Learn

about
the

different types of corrosion and how to protect against them.

1. Introduction
Metals react with the environment, producing corrosion products similar to the original ore from which the metal
was obtained. Corrosion processes are electro-chemical reactions taking place at the surface of the metal.
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Corrosion products (rust) may act as a barrier between the metal and its surroundings, slowing down the corrosion
rate. In some cases this barrier very effectively retards corrosion. This is called passivation. Passivation can
increase the corrosion resistance of metal remarkably.
Dry corrosion
At room temperature, most metals carry a thin oxide layer as a result of the reaction of metals with oxygen in the
atmosphere. Increase of temperature may cause formation of a heavier layer, or the layer may detach.
Zinc and zinc coatings carry a fairly protective zinc hydroxide or carbonate layer (zinc patina) which increases in
thickness very slowly. Aluminium carries a thin, highly protective oxide layer.
Some corrosion takes place even under completely dry conditions.
Wet corrosion
Wet corrosion takes place in environments where the relative humidity exceeds 60 %. The corrosion may be
uniform destruction of the metal surface or localised destruction (pitting, stress corrosion cracking). The corrosion
can be concentrated adjacent to a more noble metal or at points where the oxygen supply is limited.
Wet corrosion is an electro-chemical phenomenon. When two metals are in contact with water solution containing
salts, an electric potential is formed between two different metals or the surfaces of the same metal with different
surface conditions. This causes the dissolution of the less noble metal. The more noble metal remains protected
but the less noble metal corrodes.
Wet corrosion is most efficient in waters containing salts, such as NaCl (e.g. marine conditions), due to the high
conductivity of the solution. Chlorides also may increase the corrosion rate of metals.

2. Corrosion types

Corrosion can be divided into different groups according to their form of occurrence.
General corrosion / uniform corrosion
Metal corrodes uniformly all over the surface.
Local corrosion
Part of the structure corrodes at a considerably higher than average rate. The categories of local corrosion are:

Pitting corrosion
the corrosion effect is concentrated on localised areas and leads to pitting.

Crevise corrosion
proceeds at locations covered by a corrosion product and other deposits (dirt or trash). Crevice corrosion
typically occurs in small cavities, gaps, recession, etc.
Galvanic corrosion
galvanic corrosion requires two different metals, constituting a corrosion cell. A structure should contain
metals that are as close to each other as possible in the galvanic electric series.
Intergranular corrosion
intergranular corrosion proceeds along the metal grain boundaries.

Selective corrosion
selective corrosion occurs, for example, when one element in an alloy dissolves faster than the others.

Combined effect of mechanical factors and corrosion

Mechanical wearing as well as static or dynamic stresses often act in combination with corrosion. The main
categories for combined effect of mechanical factors and corrosion are:

Stress corrosion
stress corrosion occurs when a metal in a corrosive environment is exposed to static stress that results in
fracture.
Corrosion fatigue
corrosion fatigue is caused by the combined effect of corrosion and varying state of stress.
Erosion corrosion
erosion corrosion is an acceleration in the rate of corrosion caused by high velocity of a liquid, or solid
impurities carried by a liquid.
Cavitation corrosion
cavitation corrosion is erosion caused by the combined effect of corrosion and the pressure caused by
the breaking of gas bubbles formed in liquid (cavitation). .
Fretting corrosion
fretting corrosion occurs between two metals rubbing against each other under corrosive conditions.

3. Classification of corrosion environments

Environments causing corrosion can be classified in different categories according to their corrosivity. The
classification is based on standard ISO 9223:1992 Corrosion of metals and alloys -- Corrosivity of atmospheres Classification. It categorizes environments on the basis of wet time, as well as sulphur dioxide and chloride
contents.
Standard ISO 9224 Corrosion of metals and alloys -- Corrosivity of atmospheres -- Guiding values for the
corrosivity categories gives the corrosion rates of steel, zinc, copper and aluminium in the first five years.
In the two following standards, the corrosivity categories are determined by the loads imposed by the atmosphere
and through immersion:
EN ISO 12944-2
Paints and varnishes -- Corrosion protection of steel structures by protective paint systems -- Part 2: Classification
of environments
ISO 14713
Protection against corrosion of iron and steel in structures -- Zinc and aluminium coatings - Guidelines
Table 1 gives examples of the environments in each corrosion category and the rates of corrosion for steel and
zinc in the first year.
Table 1.Categories of environment
Corrosivity Environment
category
(guiding examples)
C1
very low

