Under certain conditions, particularly involving high concentrations of chlorides (such as sodium chloride in sea water), moderately high temperatures and exacerbated by low pH (ie acidic conditions), very localised corrosion can occur leading to perforation of pipes and fittings etc. This is not related to published corrosion data as it is an extremely localised and severe corrosion which can penetrate right through the cross section of the component. Grades high in chromium, and particularly molybdenum and nitrogen, are more resistant to pitting corrosion.
Pitting Resistance Equivalent number (PRE)
The Pitting Resistance Equivalent number (PRE) has been found to give a good indication of the pitting resistance of stainless steels. The PRE can be calculated as:
PRE = %Cr + 3.3 x %Mo + 16 x %N
One reason why pitting corrosion is so serious is that once a pit is initiated there is a strong tendency for it to continue to grow, even although the majority of the surrounding steel is still untouched.
The tendency for a particular steel to be attacked by pitting corrosion can be evaluated in the laboratory. A number of standard tests have been devised, the most common of which is that given in ASTM G48. A graph can be drawn giving the temperature at which pitting corrosion is likely to occur, as shown in Figure beside.
This is based on a standard ferric chloride laboratory test, but does predict outcomes in many service conditions.
Source: http://www.azom.com/details.asp?articleID=1177
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