High speed straining electrode technique has been applied for measuring the current for dissolution and film formation at newly created surfaces of iron, nickel, and several kinds of Fe-Cr-Ni alloys in boiling 46% MgCl₂ solution. Nickel content of alloys changes keeping chromium content between 15% to 19%. It is found that the maximum current density observed upon straining for a given amount at the corrosion potential decreases with the increase of nickel content. The film formation kinetics is described as i=k/(Q-Q₀)^m, where i is the current density, Q is the amount of charge involved and Q₀, k and m are constants. The value, m, also decreases with the increase of nickel content, exhibiting the highest value for 304 type steel. The higher susceptibility of alloys to stress corrosion cracking is closely connected with the higher maximum current density corresponding to the higher dissolution rate at fresh surfaces and also the larger value of m which indicates the higher inhibitive action of film against successive dissolution.

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The grooving corrosion mechanism of an ordinary electric resistance welded (ERW) carbon steel pipe in sea water was investigated using electrochemical and microscopic observation techniques. Deep grooving corrosion was observed to occur at the narrow welded parts after rotating immersion test in sea water at 40°C for one year, and no grooving corrosion was observed in the matrix of the pipe. Potential measurement using micro-electrode showed that the potential of the weld was ca. 70mV lower than that of the matrix of the pipe. Optical and scanning electron microscopy revealed that the non-metallic inclusions, mostly manganese sulfide, were arranged connectedly at the outside and inside surfaces of the weld due to the concentrated and exposed metal flows. Observation of the grooving formation in 3% NaCl at 40°C using scanning electron microscope revealed that corrosion pits were developed in the initial stages around manganese sulfide inclusions stretched in the weld, and later the pits were enlarged by chemical dissolution of the manganese sulfide inclusions due to the lowering of pH in pits or by mechanically washing out of the inclusions. As these local corrosion pits were formed by most of the connected manganese sulfide inclusions in the weld, they grew up to the deep groove with the aid of macro-cell formed between the narrow weld and the matrix (wide cathode) of the pipe. On the other hand, in the matrix of the pipe corrosion pits were not developed around manganese sulfide inclusions. It is concluded that the manganese sulfide inclusions in the weld are accompanied by sulfur rich parts caused by rapid heating and cooling, and the sulfur rich parts are dissolved preferentially by the action of the local cell with manganese sulfide inclusions.

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The effect of sulphate reducing bacteria on corrosion and cathodic protection of a steel has been studied in silt obtained from the bottom of Sumida River and in an inoculated midium consisted of peptone, yeast extract, MgSO₄, Na₂SO₄, NaCl, lactic acid and water. Sulphate reducing bacteria accelerates corrosion by depolarizing cathodic reaction. The film formed on the steel surface through reaction with hydrogen sulphide, a metabolic product of the bacteria, is essentially protective but it tends to break down locally to form galvanic cells which accelerate corrosion. The critical potential for complete cathodic protection of the steel is -0.9V (SCE) which is approximately 0.1V more negative than the ordinary protection potential in the absence of the bacteria.

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