濃硫酸中における耐候性鋼の腐食特性

Abstract

The atmospheric corrosion of steel is thought to proceed in a concentrated sulfuric acid environment, because ferrous sulfate probably formed by its reaction with SO2 in atmosphere exists under the rust layer and is not oxidized immediately by oxygen supplied through the rust layer. Thus the corrosion characteristics of low-alloy steel in 98% sulfuric acid was investigated in comparison with carbon steel. The conclusions obtained were as follows:(1) The results of the rotating immersion test in 98% sulfuric acid showed that the corrosion rate of low-alloy steel was remarkably lowered with the lapse of time after four weeks of immersion. This behavior is quite different from that of carbon steel.(2) The anodic polarization of low-alloy steel was nearly the same as that of carbon steel in the early stage of immersion. However, as the corrosion progressed, the anodic reaction of low-alloy steel was suppressed more remarkably than that of carbon steel.(3) Corrosion products of both low-alloy steel and carbon steel were FeSO4 n H2O. However, from the analytical results by the X-ray microanalyzer, it was found for low-alloy steel that there was a very thin oxide film containing chromium, copper and other alloying elements in high concentrations at the alloy/iron sulfate interface.(4) Addition of Cl− ion into 98% sulfuric acid lowered the anodic polarization of steel. The role of Cl− ion is thought to destruct the stability of the passive oxide films. The passive film formed on low-alloy steel is more stable than that of carbon steel.From the above experimental results, the corrosion characteristics of steel in concentrated sulfuric acid is quite similar to that of atmospheric corrosion. It is certain that the very thin oxide film existing between the iron sulfate layer and the substrate metal plays a predominant role in the corrosion of steels.