INTERNAL STRESS OF CHROME COATINGS ELECTRODEPOSITED FROM SOLUTIONS OF CHROMIUM (III) SALTS

Abstract

Experimental data on internal stress in chromium coatings electrodeposited from Cr(III)-solutions have been obtained. Tensile stress 250-300 MPa is observed in thin coatings with thickness was below 1-3 μm. The internal stress is practically independent on current density and pH of the solution. A reduction in the plate thickness corresponding to σ=0 is observed, when pH has changed from 1.2 to 1.6 and current density from 30 to 35 A/dm2 at higher values of pH and current density compressive stress is observed and increases the difference in the stress values at greater thicknesses of the coatings: ((– 10) - (– 50) MPa). These results have been compared with similar data for the coatings deposited from Cr(VI) solutions including the effects of the non-metal inclusions in the coatings (for Cr(III)-bath H – 10.5 - 13 at. % (26 - 32 ml H2 / g Cr), O – 0.06-0.6 at. %, С – 0.04-7.8 at. %; for Cr(VI)-bath H – 1.8 at. %, O – 1.2 at. %, С – 0 at. %)). Compressive stress, related with the inclusions (including carbides) is the major cause, limiting the possibility to improve the characteristics of chromium coatings and to deposit coatings of the quality comparable to that of chrome plated from Cr(VI) baths.