How many characters do you know about the stainless steel? Here we
introduce one character of the stainless steel.
Although one of the main reasons why stainless steels are used is corrosion
resistance, they do in fact suffer from certain types of corrosion in some
environments and care must be taken to select a grade which will be suitable for
the application. Corrosion can cause a variety of problems, depending on the
• Perforation such as of tanks and pipes, which allows leakage of fluids or
• Loss of strength where the cross section of structural members is reduced
by corrosion, leading to a loss of strength of the structure and subsequent
• Degradation of appearance, where corrosion products or pitting can
detract from a decorative surface finish,
• Finally, corrosion can produce scale or rust which can contaminate the
material being handled; this particularly applies in the case of food processing
Corrosion of stainless steels can be categorised as one of:-
• General Corrosion
• Pitting Corrosion
• Crevice Corrosion
• Stress Corrosion Cracking
• Sulphide Stress Corrosion Cracking
• Intergranular Corrosion
• Galvanic Corrosion
• Contact Corrosion
Corrosion whereby there is a general uniform removal of material, by
dissolution, eg when stainless steel is used in chemical plant for containing
strong acids. Design in this instance is based on published data to predict the
life of the component.
Published data list the removal of metal over a year. Tables of resistance
to various chemicals are published by various organisations and a very large
collection of charts, lists, recommendations and technical papers are available
though stainless steel manufacturers and suppliers.
Under certain conditions, particularly involving high concentrations of
chlorides (such as sodium chloride in sea water), moderately high temperatures
and exacerbated by low pH (ie acidic conditions), very localised corrosion can
occur leading to perforation of pipes and fittings etc. This is not related to
published corrosion data as it is an extremely localised and severe corrosion
which can penetrate right through the cross section of the component. Grades
high in chromium, and particularly molybdenum and nitrogen, are more resistant
to pitting corrosion.
Pitting Resistance Equivalent number (PRE)
The Pitting Resistance Equivalent number (PRE) has been found to give a
good indication of the pitting resistance of stainless steels. The PRE can be
PRE = %Cr + 3.3 x %Mo + 16 x %N
One reason why pitting corrosion is so serious is that once a pit is
initiated there is a strong tendency for it to continue to grow, even although
the majority of the surrounding steel is still untouched.
The tendency for a particular steel to be attacked by pitting corrosion can
be evaluated in the laboratory. A number of standard tests have been devised,
the most common of which is that given in ASTM G48. A graph can be drawn giving
the temperature at which pitting corrosion is likely to occur, as shown in
Figure 1. Temperature at which pitting corrosion is likely to occur
This is based on a standard ferric chloride laboratory test, but does
predict outcomes in many service conditions.
The corrosion resistance of a stainless steel is dependent on the presence
of a protective oxide layer on its surface, but it is possible under certain
conditions for this oxide layer to break down, for example in reducing acids, or
in some types of combustion where the atmosphere is reducing. Areas where the
oxide layer can break down can also sometimes be the result of the way
components are designed, for example under gaskets, in sharp re-entrant corners
or associated with incomplete weld penetration or overlapping surfaces. These
can all form crevices which can promote corrosion. To function as a corrosion
site, a crevice has to be of sufficient width to permit entry of the corrodent,
but sufficiently narrow to ensure that the corrodent remains stagnant.
Accordingly crevice corrosion usually occurs in gaps a few micrometres wide, and
is not found in grooves or slots in which circulation of the corrodent is
possible. This problem can often be overcome by paying attention to the design
of the component, in particular to avoiding formation of crevices or at least
keeping them as open as possible. Crevice corrosion is a very similar mechanism
to pitting corrosion; alloys resistant to one are generally resistant to both.
Crevice corrosion can be viewed as a more severe form of pitting corrosion as it
will occur at significantly lower temperatures than does pitting.
Stress Corrosion Cracking (SCC)
Under the combined effects of stress and certain corrosive environments
stainless steels can be subject to this very rapid and severe form of corrosion.
