Rust on 17-4ph

Q. I’m hoping you can help me. I have parts that are made of 17-4PH stainless steel per AMS 5643, approximately ½ inch diameter by 1 inch long. They’re rusting when our customer uses them, but products we have in our inventory from the same lot show no signs of rust. We machine the parts in-house using carbide tooling and then send them to a heat treater and thermal treater—H1025 per AMS 6875. Then they’re sent out for bead blasting and are passivated per ASTM A 967. Could the customer’s rust be the thermal treating or just poor passivation, and to what extent can these parts be expected to rust if they are processed correctly but subjected to extreme conditions? M.K.

A. Precipitation-hardening (PH) grades of stainless steel may be somewhat more difficult to passivate than the more traditional 300-series materials due to their more complex microstructure and the subsequent heat treatment necessary in order to achieve the required strength. You reference that the parts are being passivated per ASTM A 967, however, you do not provide more details on the actual process. That ASTM standard allows two broad categories of passivating chemical, nitric and citric acids. I don’t have any direct experience passivating precipitation-hardening stainless, but some of the arguments I reviewed indicate citric acid passivation would be better suited for this job than nitric acid passivation would be.

Additionally, I would encourage you to investigate the actual point at which these parts are first starting to show rust. Does your customer receive them directly from one of your jobshops? If so, you may have to work with each shop to identify at what point in the process the rust first starts to form. For instance, if some rust is already formed by the time it reaches your passivating shop, the citric acid will not do much to remove what’s already there. A pickling process would be necessary to remove this. I would suggest starting with your heat treat shop to ensure they are doing the heat treating per your specifications (time, temperature and atmosphere) and check to see if parts are leaving their facility with any signs of rust or received by the next shop in a rusty condition.