クロムメッキの電着応力とその他の性質

電着物の応力に関する研究 (第7報) -クロムの電着応力 (その2)

抄録

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.