Testing Zinc Coating on Expanded Metal

[pog451]

Hi all,

I hope Ive hit the right forum here.

We build products which consist of a media encased in a sheet of zinc-coated expanded metal. Recently we had a customer NCR stating the metal had rusted and on inspection we found the coating was something like 4-6? rather than the required min 12?.

Our supplier has done salt-spray tests on the metal, but I am not satisfied with that as a running QC measure. They also let test plates run through the process so we can do a cut and polish thickness test, but claim that the thicknesses achieved on the test plates do not accurately reflect the thicknesses of the actual coatings.

Does anyone have a clever idea of a running (non destructive on the parts or destructive on a test plate) test we can do to assure coating quality?

I realise this is a process control issue and any testing is just a Band-Aid, but if anyone has a clever idea, id like to hear it.

Thanks!

 

[outdoorsNW]

I would look into X-ray fluorescence (XRF).

I am familiar with its use in electronics to check the thickness of various plating types over copper. XRF is a non-destructive test that can work with a small sample area. In electronics, XRF is used to measure plating from under 0.1 micron to over 25 microns thick.

A circuit board can be placed under the machine to check plating thickness (plating is used to ensure the surface will solder). There are no safety concerns with part of the board being outside the machine, so you should not have a problem with larger flat product.

I am aware of portable units that can be brought to the center of large objects, but have no first hand knowledge of how well they work.

I am assuming XRF can be used for zinc over steel. I have no experience with this combination of metals.

Tom

 

[Jim Wynne]

How large is this thing? When you say "zinc-coated" do you mean that the expanded metal is galvanized, or are pieces being electroplated?

 

[allan-M]

We apply zinc and a zinc-aluminum alloy on sheet steel on our production lines. A key quality measure is how much of the zinc we have on the steel substrate. This is simply flat sheet, so it is measured in oz/sq ft.
There's a widely used ASTM standard A-90 we use, and it is not limited to sheet steel.
The method involves weighing a sample, stripping off the coating with acid, then reweighing

 

[Dooglas]

We have an electronic coating thickness gage for measuring powder coat thickness on metal. It is made by a company called Defelsko. I quickly browsed their website and they have a walk through discussion of options available for measuring the thickness of galvanized coatings on steel. It is a non-destructible option, but I do not know if it can be used to evaluate the thickness of all zinc coatings and the expanded metal surface is not really large or flat and could pose an issue for the measurement devices. Gages can be expensive: costs are from $300-$800.

 

[pog451]

Thanks for the feedback guys.

The sheets are something like 1,5m x 1m and are being electroplated. Because they are expanded metal it isn't a banal exercise to test them directly, and the values achieved using test plates have been (apparently) unreliable (although I checking that).

Im definitely going to get the supplier to buy a coating thickness gauge but that ASTM A-90 look like it may be exactly what I am looking for for doing long-term spot-checks.

As a European company we don't immediately look to ASTM to solve our issues, but this could work.

Ive used XRF before myself, but as we are talking about a supplier in India, I think the investment would be overdoing it.

Thanks!

 

[allan-M]

>As a European company we don't immediately look to ASTM to solve our issues, but this could work.

I understand.
I feel sure there are european equivalents. I would go to steel company web sites where they have specifications for their galvanized sheet steel and see what standard they refer to for measuring the coating weight of their product. Perhaps EN ISO 1461?

 

[David-D]

I don't intend to sound ignorant but are you applying the coating before or after the expansion of the steel? I'd assume if it is prior than when you expand it you'd be exposing many unprotected surfaces. If it is after expansion, is the coating accumulating uniformly or are there certain areas (corners?) which do not recieve as good of coverage and thus are more suseptable to corrosion even if the average coating weight is ok?.

Either way, a couple of thoughts. Can you run a correlation study so that the coupons do have good relationship to the steel? Maybe you need 1.5 to 1 or 2 to 1 because how it accumulates? Can you build a test coupon into your expanded steel sheet that you can then cut off and test for appropriate coating? Perhaps best of all, can you have them identify the process controls necessary to ensure you're getting what you need (via a DOE?) but that might be easier in conjunction with the others.

David