The passivation of stainless steel is a post-heat-treatment process with the end goal of improving corrosion resistance by restoring the protective oxide film on the part surface. Its advantages include:
- Enhanced corrosion resistance
- Cleaning of the part surface
- Chemical polishing (deburring) of the surface
- Improved and extended component life
Passivation can make the difference between the satisfactory performance of a material in a given application and premature component failure. As with most manufacturing operations, incorrectly performed passivation can promote corrosion.
The Passivation Process
The following steps are common in most passivation processes:
- Cleaning of the parts prior to passivation
- Validation testing to confirm that the surface is “clean”
- Placing the part in an acidic bath with controlled temperature and chemistry (Note: These process parameters are typically dictated by specification or customer requirement.) Temperatures typically range from ambient to about 60ºC and passivation times are generally around 20-30 minutes.
- Neutralization in a bath of aqueous alkaline solution (e.g., NaOH)
- Rinsing with clean water
- Testing (e.g., exposure to humidity, elevated temperature, a rusting agent (salt spray) or some combination of the three).
Goal of Passivation
The passivation process is designed to return the surface of a stainless steel (or other metal) to near original condition. Passivation produces a thin transparent oxide layer (4Cr + 3O2 → 2Cr2O3) acting as a barrier to prevent oxygen and moisture from reaching the base metal beneath. It is important to note that passivation should not change the outward appearance of the component being treated.
Contaminants such as dirt or metal particles (typically iron) from cutting tools or other manufacturing operations (e.g., grinding) may be transferred to the surface of the stainless steel parts during machining. If not removed, these foreign particles will reduce the effectiveness of the original protective oxide, making it more susceptible to environmental factors and providing sites for preferential corrosive attack. Passivation can also be accomplished in many cases by electropolishing.
We will discuss more about the passivation process itself in part 2.