Problems electropolishing stainless castings (15-5PH)
Q. We have had constant quality problems from vendors who electropolish our 15-5PH stainless investment castings. Sometimes the parts come out fine, but other times the surface is very frosted or pitted. The outcome is almost random, and bad results end up in several thousand dollars worth of scrap. It makes you want to pull your hair out.
I have tried to work with our vendors to set up some controlled experiments and narrow down the sources of inconsistency. However, both of my vendors refuse to work with us, stating that the process is "secret" and I am not allowed to work with them. This is ridiculous.
I have some background in the field and have read many publications on electropolishing, but without a laboratory set up, I cannot determine the problem exactly. My questions are as follows:
1) Are investment castings always inconsistent in polishing, and if so, what causes this randomness?
2) When we heat treat the parts, (1025 °F for 4 hours), the stainless turns brown. I believe this may be a nitride coating from nitrogen present in the air. Would this thin brown tinge be a source of our problems?
3) When the parts are machined, we use a coolant. We then clean the parts very carefully before heat treating. However, I am wondering if any residue from the coolant is diffusing into the metal when heat treated, and then comes out during electropolishing. Is this reasonable?
4) How hard is it to make a small laboratory set up. I commonly find 15 gallon containers of electropolishing solution, but is there a place to buy 1 gallon?
Thanks for any help.
marine hardware - Mystic, Connecticut
A. Any pre-existing surface condition on the castings is critical to the resulting electropolish finish. This does not preclude that your vendors are causing the problem, which is always a suspicion when they refuse to work with you.
Investment castings can have surface residue from the casting process, in addition to porosity and inclusions that can affect the polishing outcome. Machining lubricant residue can result oxide films, especially after heat treating, which are also potential problems. I don't think that the machining lubricants are diffusing into the metal, nor is the metal picking up significant nitrogen from the air at 1025 °F.
Laboratory analysis of the surface physical and chemical condition can provide you with much information about your processes and possible affects on electropolish results. We have had some good successes in using surface analysis to troubleshoot electropolishing problems. The analytical plan could be tricky if the problem is truly random.
A. For what it is worth I concur with Larry. It is important however to realize that everything that has been done to the material will have a effect on the electropolish process.
Most people understand that e/p is generally the last process that the part will receive prior to shipping. The electropolish process is so affected by the other treatments that in order to maintain any degree of repeatability in the finished product all facets must be documented. If the heat treat changes or the type of soak or the material isn't exactly the same every time the results will almost assuredly be reflected in the electropolish. I agree with the comment about your vendors not working with you. Electropolishing isn't rocket science. Anyone who tells you their process is proprietary is pulling your chain.Greg Godbey
electropolish - Fountain Valley California
Strange black residue appears on 15-5PH stainless after electropolishing(1999)
Q. Strange residue left on stainless after electropolishing I have a large number of 15-5PH stainless castings electropolished by a vendor, and frequently there appears a strange scale-like residue left on rougher sections of the parts after we receive them back from the vendor. This residue is brownish black and can be physically scraped off with a knife, but not without a lot of work. The residue is generally found in patches and appears to be perhaps a few thousands of an inch thick. It is really stuck on the parts almost as though it were baked on enamel. Most of the time, it is located in interior slots, blind holes or cavities of the casting. Never has it appeared on any pre-machined surfaces. Only on cast surfaces and mostly in slots or interior surfaces.
The vendor has demonstrated that if the parts are left in the electropolishing tank for long enough, this residue eventually disappears. However, the parts are then overpolished and are well beyond tolerances. As an initial suggestion, the vendor thought that the parts had some oil or other impurity on them before they were heat treated. We then took some raw parts and boiled them in approx. 10 Molar sodium hydroxide solution for 45 minutes. The parts were about as oil free as they could get. These parts were heat treated (1025 °F in air furnace) and electropolished. The same black residue still randomly appeared on the casting after electropolishing, although perhaps not quite as much.
The next suggestion was that the air furnace was creating some type of oxide scale on the parts, and this scale was what was responsible for the residue. We next took some raw parts, boiled them in the sodium hydroxide solution as before, and had them heat treated in a vacuum furnace. Titanium chips were packed around the parts to insure that every last trace of oxygen had been removed. The procedure was very expensive, but little to zero oxidizing occurred. It was hard to tell that some parts had even been heat treated. A sample of these parts were next electropolished, and the same residue again appeared although now to a seemingly lesser extent. Another sample of the parts were taken from the vacuum furnace and additionally glass beaded to insure that absolutely no anything was present on the surface. These ultra clean parts were electropolished, and again the black residue appeared to the same extent as the non glass beaded parts.
Next, I took some of the residue itself and tried to dissolve it in various solvents. Solvent #1 was MEK / methyl ethyl ketone. The residue did not dissolve. Solvent #2 was methylene chloride. Again, very little to no dissolving took place. Solvent #3 was Toluene with no dissolving. Solvent #4 was approx. 20% nitric acid at room temperature. The residue seemed to dissolve slightly, and definitely softened up. Solvent #5 was a 45% nitric acid bath at 130 °F for 30 minutes. The residue dissolved quite a bit. The bath time was then increased to 45 minutes, and at this point, at least half of the residue on the parts did indeed dissolve away. However, this treatment was very harsh on both equipment and operator and was not something we could do on a production basis.
1) What is this residue and how can we prevent it from occurring?
2) Is there any easier way to dissolve away this residue short of the 45% nitric bath?
Any responses would be greatly appreciated. Thanks. Sincerely,Tim Tylaska
marine hardware - Mystic, Connecticut
A. Dear Mr. Tylaska, The clue to the problem here is the fact that this smut occurs to quote you "Most of the time, it is located in interior slots, blind holes or cavities of the casting". These are areas with low current densities compared to the rest of the part. The smut is actually residual iron left on the surface since most electropolishing solutions attack the other elements such as nickel or chrome faster than they do the iron. Additionally, before I offer one possible solution, I would like to point out that any surface clean enough to hold a water break free surface is clean enough to electropolish, and boiling in caustic in not required. A suitable electrocleaner should do the job. That said I believe the use of auxiliary cathodes located near these areas on a separate power supply will solve the problem. If for whatever reason that does not seem feasible, then increasing the solution agitation should help. The other part of the solution to the problem is in your hands, not the electropolishers'. Give the rough areas a light sanding to eliminate this roughness before they are polished. Electropolishing reacts much more favorably to removing sanding marks than to casting roughness.Anthony J. Covey
electroformer - Corona, California
February 15, 2013
I own a small investment casting foundry in the UK and experience the same difficulties with Electropolishing. The results are so inconsistent no matter what supplier we use. We solution anneal our parts with nitrogen and 4% natural gas atmosphere. The castings have a light greenish brown tinge when finished. Even with parts cast in the same alloy batch, heat treated at the same time and finished in the same shot blasting machine, there are mixed results. We suspect that the shot blasting process may be trapping oxides and crud under the surface, peening over, and not then being eroded away completely by the Electropolishing process.
The inconsistent results have driven us to proposing us to offer Electropolishing followed by glass bead as an alternative finish.
If anyone has any suggestions please post a response.
- Devon UK
A. We have some experience in ss casting electropolishing.
It might work in Universal Electrolyte. Contact us for samples testing.
A. Hi Anna,
many thanks for the suggestion.
We have recently subcontracted a batch to vacuum annealing.
The results look very good; we will continue to undertake the heat treatment via this vacuum process and feedback our findings. I will also send a few samples to you for processing.
- Devon UK
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