The surface of commercially pure aluminum and aluminum alloy were darkened to dark gray or brownish dark gray by boiling well-water with a pH of 6.8, but in distilled water the surface colo rremained in light gray.
Surface oxide layeres, naturally formed in room atmosphere, does not prevent the darkening.
Blackening occurs in neutral or alkaline water.
In distilled water containing magnesium, calcium, sodium, or iron salt, aluminum was not darkened, but in some cases the degree of the luster decreased. The mutual action of the salts may play an important role in darkening.
Blackening seems to be caused by adsorption of some substances contained in water.

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The surfaces of steels (Cr-Mo-carburized steel) quenched into several kinds of oil in argon gas atmosphere were studied by electron diffraction method. In many cases, oxide in the form of Fe₃O₄ or FeO⋅Cr₂O₃ is found on the surface of steel specimens quenched into oil. FeS is, however, found in case of 90-turbine oil containing 1%-dibenzyl-disulfide. On the other hand, the diffraction pattern could not be taken on account of adhesion or scorching of organic matters in oil to steel specimen. After all, the causes of tarnish or discoloration of steel surface may be scorching of organic matters or formation of oxide or sulfide. Oxide is produced owing to the existence of oxygen dissolved or combined in oil. If sulfur is present in oil, oxidation is so much accelerated that steel surface is tarnished even if oxygen is too little to act as the cause of tarnish. The effect of sulfur changes according to the chemical activity of sulfur compounds. Thus, sulfide is produced under the existence of the active compounds like dibenzyl-disulfide.

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80 specimens of different microstructure were prepared from arc-welded steel plates, which were welded with mildsteel, 1.5%Cr-Mo steel, 5%Cr steel, 13%Cr steel and 18Cr-8Ni steel deposited metals, under the adequate welding conditions. The mechanical properties of specimens and their defects were inspected by means of X-ray and supersonic waves.
Then, the loss by corrosion, volume of hydrogen produced, and their electrode potential in 10% H₂SO₄, HCl, at the welded and annealed states, were measured. Welded strain was also studied.
The results are as follows;
1) The corrosion loss, and fuming hydrogen volume are changed remarkably according to the microstructure.
2) It is not so efficient to use 1.5%Cr-Mo steel as the deposited metal for corrosion prevention.
3) It is natural that the corrosion resistance of bead part is much increased when 18-8 steel is used as the deposited metal.
4) The corrosion resistance of high sulphur mild steel plate is low as compared with low sulphur mild steel plate.

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It is sometimes observed that the rust of the base metal appears on the surface through probable pinholes of the rust-proof coating.
This report is intended to explain the method to detect pinholes by the use of electrolysis of ZnSO₄ solution.
As a result, it was known that the electrolysis for pinhole detection should be made with the bath having undermentioned condition (A), and procedures (B) should be followed in order to eliminate the pinholes.
(A) Condition of electrolysis.
Bath composition-10% ZnSO₄ sol.
Bath temperature-Room temp.
Time-3-5sec.
Terminal voltage-3V D. C.
(B) B-1. Thickness of coating should be increased.
B-2. Slow vapourized thinner should be used.
B-3. Setting time should be extended.

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