The stresses must be tensile and can result from loads applied in service, or
stresses set up by the type of assembly e.g. interference fits of pins in holes,
or from residual stresses resulting from the method of fabrication such as cold
working. The most damaging environment is a solution of chlorides in water such
as sea water, particularly at elevated temperatures. As a consequence stainless
steels are limited in their application for holding hot waters (above about
50°C) containing even trace amounts of chlorides (more than a few parts per
million). This form of corrosion is only applicable to the austenitic group of
steels and is related to the nickel content. Grade 316 is not significantly more
resistant to SCC than is 304. The duplex stainless steels are much more
resistant to SCC than are the austenitic grades, with grade 2205 being virtually
immune at temperatures up to about 150°C, and the super duplex grades are more
resistant again. The ferritic grades do not generally suffer from this problem
In some instances it has been found possible to improve resistance to SCC
by applying a compressive stress to the component at risk; this can be done by
shot peening the surface for instance. Another alternative is to ensure the
product is free of tensile stresses by annealing as a final operation. These
solutions to the problem have been successful in some cases, but need to be very
carefully evaluated, as it may be very difficult to guarantee the absence of
residual or applied tensile stresses.
From a practical standpoint, Grade 304 may be adequate under certain
conditions. For instance, Grade 304 is being used in water containing 100 – 300
parts per million (ppm) chlorides at moderate temperatures. Trying to establish
limits can be risky because wet/dry conditions can concentrate chlorides and
increase the probability of stress corrosion cracking. The chloride content of
seawater is about 2% (20,000 ppm). Seawater above 50°C is encountered in
applications such as heat exchangers for coastal power stations.
Recently there have been a small number of instances of chloride stress
corrosion failures at lower temperatures than previously thought possible. These
have occurred in the warm, moist atmosphere above indoor chlorinated swimming
pools where stainless steel (generally Grade 316) fixtures are often used to
suspend items such as ventilation ducting. Temperatures as low as 30 to 40°C
have been involved. There have also been failures due to stress corrosion at
higher temperatures with chloride levels as low as 10 ppm. This very serious
problem is not yet fully understood.
Sulphide Stress Corrosion Cracking (SSC)
Of greatest importance to many users in the oil and gas industry is the
material’s resistance to sulphide stress corrosion cracking. The mechanism of
SSC has not been defined unambiguously but involves the conjoint action of
chloride and hydrogen sulphide, requires the presence of a tensile stress and
has a non-linear relationship with temperature.
The three main factors are Stress Level, Environment and Temperature.
A threshold stress can sometimes can be identified for each material –
environment combination. Some published data show a continuous fall of threshold
stress with increasing H2S levels. To guard against SSC NACE specification
MR0175 for sulphide environments limits the common austenitic grades to 22HRC
The principal agents being chloride, hydrogen sulphide and pH. There is
synergism between these effects, with an apparently inhibiting effect of
sulphide at high H2S levels.
With increasing temperature, the contribution of chloride increases but the
effect of hydrogen decreases due to its increased mobility in the ferrite
matrix. The net result is a maximum susceptibility in the region 60-100°C. A
number of secondary factors have also been identified, including amount of
ferrite, surface condition, presence of cold work and heat tint at welds.
Intergranular corrosion is a form of relatively rapid and localised
corrosion associated with a defective microstructure known as carbide
precipitation. When austenitic steels have been exposed for a period of time in
the range of approximately 425 to 850°C, or when the steel has been heated to
higher temperatures and allowed to cool through that temperature range at a
relatively slow rate (such as occurs after welding or air cooling after
annealing), the chromium and carbon in the steel combine to form chromium
carbide particles along the grain boundaries throughout the steel. Formation of
these carbide particles in the grain boundaries depletes the surrounding metal
of chromium and reduces its corrosion resistance, allowing the steel to corrode
preferentially along the grain boundaries. Steel in this condition is said to be
It should be noted that carbide precipitation depends upon carbon content,
temperature and time at temperature. The most critical temperature range is
around 700°C, at which 0.06% carbon steels will precipitate carbides in about 2
minutes, whereas 0.02% carbon steels are effectively immune from this
It is possible to reclaim steel which suffers from carbide precipitation by
heating it above 1000°C, followed by water quenching to retain the carbon and
chromium in solution and so prevent the formation of carbides. Most structures
which are welded or heated cannot be given this heat treatment and therefore
special grades of steel have been designed to avoid this problem. These are the
stabilised grades 321 (stabilised with titanium) and 347 (stabilised with
niobium). Titanium and niobium each have much higher affinities for carbon than
chromium and therefore titanium carbides, niobium carbides and tantalum carbides
form instead of chromium carbides, leaving the chromium in solution and ensuring
full corrosion resistance.
