Abstract
Analytical and electrochemical methods were used to study the dissolution behavior of annealed SUS 304 stainless steel in the descaling process in order to determine the annealing conditions needed to make scale easily descalable. The following conclusions are advanced:(1) With anodic electrolysis in 20% Na2SO4 solution (pH 3), oxides of Cr, Mn and Fe in the outer layer of the scale are dissolved relatively quickly, but Fe in the inner layer is dissolved at a considerably low rate. The amount of dissolved Fe increases markedly at a potential above 1.4V vs. SCE, which is nobler than that of Cr and Mn by 0.2∼0.3V. Cathodic electrolysis is of no use in dissolving these oxides. Si oxide in the inner layer of the scale is not dissolved by electrolysis in 20% Na2SO4 solution and obstructs the dissolution of other coexisting elements such as Cr and Fe, but it is dissolved by immersion in nitric-hydrofluoric acid.(2) To reduce the time required for descaling, it is important to lower the Si and Fe content in the scale and raise the Cr and Mn content, while making the scale thinner.
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