Crystalline Al2O3 at room temperature is hardly soluble in any solvent, even concentrated acid or alkali, for kinetic reasons. Only amorphous Al(OH)3 may be soluble. Al2O3 only reacts with as strong solvent as melted NaOH.

aluminium  oxyde is soluble in the organic solvent  non polar (diethyl ether , ethanol (95 %))

It's soluble in most of the mineral acids:

for e.g.  Al2O3 + 6HCl = 2 AlCl3 + 3 H2O.

Aluminium oxide is definitely soluble in citric acid (a weak organic acid with the formula C6H8O7). But one cannot generalize since Al2O3 is insoluble in most organic solvents.

Can very low concentrations of acid work? Is there any threshold?

Crystalline Al2O3 at room temperature is hardly soluble in any solvent, even concentrated acid or alkali, for kinetic reasons. Only amorphous Al(OH)3 may be soluble. Al2O3 only reacts with as strong solvent as melted NaOH.

Would isopropylalcohol work and at what temperatures

Al2Ｏ3　can be dissolved in HF. i dispersed Al2O3 in different alcohol's for coating, i don't think it will dissolve.

You did not state which form of AL-Oxide you would like to dissolve:

gamma-Al2O3 is a hygroscopic powder (white and fluffy) insoluble in water, but in strong acids and bases.  Transition temperature approx. 800° to form unsoluble (in strong acids and bases as well) alpha-Al2O3. 

to be honest: From https://en.wikipedia.org/wiki/Aluminium_oxide:  

"Aluminium oxide is an amphoteric substance, meaning it can react with both acids and bases, such as hydrofluoric acid and sodium hydroxide, acting as an acid with a base and a base with an acid, neutralising the other and producing a salt.

Al2O3 + 6 HF → 2 AlF3 + 3 H2O

Al2O3 + 2 NaOH + 3 H2O → 2 NaAl(OH)4 (sodium Aluminate) "

 Insoluble in water, "Practically insoluble" in Ethanol, insoluble in diethyl ether. "

Found in: https://pubchem.ncbi.nlm.nih.gov/compound/aluminum_oxide#section=Melting-Point  , Section 4.32.6. Solubility:

Difficult solubility in mineral acids and strong alkali

Lewis, R.J. Sr.; Hawley's Condensed Chemical Dictionary 15th Edition. John Wiley & Sons, Inc. New York, NY 2007., p. 49  cited from: https://toxnet.nlm.nih.gov/cgi-bin/sis/search2/r?dbs+hsdb:@term+@rn+@rel+1344-28-1 (CTRL-F 'solubil').

Unrelated to serious research - I've bungee (shock cord, Occy strap)  "break-down" tent poles of aluminum.  Several section joints are stuck together.  Since the sections of the poles have been joined together for at least a year, and assuming that the offending agent is oxidized aluminum, Al2O3, I would appreciate suggestions on some mild solvent and temperature that might dislodge the pole joints.  Keep in mind that the inside of the hollow poles is a bungee material, probably of cloth wrapped elastic.  (I know how to prevent the stuck joints in the future:  clean, lubricate, and separate the pole sections immediately after use.)

would methanol do the job?

• Regarding the solubility of Al2O3 in alcohols, one person above has written that ethanol (95%) will dissolve Al2O3, but another has quoted a reliable source which reports that Al2O3 is practically insoluble in ethanol.
• Another person reports that he has dispersed Al2O3 with various alcohols.
• Finally, one person reports that citric acid will definitely dissolve Al2O3.

Though temperature and concentration may not be high enough, I will try lemon juice, which among citrus fruits appears to have the highest concentration of citric acid, up to 8% compared to dry fruit weight.  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2637791/

Alternatively I can try to twist and pull the pole tubes apart with tools, or buy a new collapsible pole if one is available for this very old tent.

I know this is a really old question, but if anyone finds this on google, aluminum oxide is a very non-reactive substance. Your best shot at dissolving it would be phosphoric acid, but that will dissolve the underlying aluminum as well, so be careful.

Also, Chromic acid will dissolve the oxide, but not the underlying aluminum, but you probably don't want to mess with that unless you know what you are doing.

The Aluminium oxide can be dissolved in ethanol with ph=12 that you can receive it by NaOH. And you can help Aluminium pourbaix diagram .

From my experience point of view, both gamma-Al2O3 and alpha-Al2O3 are insoluble in either acidic or alkali solutions, probably because of kinetic reasons.

Aluminium oxide is soluble in aromatic organic solvents. An aromatic solvent is a solvent that contains an aromatic hydrocarbon such as naptha, toluene or xylene. Aromatic solvents are primarily used as solvents and diluents in various industrial fields. Aromatic solvents such as toluene are commonly used in paints, varnishes, adhesives and as chemical intermediates.

Dear James Hollister, okay, there have 2 years passed since you posted your suggestions how you would start with your problem....had you have success in treating the " "break-down" tent poles of aluminum "?? [ see your answers [009] and [011] above]. IF you applied successfully some chemical or natural acidic solution (like lemon juice), would you mind to share it publicly? Thank you in advance, best regards, W.M.

The question " What is aluminum oxide soluble in?" Was a not exactly my question. That question appears to have originated four or more years ago. Apparently Research Gate has associated my question with Adhiraj Dasgupta's question. My question related to aluminum tent poles stuck together which have shock cord inside hollow aluminum tubes that are stuck together (See above).

Thank you all for your answers and questions. I do not have access to some of the recommended "solvents." Other suggestions may be entirely too reactive for the future function of the poles, especially high temperatures and/or HF or high pH NaOH. Others are clearly useless (alcohols). I can't remember if I tried lemon juice. I did not try mineral acids or any bases. I am aware of the amphoteric nature of aluminum oxide and what organic solvents are.

I opted to replace the item from the manufacturer.

Once again, thank you.

I agree with James Hollister

How do you descale Aluminum Alloys and what type of chemical is the best?

sir/madam

The aluminum oxide is soluble or not in formic acid?

Please kindly answer my question.