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Brass Cannon 'Spots'

  • 1.  Brass Cannon 'Spots'

    Posted 11-03-2020 15:24
    This is a bit off-the-wall, but I was passed this question from a hobbyist who is faced with 'spots' - I assume its corrosion on his brass cannon.  I do not have details on why he has a brass cannon and how often he fires it, but he provided this account and some pictures.  I have no experience with brass or cannons so thoughts appreciated.  Arm-chair metallurgy encouraged :)

    - - - 

    ... I appreciate you taking interest in my dilemma.  
    If possible, I would appreciate if we could find out cause and any possible solution to prevent spotting from reappearing.
    1.  I have had the brass cannons for 8 months with no problems with finish(probably cast in last 2 years) and only after firing cannon using black powder -then cleaning/washing it, does problem appear.
    2. Once spotting begins, you can remove the spots completely without much effort with Brasso, but will continually reappear over the next 1-2 days.
    3. Does not feel raised.
    Thanks for your assistance.
    ...Inline image
    Inline image

    John Shingledecker FASM
    Electric Power Research Institute
    Charlotte NC
    (865) 201-1252

  • 2.  RE: Brass Cannon 'Spots'

    Posted 11-04-2020 00:37
    The cannon may have had a protective coating that was removed after firing and cleaning. The coating prevented tarnish and corrosion, but the brass is now unprotected and free to react with the environment. Sulfur in black powder would likely be a bad actor with the brass. 

    It would be interesting to know if the spots show up at the same location and shape after cleaning. If the spots truly reappear, then there could be segregation in the brass making local areas more susceptible to discoloration.

    Larry Hanke FASM
    Principal Engineer
    Materials Evaluation And Engineering Inc
    Minneapolis MN

  • 3.  RE: Brass Cannon 'Spots'

    Posted 11-04-2020 01:19
    That's interesting.  This will definitely be "arm-chair" metallurgy....

    They were cast in the last two years?  The number of foundries in the US that do brass has been decreasing for decades (actually probably true for ferrous (iron & steel) as well).  What I do have experience in, is that communications with overseas foundries can be problematic. Issues with quality and other factors are even more likely as a result.  For instance, does the manufacturer know that the cannon will actually be fired, as opposed to being decorative?  What quality standards were involved to make sure that the cannon are safe for this use, since such have not been used for actual warfare for hundreds of years?  "Takes a nice polish" isn't the same as safe to house explosive charges....  I am not an explosives expert but I hope the owner is familiar with the differences between historical and modern explosive powders.  And I wouldn't stand within fifty yards of a hobbyist-produced cannon.

    Possible causes of staining: Local deposits of corrosive species after use, from the external smoke cloud, or from gases being driven through pores.  Porosity that captures chemical species from firing, or just corrodents from the cleaning solutions.  Local alloy differences from poor casting practice that result in galvanic corrosion during cleaning (areas that corrode or form films because of battery effect = galvanic cells between areas with different chemical compositions).

    What would I suggest?  Have a metallurgist actually look at the piece in question.  Examinations might involve direct visual, surface microscope, and other methods of looking for porosity such as leak testing, dye penetrant, or even radiographic ("x-ray" though it might involve using a source rather than an x-ray tube depending on the details).  It's probably not worth paying for localized chemical analysis, as any significant flaws found with the other methods must result in condemnation of the cannon.  Expect to pay for at least a couple of hours of engineer/inspector time.  Don't expect a warranty of usability!  Will imperfections be found?  Certainly.  Are they bad enough to be considered flaws?  Who knows?

    If no problems are found after a round of investigation, advise the owner to give the piece a few good coats of clear coat after it has been polished.  Any spots that recur after that are coming from the inside through-wall, and are again probably grounds for condemnation.

    I hope this is useful.

  • 4.  RE: Brass Cannon 'Spots'

    Posted 11-04-2020 15:08
    My best guess is that this brass cannon has an undiscernible lacquer coating that was applied perhaps when manufactured.   Since then, possibly because of handling, humidity and human sweat salt got through tiny porous defects in the lacquer and caused corrosion of the brass.  This corrosion product then lifted the lacquer coating and continued to propagate under the coating to corrode the brass. Light diffraction through the coating makes this appear black. Black spots under lacquer coated home furnishings is a common problem.

    Leslie Loushin
    Principal Technical Consultant
    Forrest Winterset Glazebrook & Associates
    Humble TX
    (281) 812-5227

  • 5.  RE: Brass Cannon 'Spots'

    Posted 11-04-2020 15:30
    Thank you all for the input (and also a phone call from an old colleague).  I think I have more than enough to provide some basic guidance.  Great to have the ASM resource available for all.

    John Shingledecker FASM
    Electric Power Research Institute
    Charlotte NC
    (865) 201-1252