Rust Removal by Electrolysis - A Detailed Illustrated Tutorial, page 9
2. How To Make a Simple Electrolysis Machine: Anode - Sacrificial Positive Electrode p9
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2) Anode - Sacrificial Positive Electrode
In electrolysis, an electrode connected to the charger's positive terminal is called anode. The anode is also called "sacrificial" or "waste" electrode because it can be expected to disintegrate and visibly decrease in size during the process. The choice of a conductive object to be used as an anode in electrolysis should be considered carefully. Below are the items I used as anodes in this tutorial: a stainless pipe fragment, a stainless tea spoon, and a 1/4-inch mild-steel plate.
Sacrificial or Waste Electrodes - Anodes
To show you which metals or metal alloys should be used for the waste electrodes, I compiled a list of proper and improper metals and metal alloys that are used for anodes in electrolysis. Just keep in mind that electrolysis reverses the electro-chemical reaction of rusting, and, therefore, anything besides iron contained in the anode's metal composition will go into the electrolyte and more or less affect the process.
Another thing to consider is that, during electrolysis, an electroplating process takes place, through which the rust is being plated onto the anode and creates an non-conductive barrier. Thick layer of accumulated rust on the sacrificial electrode nearly stops the current flow through the electrolyte. So any metal that allows lots of rust to stick is a liability.
PROPER AND IMPROPER METALS AND METAL ALLOYS USED FOR ANODES IN ELECTROLYSIS:
1) Platinum is corrosion-resistant, does not dissolve in electrolyte producing toxic substances or contributing alloyed metals to the cathode, and does not allow rust to stick - no cleaning or attention is required during electrolytic de-rusting. Platinum is the best choice for the anodes if you can get your hands on it.
2) Graphite is an electrically conductive mineral (one of four structural modifications of Carbon) named in 1789 for its use in pencils. It is corrosion-resistant, does not dissolve in electrolyte producing toxic substances or contributing alloyed metals to the cathode, and does not allow rust to stick, so no cleaning or attention is required during the process. Graphite is much cheaper and more accessible (scrap and worn pieces can be bought on eBay) than platinum; thus, being next to the best choice!
3) Stainless Steel (contains ≥10.5% chromium) is corrosion-resistant due to high chromium content, and does not allow much of rust to stick, so it requires less cleaning. Stainless steel does not visibly deteriorate during electrolysis but it slowly dissolves chromium into the electrolyte producing two chromium compounds: trivalent chromium and hexavalent chromium. While trivalent chromium is relatively harmless, hexavalent chromium is toxic, carcinogenic and poisonous.
HEXAVALENT CHROMIUM is a highly controlled environmental pollutant! Under most electrolysis conditions the amount of hexavalent chromium generated is small so the acute effects are not generally a problem provided one takes normal precautions when using the stainless steel anodes in an electrolysis setup. To avoid both environmental and personal health damage, stainless steel anodes MUST NOT be used if the electrolytic de-rusting is performed on a large scale!
4) Mild Steel (≤2.1% carbon; low alloys: ≤0.40% copper, ≤1.65% manganese, ≤0.60% silicon) deteriorates, rusts, and allows rust to stick - regular cleaning (once in 3-4 hours) by a wire brush is required to deliver current during electrolysis. Overall, even though mild steel is not as efficient as stainless steel, it is a good choice for the anodes as it is cheap, environmentally safe and readily available (it is widely used for automotive repairs). 1/8" or 1/4" mild steel plates are the most suitable. Various shapes of anodes can be cut out of the mild steel sheets to suit any size and shape of an object to be derusted and to fit into a container with electrolyte.
5) Galvanized Mild Steel is mild steel coated primarily with zinc (zinc actually permanently becomes a part of steel). Galvanized steel also can be coated with nickel, copper, etc. If zinc and/or other coat metals get into the electrolyte, they will form compounds which, in their turn, will contribute alloyed metals to the cathode - some plating of the iron object being de-rusted may occur. The galvanized steel anodes SHOULD NOT be used in electrolysis.
6) Brass anode will deposit copper onto the iron object being de-rusted due to anodic dissolution; thus, accelerating rusting of the object. The brass anodes and anodes of other copper alloys SHOULD NOT be used in electrolysis.
7) Aluminum quickly deteriorates and allows rust to stick during electrolysis, so frequent cleaning and attention are required. Aluminum anodes SHOULD NOT be used in electrolysis. For example, aluminum foil is absolutely out of question as it will fast disappear within a few minutes of the process!
1) The golden rule of electrolysis states: the sacrificial anode must be equal (or exceed) in area to the piece being derusted. And this piece must be surrounded by anodes (see more details in the "Line of Sight" section on page 20).
2) Mild steel plates are often coated in a film of oil, and this protective layer should be removed with soap and water to get rid of any surface contaminants.
3) Make sure that each steel plate fits around the interior of the container, and part of each steel plate protrudes above the electrolyte level to enable a connection to the power supply to be made.
4) Use steel plates with rough surfaces, rather than smooth, preferably placed a little upward, so the oxygen gas bubbles could free themselves easier, not clinging to the anode causing it to be non productive.
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