Citric Acid Passivation of Stainless Steel

While it’s a wonder of natural chemistry in action, this chromium oxide passive layer is not free from practical concerns.  The passivation process takes some time to occur naturally, and is nearly impossible to inspect for assurance that it has evenly developed at all.  Also, normal fabrication processes as well as everyday wear-and-tear will destroy this passive layer anywhere that high physical force, heat, electrical current, certain chemical reactions, or abrasion may occur on a stainless steel body.  For these reasons, intentional processes have been developed to spur generation of the passive layer on-demand, which we refer to as Chemical Passivation.

Industrial chemical passivation is performed in several steps:

1. Alkaline Wash – a thorough washing process is performed on the stainless steel materials to be passivated, which strips away all contaminants on the materials whose presence would interrupt the even formation of a new passive layer.  Washing removes all oil, grease, coolants, polishing compounds,  and other foreign matter on the part, otherwise corrosion may continue to develop where these soils remain.
2. Rinse – after washing, all residual trace chemicals are removed by rinsing the part in clean, neutral water.  Water sources must be selected intentionally so as not to reintroduce any contaminants to the cleaned part.
3. Acid Bath Passivation – the stainless steel part is now introduced into an acid bath where passivation will occur.  The acid bath is made up of one or more chemicals (discussed below), diluted to a specific concentration, and often heated as well.  The part is submerged in the bath for a set amount of time as the passivation process takes place.
4. Rinse – upon completion of the passivation bath, the part is removed and rinsed again.  As with the initial rinse, this rinse must thoroughly remove all trace  residues, and may include a separate chemical treatment step that aids in rinsing and neutralization.  Water must be of high quality and have no more than 200 ppm maximum total solids.
5. Dry – after final rinsing, the part is dried off by recirculated hot air with optional vacuum dry, completing the passivation process.

Upon completion of the passivation process, it is relatively certain that the stainless materials have formed an even, continuous chromium oxide passive layer across all surfaces.  Ensuring that the passive layer has formed can be difficult.  Testing procedures do exist, however most passivation tests measure the presence of free iron on a stainless steel surface, not the passive layer itself.  Testing for free iron is rooted in the assumption that if excessive iron is present on an SS surface, that the passive layer must be compromised and in need of replacement.  Since in most cases we cannot practically inspect that the passive layer itself has formed, users typically rely on established procedures backed by standardized lab testing which strictly specifies the chemical type, chemical concentrations, temperatures, and amount of time used in the chemical passivation process.

Though we just stated that chemical passivation processes are strictly controlled and standardized, there is one large variable that is left to user discretion, and in fact has become a hotly debated topic over the last decade – that is, what specific chemical to use for the best overall results.  Let’s dive into that debate next!