Manganese Phosphate Components have a red cast to them

A. What was the length of time between titration for free acid and processing the parts?

I suspect your free acid is high and the red color is red iron oxide., or the orange valence of manganese.

Do this. Start heating the bath, when you get to 185, right at tank side (not down the street in the lab), right at tank side, titrate for total and free, start adding Manganous Carbonate until the ratio gets to 7.5 to 1, by now you are at about 195, start phosphating, hold the temp below 200.

When thru, turn off the heat. Next time, titrate at the tank side just before processing. If you titrate on Monday and process on Tuesday the free acid will be sky high. Keep heat off when not in use.

A. Good, 30 minutes is probably tight enough. You see, at operating temperature with no work in the tank the free acid forms very fast. Also the Manganous Carbonate works immediately. So. Titrate at the tank when about 185, add Manganous Carbonate (the best), or Sodium Carbonate (which makes the most sludge), and then titrate again, and keep doing that until you get to ratio, then load and keep loading until the work is finished, then cool down, take two aspirin and call me in the morning.

A. One problem we've experienced along these lines is that large parts take longer to come up to temperature so while the temperature of the bulk part is rising, the phosphate solution at the surface of the cool part is less than 180 °F. so the part undergoes phosphoric acid etching prior to phosphating.

Red phosphate is also often noted on heat treated parts and, in that case, not altogether avoidable.

Good luck at the Ars!

A. Just like to add to Milt Stevenson Jr's point - if you heat the rinse stage before the manganese phosphate bath, then the cooling down effect and thus the phos. acid etching is avoided, since the parts are already at the required temperature.

A. I said TITRATE when you get up to 185 °F. By the time you adjust the total free the solution will be up to 195, at which time start loading the tank. I did not say to phosphate at 185.

Regarding pre-heating, do that in the grain refiner pretreatment step which is less corrosive (rusting) than plain how water

A. I am 'bout ready to give up and blame the alloy, but first let's try two things more: (1)Get the T/F ratio down even lower and (2) try aluminum oxide blasting instead of steel shot(thinking that the residual aluminum left on the surface will dissolve and float off and yet do no harm to the solution).

A. Dear Sir,

You probably have copper poison at one of your baths in the line. I believe it is in the etching bath, but it can be in the cleaning bath or in the Phosphating bath itself. It is enough that there is more the 10 mg/lt of copper to give the red color. Send samples of all the bathes to copper analysis.

A. I have been using this process for about 15 years and have run into this problem many times. I have learned that the acid ratio is a culprit a good part of the time and that a free acid of 2.0 - 2.5 with a T/F ratio of 5.0 -6.0 works well. Also a bath temp of 195-200 °F is best. But, there is also another factor that must be considered, iron content. Iron content should be above 2.0 during titration. I have also found that not all metals will react the same, i.e., mild steel will park while hardened 4140 turns red. This could be caused by the heat-treating process, i.e., the carbon is being burned out of the surface. If your blasting is not penetrating this area then you may need to re-carburize the part.

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A. Hi Jimmy. For decorative finishes like chrome plating, whole booklets have been written about proper lighting for inspection. But for functional coatings like manganese phosphate, I'd be surprised if anything like that exists ...

It starts, however, with the specification. Your parts should be processed to some specification or other, and a primary job of inspection is to determine whether the parts comply with that specification. Production, inspection, and customer might debate qualitative parameters like "uniform appearance", but should never be debating whether parts comply with a completely unspecified parameter :-)

If the spec contains any wording relative to appearance, you must determine whether the parts comply with that wording, and this generally isn't possible to determine in the dark, but shouldn't be determined with special lighting (flashlight/torch) unless the spec says so, which it probably doesn't.

A second issue is that discoloration is often an early sign that a process is out of control or is getting out of control; so if your inspection by flashlight offers actionable information toward restoring control, rather than just a pass/fail judgement, the production department should find it worthwhile.

Interesting question; I hope that people who have wrestled with similar questions will chime in. Good luck.

Regards,

A. I am dealing with this exact situation right now. Will keep you updated...