Is Electropolishing Necessary When a 1.0 or Greater CR/Fe Ratio Can Be Achieved by Passivation Alone?

It depends on what the desired outcome is.

Chemical passivation will attain a chrome to iron ratio above 1.0 (Cr/Fe), without producing any measurable change in the finish characteristics of the surface while Electropolishing (EP) is designed to remove surface damage from mechanical polishing (MP) and produce a more cleanable featureless smooth surface finish (Fig. 1 & 2).
 
The need to electropolish a surface is dependent upon the desirable surface finish requirements, while passivation is required by all austenitic stainless surfaces to improve their corrosion resistance, whether they are electropolished or not. Formation of a passive film on the surface does not require electropolishing.
 
 After electropolishing Micro view - Astro PakBefore electropolishing Micro view - Astro Pak

EP’d surface (Fig. 1)                      MP’d surface (Fig. 2)

 
On the other hand, chemical passivation will improve the corrosion resistance of all austenitic stainless steel surfaces, no matter what the surface profile or roughness condition. It chemically removes the iron and iron oxide from the surface and leaves the chromium oxide at the surface to protect the alloy.
 
The effectiveness of the passivation process can be quantified or measured in terms of the Cr/Fe (chrome to iron) ratio. The more chromium in the surface, the more corrosion resistance will be present. Chemical cleaning and passivation will improve the surface corrosion resistance and remove surface contamination, but will not remove surface area, roughness, or cold work damage from polishing.
 
Chemical passivation is required after electropolishing, since electropolishing passivates the surface only to a condition typically attained by phosphoric acid. Passivation with citric acid based chelant systems or nitric acid will effectively improve the Cr/Fe ratio and double the corrosion resistance of an electropolished surface alone (Chart #1).
 
Comparing the chemical process of passivation to electropolishing is difficult since they produce two different outcomes.

Chart #1

Sample TestedXPS or ESCA Analysis (CrOx/FeOx)Cyclic PolarizationAES (Auger) Analysis (Cr/Fe)
Mill Finish - Passivated2.145+1.2
Electropolished3.0201.7
EP + Nitric Pass4.392.0
EP + Citric Acid Passivation 4.882.2
 
Passivation improves the chemistry of the surface (increases the Cr/Fe ratio) while electropolishing removes surface damage, improves the surface profile and cleanability, or smoothes the surface. While electropolishing passivates the surface, it does not meet the Cr/Fe ratio levels attained by comparative passivation processes.
 
In summary, if the goal is to attain a set Cr/Fe ratio -then passivation is the process to use but if a cleanable corrosion resistant surface is desired, then electropolishing followed by chemical passivation is the best choice. I hope this helps answer the question.
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Astro Pak Consultant, Mr. Roll serves as the primary senior technical advisor for corrosion, surface chemistry and stainless steel Passivation. With over 35 years of experience in chemical processing, Daryl has been published in MICRO, UltraPure Water Journal and Chemical Engineering for his papers on passivation and rouge control. He is a participant on the ASME BPE Subcommittees for Surface Finish and Materials of Construction requirements and a leading contributor for the Rouge and Passivation Task Groups. Mr. Roll holds a B.A. in Chemistry and Earth Science from the California State University of Fullerton and a Professional Engineer's license from the State of California.

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