True rust removal by a chemist - acids compared 

Agreed. Bottom line: Hydrochloric to remove rust...Phosphoric to convert rust. I like to grind flush and then use phosphoric to convert rust that exists below the surface plane. Note, if you rinse phosphoric while wet you can apply epoxy primer. If you let it dry, epoxy will not adhere.

Please also explain the formula for (vinegar) acetic acid with iron and iron oxide, and the pros and cons for using it.

​@@Mavrik9000 both iron and iron oxide will forms iron acetate, similar to HCl but slower reaction. Pros: safe to skin and cheap. Cons: slower reaction and requires cleaning / neutralizing afterward.

@@evadevries2952 I have used Hydrochloric acid to remove the rust off the hood of my truck because I assumed acid was acid and I always use Phosphoric, anyway the hood rusted so bad in less than a year I had to redo it with Phosphoric acid

unlike with the other acids, the chloride ions can easily subsitute the iron oxide and change the cordination complex structure which then becomes more soluble. this is why you can add table salt to almost any acid to speed up its ability to disolve rust. not sure if the chloride ions can imbed themselfs the bare metal and migrate below the surface layer. boiling water actually converts the iron to a black oxide and is the least damaging way i know of to "remove rust" because you actually convert it into a passivized surface layer. when it comes to pasivation like with phosphoric acid, i believe you actually want the most concentrated solution possible. this is definatly the case with fuming nitric acid which is less reactive than dilute nitric acid because of pasivation

They are weaker acids, so what's the point?

Id usde hydrochloric, wash tat off then use phosphoric to convert any remaining...?

@@firmbutton6485 Yes, I think Phosphoric Acid is a good option.

@@firmbutton6485 Agreed.. Hydrochloric actually reacts through the rust (complex hydrated oxide of iron) and reacts directly with the iron, this causing the rust to 'fall off.' The phosphoric will react, albeit slowly, with the actual rust, and will also react with the iron to form a 'skin' of iron phosphate which is a blackish colour, and is semi-impervious. That is why it is the rust treatment.. but generally you need to remove the worst flaking rust first when using Phosphoric acid.

@@jdsmort Hi, sorry to bother you, but is it a good option to use hcl first, then phoshopic / acetic and then a zinc spraying to prevent the rust from coming back, or does the phosphoric create a protecting barrier already? Because the acetic certainly doesnt, atleast at white vinager concentrations ahah And how do you clean hcl / phosphoric acid off metal parts? Simply using water? Or is there a possibility can they react with it, despite the low quantity of acid already present on the surface?

@@embuscadochifreinexplicaca5960 Hcl is just Hydrochloric acid... and can be cleaned with water, If you want to neutralise it you can wash with a mix of baking soda in water... ends up making salt.... Phosphoric is a weak acid and can be just washed off.. with parts it is easiest to wash off with boiling water which then allows the part to dry quickly. Acids don't react with water, they just get diluted more.

I totally agree. This is my go to method for any type of conservation work. Where there is a lot of paint to come off with the rust, I used to use sodium hydroxide [lye] as the electrolyte. Works very well at removing paint and rust but careful as I was, I always ended up with some small alkali burns from splashes. I now use commercial paint stripper, followed by washing soda electrolysis

+Asep Saepudin good point. Phosphorus treatments are also often used to prime before painting. See for example my failing attempt to use POR - 15 paint directly over rust. The missing chemical was a phosphate primer

Ken Sweden I apply boiled linseed oil after acid has dried on the underside of my van an the rust never returns

Many folks are looking for what you mention, and spend big bucks on anti-rust paintings that are sometimes useless. If you by any chance are in northern Europe, and visit a historical site with old cannons, you may wonder how they preserve these iron cannons safely for generations. The answer is a combination of what you mention with lead oxide, only allowed with special permits at cultural restoration sites. I will go into paints and preservation in later videos. In your case the linseed oil has likely crystallized as an impermeable layer towards water and works even without the metal oxide. Great input, and very interesting. Thanks for your comment.

