As one of the series of studies on hot-dip aluminum coating on steel, the mutual diffusion between molten aluminum and solid steels was studied. Holes with 10mm diameter were bored in the center of 16mm∅×15mm cylindrical steel specimens (0.02, 0.11, 0.45, 0.83% C steel, 2% Cr-1% Mo Steel, 13 Cr and 18-8 stainless steel), into which various aluminum alloys rods (99.99% Al, 98% Al, 98% Al+5% Si and 98% Al+5% Si+0.2% Be) with nearly the same diameter as the hole were tightly inserted respectively. These combined specimens were heated in vacuum at 680, 720, 760 and 800°C for 6 hours, then cooled, and the phenomena of mutual diffusion between the molten aluminum alloys and solid steels were investigated. The composition of aluminum and its alloys except 99.99% Al described above is the same as that of the molten bath used in the previous research on aluminizing.
In the case of 99.99% Al and 98% Al, aluminum diffuses deepest into every steel, and the lower the carbon in steel, the deeper grows the tongue of Fe-Al alloy layer into the steel. With molten Al+Si and Al+Si+Be alloy, the tongue does not grow into each steel and an inhibiting action of Si and Be against diffusion of aluminum into steel, instead of which the tongue grows remarkably into molten aluminum is recognized. Moreover many lumps of Fe-Al alloy exist in aluminum and remarkable dissolution of iron into molten aluminum is also recognized.
Generally, the diffusion constant of 98% Al to each steel is larger than that of other aluminum or its alloys and that of Al+Si+Be is minimum. The diffusion depth is minimum in 13 Cr and 18-8 steel. In the reaction of each carbon steel with molten aluminum, the activation energy increases rapidly with the increase of carbon content from 0.02 to 0.11%, but is nearly constant regardless of the more increase of carbon content. With 99.99% Al and 98% Al, the activation energy is about 40, 000cal/mol, with Al+Si alloy about 20, 000cal/mol and with Al+Si+Be alloy even much smaller.
From these investigations the inhibiting action of Si and Be on the diffusion of aluminum into steel was comfirmed, but it is necessary to care that Si or Be accelerates the dissolution of iron into molten aluminum bath.

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Recently, the techniques of bright heat treatment of steels were developed, but alloy steels especially chromium bearing steel were easily oxidized forming dense scales, the removement of which was very difficult. The relation between chromium content of steel, shown in Table 1, and atmosphere oxygen potential with various H₂O bearing hydrogen gas at 800-1000°C and 1-5 hours, was studied. The results are shown in Table 2 and Fig. 2-6. The degree of oxidation are shown in weight change of specimen per unit surface area. The surface change are considered concerning thermodynamical data.

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Peculiarities of plating bath prepared by cuprous carbonate (basic) and monoethanolamine were studied. The main constituent of this bath was found, by both absorption spectrum of electrolyte and paper iono-phoresis, to be an organic coordination compound in which 2 moles of monoethanolamine and 2 moles of H₂O were coordinated to Cu^II. This coordination compound was decomposed easily by a large amount of water. Suitable concentration of the complex in the bath should be more than 0.5M/l, and suitable plating current density at room temperature was less than 1.5 A/dm². Cathode current effeciency in these conditions was nearly 100%.
Cathode polarization potentials vs current dencities, and pH value and specific conductivity of the electrolyte were measured, and the effect of various additives was also investigated.

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The coloration of high purity Al-Cu and Al-Ti alloys by boiling water were examined.
All specimens were blackened to dark gray or grayish black color by boiling well-water, and then some of them were tinged with brown, blue or purple slightly. However, the relationship between the surface color and impurity in aluminum could not be found.
Copper present in aluminum accelerates blackening, and titanium in aluminum also induces blackening. But the effect of the latter is not so remarkable than the former.
Silicate soluble in water darkened the aluminum alloy fairly. The co-existence of silicate and carbonate or sulphate, especially sodium meta-silicate and aluminum potassium sulphate, increase the degree of darkening, though concentrations of two salts which show the maximum effect seem to change according to the kind of alloying elements in aluminum.

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The effects of various kinds of organic addition agents on the appearance of black chromium electroplating were investigated.
It was found that urea was a good addition agent for black chomium plating, and the following bath composition was recommended:
{CrO₃ 2.5-4M/l CH₃COOH 0.05M/l CO(NH₂)₂ 0.05M/l
From the bath composition mentioned above the chromium plating, of real black, instead of grayish, was obtained at the current density higher than 50A/dm² and at the temperature lower than 25°C.

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General properties of the black chromium electrodeposit obtained from the chromic acid bath (CrO₃ 2.5-4M/l, CH₃COOH 0.05M/l, CO(NH₂)₂ 0.05M/l) were investigated. The surface and cross-section of these deposits were examined microscopically. The apparent specific gravity was measured as compared with the other chromium electrodeposits. The structure of these deposits were analysed by the Xray diffraction method. The analytical method of urea contained in the chromic acid bath were established.
(1) The black chromium electrodeposit are grain-like having large cracks.
(2) The apparent specific gravity of the deposit is lighter than the other chromium electrodeposits and the Cr content of the deposit is about 70%, which suggests that the black chromium electrodeposit consists partly of Cr oxide.
(3) The Xray diffraction pattern shows that these deposits consist of the two different types of the stracture, i. e. B. C. C. and H. C. P.
(4) The gravimetric method using Xanthydrol is suitable for the analysis of urea in the black chromium plating bath.

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