Passivation of Stainless Steel

What is Passivation?

Passivation is a metal finishing process used to prevent corrosion. For  stainless steel, the passivation process involves the use of nitric or citric acid to remove free iron from the surface of the part.

Passivation removes embedded contaminants and returns the part to its original corrosion specifications. Though passivation can improve the corrosion resistance of certain stainless steel alloys, it does not eliminate imperfections like micro cracks, burrs, heat tint and oxide scale.

Passivation and Laser Marking

All implantable and reusable medical devices for sale in the United States must bear a Unique Device Identifier (UDI) code. Medical devices are most commonly made from 300-series and 17-series stainless steels due to their inherent mechanical strength, corrosion resistance and machinability. The naturally occurring chromium oxide passive layer that forms on the surface of these steels resists corrosion during sterilization and is inert to the human body. Fiber lasers have long been the laser of choice for making crisp, clear marks on these stainless steels, but the successful processing window is extremely small and the operating wavelength tends to generate excessive heat which may damage sensitive electronics. In simpler terms, when the surface of stainless steel is heated high enough, the surface oxide layer formed can become brittle and prone to corrosion. This heat also damages the passive layer necessitating a post-process passivation step to restore it. The problem is that this passivation step has the unintended consequence of severely degrading the marks.

Since laser marking must take place prior to passivation in a production line, a robust and permanent laser marking method is required: Picosecond IR Laser Marking.