3.

8 PAINTING & CORROSION

 Corrosion
• Corrosion is the environmental degradation of a
metal. Metal react with the gases and the liquids
of the environment and produce its oxide,
hydrate oxide or sulphate on its surfaces.
• Steel produce a oxide layer which is brittle and
having a deferent coefficient of thermal
expansion than steel. Hence it does not act as a
protective layer around the mettle. This layer of
oxide flakes off to expose a rougher steel surface
and expedite the corrosion
 CORROSION

• Corrosion is the decaying of metal by chemical

or electro chemical process with the
environment.
 EROSION

• Erosion is the decaying of metal due to

abrasive action of liquid or gas. (It is a purely
mechanical reaction with no chemical reaction
involved).
 Corrosion

Chemical Electro chemical

corrosion corrosion
 Corrosion Triangle

• For corrosion it requires to have metal water

and oxygen (air). This is known as the
corrosion triangle. By eliminating water or air
will stop the reaction
 GALVANIC SERIES

• K
• Na
• Ca More Noble Metals (Anode)=decay fast
• Mg
• Al
• Zn
• Cr
• Fe
• Sn
• Pb
• Cu
• Ni
• Br Less Noble Metals (Cathode) = Nodecay
• Ag
• Pt
• Au
• The metal places on top of the series are nobler
than the one placed below them. These more
noble metals acts as anodes in relation to the
less noble metal which acts as cathodes also the
sea water acts like electrolyte. This will form a
galvanic cell.When considering ship less noble
metal (Ni, Br propeller) catholically protected
while the more noble metal (Fe Ships Hull)
becomes an anode and gets corroded away
• As shown in the diagram when the metal is
exposed to the atmosphere by discontinuation
of protective coating the metal becomes the
anodic area of the cell and corrode deeper
and deeper. This is known as pitting.
 3.8.1.2 CHEMICAL CORROSION

• The basic cause for chemical corrosion is the

metal is having their spontaneous tendency to
return to their stable state.
• Fe + 02 + H 2 0 = Rust
• For chemical corrosion oxygen & water needs
to be present.
3.8.1.3 ELECTRO CHEMICAL CORROSION

• This occurs at a faster rate. A galvanic cell

forms when two Dissimilar metal’s presence at
a electrolyte. (Electrolyte is media which is
capable of transporting ions (e) or current.)
3.8.2 CORROSION CONTROL
 3.8.2.1 PAINTING
• Before painting the steel plate should be free of rust &
exposed the bear metal. Barrier or blast primer (which
contains Zn) to be applied to the metal to give cathodic
protection. Several under coat with a top coat to be applied
to prevent water & Oxygen been contact with the metal.
• Painting is the most common and simple way of preventing
corrosion. Effectiveness of this depends on the prior surface
preparation and the quality of the paints used.
• Primer coat of paint applies immediately after preparation of
the surfaces and followed by secondary primer coats and
finish coats according to the painting scheme.
• Composition of paints.
• Pigment, Binding agent (vehicle), Solvent
 3.8.2.2 CATHODIC PROTECTION

• The technique can only be use where the

concern metal are immersed in a electrolyte.
There are two types of cathodic protection
systems are used in modern ships
• 1. Sacrificial Anode System
• 2. Impress Current system
 3.8.3 Different types of marine paints
• (a)Bitumen or pitch
• Used as a superstructure paint but more commonly for internal
surfaces exposed to a high degree of wetness such as fresh
water tanks.
• (b)Chlorinated rubber
• This paint is used where good chemical and water resistance
coatings are required. The paint dries by the evaporation of the
solvents. It is often used when the ships are being built in winter
or poor climatic conditions.
• (c)Coal tar epoxy
• A two pack paint in which the components are pigments
epoxies resins and curing agents. The paint provides hard thick
films in which the chemical resistance qualities of epoxy resin
are combined with the water impermeability of coal tar.
• (d)Epoxy
• The epoxy resin paints have extremely good chemical,
water ,and abrasion resistance qualities but are very
expensive. They are generally two pack consisting of an epoxy
base and a hardener or curing agent. The paint are very hard
when cured and give long term protection against corrosion.
• (e)Polyurethane
• Usually two pack paint. These paints have many
good qualities abrasion resistance ,hardness, a high gloss,
chemical resistance, and when well cured have very good
water and weather resistance. Used for tank coatings but
white polyurethane is good external superstructure paint as it
remains bright for long periods and is not subject to rapid
yellowing.
• (f)Primers
• Steel plates must be protected from
corrosion while the ship is being built and
prefabrication primers must also be rapid drying,
have non toxic vapor ,not affect weld quality or
speed. be suitable for spray application, and be
suitable for first coat of ship’s paint system.
Mariners must ensure that any primers used are
compatible with following coatings, e.g. vinyl
primer must be used underneath a vinyl topcoat
Methods of preparing areas of hull and
superstructure for painting.
• If the paint surface has broken down to a
significant degree then the complete surface
should be cleaned to bare metal. The chipping
hammer and wire brush method does not
produce a satisfactory surface for the
adhesion of the modern paints.
The modern chipping methods are
• (a) Power wire brushing
• (b) Power disking
• (c) Air hammer
• (d) High pressure water blasting
• (e) Sand blasting
• (f ) Heating
• (g) Pickling
3.8.3.1 SHIP’S PAINTING SYSTEMS

