corrosion of stainless steel - Damstahl

Chapter 6 - Corrosion of Stainless Steel
Figure 6.21:
Crevice corrosion in press fitting (left) and tubes (right) of 4404 as a result
of bacterial growth in tap water installation. The corrosion arose in water
with a temperature of 45-50°C with less than 100 ppm of chloride, and it
should be noted that the corrosion (the light grey areas on the right) has
commenced in the narrowest places where the sleeve has been clamped
around the tube.
The effect of bacteria and biolayers is not, however, limited to sewage
plants. All non-sterile media (i.e. nearly all media!) have the capacity for
bacterial influence on the corrosion conditions, and even otherwise clean
drinking water can hold unpleasant surprises. As the press fitting system
was introduced more than 20 years ago, all pipes and fittings were made of
standard 4301 which should in theory be enough for cold tap water at least
with chloride under 100-150 mg/l (see Figure 6.10 and Figure 6.18). This
was not the case, and modern press fittings are all made of the superior,
acid-resistant steel (4404). This works in by far most cases, but there are
rare examples of bacteria (or combinations of bacteria) that are so aggressive
that even acid-resistant steel will suffer from crevice corrosion.
Molybdenum
is good
against crevice
corrosion
6.3.2 Combating Crevice Corrosion
Due to the slightly odd environmental conditions, the time factor, the effect
of microbiology and the impossible task of creating a 'well-defined crevice',
crevice corrosion is one of the most difficult forms of corrosion to fight. The
alloying elements that best fight crevice corrosion are usually the same as
the ones that are useful against pitting corrosion. A ranking of the various
types of steel will therefore look very much like Table 6.1.
But there are certain differences between pitting corrosion and crevice
corrosion. Molybdenum passivates significantly better in oxygen-poor media
(and thereby crevices) than chromium, and the steel's molybdenum content
should therefore be expected to mean more in crevice corrosion than pitting
corrosion. From experience it can be seen that molybdenum is therefore even
more important in combating crevice corrosion than pitting corrosion, and
it should therefore be expected that the PREN factor between chromium and
molybdenum in crevice corrosion would be higher than the 3.3 described
in Eq. 6.1 and used in Table 6.1.
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