Flash rust after drying in oven

Talk to your phosphate vendor. They should sell a rust inhibitor for your final rinse tank that will solve your problem. It will also dry quicker if you use about 5% to 10% commercial isopropyl alcohol ⇦this on eBay or Amazon [affil links] (IPA) in the final rinse in your waste water system can stand the change in BOD/COD. It is not a fire haz at this strength! Also look into using an air knife to knock off the bulk of the water as soon as it comes out of the last tank. High pressure air works well, but OSHA will have a cow. Get it into the oven quicker will also help.

Gregg, in general, you may deal with two types of rusts without detail information. 1) if the drying temperature is higher behind 350 °F, you may see the yellowish head rust on CRS. 2) if your phosphate tank was not operated properly, you may see induced flash rust by phosphating chemical, which could be seen anywhere from right out of bath to initial oven dry due to moisture level.
In order to identify where and what, I am interesting to know 1) the condition of your iron phosphate tank, such as %, pH, T, and t; 2) your oven temperature and time.

This is a common problem.

Have you checked your incoming rinse water quality? Tap water can vary from place to place, and it can cause problems. Salts and very high/low pH can accelerate flash rusting. If your tap water is bad, a simple in-line DI system (the cartridge type) can work wonders. One "2-for-1" solution is to rinse the parts with DI as they exit the tank and have the water cascade into the tank. That way, your parts see very pure water while you still get to overflow your tank.

Another solution is to run your final rinse at a slightly acidic pH. Some people will actually use a very small amount of the phosphate concentrate to keep a slightly acidic tank.

You might want to also consider a rinse aid in the final tank that will help the water sheet off quickly. These are used at very low concentrations, but you have to keep adding it (as with the acidic solution) since you should be overflowing your rinse.

Make sure your oven is drying things off quickly. If it takes a long time to dry, you're just subjecting a wet part to a warm environment which will accelerate rust formation.

If the flash rust appears in a certain area or is more prevalent among certain parts, make sure that you're adequately cleaning those parts. Also, look for dripping from conveyor lines/transfer equipment that can leave deposits and cause flash rusting in certain areas.

Is this a new problem? It looks like your Phosphate concentration is too high and your immersion is too long. Try 2.0 to 2.5 and about a minute to a minute and a half, keep the pH at about 5.5 to 6.0 range and see if that helps. You didn't say whether or not the phosphate stage is heated, if it is keep the temp under 110 degrees. The ppm of the final rinse should be as low as possible, in the less than 50 range. I doubt the condition of the tanks is a factor unless they are rusting on the inside.

Hi Gregg,

A phosphated surface isn't supposed to corrode, so there must be something happening in your line that is causing a corrosive environment. There are several possible causes.

What is the pH of Stage 4? It will naturally be slightly acidic because of phosphating solution carrying over from Stage 3. You say that the Stage 4 rinse is overflowing. If the overflow is not sufficient, then the phosphated work is being rinsed in an environment that is too acidic and flash rust can occur. Try increasing the overflow.

A low pH can also be cured by adding a small amount of alkali to Stage 4 periodically. This isn't a great solution because the amount of alkali has to be added carefully or your pH will go very high. And you have to stay on top of this constantly. Another alternative is to add some kind of corrosion inhibitor to Stage 4. Again, this is labor-intensive and will require diligence to keep the flash rust away.

How long are the parts wet after rinsing in Stage 4? The longer that they remain wet with acidic rinsewater, the better the chance of flash rust forming. If it is possible to shorten the time needed to reach the oven, then that might help. If not, then perhaps an air knife can be added to the line right after Stage 4 to blow off most of the rinsewater before dry-off. Whatever you can do to get the parts dry as quickly as possible will help prevent flash rusting.

Hope this helps.