If you see a piece of raw steel that isn’t actively rusting, that can only mean one of two things: it’s either covered in oil, or there’s a conversion coating on it.
Folks who don’t work with metal may not realize just how oily raw steel can be. Greases and oils can be invisible to the naked eye, particularly if the part is fresh from the fabricator and comes in looking bright, shiny, and “clean” – but don’t be fooled: they’re there. Anyone who has moved steel parts by hand can show you their black palms in confirmation.
Those oils are a blessing and a curse.
On one hand, oils and greases (“organics”) keep steel parts from rusting. They act as a protective blanket against the ravages of moisture and oxygen. As Bill Townsend, chemical expert and frequent guest on KaserCast puts it, steel wants to return to its natural state – i.e., corrode into the ground from which it came. You’d be amazed at the speed with which that process occurs, once the oils are removed.
On the other hand, organics cause significant adhesion problems for both powder and liquid coatings. Therefore, they must be removed before the part is coated.
Coaters like me are left threading a very fine needle. We have to strip the organic security blanket, and replace it with a conversion coating that keeps steel from rusting while encouraging liquid or powder coating to adhere.
Speed is of the essence. The longer the steel is left completely bare (meaning neither oily nor conversion coated), the rustier it gets, and the worse the outcome for any future coatings we apply.
You may be asking, “Why not just powder coat the part immediately after washing it, then?”
Put simply, powder coating is porous. It will not fully protect steel parts from moisture, and therefore it will not fully protect steel parts from corrosion. Only conversion coating can do that.
In summary, two things must happen to steel parts before they’re coated, and they must happen in this order: 1. The oils have to be removed, meaning the part must be cleaned; and 2. The pretreatment operator must create a conversion coating in order to protect the part from corrosion, even after the powder coating is applied.
Anything less is improper pretreatment, and your part will suffer the consequences.