Indoor spaces with occasional condensation

Outdoor air: inland rural

C2
low

Dry indoor spaces

C3
medium

Indoor spaces with high moisture content, not

much impurities
Outdoor air: inland urban, mildly saline

C4
high

Indoor spaces: chemical industry, swimming

pools, seaside docks.
Outdoor air: inland industrial plants, seaside
urban areas

C5-I
very high

Outdoor air: very humid industrial atmosphere

C5-M
very high

Outdoor air: saline seaside atmosphere

Corrosivity
category

Rate of corrosion in the first year

ISO 9223
EN ISO 14713
and EN ISO
12944-2
Carbon steel Zinc
m/year
m/year

Zinc
m/year

C1
very low

<= 1,3

<= 0,1

<= 0,1

C2
low

1,3 - 25

0,1 - 0,7

0,1 - 0,7

C3
medium

25 - 50

0,7 - 2,1

0,7 - 2

C4
high

50 - 80

2,1 - 4,2

2-4

C5-I
very high

80 - 200

4,2 - 8,4

4-8

C5-M
very high

80 - 200

4,2 - 8,4

4-8

Table 2. Categories of immersed and buried structures.

Category of Environment
environment (guiding examples)
lm1

Fresh water: river constructions, water power

plants

lm2

Sea water in temperature zones: Structures in

harbour areas

lm3

Soil: Buried tanks, steel piles, steel pipes

Category of Rate of corrosion in the first year

environment
ISO 9223
EN ISO 14713
and EN ISO
12944-2
Carbon steel Zinc
m/year
m/year

Zinc
m/year

lm1
lm2
lm3

10 - 20

4. Corrosion protection
The most common surface protection methods for steel are:

anti corrosive paint coating

hot dip galvanizing

electroplating

spray galvanizing

chromium plating

aluminium spraying

rubberising

coil coating of sheet steel

The most common method of protecting steel construction products is hot dip galvanising. Very simply, the process
involves coating the surface of the steel with a corrosion-resistant metal, usually zinc or an aluminium/zinc alloy.
Zinc and zinc-based coatings protect steel in two ways. Like paint, they provide barrier protection. Secondly, they
provide galvanic protection, i.e., zinc will sacrifice itself to protect steel.
In addition to the metallic coating, many flat steel construction products such as cladding and roofing products
have anorganic topcoat for increased durability and enhanced appearance. A range of different coatings is
available depending upon the product and the application. Coating thicknesses vary from 25 to 200 m.
Anticorrosive painting
Paints are barrier coatings that, when applied and used properly, give sufficient corrosion protection to steel for
many common applications. They are, however, not impervious to moisture, and rust can occur under even a
perfectly applied paint if exposure time to moisture is long enough. Nevertheless, surface cleanliness and surface
preparation are essential for good protection by anticorrosive paints. Surface preparation and corrosion protection
of steel by protective paint systems are addressed in many standards.
Pretreatment:
The surface to be painted must be completely clean before painting. The standards for inspection of steel surface
cleanliness are: covered in ISO 8501-1 and ISO 8501-2
The cleanliness of the surface can be estimated according to standard:
ISO 8502 parts 1 ... 9 and parts 11 ... 12.
which covers the preparation of steel substrates before application of paints and related products and the tests for
the assessment of surface cleanliness.
The roughness of the steel surface influences the adhesion of the paint and the corrosion protection. Surface
roughness can be estimated according to:
ISO 8503-1 parts 1 ... 5.
which describes the preparation of steel substrates before application of paints and related products and the
surface roughness characteristics of blast-cleaned steel substrates.
The pre-treatment methods for steel surfaces are given in standard:
ISO 8504 parts 1 ... 3.
which covers the preparation of steel substrates before application of paints and related products -- Surface
preparation methods.
Information of the blast-cleaning abrasives used in surface preparation is given in the standard:
ISO 11124 parts 1 ... 4.
covering the preparation of steel substrates before application of paints and related products -- Specifications for
metallic blast-cleaning abrasives; and
ISO 11126 parts 1, 3 ... 10
which is for the preparation of steel substrates before application of paints and related products -- Specifications
for non-metallic blast-cleaning abrasives.
Protective paint systems
The protective paint systems are addressed in:
ISO 12944-1.
Paints and varnishes which classifies protective paint systems by durability. The durability class does not imply any
guarantee period but the expected serviceable life before repainting for maintenance.

