Abstract
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 catholic activity, the spontaneously passivated film consists of remarkably concentrated chromium ions along with some molybdenum ions.
