Stainless Steel Pretreatments before Powder Coating

A. Where to begin? I've been a powder coat process engineer for 20 years and I find this question to be absolutely incredible unless you are only talking about a panel or two. "Break" the surface tension of the stainless with an abrasive so that the powder has something to stick to then spray away.

A. As SS is one of the most difficult to obtain adhesion with PC, evaluate a product called "TWO Pack Etch Primer". The acid content will help with adhesion and the anti corrosive pigment content with weathering. Purchase from your local Auto supply stores or local auto body shop.

A. You can either air-dry or bake the primer at 180-200 °C. This primer was originally formulated by the US Navy in 1940's and is a universally recognised formula. It has excellent adhesion to most metals and therefore improves the adhesion properties of most following coatings, liquid and powder, with the additional benefit of preventing some corrosion creep. However, it use has declined (but as far as I am aware not banned) due to the fact that it contains chromate pigments and an environmentally non-friendly solvent blend of low flash solvents. If you do wish to evaluate this material make sure you purchase the two pack variety and only apply a very thin coat (but still wet) otherwise you may lose adhesion. As with any new process you must carry out trials to satisfy you and your client that the system meets all requirements... maybe include the additional process suggest by Sheldon.

A. Because of high flexibility and surface tension, coating on stainless steel is often avoided, Stainless steel generally is aesthetically pleasing so I wonder why anybody would want to coat it, And if somebody wants to coat it any way why not go in for Mild steel (Ms steel) which is generally 5-6x times cheaper (or even aluminum) rather than stainless steel...Why do you think its called STAIN-LESS steel

A. We have occasionally over passivated/etched 304 stainless in our passivating tanks prior to powder coating.

A. To meaningfully assist you any one of us would require more detail.
A chem film on Aluminium is not particularly enlightening as to the process?
303 is not a particularly spectacular performer in the corrosion stakes.
What performances are you looking for?
Please advise.
Regards,
Bill

A. I feel sure this response will draw critics but note that no-one else has contributed to your enquiry to date so I advance my modest advice:
With Aluminium there has been no significant advance over a true chromate conversion coating of which I am aware especially with regard to "self healing properties".
With your passivated Stainless although it is an inferior corrosion resistant grade of Stainless you are face with a quandary.
Is the Powder grade you intend to apply a sufficient Corrosion Barrier? if so blast the passivate off, provide a key and apply the powder.
If not, leave the passivate on and apply the lower grade powder.
Hope this helps,
Regards,
Bill

A. I'm a passivation guy, not a powder coating guy, but I have had the question put to me a few times, and I suspect that powder coating on stainless has more to do with the surface finish (roughness) than it does whether or not the part has been passivated. As such, if there are adhesion problems, I would see if a sandblasting/bead blasting operation improves the situation. If yes, they should move the passivation step to after the blasting or just skip it, since passivating before blasting is pointless.

A. My self Prabhakar Deshpande, I am in Sheet metal fabrication field since last 40 years. I totally agree with Mr Kremer's views on pretreatment for Stainless steel. Basically Stainless steel is powder coated only to ensure better life for the product, if at all powder coating fails.

The only effective solution for Pretreatment prior to powder coating/painting on stainless steel is sand blasting or glass beading. One has to be careful while blasting as Stainless steel tends to distort as it is a bad conductor of heat.

Micheline,
24 hours is usually overkill, 2-3 hours is generally plenty for the passive oxide layer to form. However, as you say, since you are painting it almost becomes a moot point. Paint is one of the options for corrosion protection on carbon steel, the corrosion protection inherent to stainless becomes superfluous when you are covering it over.