Pitting is likely to occur when the alloy content
is not adequately high for the corrosive environment.
One indicator is the PREN (Pitting Resistance Eqivalent
Number, defined as
%Cr + 3.3 x %Mo + 16 x %N) value.
For example, in cold flowing seawater:
17%Cr (PREN 18) and 18Cr8Ni (PREN 20) will pit.
18Cr12Ni2.5Mo (PREN 26) is only marginal.
Super-austenitics and super-duplex can perform well.
Warmer seawaters require higher PRENs. Other factors,
such as stagnation, fouling, crevices, etc. may affect
performance.
Corrosion fatigue performance of different stainless
steels vary considerably and a significant factor is
the notch effect caused by pitting. Steels with high
PREN values can give unnotched corrosion fatigue values
in seawater approaching those in air, whereas lower
PREN
steels drop to around 25% of their air values.
Corrosion fatigue resistance may be reduced by crevices
and improved by appropriate galvanic or cathodic protection.
