Preventing Flash Rust

Question:

I have recently set up an acid derusting system for removing flash rust from production parts that have had electrocoat removed via a burn-off oven. The parts coming out of the acid tank are beautiful. However, when they are put into the rinse tank they flash rust immediately. I have tried water as well as water with caustic added, but I am still experiencing the same problem. Any suggestions? M.G.

Answer:

The part's surface is very clean and active after the acid dipping. I have experienced similar problems and find that very thorough rinsing, especially with hot water, tends to promote the flash rust even more quickly. I would suggest using an ambient temperature rinse tank that does not have a very high overflow rate, and then following with a water-based rust inhibitor.

Most of these water-based rust inhibitors are slightly alkaline and contain simple and/or complex amines. This will make the surface less active and provide short-term, indoor rust protection. The rust inhibitor should be applied at elevated temperature (probably 160-190F) in order to flash dry quickly. You will also want to blow out standing liquid from any recessed areas or parts that do not dry easily. The inhibitor works best in a thin film application, not pooled in the corner of a part, having to wait a long time for it to dry at room temperature.

I would discuss rust inhibitors with your chemical supplier. If you do not have a regular chemical supplier or one that only handles primary chemicals such as sulfuric acid, sodium hydroxide, etc., you will need to get in contact with a chemical supplier who blends products especially for finishing purposes. Go to the Suppliers page at www.pfonline.com or look under Rust Preventative Chemicals and Oils on page 568 of the Products Finishing 2001 Directory and Technology Guide.

Although not nearly as likely, another thing to look out for would be other contaminants in the acid and rinse tanks. Are you processing anything else through these chemicals? Other dissolved ions may set up the surface for accelerated rusting.