Powder coating problems on CR/HR steel

Try acid water dip etch /drain (pickling)before Cadplating

Sounds like contamination that, combined with the humidity, becomes a problem.

You need to make sure that the coating happens right after the cleaning, degreasing and or shotblasting process. Also mechanically clean the part after shotblasting to remove the shot dust. If you stop at the end of the day and there is a batch of cleaned uncoated stock waiting till the morning, all sorts of things can happen to it during the cold night. It does not take long to oxidise even a small part of it.

Also the shotblast cabinet needs to be dry, if not it is possible that you have coroded particles mixed in with the silicum or alu shot, which would embed into the part. This would not negatively affect the part till this corosion has had some time to do its evil stuff. That is perhaps why they fail after some time and dot imediately.

Hope this gives you some new ideas.

Case491

need afew more facts.

Sand blasting, not shotblasting with abrasive steel shot?

Is the peeling happening more on HR as compared to CR? I can see adhesion issues on HR due to adhered oxide scale or other contamination under the plating.

You're really doing Cadmium plating? Yikes?

I'm not familiar with the hot water sealing process, so more details would be helpful.

milo

Please try phosphating ( Seven tank process) prior to powder coating it might be help to solve your problem.

Sounds like an electrochemical current could be the source of your problem, especially if the surface is not coated immediately prior to coating. Any humidity in the air (especially if there are any hygroscopic salts left on the surface) may cause the deliquescence of the salts resulting in formation of a corrosion cell, that might cause loosening of the coating over time.

Brilliant answer even though I had to look up that long word

one encyclopedia says:

the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution. Deliquescence occurs when the vapour pressure of the solution that is formed is less than the partial pressure of water vapour in the air. All soluble salts will deliquesce if the air is sufficiently humid. A substance that absorbs…

I could not have said it better

I forgot to mention that most silica shot media is hygroscopic and will act exactly as described above. Always clean, clean and clean before coating. If left make sure that no air and moisture can get to it.

Hello there,

First thing first, before doing any kind of recommendation we have to pinpoint the problem. What kind of powder paint are you using? (epoxy, hybrid, polyester TGIC or urethane etc...). Low bake or normal bake? Do you always use the same supplier or you switch sometimes? What's the thickness of your coating system? Application is done: by human or automatic gun (tribo disc)? Batch oven or assembly line?

2nd take a look a your cleaning system: is it a 3, 4, 5 stage? are your baths well balance? Your baths are well maintain and clean at the right temperature as per manufacturer's recommendations right? Use the W.A.T.C.H. system:

W= water

A=Action

T=time

C=chemical

H=heat

There exists a tightly fettered interrelationship between cleaning, phosphatizing (plating) and the ultimate performance of the powder coating. WITHOUT a complete cleaning, conversion coatings will not perform, without proper conversion coating the paint will loose performance.

You mention rainy season and humid conditions:High moisture content will certainly result in poor electrostatic efficiency and in extreme condition will affect the appeareance and performance of the baked coating film. Have you seen a change in the appearance of your parts?

Do you have problem with only one color or one particular type of material or part? do you reclaim powder?

Have you notice if you got poor impact resistance / flexibility?

Contamination: by dust or other airborne particules, do you have unusual or specific activity at this time of the year (rain season) that can affect your paint? Compressed air supply: using a clean white cloth check for oil, water or other contaminant, drain filter/separator, check filter and regulator setting. Air dryer should remains on at all times to prevent moisture. Check feed hopper, pumps feed hoses collector color module, rotary sieve etc...

Storage: do you have control temperature (80oF (27oC) or less) room, avoid open package on shop floors to preclude possible moisture absorption and high risk of contanimation. Precondition powder prior to spray application: these techniques will break up the powder if minor agglomeration has occured in the package.

Minimize the amount of powder coating material held on the shop floor if temperature and humidity of application are not controlled.

If it pass the test initially (assuming you are doing the test correctly, BTW which standard test are you using?) you may face poor penetration: check for poor ground, voltage too high, powder velocity too high, spray pattern too wide... poor electrostatic charging of the powder. Do you face massive delamination or only some spots?

Poor adhesion: Poor cleaning or pretreatment (check chemical, cleaning solution), change in substrate, under cured (increase oven temp. dwell time in the oven). Change in the powder resin or type (check with supplier).

If these basic points cannot give better results (or any answers) we have to dig further in your cleaning/painting process...in the next post...let us know the result.