The Nickel and Chromium Plating of Tinplate

SynopsisTinplate, as a base for chromium plated sheet required for fabrication, offers the advantage over steel of being easily polished, but difficulties have arisen, especially with coke grades of tinplate, in obtaining nickel deposits sufficiently adherent to withstand chromium plating and fabrication without flaking.These difficulties are attributed to two effects:—(1) the mechanical effect during straining of variations in thickness of the tin coating, (2) the effects of hydrogen produced during plating, particularly the disruptive effect produced by the conversion of atomic hydrogen (diffusing through the nickel) into gas at the site of discontinuities (foreign particles, fissures, etc.) at the interface—nickel/tin.The diffusion of hydrogen to the tin surface can be prevented by depositing a film of copper before nickel plating. Tinplate which has been coated with a composite deposit of copper, nickel and chromium can be fabricated without flaking of the deposit. If the deposit is brittle, however, it is liable to crack on deformation, giving an unsightly appearance. Certain nickel deposits may be seriously embrittled by chromium plating.The porosity of composite deposits of copper, nickel and chromium on tinplate is considerably lower than on steel of average quality, but their protective value may be destroyed at bends formed during fabrication especially with coke grades of tinplate. Tinplate of charcoal quality offers the advantages of superior finish, better adhesion of the deposit and a lower tendency for the protective value of the coating to be destroyed on deformation.Tinplate may also be satisfactorily plated with chromium alone. The plated sheet can be pressed without flaking of the coating, the protective value of which is less readily reduced by deformation than that of the composite nickel-chromium coatings, but it is more difficult to obtain a good finish in the absence of a nickel undercoating.