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Thread: What causes pitting on a watch case?

  1. #1
    Master IAmATeaf's Avatar
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    What causes pitting on a watch case?

    Was admiring a watch on SC as noticed mention of pitting, pictures showed it to be in the rear under the area of the case back.

    Is this caused simply by a build up of grime and gunk or is something else in play?

  2. #2
    Master paneristi372's Avatar
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    From what I have been led to believe. It is some reaction between the old rubber gasket and the Stainless Steel. Seemingly there is something in the rubber they used to use that takes the natural protective coating off the stainless steel which them leaves it susceptible to corrosion.

  3. #3
    Salt water can also cause it. Particularly to older types of stainless steel, apparently.

  4. #4
    Quote Originally Posted by Bravo73 View Post
    Salt water can also cause it. Particularly to older types of stainless steel, apparently.
    That's handy on a stainless steel diver 😂

  5. #5
    Craftsman earlofsodbury's Avatar
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    A lot of pitting is an electrolytic reaction between the metal (or metals, which is always worse) of the watch plus the gruesome mix of water, oils, fats, salts and acids that human skin naturally produces in abundance. Tanned leather adds its own suite of chemicals to the mix if you wear such a strap. Seems to be much worse in some people than others too - my old man could have corroded a solid diamond watch, ordinary "stainless" steel lasted a very few years...

  6. #6
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    Stainless steel pitting and corrosion is complex. There’s a phenomena known as crevice corrosion, which occurs when some organic materials are in contact with stainless steel in a stationary environment......which is exactly what you get in the area around a rubber caseback seal when the seal material decomposes in the way 60s rubber gaskets often did. This causes the deep black pitting that’s often observed in the area where the seal has made contact iver a period of many years. I’ve seen this in the chemical industry, it’s often difficult to predict or simulate in testing. I did a fair amount of corrosion trial work over the years involving 316 stainless steel and sometimes the results were surprising.

    Stainless steel doesn’t like aqueous chloride solutions and can be affected by stress corrosion cracking, yet I’ve seen plenty if examples where the metal isn’t affected. However, I’ve seen cracked links on old Omega ricegrain bracelets where the fixed links join, but this may be caused by the corrosion products creating mechanical stress and splitting the metal......either way its something I always look for in old bracelets.

    Corrosion in the gasket area isn’t usually disastrous, if its in the caseback area a new O ring can usually achieve a seal as it compresses and sometimes a slightly thicker one helps. A light smear of silicone grease ensures the seal area is hydrophobic, which also helps. Corrosion around the glass seal is more of a problem, especially where an acrylic armoured crystal is fitted because the acrylic plastic can’t seal against the irregular surface. One trick I’ve used is to heat the case very carefully after fitting, 70-80degrees is enough, which helps the crystal material mould itself to the rough surface, this can take a vintage watch from zero WR to 1 or 2 bar if it works, but I wouldn’t advise anyone to try it on a built up watch. As a last resort a thin bead of adhesive can be applied but I don’t like doing this.
    Last edited by walkerwek1958; 9th December 2019 at 14:44.

  7. #7

    What causes pitting on a watch case?

    The nitric acid which is produced from the degradation of nitrile rubber o-rings.

  8. #8
    Master
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    Great post Paul. Interesting reading!

    Quote Originally Posted by walkerwek1958 View Post
    Stainless steel pitting and corrosion is complex. Thereís a phenomena known as crevice corrosion, which occurs when some organic materials are in contact with stainless steel in a stationary environment......which is exactly what you get in the area around a rubber caseback seal when the seal material decomposes in the way 60s rubber gaskets often did. This causes the deep black pitting thatís often observed in the area where the seal has made contact iver a period of many years. Iíve seen this in the chemical industry, itís often difficult to predict or simulate in testing. I did a fair amount of corrosion trial work over the years involving 316 stainless steel and sometimes the results were surprising.

    Stainless steel doesnít like aqueous chloride solutions and can be affected by stress corrosion cracking, yet Iíve seen plenty if examples where the metal isnít affected. However, Iíve seen cracked links on old Omega ricegrain bracelets where the fixed links join, but this may be caused by the corrosion products creating mechanical stress and splitting the metal......either way its something I always look for in old bracelets.

    Corrosion in the gasket area isnít usually disastrous, if its in the caseback area a new O ring can usually achieve a seal as it compresses and sometimes a slightly thicker one helps. A light smear of silicone grease ensures the seal area is hydrophobic, which also helps. Corrosion around the glass seal is more of a problem, especially where an acrylic armoured crystal is fitted because the acrylic plastic canít seal against the irregular surface. One trick Iíve used is to heat the case very carefully after fitting, 70-80degrees is enough, which helps the crystal material mould itself to the rough surface, this can take a vintage watch from zero WR to 1 or 2 bar if it works, but I wouldnít advise anyone to try it on a built up watch. As a last resort a thin bead of adhesive can be applied but I donít like doing this.

