White Patches appearing on Chromate Passivated Zinc
We are bright zinc plating from medium cyanide bath mild steel parts which are cylindrical in shape with through bore and having hexagonal faces. While drying the plated parts with chromate yellow passivation (iridescent) in a centrifugal dryer at 60° C the dried parts have white patches on the hexagonal surface. Can the white patches be avoided? Can anyone with similar experience help? Thanks.
Bangalore, Karnataka, India
First of three simultaneous responses-- (2000)
A. Dr Mr Ramajayam,
The fact that only the hexagonal surfaces seem to have the white patches seems to suggest that this is the classic problem of plating components with only partially machined surfaces.
The hexagonal surfaces may be the un-machined portion having surface defects which bleed after plating which causes the white patches.
Try stress relieving at 400 °C after degreasing and pickling and before plating. Consider using Acid Zinc plating instead of Cyanide Zinc.
Best of luck,
Bangalore, Karnataka, India
Second of three simultaneous responses-- (2000)
A. Dear Subramanian,
60 °C is too hot. Recommend 42-44 °C. The yellow (any chromate) will degrade at that temperature. The hexagon ends are where the water will tend to accumulate. Be sure the final rinses following chromating are clean. The ends are probably very smooth metal as well. If the parts are going through a hot water rinse prior to spin dry, if this hot water is contaminated, it is adding to the problem. If the basket is full, reduce the quantity of parts.
Hope this helps.Bill Hemp
tech svc. w/ chemical supplier - Grand Rapids, Michigan
Third of three simultaneous responses-- (2000)
A. I think that your first step should be determining if the white spots are from salt build up in the rinses or corrosion of the zinc layer.
Since it appears as though this problem occurs during the drying step, I would think that you should take a look at the final rinses first. I've seen high solids contents in the final rinses cause this type of spotting. If the problem is corrosion of the zinc layer, look at the chromate pre-dip (if you use one) to make sure you are neutralizing any residual plating solution. Check the pH of the rinses also to make sure that your chromating solution is also being neutralized.
One final suggestion, check the porosity of your substrate. If your substrate is porous you might be trapping solution that bleeds back over time and causes the white spots. Your rinses should be taking care of the problem.Robert Holderman
- Tulsa, Oklahoma
The above problem was posed by me. I thank all the participants for the suggestions. But one fact which I have not given is that if the plated parts are not dried in a centrifugal drier but just air dried with a hot/cold air blower, the white patches are minimal. So, could it be due to abrasion effects between the parts under centrifugal force in a centrifugal drier? But the parts appear very dull and somewhat of faded color. Thanks.
Can the white patches that normally appear on yellow passivation be avoided by resorting to olive-drab or black passivation after zinc plating?
Bangalore, Karnataka, India
A. You described white patches and the fading of the plating color. That looks like corrosion to me. My guess is that for some reason, the corrosion protection of the chromate is being used up. I am just finishing up salt spray testing on some parts that behaved the same way, but as a result of 300 hours of salt spray.
I am not even going to try and imagine why it is happening or how to fix it.
Rochester Hills, Michigan
December 1, 2012
A. Dear Subramanian
Please [determine] the chrome content in the passivation bath with help of AAS (Apart from the regular concentration analysis). If the chrome content is low, there will be a chance of white patches.
- bangalore, India
September 27, 2012
Q. We are having problems with passivation after Zinc plating. After we plate the item we rinse it in cold running water and then rinse with deionized water. Finally neutralize it with dilute nitric acid and the golden passivation is done with Chromic acid. Items often exhibit whitish area near the edges like a finger print. Also we noted that the rinsing water we use is turbid and has a slightly whitish appearance. Is that the cause of the problem or is it something else?Yaseer Zoha
Electroplating Shop - Karachi, Pakistan
A. Water with distilled water, a paper tape pH meter (indicator Alkaline) and tighten the hexagonal hole. If the measured pH is alkaline, leaching is occurring yellow chromate. If this occurs before chromating leave the part in hot water (90 °C) for 5 minutes and then chromate. Good luckRogerio Linares
- Limeira SP Brasil
A. Dear Sir,
Yellow iridescent Chromate Passivation shows this problem quite often. Zinc deposit from Cyanide zinc bath needs to to thoroughly rinsed and activated /neutralized using well maintained Nitric acid dip, otherwise alkaline residue on surface will adversely affect the adhesion of Passivation film.
Secondly concentrated passivation solution forms thicker passivation film which has poor adhesion to zinc. Using dilute Passivation solution or reducing the dip time may help to some degree. Wet passivation film is fragile in nature and can get damaged during centrifugal drying.
Good luck, Thanks.
- Mumbai, India
This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site