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Iron contamination of 316 weldments

Hello

I am a manufacture of weldments used in the enviomental water treatment industry. Everthing we make is of the 316 or 316L material. Our problem is Iron contaminaition. We constantly get rejects called rust spots. These parts are kpt inside in a good environment. I can't figure out where it comes from or if it is something in the base metal. We run furoxit (misspelled) test and fail. We do sand blast parts with garnet (virgin) and system only runs stainless. We also passivate after blast. Can you suggest how to stop this. Or maybe a solution that would prevent this.

Bob Smith
machine parts - Hampton, Virginia
2005

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What is your post-welding cleaning procedure? Are you removing the heat tint by grinding or pickling? Do you do an overall passivation of the entire surface after heat tint is removed?

I find passivation of stainless steel is much more than a yes/no issue. There are degrees of passivation just as there are degrees of corrosive exposure. The challenge is to match your cleaning procedure with your specific corrosive environment.

At minimum, the heat tint from welding must be removed. Grinding/polishing is effective but may result in smearing of the metal. This may push contaminated material into the base metal. I prefer using an electrolytic pickle rather than Nitric/HF as it is much safer and results in a better chrome/iron ratio. Follow that up with a citric-style passivation. Water quality is important both in your pre-operational service as well as when the stainless steel equipment is in use. What is the chlorine content?

Todd Turner
- Monroe, Louisiana
2005