I have discovered that our largest customer uses chlorinated paraffins in its cutting oils. My lab tells me that chlorine is present on the surfaces of the parts I am zinc-phosphating (five stages/dip immersion). About 4% of the parts show weak adhesion when they are coated with an air-drying black liquid alkyd. What happens when these paraffins are used? Can they cause this problem? How do I best remove them from the parts? M.H.
Chlorinated paraffins are added to metalworking lubricants as extreme pressure additives. They actually react with the surface under conditions of high temperature and pressure to provide maximum lubrication. While they are excellent lubricant additives, they can be very difficult to remove. If your lab is indicating residual chlorine on the parts, it means that not all the lubricant is removed by your cleaning process.
Residual lubricant (chlorinated or otherwise) will make the formation of a uniform phosphate coating difficult. The acid in the phosphate step will attack the base metal and start to develop a uniform phosphate crystal structure. Areas where there is residual lubricant will be difficult or impossible to coat since it will act as a barrier between the phosphating solution and the base metal. If the lubricant residue is light enough, the acid may undercut the base metal from around it and remove it, but the extra time it takes to do this will mean less time spent developing the phosphate crystal structure. Therefore, the part may be fully coated; but it will be spotty with areas of low and high phosphate coating weights. These irregularities, spottiness and possible lubricant residue will be enough to create adhesion problems.
To resolve the situation, attention should be focused on the cleaning step and/or, if possible, the customer’s cutting oil. Time, temperature and cleaner concentration should be maintained and checked every shift in the cleaning stage. It is important that this step be optimized for cleaning efficiency. You also may want to contact your pretreatment chemical supplier to see if it has any recommendations about your existing line or can suggest an alternate cleaner.
If you have gone through all these steps and still cannot remove the cutting oil, you need to talk with your customer. Depending on the material and the operation, it is possible that it could use a lubricant with boundary additives that would work sufficiently and be much easier to clean. It is also important to minimize the time between their application of the oil and your removal of it. With many oils, the longer they sit, the more difficult they are to remove.