Q: I operate a stainless steel pickling line. I very rarely get a grease-like substance clinging to the surfaces of parts after I remove them from the pickling bath. The parts are washed down with running hot water before processing. When they come out, it looks as if some black grease has attached to the parts. This only happens maybe once a month. I suspect that the stamping house is using a non-water-based stamping lubricant. We can have baskets full of parts sitting right next to the affected parts and the other ones will have no grease on them. Any help would be appreciated. A.J.
A. It is not clear to me what the steps are in your pickling line and what the source of all your parts is. In general, if you have parts coming from multiple locations (as you probably do), then it would be expected that they would have different contaminants on the surface.
Those contaminants could run the gamut from a light, water-displacing oil to a heavy, waxy drawing lubricant used in forming the parts. Additionally, there could be particulate residue from machining or grinding left on the surface.
If this is the case, you need to have a pickling line that includes a cleaning step before pickling. It is likely that one or more of your customers is using an organic-based lubricant that could have the possibility of breaking down while in the pickling tank. An organic residue is often dark like this and is a result of the chemical degradation of an oil or other organic source.
It is unrealistic for your customers to all use a similar type of lubricant, so I think it would increase your flexibility and decrease scrap or rework potential if you were to include an alkaline cleaner and rinse that precedes your pickling tank. Additional benefits would be increased life of the pickling tank due to fewer contaminants being dragged in, as well as a higher-quality finish since the pickling will take place evenly, rather than through an irregular residue on the surface before it gets to its primary job of pickling the stainless surface.
An aggressive but effective bright dip to clean most copper alloys is a combined nitric and sulfuric acid operated at room temperature. Concentrations could be about 20% by volume of each acid. If you need a less aggressive cleaning, a citric acid solution would provide a clean surface with much less in the way of hazardous chemical exposure to your employees and a less hazardous solution to dispose of when it comes time to dump the bath.
Somewhere upstream in this line should be a cleaning step to ensure removal of any grease and oil before a bright dip; otherwise, the acids will not etch the surfaces uniformly. The residual bright dip should not be left on the surface in any circumstance (regardless if it is the nitric/sulfuric combination or the milder citric acid cleaner) since that will lead to additional etching and eventual staining as it dries on the surface.
The final steps should be a clean water rinse and, depending on your water quality, possibly a deionized or reverse osmosis water rinse to insure the water does not lead to spotting after drying.