Comparing Passivation and Electropolishing

Lead In Photo Surface Corrosion

A 1.0 or greater Cr/Fe ratio can be achieved by passivation alone. If a 1.0 Cr/Fe ratio is all that is needed, then electropolishing is not necessary. 

The 1.0 or greater Cr/Fe ratio indicated in the ASME-BPE standard is a minimal requirement. The best passive and corrosion-resistant surfaces will have a Cr/Fe ratio in excess of 1.5/1, achievable by passivation alone. In most cases, the pharmaceutical industry requires a 15-25Ra value, typically achieved through a mechanical polishing procedure. This procedure can cause problems experienced with the formation of “gray residue” and Class 1 rouge, a form of discoloration and corrosion. 

Mechanical polishing is a hand sanding process that uses various forms of abrasive media to remove scratches, gouges, and other damage from the surface of materials. The media is applied to the surface using hand-held power equipment, resulting in surfaces compliant with ASME-BPE surface finish standards. However, mechanical polishing is actually damaging the surface of stainless steel, leaving behind scratches and contamination. This damaged surface is known as the “Beilby layer” and is usually in the range of .0003” to .0005” in depth.

Electropolishing is designed to remove surface damage from mechanical polishing and produce a more cleanable, featureless, smooth surface finish. 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.

A study done by J. Wulff looked at three distinct surfaces: honed, ground (or mechanically polished), and electropolished. It found that the honed surface had up to three distinct layers and the mechanically polished surface had up to seven distinct layers, while the electropolished surface demonstrated only one layer of pure austenite.

In mechanical polishing illustration, layers of the material are folded over on the surface. Studies have shown that underneath the “folds” there are trapped particles of abrasives, oxides, polishing compounds, dyes, greases, and other contaminants all embedded in the distorted crystal structure. Studies further show that no amount of cleaning or passivation can remove these contaminants. Only when the system is placed into service, with normal operating conditions of heating and cooling cycles, does the material expand allowing these contaminants to release onto the surface and into the product.

Stainless steel is an alloy that contains approximately 64% iron, so the grinding dust released during this process contains iron particles that are distributed and then deposited downstream on piping and equipment walls, contributing to Class 1 rouge. 

Electropolishing offers the ultimate product contact surface by providing an optimum micro-surface finish, a reduction in total surface area, and providing pure alloy without contamination or damage at the material’s product contact interface surface. Electropolished surfaces offer optimum cleanability, sterility, corrosion resistance, and a reduction to rouge formation.

During the electropolish process, approximately .0005" of material is actually removed from the surface of the steel. This ultimately removes all of the damaged layer and subsequent contaminants trapped under the smeared material on mechanically polished surfaces.

In addition to the obvious benefits to the surface via the electropolish process, ASTM B-912-02 specification recognizes electropolishing and electrochemical cleaning as an acceptable form of passivation. In order to meet ASTM-B-912-02, a nitric or citric acid and water passivation solution is applied at ambient temperature to a surface and is a very fast and effective alternative to conventional passivation procedures. Following passivation, a final rinse using deionized (DI) water at ambient temperature is performed. The duration of the rinsing process will be determined by testing the water to ensure that the effluent conductivity is within 1μS of the influent. 

Chemical passivation is required after electropolishing, since electropolishing passivates the surface only to a condition typically attained by phosphoric acid. Chemical passivation is passivation with citric acid based chelant systems or nitric acid. It will effectively improve the Cr/Fe ratio and double the corrosion resistance of an electropolished surface alone. Chemical passivation will improve the corrosion resistance of all austenitic stainless steel surfaces, no matter 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 passivation will attain a chrome to iron ratio above 1.0 (Cr/Fe), without producing any measureable change in the finish characteristics of the surface. 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.

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.

 Although electropolishing passivates the surface, it doesn’t meet the Cr/Fe ratio levels attained by comparative passivation processes. If attainment of a set Cr/Fe ratio is the goal, passivation is the process to use. If a cleanable corrosion resistant surface is desired, then electropolishing followed by chemical passivation is the best choice.

Processors must be more concerned with product contact surfaces well beyond the Cr/Fe ratio. By proper material selection and surface conditions, the actual need for repetitive passivation treatments to correct iron contamination and cleaning inefficiencies could be reduced.