Marcus P. Corrosion mechanisms in theory and practice

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remove it from the surface by dissolution of the cluster formed. More recently, the

molybdenum-induced removal of adsorbed sulfur has been observed in measurements

performed on single-crystal alloys with chemical compositions close to the

compositions of type 304 and 316 stainless steels, i.e., without and with molybdenum,

respectively [7]. After preadsorbing sulfur on the surfaces of the two alloys, the sulfur

coverage was measured as a function of time of exposure of the alloys to 0.05 M

at the corrosion potential. The results are shown in Figure 11. The monolayer

of sulfur adsorbed on the 304-type stainless steel is stable in 0.05 M H

at the

corrosion potential, whereas, owing to the presence of molybdenum, the desorption

of sulfur is observed on the 316-type stainless steel. The model used above gives a

fit of the experimental data for k ~ 1, instead of k = 2 for the Ni-Mo alloys. This

suggests that chromium lowers the number of Mo atoms required to promote the

dissolution of sulfur.

The Role of Chromium

The role played by alloyed Cr in the presence of sulfur has been investigated in detail

on Ni-based alloys with various concentrations of Cr in the range 8 to 34% (a range

of concentration within which alloys 600 and 690 are situated) [5,6,18]. The combined

electrochemical, radiochemical (

S), and spectroscopic (XPS) measurements

300 Marcus

Figure 11 Mo-induced removal of sulfur adsorbed on a single-crystal alloy of composition

close to 316 stainless steel. The measurements on an alloy of composition close to 304

stainless steel are shown for comparison. The coverage of sulfur was measured with

radioactive sulfur (

S).

performed with sulfur preadsorbed on the surface and with bulk sulfur (introduced

in the alloy in controlled amounts by bulk diffusion of S at high temperature)

revealed that Cr allows the surface to be passivated under conditions in which the

passivation would be precluded by sulfur if there were no Cr. The surface analysis by

XPS [18] revealed the coexistence of nickel sulfide and chromium oxide on the

surface. The mechanism of this counteracting effect of Cr is totally different from the

mechanism of the Mo effect discussed earlier. With Cr the desorption of sulfur is not

the key factor. What Cr does is to react selectively with oxygen (i.e., with OH or

O) to give Cr oxide, whereas Ni reacts with sulfur. This leads to the coalescence of

S in small nickel sulfide islands that can be covered by the lateral growth of the

chromium oxide islands. In other words, the antagonistic effects of Cr and S are

based on the fact that, on the considered alloys, Cr acts as an oxide former whereas

Ni acts as a sulfide former. This competitive mechanism is represented schematically

in Figure 12. In agreement with this mechanism, it has been observed that H

which accelerates the anodic dissolution of chromium, has no influence on passivated

chromium [37].

IMPLICATIONS IN AREAS OF PRACTICAL IMPORTANCE

There is much evidence for the damaging effect of sulfur species in a wide range of

corrosion-related service failures. The relation between the sulfur-induced corrosion

mechanisms presented in this chapter and the implications in areas of practical

importance can be rationalized on the basis of (a) the source of sulfur, (b) the

transport process to the metal surface, and (c) the conditions of the reduction (or

Sulfur-Assisted Corrosion Mechanisms 301

Figure 12 The mechanism of the antagonistic effects of chromium and sulfur on the

corrosion resistance of Ni-Cr-Fe alloys.

oxidation) of the sulfur species into the harmful chemical state of sulfur, i.e., the

adsorbed (chemisorbed) sulfur (or the sulfide if the concentration of the sulfur

species is high). Table 2 summarizes some of these considerations and gives a few

examples of practical areas in which detrimental effects of sulfur have been identified,

e.g., oil and gas, pulp and paper industries, power plants (high-temperature water

reactors), and atmospheric corrosion. Further details can be found in the other

chapters of this book.

