Powdercoating Failures - Blistering

[czeilenga]

Hello,
We have cold-rolled ASTM A1011 SS GR33 powdercoated by an outside vendor. Recent review of our inventory shows that the powdercoating is blistering off of the metal and after scraping the powdercoating off there is red rust underneath. Some of these blisters look like round droplets of water.

The initial reaction is that the metal was rusty before the powdercoating process. Is it possible that this failure could be due to poor pretreatment of the metal during the coating process?

I'm not familiar enough with the coating process to know, and want to educate myself before I contact the vendor directly.

 

[TVP]

Yes it is possible. Even if the rusting occurred after the blistering, the problem is likely due to poor initial cleaning and/or pretreatment.

 

[czeilenga]

Is there a way to determine at what point in the process the rusting occurred? I am trying to narrow down if this issue occurred while the product was sitting in our inventory prior to the powdercoating process.

 

[boothuk]

Czeilenga,

As per most failures that are related to powder coatings, the pre-treatment of the surface is generally the issue and should always be the first port of call.

You indicate that the substrate was rusty prior to application. Is this the case?

Have you used any chemical pre-treatment to etch / anodise the surface prior to applying the topcoat powder?

Also, what is the porosity of the powder coating like? What DFT have you sprayed to? Is there any microcracking on the surface due to stress?

There are many variables that will reduce the resilience of powder coatings but first & foremost, your material pre-treatment must be in line with the manufacturers recommendations.

 

[tnatale]

Hot rolled steel is typically more prone to this kind of problem than CRS. (You often get a scale in the HRS).

It is difficult to idnetify the probelm based on the limited amount of info that you gave, but a couple of thoughts:

- Examine the raw stock for signs of corrosion, this means looking closely at the surface, the scale can sometimes have somewhat good adhesion to the steel, but become lose after processing

- You may need to lengthen the time for the steel in the phosphate bath (to allow the phosphate to penetrate the scale and get to the base metal)

- Are you laser cutting any of the steel? If so, you have to either mechanically clean that edge or chemically strip (again a problem with a scale being formed during cutting)

- Talk to the tech rep for your pretreatment chemicals and nake sure he is aware that you are processing HRS.

 

[gr2vessels]

czeilenga,
My two cents;- since you are not powder coating specialist, you have contracted someone claiming he is the specialist and familiar with the coating process, including the surface preparation.
Regardless of how you'd store the product, the coating specialist was suppose to do his job, ie do the surface preparation in accordance with the metal you sent to him and he was suppose to charge you any extra for additional preparation he had to do in excess what agreed in your contract.
If you try to over-engineer the coating process outside of your expertise, that's the best way for you to take the blame for poor coating workmanship. Unless is your uncle the powder coating specialist, you make sure the contractor will provide you with a sound engineering explanation why should you accept his poor wormanship. Don't do the engineering work for them, they are not paying you for that, are they?
Cheers.
gr2vessels