The word “passivation” is one that is thrown around quite a bit in the metal finishing and cleaning world and seems to have a world of different meanings to different people. Although the word passivation is normally associated with stainless steel, there are several other metals that can be “passivated” in one way or another. The root of the word passivation, of course, relates to the passivity of the metal. In simple terms, the passivated metal is protected by a passivation layer from reaction with oxygen, water, chlorine ions and other sources of potential corrosion. Our friends at Wikipedia have some great information on passivation of metals including stainless steel for those who are interested in learning more at Passivation.
Clearly, however, I wouldn’t be writing this blog if Wikipedia had covered everything. In metal finishing and cleaning there are a few different ways that we use the word passivation.
- Stainless Steel equipment is passivated during or after fabrication. Stainless steel itself is naturally passivating as it forms a protective corrosive layer spontaneously on exposure to air and water. In real life, however, it is difficult if not impossible to prevent contamination of stainless steel as it is fabricated into tanks, tubes and other geometries required to make an industrial cleaning machine. The tools used in fabrication are seldom corrosion resistant themselves and, therefore, leave small amounts of contamination behind as a result of abrasion during bending, sawing, graining (sanding), cutting and other fabrication procedures. Even tools supposedly dedicated to only the fabrication of stainless steel can leave deposits behind and, in fact, even pristine stainless steel may have exposed grain boundaries that are subject to corrosion. Passivation is accomplished by washing completed fabrications with acid, usually Nitric Acid, which will dissolve and remove any remaining non-passive contaminants which might other wise spawn corrosion.
- “Passivation” has also become the designator for any process using Nitric and, sometimes, Citric Acid to alter the surface conditions of a workpiece. The passivation process usually involves cleaning as a first step. The purpose of cleaning is to remove oil, grease and other surface contaminants that would not be removed otherwise by the acid passivation process. Most hydrocarbons are at least partially resistant to acids. Without a pre-clean, the acid would not be able to reach and passivate all surfaces. Precleaning is usually followed by a rinse to prevent contamination of the passivating acid by the cleaning chemistry. Subsequent acid passivation is usually accomplished using a series of acid tanks. As the acid in the first tank is depleted, it is discarded and replaced with the contents of the second. The contents of successive acid tanks is moved forward one step in the process. Acid passivation is normally followed by one or more water or slightly caustic rinses (to counter-act any remaining acid) followed by drying.
- In a slight misnomer, “passivation” has also become associated with processes that perform the normal function of metallic passivation but also perform another perhaps even more critical function of removing bacteriological residues from workpieces. Although both functions are intended and desired, the degree of effectiveness of a particular set of process parameters may vary depending on specific conditions and expectations. Eliminating biological contaminants usually requires heat.
As you can see, the word passivation means different things to different people. The expectations for any specification of a “passivation” process should be fully understood prior to designing equipment to perform the process. Cleaning, although an important step in the passivation process, is not capable of true passivation in most instances unless and acid is involved.
Footnote – In the process of preparing this blog for posting I, as usual, ran it through the Word Press spellchecker which insisted that I use the word passivisation instead of passivation. Further research revealed that the word passivisation does not exist although perhaps it should. But that’s a subject for another blog.
– FJF –