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.

Date of Award	Feb 1972
Original language	English
Supervisor	D.J. Arrowsmith (Supervisor)