The corrosion behavior of Ni-Cr-Mo ternary alloys in hot concentrated sulfuric acids with active carbon (part 2)

The detrimental effect of active carbon on dew point corrosion by carbon-containing sulfuric acids was investigated using Ni-Cr-Mo alloys in 60 and 80% H₂SO₄ with and without active carbon at 120° C. The addition of active carbon leads to an increase in the dissolution rate of the alloys over a wide potential rage from -900 to 1200 mV vs. SCE, along with an increase in current densities of both anodic and cathodic reactions. Active carbon decomposes adsorbed molecular Sulfuric acid into active oxygen and sulfur dioxide, the latter of which is identified by gas chromatography. The reaction of active carbon with sulfuric acid also forms sulfonic acids. These reactions lead to a decrease in the intrinsic concentration of sulfuric acid and an increase in the corrosiveness in addition to acceleration of cathodic reaction. Spontaneous passivation in carbon-containing 80% H₂SO₄ occurs by the formation of a remarkably chromium-enriched passive film containing about 20 cationic % of molybdenum.