Page 29 - 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
process is stopped. Hence both ORP, [Fe3+]/[Fe2+] and etch rate are kept within a narrow range controlled by the pre-set limits.
Dissolution of alloy steels
So far, the constant etchant pool concept has only been discussed for PCM of carbon steels but with alloy steels, for instance 18/8 stainless steels, metals such as chromium and nickel will also dissolve in the etchant and be detectable in solution as hydrated Cr3+ and Ni2+ ions.
Spectroscopic analysis of the etchant can be used to measure [Ni2+] on-line and at low cost[7]. [Ni2+] in ferric chloride etchant needs to be maintained < 15-25 g/l (dependent on etched metal) to ensure that the etched surface remains smooth and does not become rough and outside manufacturing specification [8]. This monitoring technique can also be utilised to measure concentrations of other heavy metals for environmental compliance when waste ferric chloride etchant needs to be landfilled at end of life.
The dissolution of 18/8 stainless steel is relatively straightforward to analyse. Only Fe3+, Fe2+, Cr3+ and Ni2+ metallic ions are found in solution and the uv/visible absorption spectra of these ions do not overlap considerably in the absorption spectrum monitored (Figure 11).
Figure 11. The final composition of the constant etchant pool.
Conclusions
The chemical complexity of a FeCl3 etchant requires on-line measurements of the following variables for monitoring and control of “a constant etchant pool”:
• Temperature (should be kept constant ± 0.5°C)
Issue 137 August 2021 PCMI Journal 29