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Old 02-08-2014, 11:20 PM   #1
SAJ
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Default rust treatments,vinegar,phoshphoric,muriatic

WARNING. This turned into bit of a diatribe. I thought I would write a little bit about vinegar treatments since it is an oft-occurring topic in relation to both rust removal and radiators. Do not read on if too detailed for you.
I am not a vinegar expert, but I did qualify in chemistry (MSc.) and then engineering.
Vinegar is dilute acetic acid, varying from about 4% to 18%. Concentrated vinegar was available in England during my childhood years (1940’s) and my father bought it in 10 gallon glass carboys to let down with water for retail vinegar sales (he was a grocer). This was brown malt vinegar. I think the acetic acid concentration will be the most important property for rust removal, and white or brown would perform similarly.
I am not giving a chemistry lesson here, but acetic acid is an organic acid, made by fermentation of sugars to get ethanol, and then oxidising this to vinegar.
Anyway, Muriatic acid (spirits of salts or hydrochloric acid) and phosphoric acid are inorganic acids and fundamentally different from Acetic in what they do to iron and rust.
First, pure acetic acid is a thickish liquid often called “glacial” acetic acid. It is very corrosive to skin and clothes. When vinegar evaporates it becomes concentrated and has the same effect as glacial acetic. So bear this in mind when you get vinegar on skin or clothes (or in a gumboot as I have seen happen in industry).
Vinegar produces “acetates” with metals and, like most acid on metals , hydrogen can be evolved and this is explosive. If you see fizzing due to the circumstances you have created, be careful, because it is probably hydrogen gas. Some chemists report this and some don’t in experiments, due to the conditions they are using.
In heating vinegar over a long period, it will become more concentrated by water evaporation. Plus, each 10 Degree C (18 Deg F) rise in temperature roughly doubles the speed of a chemical reaction. So, hot vinegar treatment is a lot more boisterous than cold.
In a hot radiator, after all rust has been dissolved, the vinegar will carry on to attack the bare steel. Cast iron is less reactive than carbon steel water pump shafts. Copper and zinc (brass) usually more reactive.
Nearly all acetate salts (“salts” are the product of acids reacting with metals- nothing to do with table salt or sodium chloride) are water soluble. Ferrous and ferric acetates (iron acetates- either or both may form, depending on ratio of rust to bare steel etc) will therefore dissolve in the water rinse.
Muriatic acid is a stronger acid and forms chlorides with metals. It will make explosive hydrogen gas more easily with iron and attack steel faster than vinegar, particularly when hot. These chlorides (ferrous and ferric) are also water soluble.
Phosphoric acid is quite different, and forms insoluble phosphates with rust. These complex phosphates stick to the iron surface and carry on to protect it, as others alluded to in previous posts. Phosphotising, Parkerising, phosphating etc utilise this property.
This reaction product (metal salt) difference is quite important to the results you get, obviously. The phosphoric reaction will be greatly slowed when the phosphate coating forms, whereas the vinegar and muriatic reactions carry on until the acid itself is all neutralised by the metals it reacted with (or there is no iron left to react with!!).
In trying to separate rusted steel components like door skins, vinegar or muriatic should be more effective than phosphoric I imagine, because the resultant salts will dissolve away from the between the layers, whereas iron phosphates will remain there.
Incidentally ferric acetate is brown/red and dissolves readily in alcohol (ethanol) too. This can be useful to dry off the water and remove the acetates formed, while neutralising remaining vinegar to form ethyl acetate. Apply with a cloth though –don’t fill the radiator with it!
There are two other important acids that can be used. They attack rust in a gentler way by complexing (chelating) it if conditions are right. Oxalic acid (salts of lemon) is very poisonous and not available here in New Zealand as wood bleach because of this. It is great for removing rust stains (toilets sinks timber) too.
Citric acid is also very effective. It is a soluble powder and non toxic. The navy use it (or used to when I was a student) to remove rust on ships. Often in combination with other enhancers. I clean my radiators with citric in combination with other additives that would be harder to get for a DIY enthusiast.
I tried to keep this brief and it may be too detailed for some. It is from my memory and past experience and may not all apply under the conditions you create. I have not had time to research it. If your experiments prove different, then keep doing what you are doing, as long as it works.
Other chemists (Steve S comes to mind) may chime in and correct me or add to all this, since I make no claims to be always right.
I am not a rust expert. My field was lubricants, and automotive chemicals and I now concentrate on manufacturing for the composites industry (fibreglass resins etc).
SAJ in NZ
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Old 02-09-2014, 09:02 AM   #2
Jim Mason
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Default Re: rust treatments,vinegar,phoshphoric,muriatic

Great post! This is what i wanted to post but was too unsure of my cchemistry (pharmacist ny trade). Thanks...
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Old 02-09-2014, 09:19 AM   #3
Ray in La Mesa
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Default Re: rust treatments,vinegar,phoshphoric,muriatic

Thanks for the detailed insight about derusting. You have layed out some parameters for many of our projects, along with some safety aspects.
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Old 02-09-2014, 09:38 AM   #4
Growley bear
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Default Re: rust treatments,vinegar,phoshphoric,muriatic

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I use vinegar for removing rust and have had great results. I also noticed on my last project some foam formed on top of the vinegar. I use a plastic container. I had no idea that this could be hydrogen. I'll still use vinegar, but you can bet that I will be much more careful with the process.
Thank you for the great information.

Chet
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