Marcus P. Corrosion mechanisms in theory and practice

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A rather convincing justification of the catalytic mechanism is reached in

terms of atomistic events, at least on conveniently oriented crystal faces. Now, in

turn, the problem remains of understanding the pH dependence of the step nucleation

probability [31]. Further investigation of iron dissolution by impedance analysis

has clearly proved that both catalytic and consecutive mechanisms are operative

with a relative contribution depending upon electrode potential and solution pH.

Advanced Study of Iron Dissolution by Impedance Techniques Early

application of impedance measurement to the anodic dissolution of iron in acidic

media demonstrated the inductive character of the faradic impedances. The reaction

scheme initially proposed [13,60] is modified according to:

116 Keddam

The initial fast chemical hydrolysis step is introduced to account for the

order of the first reaction with respect to OH

–

confirming the results of previous

authors [13]. This modification, introduced later [71,72], implies clearly that the

origin of chemisorbed hydroxyls is the dissociation of water.

Steady-state current-voltage characteristics and faradic impedance associated

with the mechanism (24) to (31) were derived and numerically simulated with the

following assumptions:

Step (29) in fast equilibrium, i.e., surface concentration of FeOH

–

proportional to

[OH

–

] and time independent

Two-dimensional (2D) surface coverage θ by (FeOH)

ads

governed by a Langmuir

adsorption isotherm

Rate constants K

and K

obeying Tafel kinetics with symmetry factors (0 < α

< 1)

but not constrained to 0.5:

With this particular mechanism, Eqs. (2) and (3) are written as

where β is the surface concentration of (FeOH)

ads

at full coverage (θ = 1).

The steady-state coverage θ

obtained by solving (32) for dθ/dt =0,

introduced in (33), gives a current-voltage relationship:

where J

and J

are the partial current densities flowing through steps (1) and (2):

= FK

[OH

–

] and J

= FK

Similarly to Eq. (6), the solution of the linearized form of (32) and (33)

yields the impedance expression

Anodic Dissolution 117

and an associated

A fair model-experiment fit is obtained for both DC and AC responses with

the same set of kinetic parameters and a value of β, 3 × 10

–9

mol cm

–2

, in good

agreement with one monolayer of adsorbed OH

–

ions on a low-index plane of the

iron lattice (2 × 10

–9

mol cm

–2

on {100}).

The next advance was achieved owing to the extension of the frequency

domain into the millihertz range by digital TFA [47]. The existence of a second

inductive loop was then established at lower frequencies. A branching reaction

mechanism was proposed by introducing in parallel with the consecutive mechanism

a catalytic dissolution path [75]. This branching reaction path was already

prefiguring the main features of the more recent models covering broader ranges

of pH and current density.

Later on, more accurate impedance measurements were reported by Lorenz

et al. [76] and Keddam et al. [61,62] in 1 M sulfate solutions at 0 ≤ pH ≤ 5 and

0 ≤ j ≤ 150 mA cm

–2

. Both groups agreed reasonably about the main impedance

shapes and particularly the presence of three relaxation processes showing up at

intermediate pH values (around 2). The models elaborated by these groups have

in common the interpretation of the frequency dependence in terms of degrees of

coverage by adsorbed species. However, very large differences lie in the basic

assumptions regarding

The actual nature of the surface species participating in the impedance response

and the use of an adhoc “surface relaxation time constant” [68]

The existence or not of a consecutive mechanism in one of the reaction routes

The ability to interpret DC and AC data by a unique model in the active, transition,

and prepassive range

A detailed discussion of the relative performances and flaws of these

approaches can be found in Ref. 62.

A systematic analytic screening of all the possible 40 reaction schemes in

which three Fe-containing surface species are involved completed by a numerical

simulation finally led to selecting the reaction pattern [20] shown below. The three

coverages determining the impedance properties in the active and transition

domains are related to Fe(I)

ads

, Fe*(I)

ads

, and Fe*(II)

ads

. The superscript (*) indicates

species involved in catalytic dissolution paths. Fe(II)

ads

is a precursor of the passive

film whose contribution is significant only near the second maxima and beyond.

Numerical simulations showed that

The K

, K

path is the consecutive mechanism early established by impedance

measurements bounded to the hertz range and covering low pH and low

current density [60].

The K

, K

path is a catalytic route mainly operative in the transition range, the

Fe*(I)

ads

coverage being responsible for the decrease of dissolution rate past

the first peak shown in Figure 5A in this range at higher pH.