ISO 12944-2.
Paints and varnishes which specifies the corrosivity categories according to the type of atmosphere and stress
caused by immersion (tables 1 and 2)
ISO 12944-3.
Paints and varnishes -- Design considerations.
ISO 12944-4.
Paints and varnishes -- Types of surface and surface preparation.
ISO 12944-5.
Paints and varnishes -- Protective paint systems. It specifies the most common types of anti-corrosive paint and
gives instructions for the selection of these for different environmental classes.
ISO 12944-6.
Paints and varnishes -- Part 6: Laboratory performance test methods.
ISO 12944-7.
Paints and varnishes -- Part 7: Execution and supervision of paint work.
ISO 12944-8.
Paints and varnishes -- Part 8: Development of specifications for new work and maintenance.
Shop primers
Shop primer is applied immediately to the blast-cleaned steel surface for temporary protection against corrosion
during fabrication, transport, installation and storage. The shop primer is then painted over with the finishing paint
system, which usually includes a new primer coat. Usually shop primer is not part of the paint system. Therefore it
may have to be removed. Products supplied with a shop primer coat can be welded.
Guidance on shop primers is given in standards EN ISO 12944-5, appendix B and EN 10238 Automatically blast
cleaned and automatically primed structural steel products.
Zinc coatings
Zinc coating can be applied by:

hot-dip galvanising

electroplating

spray galvanizing

zinc-rich paint
The atmospheric corrosion resistance of a zinc coating is a linear function of its thickness. For example, a 20 m
thick coating will last twice a long a 10 m coating in a given environment. Hot-dip zinc coating (hot dip
galvanizing) is the most common method of zinc coating for steel structures. Table 3 gives typical properties of
different zinc coatings.
Table 3. Comparison of zinc coatings.
Coating thickness
Hot dip zinc
coating

Normally 50 to 100 m (up to 250 m) thick.

Continuously coated steel sheet 10 to 30
m.

Electroplating Usually 5 to 15 m. Thick coats cannot be

produced economically.
Zinc spraying Coat thickness varies, typically 80 to 150 m
(seldom exceeds 250 m)
Zinc-rich paint One coat about 10 to 60 m.
Adhesion of zinc to steel
Hot dip zinc
coating

Metallurgical bonding

Electroplating Interatomic bonding/mechanical adhesion

Zinc spraying Mechanical adhesion. Good if shot blasting
has been carried out correctly

Zinc-rich paint Depends on binder and carefulness of shot

blasting
Structure of the coat
Hot dip zinc
coating

Piece galvanizing: Zinc-iron layers coat plus

pure outerzinc layer. Continuous galvanizing:
very thin iron-alluminium-zinc layer, pure zinc
layer (99 %)

Electroplating Entirely pure zinc

Zinc spraying The coating is built up from droplets of pure
zinc. It is slightly oxidized and porous
Zinc-rich paint The best products have about 90 weight-%
zinc in the paint
Evenness and continuity
Hot dip zinc
coating

Good. Some excessive zinc runnings from

the batch process

Electroplating Even, depending on the efficiency of bath

Zinc spraying Depends on operators skills. The coating is
porous, but the pores are quickly filled with
zinc salts and after that the coating is
compact.
Zinc-rich paint Good. Pores, if any, are filled with reaction
products.
Pretreatment
Hot-dip zinc
coating

Piece galvanizing: degreasing and acid

pickling.Continuous galvanizing: cleaning
in annealing furnace.

Electroplating

Degreasing and acid pickling

Zinc spraying

Shotblasting (minimum Sa3)

Zinc-rich paint

Shotblasting (Sa2 to Sa3)

Corrosion resistance
Hot dip zinc
coating

Good

Electroplating

Limited (depending on coating thickness)

Zinc spraying

Good

Zinc-rich paint

Limited.

