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Electropolishing problem: Pitting & orange peel on 316SS

Q. What are the principal reasons for the following two types of defects encountered when electropolishing T316?- orange peel- rippling (streaking)

Richard C Dibling
- Melbourn, Royston, Herts, UK
1998

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Q. Subject Matter is EP of a large (in excess of 12K liters) vessel that is SS316L material. When a surface finish of 10 ra or less is achieved is there a limit of how many times the material can withstand an EP? If during the mechanical polish of the vessel, vertical ribbon marks became evident could the EP process result in an unidirectional pattern that is stress-free, microscopically smooth and highly reflective? Sincerely, Frank Ventre

Francesco Ventre
genetics - Andover, Massachusetts
2000

Q. Lastly a question: Is there a way to get a consistent finish on a vessel be it 2" or 30" without jig marks or any of the above details?.

David Lang
vessel manufacture - Southampton, United Kingdom
2005

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Multiple threads merged: please forgive chronology errors :-)

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Q. We are an OEM of equipment for the Pharmaceutical industry. Our components are made of welded 316-316L SS sheet metal, bar or tubing parts which are polished to 20µin as RA and then electropolished. Components made of sheet metal are always good looking (< 20µin)after electropolishing. However, some parts made of tubing or bar loose their finish (> 20µin). Finish as some light pitting or orange peel. We made some test and our conclusions lead us toward a material defect: we tested many sample of different heat number and, every time the result was the same. One MTR was always good and the other one bad. Is this possible? Can the material be the cause of this bad finish? We use ASTM A249, ASTM A269, ASTM A270. We also subcontract our electropolishing. On the other hand can the electropolishing process in cause?

Eric Potvin
oem for pharmaceuticals - QuÈbec, Canada
2005

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Q. T304 is not an option for us. Our customers request 316/316L. Does the added sulfur shows on the MTR? We just received a sample (see picture) where the good part had a sulfur content of 0.003% and the bad part a content of 0.013%. We also had occurrence of the problem with sulfur content of 0.007%.

Eric Potvin [returning]
oem for pharmaceuticals - Beauport, Quebec, Canada
2005

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A. They are normal residual sulfur levels. The range 0.002 to 0.02 percent is typical. That is, sulfur was not added during manufacture to confer free-machining properties. Free-machining sulfur levels are typically 0.2-0.3 percent.So the cause of your problem would seem to be somewhere else.Jeffrey Holmes says that T304 (I assume from the context that the "T" designation denotes the sulfurised free-machining version) is OK. The significant difference between 316 and 304 is that 316 contains 2 to 2.5 percent molybdenum but 304 contains negligible molybdenum (usually less than 0.1 percent). Perhaps the problem is related to the homogeneity of the molybdenum distribution, which would depend on the amount of deformation (reduction in cross-sectional area) of the bar during its rolling and the original size of the bar/billet/ingot that was the rolling stock. Of course you have no control over such aspects and indeed it's likely that such information could not readily be obtained. So maybe you need to stick with a supplier whose product electropolishes OK and accept that the reason for the difference isn't known.I'm a metallurgist primarily concerned with bulk properties of steels rather than surface finishing aspects - I cannot conceive that normal residual sulfur levels could segregate to give the problem you experience. Bill Reynolds [deceased] consultant metallurgist - Ballarat, Victoria, Australia We sadly relate the news that Bill passed away on Jan. 29, 2010. 2005 A. Check with your supplier and see what they can offer as far as low sulfur. I'm not certain what is available.There have been reports that extended precleaning (an hour or longer) in hot alkaline cleaner helps to minimize this problem. We have not been able to see that this helps, but others have reported some success with extended precleaning. It's worth a try. Other than that, and using the minimum electropolish time, I have no other suggestions. This is really a material problem, not an electropolishing problem. Jeffrey Holmes, CEF Spartanburg, South Carolina 2005

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April 29, 2008

Q. I need to find out why stainless steel parts are pitting and orange peeling in an electropolish process. Sometimes the defects are localized.

Sean Casarotti
OEM with inhouse EP - Yreka, California, USA

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August 21, 2013

Q. I have an application where I am trying to mirror polish SUS 316L. During trials using only lapping to compare the surface finish, I am discovering that pitting is occurring. I then sourced the same material from multiple alloy vendors and polished them using multiple types of abrasive slurries to the same result. However, there is a significant difference in the severity by source.