If you had prepped it with phosphoric acid prior would por15 have worked? or another way to word the question is phosphoric acid used to create a phosphate primer.

hopelessDiy ...possibly it would but the one component POR - 15 is still a one component paint so it does not cross-link like 2 component paints and is therefore always inferior in mechanical strength. This is the reason all fibre composite aimed for performance parts and airplanes etc always are impregnated fibres with 2-component thermosets. The thermosetting is the reaction between the components, making it insoluble, and rock hard. Sorry for late reply, missed this one somehow. Thanks for input and good comment. 👍

I will try this as a separate experiment. Thx for comment

Good input. Thanks for your comment!

Thank you for mentioning hydrogen embrittlement from - something I as an everyman will now be checking into. My plan had been to use a brief HCl bath to remove the bulk of rust from some old tools etc., rinse in water, then finish with a H3PO4 soak to convert the rest and protect the pieces. This is a process mentioned in the comments on another video as being used by a Biomedical Repair Technician, but as my application does include tools the embrittlement is a concern. I'd welcome any further thoughts on this. Thanks again!

A method I’ve heard should be great is to heat up the metal piece to almost starting to become red, and then drop it into motor oil. I see your problem with hydrochloric acid but you should not have that big problem with the phosphoric acid

Yes the chloride ions have a catalytic effect on the irons ability to rust. It is also true that phosphoric acid will leave a phosphorus iron layer on top of the acid cleaned iron. This layer is, however, extremely porous although not visible to the naked eye, meaning that water will penetrate though it in humid environment, and the rust will continue under the iron phosphor layer. It’s therefore always needed to create a barrier agains the humidity, by painting the iron phosphorus layer, for example. The more impermeable this layer is, the better rust protection you will have. Some people would advocate that you prime your part with a zinc primer, a paint that will let the zinc oxidize first, before the rust start forming. A top seal that really keep the water out is always the best protection. That’s why the best anti rust paints used on 17 century preserved cannons in historic castles are preserved with some tar like thick black paint. It is not for sale because it is based on lead containing formulations. I hope this helps, and have a good one!

....... off to collect a couple of million ants .... brb :

I think there is a classic amine someone mentioned here in the comments

Me too

Try oxalic acid.

And it isn't as polluting to the environment as Nitrates and Phosphates formed when using their respective acids.

Another top comment in 10 hours, bringing more information to the topic. 👍 As said here, the bubbles are from the pure iron reacting. I believe a concern that has come up is the possible hydrogen embrittlement that some YT watchers have heard about, especially for applications relying on mechanics strength. I have little quantitive information on this apart from that rust dissolves several orders of magnitude faster than the iron, and that some texts state that it is reversing on heating. If anyone wonders - lug nuts wheel fasteners and other similar applications - just exchange them to NEW ones.

Good point

Good comment. I will take one of the bolts and leave it in there for weeks. You will have an answer. However, in here the rust is so porous and sensitive to the acid that once the rust is gone, what you see is an almost dead reaction (judging from the gas evolution as bubbles)

I've used phosphoric acid for rust removal for over 30 years. I bought a gallon of 97% liquid and started experimenting with different concentrations of only acid, and then with the addition of 2-butoxyethanol, which I found listed as a component of "metal prep" used here in the US in autobody shops to treat surface rust. 2-butoxyethanol is a surfactant used in cleaning products. The saddest day of my experimenting life was the day after I put a brand new, but surface rusted clutch pressure plate and a perfectly functional but slightly rusty clutch cable for a VW-Porsche 914 in a 5 gallon bucket with 15% acid (the concentration listed on the commercial metal prep) and left it to sit over night. In less than 8 hours, the acid had completely dissolved all the thin steel parts. The spring steel that held the inner contact plate to the outer cover and flexed when the clutch fingers were depressed was gone, and the only thing left of the cable was the coiled housing, inner nylon liner, outer black rubber cover, and the cable ends. Don't leave thin steel in acid for hours.

You are on to something Andy. On my car sulfuric acid spilled off my battery onto its holder. It ate the paint off and created a lot of rust in the area. Now I'm using phosphoric acid to clean the area up and repainting.

Ken Sweden is there an update on this?

This has been tested by a youtuber somewhere and the answer is yes. If you leave it in for months then it dissolves the metal.

Coming up as a test. Best ideas gets evaluated in the lab. Thanks.

