An Investigation of Electrodeposited Alloys for Protection of Steel Aircraft Parts

Research was conducted to develop electrodeposited coatings sup erior to Zn and Cd plate for aircraft steel parts, particularly in tropical regions. A 50% Mn-50% Zn coating, superior to Zn under wet-dry cabinet conditions, was prepared by heat-treating a duplex coating of Mn and Zn electroplates to cause interdiffusion. Diffusion coatings of Mn-Sn alloys containing 40 to 60% Mn showed better inherent corrosion resistance than pure Mn. No sound Zn-Ag codeposits were obtained with either an iodide or a thiosulfate solution. Mn-Ni codeposits having a low Mn content were obtained from NH sub 4 salt solutions. Evidence of diffusion was found after heat-treating Mn and Ni duplex plates; after 21 hrs., a strong Fe pattern and an unknown phase were found. Slight interdiffusion of Zn and Cr electroplates was indicated. No interdiffusion was detected of Cd and Cr or of Mn and Cd electroplates. Nonadherent, coarsely crystalline deposits, containing mainly Pb, resulted from attempts to codeposit Zn and Pb. Zn-Ni-coated 4130 steel panels rusted rapidly in the wet-dry test; the rate of rusting decreased by increasing the Ni content from 25 to 50%. Both Cd-Sn diffusion and codeposited coatings containing about 50% Cd were comparable in wet-dry behavior to pure Cd coatings; Cd-Ag codeposited coatings rusted early in the wet-dry test, but subsequent rusting progressed slowly. A nonsacrificial- type Fe-Cr alloy containing 6% Fe was not superior to Cr-plated panels in the wet-dry cabinet. A method is proposed for rating coatings after wet-dry exposure. An x-ray diffraction study of Mn-Zn corrosion See Also ATI-22138 and ATI-64025.