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Thread: Black dust on SS surface

  1. #1
    Join Date
    Oct 2002
    Location
    Sweden
    Posts
    134

    Black dust on SS surface

    I have noticed a black dust on the bottom of new SS tanks. The new tanks are passivated with nitric acid. As instructed I let the tanks air dry. After that I have cleaned and sanitized (CIP) the tanks with a mix of caustic and clorine. Now I have noticed a black dust when wiping the bottom of the tank. I thought it could be carbon which comes from the stainless steel (304) or ?

    If it is a result from the nitric acid treatment the CIP might not work so well on the bottom of the tank.

    I have not seen this dust on used tanks which are treated in the same manner with nitric acid.

  2. #2
    Join Date
    Oct 2002
    Location
    Tadcaster, Yorkshire, UK
    Posts
    1,097
    When new tanks are welded up, there is a ridge of weld left at the join. This weld is then ground back to leave a reasonably smooth surface. If I remember correctly, this is normally to 150 grit (whatever that means exactly) It is not a smooth as the rolled steel surface but is smooth enough to be hygieninc and reduce the risk of stress corrosion cracking developing in the micro pores around the edges of the weld.

    The black dust you are seeing is indeed likely to be iron oxide, due to the oxidation of the particles as they are ground away. Over an extended period of time, this oxide will be washed off / dissolved in the beer / detergent, which is why you won't have seen it in second hand tanks even if they have not been speciallly cleaned.

    Simple answer - hand clean and degrease. Hand clean ( I am assuming you can get in and out of the the tank safely and reach all parts), Then degrease using high temperature caustic and or special degreasing additive (potassium hydroxide based). Your chemical supplier should be able to provide you with suitable material. Normal degreasing regime woul be 80 degree C degreasant at 1.5% for one hour. Rinse off, and then if you wish, passivate with nitric acid (usually obtained as a phosphoric / nitric acid detergent mix) at 5%, agin for an hour or so. Rinse off and carry out a normal clean before use.

    Cheers

    Dick
    dick

  3. #3
    Join Date
    Oct 2002
    Location
    Tadcaster, Yorkshire, UK
    Posts
    1,097
    Forgot to mention, if using chlorine, 150 ppm max with a minimum pH of 12. Otherwise you risk getting chlorine release, which will attack the stainless steel, leading to pitting and possibly stress corrosion cracking - which can irreperably an catastophically damage both pipe and tanks.

    Cheers
    dick

  4. #4
    Join Date
    Oct 2002
    Location
    Tadcaster, Yorkshire, UK
    Posts
    1,097
    I think I got the black dust wrong. Most of it will be from the carborundum disc / pad used to grind back the stainless - hence the reason it doesn't dissolve !!

    Cheers
    dick

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