There are a lot of simple ways to soak off rust. Here is a comparison of several of these. All the items used in these tests were totally covered in rust before treating.
On the left one part molasses to six parts water for one week. On the right citric acid in water for two days.
This is after one day in distilled white vinegar.
All of these items where too rusty to just rub the rust off with some turpentine or penetrating oil.
Vinegar makes little bubbles while it works.
Metal comes out with a black residue that rubs off easily.
This rubs off and you have a flat grey finish.
So now you have the basic idea. Soaking in some mixtures will remove rust faster than steel is removed. With just a little work, the rust can be rubbed right off.
Make sure that the metal you treat is fully submerged. The borderline between wet and dry can be eaten into and pitted badly by most of these treatments.
So here are a few notes on the different methods. Please be aware that while some of these materials start out being food safe, they can be used to produce materials that are not food safe. Also be aware that some food safe materials can cause issues when skin is exposed to them. I prefer to use food safe materials, but have learned, due to a very serious and painful injury caused by apple juice, that care must still be taken.
I am not kidding about the apple juice. I have scars.
Vinegar is food safe and easy to get. It will help clean and sharpen files. Vinegar will leave a grey finish that is a type of rust crystal that adds a small degree of rust resistance. A lot of old kitchen knives have this finish. I usually leave stuff in vinegar overnight.
Citric Acid is also food safe and used to pickle. I get mine in a 7 oz bag for $3 at an Indian or Pakistani Grocery.
I add enough water to the citric acid to let it fully dissolve. Then I drop rusty metal in it. Rust may take less than a day to be removed.
Molasses is slow, may take a week or so. I mix one part to 6 parts of water. Molasses won’t remove as much steel if you leave it in the bath too long. It seems to be the most gentle on steel.
Phosphoric Acid is one of my favorites, and is considered one of the milder acids available for cleaning cement. Some hardware and some office supplies will stock it. When I use it, it performs a double service. First I use it to remove zinc from steel that I plan to weld or use in my forge. Zinc vapors can cause horrible organ damage so I remove it before I use scrap that is galvanized.
After Phosphoric Acid has removed zinc it becomes a very valuable rust remover. As it removes rust it parkerizes the steel and lends it a touch of rust resistance and a flat grey finish. After using Oxpho Blue on parkerized steel, the result is quite rust resistant and scratch resistant black steel.
The top and third from the top tools in the picture below were treated that way.
Phosphoric acid stops bubbling when the rust is gone and the steel is treated.
As a clarification, All of these recipes use materials that are not going to be bad environmentally, diluted and in the quantities a home user is likely to use. However, in some steels there are elements that can be less healthy when removed from a solid and impermeable matrix. Some toxic metals can become much easier to absorb after acidic interaction. As an example, nickel salts are carcinogenic.