Factors affecting initiation of pitting corrosion in super martensitic stainless steel weldments

In the present investigation optical microscopy, in combination with sputte red neutral mass spectrometry (SNMS), has been used to examine the conditio ns for initiation of pitting corrosion in the heat affected zone (HAZ) of t wo super martensitic (SM) stainless steel weldments. It is shown that the c orrosion resistance depends mainly on the nature of the surface oxide, as d etermined by the oxygen potential in the shielding gas and the HAZ temperat ure-time relationship, and less strongly on the underlying microstructure. In the absence of HS the initiation is associated with spalling of the iron enriched chromium oxide within a narrow region approximately 6 mm from the fusion line, where the contaminated oxide laver is thinnest. As H2S is int roduced, the region close to the fusion boundary becomes susceptible to loc alised corrosion because of the more extensive metal oxidation. Thus. the h igh temperature oxidation anti iron oxide scale formation accompanying the welding operation appear to he the main factors affecting pitting corrosion initiation in SM stainless steel weldments.