Chemistry by source
Bad: 0.011%C 0.49%Si 1.21%Mn 0.033%P 0.002%S 10.8%Ni 16.8%Cr 2.06%Mo 0.03%N
Less Bad: 0.03%C 0.96%Si 1.91%Mn 0.04%P 0.027%S 12.5%Ni 17.5%Cr 2.2%Mo 0.012%N 0.2%Cu
OK: 0.023%C 0.68%Si 0.95%Mn 0.031%P 0.002%S 12.1%Ni 17.33%Cr 2.07%Mo

I approached one of the polishing companies in Germany and they attributed the pitting to the moly and said that it is impossible to eliminate the problem. Not satisfied, I began digging and found a thread on this website describing a similar issue related to electropolishing. Based on what I have read, the moly or the sulfur is an issue. I checked the certs for my three sources and there is no correlation to %Mo, not that there couldn't be a distribution/segregation factor.

On the thread that led me here, sulfur was suggested as the culprit. I also found a white paper that described a tendency for Mn and So get together and form MnS surface inclusions. My middle sample has very high sulfur, however, my worst sample has very low sulfur. No correlation. Manganese on the other hand does correlate. When I plot Mn, S, Cr, and Ni, if there is any conclusion to be drawn visually, it is to keep Mn AND S as low as possible and to keep Cr and Ni on the higher side of the spec limits.

Hopefully someone can point me in the right direction because I can't find any information on pitting as a result of lapping or physical polishing. I can only find information as it related to electrical and chemical attack and any parallels are speculative.

Craig Horton
- Cupertino, California, USA

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Q. Yes. As noted above, my method of polishing for the trials was lapping. We use a diamond abrasive for this method and we have a high degree of proficiency lapping aluminum. Stainless is a material that we have done less work with historically.

To give a little more info. My part has no flat surfaces. It is model driven and constructed of complex curved surfaces. Therefore, lapping is not my long-term solution unless I use a CNC or CAM based method. Electropolishing is off the table for polishing at this point. My samples looked terrible and I need a mirror finish. Hand buffing is also not an option because my tolerance on this part is ± 10 microns. The process that I have had the best results with is "drag finishing" in a machine that uses a dry powdered media to drag the part through a tank in a motion similar to a rotating carnival ride. However, there was some orange peel that I suspect was caused by too much heat being generated.

To step back though, my understanding is that this is a material problem, not a mechanical process problem. However, I have not been able to nail down the culprit.

Craig Horton [returning]
- Cupertino, California, USA
September 11, 2013

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Q. I have a similar issue with electropolishing 316 stainless steel. We see what could be called pitting or orange peel or frosting on what seems to be a random basis, although it seems to occur more frequently when rework is required.

Sometime back our metals supplier switched from Reliance to Thyssen (unfortunately something I have almost zero say in). After the switch, outside machine shops as well as our own internal machine shop reported that sometimes they get what they describe as "gummy" material. I suspect this means the material is soft, but I'm still waiting on samples to send to a metallurgical lab. They have to adjust their feeds/speeds on the CNCs from lot to lot based on how the material machines. I'm wondering if this "gummy" material contains higher or lower sulfur and/or molybdenum levels that could explain the defects in electropolishing. As mentioned previously we tend to see this defect on planar rather than radial surfaces, suggesting it is indeed a materials issue.

Our electropolishing vendor is convinced that the problem is due to use of coolants containing chlorinated paraffins and/or sulfur/sulfides. He says these contaminates can become embedded in the steel and react poorly with his process(es). This makes some sense given that manual/hand polishing prior to electropolish tends to fix the problem, but we don't want to have to add additional steps to this already expensive process. The parts are also passivated after electropolish which from all the reading I've done seems unnecessary.

I think that potentially multiple factors are at work here but I wanted to jump into this thread and see if I could gain some more understanding of the problem and it's cause(s). Why exactly are sulfur and molybdenum a problem, etc.? Any help would be greatly appreciated.

Matt Coppler
- Clovis, California USA
June 25, 2014