The applications of chromium diffusion coating on nickel superalloys generally used in gas turbine blades were studied.
The studies were discussed with respect to the chemical reactions on the metal surface.
The surface reactions for chromium deposition was assumed to be one of the following three types of reactions: (1) exchange reaction, (2) hydrogen reduction, (3) thermal decomposition;
CrCl₂+Me_??_MeCl₂+Cr……(1)
CrCl₂+H₂_??_2HCl+Cr……(2)
CrCl₂_??_Cl₂+Cr……(3)
As the results of calculation of deposition rate of chromium in the equilibrium state and of simple experiments for confirmatory reaction, the following conclusions were drawn.
(1) The effect of hydrogen reduction was most powerful in chromium diffusion coating on nickel superalloys.
(2) The rate of hydrogenation was an important factor in chromium deposition.

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The cathodic polarization characteristics of chromium plating baths, to which various anions had been added, were measured by potentiostatic method for the investigation of their cathodic reactions.
The following results were obtained:
1) The cathodic reaction of black chromium plating bath in the potential range of the 4 th branch consisted of electrodeposition of hydrated chromium oxide film and black chromium plating as seen in pure chromic acid solution.
2) Metallic chromium was directly deposited from the stage of chromate film without parsing through the stages of hydrated chromium oxide film and black chromium plating.
3) The deposition process and properties of the deposits were varied according to kind and amount of anions added to the chromium plating bath; and also they depended upon the conditions of electrolysis.

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Local corrosion occurred in a long-period anodizing of aluminum specimens (Al P3-1/2H) in 100g/l H₂SO₄ containing 15g/l of H₂C₂O₄.
After anodizing, the cross-sections of specimens were observed by microscopy. The normal layer of the film was referred to as the 1st layer. An abnormal layer (the 2nd layer), having a considerably lower hardness value, was observed on the side of the base metal by microscopy.
The results of the experiments were as follows:
1) When the anodizing time was constant, the abnormal layer was likely to occur at lower bath temperature and under high current density (or in either of the both cases).
2) When the local corrosion occurred, fluctuation appeared in V-T (bath voltage-anodizing time) curve. Then, a new layer (the 3rd layer) was produced and the bath voltage tended to be increased again.
3) It was shown by electron microscopy that the cell size was the largest in the 2nd layer and was smaller in the 3rd layer approximate to the normal layer.
4) Several V-T curves were studied at various bath temperatures and under various current densities. The bath voltage was rapidly dropped down from the maximum value under such a high current density as 8amp./dm², independent of the bath temperature. Four peaks appeared on the V-T curve at 20°C under 4amp./dm² and the layers corresponding to the peaks were also observed in the cross-section of the film.
5) It was effective in preventing local corrosion of the anode, when the current density was decreased at the bath voltage approximate to the maximum value.

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