Good suggestion from RU-vidr above. Thanks. Here's the video: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-C08l2V7PjMI.html

Also works for cleaning mill scale off hot rolled steel.

A great comment. Thanks. Do you have any links on time-temp correlations for different carbon content? I see that both HCL and sulphuric are used , but as you said the sulphuric probably needs a bump in temperature. I did cut up a nail that I had in the HCL for 1 week and with that long exposure, that acid had penetrated as rays along what I believe is possibly some more carbon rich boundaries. In direction along the nail.

Can't say I have tried all the other acids, but tannic acid acts as a converter rather than a remover of rust ! The iron oxides [rust] are converted to ferric tannate which is a black compound, which is durable, very stable and deters further rusting. I use this on a regular basis in conserving old tools and artifacts. Tannic acid is very safe to work with and non poisonous - I know since I consume a lot in the form of common or garden tea. Tannic acid is used in many commercial rust converters.

Yes, that would happen as soon as rust is completely removed by any method. I think so? Flash rust is a reality.

@@facereader99 it will flash rust quickly after removal with hydrochloric acid, but a quick follow with phosphoric acid, will remove any rust that develops, and create a phosphate later on the metal, preventing any further rust for quite a while.

Yea flashing is going to happen pretty much with any method. Unless it has some kind of rust inhibitor in the formula

Thank you. I appreciate your kind encouragement!

@@Ken_Sweden can you mentioned the ph of HCL Acid and Glacial Acetic acid, thanks

My opinion, in that case, is to use only the required acid strength but to add some small stainless steel balls, about a coffee cup, and then let them work their ways around with the acid. 3-5 mm would do the job perfectly. If you have to go all the way to HCl, then you will inevitably have some flash rust forming when it dries, and so the chloride ions need to be removed. I would then flush it with baking soda (as many spoons you can add to a water solution while still having a good liquid) and finally, I would do a rinse with the phosphoric acid. this last step will give you very nice sturdy priming of your inner tank surface although if properly cleaned from previous rust, it will only be there as an invisible layer that prevents further rust from forming.

Dot Matrix sure you would 😂😂 But then you might die (or get very sick) of the iron overdose.

Correct for sulphuric and hydrochloric acid!

Is that true??? Now I am searching for a good rust remover for construction (TMT) steel rods . I thought hydrochloric acid is best to remove rust. But if Hcl weakens the steel , what are the alternative ways to remove the rust??? Please give a solution for my problem... 🙏

@@praja7491 Rust converter works best imho , works amazing. Citric acid works well if you want to clean small parts , I submerge them in a warm citric acid solution and in 2 hours they are ready 👌👌👌 I keep the solution warm with an electric heater

@@dimmacommunication 👍 I will definitely try it... Thank you... 🙏

Citric acid is a good choice or even better limonene based solvents.

Oxalis acid 👍

I compared the vol% for simplicity. Comparing molarities would require me to roughly half the concentration of the hydrochloric since the rest end up on about 5-7 moles per liter, which on dissociation into ions will provide different number of moles hydronuim ions in the solutions as they would all completely dissociate at these pH. The active ‘rust dissolving power’ is however also affected by the nature of the counter ions po43-, cl-, so42-, no3- etc and the possible formation of soluble or insoluble byproducts at the surface of the rust. The phosphate may form some insoluble iron phosphate while the different forms of the soluble chloride ions are very corrosive in themselves. The released iron ions from the mixtures of fe2O3 (and iron hydroxides formed as intermediates) also need to be transported away from the surface upon the dissolution, which makes the action (liquid movements) going on at the surface also affect the outcome. Now, the presence of hydrogen and formation of gas at the iron surface is probably similar when the rust has allowed the pure iron to be exposed so I would guess the bubbles and movements of the liquid to be similar as the shape of the nails are the same, although it may take different times to expose the iron and we are not really pressure washing the nails with the different acids... So my honest answer is that I cannot safely state that the outcome would have been exactly the same if I used half the concentration of hydrochloric acid and compared that with the same as used concentration of phosphoric acid, while heavily shaking the nails in a plastic reaction vessel. However, I guess is that hydrochloride acid will still be the most effective, even using half in concentration while the others may line up differently.

Amines? Interesting comparison, and I may do it for educational purposes.

