<VV> Muriatic Acid Vs. Phosphoric Acid

Tony Underwood tony.underwood at cox.net
Mon Oct 3 00:31:53 EDT 2011

At 10:41 AM 10/2/2011, Harry Yarnell wrote:
>I've used muriatic acid diluted 50% with water.
>It won't remove grease or oil, but WILL remove rust. If left unattended, it
>will continue to eat the steel. Nasty stuff, so be careful.
>Wash with water, but treat it as soon as it dries, as what you just
>de-rusted, will start rusting immediately'
>Purchase at building supply stores, as it's used to wash brick walls.

The stuff is simple HCL, aka hydrochloric acid, cut with water to 
form ~28 % solution, depending on whose brand you buy.   It's a 
ferocious rust eater but actually isn't THAT bad on nonoxidized iron 
or steel.   Its assaults on nonrusted ferrous metals isn't bad enough 
to worry over if you wash off the stuff once the rust is 
gone.   Harry's right about treating the derusted stuff immediately 
or it WILL start to flash rust quickly since the acid leaves the 
metal absolutely vulnerable to oxidation again.

It's nowhere close to being as rough on steel as nitric acid... which 
will positively destroy steel faster than you can imagine but ignores 
aluminum etc.   HCL isn't so bad on steel but is sudden foaming 
sputtering fuming horror movie destruction on aluminum and other 
nonferrous metals.   A mixture of nitric acid and hydrochloric acid 
will dissolve glass, gold, and just about everything else except for 
certain plastic compounds, which is what the stuff is stored in when 
it's not being used to destroy something hard and metallic.

HCL is very useful as a derusting tool.  I've been using it forever 
with good results, but be careful with the stuff and make sure you 
stick with muriatic acid solution and not pure HCL which IS 
dangerous.   The pure stuff will dissolve flesh fast enough to make 
it HURT quickly.   ...damhik.

I mix a bit of dishwashing detergent with HCL solution to help it get 
past grease and oil which soaks into the rust pores and keeps the 
acid from doing its thing.

Likewise, pickling the work piece also helps a LOT in prepping it for 
acid derusting.  A bath of sodium hydroxide (aka Lye in water) will 
strip paint, grease, etc off most stuff but be careful not to get any 
of it on aluminum or copper or brass or zinc etc (in other words, 
stick to iron-steel for pickling).


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