Powdercoat Questions left unanswered in 2011

A. The issue of "outgassing" (also known by several other names) is a mystery, but one with as many pet theories about it as theorists!
We have secured 2.5 years funding for a serious research program into this, and have just recruited the person who will conduct the research. We have the cooperation of a university and a major coatings manufacturer in the research, which will start in January.
(It has been delayed about 6 months due to governmental slow decision making in uncertain economic times).
I'll be encouraging the researcher, once he starts to use this forum to both share info and to seek observations, opinions, etc.

A. I agree that laboratories are not the best place to solve industrial problems, and that's why the research will be done in our plant, (but with analysis and inspection backup from University etc.)

The factory is where the problem is, and where the solution must be implemented, so in an operating industrial environment is where this research will be conducted.

A. Hi,

I have worked for 5 years in an industry where we have to Powder coat on Hot dip Galvanized substrate. It was always a struggle against this "Out-gassing" issue.

The best way we have found was preheating the substrate at least 25° more than the max curing temperature (as per powder supplier). Also we keep the substrate inside the oven for longer duration at this temperature (we use batch oven for preheating). The greater the thickness of the material, the time will be greater correspondingly. After this operation we carry out pretreatment and normal drying before powder coating.

With this procedure we found the outgassing issue reduced up to 75%. But there are some materials, in spite of this procedure, that continue to release gas even on recoated substrates.

Different steel substrates at different HDG baths may give different results. Kindly share the experience if working in typical field.

Regards

A. Our researcher has started this week, and while it will take a little while to get up to speed with our systems, process and procedures and other new job matters, I expect him to be on this case quickly. I don't anticipate a quick answer. There's funding for up to 2.5 years research.
We have identified 13 variables that seem to contribute to this issue, and guess that most need eliminating somehow, leaving the most influential variables needing controlled.

On so called "degassing" (preheating +25° C), for those using chromate or phosphate conversions, this is not a sensible solution, as it destroys the pretreatment. In any case it isn't the whole answer, as Bill has said.

A. Preheating or special powders do not represent a definitive solution to the problem, there are too many variables. I think the only way to prevent this kind of defects is reduce the phenomenon itself by using less reactive steel grades for hot dip galvanizing in order to minimize the dendritic grow of the zeta phase and promote the presence of the final pure zinc phase.

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"...by using less reactive steel grades for hot dip galvanizing in order to minimize the dendritic grow of the zeta phase and promote the presence of the final pure zinc phase..."

That's an interesting point, Anna. Do you have some experimental evidence that this is a gas evolving from the Fe/Zn alloy part of the galvanizing layers?
Is there a higher incidence of gassing in reactive steels?
We have not been able to correlate reactive and gassing yet.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

January 30, 2012

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February 1, 2012

Hi Geoff,
The statement I made in the previous post is based both on experience in powder coating hot dip galvanized items and on metallographic microscopy observations of cross sections of coated samples.
I work for a powder coater and we sometimes coat huge hot dip galvanised girders; the steel used for such items is often a class C steel (according to afnor NF-A-35-503) with Si concentrations near to the upper limit of this class, and due to the thickness of the girders and the Si content of the substrate we always have to deal with zinc coating up to 300 microns in thickness, very fragile and with little or no pure zinc phase on top of the coating.

Anna Telk
- Berlin

A. Geoff,

In our practical case we preheated the substrate and removed from the conveyor. Pre-treatment (phosphate or chromate) carried out after preheating. This eliminated heat effect on the treatment.

Our success rate was good even though there is a double handling of the product. We could not totally eradicate the problem but avoided rework to the maximum extent.

We have seen some metal structure created more gas issue in spite of above process. As everyone confirmed, this is a combination of many variables (steel, thickness, Zn thickness, etc.). Waiting for your further inputs.

A. Hi William.

As far as the U.S. government is concerned, MIL-DTL-5541 TCP trivalent chrome conversion coatings on aluminum are the full equivalent of hex chrome, even though you are correct of course that: they do not offer self-healing properties; they are not as robust & trouble-free; they are less reliable; they are not even visible such that an operator or QA inspector can catch defective parts which missed chromating.

Regards,

A. Re. outgassing in galvanized steel substrates under powder coating:

We've had a materials scientist working on this issue for nearly a year. Its part funded by a government research grant.
The answer is not yet defined by this research, but some interesting factors have been uncovered. The steel chemistry is one of the apparently important factors, perhaps surprisingly. Another is the surface treatment of the zinc.
As this shapes up, we'll advise more, but a large randomised multifactorial trial is underway.

A. Geoff,
I observe the recent release of ASTM D7803.
I point this out just in case your own research programme might have hindered you from observing other efforts.
I also am aware that is unlikely, but just in case.
Regards,
Bill

A. Bill,

You might remember that I mentioned in this forum doing some research on outgassing.
In the UK funded by a Government outfit called the Technology Strategy Board, there are research grants available to part fund research in industry.
We applied for and got one such, a 20 month assignment to research the outgassing question. We're about 3/4 way through that time, during which we have had a scientist, supported by a university based in our place. He had conducted a number of trials, investigations and analysis. As suspected there turns out to be several contributing factors, and several categories of gassing showing several mechanisms. Some of these are far better understood, some still quite mysterious. One aspect that immediately came up was that good practices that management think are happening don't always, and that shortcuts are sometimes taken without authorisation. So a benefit to our company has been to sort those out. Next is to classify the mechanisms, and look at them to gain better understanding.
The outcome of this, in about June / July this year will be a full report on this topic, with the findings.
Its premature for me to say any conclusions, but I can say that there's no magic fix for the problem, rather the outcome will be a list of practice aspects, and material attributes that give rise to increased frequency.
One perhaps surprising aspect is steel chemistry. This is known in galvanizing to have an effect on galvanizing, but often not known that this effect moves on to powder coating on top of that galvanizing. Certain silicon levels lead to "reactive steel", which produces quite porous galvanising, which leads to moisture entrapment at a microscopic level which then leads to outgassing.
But these are not the only sources of problem.

Thats a long round-about way to say "watch this space".
Geoff