Do I need to 'neutralise' iron/steel/metal with a base after removing rust with acid?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

As a side question, is there any recommendation on the best possible ways to prevent flash rust?

I own a company that builds parts washing equipment. The machines use water and alkaline cleaning solutions. They are bare steel on the inside and they don't rust even when they are dry so long as the solution pH is above 10/10.5. We have machines that are over 30 years old in the field with no rust. We have stored machines for months without rust so long as the wash solution dried on the bare steel and isn't washed/rinsed off. I suspect the Bicarb rinse is meant to raise the pH above neutral which will protect the steel somewhat better than leaving it acidic when it dries until a protective coating is applied. The pH of sodium bicarbonate is not very high - around 8.5. I would use NaOH as the neutralizer and make sure the final rinse ends up with a pH above 10. Once your tank dries it won't flash rust or show any signs of rusting for days.

Regarding water based corrosion inhibitors: In our shop we use water and 1000ppm sodium nitrate as the rust inhibitor for all our testing. It is effective at preventing rusting of the machines before shipping. The machines use hundreds of or even thousands of gallons of water. The steel sheets get wet and dry many times during our tests. Usually the shop people don't accurately measure the nitrate as it only takes a very small amount to prevent rust. If they see rust they add another couple of pounds to the test solution. The canning industry sodium nitrate to prevent rust in canning before the inside of cans are coated. It is also used in hotdogs and other meats as a preservative. It works best with solution pH of 9. Our shop is in the Midwest and only during the most humid months of July and Aug do we sometimes see light surface rust on our steel.