Scales formed on Ni-Cr-base alloys and some practical heat-resisting alloys were investigated by X-ray diffraction method. The following results were obtained. (1) Alloying element, molybdenum, titanium and tungsten formed respectively NiOMoO₃, NiOTiO₂ and NiOWO₃ in the scales of Ni-Cr-base alloys above 1, 000°C. (2) Cobalt and aluminum addition hindered the formation of these double oxides. (3) On Ni-Cr-base alloys containing titanium, formation of TiO₂ was observed in higher content of titanium, and at higher temperature, (4) On Ni-Cr-base alloys containing niobium, CrNbO₄ was formed above 1, 000°C, and NiONb₂O₅ was not formed. On Co-base alloys containing niobium, CrNbO₄ was also formed.

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During the acid cleaning of various equipments, one of the troubles frequently encountered is acid attack on construction materials. Therefore, care should be taken to protect the equipment from acid attack during acid cleaning.
Although the acid attack on steel has been studied for many years, there seems to be very little study concerning the acid attack on copper alloys, because the corrosion rates of copper alloys are considered to be comparatively low in the non-oxidizing acid solutions such as hydrochloric acid.
However, the acid solution containing oxidizing agents such as oxygen, ferric or cupric ion, etc. may cause attack.
In this study, we have investigated the effects of acid concentration, temperature, gas atmosphere and oxidizing ions such as ferric or cupric ion on the corrosion of copper alloys in hydrochloric acid solutions.
As a result, it has been found that in the presence of oxidizing ions, the inhibition effect of inhibitors is markedly decreased, and the corrosion rates of copper alloys are greatly accelerated in the inhibited hydrochloric acid solution.
The tendency is more remarkable when air is bubbled through the solution.
On the hand, in the absence of oxidizing ions, the corrosion of copper alloys in hydrochloric acid can be prevented by inhibitors even when air is bubbled through the solution.

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In the previous report, it has been revealed that the acid attack on copper alloys during acid cleaning is mainly accelerated by oxidizing ions such as ferric or cupric ion in the inhibited hydrochloric acid solutions.
In the case of coupling dissimilar metals, there is the possibility of attack caused by galvanic action. When coupled, the anodic metal will corrode faster and afford some protection to the other.
In this study, we have investigated the effects of inhibitor, gas atmosphere and oxidizing ions such as ferric or cupric ion on the corrosion of steel-copper alloy couple in hydrochloric acid solution.
The results obtained are as follows.
(1) The greater the corrodibility of hydrochloric acid is, the higher the effect of galvanic action on the corrosion of steel-copper alloy couple is.
In the noninhibited hydrochloric acid solution containing oxidizing ions, the corrosion rate of steel coupled to copper alloy is greater than that of uncoupled steel and much greater when air is bubbled through the solution.
(2) But, in the inhibited hydrochloric acid solution, the effect of galvanic action is decreased, and the corrosion rates of coupled steel and copper alloy, whether in the presence of oxidizing ions or not, are approximately the same as those for uncoupled steel and copper alloy.
(3) Also, it has been verified that the cathodic protection is very effective for prevention of acid attack on such galvanic couples as steel-stainless steel system and steel-copper alloy system, in the same way as it is effective in the case uncoupled steel.

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Experiments were carried out on the relation between corrosion rate and flow velocity using ZnNa polyphosphate and Na polyphosphate. In addition, to know how to dissolve out, dissolution tests were made. These results were as follows.
(1) Corrosion preventive effects of ZnNa polyphosphate and Na polyphosphate used with Zn²⁺ are nearly same.
(2) In the case of low concentration, the faster flow velocity the lower corrosion rate becomes, but in high concentration, the corrosion rate is nearly constant.
(3) In the case of Na polyphosphate only, satisfactory results cannot be obtained when it is contained in high concentration.
(4) In order to get inhibitor-treated water, it is convenient to pass water through a tank filled with grained ZnNa polyphosphate.

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