemicals stored away from light and extremes of temperature and the other half
with penetrant chemicals in daily use. Any difference in the appearance of the two
halves signifies a problem with one or all of the chemicals in use.
The most common type of test piece is a chrome plated stainless steel panel
often referred to as a TAM panel. A number of star shaped artificial cracks of
increasing severity are induced by pressing a ball into the obverse side. Different
penetrant systems will reveal different numbers of cracks depending upon the
sensitivity level. A reduction in the number of cracks revealed will indicate a change in
the penetrant chemicals. A comparitor is sometimes used, made as a mirror image to
the panel, from a strippable transfer lacquer. One half of the block is often a grit
blasted panel used to check washability and remover efficiency.
It is essential that test pieces of any kind are thoroughly cleaned and protected
after use. Cleaning in a hot liquid and vapour degreaser is usual. Protection is
normally effected by immersion in clean solvent before next use the test piece must
be thoroughly dried.

Equipment Checks TECHNOLOGY

9.2 CONTROL CHECKS
9.2.1 WATER WASH TEMPERATURE AND PRESSURE
If water washing is being used, the temperature of the water should be

maintained at a constant between 5°c and 50°c. The pressures of the water and air
must be as low as possible. These figures conform to BS EN 571 and may vary
between specifications. A daily check is recommended.
9.2.2 COLOUR INTENSITY
Every week the colour intensity of the penetrant material should be checked.
For colour contrast penetrants a 5% solution of used penetrant mixed with
dichloromethane should be compared against a lighted white background with a
similar solution of new check material. Any significant difference in colour should be
investigated.
If the penetrant is fluorescent the brightness of a 55mm x 55mm filter paper

soaked in 5% solution of used penetrant and dichloromethane is checked in the UV
light monitor. The filter paper is dried and placed on the white light sensor of the
monitor. An UV-A lamp, placed at a fixed distance is shone on the filter paper and the
white light generated from the paper is noted. A similarly prepared filter paper using
new check penetrant is also checked for brightness and the readings from both are
compared. Any significant difference in brightness should be investigated. BS6443
annex B.7.1 and B.7.2 give specific details.
9.2.3 PENETRANT REMOVER CHECK
The grit blasted surface adjacent to the chrome-plated strip is sometimes used
to check washability and remover efficiency. However, a simple check of hydrophilic
remover for contamination by penetrant can be carried out each day before work is
started. While the liquid is still, shine the UV lamp on the surface. Any yellow/green
fluorescence indicates that the material is saturated and should be discarded BS6443
appendix B.5.3. describes a remover check.
If a check of the concentration is called for then a refractometer must be used.
9.2.4 DEVELOPER CHECK
Dry powder developer must remain light, fluffy and dry. It can become
contaminated with penetrant carried into the cabinet from test pieces. Therefore a UV
light should be shone at the developer weekly and any affected developer must be
removed.

Aqueous suspendible developers must be checked for specific gravity regularly
and/or by a settlement check.
9.2.5 UV LAMP OUTPUT CHECK
Mercury vapour lamps deteriorate with age and in proportion to the number of
times the arc is struck. Therefore it is best to leave the lamp on all day rather than
switch it on and off. Every month the output should be checked using a monitor. The
output of the lamp should be recorded at a distance of 400mm, which will reveal
deterioration in output (BS4489 paragraph 6.1 refers). The output at the work surface
must exceed 0.5mW/cm2 according to BS 4489 and 1mW/cm2 according to BS EN
571.
9.2.6 UV MONITOR CHECK
Every year the monitor, which checks the UV lamps, must be returned to the
manufacturer or the National Physical Laboratory for calibration.
9.2.7 WATER REMOVABLE PENETRANT, WATER TOLERANCE

CHECK
Contamination of penetrants with water is a problem when water removable

penetrant is used. A simple monthly check will prove that the penetrant is fit to use.
Ascertain from the manufacturer the water tolerance of the penetrant in use. Take a
sample of the used material and add water up to 50% of the tolerance. If the penetrant
goes milky and fails to clear it must already have had at least 50% of the allowed
water in it. Therefore it would be wise to change the bath.
9.3 MAINTENANCE CHECKS
Included in this section are some common sense checks, which should be
applied. The list is not exhaustive; other checks may be necessary in specific
systems.
9.3.1 TANK LEVELS
Each day the penetrant tanks must be replenished. Some specifications require
that permanent marks be put on the tanks showing maximum and minimum levels.


