AUSTRALIAN STAINLESS STEEL DEVELOPMENT ASSOCIATION
GENERAL CORROSION RESISTANCE
The normal state for stainless
ASSDA
Technical FAQ No 8 8
Stainless steels resist corrosion because they have a self- Most of the following graphs are from the Outokumpu Corrosion
repairing “passive” oxide film on the surface. As long Handbook. The specific alloy compositions are tabulated in that
as there is sufficient oxygen to maintain this film and Handbook and in the Appendix of this FAQ.
provided that the level of corrosives is below the steel’s However, a series of graphs each showing the results for one
capacity of the particular material to repair itself, no material over the full range of concentrations and temperatures
corrosion occurs. If there is too high a level of (say) is cumbersome and so multi-material plots are used for the initial
chlorides, pitting occurs. As an example, 316 works material selection. Titanium is frequently included because of the
well in tap water (<250ppm) all over Australia, but will widespread expectation that it is the “super” solution – although
rapidly corrode in seawater because seawater has very the data shows this is not always correct.
high chloride levels (20,000ppm). The two graphs below show data for austenitic and duplex
stainless grades in pure sulphuric acid. However, only the 0.1mm/
If there is not enough oxygen and the local corrosives are not high year lines are drawn for each alloy because it is assumed that a
enough to cause pitting, then general corrosion can occur. This
loss of 0.1mm/year would be acceptable for continuous exposure
might happen in a crevice (which has very limited oxygen) or in a
strong, reducing acid (such as mid concentrations of sulphuric during 365 days per year. This assumption may not be acceptable
acid). General corrosion can occur when there are stray currents if, for example, the process using the acid required very low iron
flowing from stainless steel to ground. This can happen in mineral levels. For each material, the temperature and concentrations of
extraction if the bonding arrangements are inadequate during pure sulphuric acid that are below the line would mean a corrosion
electrowinning. General corrosion may also occur from galvanic rate of less than 0.1mm/year.
effects, e.g. if a conductive carbon gasket is used on stainless steel
in an aggressive environment.
QUANTIFYING CORROSION RESISTANCE
For circumstances where general corrosion is expected, graphs
are available called iso-corrosion curves. They plot the effect
of a single chemical and corrosion rate for temperature against
concentration. An example is the graph below of a 42% nickel
alloy 825 in pure sulphuric acid with air access. This graph shows
that the corrosion rate increases with temperature and that
provided the temperature is less then ~45°C and a corrosion rate
of 0.13mm/year is acceptable, alloy 825 would be suitable for any
concentration of pure sulphuric acid. The boiling point curve is
often included to show the limits of data at atmospheric pressure.
These rates are a hard conversion from inch units (0.001 inch per year),
whereas metric data refers to <0.1mm/year, <0.5mm/year and >0.5mm/
year and may include cautions about risks of pitting or cracking.
WHAT ABOUT IMPURITIES OR ADDITIVES?
The graphs below show (and note the temperature scale changes
from earlier graphs) the dramatic reduction in corrosion resistance
when 200mg/L of chlorides are added to sulphuric acid or ten
times that amount, i.e. 2,000mg/L. The heavily reducing range
from about 40% to 60% acid concentration defeats even the high
nickel 904L and 254/654 grades.
Nevertheless, a number of grades are potentially suitable for
concentrations below 20% sulphuric even with significant
chlorides. However, the graphs also show that at the other end of
the concentration scale, the oxidising conditions, which occur for
sulphuric acid above about 90%, are extremely aggressive if the
acid is impure.
The data in this section is intended to show that while these iso-
corrosion graphs are useful in predicting corrosion rates for specific
pure compounds, the addition of aggressive ions, oxidisers or
crevice conditions require more detailed consideration.
MATERIALS SELECTION FOR OTHER CHEMICALS
A very common chemical is phosphoric acid, which is used in
cleaning, pre-treatments, food preparation and a host of other
applications. It requires increasing chemical resistance with high
temperatures and concentrations. For pure phosphoric acid, the
iso-corrosion curves show a progression from ferritic 444, through
the austenitic 304, 316, 317 to 904L. This is not an oxidising acid
so although it removes iron contamination, it does not strengthen
Some additives act as inhibitors to corrosion and this can be critical
the passive film on stainless steels.
in selecting suitable materials for mineral extraction processes.
For example, the graph below shows that adding iron ions to Phosphoric acid is frequently associated with chloride or fluoride
sulphuric acid improves the resistance of 316. Adding oxidising ions especially in production from rock phosphate. The variation in
cupric ions has a similar effect but as with any inhibitor, attack composition in this wet process acid (WPA) means that iso-corrosion
can occur in crevices where the inhibitors may be used up. And plots are of limited use. However, with thermally produced acid
despite the requirement for oxidising conditions to ensure stability and various impurities, a plot of corrosion rate vs. contaminant ion
of the stainless steel’s passive layer, it is possible to add too much concentration may be used instead of an iso-corrosion graph – in
oxidant as shown by the positive effect of small additions of this case chlorides with the 2.5% molybdenum version of 316.
chromic acid followed by a reduction in corrosion resistance if This data is for exposure 24 hours a day, 365 days a year. Note that
more chromic acid is added. It is relatively common to refer to the while the two graphs do not overlap, the trends of these different
redox potential (rather than concentrations of oxidising ions) if the experimental plots do not exactly match, i.e. iso-corrosion curves
chemistry is not simple. provide trend data and not precise values.
ALKALIS
As shown by the plot, austenitic stainless steels are resistant to
general corrosion for all concentrations of sodium hydroxide and,
for high concentrations, the usual problem is lack of solubility.
However, at near boiling temperatures, austenitic stainless steels
(and especially those with extensive chromium carbide precipitates)
are susceptible to cracking as shown by the shaded area.
SUMMARY
If you intend to use a stainless steel with a new, relatively pure
chemical, iso-corrosion curves offer an initial guide to the
temperature and concentration limits against general attack. If
ACIDS FOR CLEANING STAINLESS STEELS there are contaminants or oxidants present, then the corrosion
susceptibility can increase or decrease significantly and specialist
Both the chelating oxalic and citric acids, and the oxidising nitric advice should be obtained.
acid, are widely used on stainless steels both for cleaning and
passivation as shown in ASTM A380 and A967. Nitric acid can be
used at elevated temperatures and low to medium concentrations
without concern for the standard austenitics. However, at high
concentrations and above ambient temperatures, they can suffer
intergranular attack, unless a low carbon grade is used. In the
same environment, molybdenum-containing grades may suffer
intergranular attack of the intermetallic phases such as sigma.
APPENDIX
TYPICAL COMPOSITION OF PRINCIPAL ELEMENTS
GRADE LABEL Cr Ni Mo Cu OTHER
410S 13Cr 13 <0.6 - - -
444 18-2 18 <1 2 - Ti + Nb
304 18-10 18 8 - - -
316 17-12-2.5 16.5 10.5 2 - N
317 18 11 3 - -
904L 19 23 4.5 1.5
6% Mo 254 20 18 6 0.6 N
654 24 22 7.3 0.5 N 0.5
2304 23 4.8 0.3 - N
2205 22 5.7 3.1 - N
2507 25 7 4 - N
825 825 20 42 3 2 Ti 0.8
The technical recommendations contained in this publication are necessarily of a general nature and should not be relied on for specific applications without first securing
competent advice. Whilst ASSDA has taken all reasonable steps to ensure the information contained herein is accurate and current, ASSDA does not warrant the accuracy or
completeness of the information and does not accept liability for errors or omissions.
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Brisbane QLD AUSTRALIA 4000
www.assda.asn.au
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