Molasses, definitely

I can tell it's iron because I could see the crystalline structure on a weld burr I knocked off. The bowl was hand planished and the flanged edge was rolled. A flat open circular base was spotted onto the bowl with what looks exactly like just a dollop of molten iron 1.5 cm across. This "dollop" had a protrustion that I broke off because the weld makes the piece off-balanced. Based on the interior rust pattern it appears it was used as a drainage pan for a potted plant.

Oh, and I'm looking for a chemical solution because heating the whole piece with a blowtorch would produce spotty effects because of the large size of the object and the subsequent heat dissipation.

Jump to the acetic acid video I made on boosting the action of white vinegar by electrolysis. It’s somewhat not stated so much in this jungle of electrolysis videos but as you run the electrolysis in acidic solutions, the hydrogen that form will form on the pure metal. That means you will form it on the clean metal of your bowl under the rust and you will crack off all the rust above it. With the vinegar you are safe for most metals and most of the action will be by the elections merging with hydrogen ions at this metal surface since what is formed is hydrogen gas bubbles that will do the job for you. One could spice the solutions with stronger acids but if you would have zinc or aluminium or other metals somewhere under the rust they would react violently with the acid. Zinc would for example fizz bubbles immediately and strongly only immersing without electricity connected, it if there was hydrochloric acid used for spicing it up. Staying with acetic acid / white vinegar keeps you safe. In the end, if it can’t take that, then there was not enough metal left anyway. However, you have to make somewhere a good contact point into the metal since rust is a poor conductor. Thanks for great input!

@@Ken_Sweden I actually did watch your video with the acetic acid electrolysis after I had written this comment and it's great because it pointed out the BIG difference in conductivity between iron oxide and iron itself. I'm definitely going to do that (maybe even today) because it's the cheapest and quickest method. I just have to buy 10 liters of vinegar because I'll be doing this in my bathtub. I'll use ordinary road salt for electrolyte because that's also pretty cheap and I have a 25kg bucket of it already.

Sounds great, just stay safe when dealing with the large water contents and electricity. It’s the Amps that are dangerous so I always run little current since it doesn’t need much to generate the hydrogen gas from the acidic solutions. Ventilation and open bath is also good. Although I have not been able to ignite it myself.

He says 30 percent for each one

How'd it go?

@@SalvageDIYers it works well. I needed far less wallpaper paste than i thought so start with a little added and work your way up. Play around with the viscosity, something between honey and yogurt enough to cling and still be wet. May as well keep it full strength concrete remover too, faster. Nice thing is it goes green when its working. You have to keep reapplying/ brushing, scrub, rinse, dry, repeat.

Within 20-30 min no eating of the iron will occur but if you leave the nails in the acid for 1-2 weeks, then the acid will start working it’s way into the iron along grain boundaries in the metal. This makes the metal iron brittle, also the process is referred to as hydrogen embrittlement. You will only be able to see this if you cut up the nail along the nails, which I have done. The short time I show on,h remove the dimension thickness of the rust. Thx for comment!

Ben Norway wow that’s a fantastic answer. Thank you very much

Yes but since it was in solution it appears as most of the black iron phosphate formed as a powder that easily was rinsed of with water. Thx for input!

That's exactly what we used in the metal plating factory around 40 years ago, evil stuff when its cooking but strips metal bare FAST....which was what we needed to prep commercial metal parts for tin/copper/nickel/chromic etc

Thanks for feedback!

tannic acid in tea is also used for rust converter on cars

HCl

No that will happen much slower and via hydrogen diffusion into the iron , making it brittle.

@@Ken_Sweden thanks man

Yep, it interacts with air(O + N) just like rust removers turn from pale to dark when the P-acid eats the oxides.

What's the ideal concentration that works best? One that works better from the more diluted rust converters?

From experience I can say it depends on metal, temperature, etc.