Two categories of sulfur species may be considered according to the origin of

the species: (a) sulfur species (molecules or ions) in the environment, e.g., SO

(gaseous), H

S (gaseous or aqueous), HS

–

, HS

–

, S

–

, HSO

–

, SO

–

, and

(b) sulfur in the material: sulfur in solid solution, sulfur segregated at the surface

or in the grain boundaries, and sulfur in sulfide inclusions. After considering the

source of sulfur, the process by which the sulfur species arrive at the surface (transport

process) has to be identified. For environmental sulfur species it is generally

diffusion in the liquid (e.g., aqueous solution). For sulfur present in the bulk metal

or alloy, it may be surface segregation by solid-state diffusion at high temperature

or anodic segregation (a process that has been defined in a preceding section). Higher

anodic dissolution rates of grain boundaries, further accelerated by sulfur (according

to the mechanism of sulfur-enhanced dissolution described earlier), result in rapid

surface enrichment of sulfur at grain boundaries exposed to the electrolyte. In the case

of sulfide inclusions, e.g., MnS, although their detrimental effect on corrosion of

steels and stainless steels has been extensively studied [30– 33,38–41], the exact

mechanism is not fully understood. The important case of sulfide inclusions in

stainless steels is discussed in detail in Chapter 10. On the basis of the

302 Marcus

Table 2 Sulfur Species that may be Present in the Environment or the Material, with the

Process of Transport to the Surface, Nature of the Surface Reaction Producing Adsorbed

Sulfur, and Examples of Areas of Practical Importance

data reviewed here, other authors have invoked the role of adsorbed sulfur to explain

the detrimental effects of sulfide inclusions in stainless steels [38–40]. It must be

pointed out that early studies of the effects of H

S addition on corrosion of stainless

steels [41] have revealed the equivalence of the effects of sulfur added in the form of

S or present in sulfide inclusions. The effects of S

–

observed in the pulp and

paper industry [23] and of sulfur species in the steam generators of pressurized water

reactors [43] have also been interpreted on the basis of the data presented in this

chapter. Attempts to investigate the transport of sulfur from the inclusions to the

surrounding surface led to the conclusion that sulfur is cathodically deposited after

dissolution of the sulfide, producing rings of sulfur around the sulfide inclusion [38].

In a study of the mechanisms of pitting corrosion of stainless steels using submicron

resolution scanning electrochemical and photoelectrochemical microscopy [42] it

was concluded that the electrodissolution of certain MnS inclusions in stainless steel

is chloride catalyzed. The enchancement of the dissolution of sulfide inclusions by

–

had also been suggested in an earlier work [40].

After the process by which sulfur is transported to the surface, the nature of the

surface reaction producing adsorbed sulfur must be considered. Sulfur may be

adsorbed by electro-oxidation of sulfides [H

S (aqueous) or HS

–

] or by electro-

eduction of sulfates (HSO

–

or SO

–

) or thiosulfates (HS

–

, S

–

The oxidative or reductive reactions and the conditions of potential and pH in

which they may take place are detailed in the following. For SO

, the surface reactions

are presented in the chapter on atmospheric corrosion (chap. 15).

Once sulfur is present on the surface in the active chemical state (i.e., adsorbed),

it has the same effects (which have already been described) irrespective of its origin.

The stage at which the surface reactions become localized is not exactly known.

It is probably strongly related to the stage at which surface defects are crucial, which

is the case for the surface reactions that produce adsorbed sulfur.

THERMODYNAMICS OF SULFUR ADSORPTION ON METAL

SURFACES IN WATER

The principle of potential-pH (Pourbaix) diagrams has been extended to the case of

bidimensional layers of elements adsorbed on metal surfaces [44,45]. It allows us to

predict the conditions of stability of adsorbed sulfur on metals immersed in water

containing dissolved sulfur species such as S, H

S, HS

–

, HS

–

, S

–

, HSO

–

, and

2–

. The calculated E-pH diagrams show that the stability domain of adsorbed sulfur

extends beyond the usually predicted range of stability of metal sulfides, and thus

adsorbed sulfur layers can exist under conditions in which no bulk sulfide is stable.