The K

, K

path is the main dissolution route at higher pH and current density

(j > 50 mA cm

–2

at pH 2 and j > 10 mA cm

–2

at pH 5) controlling the second

peak in Figure 5A.

Both catalytic paths introduced in this model were previously proposed by

Lorenz et al. [78–81] for interpreting the prepassive dissolution range.

Comparison of Figure 5A and B at pH 5 illustrates the outstanding power of

impedance measurements associated with modeling for mechanistic analysis.

The pH dependence of the rate constants proves that chemical bonds of Fe

atoms with solution species contribute to a determining extent to the generation of

active sites of dissolution, even in atomistic models where surface lattice features

(kinks, steps, terraces, etc.) are generally put forward. Large similarities of glassy

metals (amorphous) with crystalline ones [83,61] must also be regarded as arguing in

favor of chemical bonds as predominant entities with respect to lattice related sites.

Further support for the existence of a branching reaction pattern was obtained

from RRDE investigations [84]. It is found, as shown in Figure 6, that the collection

efficiency N for the oxidation to the trivalent state of the divalent Fe species

released by the dissolution of the iron disk decreases with pH and current density.

This is interpreted by paths K

and K

producing less reactive Fe(II) species

than K

does. In agreement with this view, the lower the rotation speed, the larger

the collection efficiency [84] because a homogeneous chemical reorganization of the

118 Keddam

Anodic Dissolution 119

Figure 5. Experimental (A) and calculated (B) current-potential curves and complex

impedance diagrams at the corresponding points labelled on I(E) curves (Frequency in Hz)

(Johnson-Matthey iron, l M sulfate, pH = 5, 25°C). Numerical simulation (B) according to

the reaction mechanism (36). From ref [61].

cation into a more reactive one is taking place between disk and ring. Such a

chemical step is formally accounted for in most reaction mechanisms by the

following step:

FeOH

+ H

→ Fe

+ H

Similar results are reported in Ref. 85 from channel flow double electrode (CFDE).

Role of Anions in the Anodic Dissolution of Iron

The specific role of the electrolyte composition at a given pH is an extremely

common experience. The contribution of anions other than hydroxyl is expected

through the acid-catalyzed or anion-ligand mechanism [24,25]. It must be emphasized

that the role of pH in polyacids, such as H

, may be obscured by the correlated

changes of electrolyte composition depending on acidity constants [86].

Specific anion dependence is expected to occur in the passive and transpassive

domains, and dissolution in the active range can be made to deviate from the

hydroxo-ligand mechanism [87] only by anions able to replace OH

–

, essentially

–

[88] and the halide ions. In the case of iron, due to the well-known passivity

breakdown and subsequent localized corrosion by halide ions and particularly Cl

–

chloride effects have been investigated extensively. Complexing anions such as

acetate have also been considered to a lesser extent.

In the case of acetate, a series of papers [71,86,89] considered the participation

of the anion from the initial reaction steps of dissolution and it was also claimed that

this anion contributes only to the dissolution of aged solid phases [90]. Later studies

[86,89] in acetate solutions at neutral and slightly acid pH reported a clear negative

order with respect to A

–

or HA. A model is derived on the basis of the participation

of HA and OH

–

, which are assumed to play a symmetric role with the formation of

120 Keddam

Figure 6. Potential dependence of the collection efficiency: influence of solution pH

(ring reaction Fe

→Fe

) for iron dissolution (Johnson-Matthey iron 1M sulfate).

Theoretical collection efficiency [37]: 0.48. Shift to cathodic potentials at increasing pH is

consistent with Figure 5. From ref [84].

(Fe(OH)

)

ads

, (Fe(OHA))

ads

, and (FeA

)

ads

. The kinetics of iron dissolution in the

active range in the presence of halide ions X

–

is largely dominated by the

competitive adsorption of X

–

[88] with the dissolution activating OH

–

. A critical

survey of the possible reaction paths in which Cl

–

competes with other anion

adsorption is given in Ref. 73. The mechanism is claimed to depend on the pH

range. The catalytic step of dissolution in Cl

–

-free media [63] is considered to

become at medium acidities (pH > 0.6):

Anodic Dissolution 121

In strongly acidic solutions the formation of the surface complex FeCl

–

and dissolution as FeCl

–

are proposed to interpret the accelerating effect of H

the dissolution rate [73].

