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I've never asked a question here before but these forums seem to have some extremely knowledgeable people so I thought I'd give it a try. I know it's a 1st grade question but I honestly don't have the first idea about this. So, I have a motorcycle, the fuel tank was completely full of rust, extremely bad. I've finally managed to get the tank clean by using a combination of things, mainly white vinegar (at least I know this is acetic acid 5%). I got this idea from watching many Youtube videos and reading online articles. I'm now at the stage where I need to get the vinegar out and clean and dry the tank (preventing as much flash rusting as I can).

My question is... without exception, all of the resources I've seen/read have said "after removing the vinegar, it's important to neutralise the fuel tank with baking soda/washing liquid" - I was planning to just follow this as it sounds good! This is apparently to 'stop the vinegar reaction/ return to a safe ph level'....I've seen how quickly flash rust takes hold and I'm wondering if using baking soda is really necessary. Surely it would need to be in the tank for a few minutes at least for it to do anything? - What exactly is this doing? Looking into this I've also read that baking soda and vinegar will create salt, this is concerning as I'll then need to make sure any salt is thoroughly removed. I could understand the idea to say, neutralise the vinegar itself before disposal (maybe?) but does the vinegar have any lasting effect on the steel after it's been removed?

As a side question, is there any recommendation on the best possible ways to prevent flash rust? I've seen/read many ideas which I think probably work to varying degrees of success, mainly to get the tank dry as quickly as possible then coat it with an oil/wd40/fogging oil/kerosene. I've just discovered water based corrosion inhibitors and was thinking these might be the ultimate best option, so after going through whatever process I need to, as a final step I would rinse with a corrosion inhibitor?

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4 Answers 4

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You might want to migrate this question to Motor Vehicle Maintenance and Repair SE for a more practical and less theoretical range of answers.

That being said, plain water is sufficient for rinsing out any remaining acetic acid. (It's a gas tank, not lab equipment.) However, unless you take further steps to stop the tank from rusting while in service, your efforts will all be wasted. The tank rusted because of water in fuel, most likely due to condensation of water from humid air as the tank "breathes". You now have bare, clean steel with no protection from future rust.

Forget the oil / kerosene / what have you. Its protective effect will last only until the first fuel-up, when the gasoline will dissolve it.

I suggest that you coat the interior of the tank with a product that is designed for the purpose, such as POR-15 Fuel Tank Sealer. I've used it. It works. It will completely coat the inside of the tank with a hard polymer coating that is impervious to fuel and water, and it will prevent any future rust.

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  • $\begingroup$ Your suggestion of the interior sealer is a good one. A tank that was "completely full of rust, extremely bad" is a leak waiting to happen. $\endgroup$ Mar 11 at 15:02
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    $\begingroup$ In case of future link rot, the "this one" link above refers to POR-15 Fuel Tank Sealer. $\endgroup$
    – MTA
    Mar 11 at 19:19
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    $\begingroup$ Thank you MTA, very useful and practical advice. That's exactly what I was thinking, The main reason for the question was due to thinking about how to avoid as must flash rusting as possible, if I have to rinse it again after using the sodium bicarbonate then it's an extra step maybe unecessary, delaying the drying process and inviting flash rust. I think a small amount of flash rust won't really be a problem. I was planning to use a tank sealer however I've heard a lot of horror stories about them pealing and clogging up the fuel filter, that's what getting the rust out is supposed to avoid $\endgroup$
    – Darren
    Mar 11 at 21:05
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    $\begingroup$ @Darren That is correct, flash rust or any light, sound rust is not a problem for the product I mentioned. Stories of coatings that peel off are most likely due to improper surface preparation: either trying to coat flaking rust or coating over an oily surface. When the surface is properly prepared, this stuff can only be removed by grinding it off. $\endgroup$
    – MTA
    Mar 11 at 21:35
  • $\begingroup$ @MTA I do suspect that poor preparation and pre existing heavy rust may be the reason some people have had poor results from using a tank sealing product. I think you've allayed my fears and I'm going to give it a try. If they really bond to the tank that well, then that's a perfect solution $\endgroup$
    – Darren
    Mar 11 at 22:23
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Apparently you mean the interior? The reaction of sodium bicarbonate with acetic acid is very rapid, slosh it around then pour it out. Being damp it will tend to get a very thin rust on it quickly unless coated with an oil as you suggest. I would not try to used corrosion inhibitor, that is a whole different science. Gasoline is not very corrosive to steel, it is stored in steel tanks in refineries and shipped in steel pipelines with no problem (Refinery tankage does rust on the bottom because water settles out of the gasoline causing corrosion.). I would think any rust would not be a problem because it should be caught by the filter. I have not looked for years but steel gas tanks were normally lead plated (terne plate) or galvanized. If you do this again, I would suggest "conversion coating", phosphate compound such as Naval Jelly. That leaves a bit of a protective coating on rusted steel.

