Electrochemical Behavior and Passive Film Properties of Hastelloy C22 Alloy, Laser-Cladding C22 Coating, and Ti–6Al–4V Alloy in Sulfuric Acid Dew-Point Corrosion Environment

The electrochemical behavior and passive film properties of Hastelloy C22 alloy, laser-cladding C22 coating, and Ti–6Al–4V alloy in sulfuric acid dew-point corrosion environment were investigated through a combination of electrochemical measurements and surface analyses. The C22 alloy and laser-cladding C22 coating exhibited similar passivation and repassivation behavior without pitting corrosion, resulting from a similar passive film with a bilayer structure consisting of a Cr2O3-dominated compact inner layer and a porous outer layer containing oxides of Mo and hydroxides of Ni and Cr. The slightly poorer corrosion resistance and higher sensitivity to localized corrosion exhibited by the C22 coating were attributed to the microscale heterogeneity of the passive film resulting from the element segregation in the microstructure introduced by the laser-cladding process. The corrosion of the TC4 alloy performed as the preference dissolution of the β phase. Compared to the C22 alloy and C22 coating, the TC4 alloy exhibited more stable passivation behavior but poorer corrosion resistance, which is attributed to a compact but less protective single-layer passive film consisting of oxides of Ti and Al. An increase in temperature degrades passive film stability and accelerates the charge transfer process.