According to other authors [91,92], Cl

–

and OH

–

would play a symmetric

role by forming (FeClOH)

–

ads

as an intermediate in a consecutive mechanism

followed by the rate-determining step

and in highly acidic solutions [92] the contribution of Cl

–

in the dissolution path

is depicted by the formation of (FeCl)

ads

and FeClH

as intermediate species.

Impedance measurements have been applied to iron dissolution in Cl

–

-con-

taining media [93–95]. Based on the mechanism for sulfate media and similarly to

earlier work [61,62], a branching pattern was proposed in (HCl/NaCl) involving

a consecutive and a catalytic dissolution path having (FeOH)

ads

as a common

bifurcation species. A third reaction path of the consecutive type with (FeClOH)

–

as an intermediate is in agreement with Ref. 91. An interpretation of the role of Cl

–

was advanced [94,95] in the framework of a progressive modification of the sulfate

mechanism [61,62] as a function of the chloride concentration, up to the sulfate-free

medium.

Mechanistic changes with pH are interpreted in a semiquantitative way as

follows: a gradual increase of Cl

–

content initially reduces the contribution of the

, K

) dissolution path in favor of a catalytic step similar to (K

), the Fe*(I)

ads

catalyst now being a Cl

–

-containing species.

Active Dissolution of Other Metals

The metals belonging to the so-called iron group (Co, Ni), which exhibit an

active-passive transition, have been far less extensively investigated than iron

itself. The state of the art can be found in Ref. 2. Little contribution is due to

impedance measurements and the mechanisms must be regarded as much less

solidly established than for iron.

Cobalt Cobalt seems to behave much similarly to iron and its dissolution is

assumed to take place through the formation of (CoOH)

ads

, a catalyst surface

species produced in an initial step of dissociative adsorption of water analogous to

Eq. (23). Regarding the structural description of the dissolution mechanism in

term of kinks at the lattice surface, cobalt and iron apparently follow the same

process in which atomic kinks are assimilated to catalytic sites (CoOH)

ads

Unfortunately, in the case of Co, microscopic evidence of the changes of surface

morphology and consequently estimation of the kink-kink distance are lacking.

Nickel The mechanism of active dissolution of nickel remains highly

controversial. It seems that there is no agreement so far even on the actual shape

of the current-voltage profiles. Two-peak or single-peak curves are reported,

depending on the potential sweeping rate, the prepolarization conditions, the

structural properties of the metal, etc. This erratic behavior is tentatively ascribed

to either the remanence of stable natural oxides on the metal surface or strong

interaction with hydrogen. It must be pointed out that experiments performed in

situ on a surface continuously refreshed by mechanical abrasion never yielded

more reproducible results.

Chromium Due essentially to its role in the corrosion resistance of stainless

steels, the anodic behavior of Cr has been far more extensively considered. An

overview can be found in Ref. 96. Early studies were based on polarization curves

[97] and potential transient experiments [98].

Most of the significant data are established by impedance [43,99,100] and

RRDE [101,102] and reveal rather poor reproducibility, probably because of

interference with hydrogen overvoltage and its metallurgical dependence.

Simultaneous dissolution as Cr

and Cr

in the active and active-passive transition

ranges [103] and no pH dependence of the dissolution rate led, in contrast to Fe group

metals, to more direct dissolution mechanisms being proposed [104]. The

following oversimplified model [105,106] was considered with the purpose of

accounting for the Cr dissolution in the description of Fe-Cr alloys [107].

By means of CFDE the existence of adsorbed hydroxylated intermediates

CrOH and CrOH

was proposed [101] in the active dissolution range. From

impedance data on polycrystalline and (100) single crystal plane [96,99],

simultaneous hydrogen evolution was introduced in the anodic behavior of Cr by

assuming a strong dependence of the anodic reaction on the H–Cr bonding.

Evidence of diffusion-controlled kinetics in the impedance was tentatively

interpreted by surface diffusion of Cr(I)

ads

, and Cr dissolution and H evolution

were assumed to be competing on terraces. Figure 7 shows the main aspects of this

approach, which attempted to elaborate a kinetic description on the basis of an

atomistic picture of the metal surface.

One of the most innovative ingredients in this model is the coexistence of H

evolution and Cr dissolution on the passive areas, but divalent state of Cr in the

passive range appears quite contradictory to available data from X-ray photoelectron

spectroscopy (XPS).

Other Metals Titanium [108–110] and copper [111] are among the more

extensively investigated metals not pertaining to the iron group or being major

constituents of stainless steels.

