The sources of the contaminant can be as follows:
- Fingerprints – Fingerprints will contaminate the steel surface by the decomposition of body oil in the fingerprints that will leave a surface carbon deposit and restrict the nitrogen diffusion into the steel surface. The result of the fingerprint deposit can be seen as a clear fingerprint on the steel surface.
- Prewash water – If the prewash water is not accurately monitored as far as water hardness is concerned, and water softener is added to the incoming wash water prior to it entering into the washing unit, the softening salt will deposit onto the steel surface and will retard the diffusion of nitrogen into the treated steel surface.
- Cutting fluids – If cutting fluids are not thoroughly cleaned from the steel surface prior to the nitride process, the residual cutting fluid that remains on the surface or in cavities can interact with the hydrogen (from the decomposition of ammonia as the nitrogen source) and has the potential to form chlorides or sulfides, which will cause a retardation of the diffusion of nitrogen into that area.
- Grease – Uncleaned surface grease can cause the steel being nitrided to have soft spots in the grease contamination area.
The wash can be completed with a commercial industrial washing machine utilizing the dunk-and-spray method with hot water at a working temperature of approximately 180-200°F. Once again, care must be taken with the washing system if additive cleaning products are added to the makeup water. Be sure that the final rinse cycle will use hot clean water.
If the components are to be ion nitrided, the water wash and rinse system need not be complex. If the components are small enough, a commercial kitchen dishwasher will suffice. One should remember that on the ramp up to the process temperature for plasma-assisted nitriding, the components are generally surface cleaned with either hydrogen atomic bombardment or a mixture of hydrogen and argon. The process is called sputter cleaning and can be likened to the process of “atomic shot blasting.”
Care should be taken with the use of argon gas as too much could lead to surface etching of the part to be nitrided. The typical argon to hydrogen is generally in the region of 90% hydrogen and a maximum of 10% argon, and the time at the sputter-clean temperature will be dependent on the degree and type of surface contamination.