Q: I am looking at a report that a major company just sent in. The steel parts are showing signs of rust after 24 hours and visible signs of rust after 48 hours in a 5% salt fog solution.
I’m somewhat new to this world of powder coating and am wondering how this is possible? Some holes and slots that have been punched out, I can somewhat understand. But when I see aluminum wheels and outdoor items holding up season after season, I would have thought the powder coating would hold up longer?
Is there a method of correlation between hours in a salt chamber to what this means to user conditions?
A: A properly applied and cured powder coating should last significantly longer than this. Metal cleaning and pretreatment is critical, as is complete coverage. This means the metal must be completely clean, with absolutely no dirt, oil, fingerprints, etc.
Furthermore, a suitable pretreatment must be employed. This can be as simple as a thorough media blast or as complex as a multi-stage phosphate or zirconium/silane process. Expected field exposure dictates the degree of pretreating necessary. In addition, if the powder coating isn’t completely cured, it will not have the necessary barrier properties to thwart the invasion of water and salt ions through the coating.
Incomplete coverage is an obvious no-no. Standard polyesters should give you a couple years of durability before showing signs of corrosion. Epoxies are significantly better, but they fade in the sunlight. The best coating systems comprise a killer cleaning/pretreatment process followed by an epoxy primer, and then an outdoor durable polyester (or even better, a fluoropolymer).
As for correlating salt spray performance to predicting outdoor durability, it’s a tricky proposition. Most powders will typically go 500 hrs in a salt fog cabinet before exhibiting any rust. I would guess this equates to somewhere around 18-24 months in a continental climate. Coastal environments are much more brutal and harder to correlate. A good epoxy over a good substrate will go 1,500-2,000 hrs in salt fog.
I think you have a metal prep and/or process condition problem. I recommend checking solvent resistance (an indicator of cure) and film thickness.