abc® Aalco is a registered trademark of Aalco Metals Ltd
© Copyright: Aalco Metals Ltd, The Hersham Centre, Hersham Green, Hersham, Surrey KT12 4HP
All Data is indicative only and must not be seen as a substitute for the full specification from which it is drawn.
In particular, the mechanical property requirements vary widely with product form and product dimensions.
For more complete details please refer to the relevant specification – The BS EN Specifications for Stainless Steel
are listed on a separate Datasheet.
1
1
Stainless Steel – Corrosion Resistance
Corrosion
Corrosion can be defined as the attack of a
metallic material by its environment. Stainless
steels all possess a high resistance to corrosion.
This resistance is conferred by the naturally
occurring chromium - rich oxide film which is
always present on the surface of stainless steel.
Although less than 130 Angstrom thick (1
Angstrom unit = 10-8 cm) this invisible film is
extremely protective as it is inert and adheres
tightly to the metal. The oxide film has the
unique property of self-repair which is
unattainable in applied films. This means that if
the film is removed or damaged or a new metal
surface is created by cutting then in the
atmosphere or other source of oxygen the
protection will be instantaneously re-
established. The more highly alloyed grades of
stainless steel possess the best corrosion
resistance and are able to withstand more
aggressive environments.
Selection of the correct grade of stainless steel
is the key to avoidance of corrosion problems.
Corrosion Resistance
Corrosion takes many different forms. Its
initiation and subsequent rate of progress is
affected in varying degrees by numerous
material and environmental factors. A
comprehensive assessment of the exact
'corrosion resistance' of a material is therefore
difficult. However, corrosion tables covering a
vast range of stainless steels and environments
are available.
Design Criteria
To achieve optimum corrosion performance care
must betaken at the design stage. In particular
design should employ smooth contours and
radiused corners whilst avoiding sharp edges
and crevices. Design should also promote
material flow and mixing to avoid localised
concentrations and/or stagnant conditions.
Other considerations are ease of cleaning and
maintenance as well as avoidance of dissimilar
metal contact.
Oxidation
Oxidation is the combination of a metal with
oxygen to form the metal oxide which occurs in
dry conditions. When this process is ongoing
the whole of the metal may be converted.
Stainless steels are oxidation resistant, even at
elevated temperatures. Special heat resisting
grades, such as type 310, are operated at
temperatures of up to 1100°C.
Forms of Corrosion
There are many different forms of corrosion.
General Corrosion is the uniform overall
attack of a component across its whole surface.
It is avoided by correct grade selection.
Pitting Corrosion is the highly localised attack
seen as small spots across a surface occurring
mainly at sites of metallurgical heterogeneity.
Particularly prevalent in chloride environments,
especially if Oxyqen is in plentiful supply, it can
lead to perforation. Higher Chromium, Nickel
and Molybdenum contents improve pitting
resistance, thus type 316 is extensively used in
such situations. Laboratory tests can be carried
out to confirm pitting resistance such as ASTM
G48-03. The Pitting Resistance Equivalent
(PRE) number can be calculated and gives a
good indication of the resistance to pitting:
Austenitic Stainless Steel
PRE = (%Cr) + (3.3 x %Mo) + (16 x %Ni)
Duplex Stainless Steel
PRE = (%Cr) + (3.3 x %Mo) + (30 x %Ni)
Atmospheric Corrosion occurs due to the
attack from oxygen, water and the pollutants
therein such as chlorides, sulphur compounds
and solids. The problem is particularly
prevalent in coastal, industrial and highly
polluted areas where necessitates the use of
type 316 is recommended for outdoor
applications in such environments.
Acid Corrosion occurs due to aggressive attack
by acids which may be accelerated by the
presence of other chemicals. A large number of
acid environments are resisted by stainless
steels - resistance to oxidising solutions is very
good provided the correct grade is used.
abc® Aalco is a registered trademark of Aalco Metals Ltd
© Copyright: Aalco Metals Ltd, The Hersham Centre, Hersham Green, Hersham, Surrey KT12 4HP
All Data is indicative only and must not be seen as a substitute for the full specification from which it is drawn.
In particular, the mechanical property requirements vary widely with product form and product dimensions.
