Time Variation of Pit Depth Distribution on Type 304 Stainless Steels in FeCl3 Solutions
Hitoshi Yashiro, Wataru Noro, Kazuo Tanno
The distribution of pit depth on type 304 stainless steels generated in FeCl3 solution has been investigated as a function of immersion time, in view of demand for experimentaldata to establish a deterministic model for their life prediction. The early depth distribution showed several peaks corresponding to those in corrosion potential which suggests wavelike generation of pits. The distribution in growing stage split at approximately 40μm which seems to be the critical depth for maturation. The matured pit can continue to grow as long as the corrosion potential exceeds repassivation potential. The profile of the whole distribution became less steep with time. Although the rate constant for increase of pit depth did not seem to be uniform, the maximum pit depth was analyzed to study the kinetics of pit growth. The depth (h) was proportional to the square root of time:
The growth rate was independent of potential and agitation. A rise in temperature caused increase in both pit density and growth rate.