Potential-pH diagrams have been calculated for sulfur adsorbed on surfaces of iron,

nickel, and chromium, in water containing sulfides or sulfates [44–46], and in water

containing sulfides and thiosulfates [47]. They are now also available for copper [48].

Principle of E-pH Diagrams for Adsorbed Species

An element A (which may be O, S, N, or H) is adsorbed on a metal M in the form

of a monoatomic layer, with a valence state equal to zero [denoted A

ads

(M)]. The

Sulfur-Assisted Corrosion Mechanisms 303

adsorption of A from a species dissolved in aqueous solution may be an electro-

oxidation or an electro-eduction reaction, depending on the valence state of A in

the dissolved species.

In contrast to adsorption in ultrahigh vacuum (UHV) and in gas phase,

described in Chapter 2, the adsorption of an atom or a molecule on a metal surface in

water involves replacement of adsorbed water molecules [H

ads

(M)] and competition

with the adsorption of oxygen [O

ads

(M)] or hydroxyl [OH

ads

(M)] resulting from the

dissociation of H

O molecules on the metal surface.

In a Langmuir model for adsorption, it is assumed that the two-dimensional

phase is an ideal solution, where water, oxygen or hydroxyl, and sulfur adsorb

competitively on the same surface sites and there are no interactions between

adsorbed species.

Under these conditions the chemical potential of each element A in the phase

adsorbed on a metal M can be expressed as follows:

304 Marcus

where θ

is the relative coverage of the surface of M by adsorbed A (0 ≤ θ

≤ 1;

= 1 for the complete monolayer of A); μ

Aads(M)

is the standard chemical potential

of A, corresponding to saturation of the surface by A. The chemical potential of

adsorbed water is also given by:

with

As an example of the method of calculation of the E-pH relations, let us consider the

adsorption on a metal of oxygen from water:

ads

(M) = O

ads

(M) + 2H

+ 2e

–

The equilibrium potential of this half-reaction is obtained by applying the Nernst

law with the chemical potentials of adsorbed oxygen and water expressed as in

Eqs. (1) and (2):

with the standard potential E° given on the standard hydrogen scale by

with μº

H+

+ μº

e–

–

μº

(g)

Electrochemical experiments and surface analyses show that the adsorbed

oxygen species in solution on most transition metals at 25°C are likely to be

hydroxyls. Thermodynamic data obtained from electrochemical experiments are

presently available only for OH

ads

on copper [49]. Therefore O

ads

is considered here

on Fe, Ni, and Cr and OH

ads

on Cu. The standard Gibbs energies of formation

(chemical potentials) for sulfur and oxygen adsorbed on metal surfaces can be

calculated [44–46] from literature thermodynamic data for reversible chemisorption

at the metal-gas interface (see Chap. 2).

E-pH Relations for the Equilibria between Dissolved and

Adsorbed Species

In water containing thiosulfates, the sulfur species to be considered are the

thermodynamically stable sulfide species H

(aq)

and HS

–

, and the metastable

thiosulfate species HS

–

and S

–

. The E-pH relations associated with the various

equilibria between water, the dissolved sulfur species and adsorbed sulfur, and

oxygen or hydroxyl adsorbed on Fe, Ni, Cr, and Cu at 25°C are as follows:

