Citric acid rust removal. How effective?

[chrishawley]

Having found out that generic milkstone remover is just as good as branded Jenolite as a phosphoric rust treatment another question arose as to the effectiveness of generic citric acid. Picture shows a heavily rusted piece immersed in citric acid solution, with a strength of about half a cup to a pint, for 24 hours at an ambient temerature of about 10 degrees C.  Rather exceeded my expectations but I don't happen to have any branded product with which to compare.

Might other members have an impressions as to whether 'generic citric' is just as good as markedly more expensive branded products?

 

[thescrapman]

I have used Generic citric acid in the past,  also known as catering product,  widely used for cutting drugs, nice and strong and Heat helps. 

Buy in 20,kg bags. 

[dougbgt6]

I have recently discovered white vinegar cleans limescale from porcelain. I quick scan of the Interweb  tells me it is equally good with rust. A gallon of vinegar and a cup of dishwasher salt to improve potency. Vinegar £8.50 on Amazon for 10 litres.

Doug

[JohnD]

The chemistry of rust removal is that any acid will convert iron oxide into iron 'acidate', in this case iron citrate.   A 'strong' acid, like sulphuric or hydrochloric will attack the native iron as much as the oxide, so 'weak' acids are used,usually organic acids like citrate.

The point about phosphoric acid is that iron phosphate is almost insoluble in water, so it remains on the part, while other acid's products will wash away.    So for heavy rust, as in the example above, use citrate/acetic etc first ( or electrolysis).

Once all the old rust is removed wash in clean water.   Flash rust will occur!  But a second treatment with phosphoric will convert that to a layer of phosphate which  will not rust any further and is  an excellent surface to cover with primer and paint.

John

[Chris A]

41 minutes ago, dougbgt6 said:

I have recently discovered white vinegar cleans limescale from porcelain

Not only but also, wherever there is scale. Here it is widely used as a cleaner, I buy mine in the supermarket in the household cleaning section in 5 or 10 litre 'bidons' at either 12 or 14 percent, its called 'ménager' to distinguish it from the cooking version. If you want to be posh there are even versions with a perfume additive 

I think, but not sure, some owners of classics flush the car rad out with it - must check up on that as I will be cleaning out and replacing the coolant in my 13/60 this year.

 

[dougbgt6]

I use washing soda for for the rad and cooling system, but don't leave it in for long, a couple of days, a couple of short runs. Washing soda eats aluminium, but there's nothing in there finely engineered to worry about.

Doug

[Unkel Kunkel]

Citric acid is cheaply available on line as food grade citric acid crystals.

From my experience, it’s effectiveness as a rust remover is temperature dependent in the sense that at 10C it is quite disappointing compared with very impressive results at 40 -50 C .( immersion heater) I have no experience of using higher temperatures.

The action is both to form Iron citrate and some chelation.

Ideally scrub the debris off the part you are de-rusting at intervals.

Clean off with de-ionised water and dry quickly with a hot air gun,  to avoid flash rusting , before the final stage of the process which is application/ immersion in phosphoric acid.

Phosphoric acid is available as Jenolite which is very expensive; also as Machine Mart’s Rust Remover which is much cheaper  (and I think  very similar)  or the solution dairy farmers use to clean their milking apparatus  milkstone disolver ( though check the ingredients - I recall looking at this some years ago and there seemed  to be formulations that aren’t phosphoric acid.

Don’t  wash off the phosphoric acid.After an hour or so, but Before it dries, re-apply it -then  dry it off with rags.Where it is black or has a white powdery deposit repeat the phosphate part of the sequence. You will need to do this part several times to achieve a stable phosphate anti rust layer.

Citrate is very much safer to use than say hydrochloric acid .The waste  solution Fe citrate and chelates is not considered environmentally damaging.

An alternative for inaccessible bits can be phosphoric acid followed by the various ‘ converters’ which are largely tannic acid based  - forms a blue black chelate which can be painted.Tannic acid is also cheaply available.The main hazard is that it stains black almost anything it comes into contact with - esp. fingers which will remain stained  for  days.

Evapo-Rust keep their list of ingredients close to their chest, but some regard it as citrate plus at least one chelating agent, possibly EDTA, buffered  with an alkali to give a near neutral pH.

Works really well - but not cheao.

 

[GrahamB]

As John states, The advantage of phosphoric acid is that it leaves a protective layer of phosphate.  I have used the Machine Mart rust remover which is phosphoric acid based and costs about £8 for a litre.

[PeteH]

Never deliberately used it on rust. But citric acid is my go to for descaling, more so as we are in a hard water area. The main advantage being that it does not attack stainless steels which many modern boilers and heat exchangers tend to be constructed of. I’ve also used it to clean copper quite successfully too. I buy mine as crystals in 2kg bags off ‘tinternet. I was considering using it to flush the cooling system cannot see that being an issue as it’s basically another heat exchanger. Phosphoric acid was used to descale the desalination plants on ships back in the 60’s. The degraded contents where flushed to Davy jones locker.

Pete

Edited March 17 by PeteH

[ahebron]

I used the juice of 12 lemons off our tree and an equal amount of warm water to clean some BMW motorbike wheels that proprietary cleaner wouldnt touch. It also removed the rust from the weights

[DanMi]

for heavily rusted components, I have successfully used cheap brick cleaner which I believe is HCL works very well, but it flash rusts fast after

[wimpus]

On 17/03/2024 at 13:17, dougbgt6 said:

I use washing soda for for the rad and cooling system, but don't leave it in for long, a couple of days, a couple of short runs. Washing soda eats aluminium, but there's nothing in there finely engineered to worry about.

Doug

I do this to for years, works so great !

I know a guy that left it in for 9 months 🫣

[daverclasper]

I have used Citric Acid powder mixed with wallpaper paste to form a gel (seems to need a minimal amount of water) for upright/areas that can't be removed from car. Best in summer temps. Also cover with cling film to stop drying out. Left it on for a few days.

Seemed to work better than Jelonite, etc

[Unkel Kunkel]

Hydrochloric acid,HCl,   as found in brick cleaner, known by  some by some by its old name of ‘muriatic acid’ , is cheap and works well -  particularly  on tools that are rusted and cement-encrusted, but it does have  some draw backs.

It is a strong acid and that means it can cause burns-  splashes  can cause eye damage.

It will cause flash- rusting ( unless the acid is removed or neutralised promptly after treating ) but it can also have a very annoying effect  on anything ferrous  within a 2 metre radius in your garage or workshop (from the very reactive HCL fumes, resulting  in red brown coat ping in a red rust. 

In theory, HCL  can cause springs and other high tensile steel alloys to become liable to  snap - because of of ‘ hydrogen embrittlement’.Certainly true of small springs and enough to deter me for use on road springs.

Citric acid is very user-friendly,  even benign , and has comparable results, - if used  warmed.

Edited March 19 by Unkel Kunkel

[Peter Truman]

A past friend here used to use a big container of Molasses it worked but what a mess to clean it up.

I used to use Swimming pool acid it worked, and there was no effect on the final paint finish. If I washed it off it quickly flash rusted