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Electroless siliconizing of Nimonic 75
Journal of the Less-Common Metals, @ Elsevier Sequoia S.A., Lausanne -
123
51 (1978) 123 - 124 Printed in the Netherlands
Letter
Electroless
siliconizing
W. J. TOMLINSON Department (Received
of Nimonic
75
and B. HOLE
of Applied
Sciences,
Lanchester
Polytechnic,
Eastlands,
Rugby
(Ct. Britain)
May 5, 1977)
There has been interest in recent years in siliconizing superalloys for protection against hot corrosion [l] . Gay and Quakernaat [Z] have recently studied the structure and mechanism of formation of an electroless silicide coating on commercially pure Ni. The object of the present work was to investigate in outline the electroless siliconizing of Nimonic 75, which is a typical superalloy and considerably more complex than Ni. The alloy (mainly Ni-18% Cr-5% Fe) was used in the as-received condition; the KClNaCl-NaF-NasSiF, melt and the coating procedure were similar to those used by Gay and Quakernaat [2]. The coating formed three distinct layers, designated A, B and C (Fig. l), and a zone labelled D in the alloy at the alloy/coating interface; zone D was shown by its differential etching to be due to a concentration difference. Felix et al. [3, 41 have shown that a silicide coating formed by chemical vapor deposition on an alloy not too dissimilar from Nimonic 75 consisted mainly of the phases (Co, Ni, Cr) Si, NiaSi and (Cr...)sSis and a Cr-Si concentration zone in the alloy; it is probable that the present A, B and C layers correspond to these phases and the D region to the concentration zone in the alloy. The number and complex composition of the regions [ 3, 41 is comparable with the formation of Y-NisSia and /3-NiaSi on pure Ni [2]. The presence of a Cr-Si zone in the alloy is particularly important; Felix et al. [3, 41 report that it acts as a barrier and prevents diffusion of Si into the alloy. The irregular narrow zone on the inside of the A layer (Fig. 1) is an etching effect caused by a concentration difference. Frequently the outer layer was very uneven but the inner layers had a relatively uniform thickness and the kinetics of formation at 960 “C of the B and C layers were linear (Fig. 2). Whilst the linear kinetics indicate that diffusion through a region is not rate controlling, it is not clear with the limited data on this complex system what the significance of the linear kinetics is. Whilst it was not possible to establish the form of the kinetics on electroless siliconizing of pure Ni, the mechanism was shown to be a combination of inward diffusion of Si and outward diffusion of Ni [2]. We may
124
20um
ALLOY
Fig. 1. Cross section of the corrosion “C for 2 h. The specimen was etched Fig. 2. Average
thickness
/
0
2
TIME,
products formed on Nimonic with Marble’s reagent.
of the B and C layers on siliconized
L
a
6
h
75 siliconized
Nimonic
75 at 960
at 960
“C.
note in connection with the coating thicknesses that mechanical polishing with Sic papers or etching the specimens before siliconizing nearly doubled the coating thickness. A microhardness survey showed the layers to have very different properties. Reichert microhardness values (N mmF2) of the layers were alloy 200, D 300, inner C 1000, outer C 1200, B 670, A 1500. The high hardness of the A layer was clearly indicated by the extensive damage observed around each indentation. In conclusion it appears that electroless siliconizing of the complex alloy Nimonic 75 produces a satisfactory and reproducible coating, but that the brittleness of the outer layer may cause problems. Acknowledgment This work was submitted by B.H. as an undergraduate C.N.A.A. B.Sc. in Physical Science.
project
for the
1 D. Chatterji, R. C. DeVries and G. Romeo, in M. G. Fontana and R. W. Staehle (eds.), Advances in Corrosion Science and Technology, Vol. 6, Plenum Press, New York, 1976, p. 63. 2 A. J. Gay and J. Quakernaat, J. Less-Common Met., 40 (1975) 21 - 28. 3 P. C. Felix and H. Beutler, in F. A. Glaski (ed.), Proc. 3rd Int. Conf. on Chemical Vapor Deposition, The Electrochem. Sot., Princeton, N. J., 1972, pp. 600 - 617. 4 P. C. Felix and E. Erdos, Werkst. Korros., 8 (1972) 627 - 636.
