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Welder/Machinist wants educating on passivation of stainless steel

I need help in passivating a stainless steel weldment. What I have is a custom rack for a customer. Material is 304 S.S. tubing welded with 316 filler. The part was semi polished with all the weld ground out. On request I added several pieces and I am sure exceeded the maximum heat interpass. And there lies my problem. I have iron oxide coming to the surface at those points. The customer declined a high polish based on expense. Was told passivating with nitric acid should cure the problem. If so, what is the formula to dip the part, and how long? Also can a solution be mixed for a brush or spray bottle?

Thanks,

Seth,

Passivating SS weldments involves handling, either very toxic and aggressive acids or experimenting with "new generation chemicals" which I am not sure are on sale to the public. For the first approach you need at least four tanks (boxes made of hard polypropylene or PVC plastic) for pickle, rinse, passivation and neutralization. Dissolved in water, 40% nitric acid (very toxic) and 6% hydrofluoric acid takes care of pickling (this last one's the chief of them all, can cut through your skin, muscle and bone if not treated at once). Passivation will proceed in 30% nitric, neutralisation in a slight alkaline mix and rinse of course in tap water. If you pass through that safe and alive, then comes the waste dump and its consequent EPA liabilities. I really don't think you want to do it. Get a professional instead. One last word, try blasting a small area with glass beads (not sand), it may satisfy both you and your customer (Not totally problem free - Much less certainly...)

Seth!

Dammit, you sure hit the NAIL on the HEAD with your statement about overheating the weld ... a major point of concern with many mfgs of stainless! Why doesn't the industry advise people about this? On welding ultra high grade NiCr alloys, the chief metallurgist at Wiggin (now called Special Metals Wiggin) exclaimed that the Moly content had 'denudified' during welding. His words. I heard him say this. But, unfortunately, if you pose this question about alloy disappearance during welding to Specialty Metals to-day,.... they will NOT reply. !

Suggestion... why not use pure Nickel as the weld rod (BUT I don't know the melting temperature) as it might/should have, I think/hope, a lower melting temperature than a 3l6 weld rod.

As Guillermo said, don't use HF. BAD NEWS! The only acid that frightens me. Perhaps use citric acid.

Re tanks, don't use PP (for nitrics) due to potential weld failure. Use PVC. Or, best solution, go and use some el-cheapo rotationally moulded tanks. These are normally made in medium density Pe but please use tank adaptors, don't weld on any fittings.

I hope that this has helped you somewhat.

Cheers!

Seth:

This is not an unusual occurrence, unfortunately. You should be able to clean up the problem of the ferrites with a citric acid based gel. This has proven to be equal to or superior to the nitric ones in passivation, once you have the weld discoloration removed mechanically or electro-chemically. Is this a one-time job, or ongoing? If it is ongoing we can recommend a number of different systems to give you good results without using hazardous HF systems.

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