85/15 Zn/Al pressure vessel coating

[spencew]

Dear All,
New to this very informative forum, and wishing for some advice.
We have several hundred pressure vessels sucessfully used for transportation of various refrigerants over many years. The carbon steel is coated with 85/15 Zn/Al, using a hot flame technique.
Our vessel supplier changed to a plasma application process, and after 1 yr of service we have found 4 containers with an internal bubbling of the coating (please see the photo's).
We have never seen this problem before.
We will thoroughly investigate this problem, but was interested to know is this is more likely to be due to an application problem (the change to plasma), or a corrosive contaminant in the refridgerant we have not experienced before?
Thanks for you help.

[MichiganMan]

Blistering like that is more likely to be indicative of an application process error than anything. It isn't impossible for it to be somehing other than application error, but it is far more likely to be either misapplication or inadequate surface preparation prior to application.

[Gordon]

Hi spencew

to the Surface Engineering Forum.

I go along somewhat with MichiganMan's comments.

Do you have any information on the plasma spray process used? Just seems to be an unusual process to use for this coating type. Can't really see any advantage in using plasma, particularly if the old flame spray coatings work without problems. Arc spray is the usual alternative, which in some cases can be beneficial.

It would be interesting to know what you find lurking under these bubbles and may give clues to what has caused the problem.

[loriolo]

Hi Spencew

I think that they changed the coating material as well as they changed to plasma process, because these internal bubbling of the coating seems a corrosion that was bound to happen because they applied a cathodic metal coating instead of ZN/Al that is anodic respect to the steel been coated, I had an experience like this, when a customer of mine applied Monel coating inside a seawater tank, I saw the same bubbling after 3 months, I agre with gordon opinion, that the best choise for Zn, Al and theirs alloys is to use Arc spray technologie...

[spencew]

Thank you all for your comments.
The 'bubbles' will be opened next Wednesday to see what can be found. The tanks are actually in France at this time. A sample of the coating material will be analysed to confirm 85/15 Zn/Al. To be honest, I believe that this will be confirmed correct.
On this basis, I also assume this would also point to an application problem with the Zn/Al supposed to be sacrificial?
I don't have the specific application details at this time. As I am far from an expert in this field, would there be significant surface prep differences required for Plasma compared to Flame spray?
What would an expert hope to conclude from an investigation, if anything?
Thanks again.
Warwick

[MichiganMan]

Assuming that analysis confirms that the material is 85/15 Zn/Al an expert should be able to give a root cause failure analysis and determine why the blisters are forming.

Also, I agree with Gordon that Plasma would be atypical for application of the anodic corrosion protection, and frankly I've never seen it done, nor do I expect to. In my experience there would be a decided cost advantage to using arc spray over plasma or combustion, and I cant imagine that Plasma would have the same quality of coating for this alloy in this service for a number of reasons that probably aren't necessary to get into detail about.