On its own stainless steel has a natural oxide “passive” layer that protects it from corrosion. However, when stainless steel is machined and fabricated, the surface is contaminated by the oils and greases used in the fabrication process. If not removed, this contamination will lead to corrosion.
The contaminants are removed by cleaning the metal followed by using acid, usually nitric acid, to remove free iron from the surface. Slowly and naturally a “passive” layer is developed once again on the surface of the stainless and the stainless is now “unreactive”. This layer is invisible and only a few molecules thick but it now provides a barrier to prevent oxygen and moisture from reaching the iron underneath. If oxygen reached the iron, it would oxidize – or rust – turning a yellowish or orange-red color and over time this would weaken the metal.
The commonly used nitric acid passivation process can quickly dissolve all the iron compounds and other trace metals on the surface and help to recreate the passive layer at the same time. The results are controlled by the time in the acid, the temperature of the acid, and the concentration of the acid – Time, Temp, and Conc.
At H&W, we passivate to standards developed and revised over the years. These standards range from military specs to aerospace material specs to customer developed specs. There are many options available based on the alloy being used or we can help the customer develop their own standard to accommodate their own product.
H&W’s Passivation Services include the following steps: racking, cleaning, etching, deoxidizing, painting, anodizing, dying, sealing, unracking, rinsing and packaging. Each can be customized for your specific metal and desired coating type.