An Evaluation of Processes for the Blackening of Precipitation-Hardened Stainless Steels

摘要

Precipitation-hardened stainless steels (corrosion-resistant materials with excellent mechanical properties) require dark, nonreflective finishes to be useful for most military applications. Work involving such finishes was conducted to determine which of the various blackening processes could be used for these materials without decreasing their inherent corrosion resistance. Corrosion tests were conducted on coated and uncoated 17-4, 17-7, 14-8Mo, and AM350 alloys in the annealed and precipitation-hardened condition. The blackening processes evaluated included black chromium plating, zinc phosphating, molten sodium dichromate, and alkaline-oxidizing treatmentds. Activated and passivated specimens were used as controls in the corrosion tests. Potentiostatic, potentiodynamic, and 5% salt spray (astm-b117) tests were used to evaluate the various specimens. Salt-spray tests showed that maximum corrosion protection was provided by black chromium and zinc phosphate coatings. All corrosion tests showed that black coatings produced by the alkaline-oxidizing process provided varying degrees of corrosion resistance, dependent upon the alloy or thermal treatment used, and that coatings produced by the molten sodium dichromate treatment provided poor corrosion protection. Black coatings on the 14-8Mo and AM350 alloys exhibited the best corrosion resistance. (Author)