Low Temperature Gaseous Nitriding of 4340 Steel for Increased Corrosion Resistance

Corrosion-related maintenance of metal alloy components such as hydraulic wing and blade fold actuation systems for carrier-based aircraft and rotary gear assemblies costs the Navy approximately $1 billion per year. Innovative and low-cost surface modifications that do not compromise mechanical properties are needed to improve the corrosion resistance of the alloys. Current surface treatment technologies offer limited effectiveness in long-term use for steel alloys. These techniques include the application of hard chromium coatings, which have toxicity issues. Nitriding and/or carburizing techniques are other traditional methods used to harden steel alloy surfaces and improve corrosion resistance. One shortcoming of these methods is that the high processing temperatures required risk degrading the mechanical properties of the steel alloy. Recently investigated methods of nitriding iron at low temperatures have demonstrated that iron can be coated with a thin layer of nickel (<100 nm) and that an iron nitride layer can be formed below the nickel coating when exposed to an ammonia atmosphere at temperatures as low as 225°C. The catalytic nickel coating affects the preferential formation of the desired iron nitride compound instead of the iron oxide (i.e., corrosion). The iron nitride layer is hard, non-porous, and chemically stable at temperatures below 400°C. This process has not yet been used commercially. In this project, Ultramet will deposit a thin (<100 nm) layer of nickel on iron and 4340 steel using chemical vapor deposition technology, which does not rely on conventional thermal activation, thereby minimizing the deposition temperature. The resulting surfaces will be nitrided in an ammonia atmosphere at 325°C and lower. The viability of chemical vapor deposition for production of a hard, wear-resistant surface on these materials will be assessed, and the ability to protect them against corrosion and retain their mechanical properties will be evaluated. Chemical vapor deposition is the best alternative for protecting steel surfaces because it is the only process that can treat steel at low temperature without altering the mechanical properties of steel.