Alloy Oxidation - Environmental Degradation of Metals - page 350

Table 6.5 Compositions of Cobalt-Based Superalloys

Composition in wt.% (balance Co)

Cr Ni Fe Mo W Ti Cb C Others

I. Wrought alloys
S 816 20 20 4 4 4 — 4 0.38 —

HA 188 22 22 1.5 — 14 — — 0.1 0.08 La

L 605 20 10 — — 15 — — 0.1 —

Mar M 918 20 20 — — — — — 0.05 7.5 Ta, 0.1 Zr

II. Cast alloys
HS 21 27 3 1 5 — — — 0.25 —

X 40 25.5 10.5 2 — 7.5 — — 0.5 0.01 B

FSX 414 29.5 10.5 2 — 7 — — 0.35 0.01 B

Mar M 509 24 10 — — 7 0.2 — 0.6 3.5 Ta, 0.5 Zr

Mar M 302 21.5 — — — 10 — — 0.85 9 Ta, B, Zr

Source : R. W. Cahn, P. Haasen, and E. J. Kramer, eds. Materials Science and Technology , Vol. 7.
VCH, Weinheim, 1992, p. 685.

for which the present discussion is limited to either pure Ni or Co or their respec-
tive alloys.
It is to be further recognized that once a deposit of Na 2 SO 4 has been formed on
the alloy surface, the extent to which the deposit affects the corrosion resistance of
the alloy will be governed by whether or not the deposit melts at the operating
temperature, how adherent it is, and the extent to which it wets the surface. Even
though a liquid deposit is generally necessary for severe hot corrosion to occur,
considerable degradation of alloys and oxidation-resistant coatings in marine gas
turbines has also been experienced under dense, thick, solid deposits. Accord-
ingly, hot corrosion processes have been subdivided into two categories: type I,
high-temperature hot corrosion (HTHC) processes, which are experienced above
the melting temperature of pure Na 2 SO 4 (1157 K), and type II, low-temperature
hot corrosion (LTHC) processes, which occur at temperatures below the melting
point of Na 2 SO 4 (i.e., in the range 873-973 K) where the deposited salt is ex-
pected to be solid [56]. The combustion gases arising from burning of coal and
other types of fuel oil contain both SO 2 and SO 3 ; accordingly, to simulate the
service environmental conditions, most of the hot corrosion tests employ either
static air or flowing oxygen

SO 2 gas mixtures over a platinum catalyst, and/

or SO 3 , since SO 2

1 / 2 O 2 s SO 3 .SO 2 or a mixture of SO 2

O 2 and/or SO 3
contains two oxidants, S and O. If one considers the thermodynamic equilibrium
between a metal and a mixture of SO 2

O 2 , only one stable compound, e.g., an
oxide or a sulfate, is the stable end product of the reaction. However, for kinetic