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The covering powers of chromium platings from both of the plating baths of low and high concentrations were studied.The following results were obtained:(1) The covering power by the Hull Cell Test was much higher in the bath of low concentration than in that of high concentration.(2) On the contrary, the covering power by the parallel or bend cathode process was lower in the bath of low concentration than in that of high concentration.(3) The above two contradictory phenomena were due to the fact that the current was likely to be concentrated along the edges of the specimen so that the current density in the center was lowered; and the lowering degree of the current density was greater in the bath of low concentration than in that of high concentration.

Activities of fluoride and foluorosilicate in chromium plating baths were studied by using a fluoride electrode, and the following results were obtained.(1) In chromium plating baths, the activity of fluoride was less than that of the additional fluoride. The reason for this was that F-+H+→HF.(2) The activity of fluoride well corresponded with the value calculated from pH of the plating bath and ionization constant of HF, 7.2×10-4.(3) The activity of fluorosilicate was quite the same as that of fluoride at concentrations of less than 10-3mol/l or at higher pH. The reason was that the fluorosilicate was completely decomposed at low concentrations, and it was decomposed to fluoride at higher pH as follows:SiF62-+6OH-→6F-+[Si(OH)6]2-(4) The difference between activities of fluoride and fluorosilicate under other conditions was due to the formation of complex salt by fluorosilicate.

The bath voltages of chromium plating were studied at various concentrations of CrO3 and under various plating conditions.The following results were obtained:(1) A higher bath voltage was required for a CrO3 bath of lower concentration. When the voltage of CrO3 250g/l bath was taken as the standard, the voltage required was about 2 times for 50g/l bath, about 4 times for 25g/l bath, and about 6 times for 12.5g/l bath at 1l beaker test.(2) Assuming that the hypothetical value of resistance voltage of chromium plating bath calculated from its conductivity is Eohm and the real value is E'ohm, the following relation was derived:k=Eohm/E'ohm=K/K'=(2.4±0.2)l1/3where K and K': Hypothetical and real cell constants, respectivelyl: Distance between electrodes(3) Accordingly, k was called cell constant ratio.

The self-regulation of sulfate concentration by the addition of Sr2+ to a chromium plating bath saturated with SrSO4 was studied. The following results were obtained: (1) Since SrSO4 is a strong acid salt, the solubility of SrSO4 in low pH solution such as a chromium plating bath was not much higher than that in its aqueous solution. (2) The SO4 concentration could freely be regulated by the addition of SrCO3 to the chromium plating bath. (3) The decrease in the solubility of SrCO3 conformed to “common-ion effect” in the chromium plating bath, but not in the aqueous solution. It would be more clear by assuming activity instead of concentration. Namely, the ionic strength was higher and steady in the chromium plating bath and was supposed to be proportional to CrO3 concentration; but it was changeable in the aqueous solution. (4) For example, the solubility of SrSO4 (as SO4) was 1.4g/l in chromium plating bath containing 50g/l of CrO3 at 55°C. When SrCO3 of 6.0g/l was added to the bath, the SO4 concentration became 0.5g/l, by which the plating bath for self-regulating SO4 was prepared.

The self-regulation of fluorosilicate concentration by the addition of K+ to a chromium plating bath saturated with K2SiF6 was studied. The following results were obtained: (1) Since K2SiF6 is a strong acid salt, the solubility of K2SiF6 in low pH solution such as a chromium plating bath was not much higher than that in its aqueous solution. (2) The decrease in the solubility of K2SiF6 conforming to “common-ion effect” did not agree to the following dissociation. K2-SiF6→2K++SiF62-. (3) The reason was explained by the following hydrolysis of K2SiF6: K2SiF6+H2O→2K++SiF5(OH)2-+HF. The dissociation of a fluorosilicate having higher solubility such as Na2Si F6 was as follows: Na2SiF6→2Na++SiF62-. Whereas, the dissociation of a fluorosilicate having lower solubility such as BaSiF6 was as follows: BaSiF6+2H2O→Ba2++SiF4(OH)22-+2HF. The results of experiments well agreed with the basis of these dissociations. (4) For example, the solubility of K2SiF6 was 10g/l in chromium plating bath containing 250g/l of CrO3 at 55°C. Therefore, the plating bath for selfregulating SiF6 was prepared by the addition of a small amount of K2CO3 to the bath. The solubility of K2SiF6 was 7g/l in low concentration chromium plating bath containing 50g/l of CrO3 at 55°C. Since the optimum concentration of K2SiF6 in this bath is 0.6g/l, 27.5g/l of K2CO3 shall be added to get the self-regulating bath. However, this composition was not adequate for the chromium plating bath owing to the increase of pH value.

