[ausev] Paint question

[m. howse]

I'm beginning to believe I'm incorrect in my recollection. Although my  
intent was to avoid the naval jelly and phosphoric acid approach as there  
are supposedly better methods available. What ever it was I used was quite  
successful but in my old age flourides can become chlorides without  
warning. Anyway, there is a good and simple while inexpensive product to  
use instead of phosphoric acid type materials if needed or wanted. Not  
that they don't work.

marv

On Sun, 30 Mar 2008 23:20:45 -0500, Chris Robison <chris at chrisrobison.org>  
wrote:

> m. edmund howse wrote:
>> Something I've used in the past on my 69 mustang turns the iron oxide
>> into an iron chloride.  It's very simple to apply and is almost
>> immediately paintable, but I've forgotten what it is. Then again I can
>> probably find it if there is any interest.
>>
>
> I don't know what will produce iron chloride as a coating, but dip a
> rusty steel or iron part in hydrochloric acid and the rust will be
> converted to iron chloride (ferric chloride) which is produced along
> with water and gets dissolved in the acid. Continue to introduce rust to
> the liquid, and I believe eventually you'll have nothing but ferric
> chloride in an aqueous solution (no acid left). I imagine it would be
> very similar to the nasty brown ferric chloride solution that you can
> buy in bottles at Fry's for etching circuit boards.
>
> In general for rust treatment, the two usual chemicals are phosphoric
> acid, and tannic acid.  Phosphoric acid leaves the metal with an iron
> phosphate coating, and tannic acid (the same stuff in tea and coffee,
> btw) creates a dark black iron tannate coating. By themselves, both
> require the rust to function, and both (especially phosphoric acid)
> should be thoroughly washed off after treatment, or they'll continue to
> etch the metal. These coatings also do not constitute a robust
> protection of the substrate and should be followed with something else
> like primer and paint, powdercoat, etc.
>
> To make this easier, some modern products include a primer as part of
> the formula. These seem to use primarily tannic acid instead of
> phosphoric in general. I speculate this may be because it's less
> reactive with the bare metal, so once the reaction is finished, it's not
> as critical to remove all of it, and whatever's left behind just remains
> in solution with the primer while it dries. At any rate, this is the
> action that's happening when a product like Rust Reformer changes color
> -- as the tannic acid changes the rust into iron tannate, the coating
> changes from white to black. When it's dry, the metal is converted,
> primed and ready to paint.
>
> Here's a page discussing tannic and phosphoric acid (and a bunch of
> other stuff, mostly relating to cleaning and preservation of found items
> from a saltwater environment):
>
> http://nautarch.tamu.edu/class/anth605/File10b.htm#Tannic
>
>
>    --chris
>
>
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