Painting Valves

[metallion]

Hey Guys!

I'm new to this forum and I've got a perplexing dilemma regarding paint adhesion to our valves. First off-our valves are Standard Low temp Carbon Steel Bodies and our valve types are:Ball valves and high pressure globe style valves.

We have been having a problem with our two-part epoxy paint "peeling" on the flanges (which are machined).We have introduced a parts washer to our paint process with a phosphate base to see if this will help with adhesion.

Any help would be greatly appreciated in resolving this issue!

 

[FinishingTalkdotcom]

What type of phosphate? Iron phosphate would be more economical, but unless the parts are painted immediately after phosphating, they could begin to flash rust. This would not promote but would decrease adhesion. If you need more storage time between phosphating and painting, may want to investigate zinc phosphating. This would also increase salt spray corrosion resistance, resulting in a better quality product. With either phosphate, make sure you analyze the chemical concentrations frequently (once per shift). If the iron or zinc phosphate concentration decreases below recommended parameters, you are left with the steel being processed through an acidic environment, unprotected, which will promote rusting.

 

[metallion]

Thanks for your input FT.com! I'll have to look into the chemicals on Monday.

Can you recommend a better paint for our valves? We apply our paint with a hvlp gravity feed Sata-Jet and I'm just not sure that our 2 part epoxy is the way to go!?

 

[kenvlach]

metallion,
The product bulletin for the paint should give specific cleaning & pretreatment instructions. Two-part epoxy is very broad category; some will need a compatible primer, some will need phosphating + primer, and some are suitable for bare metal (self-priming).

The initial peeling was perhaps due to inadequate cleaning of cutting lubricant from the machined flanges, but it would take a little bit of testing to confirm. Do the cleaned surfaces produce a water break-free surface when rinsed in high purity water? A smooth sheet of water, without beading or breaks, is a good indicator of freedom from oils.

"We have introduced a parts washer...with a phosphate base..."
Sounds like an alkaline phosphate cleaner, trisodium phosphate or similar. Best used warm (~70 [sup]o[/sup]C) or per instructions & rinsed in very clean (low TDS) hot water to avoid residues & aid in drying prior to painting. The alkaline processes are much better than the acidic ones in not causing the flash rusting warned about by FinishingTalkdotcom.

 

[metallion]

kenvlach,

Thank you for your post. Yes,it's an alkaline phosphate cleaner! This washer has recently been introduced in our process,so we're hoping that this is indeed the answer to our peeling issue.

I was of the impression that the peeling was due to the fact that we are coating over a machined surface. We decided that shot blasting may be an option-something for the primer to bind to...any thoughts?

 

[kenvlach]

Shot blasting is usually done to remove rust; the help with mechanical adhesion of coatings is secondary. But for a flange, maybe the need for a smooth surface precludes shot blasting. Maybe some code applies for the surface finish?
Anyway, if blasting, necessary to preclean to avoid embedding contamination.

But, you still need the proper chemistry -- a primer or self-primer for bare metal. It's true that phosphating is often used as a pretreatment for mass production (good adhesion for primer), but it is a messy process (produces sludge in bottom of phophating tank) & requires a lot of energy for heating. Also, is phosphating permitted inside the valves?

If your paint's bulletin says OK for bare metal, the improved cleaning may suffice.

 

[metallion]

Once again,thank-you kenvlach!

I will def. put your suggestions to use. Yes,we are using a primer (epoxy)and,no-phosphate is not permitted in the valves- so we use magnet covers on the flanges during the shot blasting process.

Cheers!

 

[KAMALBENKHDOURA]

Mr. Metallion

The best coating for valve body is fusion bobded epoxy coating and for the ball is better to protected with rubber lining.

Regards
Kamal