corrosion of stainless steel - Damstahl

Chapter 6 - Corrosion of Stainless Steel
Figure 6.9:
Pitting corrosion in 0.5 mm thick, stainless steel 4301 sheet after only
a few days immersed in a mixture of salt (NaCl) and hydrogenperoxide
(H 2
O 2
). While 99% of the surface is completely unaffected, four day's of
corrosion was sufficient to produce penetration. The picture on the right is
a microscope photo of the same item; the penetration is right in the center.
6.2.1 Environmental factors in pitting corrosion
General corrosion is, as described above, a type of corrosion which is always
taking place. Depending on the type of acid, pH, temperature, impurities,
type of steel and other factors, the corrosion will take place at a faster or
slower pace, but it will never be zero. Compared to general corrosion, pitting
corrosion is much more an either/or type of corrosion. Either things go
incredibly well or else they go very, very badly. There is nothing in between.
The environmental factors influencing the risk of pitting corrosion for a given
type of stainless steel are usually the following:
Chloride concentration
Temperature
Corrosion potential (type and concentration of oxidants, the
cathode reaction)
pH (acidity)
In general the risk of pitting corrosion rises with increasing chloride
concentration, increasing temperature, increasing corrosion potential (cathode
reaction) and decreasing pH (more acidic solution).
6.2.2 Critical Pitting Temperature (CPT)
The interaction between, in particular, chloride concentration and
temperature can be measured using the 'critical pitting temperature' (CPT).
For a certain type of steel in a certain medium (and at a determined corrosion
potential), the CPT is the temperature above which pitting corrosion will
take place. In practice, CPT is measured by exposing a steel electrode to
the medium concerned. The temperature is slowly increased step by step,
and the temperature at which pitting corrosion occurs is defined as CPT.
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