Surface Finishing Tutorial

#031 Electrolytic Etching - Electrolytic Machining - 2

Category : Metal etching

February26, 2010

(4) Current efficiency and process speed of electrolytic machining

Processing speed of the electrolytic machining can theoretically be obtained using Faraday's law of electrolysis. However, it is necessary that the following conditions are met.

(1)	Etching produced ion valence is confirmed.
(2)	The only electrolytic reaction is the elusion of metal at the anode.
(3)	Metal is removed only by electrolytic elusion, and not by collapsing. Metal is not to collapse in lumps.

In general, the actual metal removal speeds may not match the calculated theoretical values. This may often be so because one or more of the conditions show above may not be fulfilled.

The electrical current efficiency can be calculated by the following equation.

Current efficiency(%)= (Actual metal removal amount x 100)/Theoretical metal removal amount

This represents a percentage of current actually used for intended purpose out of the total current flowing through the electrolyte.

In actual practices the amount of actual metal removal amount may be larger than the theoretical value, and the current efficiency may seem to exceed 100%. The reason for this may be...

(1)	The actual metal ion valence eluted was smaller than the theoretical value used for the calculation.
(2)	The metal may have experienced some anodic collapses. That is, instead of the metal electrolytically eluting as individual atoms, it collapsed in atomic clusters.

On the other hand, if the actual metal removal amount may be smaller than the theoretical amount, and the current efficiency may seem lower than 100%, the following may be the cause.

(1)	The actual elusion may take place at larger valence than the theoretical.
(2)	There may be sub-processes other than the electrolytic elusion of the metal that may be consuming the current.

One of the most problematic sub reactionary processes is anode gas generation. The overvoltage level of the anode increases as the current density increases. When this overvoltage increases enough and the anode's electrical potential reaches a point to allow oxygen generation, a part of the electrical current will be consumed for this gas generation.
As can be seen so far, increasing the anode current density to hasten the machining speed past a certain point will result not only in metal elusion but also gas generation, and the etching efficiency degrades and machining speed decreases.

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