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October 17, 2017, 08:56:12 PM
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   Author  Topic: Rust removal using phosphoric acid  (Read 934 times)
Green and Gold
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Rust removal using phosphoric acid
« on: February 24, 2016, 08:54:10 AM »
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Another option for rust removal is via phosphoric acid.    It’s the active ingredient in products like automotive rust converter, commercial gas tank liner rehab kits, and certain metal pickling solutions.  (Phosphoric,  NOT  hydrochloric/muriatic)
If you’ve got a farm supply store nearby, Milkhouse brand sells a milk stone remover and acid rinse.  Label says it contains a 75% solution of phosphoric acid,  slightly more than ½ of the gallon is acid, and the other is water and dye for $12.50 or so per gallon.
Recently did a motorcycle tank, a few rusty tools and a test spark plug that had been left in the back of a sled for a decade or two.  All work was done in a cool 38 to 45 degree workshop in Feb; certainly a less than ideal time if the goal is to be quick.    Action of the acid is slow when cold; it took 4-10 days for the rust to be gone.  Process left a coating of converted oxide that needs to be flushed.   
Instead of using BBs, nuts and bolts, screws or the like to dislodge crud inside a tank, ice shards work wonders.    They are sharp and you don’t have to worry about removal as they just melt. 
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Re:Rust removal using phosphoric acid
« Reply #1 on: February 24, 2016, 11:17:08 AM »
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That's interesting, thanks for sharing.  Thats yet another chemical soak method.  I have yet to try the Oxalic Acid method. That will happen this summer.  I like the idea of scrubbing the inside of the tank with the ice shards. Only problem with that is where the heck would any of us find any THIS winter 
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Re:Rust removal using phosphoric acid
« Reply #2 on: February 24, 2016, 11:20:40 AM »
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Any rust removal tips is great for us fellows on this board. Rust is some thing we see plenty of and constantly deal with.
Al,  did you use the stuff straight out of jug or did you cut it more with water ??
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Re:Rust removal using phosphoric acid
« Reply #3 on: February 25, 2016, 03:56:13 PM »
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Bones: I was experimenting with the mixture/dilution ratios.  I tried straight from the bottle, a 60/40 ratio, and then a 1 (acid) to 4(water) in order to fill the gas tank to the top.    It all worked, even at the 1:4 ratio.    I kept a spark plug in the 60/40 solution for the entire 10 day period the tank was soaking and after all the rust was gone after 3 days not much happened to the non-rusted metal.    I also dropped a rusty screwdriver blade into the 1:4 (1 part acid, 4 parts water) mix and the rust was gone in about 3 days.  All of this was in a workshop where the temp was averaging about 40 degrees.  I imagine it might  be more aggressive at 70..... 

I used a commercial tank liner kit in the early 90s and they used a dilution rate of 1 quart to 3 gallons.    It did a good job removing all rust, but the lining material eventually blistered from the tank walls.   
« Last Edit: February 25, 2016, 03:58:27 PM by Green and Gold » Report to moderator Logged
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Re:Rust removal using phosphoric acid
« Reply #4 on: March 21, 2016, 07:34:40 AM »
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Update:
Someone gave me a well-used 86 Suzuki ATV and used the parts as a test for rust removal.  Put the 4 gallon acid mix into a small concrete mixing tub.  Soaked the ski plate and drum brake shroud which were over 90% rusted; chain guards, brake drum shroud, and trailer hitch support bar with large  rust patches; and a section of a gas tank shroud that was completely rusted through in spots in the used 1:4 solution.  Air temp was 60 degrees one day, 50 the next two.    All of the exposed rust was removed in 60 to 72 hours.  However there remained a few spots where the rust had started under existing paint.    Looks like it took a while for the acid to start lifting the paint. Either that or I didn’t get 100% of the surface grease/crud removed before immersion, although I did scrub all the stuff well beforehand.
IMHO, the best thing about this is it gets into all the nooks, crannies and pits that anything short of media blasting can’t reach.       
Acid strips anodizing pretty effectively too.       
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Re:Rust removal using phosphoric acid
« Reply #5 on: October 9, 2017, 06:57:02 AM »
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I knew there was a better acid to use than muriatic, which I have tried with dismal results. I have a very low serial number chalet (146!) to try this on. Thanks for the heads up! Rick
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Re:Rust removal using phosphoric acid
« Reply #6 on: October 9, 2017, 06:29:14 PM »
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I use POR-15 Metal Ready.

qr.por15.com/QR-assets/downloads/POR15/MetalReadyDescription.pdf

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