Page 20 - PCMI Summer Journal 2021

The Chemistry and Control of Etching Ferrous Metals with Ferric Chloride Solutions: The Concept of a Constant Etchant Pool Presented by: David M. Allen, Emeritus Professor of Microengineering, Cranfield University, UK
The chemistry of etching with ferric chloride
In the etching of carbon steel (predominantly iron), the metal is dissolved by oxidation,
i.e.
iron (Fe) + oxidising agent (etchant) → soluble ferrous (Fe2+) salt + reduced etchant
The oxidising agent in this case is ferric chloride (FeCl3) and the reduced etchant is ferrous chloride (FeCl2).
Chemically:
Adding the above two equations:
In other words:
Fe + 2Fe3+ → 3Fe2+ (overall reaction) Fe + 2FeCl3 → 3FeCl2
Fe → Fe2+ + 2e- (metal oxidation)
2Fe3+ + 2e- → 2Fe2+ (reduction of the ferric chloride etchant)
Chemical components of a commercial ferric chloride etchant
Aqueous solution of ferric chloride
The concept of a constant ferric chloride etchant pool is more complex than it appears. The simplistic concept is the diagram illustrated in Figure 1, where the etchant pool comprises only ferric chloride dissolved in water.
Figure 1. Ferric chloride dissolved in water.
However, when ferric chloride dissolves in water, it undergoes a natural hydrolysis, i.e. FeCl3 + 3H2O  Fe(OH)3 + 3HCl
Issue 137 August 2021 PCMI Journal 20