@@danielberei3021 I'm guessing the cooler it is, the higher the concentration, and vice versa? Thanks

Well I haven't thought of that, but a higher temperature makes things faster I guess. Higher concentration doesn't always work. Some steels get a superb grey finish, the rust is easily removed on some, and others you can scrub all day and rust will still be there in a black layer(mostly on mild steels). If it's a black layer, it's paintable and it will last I can assure you that it won't flake. Done several car underbodies and painted them with hammerite garage door. It will last for yonks

The hydrochloric is killing the rust like no other acid but it seems to want to come back so check out my last video, just today, on how I do it. The phosphoric also lets a small amount of red dish come back but then I get it with the tannic acid. What I don’t say in the new video is that the tannic acid makes a fascinating surface that you see as only matt black in the video but in reality, if you zoom in on this surface, it’s like smooth and soft feeling when dried. I have had very good experience with this surface, every time, regardless what I paint, it always suck up the paint and levels the paint very evenly. Then the paint sits rock hard, of course best for epoxy, and even better for powder coated epoxy, I’m just amazed myself. It doesn’t matter which tannic acid you use, or rust converter, but it has to be that tannic acid... I have tried several brands and no difference, some are more yellowish. Just one this more, you see that your tannic acid is active when you brush it on, and for a split second your surface show as purple in color, very strange but that is always the same, then with more layers it becomes black as in the videos.

I think you are right. For more severe rust I nowadays use the hydrochloric, then I neutralize with sodium hydroxide toilet cleaner and in the end, if there is any signs of anything left in red brownish possible flash rust, then I do a final dip in phosphorus. Almost like you say. Thx for comment

+Jozsef Izsak yes, in the long run the action will be the same as long as you have an excess vinegar acid. Vinegar is only about 5% concentration of acetic acid so it might take awhile. Thx f good comment!

+Gabriel Erdelyi To be honest you won’t see much difference since the iron reacting with the phosphate will be full converted also with the 30% you see in the video. It’s always very much excess of phosphate to rust in the acid. It may go a bit faster but you would really have to time it to notice the difference. For your anchor chain you may have an issue because the iron phosphate forming is like a porous layer, a good paint base coat, but with limited mechanical strength. Not to be negative but I would guess the chances it wears off are quite significant. I hope this helps and all the best!

Google conversion coating and Mil en.m.wikipedia.org/wiki/MIL-DTL-5541. 🙏

+Robert Mintz excellent input!

Good input. I have to correct some aspects of your statements for our followers. The HCl is still bubbling as long the acid is reacting with the iron, and although here suggested that phosphoric acid would be different in terms of reactivity, it is wrong. The phosphoric acid leaves a porous iron phosphorous coating, which is black and covers your part however, since it is porous, the hydrogen is still reacting under the coating. This is meaning that if you leave your rusty iron part in the acid for 3 weeks, also with the phosphoric acid, the part would be completely eroded. You can try it, and you will find out this yourself. Now, phosphoric acid is a weaker acid than hydrochloric acid, so everything will take a longer time, but as with all acids, if you raise the temperature 10 degrees, that is equivalent to a double concentration of your acid, so the process can be speeded up. The maximum concentration of phosphoric acid is about 85%, whereas hydrochloric is 37%, so the hydrochloric reacts vigorously with water and even the water in the air. This means that everything above 37% will automatically go into a gas (read the concentration on the bottle, and you will find that there is nothing like 100% HCl). Be aware of this when you open a bottle because it will smoke out in your face as concentrated hydrochloric gas acid gas, while 85% phosphoric acid is a liquid, and no problem opening the bottle. In other words, extreme differences in reactivity. In practical terms, this means that your rust will dissolve at least 10 times faster with hydrochloric acid compared to phosphoric (rough statement and relative to the density of the rust you have). Now, the major benefit of phosphoric acid is that it will provide you with a good surface for painting. The phosphoric acid counter ions, i.e., the phosphorous ion, are not catalyzing further rusting as the chloride (called hypochlorite ion) will do. In fact, all hydrochloric-treated iron must immediately be neutralized completely on the surface, which could be difficult for a DIY person at home to verify. This means that in the worst case, you have hydrochloric remains left, and you paint over it, only to find that you have put an acidic rust-promoting layer between your paint and your iron part. Terrible, of course, so here one can find a good reason to stay away from the hydrochloric acid and instead go for the phosphoric acid. However, always expose your parts to minimal acidic conditions because every acid can bring hydrogen embrittlement of your part, meaning that it will fall apart and crack much more easily than a new part. Some studies say that hydrogen embrittlement can be avoided by heat treatment of the iron after acid treatment. In reality, at this point, most people would go and buy new things to replace the original item. Because, on top of the rust elimination process with acid, you would need a several 100s k USD microscope to verify if you have removed diffusive residual pathways in your iron (by any heating activities). You would also need to do mechanical testing if you are going to use your things for space missions. Cheers.

