Stainless Steels and Alloys: Why They Resist Corrosion
and How They Fail*
The trend observed in figure 11 is similar with Figure
10. Both figures revealed that SAF2304 is less resistant to pitting than
the other two. Simple and rapid measurements of this kind can be used to
evaluate and select the right grade of stainless steel for a specified
application. Critical temperature, critical pitting potential and the protection
potential can be determined in a given process condition such that pitting
corrosion of stainless steels and alloys can be minimized or avoided.
It was reported that the pitting potential for type 304
stainless steel is directly proportional to the concentration of chloride
ion on the semi-log scale [12-14]:
Epit= A + B log[Cl-]
At the design stage, knowledge of the nature of environment
and particularly the concentration level of chloride ions would be useful
in assessing the suitability of a specific grade of stainless steel for
the intended application.
In addition to the environmental considerations, the pitting
and crevice corrosion resistance of stainless steels is also determined
by alloying elements such as chomium, molybdenum, nitrogen and tungsten.
The synergistic effects of these alloying elements help to stabilise the
passive film, and in case of breakdown, rapid repassivation can take place
to heal the damaged area. This compositional effect is commonly represented
by the "pitting resistance equivalent number, PREN":
PREN = Cr% + 3.3x(Mo%) + 16 x (N%) + 1.65 (W%)
The numerical value derived from the chemical compositions
of a specific grade of stainless steel is sometimes used as an indication
to the pitting and crevice corrosion resistance. For example, a PREN value
of 40 is required for a stainless steel to be resistant to localised corrosion
in deoxygenated sea water. Super duplex and super austenitic stainless
steels (6Mo) meet this compositional requirement and in fact are resistant
to pitting and crevice corrosion in deoxygenated sea water.
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