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Flash rust after drying in oven





February 17, 2009

I have a small metal cabinet manufacturing company in which we are having flash rust appearing on our cold rolled sheet steel after we dry it off in our ovens.
We are using a four stage dip tank system. First stage is kept at 140 deg. using a high alkaline degreaser--2nd stage is fresh water rinse with constant overflow--3rd stage is an iron phosphate at 3-4 oz per gallon of water--immersed for approx. 3-5 minutes. Our tanks are 300 gal capacity.
Fourth stage is room temp. tap water rinse with constant overflow.
The dip tanks are approx. 60 years old made of 1/2" steel plates. They are very heavily coated w/ paint, etc..... on the outside and they do have a coating of "something" on the inside of the tanks that cannot be scraped off--I have been told that the quality of these tanks may be the problem. Now we have to rub or wipe all parts down after they come out of the oven to remove the flash rust--very time consuming, Any help?

Gregg Baslee
plant mgr. - Kansas City, Missouri



simultaneous replies

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 [on eBay or Amazon] (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.

James Watts
- Navarre, Florida



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.

Jun Zhang
- Cincinnati, Ohio



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.

Christian Restifo
- Pittsburgh, Pennsylvania



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.

Sheldon Taylor
Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina




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.

George Gorecki
- Naperville, Illinois



March 4, 2009

Sir:
I have zero experience in phosphating. However I have done many rinsing experiments. My experience is that the steel product must not be allowed to dry between rinsings and a pH lower than 5 prevents rusting. Also the hotter the steel the faster it rusts.
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA



March 5, 2009

Sir:
In my previous response I left out two things. (1) Not only does a pH below 5 prevent rusting of pickled steel, but also a pH above about 8 also prevents rusting. (2) A certain surfactant at between 1 part and 3 parts per 10,000 prevents rusting of pickled steel.
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA


what temperature need for drying phosphate in oven?

Verya Rezaei
- Bandar Abbas, Iran
May 21, 2011



Hi, Verya

Is this a question for Gregg, to which you need the answer in order to help him? Or do you have a problem of your own with flash rusting of a phosphate coating on your parts? Or are you misunderstanding this thread, thinking that we are talking about the preparation of phosphate-based bulk supplies? Please spend at least several sentences describing your situation. Thanks.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 22, 2011




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