Summary for white rust on aluminum Discussion
white rust on powder coated aluminum parts is discussed as a pre-coating contamination and corrosion issue, especially on aluminum die castings in rainy season. Rakesh Sharma first asked whether the white rust was present before or after coating; Gurvin Singh replied that if it is before coating, a proper pretreatment line and chromatising before powder coating should help. When Rakesh clarified that the white rust appears before pretreatment and is not removed by the PT sequence, Prabhakar Gupta explained that white rust on aluminum components often comes from contact with water, particularly hard water. He recommended removing it before chromating or painting, usually with a light acid dip, and suggested a 5% oxalic acid dip for 3-5 minutes followed by an alkali dip and chromatising, while warning that the acid must be chosen carefully for the alloy and any other metals present.
Q. Sir,
We face the problem of white spot on aluminum die casted (LM-24)in rainy season. How can we overcome this problem. We are in process of powder coating on aluminum alloy Lm-24.
RAKESH SHARMA
– DELHI, INDIA
2003
A. Rakesh:
You have to give some more information. Do you have this white rust before coating or after coating?
I assume you have this before coating and would like to remove it before coating to get a proper finish. If that is correct – are you doing pretreatment before coating?
You need to do chromatising before powder coating aluminium. A proper pre treatment line will take care of your woes.
Hope this helps.
Gurvin Singh
Mohali, Punjab, India
2003
Q. Dear Gurvin Singh,
We observe the white rust on parts before pretreatment which is not removed by the PT. We are adopting Vibro finishing – degreasing – rinse – chromate – rinse – dry.
Rakesh Sharma
– Gugoan, Harayana, India
2003
A. Dear Rakesh,
White rust on Al components occurs when components come in contact with water particularly hard water. Before chromating or any pretreatment and painting it is essential to remove it. Mostly light acid dips are recommended to remove white rust. The acid has to be selected carefully considering the composite structure, metal composition and other metals incorporated in component. A light (5% Solution) of Oxalic acid dip for 3-5 minutes followed by alkali dip and cromatising will give satisfactory results.
PRABHAKAR GUPTA
– Kanpur, U.P., India
2007
Q. WE ARE AN O.E.M TO A VERY REPUTED CEILING FAN MANUFACTURER FOR LAST 6 YEARS. JUST 15 DAYS AGO WE ARE GOING THROUGH A VERY DIFFICULT PROBLEM. WE USE ADC12 AS RAW MATERIAL. POWDER COATING IS DONE ON OUR COVERS; IT GETS SHRUNK IN CERTAIN AREAS IN A KIND OF PIN HOLE OR CLOUD. WHEN WE PEELED THE PAINT WE DISCOVERED WHITE SPOTS BENEATH IT ON SURFACE. PLEASE GUIDE WHAT THE PROBLEM IS AND ITS REMEDY. WE HAVE SAME PROBLEM ELSEWHERE TOO.
PRATEEK GUPTA
– HARIDWAR, UTTRAKHAND, INDIA
November 24, 2012
A. Hi Prateek. I doubt that any kind of outgassing or related problem could produce white rust; there doesn’t seem to be enough time or the right conditions for it. I’m quite confident that these parts exhibit the problem before they are powder coated.
Therefore, maybe if you offer a reward to the painters for how many defective parts they remove before coating, rather than for how many parts they coat, the problem can begin to be solved. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
Need quick confidential answers? $25
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January 30, 2013
Q. Dear Sir,
We are shock absorber manufacturing company and doing powder coating to aluminium casting two wheeler front forks. The powders used are metallic black and silver, and we observed white patches on cured parts and craters. The defect description is as below – black spot at center white patches are and surrounded by white patch. It is appearing as oil patches. Kindly provide suggestion to address problem. We have adjusted gun vol to 40 to 50 kv, cleaning of metallic tip after every 5 hangers etc.
Aniruddha Marathe
– Pune, Maharashtra, India
April 14, 2013
A. The photo is a little indistinct.
The faults look a little like fingerprint marks.
Maybe your operators are handling the pre-treated parts without clean gloves?
I would also check that the air supply to your application equipment is clean and dry.
I would also check that your powder is properly agitated.
Metallics really must have fluidised bed hoppers rather than vibratory ones.
I hope this helps .
Bill
William Doherty
Trainer – Salamander Bay, Australia
adv.
