Page 18 - PCMI Journal December 2020

Manufacturing Bespoke Aesthetic and Functional Surfaces via Photochemical Machining, Electrolytic Photoetching and Electrolytic Photopolishing Presented by: David M. Allen, Emeritus Professor of Microengineering, Cranfield University, UK
material is etched away, the half-etch surface becomes smoother as the corrosion-resistant oxide layer on the surface is removed, exposing the true alloy metallurgy underneath.
It is possible to explore the effect of the etch-resistant oxide layer on stainless steel by growing the oxide layer thickness artificially through passivation, for example, in nitric acid. By examination of Figure 1, the difference in half-etch surface finish when shallow etching non-passivated and passivated stainless steel can be easily seen [1].
Figure 1(left) SEM (x300) of slot etched 28 μm deep in untreated hard-rolled AISI 302 stainless steel and (right) SEM (x300) of slot etched 27 μm deep in strongly-passivated hard-rolled AISI 302 stainless steel.
The high surface roughness of an initial shallow etch (Peak Ra) is demonstrated in Figure 2a. As etching proceeds deeper into the metal, the surface roughness reduces to a value that is termed the Characteristic Ra. As shown in Figure 2b, the Peak Ra and the Characteristic Ra merge when the etch temperature is ≥ 50°C [2].
Figure 2. Effects of (a, left) depth of etch and (b, right) etch temperature on surface finish of AISI304etchedin40.7°BéFeCl3 with0.27%freeHCl[2].
Issue 136 December 2020 PCMI Journal 18