Recent Developments

Abstract

Electrodedosited metal coatings have internal stresses that can be either tensile or compressive. When in tension a coating is striving to contract inwards but is prevented from doing so by the strength of its bond to the metal base, it has tendency to crack, exposing the base to corr-osion. The Tin Research Institute have reported that they have made a tin nickel electroplate that has residual compressive stress when deposited under normal conditions. Research workers at the Institute have been able to deposit the coatings under varying states of compression and tension so that the whole subject has been reviewed. In the tests various specimens were produced with coats 0.0002 in thick on steel panels. These were exposed out-doors for two weeks under conditions that encouraged cond-ensation but prevented direct access to rain. A steady improvement in corrosion resistance as the tensile stress diminished, passed through zero and increased on the compression side, was measured. Also, where a coating is subject to bending, a compressive stress is an advantage, because relatively small amounts of deformation might well crack a sample in tension but a compressive stress has first to be neutralized before tension occurs.