Standards
Hot dip zinc
coating

EN ISO 1461, EN ISO 14713, ISO 3575

(coated sheet)

Electroplating

ISO 2081

Zinc spraying
Zinc-rich paint
Notes
Hot dip zinc
coating

The maximum size of the object to be dipped

depends on the zinc bath. Reversing dipping
can be used to handle long objects. The
objects should be appropriately designed to
allow successful hot dip zinc coating.

Electroplating Zinc pot dimensions set the limits. Usually for

small components of simple shape. Suitable
for sheet and wire. No heat is developed in
the process.
Zinc spraying Size and form unlimited. Economical for

objects that weigh a lot in proportion to

surface area. Uneconomical for network
structures. Less accessible spots limit its use.
Best method for producing thick coatings.
Zinc-rich
Suitable for the same applications as painting
paint
in general. Narrow places present problems.
The atmospehric corrosion rate of zinc is approximately ten times slower than that of steel. The corrosion rate of
zinc is:

rural atmosphere: < 1 m/year

urban atmosphere: 2 m/year

industrial atmosphere: 2 ... 10 m/year

marine atmosphere: 2 m/year

The life expectancies for zinc coatings under different conditions are presented in figure 1.

Figure 1. Life expectancies for zinc coatings.

Stainless steels
Stainless steelsare the most corrosion resistant steels
used in construction. Stainless steel contains a
minimum of 11% chromium that produces a thin
protective oxide film on the surface that protects the
material from corrosion. If damaged, this protective
layer simply re-forms. Stainless steel is rarely used
for structural steel but is used in some specific
structural products such as lintels and masonry
support systems. The most common use of stainless
steel is for building roofing and cladding and internal
applications such as escalators, doors, railings, etc.

<< Pre

5. Corrosion allowance
A steel structure that will not be protected against corrosion by painting or zinc coating can be made to run its
planned service life by adding a corrosion allowance to its material thicknesses, determined according to the
service conditions.
Corrosion in air:
Humidity, temperature, rain, wind, impurities and metal wet times have an effect on the corrosion rate. Corrosion
occurs when the relative humidity of the air is 70 to 80 %. Corrosion reaction is possible generally when the
temperature is above 0 C and the relative humidity is over 80 % (the surface is wet). Air impurities that dissolve in
condensed water or rain water may accelerate corrosion. Settling of dust and dirt on the metal surface accelerates
atmospheric corrosion.
Information about steel corrosion rates in different atmospheres is given in table 4. It should be noted that localised
corrosion can occur, which can greatly exceed the corrosion rates given in the table.
Table 4. The corrosion rate of steel in different atmospheres (uniform corrosion).
Atmosphere
Corrosion rate
(m/year)
Rural

4 - 60

Urban

30 - 70

Industrial

40 - 160

Marine
60 - 170
Corrosion rates of steel in water and soil are given in different information sources.
Weathering steels

Weathering steels are high strength, low alloy, weldable structural steels that possess good weather resistance in
many atmospheric conditions without the need for protective coatings. They contain up to 2.5% alloying elements,
e.g. chromium, copper and nickel. On exposure to air, a protective rust patina forms that adheres to the surface of
the steel. This layer causes the rate of corrosion to slow so that after 2-5 years, corrosion almost ceases.
Requirement for the formation of the protective corrosion product layer is regular wetting and curing of the surface.
Long wet periods may prevent the formation of the protective layer.
Wet environments, immersed or buried conditions are unsuitable for weathering steels.
Consideration for use of weathering steels:

the actual loss varies with the environment. For long-life corrosion allowance must be considered

crevices and water/dirt traps should be avoided

rust stains may run to adjacent surfaces and cause staining

fasteners should be made of weathering steel

specific low alloy welding rods should be used

for an even weathering result, surface blasting may be necessary

weathering steels are unsuitable for use in marine and aggressive industrial environments.

References
ESDEP WG 4A, Protection: Corrosion, Lecture 4A.1: General Corrosion
Erkki Huhdankoski & working group. Rautaruukki steels under critical conditions 2000