123
51 (1978) 123 - 124 Printed in the Netherlands
Letter
Electroless
siliconizing
W. J. TOMLINSON Department (Received
of Nimonic
75
and B. HOLE
of Applied
Sciences,
Lanchester
Polytechnic,
Eastlands,
Rugby
(Ct. Britain)
May 5, 1977)
There has been interest in recent years in siliconizing superalloys for protection against hot corrosion [l] . Gay and Quakernaat [Z] have recently studied the structure and mechanism of formation of an electroless silicide coating on commercially pure Ni. The object of the present work was to investigate in outline the electroless siliconizing of Nimonic 75, which is a typical superalloy and considerably more complex than Ni. The alloy (mainly Ni-18% Cr-5% Fe) was used in the as-received condition; the KClNaCl-NaF-NasSiF, melt and the coating procedure were similar to those used by Gay and Quakernaat [2]. The coating formed three distinct layers, designated A, B and C (Fig. l), and a zone labelled D in the alloy at the alloy/coating interface; zone D was shown by its differential etching to be due to a concentration difference. Felix et al. [3, 41 have shown that a silicide coating formed by chemical vapor deposition on an alloy not too dissimilar from Nimonic 75 consisted mainly of the phases (Co, Ni, Cr) Si, NiaSi and (Cr...)sSis and a Cr-Si concentration zone in the alloy; it is probable that the present A, B and C layers correspond to these phases and the D region to the concentration zone in the alloy. The number and complex composition of the regions [ 3, 41 is comparable with the formation of Y-NisSia and /3-NiaSi on pure Ni [2]. The presence of a Cr-Si zone in the alloy is particularly important; Felix et al. [3, 41 report that it acts as a barrier and prevents diffusion of Si into the alloy. The irregular narrow zone on the inside of the A layer (Fig. 1) is an etching effect caused by a concentration difference. Frequently the outer layer was very uneven but the inner layers had a relatively uniform thickness and the kinetics of formation at 960 “C of the B and C layers were linear (Fig. 2). Whilst the linear kinetics indicate that diffusion through a region is not rate controlling, it is not clear with the limited data on this complex system what the significance of the linear kinetics is. Whilst it was not possible to establish the form of the kinetics on electroless siliconizing of pure Ni, the mechanism was shown to be a combination of inward diffusion of Si and outward diffusion of Ni [2]. We may
124
20um
ALLOY
Fig. 1. Cross section of the corrosion “C for 2 h. The specimen was etched Fig. 2. Average
thickness
/
0
2
TIME,
products formed on Nimonic with Marble’s reagent.
of the B and C layers on siliconized
L
a
6
h
75 siliconized
Nimonic
75 at 960
at 960
“C.
note in connection with the coating thicknesses that mechanical polishing with Sic papers or etching the specimens before siliconizing nearly doubled the coating thickness. A microhardness survey showed the layers to have very different properties. Reichert microhardness values (N mmF2) of the layers were alloy 200, D 300, inner C 1000, outer C 1200, B 670, A 1500. The high hardness of the A layer was clearly indicated by the extensive damage observed around each indentation. In conclusion it appears that electroless siliconizing of the complex alloy Nimonic 75 produces a satisfactory and reproducible coating, but that the brittleness of the outer layer may cause problems. Acknowledgment This work was submitted by B.H. as an undergraduate C.N.A.A. B.Sc. in Physical Science.
project
for the
1 D. Chatterji, R. C. DeVries and G. Romeo, in M. G. Fontana and R. W. Staehle (eds.), Advances in Corrosion Science and Technology, Vol. 6, Plenum Press, New York, 1976, p. 63. 2 A. J. Gay and J. Quakernaat, J. Less-Common Met., 40 (1975) 21 - 28. 3 P. C. Felix and H. Beutler, in F. A. Glaski (ed.), Proc. 3rd Int. Conf. on Chemical Vapor Deposition, The Electrochem. Sot., Princeton, N. J., 1972, pp. 600 - 617. 4 P. C. Felix and E. Erdos, Werkst. Korros., 8 (1972) 627 - 636.