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  1. #16
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    May 2008
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    Sydney
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    I went for a walk at the beach this morning and thought of this thread when walking down the stairs. The railings are all stainless and I noted the railings that get touched were shiny but the lower parts of the railings that do not get touched were showing tea stain.

    The shiny parts were clean and the tea stain parts were dirty so I would agree with an earlier comment that clean stainless will not corrode as the protective oxide layer can be formed.

    Regards
    Pete

  2. #17
    Join Date
    Jan 2011
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    Far West Wimmera
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    Quote Originally Posted by Graziano View Post
    I looked up stainless passivation earlier this year and found an article that said to use 30% nitric acid in water. It made the steel shot blasted 316 that had gone tea brown after a day in the rain, go silver white in appearance and it's been fine for the last 5 months or so. I did several coats and rinsed off in between with the hose so the it didn't dry out with acid on it. Nitric is an oxidising acid apparently.
    Nitric acid is only the first step as far as I am aware although there are some occasions when it is all that is able to be used. We had some 90 and 45kl tanks at work that were staining and these were treated by cleaning with nitric acid. It was a warranty job. The whole area was flagged off. I was asked if I wanted the drums the nitric acid came in last year as a big cleanup was happening. When I had a look at them I said why not. I was neutralising them anyway. They are a 20l and a 25l normal oil type drum but made of stainless steel! We also had some rotary fermenters which came from France as deck cargo on a ship. As they were not covered they received salt spray on the outside surface. These also had to be treated the same way.

    I think I have some nitric acid. I might give it a try and see what happens.

    Dean

  3. #18
    Join Date
    Oct 2011
    Location
    Adelaide
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    52
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    As I now have the power supply, I thought I'd follow up on this one.

    Some months/ years ago I splashed out on a kit by a mob called Retropol (RETROPOL ELECTROPOLISHING SYSTEMS - HOME) - no relation except as a customer.
    Basically they supply an electrolyte mix and brushes and you use your TIG welder as a power supply. The electrolyte is a mixture of phosphoric and citric acid. If nothing else nicer than hydrofluoric...
    Stuart, the business owner is keen for users to get a good result and answered my questions promptly when I emailed (including a few tips on improving my TIG technique...).

    Using the Weldskill box to drive it, here is a before and after on some 0.7mm 304. This is actually the back of the joint as the front was ground flush.
    P1040066.JPG P1040069.JPG
    As you can see the electrolysis has removed the surface discolouration and the light oxide on the weld. Some of the heavy stuff is still there so that will have to be removed mechanically if that is a concern - Those seams are around 30mm long, so not something that I'm losing sleep over at the moment as that is caused by my technique (too much heat) and joint design (awkward) rather than the product. Current requirements are only around 5A or so, so if you had some electrical smarts you could even make a dedicated power supply. Application is just dipping the brush in the solution and then brushing it over the seam (as videos on the website show), neutralise and then I rinse off in water.

    While if you were doing dedicated production work I'd suggest a proper unit, but for occasional use probably worth considering. The only issue I had with it all is that the brush tends to soften from the heat put through it so needs to rest occasionally. While the brush adapter is designed to go in a TIG torch the Weldskill has a tong type electrode holder which worked equally well.

    Michael

  4. #19
    Join Date
    Jan 2015
    Location
    Sydney
    Posts
    203

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    Sort of related.
    How would one go about pickling inaide a tank? Example lets says a stainless 20l water tank for consumption. Ovbiously purged but how would you clean the inside weld areas?


    Sent from my SM-G900I using Tapatalk

  5. #20
    Join Date
    Oct 2011
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    Adelaide
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    The way we used to do tanks for hot water heaters (big domestic things) was slosh around a hydrofluoric/ nitric(?) acid mix for say 5 minutes and then drain it out.

    If it is a tank of your design then I'd leave an access panel big enough to get in (probably necessary so it can be cleaned out every so often anyway)

    Michael

  6. #21
    Join Date
    Jan 2015
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    Sydney
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    Can you"thin" out pickling paste? Thats all i got on hand.

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  7. #22
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    Oct 2011
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    Adelaide
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    The acids that we used were in the liquid form, so I guess paste could be thinned.
    The big thing about pickling pastes and the reason I don't want anything to do with them is that they are very dangerous things - the hydrofluoric acid attacks calcium in your body, but you don't get a burning sensation from a spill, so the only defences are PPE and vigilance.
    They were one of those things that made my skin crawl when I was near them.

    Michael

  8. #23
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    Jan 2011
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    Far West Wimmera
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    I recall something about a tig welder, citric acid and phosphoric acid just recently. I was looking at a website that sold the acid mix and some other stuff from memory. The tig welder provided the current somehow.

    Dean

  9. #24
    Join Date
    Oct 2011
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    Possibly post 18 of this thread?

    Michael

  10. #25
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    Jan 2011
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    That may explain why I could not find it anywhere else.

    Dean

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