White rust in zinc plating with blue passivation

Hi, Byju. Traditional old-fashioned blue chromates were rated only 24 hours, not 96 hours. If you are trying to replace this with home-brew trivalent coating without a topcoat, it's possible that it could fail in 7 hours if not operated perfectly.

We don't whether this process ran perfectly for years until yesterday at one extreme, or whether your shop has never successfully plated any parts before at the other extreme -- but inadequate chromating would be my first guess. Please tell us more about your chromating process. Good luck.

Ted is right, you cannot get 96 hours in salt spray test with a thin blue chromate. You can get it with a thick trivalent chromate with a sealer. Sara Michaeli Tel-Aviv-Yafo, Israel 2005 There are trivalent chromates available with or without top coat to meet your requirement of 96 hour sst life. These process are already proved in large scale production level. Contact chemical manufacturer. If you prefer to use hexavalent, use top coat to enhance corrosion. Packing: avoid polythene and use cb box. P. Gurumoorthi electroplating process chemicals - Chennai, Tamilnadu, India 2005

Byju/Ted

Yes. It is possible to get 96 hrs white rust SST. Use trivalent blue from reputed suppliers. They claim 150 hrs but 100 hrs is achievable. give a top coat- 150 hrs is minimum and can stretch to 200 hrs.

By the way Byju, what process have you adopted such as cyanide/Noncyanide/Acid chloride Zinc.

If you are more specific I can help you in a better way.

Byju, it so happened that you belong from my home town and that is very exciting. WHAT A SMALL WORLD !

Thanks, P; thanks, TK. Yes, the pressure to develop good RoHS-compliant coatings has indeed resulted in the development of trivalent chromates that offer more salt spray resistance than the 24-hour requirement of older specs. I appreciate the update.

But the question wasn't really whether it is possible to purchase a trivalent chromate that is rated for 96+ hours; it is why he is getting only 7 hours, and my first guess remains that, either due to low quality of the system or poor operation (including inadequate rinsing of the zinc plating), his chromating is the problem. His next issue indeed might be to find out from the supplier (if this is not home-brew) what this particular chromate is "supposed to" be capable of when well operated. Thanks!

Ted,
I guess Byju is using traditional Hex chromate with out topcoat. Improper conc. and dip time may have resulted in thin layer and very low SST result. I would recommend Trivalent blue chromate with topcoat with proper process control as follows:
1. Zinc plate
2. Drag-out
3. rinse
4. rinse
5. Nitric Acid dip(0.5-1.0%v/v if plating bath is Acid chloride. up to 1.5% if Alkaline or cyanide bath. Dip time 5 to 10 secs.(Hourly monitoring of the strength of HNO3 found useful)
6. rinse
7. Tri-blue chromate -pH(most important), conc. and dip time has to be strictly followed as per Data Sheet
8. rinse
9. rinse(preferably hot water at 50-60 degree)
10. Top coat (pH, Concentration, and dip time to be followed as per data sheet.
11. Hot air dry.
12.Do not handle or pack till 24 hrs. Subject to SST only after 24 hours curing.
All rinses have to be changed frequently. Continuous flow rinse preferable.

Good luck, Byju !

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Ed. note: thanks again, TK.

Hi, Shay

I doubt that the problem is in the chromate step. My guess is that the zinc plating is too thin because the hardware items are harder steel (higher carbon) than the steel plates. Cyanide zinc is known for poorly plating on high carbon steel due to an issue with hydrogen overvoltage. Please check the thickness of the zinc plating. If you decide to apply a sealer, yes, there should be rinsing after the passivate, and slightly warm would be good -- but 50 °C is probably too hot and will wash away some of the conversion coating.

Regards,

Dear TED, thanks for the time you took to answer to my question; I find your site excellent and resourceful.

My process: RoHS on cyanide Zn coated bolts.

Since cyanide Zn process does not plate well on high carbon containing steel,
1. Would you suggest alkaline Zn or FeZn plating? I have access to these processes!
2. You claimed that the Zn plating was too thin, what would be the max coat that will not affect the screwing functionality of the bolts and nuts?
3. With respect to the sealer could you recommend a good sealer?

I wish to thank you for the excellent site you maintain. I have spent hours reading the comments. This was of substantial help.

Kind regards
Shay

Hi again Shay.

1. Acid zinc is usually used for high carbon steel. (In the old days a flash of cadmium was done before the cyanide zinc, but that is not RoHS compatible). I suspect that alkaline zinc will suffer the same issue as cyanide zinc, but I don't know about the zinc-iron.

2. I doubt that even .0005" would be an interference problem, but I'd bet you don't even have .0002". Bolts and nuts have a lot of surface area so they require high amperage or extended plating time. Half an hour would be an average plating time, and up to an hour is sometimes used for .0005"

3. I believe in using proprietary products. Whatever supplier is providing the trivalent chromate should offer a compatible sealer.

Good luck. Regards,

A. Dear Mr Basavaraja,

adv.
In response to your RFQ for a plater at Bangalore, I offer the services of my plating shops. Please get in touch.

A. Hi Ajay. Please try to use the site's search engine because each of your questions is the topic for a long discussion thread here or a chapter in a plating text. Please find the best thread about each of your topics, as they have all been discussed at some length and it will probably be better to read the multiple opinions already expressed than to rely on a single quick and partial answer. Good luck.

Regards,

A. Hi Athul.

We appended your inquiry to a thread which may begin to answer it for you. There are dozens more threads on the topic here if you try the search engine. You may alternately wish to retain a plating consultant, as it does take either a very long time and/or highly detailed knowledge to sort through the many open questions that your inquiry implies: substrate condition, thickness of plating, adequacy of the chromate conversion coating, drying, storage conditions, expected life, environmental conditions in your shop, etc. Good luck.

Or pick what you guess is the biggest cause of the problem, tell us about it, and we can hash it around here. Thanks.

Regards,

? Hi Varun. May we infer that after the zinc plating you have a trivalent chromate conversion coating and a sealer and good rinsing? Have you been plating for a long time and this issue suddenly popped up? Thanks.

Regards,

A. Dear Varun,

Normally the problem is because of poor rinsing and holding design in the jig.

Improve the rinsing, and design the part holding method to avoid solution carry-over.

Thanks

A. Hi Pandu. In addition to this page we have four or five other threads on the subject of white spots on zinc plating which you might want to review by trying the search term "acid zinc white spots" in the google custom search engine. But please send some photos of the problem to for posting here so we can see what you are referring to and narrow down the possibilities. Thanks!

Regards,