First we'll look at the word:
The word "electrolysis" can be broken into two parts: electro and lysis. 'Lysis' means to break down, or break a bond, to "tear apart". 'Electro' roughly means electric or electric current, electricity. So the word electrolysis is a combination of two terms and means to break a bond or take apart using electricity. I believe they are Latin terms, lysis coming from a Greek word meaning to "release". So it is literally breaking a bond, or releasing by means of electricity - using an electric current to take something apart. In this case, we do want to take something apart - we want to break "rust" down and get good iron back.
Rust is a combination of iron and oxygen. In the process of electrolysis as I use it, we use electricity to break the bond between oxygen and iron atoms. This is a very simple explanation, if you wish, you can find more complex details in science books, or on the Internet, etc. There is a lot more that happens than can be explained here, but these are the basics. Atoms are partially made up of electrically charged particles - electrons with a negative charge and protons with a positive charge. It's these electric charges that cause certain atoms to want to "stick together" in certain combinations. Oxygen and the iron in the steel stick together to form rust. Electrolysis will help to take them apart again. Atoms can lose or gain electrons. This gives them a positive or negative charge because there is no longer a balance between the number of electrons and protons in an atom. Electrons can be "bumped" from one atom to another. The solution made with water and washing soda is simply to make a safe, simple solution that will allow the electricity to work through it and not interfere with the process or produce chemicals that would be unsafe. Oxidation is a chemical reaction where something gives up electrons. Reduction is when something accepts electrons. Oxygen likes to be "reduced". When oxygen is reduced, or accepts electrons, it makes oxide, O--. If we put oxygen together with iron metal, the iron is oxidized (gives electrons to the oxygen) and the oxygen is reduced (accepts the electrons lost from iron). The result is one form of rust, ferric oxide, Fe2O3. Whenever something is oxidized, something else must be reduced. Electrons must come from somewhere (oxidation), to go somewhere (reduction). With the process of electrolysis, we are going to reverse the process to "break the bond" between metal and oxygen using electricity.
We will use metal rods called "electrodes" to help in this process. The electrodes will be submerged in the water and washing soda solution. One electrode will be called the "anode" because it will be caused to have a positive electric charge. The anode is hooked to the positive wire of the battery charger. The positive wire accepts electrons. If the positive wire is accepting electrons something is losing electrons (oxidizing). When 12 volts is applied to the anode, water is oxidized at the anode surface and gives electrons up to the anode. The product is oxygen. The bubbles you see coming from the anode are oxygen that resulted from the oxidation of water. The other electrode is called the Cathode. In our case, the part we want to remove the rust from will be the cathode. Cathode is the word for an electrode with a negative charge. The cathode is connected to the negative wire of the battery charger. The negative wire supplies electrons. So something must gain electrons at the cathode (reduction). Two things are reduced at the cathode, water and the rusty iron. The reduction of water produces hydrogen. The bubbles coming from the cathode are hydrogen gas. (The fuel for the space shuttle is hydrogen and oxygen. Rust electrolysis should be done in a well ventilated area so that explosive concentrations of hydrogen and oxygen are not reached.) The rusty metal takes on electrons and is no longer attracted to the oxygen atoms, and the bond is broken. The rust is "reduced". If this process is done on a large scale (for example, the trailer frame shown on my pages) a lot of water is lost to "reduction" in the process and needs to be replenished. A lot of oxygen and hydrogen are formed. This same process is used to keep a supply of oxygen in submarines. The hydrogen is disposed of for safety reasons.
During electrolysis the rust turns from orange to black. In most cases, the
rust next to the iron is reduced to iron metal. This reduced iron will form
a somewhat porous layer of new iron on the metal object being cleaned. After
electrolysis the iron object will rust very quickly unless it is protected
because this porous layer of new iron has a high surface area. The rest of
the rust may reduce to a variety of compounds depending on the compounds in
the original rust and the details of the electrolysis. Typically the black
stuff that can be rubbed off after electrolysis is a mixture of iron metal
and magnetite, Fe3O4 , an oxide of iron. Magnetite is an intermediate product
in the reduction of rust to iron metal. It is the black stuff in magnetic
So once your parts are cleaned or removed from the solution, you'll want to rinse and brush them off to remove the loose iron, dry them quickly and completely, and protect them with primer or other rust preventative.