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topic 3921

Rust on pickled & passivated 304 stainless parts


(1999)

We use 304 stainless steel square tubing to manufacture frames for some of our refrigeration units. These frames are "pickled and passivated" (p/p) by a local company. We still see rust form on these frames and also the exterior SS panels that are attached to the frame. We have also P/P a large sheet metal parts (304) that started rusting soon after installation. these parts are installed in a unit with water entering and leaving constantly. Many of our older units show little problem with rust even w/o P/P. we started seeing this problem three years ago and started P/P to try and cure the problem but we haven't got a good solution for it yet. Has anyone else seen this type of problem after P/P and what is the correct cure?

Thanks for the help J

jay arendt
refrigeration - Denton Texas


First of two simultaneous responses--

-

Hi Jay , If your Stainless Steel is rusting then the passivation process being used is not doing its job , without going into a long and complicated explanation the only process that gives consistent results in passivating Stainless Steel is electropolishing . Contact Dan Weaver at Team Metal Finishing in Georgia they use and market an Aussie developed almost benign electropolishing process that is streets ahead of anything else available ( I maybe a little biased because of my origin ! ) regards John . C . Tenison - Woods

John Tenison-Woods
John Tenison - Woods
- Victoria Australia


Second of two simultaneous responses--

(1999)

You cannot really trouble shoot a problem very well without watching the process.

My guess is that the processor is using a long pickle cycle, probably hydrochloric acid, that is probably somewhat contaminated with iron. Next, he is probably very slow getting into the rinse tanks. Next, the rinse is possibly not clean enough. Finally, the passivation is probably the speck time, which is frequently not long enough.

Look at this sites passivation related letters in the last two years. Some will be eye openers.

Quality of your SS may have changed, even from the same vendor. There is a lot more "import" SS in this country now than in the past and some of it at best marginally meets specifications (maybe).

Your process may have changed. Exactly the same cutting fluids? Using resharps rather than new? Different press? Using the same equipment for steel, now, when you did not do it as much in the past?

Lots and lots of possibilities.

James Watts
- Navarre, Florida



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