it was shown in Figure 3 that there is a correlation, for many metals, between the
heat of chemisorption and the heat of formation of the bulk sulfide that fits
approximately the relation ΔH
ad
= 1.25ΔH
f
(1/y M
ν
S
ν
). The fact that chemisorbed
sulfur is more stable than the three-dimensional sulfide has important consequences
for corrosion processes. Indeed, sulfide on the surface may be dissolved while a
monolayer or a fraction of it remains on the surface and changes, as described later
in sulfur-assisted corrosion mechanisms (Chapter 9). The metal-sulfur bond strength
is easily obtained from the heat of adsorption. The nickel-sulfur bond strength is
–464 kJ mol
–1
, based on –217 kJ mol
–1
as the heat of formation of monoatomic
gaseous sulfur. In the same way, the S–Fe bond strength is found to be –414 kJ mol
–1
.
The heat of segregation of sulfur onto metal surfaces is generally markedly
exothermic. Very low levels of sulfur dissolved in the bulk equilibrate with a very
high surface coverage. Although segregation is an exothermic process, it takes
place only at high enough temperatures to allow the diffusion of sulfur in the solid
state. Usually temperatures above 600°C are required for sulfur to diffuse to the
surface; at 800°C it takes a few minutes to obtain the saturation of the surface by
sulfur adsorbed on a nickel sample containing ~ 10 ppm sulfur. The diffusion
coefficient of sulfur in nickel is [36]
D(1073 – 1498 K) = 1.4 exp(–218.6/RT)
Similarly, sulfur may enrich grain boundaries even when present in the metal at
very low concentration. Values of the heat of grain boundary segregation are
shown in Table 5. One observes that the sulfur binding energy varies in the order
free surface > grain boundaries > solid solution.
Grain boundary segregation of nonmetallic impurities influences many
chemical and mechanical properties [37]. It increases the metal brittleness, favors
hydrogen embrittlement, decreases the intergranular fatigue strength or the creep
rupture life, and favors intergranular corrosion and stress corrosion cracking. In
this respect, sulfur is one of the most detrimental elements. As shown on bicrystals
of nickel, a close correlation exists between the intergranular sulfur segregation
and the intergranular corrosion [38]. At 625°C, the intergranular segregation was
observed for a sample with a sulfur content as low as 0.0005%. Thermodynamic
data on sulfur dissolution in pure metals and alloys and on sulfur diffusion have
been reviewed in Ref. 39.
Halogens
Halogen chemisorption on metals is of special importance in corrosion. This subject
has been reviewed by various authors [40–42]. At room temperature, halogen
molecules are dissociatively adsorbed on transition metals and, depending on the
system and the halogen pressure, the reaction may proceed beyond the adsorption
stage to give halogenide. For a given metal, the halogen binding energy decreases
in the order F > Cl > Br > I. Data at low coverages deduced from desorption kinetics
or from the heat of formation of halogenides are shown for Fe in Table 6.
For the refractory metals Nb, Mo, Ta, and W, the heat of adsorption of F and
Cl at low coverage was observed to be nearly identical 450 ± 20 kJ mol
–1
[45]
whatever the metal or the surface orientation. For Br on Nb and Mo the same value
Adsorption From Gas Phase 27
Copyright © 2002 Marcel Dekker, Inc.