定電位電解法による銅-ニッケル-クロムメッキの耐食性の評価

抄録

The evaluation by electrochemical study was made of the corrosion resistance of copper-nickel-chromium electroplates. Polarization curves were obtained in acid sulfate solution with or without chloride ion by the potentiostatic method. The metal specimens examined in the present study were as follows:
(a) mild steel (b) copper plated from cyanide bath (c) sulfur containing bright nickel electroplate (d) sulfur-free semi-bright nickel electroplate (e) chromium plated from conventional bath (f) contact electrode in which cross section of the electroplated articles was used as a working electrode (g) coupling electrode in which two metal electroplates were coupled with each other externally without any contact between them in the electrolyte (h) electroplated nickel-chromium plate.
In general, bright nickels obtained from Watts type bath with sulfur containing addition agents such as 1, 5-naphthalene disulfonate, saccharin, p-toluene sulfonamide and thiourea were found to be more active than sulfur-free semi-bright nickels. With increased sulfur content in the nickel deposits, the polarization curves were shifted to less noble potentials and the rate of corrosion of nickels increased remarkably inhibiting the passivation of nickel.
It is found that chromium electrodeposits tend to become passive at less noble potentials than nickel in acid sulfate solution and the dissolution of chromium is very small compared with that of nickel at the potentials of -0.34∼+0.10V vs. SCE. Corrosion of nickel-chromium electroplates at the potential region of the active state of nickel might correspond to the anodic dissolution of nickel through the pores or cracks in the chromium coating.
By using the contact electrode and also coupling electrode systems in the measurement of polarization curves, it can be predicted that sulfur containing nickel plating on a sulfur-free semi-bright nickel will dissolve in preference to the semi-bright nickel exposed at coating defects and may result in the effective catholic protection of the electroplated articles owing to the retardation of the rate of penetration into the double-layer nickel coatings. However, rapid corrosion penetration of the underlying nickel layer can be expected if the two layers do not have sufficiently different sulfur contents.