Response to Spray Wand Phosphatizing

Comment: The following was a response from a reader of the column. He elaborated on the question of spray wand phosphatizing. I did note that the pH of the readers cleaning system was far too low and would have to be raised to at least 4.0 to do an adequate job coating the parts.
Additionally, I had also received other feedback on the logic of “top-up” cleaning and “top-down” rinsing. B.D. elaborates on the reasoning. Makes sense to me.

Q. I read in the June 2006 addition of the Parts Cleaning Clinic your answers to the Phosphate Cleaning Systems question, where someone was using a spray wand phosphatizer at a pH of 2.0 and wanted to increase their quality. Hopefully, I can add to your answers:

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1) The optimum pH for phosphatizing systems depends on the accelerator, however a good rule of thumb is to keep the pH between 4.5 and 5.0. At too low a pH, all one is doing is pickling off the metal and it’s not being redeposited as an iron phosphate coating and at too high a pH, no pickling occurs at all. Spray wand iron phosphatizers are normally formulated so that pH adjustment isn’t necessary. One can just dilute and go. So, if the pH is around 2.0, the pH needs to be increased to between 4.5 and 5.0 with caustic soda or soda ash.

2) The reason cleaning is done from the bottom up is to ensure there are no streaks on the part. If one goes from the top down, the chemical will drip or run down which can leave streaks on the part. Normally, that’s not a big deal with painted parts but with some paints those streaks will telegraph through. Therefore, clean from the bottom up and rinse from the top down.

3) I agree that the chemical supplier will have all that information.

4) I agree that the metal will not reach the solution temperature in the spray time allotted. BD