[ausev] Paint question

[Chris Robison]

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