Phosphate Cleaning Systems

Ask an Expert From: Products Finishing,

Posted on: 6/1/2006

The issues I currently have are as follows: (1) I have a pH level of 2.0, that I think is causing flash rust to occur and in turn some slight delamination of paint during scribe testing. What is suggested to control pH? (2) I’ve read that when cleaning you should wash from bottom to top; this seems counter-intuitive to me. Can someone explain? (3) I’m sure the wheel has been invented on controls and work instructions for manual phosphating so does anyone have any links to a location that I can get our’s off of? (4) Does the metal temperature need to reach a certain temperature for the coating reaction to take place or only the solution?

Q. We currently use an iron phosphate one-step cleaner applied manually through a high-pressure wand.  The soap solution is applied at medium pressure and a clean water rinse is applied at high pressure.  Most of the material we are treating is hot-rolled, welded and extremely grimy.  

Researching, I noticed we were keeping zero controls on anything we were doing so I started from the ground up. We developed work instructions on how to spray the 20-ft containers we wash, top to bottom, with a clean water rinse. We also started keeping titration and pH levels on a daily basis. The issues I currently have are as follows:

  1. I have a pH level of 2.0, that I think is causing flash rust to occur and in turn some slight delamination of paint during scribe testing.  What is suggested to control pH?

  2. I’ve read that when cleaning you should wash from bottom to top; this seems counter-intuitive to me. Can someone explain?

  3. I’m sure the wheel has been invented on controls and work instructions for manual phosphating so does anyone have any links to a location that I can get our’s off of? 

  4. Does the metal temperature need to reach a certain temperature for the coating reaction to take place or only the solution? E.P.

 

A. You certainly have tackled the problem in an organized way and can see you have instituted some controls on your process. It will yield progress in the future.

Spray wand phosphate systems are designed to clean and phosphate in a single step. The formulation of these products is such that the chemistry can only provide a moderate level of grease and oil removal. Since you mention how dirty the containers are, it could be possible that at some point in the future, you may have to consider a separate cleaning process prior to the iron phosphate. This could be necessary in order to better remove the oily contaminants from the surface prior to phosphating.

To answer your questions:

  1. a pH of 2.0 seems far too acidic for a spray phosphate system. Most iron phosphate systems operate around a pH of 4.0. This is significantly higher than the pH 2.0 you are operating at considering pH is a logarithmic scale. What you describe is likely happening. Due to the very low pH, very little phosphate is allowed to form on the surface and as a result, flash rusting will occur. Painting over a surface with flash rust is a less than ideal substrate, which is likely to result in the paint delamination problem you describe. Whoever is supplying your phosphating chemicals should be able to provide you the optimal pH for their operating system (which will be higher than 2.0). If not, I would suggest finding a new supplier. Several potential candidates can be found on the PRODUCTS FINISHING Find Suppliers Page.

  2. Washing from bottom to top does seem counter intuitive. The only possible explanation I can think of would be if washing from top to bottom concentrated so much contaminant on the base of the container that the cleaner/phosphate chemical would no longer be able to cut it. Washing/coating from the bottom to top may provide you with a more “even” surface as far as contamination goes. As mentioned above, the cleaning ability of these systems is generally limited. If you do end up with a single cleaning step, it would probably be best to start with the top and work your way to the bottom.

  3. Regarding process controls and work instructions for the system, again, your chemical supplier should be able to provide you with at least a framework that you can incorporate into your quality system. If not, you should seek another supplier.

  4. The metal will never reach the solution temperature due to the evaporative cooling taking place when spraying the solution and the thermal conductivity of the metal which will help in dissipating the heat. Generally these solutions do not operate at a magic temperature, but simply will require a longer spray time the further you are below the recommended operating temperature of the system. For instance, if the supplier recommends one minute at 120°F, you may have to spend about two minutes if you are operating at only about 100°F.


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