Pitting corrosion was found on interior walls of boiler tubes made of carbon steel in a thermal power station which has been operated for about three years. The interior walls of tubes were covered by a scale of Potter-Mann type, double magnetite layers. Almost all the pits were filled with the magnetite scale and could not be detected unless the scale was removed. In the thick scale formed on the interior wall of tube, whose outer surface was faced to flame, metallic copper particles, like markers, were embeded in near one-half of the over-all magnetite thickness, forming nearly a line paralel to the steel surface. On the contrary, the metallic copper particles existed in the surface of thin scale on the opposite interior wall of the same tube, whose outer surface was embeded in the boiler wall. The metallic copper in the thick scale seemed to be formed in the early stage of operation by reduction of copper ions which were produced by corrosion of condenser tubes of copper alloy. The metallic copper particles were often found in the center of the scale inside the pits. It was concluded from the detailed examination that pitting corrosion took place not during the operation of boiler but during pickling and/or other treatments before the commencement of operation. Because the scale/steel interface was smoothed with scale thickening, pitting corrosion thus developed assumes not to be dangerous for the operation of boiler.

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In the present study, effects of matrix grain size and its growth during oxidation upon scaling behaviours in 760 Torr oxygen at 1000°C of iron-18%chromium-manganese alloys were examined by thermogravimetric methods, X-ray diffraction and electron-probe micro-analysis. When the alloy containing 0.19% manganese was oxidized, breakaway oxidation was caused by the disruption of a Cr₂O₃ rich protective scale and a thick stratified scale was formed containing α-Fe₂O₃ and Fe, Cr spinel oxide. However, the breakaway was not observed by the several oxidation runs for 5hr of fine grained iron-18%chromium-1.50%manganese alloys, because MnCr₂O₄ scale which has great plasticity was produced uniformly on the alloys at the initial stage. When coarse grained samples containing 1.50% manganese was oxidized, MnCr₂O₄ developed unevenly at the alloy/oxide interface, which induced the stress in the protective scale and breakaway phenomena were sometimes observed.

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The effects of Cl^- and Fe³⁺ on the dissolution of stainless steels in 5-25%H₂SO₄ have been studied at 75°C. The dissolution of types 304 and 321 is retarded by 100-5, 000ppm Cl^-. The dissolution of type 316, on the other hand, increases with the Cl^- concentration in the range of 100-1, 500ppm and decreases on further additions of Cl^- up to 5, 000ppm. These effects of Cl^- can be explained by the change of cathodic polarization behavior. The simultaneous presence of Cl^- and Fe³⁺ above certain concentrations causes pitting, general dissolution and the combination thereof depending on their concentrations. The last form of corrosion is the result of the change of the corrosion potential from the pitting region to the active region, caused by the change of the anodic polarization curve as pits develop.

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Corrosion inhibitors for metals were classified into three general types, oxidation, deposition, and adsorption ones. Mechanisms of corrosion inhibition with these types of inhibitors were explained by using polarization curves. Applications of the inhibitors in aerated neutral solutions, acid and basic media, oil-containing systems, and atmospheric environments were also described.

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