Environmentally assisted cracking of nickel-chromium-phosphorus amorphous alloy coated 4340 steels

Abstract

Surface modification of engineering materials has become more popular due to the scarcity of alloying elements like nickel and chromium and the increasing need for stronger and more corrosion resistant materials. On the other hand, nickel-chromium based amorphous alloys prepared by melt-spin have proven mechanical and corrosion properties but are too thin and too narrow to have any engineering usage. Therefore, surface modification by electrodeposit a thin layer of nickel-chromium based amorphous alloys on ordinary steels can be an economical and effective way to improve their properties. In this study, nickel-chromium-phosphorus amorphous alloys have been successfully electrodeposited with sulphate-base solutions on both copper foil and quenched and tempered 4340 steels. The deposits are thick (over 65 μm), strong, crack-free, and very corrosion resistant. Electrolyte compositions and plating conditions for obtaining optimum alloy combination have also been established. The plated steels specimens were environmentally evaluated by slow strain rate tests and corrosion fatigue tests in salt and acid solutions under both anodic and cathodic controls. Some test solutions also contain arsenic to promote hydrogen absorption in the steels. Test results indicate the amorphous alloy coatings acted as barriers to hydrogen entry and increased the fatigue life of the steel specimens. The coatings also improved the corrosion resistance of the steels in the anodic condition and increased their time to failure. However, protection offered by the coating only occurred when the steel specimens were not near plastic deformation. When yielding started, cracks and debonding developed in the coatings and destroyed their effectiveness.