As part of a program to develop a stainless steel (SS) with superior resistance to stress corrosion cracking (SCC) and intergranular attack (IGA), an investigation was initiated to identify possible ways to improve the properties of 18Cr-8Ni SSs in regard to both these’ forms of corrosion. This paper discusses one aspect of the general program, the influence of silicon additions on the general and localized corrosion behavior of 18Cr-8Ni-(bal)Fe-type alloys in dilute-acid media.
The results show that increases in silicon from 0 to 2.85 wt% caused a fourfold increase in the general corrosion rate for alloys in the solution-annealed condition. Subsequent additions of 3.73 and 4.45 wt% silicon resulted in a significant decrease of up to ten times in corrosion rate compared with the base alloy. For alloys in the sensitized condition, increasing the silicon content resulted in a general lowering of Ic ~26 to 1 to 3 mA/cm2, along with a decrease in the passive corrosion rate. Analysis of electrochemical polarization curves indicated that the beneficial effect of silicon on the corrosion rates of annealed alloys results from changes in the kinetics of the cathodic partial process. Furthermore, the beneficial effects observed in the sensitized alloys result from changes in the anodic Tafel constant βa.
These data indicate the significant potential improvements in the corrosion behavior of 18Cr-8Ni type SSs having controlled amounts of δ-ferrite (stabilized by silicon addition up to 5%) in 0.5 M H2SO4.