For more complete details please refer to the relevant specification – The BS EN Specifications for Stainless Steel
are listed on a separate Datasheet.
2
2
Forms of Corrosion continued….
Galvanic Corrosion covers situations where
attack is caused by a potential difference. This
potential difference can be set up in a number
of ways including contact between dissimilar
metals in an aqueous or conducting solution,
differential aeration (variation in oxygen
concentration) and local variations in
concentration of the solution. Correct material
selection and good design can eliminate this.
Waterline Corrosion is a form of galvanic
corrosion taking place at the surface of a liquid
in which the stainless steel is partially
immersed.
Crevice Corrosion occurs in crevices such as
joints, cavities, holes, corners, grooves, slots,
gaskets and gaps between components. It is
caused by the breakdown on the protective
oxide layer, normally in reducing environments
and can be seen as another types of galvanic
corrosion. Good design should be used to
eliminate such crevices and thus avoid this
phenomenon.
Stress Corrosion Cracking (SCC) can occur in
austenitic stainless steels when they are
operated under tensional stress in chloride
environments at temperatures in excess of
about 60°C. The stress could arise through in-
service loading, pressurisation of pipework and
vessels or as residual stress from cold working.
Nickel content assists resistance to SCC and
thus grade 316 is more resistant to this form of
attack than 304, whilst the duplex grades such
as 2205 perform very well. Ferritic stainless
steels are immune to this form of attack.
Sulphide Stress Corrosion (SSC) is another
form of Stress Corrosion Cracking that can
occur in environments containing both chlorides
and Hydrogen Sulphide (H2S). It is of particular
concern in the off-shore oil and gas industry.
Bacterial Corrosion occurs due to the
presence and activity of certain types of
bacteria and tends to be localised, for example
in crevices. It is overcome by good design,
continuous flow and regular cleaning.
Inter-Granular Corrosion in austenitic
stainless steels is a rapid and localized
phenomenon. Called sensitization, it is caused
by Chromium Carbide precipitation at grain
boundaries which depletes the surrounding area
of chromium thus reducing its corrosion
resistance. This may be caused by incorrect
heat treatment, heat input during welding,
service in the temperature range 450 to 850°C
or service at a higher temperature and slow
cooling through this range. Modern
steelmaking, correct grade selection and post
weld heat treatment prevents this occurring.
Various types of stainless steel can be
considered to avoid the problem including extra
low carbon grades and titanium (321 & 316Ti)
or niobium (347) bearing grades.
Weld Decay is a form of intergranular
corrosion occurring in the heat-affected zone of
the parent metal parallel to the weld.
Susceptibility to this attack is assessed using
one of these standard tests:
Test as given in BS1449/BS1501 using boiling
copper sulphate/sulphuric acid,
Test as given in ASTM A262 Practice C using
boiling nitric acid. Low carbon grades perform
better in the more severe (latter) test.
Fretting Corrosion which can also be called
corrosion-abrasion is caused by continuous
removal of corrosion product due to surfaces
rubbing together, which leads to progressive
wasting of material. Stainless steels do not
suffer from this form of attack.
Pickling and Passivation
To achieve optimum corrosion performance
stainless steel must go into service fully
passivated. Its protective surface oxide layer
must be intact and uncontaminated. In
particular weld tint and any fabrication debris
must be removed. It is particularly important to
remove any Carbon or mild steel contamination.
This is done using proprietary solutions,
chemical and pastes based on Nitric Acid or a
mixture of Nitric and Hydrofluoric Acids.
This information is based on our present knowledge and is given in
good faith. However, no liability will be accepted by the Company
is respect of any action taken by any third party in reliance thereon
As the products detailed may be used for a wide variety of purposes
and as the Company has no control over their use; the Company
specifically excludes all conditions or warranties expressed or
implied by statute or otherwise as to dimensions, properties and/or
fitness for any particular purpose. Any advice given by the
Company to any third party is given for that party’s assistance only
and without liability on the part of the Company.
Any contract between the Company and a customer will be subject
to the Company’s Conditions of Sale. The extent of the Company’s
liabilities to any customer is clearly set out in those Conditions; a
copy of which is available on request.