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White and black rust on zinc plating

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⇦ (tip: readers rarely show much interest in abstract questions, but people's actual situations usually prompt responses)   smiley face

Q. What is reason and significance of Black corrosion spots in Zinc coated Steel part?

What is their difference and which one should appear first, if both White and Black corrosion is there?

Eswaran Murugesan
Student - Bengaluru, India
September 13, 2022

⇩ Closely related postings, oldest first ⇩

Q. I would like to know whether, during Salt Spray Test, black spots on zinc plated parts (on steel or Zamak) are considered as white rust or not? If there are only two options to choose -- White Rust or Red Rust -- then which are black spots considered? What is the standard saying about this?

Chetan Anand
vehicles - NOIDA, U.P., INDIA

? When did you observe this black spot after galvanizing or during salt spray test?


Syed Kazmi
- Dubai

A. Corrosion products of zinc are always white in colour. Black and gray spots sometimes develop during the testing are not to be taken as a failure. They are caused by some impurities like iron in deposit or passivation film. But they indicate weaker spots on coated surface.


D.T. Thakur
Metal finisher. - Mumbai, India

"Zinc Electroplating"
by Muralidhara & Venkatesh

on AbeBooks

or Amazon

(affil links)

thumbs up sign Hi Chetan. We thank Thakur for his answer to your question, which I believe is correct. But something to remember is that the salt spray test specifications like ASTM B117 [affil link] don't say what constitutes failure, the plating specifications like ASTM B633 [affil link] do. So when you ask "What is the standard saying about this?", we must reply "What standard are you plating to?" :-)

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

Q. I am Ana, an engineer for a metal fasteners manufacturing company. I used to be with a PCB manufacturing industry which uses different metal finishing so this is something new to me. I got a question on screws electroplated with zinc then coated with yellow chromate. We observed white and black spots on these products, it came out good after plating but after 2-3 weeks when it is pulled out from stock these defects occur. We follow simple process for this product type, after electroplating zinc on the items, the surface will be passivated, then chromated. After chromate, this will be rinsed with water then dried in our spin dryer. Drying time is 4 minutes.
Does zinc-yellow chromate finish require further treatment after chromating? Or can we still remove those white or black spots white having to strip and re-plate?

I would appreciate any help you could render to me.

Ana Ferrer
plating shop employee - Philippines
June 3, 2008

A. Ana,

I think you need to check on your pre treatment process.

The degreaser. What is the concentration?
De-ruster- the HCl concentration and have you change it lately
The electrocleaner.

do you have activation prior to zinc plating?

Usually the white rust with black spots are common pre treatment problem.

hope it helps.

Barry M. Umacob
- Philippines

A. Black spot and white spots may be the starting of corrosion. Black spots may be yellow passivation break down and white spots may be zinc corrosion. This may be due to improper drying, improper rinsing. Because in hexavalent this should not happen.

Suresh Reddy
- Bangalore, India

A. Please check the operating conditions of your plating bath and do proper rinses before and after passivation.

Shoban Kesarapu
Shoban Kesarapu
plating chemicals supplier - Secunderabad, A.P., India

Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)

Q. We are doing zinc plating with blue passivation for 2 wheeler components. We observed development of white rust (black dots also) on the surface after 48 hrs of salt spray test. We checked plating thickness and it is measured above 15 microns.

Please suggest any process improvement to achieve the above requirement and possible causes of occurring of this defect.

shrikant bhagat
Shrikant Bhagat
- Pune, Maharashtra, India
May 24, 2009

"Zinc Plating"
by Herb Geduld

on AbeBooks

or Amazon

(affil links)

A. Mr Bhagat

There is nothing called BLACK rust on zinc after Passivation in SST.

Black SPOTS occur and are the result of either loose zinc floating particles deposited and then broken off, OR small pits holding trapped plating solution. Both show up as BLACK spots and are a definite plating defect which at times causes Premature failure in SST.

At times Black spots are only caused by small co deposits of non metallic particles of say sand or silica or even dust that are in the plating solution due to a dust storm or any dust from a ledge falling into a tank in tiny non-observed amounts and are plated on the surface and do not passivate but do get attacked in SST.

Proper filtration and dust prevention and bagged anodes are your real answer in your Plating stage..Good Housekeeping. Good rinsing Practice. Good Filtration of TRIVALENT Chromate baths. Complete and good pretreatment.
These are basic plating essentials your Plating Chemistry supplier can help with.

White rust indicates failure of the Chromate film.

Red rust indicates failure of the Plated deposit when the corrosion hits the Mild steel base metal.<

Asif Nurie [deceased] [deceased]
- New Delhi, India
With deep sadness we acknowledge the passing of Asif on Jan 24, 2016

Q. If black rust is observed in salt spray, is this called failure if there is no observation of white or red rust?

Process is Zinc Plating. How to decide at this condition?

June 8, 2015

"Zinc Electroplating"
by Muralidhara & Venkatesh

on AbeBooks

or Amazon

(affil links)

A. Hi Sarfaraz. Asif has explained that there is nothing called black rust in this situation, but suggested that black "spots" can be a plating defect. You should try to determine what the spots actually are, and what is causing the spots, and he suggested a couple of possible causes.

Leaving semantics aside, black "spots" generally are not considered a salt spray failure; however, we must know what specification you are plating to because many specifications define failure. You will see some opinions about this subject on thread 35573, but again we must ask what spec are you plating to and what are these black "spots"? Can you examine them with a loupe and/or a microscope? Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

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