Is there a maximum thickness limitation for Zinc coating per ASTM B633?

A. That is very reasonable unless there is a "fit" problem for the final assembly. To get at least 5, you have to exceed it by a rational factor or risk more QC inspections and more rejections. More is better until it is too much for your purpose.
You have the option of specifying the max amount in your Purchase Order.
If I do your work, I will charge a lot more for 5 to 6 than 5 to 8. Above 8, it is costing me money, so I do not want to exceed that if at all possible.

Hi Helly. James' answer is excellent, but there is a typo in your question. You are certainly not receiving plating of 0.08", as that is an unheard of zinc plating thickness. Even a hundred times thinner at 0.0008" would be quite heavy for zinc electroplating. You probably mean, as James assumed, 8 µm?

Regards,

A. Hi Manny. James Watts has already answered a very similar question; it does not sound like a problem. But lest I be misreading your question, you certainly cannot issue a certificate of compliance to a spec unless you have a copy in your possession because there is more to compliance than thickness. Good luck.

Regards,

A. Hi again, Manny. Zinc plating is often done for corrosion resistance, which tends to be proportional to plating thickness; so it is common to specify a minimum plating thickness. There is no "generally understood tolerance" on plating thickness. My pdf of ASTM B633 got corrupted and my books of standards are in storage, so I can't tell you what ASTM B633 says -- you'll have to read your copy and tell me instead -- but I'm confident there is no implication that the maximum thickness of one classification cannot exceed the minimum thickness for the next. Often, the thickness of zinc plating will be specified something like "x µm minimum thickness on all surfaces touchable by a 3/4"dia. ball, and all non-touchable visible surfaces covered" -- but it's all only what the customer specifies.

We frequently tell students who post on these pages that we're happy to help them with the answers but we simply can't help them with the questions. That is, if they don't understand what the teacher wants, clarification has to come from the teacher, not with a guess from us. I think we're in a similar position here :-)

Your customer can specify anything he chooses, irrespective of ASTM B633 or any other standard, such as "per ASTM B633 except minimum thickness of 50.8 µm", and your job is to comply or decline to quote. The designer seems to wants 50.8 µm of zinc plating somewhere, somehow, but neither you nor the buyer seem to quite know what he means by that. ASTM B633 can't clarify it, and reders can't clarify it :-(

50.8 µm is outrageously heavy for zinc electroplating and I personally think the designer should probably talk to a metal finishing consultant about whether mechanical plating, hot dip galvanizing, or dip-spin zinc rich coatings might be a more appropriate finish for his needs, rather than you and the buyer scratching your heads worrying about maximum allowable thickness when the specified minimum thickness is already probably impractical :-)

Best of luck.

Regards,

A. Hi cousin Amed. This is your second question on the same theme of post treating pre-galvanized EMT (you also posted on thread 55350), but we don't seem to be getting to what is actually going on ... because I don't understand why you keep talking about post-treating pre-galvanized material you have bought, when I have never heard of such a thing (I'm not saying it can't be done or has never ever been done, but I spent over 50 years in this industry and never saw it, so it is very far from routine).

Whatever chromate post-treatment is required is supposed to be done by the galvanizer. Sorry, 60 microns is probably not a practical thickness for electroplating and to my knowledge your can't mechanically plate sheets of steel. Can we make a guess that the material is getting white rust and the seller is trying to convince you that it was your job to prevent this via a post-treatment? I don't think it is.

If you are manufacturing the EMT conduit yourself in-house from sheet, you can use bare steel and then galvanize or electroplate it and chromate conversion coat it after fabrication, but you will not get zinc on the inside by electroplating, and as mentioned, I don't think you'll get 60 microns.

Regards,

A. Thanks for the kind words, Amed. I now understand what you want to do and why. Sorry, though, I don't have actual experience in the manufacturing of EMT from pre-galvanized sheet, so I would not be familiar with the usual thickness standards and pre-galv specs, nor the relevant ASTM, ISO, UL, CSI, etc. standards for the product line.

But the thing is, metal finishers (especially high production shops) look for robust processes, not processes which propose to improve the corrosion resistance by some vague amount on some variable percentage of their production. For example, a process which will work if the parts happen to be clean but which will fail if the parts happen to be dirty isn't of much value. A process which will work on the existing zinc coating if it's relatively fresh, and in the area far from the weld zone, but not near the weld zone and not if the zinc coating has aged for 2 months isn't very useful to you.

That's the problem with trying to install a simple chromate conversion line: Sure, it might help sometimes; some of your EMT and some of your diecast fittings may be good (but some will not be). The correct answer is quite a bit more complex and involves installing a zinc electroplating plus chromate conversion coating line which incorporates alkaline electrocleaning, acid activation, zinc electroplating, chromate conversion coating, and drying.

Before planning an installation, I suggest that you do some development work, either with a local electroplating shop or a plating process supplier, and examine the quality of work that results from trying to just chromate conversion coat & silane treat the EMT vs. the results from alkaline cleaning, acid activating, zinc electroplating, and chromate conversion coating & silane treating it. Input from readers with EMT experience is welcomed.

Regards,