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What kind of chemistry is used for corrosion protection?

Discussion in 'Bodywork' started by Uky, May 3, 2017.

  1. Uky

    Uky Active Member

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    As far as I know, there are two kinds of corrosion protection out there:
    Bitumen based and Wax based.

    And mixing them is a big NO.

    But what kind does Mercedes use in vehicles today?
    On cars sold in some countries, there is some extra protection.

    Code is "1U4 Vehicle waxing with additional scope"

    Does anyone know more in detail what products are used in the factory?
     
  2. PobodY

    PobodY Hardcore MB Enthusiast

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    That's not really chemistry so much as a physical barrier to prevent oxidation. - If you wanted a chemical solution, why not something similar to marine applications; a sacrificial zinc cathode?
    (Yes, I realise that's essentially galvanising.)

    Out of interest, why can't you mix bitumen and wax? To an inorganic chemist the difference is just the size of the organic molecule, but I suspect there's a real organic chemistry answer (like you can't mix mineral oils with synthetic waxes).

    I would have thought that Mercedes just powder coat the monocoque during production.
     
  3. hotrodder

    hotrodder Hardcore MB Enthusiast

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    Bitumen and wax based systems are sometimes mixed for example https://www.smithandallan.com/documents/SDSHM126%20Hammerite%20Waxoyl%202.pdf but generally are used in slightly different ways... waxes are injected/sprayed inside of box sections/panels and anywhere else that can't be reached by thicker underseal whether it's a traditional bitumen based goop or a modern stone chip paint. Cavity waxes can creep into seams and also self heal because they take soooo long to dry out but on exposed areas don't stay put as well

    AIUI the idea behind bitumen based underseals is was that gave more protection than 'stone chip paints' could back in the day i.e. more flexible = less damage from stone chips and so on. Also provides some sound deadening as does modern rubberised(?) stuff. While it was better than paint at resisting stone chips it eventually it dries out enough for 'weather to get behind it' and then it traps moisture* accelerating rot. When i had the subframe out of my s124 the origional rubbery underseal looked perfect but could be easily peeled off in sheets in places so after getting rid of all the loose bits we went nuts with with some fresh stuff (can't remember if it was a stone chip like Upol gravitex or some 3m schutz, whatever was to hand in my mates w/shop)

    Traditional bitumen based or modern versions are, like stone chip paints, all sacrifical coatings and need maintenance/reapplication from time to time hence w/shop manuals including stuff like http://www.w124performance.com/service/w124CD2/Program/Chassis/97-200.pdf which most of us never bother with

    AIUI MB were very late to the party (and still haven't quite mastered things?) but pretty much all car manufacturers have been electrophoretic coating bodyshells for eons now (google 'e coating car body'). E coating is kinda, sorta, loosly akin to both powdercoating (the electostatic bit) and galvanising** and also provides a better 'key' for the regular primer and paint to stick to [/gross generalisation]


    * same thing can happen with powder coating i.e. while tougher than traditional paint it can fail in a way that's not as obvious as stone chips but lets moisture in and then accelerates corrosion because the part never dries out properly

    ** 'proper' (hot dip) galvanising involves a 'kin large tank of molten zinc (potential distortion to anything dipped in it) and waaaaay thicker coatings which is fine for some railings but not if you want to finish with shiny paint or want the doors etc to fit afterwoods
     
    Last edited: May 4, 2017
  4. OP
    OP
    Uky

    Uky Active Member

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    What I was referring to in my first post was that if you use Bitumen based "chemistry" on a vehicle that has a wax based protection added from the factory, the Bitumen will dissolve the wax protection. I've done it without knowing what went wrong and I remember what it looked like.

    Yes - Mercedes was v e r y late to the party. I remember my CL500 (215) from 2000 and that car had no protection what so ever.

    Thanks for the information.
     
  5. hotrodder

    hotrodder Hardcore MB Enthusiast

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    Same thing happens if you reapply wax based protection (like you're supposed to regularly, see the link i posted)... the solvents in the 'fresh' wax will effect/at least partially dissolve what's left of the old stuff because the solvents are necessary to thin it enough for application. Same deal when detailers valeters prattle on about layering waxes etc [/tongue slightly in cheek]... marketing BS bangs on about the wax being 100% carnuba which is rock hard and would scratch things if you tried to rub it on paint without first blending it with loads of other stuff including softer waxes and solvents. The origional idea behind waxing twice was to ensure even coverage, eat a thesaurus and you can charge more for 'layering' and claim it gives more protection when in reality it just takes longer compared to doing it properly once
     
  6. PobodY

    PobodY Hardcore MB Enthusiast

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    In fairness, you'd probably still melt all the wax if all you used to apply bitumen was heat.
     
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