The Corrosion Behavior of Ni-Cr-Mo Ternary Alloys in Hot Concentrated Sulfuric Acids with Active Carbon (Part 3)

The Roles of Small Addition of Tantalum to Ni-Cr-Mo Alloys

Abstract

Small addition of tantalum to Ni-20Cr-20Mo alloy improves the corrosion resistance to sulfuric acid dew point corrosion. The roles of tantalum in enhancing the corrosion resistance in 60 and 80% H₂SO₄ with active carbon at 120°C was investigated. Dissolution of the alloy containing 2wt% tantalum suppresses in the active region and the alloy is caused to passivate spontaneously. The passive current of the alloy is lower than that of the tantalum free alloy. XPS analysis was carried out to examine the surface films formed on these alloys immersed or polarized in the acids with active carbon. The cationic fraction of tantalum in the surface films on the alloy containing tantalum is so low as it is about 0.02. However, the ratio of O^2-/total oxygen in the surface films on the alloy containing tantalum is higher than the ratio on the tantalum free alloy. It is as a result of enhancement of the surface films by the formation of composite chromium-tantalum oxy-hydroxides, which have higher stability.