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    $\begingroup$ Hi blacksmith and thank you very much for your answer. Good point, I do indeed mean the inside of the tank. That's useful to know that I can just slosh it around and then get it straight back out. I'm still wondering what this is actually doing? I know the sodium bicarbonate will react with the vinegar but there will be minimal vinegar remaining in the tank. Is the intention to make sure the vinegar doesn't mix with the fuel that will be going in or is the purpose to restore the steel somehow? I'll definitely think about other solutions and look into Navel Jelly if I ever do this again. $\endgroup$
    – Darren
    Mar 10 at 22:00
  • $\begingroup$ It's called Naval Jelly, not Navel Jelly. :D BTW, a lot of carbonated drinks such as Coca-Cola also contain phosphoric acid and can be used as makeshift rust removers in a pinch. Just make sure to use the sugar-free kind if possible, to avoid the risk of gunking up your fuel tank with leftover sugar residue. $\endgroup$ Mar 11 at 15:42
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Rust is a spot phenomena. It does not proceed on the whole surface of iron and steel plates. Rust starts at and around surface microcrystals of "impurities". When started, it goes on around the same microcrystal, digging deeper and deeper at the same point. The metallic surfaces more than about $1$ millimeter away form the spot are nearly not oxidized. As these cracks and clefts may be thin and deep, but contains acidic solutions, it is difficult to remove these acidic residues by simple washing with water. It is much more efficient to use bicarbonate powder that may enter the cracks. Corrosion will be stopped. And if too much has been used, it won't bother the metallic piece.

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  • $\begingroup$ Hi Maurice and thanks for your very in depth answer. So to summarise in layman's terms, the purpose of the flush with sodium bicarbonate is to remove/neutralise any remaining trace of acid, and is NOT to counter any effect the acid had on the steel? $\endgroup$
    – Darren
    Mar 11 at 10:50
  • $\begingroup$ @Darren. You are right. Bicarbonate neutralizes any remaining trace of acid. $\endgroup$
    – Maurice
    Mar 11 at 16:46
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    $\begingroup$ I'd say the "trick" with the bicarbonate for removing the acidity is that it doesn't need to enter the cracks - it will remove acid equivalents in the bulk solution, and protons are much faster transferred from anywhere (including the cracks) compared to ions actually moving by diffusion. However, I'd think OP will want to get rid of the remaining electrolytes as well. Meaning they anyways need to rinse thoroughly and wait for the stuff to diffuse out of the pores. $\endgroup$ Mar 11 at 17:12
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As @MTA already pointed out, you will get moisture again into the interior of the tank, and therefore need to apply an appropriate coating.

The conclusion from that wrt. to your question of how to prepare/rinse the tank now is: you need to follow the instructions of the actual sealing product you use. My guess would be that it likely requires the tank interior to be dry and free of any oil/fat/grease, and may suggest to use certain primers first. And, even if not specified, you'll also not want to have residual acid nor salt.

Some considerations:

  • Bicarbonate ($\ce{NaHCO_3}$) removes residual acidity by reacting with the acetic acid producing sodium acetate and $\ce{CO_2}$. The $\ce{CO_2}$ "contains" the acidity and as gas can quite easily be gotten out of the tank. This reaction is very fast, since it does not require diffusion of the (large) acetate ions but only proton transfer.
  • However, you'll have salt left (both excess sodium bicarbonate and sodium acetate) which you want to rinse out: in case of future moisture they'd give you a more corrosive mixture than only the pure water.
    Rinsing is most efficient if you do it often (smallish amounts of water each time are sufficient, but you want to be sure to reach all the inner surface area). This process requires the salt ions to travel (diffusion), so is somewhat slower than the acid-base reaction above.
  • (Personally, I'm not sure I'd have used acetic acid in the first place)

  • Also, it may be worth while considering to use the high vapor pressure of the excess acetic acid (dry in a warm place, ideally with a ventilator) rather than introducing more electrolytes via the bicarbonate. You'd still need to rinse afterwards to remove acetate, though.

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  • $\begingroup$ Thanks cbeleites, future rusting is the biggest concern and there doesn't seem to be any 'perfect' solution to that. It seems most people just keep the tank fully topped up with fuel but when the bike is in use, it's hardly practical to keep refueling after every journey. The tank sealer would be ideal but I've heard many people say they wish they hadn't used one as the ethanol eventually breaks through and causes the seal to peel and clog up the fuel filter, so I'm in two minds about that. I probably wouldn't use acetic acid again. I just wanted the rust out and I know nothing about chemistry $\endgroup$
    – Darren
    Mar 11 at 21:20
  • $\begingroup$ @Darren, rust will also clog up the filter... make sure the product is suitable for gasoline tanks, if it doesn't clearly say so, ask whether that also applies to high ethanol content. However, if you seal in any moisture, the tank will go on rusting. If that happens, there is no way to prevent the "peeling", because you could also say that the tank peels away from the sealer... So make really sure that the interior is entirely dry and properly primed. $\endgroup$ Mar 11 at 22:13
  • $\begingroup$ Yes exactly, but it would be rather annoying if the product that was intended to stop the rust forming, and then clogging the filter gave you the same or a worse problem. MTA has mentioned the tank sealers are solid if used properly with proper prep as you say. I think the complaints about them may source from improper preparation. I think I'm going to give it a try. $\endgroup$
    – Darren
    Mar 11 at 22:39
  • $\begingroup$ @Darren: IDK about the relative risk of clogging the filter, but a sealant should have a much lower risk of fuel leaks, as another answer pointed out. You already know that a lot of the metal in the tank wall is rusted away, so further rust could go all the way through. $\endgroup$ Mar 11 at 23:57

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