Sporadic Rust

Q. We have a single-stage spray wash system that we use on large cold-rolled steel sheet and hot-rooled, pickled and oiled tube fabrications before powder coating. The wash runs at ~145°F. We wash to water shed.

We are experiencing a percolating rust problem, sometimes months after powder coat. When we peel off a blister, the rust has a definite mottled square pattern.

The chemical we’re using is an iron phosphate cleaner. But I see the tech sheet specifies it for steam or immersion. Our rep assures us it will work with the pressure wash.

We have not experienced a rust issue except for the seasonal heat and humidity but now have sporadic rusted parts all year round. Any thoughts? T.A.

 

A. I would tend to agree that if your cleaning chemistry is formulated for steam and immersion, it should work satisfactorily in a spray application. But, as always, if your chemical supplier cannot satisfactorily answer your questions and give you some guidance, then you need to find one who can.

While you continue to work through the problem yourself, I would suggest taking a look at the line to see what has changed since this problem first appeared. Do you have different operators, equipment, chemicals (even upstream, such as lubricants) or parts/materials?

I would also take a close look at the process control information you have on the line. Your supplier should have set you up with equipment and procedures to collect data on temperature, pH, rinsewater conductivity and process chemical concentrations.

In your case, I would focus on the pH of the system over time. If pH shows a general drift downward, this could be a source of flash rust for your component parts prior to powder coating. Additionally, you could check to make sure parts are adequately dried and contain no recessed or trapping areas that may hold moisture.