Page 22 - Demo

Issue 143 August 2024 PCMI Journal 22
PCM of Difficult-to-Etch Metals and Alloys: Nickel- and Cobalt-Based Superalloys
Emeritus Professor David Allen, Cranfield University
Figure 1. Etched Inconel 617 showing (left) passive surface and (right) active
surface after etching for the same time (Photographs courtesy of Heatric Division
of Parker).
Alternatively, Inconel 617 should be etched in higher Bé, concentrated ferric
chloride solutions and at higher temperatures if possible.
Etchability ratings of Inconel alloys according to analysis of the published data
are therefore as follows: Inconel X-750 (easiest to etch with an equivalent
etchability to AISI 316 stainless steel for example), followed by Inconels 600, 601,
617, 718 and 625 (the most difficult to etch). It is therefore desirable to
formulate suitable etchants with higher oxidising power to etch Inconel 625.
Such etchants may be found in the chemical milling industry applied to
manufacturing of aerospace parts from a wide range of nickel-based superalloys
(Table 7). These etchant formulations can include aqua regia, an aggressive
etchant that can etch gold and platinum, and the extremely dangerous
hydrofluoric acid (HF) with its associated Health, Safety and Environment
impacts [15].
General Motors, USA [16] patented an etchant comprising “…an aqueous
bath…consisting essentially of 14% to 18% of ≈42° Baumé ferric chloride
solution, ≈42% to 52% concentrated hydrochloric acid, ≈0.5% to 4%
concentrated nitric acid, all proportions by volume, and antimony trichloride to
at least substantially maintain surface smoothness.” This etchant was used to
etch Hastelloy C-276, Waspaloy, René 41 and Inconel X-750.
North American Aviation, USA [17] patented use of a ferric chloride aqua regia
type etchant comprising nitric acid, hydrochloric acid, ferric chloride, chromic
chloride and a surface-active wetting agent. This etchant was used to etch
Hastelloy R-235, Hastelloy X, Waspaloy, René 41, Udimet 500, Udimet 700 and
Inconel 700.