corrosion of stainless steel - Damstahl

Chapter 6 - Corrosion of Stainless Steel
6.1.2 Environmental Factors in Acids
The risk of general corrosion of stainless steels exposed to acids normally
depends on:
The acid type and concentration (acidity, pH)
The acid's oxidation capacity
Temperature
Type and concentration of impurities
Most often: The
higher the acid
concentration, the
worse the corrosion
Oxidising and
non-oxidising
acids
The acid's concentration is not as unequivocal as might be expected. For
most types of acid, higher concentrations generally mean greater weight loss
of the steel. However, at extremely high acid concentrations - e.g. > 90% of
sulphuric acid (H 2
SO 4
) - the lack of water actually results in the acid failing
to function properly as an acid, which is to say that water is thus the limiting
factor. This is only the case, however, with sulphuric acid; for almost all other
acids, the following general rule applies: The higher the concentration, the
worse the corrosion.
For all non-passivating alloys, it generally follows that the more active the
cathode reaction (see Chapter 5), the worse the corrosion, though this is
not necessarily the case with passive alloys. The formation of passivating
chromium oxides requires a certain oxidising effect, and, up to a certain
limit, stainless steel thus performs better in oxidising acids than in nonoxidising
acids.
This effect is illustrated in Figure 5.8 on page 64, where the strong, nonoxidising
acids (sulphuric acid, phosphoric acid, etc.) will typically make
the steel fall into the yellow ”active area” with resulting high corrosion
rates. An oxidising acid, on the other hand, such as a pure nitric acid or
peracetic acid will make the steel jump up into the green ”passive area”
where the corrosion rate is far lower despite the higher potential (and thereby
theoretically greater driving force). The corrosion rate is not only governed
by energy, but just as much by kinetics.
This effect is clearly seen with stainless steel in sulphuric acid (see
Figure 6.3) where even small amounts of the highly oxidising chromic acid,
CrO 3
, has a strong inhibiting effect on the corrosion of 4307 steel. This is
seen by the curve's high location in relation to the pure sulphuric acid. If the
concentration of CrO 3
is increased beyond the indicated 0.2-0.5%, the curve
will fall again as a sign of ”overdosing”. Other oxidants such as hydrogen
peroxide (H 2
O 2
) has the same beneficial effect and can therefore be used as
a corrosion inhibitor in sulphuric acid. Perioxide-inhibited sulphuric acid
is often used as a substitute for nitric acid in applications where remains
of nitrates (NO 3–
) for various reasons are unwelcome, e.g. in the cleaning
of milk tanks.
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