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  1. #1
    atetsade Guest

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    bleach of the common variety of sodium hypochlorite which releases chlorine gas

    why does it corrode stainless steel so aggressively? what is the reason for the pitting nature of the dark corrosion?

    what is the effect of removing the stainless steel's oxidic coating.. is this what causes the deep immediate corrosion of bleach on stainless steel?

  2. #2
    Forrest Addy is online now Diamond
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    Yours is a question for a chemist specializing in corrosion. I know bleach contains about 3% sodium hypoclorite cause it says so on the bottle in my laundry. I presume that means NaHClO3. I do know that clorite -ClO3 radicals are chemically very active, love to make metal salts out of metals, and they fortify the effect of detergents.

    Cloride corrosion is a serious matter for stainlesses and a good reason why it's seldom used in salt water submergence except in expensive rich alloys.

    Not even the most exotic austinetic stainlesses last forever in a bleach evironment. That's one of them facts of life.

    Durability depends on the stainless alloy. Some alloys are intended for service in sulfite and clorite solutions. The paper pulp industry uses processes that features both environments. Maybe research in that direction will lead you to the magic alloy if that's the stimulus for your inquiry.

  3. #3
    MBensema is offline Hot Rolled
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    I'm off to Eastec, so this is only a brief explaination. Chlorine eats through the chromium oxide coating that protects stainless steel very quickly. Once the coating is removed in some parts and not others, galvanic corrosion starts and pittings form. It's important to have oxygen in the fluid to keep the chromium oxide coating replentished.

    Another form is creavice corrosion. Same problem, the chromium oxide coating gets removed and since there is not any oxygen in the creavice to replace the oxide coating, corrosion starts. This is the form of corrosion in the pittings themselves as well as any corners, etc. where the fluid cannot flow.

    304 and 316 are not all that good for salt water applications. Duplex stainless steels are much better choices, 2205 being one of them. For even higher corrosion protection, super duplex steels were developed.

  4. #4
    bnelson is offline Senior Member
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    These days bleach is about 6% sodium hypochlorite (NaClO), +/- sodium hydroxide (lye). It's very alkaline, so you would think it would protect agains rust.

    No so, for two reasons. Hypochlorite ion is a powerful oxidizer which can transfer monatomic oxygen (as reactive as a free radical) to iron through a crevice in any protective coating, forming iron oxide. Once there, that small seed of oxide allows rusting to continue as it naturally does.

    Secondly, hypochlorite reacts with the carbon dioxide in the air to release small amounts of nascent chlorine. This is also monatomic, and a much stronger oxidizer even than monatomic oxygen. It will pretty much attack any metal except gold. Ditto for the chromium oxide coatings (converts to soluble chromic chloride).

  5. #5
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    JRIowa is offline Diamond
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    MBensema,
    The duplex stainless won't even hold up to chlorides. Hastaloy C would be a better choice.

    JR

  6. #6
    andy pullen is offline Hot Rolled
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    I've done some work for Clorox over the years. Alot of the equipment in their plant is stainless. But there is also alot of titanium. The titanium seems to hold up better in that environment.

    I've also done work on Navy periscope and antenna masts for submarines. There's not much stainless in those assemblies. The stainless that's in them is 304/316L. But there is alot of monel. Mostly K-500 which is tough stuff. The I-beam that is the main support for the mast is K-monel. I would not even venture a guess how much that piece of material costs. There's even quite a bit of 6-4 titanium in some of the masts.

    We would do the refurbs on the masts as well as build new ones. When they came into the shop, there was salt powder all over everything. And the stainless parts are the only ones I remember being scrapped out for corrosion.

    Forrest has probably seen some of the same things in his time in the shipyard.

    Andy Pullen

  7. #7
    Evan is offline Titanium
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    You want a nightmare corrosion problem? Stainless steel looks positively invulnerable compared to aluminum. I used to work on float planes and amphibs in salt water service. It really is a different type of metal work than found in most shops. The alclad sheet has to be handled with gloves and a single scratch through the cladding can scrap a very expensive part. The shop has to be kept clean enough to eat off of. That was one of the things I liked most about aircraft work, especially helicopters. Nice clean working conditions.

  8. #8
    Too_Many_Tools is offline Stainless
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    My little lesson in life on this subject occurred when I placed some surplus but very expensive stainless steel surgical tools in a bleach solution and came back in a week to retrieve them.

    As you mentioned, any break in the chrome plating resulted in extensive and very destructive corrosion to the stainless steel.

    TMT


  9. #9
    atetsade Guest

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    TMT

    my life lesson was with all my silverwear. thank god it was the bleach. I thought it was just my kitchen sink.

    I thought I was going to have to invest in silverwear made of more noble a metal than 18/8.

    well who the hell told me to use bleach anyway.

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