Pitting corrosion steel plates

Discussion in 'Materials' started by naserrishehri, Dec 22, 2022.

  1. naserrishehri
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    naserrishehri Senior Member

    Hello friends
    We have many steel sheets 12x2.4m with deep pitting in our shipyard , is there any grinding machine to reduce the thickness of plates up to a smooth surface ?
    May it change the mechanical properties and grade of steel plates?
     
  2. gonzo
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    gonzo Senior Member

    Grinding will probably cost more than new plates. What is the thickness of the plates, depth of pitting, percentage of surface that is pitted? If this is only a cosmetic issue, it may be possible to used them where they don't show.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    What is the depth of the pitting v thickness?
    You need to be careful not to grind away too much parent metal....if it is all over the plates, I would scrap it and not use it.
     
  4. naserrishehri
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    naserrishehri Senior Member

    For example if the plate has 10mm thickness and maximum pitting is 2mm then we want to grind whole plate 2mm and use it as plate 8mm.
    Is it possible?
     
  5. naserrishehri
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    naserrishehri Senior Member

    There are many plates with different thickness between 10 to 30mm.50 percent of each plate has pitting.
     
  6. gonzo
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    gonzo Senior Member

    It is possible, but more expensive than new plate. There are applications where it would be acceptable. For example, to patch sheet pile on a waterfront.
     
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  7. TANSL
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    TANSL Senior Member

    Shot blasting, for a shipyard, is not that expensive. All plates should go through the sandblaster.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    This sounds like poor quality control from the supplier, or the shipyard storage facility.
    If it is in this condition, delivered from the mill, send it back as non-compliant.
    If it is yard storage, then you have a serious issue with the storage of metals in the yard....possible cross contamination too.
    Either way, it sounds like the steel, is poor quality and should not be used.

    Shotblasting is used for general surface removal of oxides and/or to create a key surface for painting.
    To remove 2mm of steel, is thousands of times greater, in thickness terms, than the microns of thickness removed in shot blasting.
    I am not aware of anyone removing 2mm of steel via shotblasting....nor anyone recommending it either.
     
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  9. naserrishehri
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    naserrishehri Senior Member

    The reason of pitting is poor storage. Pittings are deep. I have heard there is a grinding machine which can remove a thick layer on flat plates.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Sounds like you have cross contamination of grinding dust etc coming in contact with the plates. And/or the plates have nor air/space between them either once removed from their packing.

    If you can find a machine that will grind away 2mm - great, good luck!
    But what you would like will only create more work and rework and lead to a less satisfactory solution.

    Your only option, most likely, would be to send to the plates back to the mill for them to remove the 2mm.
    This is a specialised bit of work you are seeking...
    And, if this is all for being built to Class, you need to get this witnessed and approved by the surveyor too!
     
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  11. seasquirt
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    seasquirt Senior Member

    Are the pits the same on both sides of the plates ? 10mm plate with minus 2mm both sides is only 6mm remaining, and not worth it I'd say, unless you owned a machine shop with cheap labour. If not being used to any standard, the deepest pitting can be welded full, tops dressed back by hand, then sent to a machine shop to be milled to minimum cleanup, or a standard thickness better, which is expensive, then shot blast the remaining few percent of shallow pitting, for painting, galv, or whatever treatment . If no milling machine, a slash grinder may be used, depending on how big the pieces are; and that will take forever. Or get a quote on its scrap value, and use that to get something more useful. It obviously doesn't like corrosive environments.
     
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  12. TANSL
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    TANSL Senior Member

    You're probably right...... Well, you're right. :oops:
     
  13. fallguy
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    fallguy Senior Member

    Doesn't grinding create a new problem? The new surface will rust differently than a mill finish.

    There are probably uses other than ships where the material could be used and get a decent lifetime; despite the pitting.

    I have made things from deeply pitted steel that will outlive me, but it would not have been suited for a ship.

    Resell for alternative use.
     
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  14. naserrishehri
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    naserrishehri Senior Member

    Yes.pittings are the same on both sides.
     

  15. Rumars
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    Rumars Senior Member

    Yes there are such machines, they are called surface grinders, you would need a vertical axis reciprocating table one, or possibly a wide belt grinder. They are used to produce flat surfaces and can be buildt in the size you mention, but we are talking about a specialized heavy industry machine, it's probably going to cost more then replacing the stock.
     
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