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Black passivation/chromate on zinc plating gets white rust

As per headline we have faced so many problem in Zinc with Black Chromating Layer, Major Problem is White rust appear in material in ware-house specially in rainy season, we have maintained 8 micron thickness of zinc in acid medium and this problem occurring mainly in Fasteners. We have taken all possible c'measure which would be somewhat responsible for white rust like:- we have control the iron ppm less than 50, fix up the dosage of Additive acc.to chemical supplier guidance in ml/1000 amp-hr, More frequent cleaning, maintained the conc. and pH, time according to chemical supplier technical data sheet, drying with maintaining the level of R.P.M < 400 along with 60 °C and digital time controller 3~4 minutes in Centrifugal-drier, then maintaining the curing time about 24 hrs. in open area,and dispatch in Closed vehicle and material pass the salt spray test at our hand & some out source lab. but always coming the problem at customer of white rust appear in packed poly-bag and failed in salt spray test. We have maintaining the spec. of white rust 72 hr. and red-rust of 240 hrs. Please suggest some technique by which we can avoid the discussed problem.

Anil Kumar Kaushik
Plating Shop Employee - Gurgaon, Haryana, India
2007

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A. As we know that the bright dip before black passivation solution contains nitric acid,which contaminates the passivation solution further if intermediate D.M.Water dips are not well maintained. Additionally after passivation, the components are to be washed thoroughly in D.M.Water before being dried in centrifugal drier at the temp. between 45 to 50 °C. The acidic character of passivation itself acts as culprit in making the passivated component susceptible for suicidal corrosion. Make sure the washing is proper/adequate.For extended life, you may use rust preventive oil application after drying & before packing. Arvinda Kumar - ghaziabad,UP, India 2007 2007 A. Anil, The corrective measures you have done, I would say will take care of your plating.There are more corrective measures required for passivation as this is mainly responsible for the result of your White Rust problem. I am imagining that you are doing Hex black as your WR requirement is 72 hrs; and at the same time your mention of reducing the RPM of centrifuge to <400 and maintaining 60 degree C makes me feel that you are doing trivalent black passivation. In either case,Please do the corrective measures as follows 1. Use DM water only for passivation bath make up. 2. Do perfect clean water(DM preferable) rinsing till the end 2. Work within the specified pH- it is most important. 3. Call your process supplier and discuss, keeping the Tech. datas of the product in front of you. Please give more details such as your process sequence. Regards T.K. Mohan plating process supplier - Mumbai, India

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Q. My question is this:
We have farmed out plating of parts.
Parts are metal and plating is black zinc. However, during pick up of delivery to plating process, parts were wetted by rain inside the delivery truck and were not wiped immediately to dry off.
2 days passed and the parts occurs white rust.

My question is what is the possible cause of this?

Almira Macatangay
Tri r - Philippines, Cavite
June 11, 2018

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Black passivation has chalking, pitting, atmospheric corrosion?

November 22, 2019

Q. I'm located in Finland and producing components for automotive industry. We have steel parts that have nice looking black passivation, but after some time (a few weeks to a few months) they are showing white corrosion or chalking (we call it 'white rust'). The 'white rust' is normally forming on areas that are contact with outside atmosphere. Surface areas that are inside the component are still looking fine. What do you think could be the remedy for this problem?

Thanks

Saku Urponen
Quality Engineer - Rauma, Finland

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Ed. note: We had enough questions about white rust on zinc plated components to make it a separate subject. So if you wish to read more, please see thread 26622, "White rust on zinc plated parts"

adv.