Electrodeposition of nickel-iron alloys having a low temperature

Abstract

There is disclosed a method of electrodepositing, on a substrate, a substantially homogeneous admixture (alloy) of nickel and iron containing substantially no iron oxides and having a low temperature expansion coefficient. The method includes the steps of forming an aqueous electrolyte solution of nickel chloride and ferrous sulfate containing ferrous ions and nickel ions present in predetermined concentrations which will yield a nickel iron alloy with predetermined proportions of nickel and iron and having a low temperature expansion coefficient, and electrodepositing the nickel and iron from the solution onto a substrate using a nickel iron anode whose proportions of nickel and iron equal or substantially equal those of the alloy to be deposited, whereby the anode is electrochemically corroded by the solution at substantially 100% efficiency to continuously and uniformly maintain the proper nickel and iron proportions in the solution during the entire electrodeposition process. When electrodepositing the iron and nickel on the substrate it is important that free oxygen be excluded from the electrolyte solution. In addition, there is disclosed an optical mirror consisting essentially of an electrodeposited nickel-iron face sheet having a relatively low temperature coefficient of expansion on a lightweight graphite backup substrate having approximately the same temperature coefficient of expansion as the electrodeposited nickel-iron face sheet.