Everything below is stole from other websites. There are about 20 or so sites on the subject.
How can I dispose of the solution?
The bath will last until it gets so disgusting that you decide it is time for a fresh one. There is nothing especially nasty about it-it's mildly basic-so disposal is not a concern, except you may not want all the crud in your drains.
Why you should not use stainless steel electrodes for electrolysis
Many people using the electrolysis method for rust reduction swear by stainless steel, stating (incorrectly) that it's not consumed, stays clean and seems safe.
Stainless steel is indeed consumed when used in the electrolysis process, although slowly. The main problem with using it is the hazardous waste it produces. Stainless steel contains chromium. The electrodes, and thus the chromium is consumed, and you end up with poisonous chromates in your electrolyte. Dumping these on the ground or down the drain is illegal. The compounds can cause severe skin problems and ultimately, cancer. Hexavalent chromate is poisonous. These compounds are not excused from hazardous waste regulations where household wastes are.
These compounds are bad enough that government regulations mandate "elimination of hexavalent chromate by 2007 for corrosion protection."
Does your electrolyte turn yellow? That's a sign of chromates
If you have been using stainless steel for the anodes (positive electrodes), wear rubber gloves when working with or near the liquids. If you need to dispose of it, allow it to evaporate into powders and dispose of the powders in sealed containers during your local "hazardous waste clean-up days".
Best bet - don't use stainless steel no matter how tempting it is.
Rust is a complicated material. Typically, it is a combination of ferrous and ferric oxides, hydroxides, and hydrated oxides and some of these compounds may be present in several crystal forms.
There is much speculation in the chemical and archeological literature about the products that form when rust is reduced in sodium carbonate. In searching for an answer, people may find a lengthy publication on the DENIX web site ( https://www.denix.osd.mil/denix/Publ...4-IRON-2.html)
. Much of the electrochemistry described is not correct and the conclusions drawn about reduction products are not in agreement with most chemical literature. It was not until 1996 that some chemists from the Swiss Federal Institute and Brookhaven National Lab did definitive work on this subject (see papers by Virtanen in J. Electrochemical Soc. 1996 and 1999). Using a sophisticated X-ray technique they determined what was going on at the cathode when iron oxide is reduced. Normally reductions occur in solution. That is, something has to dissolve before it can be reduced. However, they found that iron oxide will conduct electrons and therefore can be reduced without going into solution. This process is referred to as solid state reduction. The ferric iron atoms in the rust begin to reduce to ferrous oxide, which initially results in a mixture of ferric and ferrous oxides. This combination is called magnetite and is often written as Fe3O4. Eventually, all the ferric oxide becomes ferrous iron. Under less powerful reducing conditions the product would be ferrous carbonate or ferrous hydroxide. However, under the extreme conditions of reduction powered by a 12 V battery charger, they found that ferrous iron can be reduced all the way to iron metal. All this chemistry can occur without any of the iron going into solution. So, based on this work, when we see the rust slowly turning black, we are seeing the formation of Fe3O4 which is black and eventually iron metal, which is also black. Finely divided iron is black, not shiny like a solid chunk of iron. All this work was done under laboratory conditions.
We wanted to find out what happened when a rusty plane iron was reduced in a bucket. We did reductions of heavily rusted iron object in sodium carbonate under conditions normally used from cleaning rusted objects. We used either a 1 or 5 % solution of sodium carbonate and a 12 volt battery charger and continued electrolysis for about 2 hours. The iron piece was dried under an oxygen free atmosphere (nitrogen). The loose black deposit on the iron surface was removed by sticking it to a piece of tape and it was analyzed by X-ray diffraction. We found that the deposit was magnetite. No iron was detected and no ferric oxides were detected in the black material that readily came off on the tape. Therefore, under our conditions, all the rust was reduced, but the reduction of what had been loose rust did not proceed all the way to iron metal. Perhaps it would have if we had continued electrolysis for a longer time. We had no way of determining whether the rust at the surface of the iron object reduced all the way to iron. We expect that at least some iron was formed at the surface, because after reduction the iron surface rapidly forms red rust (ferric oxide) if it is not quickly dried. Magnetite does not rapidly rust, but finely divided iron will form rust in just a few minutes if it is wet. We conclude, based on our work and that of Virtanen, that rust reduction under the conditions normally used for cleaning, results in the formation of magnetite and possibly some iron metal.
The other chemistry that occurs is the electrolysis of water. At the anode water is oxidized according to this equation
2 H2O = O2 + 4H+ + 4e-
The H+ formed is quickly neutralized by the carbonate to make carbon dioxide. So, some of the bubbles at the anode may be carbon dioxide as well as oxygen. At the cathode water is reduced
H2O + 2e- = H2 + 2OH-
It is important that any copper connector to the anode not touch the solution. If it does, copper will oxidize to cupric ion, Cu++. The connector will be destroyed. Most of the copper ions formed should precipitate as copper carbonate or copper hydroxide, but if any of this dissolved copper reaches the cathode it will be reduced to copper metal on the iron object. Its presence will promote rapid rerusting.
I have not heard of anyone claiming that the washing soda and iron will form a toxic by product. Solutions made from something other than Sodium Carbonate would cause a problem. Do you have a link or a resource you can post?