It is well known that the hardness of electroless Ni-P deposit remarkably increases after heating at about 400°C. However, the knowledge of the above mechanism has scarcely been reported.
In these experiments, the metallographical phase changes of electroless Ni-P deposit were investigated by thermal and crystallographical analyses in comparison with the corresponding changes of Ni-P powder compacts.
The results obtained were as follows:
(1) In the curves for thermal analysis of electroless Ni-P deposit and Ni-P compact, endothermic and exothermic changes could be observed.
(2) Judging from the X-ray diffraction patterns for both electroless Ni-P deposit and powder compact, it was possible to confirm that nickel phosphide had been formed at the temperature of the exothermic change. It could be explained that the exothermic changes would be attributed to the heat of formation of nickel phosphide.
Consequently, it was concluded that the Ni-P deposit would be the mixture of Ni and P, and the remarkable increase of hardness would be due to the formation of nickel phosphide.
(3) It was considered that the endothermic changes would be the heat of vaporization of yellow phosphorus in Ni-P deposit or the heat of sublimation of red phosphorus in Ni-P powder compact.

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The phase changes of electrolytic Ni-P deposits during heat treatment are reported in this paper. Ni-13% P deposits were prepared by controlling the compositions of bath. The experimental methods in thermal and X-ray analyses were similar to those in the previous report, except for additional magnetic analysis.
The results obtained were as follows:
(1) The change of hardness in the heat treatment of electrolytic Ni-P deposits showed similar behavior to that of electroless Ni-P deposits.
(2) The evolution of heat occurred at 360°C in the heat treatment of the electrolytic deposit, which was higher than that temperature found in electroless deposit. The above result showed that phosphorus in electrolytic deposit would be like red phosphorus.
(3) The formation of nickel phosphide during that treatment at 350°C was also identified by X-ray analysis.
(4) Susceptibility of electrolytic Ni-P deposit at room temperature was smaller as compared with electrolytic pure Ni deposit. It was observed in the curve for magnetic analysis in the heated state that the intensity began to increase at 200°C followed by the following three regions: decrease at 300-340°C, rapid increase at 350-360°C, and final decrease above 360°C.

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The adsorption behavior of addition agents used for electroplating of copper in acid copper sulfate bath was studied on a dropping mercury electrode relating to their concentration and electrode potential by measuring differential capacity of electrical double layer.
The potentials were measured by using Hg/Hg₂SO₄, 1N H₂SO₄ as a reference electrode. The zero charge potential of Hg electrode in 1N H₂SO₄ was estimated as zero volt for further discussion.
By the addition of thiourea (TU) or acetyl thiourea to 1N H₂SO₄, new peaks of capacity appeared at +0.6V and +0.75V.
When phenylthiourea was added to the same solution, the maximum of differential capacity at +0.15V appeared to decrease considerably.
These effects could be explained by the adsorption of TU and its derivatives on the surface of mercury electrode at rather positive potential.
Sodium β-naphthalene sulfonate (NS) and trisodium 1, 3, 5-naphthalene sulfonate had the similar effects on capacity-potential (C-E) curve, in which a minimum of differential capacity was observed at +0.3V and a maximum of that at -0.3V in the both agents. However, disodium 1, 5-naphthalene sulfonate gave a minimum at +0.3V and a maximum at 0V.
When TU and NS were concurrently added to 1N H₂SO₄, C-E curve was found as the sum of the each individual curve of each single addition agent. Therefore, it was concluded that there was no specific interaction between the adsorptions of TU and NS.
When benzotriazole was added to 1N H₂SO₄, the maximum at +0.15V in C-E curve remarkably decreased. By the addition of polyacrylamide, the differential capacity of electrical double layer noticeably increased at E>+0.55V and the maximum at +0.15V decreased.
It was found that methyl-isothiourea, glycine, and l-cystine had no effects on C-E curve in 1N H₂SO₄.

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Diffusion coating was conducted on sintered iron parts with pure zinc powder by packing method. Coatability and atmospheric corrosion resistance of the coated products were discussed. For obtaining the treated layer corresponding to the 3rd Grade of the Specification of Hot-Dipped Zinc Coatings (JIS H-8641), having the coating weight of more than 600g/m², the time of 1-1.5 hat 500°C would be enough for the treatment, which gave satisfactory results in accelerated corrosion test and exposure test.
The coatability test was conducted by dipping the specimen in oil and determining the weight increase by oil impregnation. The results showed that the pores of sintered body were perfectly sealed and corrosion resistance was excellent when the weight increase by oil impregnation was less than 200g/cm². The results of the present experiments were summarized as follows.
(1) When the temperature of treatment was higher than m.p. of zinc, the thickness of the treated layer was increased almost in a straight line with the time, but the deviation was also became larger. However, in the treatment at lower than m.p. of zinc, the relation of the thickness and the time was parabolic, and the deviation was comparatively smaller.
(2) The thickness of the treated layer of sintered iron parts was smaller than that of low carbon steel treated under the same conditions.
(3) The treatment of zinc diffusion coating did not depend upon the effects of furnace atmospheres. For example, no difference of the results was observed between the treatments under hydrogen and the air atmospheres.
(4) The relation between the weight increase and the thickness of treated layer by zinc diffusion coating was not expressed by a straight line. In case of sintered parts, it was supposed to be due to the impregnation of zinc into the pores. In case of the specimen of low carbon steel, the density of treated layer determined by the above curve was 4-5g/cm³ and the treated layer was porous.
(5) Some specimens which showed weight increase of 200g/cm² by coatability test gave approximately 10% of intercommunicating porosity in powder metallurgy, which was proved to be very successfully sealed.
(6) Atmospheric corrosion resistance of sintered iron parts treated with zinc diffusion coating was proved to be excellent by the results of salt spray test and indoor exposure test. For obtaining the treated layer corresponding to the 3rd Grade of the. Specification of Hot-Dipped Zinc Coatings, having the largest coating weight (more than 600g/m² in average), the treatment at 500°C for 1.5h would be enough. The coatability of these specimens was expressed by the weight increase of less than 100g/cm² by oil impregnation.

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