Removing Rust

Question:

I am currently using a cyanide-based, unheated, electrolytic derust tank. I am interested in finding an acceptable substitute for the cyanide. I prefer a non-acidic product and can add heat to the tank as necessary. I have a lot of salesmen giving me advice, but I want to know from an expert what the recommendation is and how will it compare with cost and effectiveness to my current process. Also, I am currently expanding my business to production electrocoaters and powder coaters. We currently use a burn-off oven, media blasting as well as an alkaline paint stripper. I have noticed that my competition uses solely burn-off ovens and a pickling solution. Can you tell me the most effective acid formulations for the pickling tanks? The ovens are wonderful for racks and hooks, but is there a new technology for removing these coatings from production parts? Thank you for your help. M.G.

Answer:

Assuming your rust problems run the gamut from light to severe, it may not be possible to eliminate a strong acid from the derusting process. I have never found “neutral” rust removers to be effective. These are typically dilute organic acids with additives that suspend the dissolved metal. One possible alternative for light to medium rust would be an alkaline derust tank. This could be used in your current tank (after thoroughly cleaning it out) but typically involves higher temperatures (up to 200F) and longer times using a product that is highly caustic. A product such as this may not be capable of removing heavier rust though.

Two other alternatives to think about, but they will cost you. Using a much more dilute acid solution, you could electrolytically remove the rust. This would involve the purchase of a rectifier, bus bars, racks, etc. Another alternative would be to try salt bath descaling. Again, this would involve the purchase of new equipment but is very effective at removing all levels of rust and produces parts of uniform quality regardless of the incoming part. Final suggestion. Have you identified the source of your rust problem? It may be much easier to prevent the rusting than allowing it to occur in the first place.

Regarding your question about the paint strippers. I agree with your observation that burn-off ovens are very effective at paint removal from hooks and racks. Have you tried them on parts? There could be two reasons why they may not work on your parts. One reason could be warpage. Some burn-off ovens operate at fairly high tempratures where finished parts can warp, especially if dissimilar metals are joined and/or a light gauge material is used. Unless a lower temperature can be run, the burn-off oven can’t be used for these parts. Another problem encountered with parts exiting a burn-off oven is the residue left behind. This can usually be easily removed in a spray wash or agitated immersion wash (or hand wipe if neither of the other two is available). The typical formulations for paint strippers can be acid based but may also contain some solvents to help speed the process. Additionally, a couple of manufacturers have recently introduced neutral paint strippers.

There are also several alternatives to chemicals and burn-off ovens for paint stripping. These include hot fluidized beds, molten salt baths, plastic media blasting and sodium bicarbonate blasting.