122 Keddam

Recent Advances in the Mechanism of Passivation by Downstream

Collector Electrode Techniques

In spite of the very elaborate approaches based on current-voltage techniques,

credible steady-state and transient mechanisms have hardly been derived for the

transition from the active to the passive range. An accurate description of the

whole set of elementary steps requires additional data on the relative contribution

of the oxidation current to the formation and growth of 2D and 3D layers and on

the release of cations in solution. Passivation mechanisms can be elaborated on the

basis of the active-passive transition in two complementary ways:

Analysis of passivation transients on an initially active surface either by applying

a steep potential jump into the passive range or by creating fresh surfaces at

constant applied potential by nonelectrochemical depassivation (chemical:

passivity breakdown; mechanical: scratching, ultrasonic waves, etc.; radiative:

laser beam impact [112,113]). These techniques have proved to be of

outstanding importance for the investigation of the mechanism of localized

corrosion associated with passivity breakdown [114,115].

Analysis of tiny changes of the anode maintained potentiostatically between

active and passive states in the negative slope of the dissolution peak.

Electrochemical impedance and frequency-resolved RRDE data are used

together.

Dissolution during Transient Passivation of Freshly Generated Iron

Surfaces Pulse depassivation of the upstream iron anode and collection of the

Fe(II) released into the solution on a downstream glassy carbon were performed

in a CFDE [116]. Transient currents I

on the Fe anode and collected currents I

for Fe(II) oxidation are shown in Figure 8a and b. After processing for double-

layer charging and collection efficiency, the charges involved in dissolution Q

and

were discriminated from the overall charge Q

supplied to the disk. The results

as a function of pH and disk potential E

are shown in the Figure 8c and 8d. The

Anodic Dissolution 123

Figure 7. The corrosion processes occurring at a chromium-sulfuric acid solution

interface. From ref [96].

124 Keddam

Figure 8. “a”: transient currents on the Fe working (upstream) electrode. “b”: detection currents on the glassy carbon collector (downstream)

electrode 1M Na

acidified to pH 3. “c”: potential dependence of the charge Q

for dissolution of ferrous cations and Q

for passive film

formation at various pH. Depassivated surface area 0.08 cm

. “d”: potential dependence of the charge ratio Q

(integral emission efficiency)

at various PH.

whole set of data was interpreted by the reaction model (40), a simplified form of

the one derived for the faradic impedance of active iron (36). The kinetic equations

for the time dependence of the two coverages by Fe(II) and Fe(III) were integrated

numerically. Considering the wide range of current densities covered by the transient,

both noncatalytic and catalytic reactions paths were considered. Only Fe(II)

species participate in the dissolution mechanism and passivation is ascribed to

Fe(III). These assumptions are justified by the large degree of (super) saturation

in ferrous species during short transients.

That is supported by the results of simulations indicating in the first milliseconds

of the transient a fast buildup of the Fe(II) coverage to a maximum of about 0.5,

which then falls off during the growth of the passive film. The higher the pH and the

applied potential, the sooner and the steeper the transition from Fe(II) to Fe(III).

Impedance and AC RRDE Study of the Active-Passive Range of Iron

Impedance and the AC component of the ring current were simultaneously measured

at a series of polarization points in the negative slope region of Fe in 1 M sulfuric

acid [117]. This region immediately prior to the Flade potential is very meaningful

for understanding the imbrication of dissolution and passivation steps. The ring

potential E

was settled at two different potentials for collecting Fe(II) by oxidation

(0.80 V/SSE) and Fe(III) by reduction (–0.40 V/SSE).

Typical data obtained for one point of the polarization curve are shown in

Figure 9. Impedance shows the well-known behavior of a passivation range

characterized by the negative zero-frequency limit of the real part consistent with

the slope of the steady-state current-voltage curve. The low-frequency capacitive

arc displays the frequency response of the film growth. Emission efficiencies N

relative to Fe(II) and Fe(III) going to the solution contain two frequency domains,

i.e., a surface charge stored in two different species. For Fe(II) the largest changes

take place in the same frequency range as for impedance. Its positive imaginary

part establishes the passivating role of the faradic surface charge with respect to the

active path of iron dissolution (see earlier). A small loop at higher frequencies is

poorly visible. It reveals a feature not detected on the impedance spectrum and lies

in the half-plane of negative imaginary parts reflecting a surface charge stored in a

Anodic Dissolution 125