Before and after.

Note condition of exterior bare metal in second photo, upper right corner, after applying paint stripper, paint thinner wash, phosphoric acid, and paint thinner wash. After painting, bare metal was coated with RPM wax.

As for the interior, just immediately wash off Evapo-Rust with paint thinner, and carefully inspect for debris around the heads of the hub bolts. Those same hub bolts may be available from AMKproducts.com if replacement needed.

Can rust be thoroughly cleaned from inside of axle housing? It was sitting outside for years. Could it be cleaned of rust good enough to trust it not to contaminate my new gears, or should I not take a chance and scrap it and buy a cleaner one? Thanks.

Just find someone with a good caustic soda hot tank. Mine is 4x8 and ive dipped housings and they come out near like a new part in three days. Probably be easier and cheaper then your time and gallons of evap.

or a vinegar concoction in a large plastic bag, drum or ?

Just find someone with a good caustic soda hot tank. Mine is 4x8 and ive dipped housings and they come out near like a new part in three days. Probably be easier and cheaper then your time and gallons of evap.

That's hard to find in the northeast thanks to the EPA, there used to be a place with a big dip tank ... can't remember it nationwide chain ... that I had the frame to my 56 chevy dipped in back in the 80's

I'd go with granulated citric acid. Cap the ends and fill it up at 30:1 ratio. A 5 lb bag is enough for 18 gallons and should only cost around $20. It only takes a couple days for the rust to disappear. I recently soaked a deck lid and the results were great.

I have never used citric acid for rust removal.

When I googled it I found these comments from chemistry.stackexchange
........

Long, long time ago I was employed to formulate rust removers for iron. We needed to be concerned with toxicity, corrosivity, cost, waste disposal, effectiveness, speed, damage to the (uncorroded) metal, and flash rusting (rapid rusting once cleaner is rinsed off).

Rust isn't just one chemical compound. It is generally hydrates of FeO
F
e
O
and Fe2O3
F
e
2
O
3
(and there are a bunch of them, even if you exclude hydroxides).

So, since different acids will have different effects with different chemicals, you won't necessarily be able to find the one magic bullet for any and all types of rust. The stronger the acid, the more metal will be attacked, unless passivation is used (there's two types chemical and electrical). Amines are also often used. Strong acids Hydrochloric, sulfuric, nitric, are all more aggressive than the weaker organic acids. Naval Jelly (which I was involved with) uses phosphoric acid, but as you noted, it coats the steel/iron. These mineral acids can be dangerous and can't just be thrown out when finished.

There are alternatives. One trick is to get the chloride, sulfate, or nitrate into solution by using a compound other than the acid. For instance, adding sodium chloride to a solution of citric acid should make the citric acid more aggressive, and yet, won't change the pH. (One reason for this is that FeCl3
FeCl3
is more soluble than the iron citrate.)
Hint: one reason citric acid works pretty well is because the salt it forms is more soluble than something like iron phosphate, so it leaves the surface allowing further attack.

Some de-icers use nitrate or calcium chloride which may also be effective, and of course some fertilizers use nitrates, too (even some for indoor plants). So, if you'd rather not work with the strong acids (unless you're a careful person, you shouldn't work with them), you could try that approach.

As far as I know, the only other acid commonly used for rust removal is oxalic acid.
I think Zud cleanser uses it, if you can find it. It's a poison, so don't be licking your fingers until you've washed them (yes, even assuming you DID wear gloves!). I don't think it is much better than citric acid. In fact, I'd guess it's worse in terms of rust removal.

Nitric acid will probably be difficult to find/buy.
Hydrochloric acid is available (last I looked) at Home Depot etc. (Look in cleaning products, drain cleaners, and cement cleaners).

Sulfuric acid, too. That is also used as battery acid, but I'm not sure if you can buy that at an auto parts store without the lead in it. Do not buy anything with lead in it! Do not use anything (like old battery acid) with lead (Pb) in it. Do not touch...you get the idea, right? Finally, look at a table of pKa values for the acids.

So, effectiveness will depend not only on acid strength, but on iron salt solubility.
Ideally, you want to avoid any oxidizing acid
(since that will dissolve (oxidize) the metal)
but in reality, a bit of metal loss helps get rid of that stubbornly clinging rust.

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