  9. #9
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    Quote Originally Posted by E_2_Right-Force View Post
    The nitric acid which is produced from the degradation of nitrile rubber o-rings.
    Iím inclined to disagree, 316 stainless steel is (supposedly) resistant to nitric acid. Had to check that because itís a a good few years since I worked with nitric acid.

    Decomposition of nitrile rubber at room temperature is unlikely, at elevated temperature hydrogen cyanide is a likely decomposition product.

    Willing to be proved wrong on this if you know better.

  10. #10
    Craftsman earlofsodbury's Avatar
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    I wonder what watch OP has in mind? i.e. - a modern watch, with a homogeneous, high quality stainless steel fabric might well take a lot of (bio)chemical abuse unpitted, but older watches that throw nickel, brass, bronze, carbon steel, low grade stainless, various multi-layer precious-metal platings etc together with poorer fit of parts and worse sealing - not so much... Two different metals plus a bit of moisture from human skin, washing, swimming = all the galvanic corrosion your heart can desire...

  11. #11
    Master
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    So is this the reason why for vintage watches, even after a service / seals replaced WR is commonly not guaranteed? Iíve often wondered.

  12. #12
    Grand Master
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    Quote Originally Posted by gcleminson View Post
    So is this the reason why for vintage watches, even after a service / seals replaced WR is commonly not guaranteed? Iíve often wondered.
    I think thatís correct, but I think itís more helpful to state whether the watch has passed a pressure test or not. No two watches are identical, some donít show pitting in seal areas but others do, there are plenty of examples where the WR is completely restored by fitting new seals or a new acrylic glass.

  13. #13
    Master IAmATeaf's Avatar
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    Thanks for all the replies, the watch I was admiring was the blue face DJ.

  14. #14
    Quote Originally Posted by walkerwek1958 View Post
    Iím inclined to disagree.
    I thought you might.

  15. #15
    Grand Master
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    Quote Originally Posted by E_2_Right-Force View Post
    I thought you might.
    I`ve never heard of nitric acid being liberated from nitrile rubber, to me it sounds unlikely, but I`m always happy to learn.

    Traces of hydrochloric acid can be released from some synthetic rubbers when they degrade, but that doesn`t apply to nitrile rubber.

  16. #16
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    I think this is the kind of pitting being described. 70ís Russian stainless steel, nitrile rubber gasket (I think). A bit of silicone grease and new seals and it seems to be safe for snorkelling at least.


  17. #17
    Grand Master
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    Quote Originally Posted by alfat33 View Post
    I think this is the kind of pitting being described. 70ís Russian stainless steel, nitrile rubber gasket (I think). A bit of silicone grease and new seals and it seems to be safe for snorkelling at least.

    I donít know what rubber was used in the 60s/70s but Iíve seen plenty of examples of this type of pitting, I donít think it would be nitrile rubber. Iíve seen square section rubber seals that had turned into a hard material that needs chipping out of the grove, and its usually accompanied by pitting. I suspect the modern watches wonít suffer, canít think Iíve seen it on anything later than early 90s. The modern nitrile rubber seals lose elasticity to some extent and become permanently compressed, but Iíve only seen one example where the rubber has visibly degraded.

  18. #18
    Master zelig's Avatar
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    What causes pitting on a watch case?

    It does not matter what the gasket is made of. It is down to the proximity of the two components and development of an occluded cell ...

    https://www.corrosionpedia.com/defin.../occluded-cell

    A similar effect can be seen if you leave a stainless spoon on a stainless sink with a thin film of water underneath - resulting in corrosion.

    Itís down to changes in the chemistry locally - the H+ ions are concentrated in the occluded regions - which effectively lowers the pH very locally - hence it becomes very acidic locally.

    The presence of chloride ions (from sweat or salt water for example) can exacerbate the situation as it locally breaks down the (protective) passive layer formed on stainless steel resulting in local pit formation.

    z
    Last edited by zelig; 10th December 2019 at 22:12.

  19. #19
    SS304 supposedly pits worse than SS316.

  20. #20
    Grand Master
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    304 stainless is generally less corrosion-resistant than 316, it's also cheaper too.

    The pitting is caused by galvanic action on a micro-scale, aided and abetted by other factors. In the past, decomposition of gaskets has created a sticky mess that seems to encourage it, but I`ve seen gaskets turn ti a hard black solid and still found pitting underneath. The good news is that it seems to be far less of a problem than it once was, or maybe the watches just aren`t old enough.

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