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January 7 - January 13, 2013

I am experiencing problems with a paint system on some vessels we built. The system is an ethyl silicate zinc primer, glass flake epoxy tie-coat and aliphatic poly top coat. The project is constructed from A516 boiler plate 1/4 in thick. Blasted to SP10 with 2.5 mil profile. Painting is being done inside a paint booth where ambient conditions clearly meet the specification. The coatings are exhibiting symptoms of solvent popping in a specific area on the exterior of the vessel. The problem only manifests itself over the back side of a weld that is on the inside of the vessel. We have made numerous adjustments to the system as well as the process to try to solve the issue. Nothing is helping. The manufactures reps are scratching their heads, and I am pulling my hair out. Any input would be welcome.

Selected Answers

From Brian Chapman of Cadillac Fabrication on January 10, 2013:
     Donald, the inside of the vessel in uncoated. Yes, the problem is occurring in the HAZ. Directly over the backside of the weld, but on the weld itself. The hardness of this particular area was not checked; however, all of the seal welds at the corners of the vessel were checked for hardness and were certified to be within the specification of the engineering firm on the project. The blast profile was not checked directly in the HAZ, but the vessel is approximately 1800 sq ft total surface area and 20 checks were performed and they all fell between 2.2 and 2.7 mils. MTR was provided to the engineering firm and accepted as being in conformance. The interesting (perplexing) part of this is the welds in question are there to secure a 4 in. C channel to the interior of the vessel to hold grating which supports the media bed. There are stitch welds on the top and bottom of the C channel, and the problem is only occurring in the welds at the top of the channel, NOT the one just 4 inches below,

From Brian Chapman of Cadillac Fabrication on January 10, 2013:
Richard, The problem is not on the weld. The weld is on the INSIDE of the vessel skin. The problem with the coatings is occuring in the HAZ on the OUTSIDE of the skin of the vessel.

From Brian Chapman of Cadillac Fabrication on January 10, 2013:
Bill, the welds are along the edge of the web on the channel.

From Dave Jurgensen of Jurgensen Enterprises International, Inc. on January 19, 2013:
     A different coating may be needed. What kind of a vessel is it? Contact me for a solution.

From Bill Patterson of Retired on January 10, 2013:
     Do the two stitch welds follow the two edges of the web of the C channel, or the two edges of a flange? If the latter, presumably more heat is required welding along the edge where the flange and web meet the plate than along the edge where only the flange is welded to the plate. The greater heat could change the properties of the steel, but I'm neither a welder nor a painter so can't carry that forward to a conclusion. Presumably, the blasting is carried out after the welding is complete, so we aren't looking at oxidation of the heated surface.

From PANKAJ MANUBHAI VYAS of A Self employed Technical Consultant to Coatings Industries. on February 5, 2013:
     I suggest simple / logical solutions to overcome this defect.  It appears that weld seams are highly porous. Can you apply a penetrating sealing lacquer coat of either Epoxy or PU , allow to penetrate and dry for sufficiently longer time? Then follow the other usual system .  Use some other alternate tie coat free of glass flakes if possible . Give longer time for the tie coat to dry. Use forced dry measures if possible by raising the temperature.  Look at the solvent combination in the coating and strike a balance between slow evaporating and faster evaporating solvent combination. Consider the prevailing weather conditions and ambient temperatures before selecting the solvent combination.  Give adequate time before top coat is applied to ensure solvents have evaporated from earlier coats.

From Brian Chapman of Cadillac Fabrication on January 21, 2013:
     Donald, you are correct. The channel is inside, the problem is on the outside, only on the weld at the top of the channel.

From Donald L Crusan of Marcellus Independent Technical Solutions on January 11, 2013:
     Brian, Trevor's comments make sense, and before I read it, I thought of buildup of some sort on the upper weld that might not be there on the lower weld because of basic physics. But if I read your comment correctly, the C channel is on the inside and the problem is on the outside with no weldments attached. Gang, from a metallurgical viewpoint, could it be possible for the C Channel to be a heat sink in one area only?

From Brian Chapman of Cadillac Fabrication on January 10, 2013:
     There is no doubt in my mind that the problem described is either air or solvent entrapment in the zinc primer. The baffling part of the equation is why it is such a localized area and not in every HAZ of all the similar welds.

From trevor neale of TF Warren Group on January 11, 2013:
Assuming that the surface preparation produced the specified profile and assuming the IOZ is a completely cured, solvent-based type, solvent entrapment appears to be the fundamental cause. It is unusual to characterize a glass-,filled coating as a tie coat, the purpose of which is seal the porous surface of the IOZ.Typical tie coats are lightly pigmnented to allow penetration into the intersticial spaces within the zinc, thus avoiding pinholes,bubbles or blisters .Over welds it is recommended practice to apply the tiecoat by brush to ensure thorough wetting of the surface.Finally, assuming solvent entrapment is the cause, preheating the areas after the zinc has cured and before sealing should ensure that any residual solvents are forced out.Good luck with solving your problem .

From richard d souza of stoncor middle east llc on January 10, 2013:
     The question itself is completely baffling. The problem appears to be related to the porosity of the weld seam, and I have seen porosity of weld seam is extremeley damaging to the performance of coating film. Coatings tend to bridge over pores; then, during the curing process, the coating can shrink and pop open, leaving a void in the coating film. Solvents contained in the coating may enter pores and force air out of the pocket, which will cause blisters or craters. Whatever is the intended service, porosity must be removed by grinding or by applying additional weld metal, or fill them up with thixotropic putty material compatible with the system after the primer coat of IOZ primer. The questions asked by the previous respondent should also be considered carefully to correct the design of construction.

From Donald L Crusan of Marcellus Independent Technical Solutions on January 9, 2013:
     Let's ask a couple of questions. First, are you coating the inside of the vessel? Second, do you mean the HAZ of the weld. Third, has anyone checked the hardness of the weld? Fourth, what is the blast profile at the HAZ? Fifth, has anyone thoroughly checked the steel MTR and did a CE? I have been finding hard spots in steel occurring more frequently the last few years.

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