• The paint system applied to any part will be

dictated by the environment to which that
part of the structure is exposed. traditionally
the painting of the external ship structure was
divided into three regions
1.Below the light load line where the plates
are continually immersed in water
• The ship’s bottom has priming coats of corrosion
inhabiting paint applied, followed by an
antifouling paint. Coatings used for steel
immersed in sea water are required to resist
alkaline conditions. The antifouling paints are
applied after the corrosion inhibiting coatings
and should not come into direct contact with the
steel hull since the copper and mercury
compounds present in the paint may cause
corrosion
 International Convention on the Control of
Harmful Anti-fouling Systems on Ships
• The International Convention on the Control of
Harmful Anti-fouling Systems on Ships will prohibit the
use of harmful organotins in anti-fouling paints used
on ships and will establish a mechanism to prevent the
potential future use of other harmful substances in
anti-fouling systems
• Anti-fouling paints are used to coat the bottoms
of ships to prevent sealife such as algae and molluses
attaching themselves to the hull – thereby slowing
down the ship and increasing fuel consumption.
• The new Convention defines “anti-fouling
systems” as “a coating, paint, surface treatment,
surface or device that is used on a ship to control
or prevent attachment of unwanted organisms”.
• In the early days of sailing ships, lime and
later arsenic were used to coat ships' hulls, until
the modern chemicals industry developed effective
anti-fouling paints using metallic compounds.
• These compounds slowly "leach" into the sea
water, killing barnacles and other marine life that
have attached to the ship. But the studies have
shown that these compounds persist in the water,
killing sealife, harming the environment and possibly
entering the food chain. One of the most effective
anti-fouling paints, developed in the 1960s, contains
the organotin tributytin (TBT), which has been
proven to cause deformations in oysters and sex
changes in whelks
2.Between the light and statutory load lines. i.e the boot
topping area where immersion in intermittent and much
abrasion occurs
• The modern practice requires a paint system
for the hull above the waterline. These
systems are often based on vinyl and alkyd
resins or on polyurethane resin paints
 3.The top side and superstructure which are exposed to an
atmosphere laden with salt spray and are subject to damage by
cargo handling and general wear and tear.
• Red lead or zinc chromate based primers are
commonly used with white finishing paints.
Linseed oil based paints which yellow on
exposure are generally avoid on modern ships.
zinc chromate paints should be used on
aluminum superstructures.
 3.8.3.2 The drying processes.
• there are essentially 2 drying mechanisms
used in marine coatings

(i) Physical drying– referring to non-

convertible coatings.
(ii) Chemical drying– referring to
convertible coatings
 3.8.3.3 Painting schedule
• The maintenance painting schedule is based
on schedule for new building. The following
points are prime important.
1-surface separation
Which includes the method of cleaning,
method of removing corrosion products and
defective paint coats.
2-type of paint to be used
 3.8.4.1 Paint surface calculations

Estimating paint coverage is a key costing factor

for both owners, vessel operators, shipyards and
contractors.
 IMPRESSED CURRENT SYSTEM
• Electric from a power unit is supplied to anodes
(unlike in the cathodic protection system these
anodes are insulated from the hull) fitted around
the hull. The electrons from the D.C power source
travel into the hull at the same rate at which ions
formed in the cathode thus preventing loss of
electrons from the hull. Therefore the iron ions are
not formed and the hull is not wasted. A reference
electrode is used to measure the potential
difference and supply a D.C Current accordingly