Painting/Powder coating cast iron: gassing problem

A. For getting better performance you need to perform the following necessary stage:

ALKALI PERMANGANATE SOLUTION CONSIST OF

100 GRAMS PER LITER Sodium Hydroxide

100 GRAMS PER LITER potassium permanganate ⇦this on eBay or Amazon [affil links]

Temperature = 90 °C

Note : Dipping time approximately 30 minutes , then rinse and dip in some acidic solution

A. I suspect that you are using a lightweight iron phosphate treatment (cleaner coater). I suggest that you consider a calcium modified zinc phosphate process which with the addition of suitable detergents can be used as a "cleaner-coater". Even better will be a Class 1 massive iron phosphate used after a micronised manganese phosphate pre-dip. 60 secs pre-dip cold followed by 5-10 mins Class 1 iron at 90 °C and 30 pts. Water wash and dry.

A. Jeff

Look at the pH of your last stage sealer (final rinse) and see if keeping the pH between 8.2-8.8 and a total or "M" alkalinity part per million range of 350-600 ppm In working at similar process as your doing I have observed a DEA or MEA ( amine type alkaline final sealer ) to improve adhesion on machine surface sides of cast iron product. Air-knife off excess water and consider heating part to help outgas off any acidic residue in the porous cast surface.

A. Maybe a different coating system that doesn't require heat, it's the heat of the curing process that's causing the problem, and I've seen castings that outgas no matter what tricks you try on them. There are powders that have a longer gel time in order to give you a better chance at eliminating the outgassing if your married to the powder coating. Your coater should be able to get it from one of his paint suppliers and maybe try that if he hasn't already.

A. I've not noticed carbon being a problem in powdercoating cast iron.
What problem symptoms are you experiencing?

A. I suspect that the problem you seek to avoid is more to do with gas entrapped in the casting than Carbon content.
My experience with CI parts is to treat them pretty much the same way as with HDG parts.
De-gas the parts by exposing them to a temperature ~20-30 °C metal temperature above powder cure temp for ~20-30 mins.
Then make sure you are using a de-gas powder (often referred to as casting grade).
The object is to drive out as much gas as possible before pre-treatment and then to extend the Gel time of the powder during the cure cycle to allow any residual gas to escape.
Hope this helps.
Regards,
Bill

A. The issue of "gassing" is an interesting one. Seems to occur almost exclusively on casting (both iron and aluminium) and HDG.
We are embarking on a research project soon to try to get to the bottom of this phenomenon.
There are many opinions as to what causes it, what gas it really is, how to avoid it, or to cause it to evolve prior to curing.
"Ant-gassing" powders gel slower, allowing time for evolved gas to escape and the coating to reform smoothly after a bubble pops though. They have (I understand) more wax in their formulation.
The common "cures" for gassing are not foolproof.
Anti-gassing powder sometimes don't cope.
"de-gassing" doesn't always work.
These factors seem to have some influence:
steel chemistry; In HDG, the thickness of zinc; The hanging orientation; the colour; the metal thickness; The pretreatment route.

I'm keen to get as many opinions as possible on this phenomenon.

Dear Geoff,
Yes we would be very pleased to get involved with your company and local university in an endeavour to discover how and why gassing occurs during the baking of a powder coating film when applied to a galvanized substrate. By including waterborne primer into the University program we may obtain some answers to why it seems no longer necessary to pre-heat or T-wash a galvanized substrate prior to powder coating.
I believe getting involved with this exercise will help us to understand why/how a waterborne primer works in this application. Could it be that the wetting agents in the formula (included to wet a metal substrate and help with pigment dispersion) allows the waterborne primer to apply evenly over a metal substrate but also allows the primer to sink into the galvanized coating and displace any air/gas that may be en-trapped within?
I understand that you will be contacting us for samples when the University is ready to begin its investigatory exercise.
Terry

A. The problem with castings is relatively easily explained, i.e., porosity.
The problem with HDG is a little more interesting.
I observe that BS 6497 calls for air quench.
I always thought this was more to do with Sodium Dichromate used in the water quench to delay White rust onset and which interferes with Powder adhesion.
Maybe air quench also allows entrapped gases to escape?
I wonder what comparative results would be achieved if the metal was IR heated rather than convection heated at the de-gassing phase?
This subject should be attracting a wider audience to provide as much independent input as possible.
The recognised contributing factors might be more fully elaborated on to assist us all as well.
For Example: how does hanging orientation affect the phenomenon?
Let's get together on this one for everyone's benefit.
Regards,
Bill

A. My Dear,

You pre-heat the substrate at about 80 °C, allow to cool to 45/50 °C, then start coating.

A. Hi Kelly. Are these bullets made of cast iron or have we crossed threads a bit? What are the bullets made of? What are you "painting" them with. I doubt that slightly improper outgassing protocol accounts for softness of the paint.

Regards,

Hi Abhijit. We appended your inquiry to a long and very informative thread on the subject. You may wish to review it, and then re-cast your questions in terms of what has been said. Good luck.

Regards,