9.3.2 EQUIPMENT CLEANLINESS
It is important that the work area is kept clean and tidy for obvious reasons.
Also in fluorescent areas the brightness of the area can be significantly affected by
spilt penetrant material. This is usually an ongoing job but formally noted daily.
9.3.3 AIRLINE CLEANLINESS
Oil and water traps in factories are frequently overlooked so it is wise to check the
cleanliness of the air supply daily. Simply direct the air jet at a strong tissue and check it
for moisture.
9.3.4 PROCESSING UNITS
A weekly check on the whole penetrant line or lines is good housekeeping and
potential health and safety hazards should be reported. Annoying things like torn or
badly hung curtains should be fixed.
9.3.5 UV LAMP MAINTENANCE
Clean each lamp and filter before checking the output value. A significant amount
of light can be lost due to dirt. Electrical safety must also be checked.
9.3.6 CLEAN TANKS
It is amazing what can get into penetrant tanks therefore a six monthly clean
out is prudent. Although it is rarely necessary to discard the material a drain cock, in
the tank side, a few centimetres above the bottom should be fitted and used to drain
off the penetrant. A second cock in the bottom of the tank can be used to drain the
residue and this small amount is often the only material that needs to be discarded.


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WORLD CENTRE FOR

PRACTICAL PENETRANT INSPECTION

TWI Ltd, Training and Examination Services

Practical Penetrant Inspection WORLD CENTRE FOR

Practical Penetrant Inspection WORLD CENTRE FOR

2.0 FLAW ENTRAPMENT EFFICIENCY

3.0 PENETRANT PROPERTIES

4.0 FLUORESCENT PENETRANTS AND THE ELECTROMAGNETIC

5.4 DEVELOPER PROPERTIES

6.0 PROCESS SEQUENCE

Practical Penetrant Inspection WORLD CENTRE FOR

6.6.2 DRY POWDER DEVELOPER APPLICATION

7.0 SYSTEM CLASSIFICATION

Practical Penetrant Inspection WORLD CENTRE FOR

8.0 CHOICE OF PENETRANT SYSTEM

9.0 EQUIPMENT CHECKS

Practical Penetrant Inspection WORLD CENTRE FOR

Practical Penetrant Inspection 1.1 WORLD CENTRE FOR

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Contact Angle Wetting Ability Droplet Shape

Less than 90o High

Greater than 90o Low

Contact Angle, Wetting Ability and Droplet Shape

Practical Penetrant Inspection 1.2 WORLD CENTRE FOR

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As viscosity is strongly influenced by temperature, so are penetrant inspections.

Practical Penetrant Inspection 1.3 WORLD CENTRE FOR

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Practical Penetrant Inspection 2.1 WORLD CENTRE FOR

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The efficiency of a penetrant inspection will be influenced by

Penetrant manufacturers will within a classification such as water washable

The degree of penetrant removal must also be considered. Components that

Practical Penetrant Inspection 2.2 WORLD CENTRE FOR

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• Availability and cost

3.1 WETTING ABILITY

The wetting ability of penetrants is an important physical property that affects

3.2 SPECIFIC GRAVITY

Specific gravity is a comparison of the density of a penetrant with the density of

Practical Penetrant Inspection 3.1 WORLD CENTRE FOR

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Volatility is characterised by the vapour pressure or boiling point of a liquid.

3.5 CHEMICALLY INERT

Penetrant materials must be as inert and non-corrosive as possible. Maximum

It is important that the penetrant be chemically compatible with the material

When new applications are evaluated, the compatibility of an organic penetrant

Viscosity relates to the thickness or body of a fluid and is a result of molecular

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3.8 SOLVENT ABILITY

3.9 TOLERANCE TO CONTAMINANTS

3.10 HEALTH HAZARD

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3.12 ELECTRICAL CONDUCTIVITY

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Ultraviolet radiation can be divided into three categories based on wavelength,