Page 30 - 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
• °Baumé (as an indicator of etchant viscosity)
• Free-acid (HCl) content
• Oxidation-reduction potential (ORP) and
• Dissolved metals.
With the ability to monitor and control such data, a constant etchant pool can be custom- designed. An optimum etchant composition can be formulated based on the quality of etched product obtained from the various etchant compositions tested. Optimised etchant compositions are frequently regarded as commercially confidential. However, controlled ferric chloride etchant variables for etching high quality products from metals such as low- carbon steel and AISI 304, 316 and 430 stainless steels have been published in the open literature [1(c)].
References
1. DM Allen, Photochemical Machining and Photoelectroforming, ISBN 978-1-5262-0188-1,
3rd reprint 2019, (a) p.62, (b) pp.106-7, (c) p.101.
2. DM Allen and HJA Almond, Characterisation of aqueous ferric chloride etchants used in
industrial photochemical machining, Journal of Materials Processing Technology, 149,
238-245, 2004.
3. DM Allen and LT Ler, The potential of oxygen for regeneration of spent ferric chloride
etchant solutions, PCMI Journal, 59, 3-7, 1995.
4. A Visser and M Junker, Influence of the hydrochloric acid content on spray etching of
stainless steel with ferric chloride solutions, PCMI Journal, 56, 15-20, 1994.
5. DM Allen and HJA Almond, Monitoring of Ferric Chloride Etchants, Final Report for PCM
Research Consortium #1, January 2001.
6. K Lauver, An overview of the operation of ferric chloride regeneration systems in
photochemical etching machines, PCMI Webinar, 26th May 2021.
7. DM Allen, HJA Almond and B Maynard, Testing of a LIGA-microspectrometer for
monitoring the dissolution of copper and stainless steel in aqueous ferric chloride solutions, Design, Test, Integration and Packaging of MEMS/MOEMS Proceedings, 483- 486, 2004.
8. DM Allen, PCM of nickel and its alloys and the measurement of Ni2+ ion concentration in ferric chloride etchant, PCMI Journal, 133, 18-32, 2019.
Issue 137 August 2021 PCMI Journal 30