Dependence of Corrosion Fatigue Crack Initiation Mechanisms on the Corrosion Behavior of Two Stainless Chromium Steels

Abstract

Abstract Corrosion fatigue experiments have been carried out in a 0.5N and 4N sodium chloride solution with pH of 6.5 at 80 C. The solution has been air saturated. Two different chromium steels were examined, a tempered martensitic steel with 12% chromium and a pure ferritic steel with 18% chromium and 2% molybdenum. The results of this investigation showed that the corrosion fatigue crack initiation mechanisms are directly related to the type of corrosion attack. Under pitting corrosion conditions, a pronounced decrease in fatigue limit has been measured. Under passive corrosion conditions, a corrosion fatigue limit exists, which could be related to the thickness of the passive surface film and the surface slip characteristics of the alloy. It has been shown that under passive corrosion conditions, the corrosion fatigue limit cannot be lower than the cyclic yield strength of the material. Nonmetallic inclusions were found to play an important role in corrosion fatigue crack initiation. In the 12% chromium steel, sulfide inclusions acted as pit initiation sites. Chemically inert niobiumcarbonitrides in the 18% chromium steel induced local corrosion attack, but only under corrosion fatigue conditions in the 4N sodium chloride solution.