Strange black residue appears on 15-5PH stainless after electropolishing

Letter 3034

Strange black residue appears on 15-5PH stainless after electropolishing

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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 appr. 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 degrees 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 [link is to product info at Amazon]. The residue did not dissolve. Solvent #2 was methylene chloride. Again, very little to no dissolving took place. Solvent #3 was Toluene [link is to product info at Amazon] with no dissolving. Solvent #4 was appr. 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 degrees 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.

My questions: 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
Tylaska Marine Hardware - Mystic, CT

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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 auxillary 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

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