Role of molybdenum in corrosion of iron-based alloys in contact with hydrogen sulfide containing solution

A sample iron molybdenum alloy with 3.4 wt% (2 at%) molybdenum, and pure iron, are exposed to hydrogen sulfide saturated saline solution for up to 56 days. In addition, their behavior under anodic polarization in the same electrolyte is investigated. The initially fast dissolution of the iron molybdenum alloy slows down significantly over time, while iron corrodes with a constant rate. The observed slow down of the corrosion rate can be described well with an exponential decay of the instantaneous corrosion rate with a time constant of (0.15 ± 0.03)/day, which implies stop in corrosion in practical terms after ≈2 weeks. Relationships are discussed between the instantaneous corrosion rate, and the time-averaged integral corrosion rate. Dissolution under anodic polarization of the iron molybdenum alloy is slower than for pure iron. While at certain times, pyrite, FeS2, is found as corrosion product, the main corrosion product is mackinawite, FeS. The latter likely contains a certain fraction of molybdenum in case of the iron molybdenum alloy. On iron molybdenum, corrosion products forming a sealing layer are observed, which slow down further corrosion. The corrosion products on iron molybdenum show better adhesion to the base material surface.