2012 Volume 53 Issue 6 Pages 1048-1055
The effect of copper (Cu) addition on the active corrosion behavior of hyper duplex stainless steels in sulfuric acid was investigated. The addition of Cu in the base alloy enhanced the resistance to general corrosion by decreasing the critical and corrosion current densities, and increasing the polarization resistance. There are two primary reasons for the considerable enhancement of the corrosion resistance of the experimental alloys containing Cu. First, the protective surface film was enriched with the noble metallic copper (Cu) due to the selective dissolution of the active metallic Cr, Fe, and Ni, and the electrochemical dissolution of the corrosion products such as iron-sulfide (FeS2), iron sulfate (FeSO4), ferrous oxide (FeO) and hydrous iron sulfate (FeSO4·7H2O). Second, chromium oxide (Cr2O3), chromium trioxide (CrO3), nickel oxide (NiO), molybdenum dioxide (MoO2), molybdenum trioxide (MoO3), and tungsten trioxide (WO3) in an oxide state, molybdenum oxy-hydroxide (MoO[OH]2) and chromium hydroxide (Cr[OH]3) in a hydro-oxide state, molybdate (MoO42−) and tungstate (WO42−) as corrosion inhibitors in an ion state, and ammonium (NH4+) elevating the pH in an ion state were increased and assisted in improving the corrosion resistance.