Experiments were conducted to find the optimum bath composition and plating condition for obtaining a bright chromium plating from CrO3 baths of concentrations lower than 50g/l.The following results were obtained:(1) The optimum bath composition for bright chromium plating was CrO3/H2SO4/Na2SiF6=100/1/1 and the concentration of CrO3 was 25-50g/l.(2) The plating conditions seemed to be almost the same as those of the normal bath of 250g/l CrO3. However, the range for brightness was narrower with the decrease in CrO3 concentration of the bath.(3) Chromate films appeared on the surface of the chromium plating from CrO3 baths of low concentrations. The films could be removed by dipping for a short period in the plating bath after deposition.

Stress of the chromium deposit obtained from various baths, at different temperature and current density, was measured with Brenner and Senderoff's contractometer, to study the relation between the stress and other properties, such as brightness, hardness, cracks and corrosion resistance.Result:(1) In chromium plating at a certain current density, the luster of the deposit changes gradually in the order from “mat” to “frosted, ” then to “bright, ” and finally to “milky, ” depending on the change of temperature. Stress of the thin deposit is very small with “mat, ” rapidly increases with “frosted, ” then becomes maximum with “bright” and decreases again with “milky.” It is supposed that the smaller the grainsize of the deposite is, the brighter is the luster.Stress of the thicker deposite becomes smaller, depending on brightness, and finally compressive at “bright.” This phenomenon can be explained by the exceptionally good micro-throwing power of chromium plating in the cracks of the deposited. Many theories have been so far discussed upon the brightness of chromium plating, but the good micro-throwing power is supposed to be one of the reasons for brightness as in the case of nickel plating.(2) The facts that its iso-hardness curve resembles its iso-stress curve and that the brighter the deposit is, the higher is the stress, suggest that the hardness of chromium deposit should have much to do with its stress.(3) Cracks produced in the cromium deposit are supposed to be caused by its initial big stress. this means that the stress is greater than the tensile strength. In fact, when the initial stress is small, few or not cracks occur. Such deposits can be obtained from the bath of concentrated CrO3, high ratio of CrO3/H2SO4, and higher temperature, which can also be considered as right plating conditions to get a deposit of good corrosion resistance.(4) Initial stress of the chromium deposit from the bath of SiF6 and CR-110 is comparatively greater than that from Sargent bath. Stress of the thicker deposit is smaller, which usually has fine cracks and brighter luster.(5) The deposit from the bath with selenic acid has smaller stress and fewer cracks, but its brightness becomes poor.

Brenner and Senderoff's Contractometer type stress meter was experimentally made reconstructing spiral type thermo regulator.Using this stress meter, the stress of electrodeposits in Watt's type bath was measured.The results is as follows:(1) Errors of stress measurement by this stress meter were within ±5%.(2) Rate of increasing the thickness of electrodeposits owing to decreasing of stress in electrodeposits were almost the same ratio as those published by Kushner or other authorities, and it was confirmed that this phenomenon is caused by increasing grain size of particles of the electrodeposits.(3) Measured electrodeposit stress did not vary largely no matter whether the base metal of spirals was made of steel, copper or brass, of different Young's modulus.(4) Thickness of the spiral of base metal may well be as large as to be enough sensibile to the pointing needle.(5) So far as the surface of base metal of spirals is concerned, in case of the mechanically polished state, the stress was not affected, but in case of the state treated by FeCl3-HCl or other chemical solutions, the first electrodeposit stress was pretty small.

Complex salt of cupper and zinc cyanide is decomposed into CuO and ZnO, by H2O2, and then dissolved by HNO3.The total amount of Cu and Zn can be determined by titrating the solution with EDTA with PAN (α-Pyridyl-β-azonaphtol) as indicator.Zn can be titrated directly by EDTA in the presence of Cu which is masked by the addition of slight excess of thiosulfate at pH 5-6, using the PAN indicator and methanol.The amount of Cu in the mixture can be calculated by the difference of the above two.

Complex salt of copper and zinc cyanide is decomposed into CuO and ZnO, by H2O2, and then dissolved in HNO3.Zinc can be titraled by EDTA in the presence of copper which is masked by adding a slight exess of thiosulfate at pH 5-6, using methanol and the PAN (α-pyridyl-β azonaphtol) indicator.Then copper is analyzed with EDTA, after breaking down the copper masked by thiosulfate, using hydrogen peroxide as fellows:2Na2S2O3+H2O2→Na2S4O6+2NaOHThe end point of above reaction is told by PAN indicator.