Short notes on: Electro–brightening

Electro – brightening-

Electrobrightening/electropolishing is the process by which we submerge a cobalt chrome framework in an acidic solution then run an electric current through the alloy to remove the outer layer at a microscopic level. This procedure creates a high shine on the surface of the alloy. Immediately after sandblasting a Co/Cr alloy substructure it is important that we don't touch it's surface again with our bare hands. This is because the residual grease on our hands will adhere to the surface of the chrome and prevent the electrolytic action. The result would be dark "spots" on the surface of the chrome.

To electrobrighten a Co/Cr substructure, suspend the chrome on an anode (here, a non-reacting cathode stainless steel hook) submerge it in the electrolytic solution, turn on the current and set the timer.

How does electrobrightening work? The electrolyte and Co/Cr substructure act as anodes (+) and a non-reactive stainless steel plate submerged in the electrolyte acts as a cathode (-). When an electric current is applied to the electrolyte it acts as a conductor and allows the current to pass through it to the cathode plate, creating an electric circuit. Metal ions (atoms or molecules with a net electric charge) on the surface of the metal framework now become positively charged (+), oxidise and are attracted towards the negative cathode (-). Electric fields naturally focus on microscopic "peaks" (micro-projections) possibly because of higher current densities in these areas. This means more material is taken from the "peaks", and less from the "valleys" (micro-depressions). This is referred to as anodic levelling (see 5 & 6 in Fig 20 below). The result is a smoother, more reflective surface which appears brighter to the eye (even though as little as 5-10 microns of metal are removed).

A by-product of oxidisation is the formation of oxygen bubbles along the surface of the framework which also contributes to the cleaning process by removing microscopic debris. 

A current of about 2.5 Amps for 6 minutes is sufficient for the average RPD alloy framework.

However, this may vary depending on the temperature and age of the electrolyte (warm electrolyte is quicker than cold and old electrolyte is less efficient). A low electric current will result in an "etched" surface and a high current rapidly reduces the thickness of the alloy. CHECK THE ELECTRIC CURRENT!

An example electrolyte solution could be made up of:

1. Ethylene glycol: 80%
2. Sulphuric acid: 15%
3. Water: 5% (Strananought, 1974, p215).