Using Sodium Chloride as an Electrolyte to Etch Aluminium
In theory this is what should be happening in this cell. But the gas coming off the anode does not have the typical chlorine smell.As electrolysis takes place it would be expected that sodium hydroxide (caustic soda) would develop in the solution and that this would further corrode the aluminium anode. The corrosion of the oxide layer would expose the metal to further corrosion from HCl as a result of the dissolution of the hydrogen gas.
With the level of corrosive potential in this cell and increasing levels of sodium hydroxide it would be expected that the anode would be very deeply corroded in a very short time and as the level of caustic soda increased so would the etching speed. This is not the case. After about 30 minutes a grey gel starts to appear that has been identified as aluminium hydroxide this appears to slow down the etching process.
As a potential explanation, It has been suggested that the salt (sodium chloride) turns into sodium hydroxide when the chlorine is released through electrolysis and this grabs the chlorine back from the aluminium chloride turning it into aluminium hydroxide, this is a gelatinous substance that slows down the etching process. The addition of a chelating agent to the electrolyte, such as EDTA or sodium gluconate, acts to slow down the formation of aluminium hydroxide. The voltage being used is very low between 1v to 0.25. From observing the activity of the bubbles very little gas is being produced.
Some Questions arising from these observations
What is the gas being produced at the anode if it is not chlorine is it oxygen?
What is being left in solution?
If sodium hydroxide is being produced is it likely to be produced in harmful levels.
Will the disposal of the solution require special treatment? Or can it be disposed of in the domestic system?
All of the by products from this process are in domestic use, sodium hydroxide is used for clearing drains, chlorine is used in swimming pools and by products from aluminium are used for cleaning and filtering the domestic water supply.