Cadmium has been used widely for coating steel structural components and threaded assemblies principally to provide protection from corrosion and in-service hydrogen embrittlement.  However, Cd is highly toxic, and the environmental and health problems resulting from its use have become a serious concern.  In order to find viable replacements for the Navy, this study was conducted as part of a leveraged effort with the Joint Cadmium Alternatives Team (JCAT).  In this study, Cd, Zn-6Ni and Zn-13Ni were electrocoated on 4340 steel specimens.  In addition, aluminum coatings were applied by both ion vapor deposition (IVD) method and also by electroplating. The relative ability of each coating to protect the substrate steel from corrosion fatigue and stress corrosion cracking was evaluated. The induced residual stresses in the coating and substrate were measured as-deposited.  Then, the specimens were subjected to tests of fatigue in air and 3.5% NaCl solution and stress corrosion cracking (SCC) in 3.5% NaCl solution.  As a reference, bare 4340 steel specimens were subjected to the identical tests. 

The residual stress was determined to be tensile in the bare specimen and compressive in the coated specimens.  Among the employed coatings, the electroplated Al resulted in the greatest magnitude and depth of peak compressive residual stress and thickest layer of compression.  All coatings were found to shorten the fatigue life in air but protect the substrate against corrosion fatigue in 3.5% NaCl solution.  The electroplated Al, IVD Al, Zn-6Ni and Zn-13Ni coatings were not as effective as the Cd coating for substrate protection.  Among the Cd replacement candidates, the electroplated Al coating provided the best overall protection of 4340 steel against fatigue and SCC in 3.5% NaCl solution, while IVD Al was next best.