Passivation vs. Electropolishing

Is electropolishing actually necessary given that a 1.0 or greater Cr/Fe ratio can be achieved by passivation alone?

The short answer? If a 1.0 Cr/Fe ratio is all you are trying to achieve, no it is not. Now for the long answer. First, 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, again achievable by passivation alone. The pharmaceutical industry, in most cases, requires a 15-25Ra value typically achieved through a mechanical polishing procedure. It is this procedure that, in my opinion, causes many of the problems experienced today with the formation of the “gray residue” and Class 1 rouge that has plagued end users for years.

Mechanical polishing, by definition, 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, the truth of the matter is that mechanical polishing is actually damaging the surface of stainless steel, leaving behind scratches and contamination. This damaged surface is known as the “Bielby Layer” and is usually in the range of .0003” to .0005” in depth. (See Figure 1.1)

Figure 1.1

Figure 1.2

The damages in the upper most grains can be seen in the illustrations above. Both are a 180 grit mechanical polished surface reading a 20Ra using a profilometer. They provide a clear indication of the actual surface left behind. In a study done by J. Wulf, this damaged layer was identified to have up to seven distinct layers. The illustration below, Figure 2, depicts these results.

Figure 2

This study looked at three distinct surfaces: honed, ground (or mechanically polished), and electropolished. The study found 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.

Looking back at the mechanical polishing illustration, figure 1.2, notice the layers of the material 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.

With stainless steel being an alloy that contains approximately 64% iron, it is only logical to conclude that the grinding dust released during this process will contain 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. (See figures 3.1 and 3.2)

Figure 3.1

Figure 3.2

In addition to offering an “UltraClean” surface, electropolishing also offers a reduction in total surface area as shown in figures 4 and 5. In these examples, samples were provided for a white light interferometric (WLI) surface analysis to look microscopically at the surface profile.

 

Figure 4 shows the cross section and surface profile on mechanical polish 11.5 Ra stainless steel sheet metal. The red line in the box shows the actual surface profile, highlighting the peaks and valleys of the surface.

 

Figure 5 shows the cross section and surface profile on mechanical polish 11.5 Ra stainless steel sheet metal followed by electropolishing to a final Ra of 2.3. The red line in the box shows the actual surface profile, displaying the lack of peaks and valleys of the surface and indicating a microscopic, featureless surface and reduction in the total surface area.

When comparing figure 4 with figure 5, the improvement to the surface is undeniable. The surface area is reduced and the damaged layer from the mechanical polishing process is eliminated, along with the sub-surface contaminants.

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.

In conclusion, 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.

 

 

One comment

  1. Morris Key says:

    Mr. Kimbrell, in your experience, might it be practical, i.e., cost effective, to electropolish cast gray iron fittings preparative to receiving a corrosion-protective coating?

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