corrosion of stainless steel - Damstahl

Chapter 6 - Corrosion of Stainless Steel
General
corrosion in
molten salts
In addition to aqueous media, general corrosion is a risk if the stainless
steel is exposed to molten salts. Just as in strong acids, liquid, chloride and
fluoride salts (e.g. fluxes for soldering, temperatures typically > 400°C) may
entail total breakdown of the protective oxide layer of the stainless steel
which will result in rapid general corrosion. General corrosion in alkaline
media or molten salts is, however, rare. Below the emphasis will therefore
be on the strong, aqueous acids. A typical example of general corrosion of
stainless steel in molten salts can be found in Figure 6.6.
Even in strong acids, stainless steel can achieve a certain level of passivity,
and in practice there is a big difference between an acid that simply thins
the passive layer, but does not entirely dissolve it, and an acid that eats away
the whole of the passive layer and immediately attacks the underlying steel.
Relatively weak and pure acids (e.g. citric acid, acetic acid or formic acid)
only thin the naturally occurring oxide layer slightly. The oxide layer is
restored with the same speed, and corrosion loss is minimal - at least when
the acids are pure. If the acids contain aggressive ions (especially chloride),
the situation may become far worse, especially at high temperatures.
Isocorrosion
diagrams
indicate
combinations
of metals and
environments
with the same
corrosion rate
6.1.1 Isocorrosion Diagrams
The corrosivity of a certain acid towards various types of stainless steel can
be read on a so-called isocorrosion diagram. An isocorrosion diagram is a set
of curves showing the acid concentration on the x axis and the temperature
on the y axis. Each curve shows the conditions that are required to peel off
precisely 0.1 mm of steel per year. Above the curve, conditions are more
corrosive than 0.1 mm/year, while under the curve conditions are milder.
An example of an isocorrosion diagram for different types of stainless steel
in sulphuric acid can be found in Figure 6.3.
The corrosion rate for a given combination of stainless steel, acid and
temperature can be determined by simple weight loss experiments. All you
need is a sheet with a known area and known weight, and after a certain
length of time in the acid the sheet is weighed again - the amount of metal
that has corroded away can then be calculated. This technique can only
be used for corrosion experiments where the loss of material is entirely or
almost entirely homogenous - such as general corrosion. For all other types
of corrosion, the weight loss technique will, at best, be an imprecise way of
measuring corrosion.
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