Another method used to overcome intergranular corrosion is to use the extra
low carbon grades such as Grades 316L and 304L; these have extremely low carbon
levels (generally less than 0.03%) and are therefore considerably more resistant
to the precipitation of carbide.
Many environments do not cause intergranular corrosion in sensitised
austenitic stainless steels, for example, glacial acetic acid at room
temperature, alkaline salt solution such as sodium carbonate, potable water and
most inland bodies of fresh water. For such environments, it would not be
necessary to be concerned about sensitisation. There is also generally no
problem in light gauge steel since it usually cools very quickly following
welding or other exposure to high temperatures.
It is also the case that the presence of grain boundary carbides is not
harmful to the high temperature strength of stainless steels. Grades which are
specifically intended for these applications often intentionally have high
carbon contents as this increases their high temperature strength and creep
resistance. These are the “H” variants such as grades 304H, 316H, 321H and 347H,
and also 310. All of these have carbon contents deliberately in the range in
which precipitation will occur.
Because corrosion is an electrochemical process involving the flow of
electric current, corrosion can be generated by a galvanic effect which arises
from the contact of dissimilar metals in an electrolyte (an electrolyte is an
electrically conductive liquid). In fact three conditions are required for
galvanic corrosion to proceed; the two metals must be widely separated on the
galvanic series (see Figure 2), they must be in electrical contact and their
surfaces must be bridged by an electrically conducting fluid. Removal of any of
these three conditions will prevent galvanic corrosion.
Figure 2. Galvanic series for metals in flowing sea water.
The obvious means of prevention is therefore to avoid mixed metal
fabrications. Frequently this is not practical, but prevention can also be by
removing the electrical contact – this can be achieved by the use of plastic or
rubber washers or sleeves, or by ensuring the absence of the electrolyte such as
by improvement to draining or by the use of protective hoods. This effect is
also dependent upon the relative areas of the dissimilar metals. If the area of
the less noble material (the anodic material, further towards the right in
Figure 2) is large compared to that of the more noble (cathodic) the corrosive
effect is greatly reduced, and may in fact become negligible. Conversely a large
area of noble metal in contact with a small area of less noble will accelerate
the galvanic corrosion rate. For example it is common practice to fasten
aluminium sheets with stainless steel screws, but aluminium screws in a large
area of stainless steel are likely to rapidly corrode.
This combines elements of pitting, crevice and galvanic corrosion, and
occurs where small particles of foreign matter, in particular carbon steel, are
left on a stainless steel surface. The attack starts as a galvanic cell – the
particle of foreign matter is anodic and hence likely to be quickly corroded
away, but in severe cases a pit may also form in the stainless steel, and
pitting corrosion can continue from this point. The most prevalent cause is
debris from nearby grinding of carbon steel, or use of tools contaminated with
carbon steel. For this reason some fabricators have dedicated stainless steel
workshops where contact with carbon steel is totally avoided.
All workshops and warehouses handling or storing stainless steels must also
be aware of this potential problem, and take precautions to prevent it.
Protective plastic, wood or carpet strips can be used to prevent contact between
stainless steel products and carbon steel storage racks. Other handling
equipment to be protected includes fork lift tynes and crane lifting fixtures.
Clean fabric slings have often been found to be a useful alternative.
Passivation and Pickling
If stainless steel does become contaminated by carbon steel debris this can
be removed by passivation with dilute nitric acid or pickling with a mix of
hydrofluoric and nitric acids.
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