This is not the typical chemistry question on this website, but I think it's an important practical question.

When I got a gas string trimmer, the woman who worked at the gardening shop told me that E10 (fuel commonly sold in the USA - 10% Ethanol, 90% gas) left sitting in a gas tank for months would be harmful. So I wonder, is this true? And if so, what about for cars and other engines (boats, tractors, etc).

This effects my life because I was considering at some point getting a plug-in hybrid car because during most weeks I would never have to use or buy fuel. However, could I be damaging this car by leaving the same fuel inside the tank for months?

There are various claims about ethanol either helping or hurting things, but I have not seen any supported by science. What is the chemical basis of these claims? A layman translation is appreciated.


Well first off, pure ethanol is hygroscopic; it attracts water, to the point that it will pull it out of the air. Ethanol and gasoline will mix, but ethanol, gasoline and water will not; the ethanol-water mixture will come out of solution and settle on the bottom of your tank. Add a little oxygen to the mix, and you get rust. However, the more common side effect of this is more immediate; turn the car on, and the fuel pump will draw the water from the bottom of the tank into the engine, where it will promptly kill it, and require a costly dry-out process.

Down here in the south, where it's hot and humid most of the year, you hear a news story or two about this every year, usually when a gas station didn't properly purge its storage tank on a regular basis. The regular addition of thousands of gallons of fresh ethanol fuel "recharges" this hygroscopic process, causing a rather large pool of water to form at the bottom of the tank which the station is supposed to drain regularly. If they don't, and you fill up when the store's tank is just a little too low, or just a little too soon after the tanker's refilled it, the pump will draw a water-gas emulsion, or even straight water, into your tank, and the water will settle to the bottom.

The usual fix for long-term storage of ethanol fuel is to add a stabilizer, which will contain a high concentration of isopropanol. The main thing that will do is form an azeotropic mixture with water, meaning that its components will evaporate at the same rate, so when the isopropanol evaporates, it takes the water with it. This same mixture, in sufficient concentration, will also burn, so while it's not the greatest thing for your engine it can at least be flushed out the normal way.

Second, gasoline, as you probably well know, isn't a pure substance; it was originally what was left over after the refinery had extracted the "useful" heavier compounds (kerosene, diesel fuel, lubricating oil, paraffin waxes) out of crude oil by fractional distillation. Nowadays it's much more carefully purpose-made by thermal decomposition of heavier alkanes, but it's still an amalgam of relatively lightweight carbon compounds (usually between 6-12 carbons, with the dividing line for "octane rating" drawn between heptane and octane), plus additives and pollutants. It contains three components of interest; sulfur (a naturally occurring pollutant and difficult to remove completely; "summer blend" gas in the U.S. is more expensive because they have to get rid of more of it to prevent smog and acid rain), oxygen (modern fuels are oxygenated to make them more clean-burning), and ethanol (added to make the fuel burn cleaner and be more sustainable to produce than neat gasoline).

In the presence of water (pulled in by the ethanol) and oxygen, sulfur naturally forms sulfuric acid by a variation of the "contact process"; it oxidizes easily to sulfur dioxide, then more slowly to sulfur trioxide, which when dissolved in water becomes sulfuric acid. This reaction, specifically the formation of sulfur trioxide, is sped up in industrial processes by heat and by contact with catalyst metals such as vanadium oxide (hence "contact process"), but it will also happen on its own without any special treatment (i.e. acid rain).

This sulfuric acid will do a lot of corrosion damage, reacting with the tank's sheet metal producing hydrates of iron sulfate. When burned, those sulfates decompose into iron oxides and sulfur dioxide again, which do more damage to your exhaust system on the way out (rust is a catalyst for more rust, and we've discussed sulfur dioxide's contribution to the party in detail). In addition, sulfuric acid is even more hygroscopic than ethanol; at concentrations in water as low as 10%, it'll attract more water. Lastly, acids and alcohols form esters, in our case ethyl sulfate, and esters and alcohols form ethers, here diethyl ether (with the sulfuric acid and water as a byproduct). These are reversible and ultimately circular reactions, which will form an equilibrium of roughly equal concentrations of the intermediates. A dehydration reaction to produce ethylene is also possible in very high acid-to-water concentrations of sulfuric acid (possible when filling up in the winter, when it's dry and the gas has more sulfur). Ethylene will form a number of intermediate products with just about anything in that gas tank, some better for your engine than others (ethylbenzene is an antiknock compound; methyl ethyl ketone is a solvent and a pollutant byproduct of combustion engines).

  • $\begingroup$ Do you have any thoughts on how long this process takes after getting a "fresh" tank of gasoline? Is this process made worse by higher ethanol mixtures? So should E85 be even worse for engines and tanks than E10? $\endgroup$ – chrishiestand Mar 8 '13 at 2:20
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    $\begingroup$ Given that E10 has been quite common in California and other states for years now, I'm surprised I haven't heard about it causing even more problems. I also wonder if car manufacturers have figured out ways to counter-act these problems. $\endgroup$ – chrishiestand Mar 8 '13 at 2:21
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    $\begingroup$ This is a great answer and a fascinating read. $\endgroup$ – Richard Terrett Mar 8 '13 at 3:40
  • $\begingroup$ Well, water in the tank has been a rather common problem around me, especially at cheaper stations. Usually though, it comes from actually pulling water (or a suspension of it, churned up by the filling tanker) from the gas station tank, where this hygroscopic change happens on a scale several thousand times that of your own fuel tank. Your better stations avoid problems by routinely draining the tank all the way, water and all, during the switch from summer to winter blend and back. $\endgroup$ – KeithS Mar 8 '13 at 15:32
  • $\begingroup$ The part about the isopropanol azeotrope is nonsense. (Yes of course water would form an azeotrope with it, also with ethanol, and even with the hydrocarbon. But that's totally irrelevant, because nothing evaporates here.) Isoprop is simply a compatibiliser between water/ethanol and hydrocarbons. $\endgroup$ – Karl Nov 27 '16 at 11:43

Actually this paper explains the water ethanol gasoline mixture effects very well. If the ethanol will harm the equipment is more dependent on how the machine was designed. The ethanol can deteriorate carburetor parts the gasoline ethanol comes into contact with if the manufacturer hasn't replaced them yet. Rust is not likely from humid air. http://epa.gov/OMS/regs/fuels/rfg/waterphs.pdf

  1. Methanol additions to gasoline in freezing climates may form a second phase in long storage. It will corrode the heck out of magnesium which is used in small portable engines.
  2. EPA sulfur restrictions now require most refinery streams to be put through hydro-treaters (high pressure H = costly) to remove sulfur. Nothing to do with summer/winter fuel.
  3. The E10 will not separate a second phase unless exposed to the atmosphere to pick water. So not a problem in a closed tank. I have a portable generator that has sat for as long as a year with no problem.
  4. In 35 years of refinery corrosion experience, I never heard of a service station tank being drained except for replacement.
  • $\begingroup$ Hello 👋 and welcome to StackExchange. Have a look at the help center to learn how this site works and what type of questions are allowed here. It's good to see you contributing to the site o your first post. Keep it up. $\endgroup$ – Pritt says Reinstate Monica Jun 3 '17 at 4:22

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