The Corrosion Behavior of Ni-Cr-Mo Ternary Alloys in Hot Concentrated Sulfuric Acids with Active Carbon (Part 1)

In order to prevent dew point corrosion by sulfuric acid, the roles of chromium and molybdenum in enhancing the corrosion resistance in 60 and 80% H₂SO₄, at 120°C were investigated. Particular attention was paid to the addition of active carbon to H₂SO₄. The chromium contents of alloys were 16 and 20% and the molybdenum contents were 17, 20, 24 and 25%. In carbon-free H₂SO₄ increasing molybdenum content gives rise to a decrease in corrosion rate and Ni-20Cr-24Mo alloy shows high corrosion resistance by spontaneous passivation forming a remarkably molybdenum-enriched passive film. In carbon-containing H₂SO₄ increasing chromium content is effective in decreasing the corrosion rate. Three Ni-20Cr-Mo alloys are spontaneously passive in 80% H₂SO₄ with carbon. Because the carbon addition increases the open circuit potential exceeding the stability limit of tetravalent molybdenum as a result of enhancement of the cathodic activity, the spontaneously passivated film consists of remarkably concentrated chromium ions along with some molybdenum ions.