Plating Adhesion Tests -- Zinc Plating on Steel

A. Yes. Search for "bend test" in this site. There are over 130 letters. You should ask for coupons plated together with your parts. They must be made of the same base material and consequent thickness. There should be no peeling after bending them at your selected angle (90 deg. or 180 deg.).

A. Jaap: I was involved in nickel plating a steel stamping that was later crimped onto another assembly. We sometimes encountered peeling problems because of process errors (chemistry, cleaning, etc.). We finally had a fixture made that manually reproduced the crimping operation. We tested a statistically selected sample to determine if that particular batch was O.K.

It is better, of course, to examine and change your process to eliminate these problems. If you are having problems with adhesion, it is probably due to poor cleaning prior to plating. An excessive amount of plating can also cause adhesion problems. I would try to pin down the cause of the problem and eliminate that.

A. Daryl is right - it is better to prevent the problem occurring than to solve it once it is there. In the majority of cases poor adhesion is caused by inadequate or wrong cleaning processes. Sort out the cleaning and the problem will go away. However, it may be a good idea to have an understanding of the bend test so you can do routine quality testing to maintain your (hopefully soon to come) newly-found higher quality of plating.

A. Poor cleaning - I doubt it! Zinc on steel is perhaps the easiest plating combination - quick clean, a little muriatic and adherent plating guaranteed. Okay, maybe it's a leaded steel or there's a little wax residue or something else out of the ordinary but my money wouldn't be on "poor cleaning".

I'd put my money on the type of zinc plating used - acid zinc works great on barrel work and looks like simulated bright nickel. At times it even has the brittleness of bright nickel and readily pops or chips when crimped. Oh, if we could only still all use cyanide zinc baths we might not even be having this conversation. Even newer alkaline, non-cyanide zinc baths are capable of undergoing crimping without lifting. As for what is the best test to use on the plated parts, I can't think of anything as severe as crimping so bend test, heat-quench, grid scribe, etc. may never create the extreme conditions seen by crimping -- my suggestion, stick to crimping! If you need something cheaper than your actual part, buy some thin wall steel tubing and cut them to length to simulate your actual part.

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A. There are numerous bend tests around, some of which are informal and some are standardised. Try
ASTM B490,
ASTM B489,
ASTM B571 or
ASTM D3359.

I also believe BS 2011 and BS 4511 may help you.

A. I have seen galvanizing expressed in a measure of hardness called a Diamond Pyramid Number (DPN). Galvanizing consists of different zinc iron alloy layers (Gamma, Delta, Zeta, Eta). The zinc iron alloy layers have different levels of hardness ranging from 250 (Gamma) to 70 (Eta-pure zinc). Your base steel can vary, but is around 159 DPN. Hope this helps.

A. Adhesion strength of galvanized coatings is not measured very often, (because it is not a problem), but one method is the pull off dolly system.

A small "dolly" is glued to the surface, and a hydraulic ram pulls it off. The pressure required to remove it is the adhesion strength. A proprietary brand device is available.

A. The measure of Zinc coating can be done in different scientific ways, though all of them are highly deficient in accuracy if the test methods and expertise are not in cohesion. It needs to standardize with respect to the coating type, process of coating and method used.

Zinc coating adhesion by Pull off Adhesion method
Zinc coating adhesion by Falling weight method
Zinc coating adhesion by Bend test

Later two are of the nature of estimate tests and needs to be compared with series of diff samples. But still remains in accurate.

Pull off adhesion test is ok, but I personally experienced several difficulties in real world testing. Is ok if properly established. Device should be selected of much higher capacity than what one needs to test. Higher coating adhesion test is full of inconsistent as the complete layer never comes off.

Regards,

A. Hi Arijit. We appended your question to a thread on the topic, and it will give you a number of ideas for checking adhesion. But firstly, what specs are you plating to? The plating specifications may reference some adhesion tests which you are expected to perform. Good luck.

Regards,

A. Hi Chundi. Your inquiry is a bit too brief for me. I do not understand what you are saying about the failure: you say have no adhesion problem even after bending 180 degrees, but you could peel the plating off with your nail. More words please.

As for the 3.1 and 3.2 procedure, what spec are you referring to?

But be aware that 40 microns is very thick for zinc plating. Most automotive plating seems to top out at 13 microns, and ASTM B633 considers 25 microns to be for "very severe" service.

Regards,