Page 17 - Demo

Issue 143 August 2024 PCMI Journal 17
PCM of Difficult-to-Etch Metals and Alloys: Nickel- and Cobalt-Based Superalloys
Emeritus Professor David Allen, Cranfield University
It is suggested that a difficult-to-etch material is one that cannot be easily etched
in a “standard” etchant of ferric chloride, i.e. one that will etch carbon steel or
copper. There is an historic (1980s) PCMI-generated, listing of metals and alloys
(Table 1) that ranges from easy-to-etch materials (Good) to difficult-to-etch
materials (Poor) [3]. The emboldened Inconel alloys and Hastelloy B are rated as
‘Good to Fair’, Udimet alloys as ‘Fair to Poor’ and Hastelloy C and René 41 as
‘Poor’. The ratings are somewhat subjective but broadly reflect the
Electromotive Force Series of Standard Oxidation-Reduction Potentials (ORPs) of
the chemical elements.
Table 1. Etchability ratings of some metals and alloys [4]
Good Good to Fair Fair to Poor Poor
Copper (rolled) AISI 215 Stainless steel Molybdenum Tungsten
Copper (electrolytic) AISI 301 Stainless steel Nichrome Hastelloy C
Beryllium copper AISI 302 Stainless steel Udimet alloys Titanium
Brass AISI 304 Stainless steel Vanadium René 41
OFHC copper AISI 305 Stainless steel Chromium Niobium (Columbium)
Phosphor bronze AISI 316 Stainless steel Gold Tantalum
90-10 copper/nickel AISI 321 Stainless steel Lead
Zinc AISI 347 Stainless steel Manganese
Carbon steel PH 15-7 Stainless steel Rhenium
Kovar PH 17-7 Stainless steel Zirconium
Nickel AISI 410 Stainless steel
Monel AISI 420 Stainless steel
Nickel silver AISI 430 Stainless steel
Magnesium Inconel alloys
Aluminium Hastelloy B
Anodised aluminium
As Visser has shown, to etch a material it is necessary to increase the etchant
concentration and/or increase temperature above its “retarding threshold” [5].
Thus, higher concentrations and temperatures are required to etch more
difficult-to-etch materials. In a ferric chloride etchant, the ratio of Fe3+/ Fe2+ must
be kept high to increase ORP . ORP is also increased when the operational
temperature is raised.
It can be confusing that Inconel 625 is also known as Alloy 625, Altemp 625,
CarTech 625, Chronin 625, Haynes 625 alloy, HPM 625, Incoloy 625, Nickel 625,
Nickelvac 625, Nicrofer 6020, Pyromet Alloy 625, Udimet 625 and VLX 625.
However, all the above are designated as Universal Number Specification (UNS)
N06625 (Table 2) and Werkstoff Number 2.4856 confirming the specific chemical