The Influence of Fluorides on the Microcracking of Electrodeposited Chromium

Abstract

Various fluorides were added to a base chromium plating solution to obtain microcracked chromium electrodeposits. Deposition was done on bright nickel plated mild steel substrates and for most of the experimental work a fresh chromium plating solution was used for each test. The crack counts were statistically analysed and in the case of Hull Cell results multiple regression procedures found quantitative relationships between crack counts and fluoride concentration together with current density. Covering Power, measured with a Hull Cell. was quantitatively influenced by both fluoride concentration and the alkali metal part of the molecule. Microcracking only occurred within a relatively narrow range of fluoride concentration and a theory is proposed. Simple compounds such as lithium fluoride did not cause microcracking when plating was done in plastic containers but the same solution electrolysed in a glass container gave a microcracked deposit. Fluorosilicates caused microcracking in either glass or plastic containers. A mechanism is suggested for the formation of reaction products of glass and fluorides, these products being similar to fluorosilicates in that they both cause microcracking. The alkali metal part of the fluoride molecule influences microcracking and quantitative relationships were found between microcracking and various parameters of the alkali metal such as atomic number. Caesium is apparently anomalous having a greater effect on microcracking than the other alkali metals. Although the type of fluoride: and the alkali metal part of the fluoride molecule influence microcracking the most important effect was found to be fluoride concentration. Since fluorides break down during electrolysis even if only slowly it is incorrect to call them catalysts and some experimental evidence has been found for the incorporation of fluorine into the electrodeposit.