Sulfur-Assisted Corrosion Mechanisms 305

Potential-pH Diagrams

The preceding equations have been used to construct the potential-pH diagrams for

sulfur and oxygen (hydroxyl) adsorbed in water containing sulfides (H

S or H) or

thiosulfates (HS

–

or S

–

) on Fe, Ni, Cr, and Cu [44–48]. The diagrams are

shown in Figures 13 to 16 at 25°C for different sulfur coverages (θ

= 0.01; 0.5; 0.99)

and a molality of dissolved sulfur m

= 10

–4

mol kg

–1

. The diagrams are superimposed

on the S-M-H

O diagrams (M = Fe, Ni, Cr, Cu), calculated for a molality of dissolved

metal m

=10

–6

mol kg

–1

These diagrams allow us to predict the E-pH conditions in which sulfur is

adsorbed on a surface of Fe, Ni, Cr, or Cu in water by oxidation of sulfides or

reduction of thiosulfates. In aqueous solution, when the potential is increased in

the anodic direction, the adsorbed water molecules are replaced by sulfur atoms

adsorbed from H

(aq)

and HS

–

. At higher potentials, sulfur is oxidized in HS

–

and replaced by adsorbed oxygen or hydroxyl. The replacement reaction is

completed within a very narrow range of potential (~ 0.06 for O

ads

and 0.08 V for

ads

). The domains of stability of the adsorbed S monolayer (for m

=10

–4

mol kg

–1

and m

=10

–6

mol kg

–1

) overlap the domains of stability of the metals (Fe, Ni, Cr,

Cu), the dissolved cations (Fe

, Ni

, Cr

, Cu

, and Cu

), and the oxides or

hydroxides (Fe

, Fe

, Ni(OH)

, NiO, Cr

, Cu

O, CuO). The stability

domains of S

ads

are significantly wider than the stability domains of the bulk metal

sulfides, and thus S

ads

is stable in E-pH regions where there is no stable metal sulfide,

which reflects the excess of stability of the chemisorbed state with respect to the

corresponding 3D compound. On this basis, detrimental effects of sulfur on the corrosion

resistance of metals are predicted even under potential and pH conditions where the

metal sulfides are not thermodynamically stable. The comparison of the behaviors of

the different metals in the presence of dissolved sulfur species shows that the

306 Marcus

Sulfur-Assisted Corrosion Mechanisms 307

Figure 13 Potential-pH diagram for sulfur adsorbed on Fe (25°C, m

= 10

–4

mol kg

–1

Figure 14 Potential-pH diagram for sulfur adsorbed on Ni (25°C, m

= 10

–4

mol kg

–1

308 Marcus

Figure 15 Potential-pH diagram for sulfur adsorbed on Cr (25°C, m

= 10

–4

mol kg

–1

Figure 16 Potential-pH diagram for sulfur adsorbed on Cu (25°C, m

= 10

–4

mol kg

–1

extent of the overlap of S

ads

with the stable metals decreases in the sequence Ni,

Fe, Cr, so the effect of thiosulfates on corrosion of these metals is expected to

decrease in the same order Ni > Fe > Cr. Prediction of S adsorption in the active

domains (i.e., during anodic dissolution) is also important because this is the

condition in which S-enhanced dissolution is experimentally observed. Another

effect of thiosulfates expected from the stability of S

ads

in the passive domains

is the blocking or retarding of passivation (or repassivation) of stainless steels.

Such diagrams are useful to assess the risk of corrosion of metals and alloys

induced by adsorbed sulfur produced by electro-oxidation of sulfides or electro-

eduction of thiosulfates.

CONCLUSION

The fundamental aspects of sulfur-induced corrosion have been reviewed. The

mechanisms have been derived from data obtained on chemically and structurally

well-defined surfaces using electrochemical and surface analysis techniques

(

S radiotracer and surface spectroscopies).

The data obtained show the direct link that exists between atomic-scale surface

reactions of sulfur and macroscopic manifestations (enhanced dissolution, blocking

or retarding of passivation, and passivity breakdown). The data provide the

fundamental basis required to rationalize the detrimental effects of sulfur species

encountered in a large number of service conditions.

REFERENCES

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Modification (E. McCafferty C.R. Clayton, and J. Oudar, eds.), The Electrochemical

Society, Pennington, NJ, 1984, p. 173.

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Materials (EFC 12), 1994, p. 17.

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Sulfur-Assisted Corrosion Mechanisms 309