@@Ken_Sweden but most diy rust removal is either for show things, like a piece for a fence or something for on their wall. Or its going to be car bodywork, wich i dont really think hydrogen embrittlement would be a problem on unless you get a real bad case of it. But stil a great test here, something that really proves the effectiveness and not some corporate BUY OUR CRAP thing. And then some more words in the comments from you and other to make it even better. And fyi, there are rust removers out there that use HCl, i dont know the brand anymore. Seemed like a strange choice to me, and you kind of confirm that in this comment. It works really fast, but how do you know if you got rid of the last bit of it? You probably dont know. And i have seen what happens when you get something acid dipped and then painted and find out afterwards the guys that did the acid dip didnt neutralise it properly. Paint fell off and the thing was rusting faster then you could imagine.(dont know what acid they used tho)

thanks @@Ken_Sweden for the comprehensive reply. What I can say, from an empirical perspective, is that Phosphoric acid always gives me a great etch and long-lasting paint afterwards. Iron phosphate is barely soluble in water whereas both ferrous and ferric chloride dissolve easily and remain acidic.

Same as hydrochloric

Noan YoBiseniss Thanks for your comment. Hydrochloric acid at 98% is a gas, that’s why I had to normalise to 30%. Of course there are many more factors to consider than the activity of the hydronium ions. However, I am not fitting any experiments to data. The data were tabulated by other scientist long before me, and were done so correctly so they are reliable. that’s why knowledge about what they mean has a value.

When I get negative comments I check if I’ve mistaken on something, or sent out wrong info. If so, I would take down video but so far I have not needed to do that. Many thanks for kind words.

You mean the hydrochloride? If so, Yes I think that’s a good approach and you will avoid the flash rust

Stan Rodgers yeah that would take even some more effort to try out all the blends...

Still, it might be good for one thing only, and not other.. HCl still beats aqua regia for iron. However it also eats pure iron. To selectively remove rust but not iron, you need to look into things like absorbtion spectra and use tuned lasers, which are extremely quick in comparison.

My guess is gravity is not your friend as the vinegar will just run off. In this application you might want something that is a gel like naval jelly.

+Mr. Webguy is right. Would be nice to have something that made it more thick.

+Noan YoBiseniss You are right, phosphorus acid reacts w released iron ions, and a black layer will form on your piece. However that has to be dried if you wish to keep it as a solid, perhaps prime for painting. I rinsed it off and only focused on the acids ability to dissolve the rust. Thanks again for your input!

That’s a tricky one because the numbers of H are not the cause for their difference in corrosive nature. Even if I’d do as you say (I assume you meant) and take 2 moles of HCL to compare with 1 moles of h2SO4, the outcome would be even more inline with the results shown. No acid would be as corrosive as the muriatic acid. Anyway, thank you for the input raising some important critics. Makes me wonder if you actually could better see these differences if everything was done in an ice mix...

@@Ken_Sweden what about HF? It seems more corrosive than HCl

I think that's because concentrated phosphoric acid is very viscous as undiluted. For the acid to function it needs of course to release the iron atoms into ions from the rust but also transport the dissolved liberated ions away from the rusty surface. Now in the case that the transportation away from the surface doesn't work, it will end up as a volumetric zone that wont have the same low pka acidic value, acting as a weaker acid, and therefore not work as good. I am not 100% certain about this but it's a suggestion off the hip, knowing that boundary conditions often have effect on diffusion. Many thx for your comment. With a reference to your comment, I would better be able to answer.

@@Ken_Sweden Hi Ken! I think, i know what you saying. I have very rusty rims, and now, going to add some water in my acid, thank you very much, and God bless..! 😊

@@rikujkoivisto just do : acid ADD to Water. That's the rule Number 1. Good luck

@@Ken_Sweden Order was not the point, but thank you anyway!

@@Ken_Sweden Some people have to learn the hard way. 🤭