Page 21 - Demo

Issue 143 August 2024 PCMI Journal 21
PCM of Difficult-to-Etch Metals and Alloys: Nickel- and Cobalt-Based Superalloys
Emeritus Professor David Allen, Cranfield University
Table 5. Chemical compositions of various Inconel alloys.
Alloy Ni
balance
Cr % Mo % Co % Nb+Ta
%
Al % Ti % C % Fe %
Inconel
600
72 min. 14-17 0 0 0 0 0 0.15 6-10
Inconel
601
58-63 21-25 0 0 0 1-1.7 0 ≤ 0.1 11-16
Inconel
617
44.5
min
20-24 8-10 10-15 0 0.8-1.5 0-0.6 0.05-
0.15
0-3
Inconel
625
58 min. 20-23 8-10 0-1 3.15-
4.15
0-0.4 0-0.4 0-0.1 0-5
Inconel
718
50-55 17-21 2.8-3.3 ≤1 4.7-5.5 0.2- 0.8 0.65-
1.15
≤ 0.08 18.5
Inconel
X-750
70 min.
incl. Co
14-17 0 1 max 0.7-1.2 0.4-1 2.25-
2.75
0.08
max
5-9
Table 6. Practical results of etching Inconel alloys in ferric chloride solutions
Inconel
alloy
Results of etching in ferric chloride (s= spray, i = immersion)
[FeCl3]
(M)
°Bé °C s/i Etch rate
(μm/minute)
R
a
(μm)
Ref.
X-750 2.91 38 45 s 5.075.07 2.15 [9]
600 2.91 38 45 s 4.24 2.00 [9]
600 4.32 ≈49 65 i Etch factor optimisation [11]
601 4.93 ≈53 47 i Process optimisation [13]
617 3.39 43 50 s ≈50 when retarding threshold at 50°C is exceeded [14]
718 2.91 38 45 s Pits [9]
718 3.70 44.5 55 i 0.20 [12]
625 2.91 38 45 s - [9]
Perhaps surprisingly, considering its 8-10% molybdenum content, Inconel 617
can be spray-etched in 3.39M (43° Bé) ferric chloride at 50°C but care needs to
be taken when etching large volumes.
As the alloy is dissolved, [Fe3+] drops below the 50°C retarding threshold required
for the alloy. The alloy surface therefore becomes passive (Figure 1). Because
there is very little iron in the alloy (<3%), [Fe3+] cannot be boosted significantly,
even via chlorine gas regeneration, as there is so little [Fe2+] byproduct available
for regeneration. Feed and bleed of fresh ferric chloride is therefore required.
This can be an expensive procedure and may not be cost-effective.