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topic 28771

Problem with red/brown iron phosphate color

A discussion started in 2004 but continuing through 2019


Q. We are currently experiencing a "problem" with the iron phosphating of our gray iron motor end bells. The parts are coming out with a red/brown (rust) color versus our typical gray color. This color difference is exhibited on the as-cast surfaces. The machined surfaces are not noticeably different from our normal process. We have experienced the problem with multiple casting vendors, although the problem appears to be more prevalent with some than others. We experienced the same problem about 6 months ago, but our manufacturing plant can't tell me what was wrong other than I did hear they changed out the chemicals despite claiming the chemical vendor had verified proper chemistry. Any ideas on possible causes/solutions and also how to rework about 5000 pieces?

Douglas Ogle
motor manufacturing - Bluffton, Indiana, USA


A. 1.The creation himself of the rust
a) The raw material already rusty the Digestion, possibly the effacement of the rust in the process of the abrasion
b) too low worth. To raise {To pick up} worth.
c) the too low concentration To increase the concentration
d) the high weight with chlorides (>250 mg/l Cl) the Analysis of the water, possibly to change the inflow of the water rinsing

2. The lack of the creation himself of the coat and
a) too low worth. To increase worth
b) too high worth. To raise {To pick up} worth.
c) the relief after the rolling on the surface of the element To consume or the mechanical tooling

3. The not uniform coat and
a) the relief after the rolling on the surface {area} of the element To consume {To corrode} or the mechanical tooling To bring the tradesman
b) blocked up breathe hard {snout} injection's To clean up the device
c) blocked up sprayer nozzles Snouts
d) inadequate removing the fat To check parameters, to add the amplifier of removing the fat or to prepare the new bath.
e) Soiling with grease secondary the exhausted Bath
f) the bath outside parameters of the work To correct

4. The deposit of the rust and
a) unsuitable worth. to correct
b) Under of the lay-off the tunnel did not become empty To change the system of the work
c) the distance between zones too long To change the product

5. The coat too pale and
a) The too little thickness of the coat To check parameters

6. The coated coat with the dust and
a) not proper worth. To check parameters
b) the too high concentration To thin the bath
c) the bath strongly drossy To prepare the new bath

Mariusz Kulczynski
- Wroclaw, Poland


A. Hi Douglas,

From what you have written it is not clear whether you are describing rusty parts or phosphated parts whose conversion coating color makes the part appear to be rusty.

If the parts are rusty, you should be able to rub the rust off onto a cloth. If it's a properly applied phosphate coating, it won't rub off.

Different types of metal surfaces may react differently in a phosphating bath, which explains differences in coating appearance. These parts are not bad or inferior, they just look different. You shouldn't have to rework these parts.

George Gorecki
- Naperville, Illinois

Gray/brown powder on iron phosphate coated parts

January 28, 2019

Q. Hi,

we have a 10-stage (spray) pretreatment system. The stages are as follows:
1-Hot water
2-Alkaline cleaner PH=11.5, con=1.7%, Temp=55 °c, P=1.2 bar
3-Alkaline cleaner PH=10, con=1%, Temp=55 °c, P=1.2 bar
6-Iron phosphate PH=4.5-5, conc=0.4%, Temp=50c, P=1.2 bar
7-Iron phosphate PH=4.5-5, conc=0.4%, Temp=50c, P=1.2 bar
9-RO Rinsing
10-RO Rinsing

The problem is that there is a slight grey/brown powder on the parts especially on the bottom (white towel test). but the parts are completely clean after stage 5 that means these powders are produced in the phosphate stages.

I have attempted to reduce the phosphate concentration and the temperature, but nothing has been changed.

How can I solve this problem?


Afshin Salmanpour
- Guilan/Iran

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