3-Stage Versus 5-Stage Washers

Q: We have been in business as a job shop powder coater now for several years. We use a three-stage washer for part preparation that includes cleaning and iron phosphating in Stage 1, followed by two rinse stages. We do all indoor-use products, so the three-stage has worked well for us. We have had opportunities to take on work that will be used outdoors, but we are not sure the current washer will deliver the necessary quality needed for outdoor products. Our chemical supplier has urged us to consider a five-stage washer so we can get a better conversion coating that has more corrosion protection. Can you tell us the difference between a five-stage and our current washer, and help us determine if it will provide the needed protection?

A: Your current washer combines the cleaning and phosphating in Stage 1. The cleaner is not as aggressive as an independent alkaline cleaning stage, and the iron phosphate coating is not as effective as an independent conversion coating stage would be. What a five-stage will do is provide an independent cleaner (usually alkaline but sometimes acid) and an independent iron phosphate stage. In addition, rinsing is typically superior. As a result, it definitely enhances performance and provides somewhat better corrosion resistance.

Understand that iron phosphate does not offer very robust corrosion protection, even in a five-stage. It helps with adhesion and protects against rapid flash rust but will not stand up to attack by aggressive corrosive conditions. If your outdoor products need superior corrosion protection, you may need to consider a transitional metal conversion coating such as zirconium oxide or, in particularly harsh environments, a zinc phosphate.

Another option to consider is double layers of powder. If you use two coats or, even better, a good primer coat followed by a good topcoat, you can probably get by with the iron phosphate, even with your three-stage washer.