Marcus P. Corrosion mechanisms in theory and practice

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Several of the reactions described have been identified in so-called photochemical

smog formation [16]. This complex set of reactions involves an initial mixture of

nonmethane organic compounds, NO, and NO

, which is photochemically

transformed into a final mixture including HNO

and O

, aldehydes, and

peroxyacetylnitrate.

Principal sources of emission of NH

are animal shelters, fertilizer production,

and cleaning detergents. In the aqueous phase, NH

establishes equilibrium with

, which results in increased pH. An important role of NH

in atmospheric

corrosion chemistry is to partly neutralize acidifying pollutants by forming particulate

(NH

)

and acid ammonium sulfates, such as NH

HSO

and(NH

)

H(SO

)

. By

increasing the pH of the aqueous phase, NH

also increases the oxidation rate of

S(IV) to S(VI), as discussed earlier.

Chlorine-Containing Compounds

Chlorides participate in atmospheric corrosion reactions mainly as aerosols through

transport from marine atmospheres. Other important sources are road deicers and

dust binders on roads, coal burning, municipal incinerators, and fingerprints.

Burning of high-chlorine coals may also result in emission of HCl, which is highly

soluble in water and strongly acidifies the aqueous phase. Cl

is emitted from

industrial processes, such as bleaching plants in pulp and paper industries and certain

metal production industries, and from cleaning detergents. Cl

can also photo-

dissociate into chlorine radicals, which react with organic compounds (RH) and

form HC1 according to:

Atmospheric Corrosion533

Other Atmospheric Compounds

In addition to the gaseous species already commented on, Table 1 includes HCHO

and HCOOH, which are important indoor corrosion stimulants (as discussed later)

and which can originate from adhesives, tobacco smoke, combustion of biomass, and

plastics, for example. A comparison between typical outdoor and indoor concentrations

of the most important gaseous corrosion stimulants (Table 1) reveals, in general, lower

levels indoors than outdoors. This is mostly due to enhanced indoor absorption of gases

and particulates and also to the retardation and damping of outdoor variations by

ventilating systems and air filtration. Exceptions are NH

and the organic species,

which, as a rule, show higher levels indoors than outdoors as a result of anthropogenic

activity.

Of utmost importance in atmospheric corrosion is the presence of particles and

aerosols (an ensemble of particles suspended in the air) of mostly chlorides, sulfates,

and nitrates. The size, shape, and chemical and physical properties of these particles

and aerosols can vary widely. A more detailed description of particles and their role

in (indoor) atmospheric corrosion is given elsewhere [20].

THE AQUEOUS LAYER

Formation of the Aqueous Layer

The reaction of water vapor with a metal surface is of paramount importance to

atmospheric corrosion. A large number of studies of initial water–metal interaction

have been made on well-defined single-crystal surfaces of pure or oxidized metals

(for a review, see Ref. 21). Water may bond in molecular form to most clean and

well-characterized metal surfaces. Through the oxygen atom it bonds to the metal

surface or to metal clusters and acts as a Lewis base, as bonding is connected with

a net charge transfer from the water molecule to the surface. Simple models of

preferred adsorption sites are based on Lewis acid-base chemistry, in which water

adsorbs on electron-deficient adsorption sites [22]. Water may also bond in dissociated

form, in which case the driving force is the formation of metal-oxygen or metal-

hydroxyl bonds. The end products resulting from water adsorption are then adsorbed

hydroxyl, atomic oxygen, and atomic hydrogen [21]. On metal oxides, water may

also adsorb in either molecular or dissociative form. The tendency to dissociate seems

to be facilitated by lattice defect sites, as observed, for instance, on monocrystalline

TiO

[23], NiO [24], and a-Fe

[25]. The monomolecular thick film of surface

hydroxyl groups formed from dissociation of water is relatively protective and

reduces the subsequent reaction rate of water [26]. The first monolayer of water

adsorbed to the hydroxylated oxide substrate appears to be highly immobile, whereas

the second and third monolayers are more randomly oriented and less immobile

[27]. The water layer adjacent to the gas phase appears to have an icelike structure

[28]. The adsorption characteristics of water are strikingly similar for many different

metals, and the exact nature of the metal oxyhydroxide seems to have only a minor

influence on water adsorption phenomena. The quantity of reversibly adsorbed

water increases with the relative humidity and with time. Table 2 presents the

approximate number of water monolayers at 25°C and steady-state conditions,

which have been experimentally determined by the quartz crystal microbalance

method on a variety of metals [28,29].

The bond strength between water and the hydroxylated metal surface is similar to

the bond strength between neighboring, hydrogen-bonded, water molecules [21]. From

this follows the possibility of water clusters on relatively homogeneous surfaces, which

is further promoted on less well-defined surfaces with highly reactive sites, such as kinks

or steps, thereby increasing the probability of anode-cathode area formation.

The aqueous phase formed acts as a solvent for gaseous constituents of the

atmosphere. Preferred sites for corrosion attack may be related to sites where water

534 Leygraf

Table 2 Approximate Number of Water Monolayers on Different Metals Versus Relative

Humidity

adsorption is favored and gaseous molecules, such as SO

and NO

, are easily

dissolved. At aqueous films thicker than about three monolayers, the properties

approach those of bulk water [30]. The relative humidity when this occurs is close

to the “critical relative humidity” [31,32], above which atmospheric corrosion rates

increase substantially and below which atmospheric corrosion is insignificant. The

critical relative humidity for different metals in the presence of SO

has been

reported to be between 50 and 90% [28].

Electrochemical Reactions

The aqueous phase also acts as a conductive medium for electrochemical reactions.

Although atmospheric corrosion is largely dependent on electrochemical reactions,

relatively few electrochemical studies have been focused on the elucidation of

these basic mechanisms. The reason seems to be obvious if one considers the dif-

ficulties in reproducing a thin aqueous film in an electrochemical experiment. The

thickness of such a film may vary with different outdoor exposure conditions,

involving the complex chemistry and photochemistry produced by atmospheric

pollutants together with solar light and resulting in the precipitation of represen-

tative corrosion products and changed transport properties. Nevertheless, systematic

electrochemical experiments have been conducted with special emphasis on

atmospheric corrosion (see, e.g., Refs. 5,33–37). In the absence of pollutants, the

most common anodic and cathodic processes in corrosion of metals exposed to a thin,

neutral, aqueous phase are:

Atmospheric Corrosion 535

Under most atmospheric corrosion conditions, the anode reaction rather than the

cathode reaction is observed to be the rate-limiting step [2]. Upon evaporation of

the aqueous layer, a film of corrosion products—consisting of metal hydroxides or

metal oxyhydroxides—may precipitate. With repeated condensation–evaporation

cycles, this film usually hinders the transport of ions through the corrosion product

or the transport of Me

from the anodic site. Hence, the anodic reaction rate is

lowered and, thereby, the atmospheric corrosion rate.

Acidifying Pollutants

In the presence of atmospheric acidifying pollutants, such as SO

, the anode reaction

is facilitated and, consequently, the total corrosion rate as well. Upon deposition of

, interaction with the aqueous phase proceeds with the following reactions:

Despite numerous studies of the important influence and role of SO

, there is as yet

no complete description of the interaction, on a molecular level, of SO

with an

oxidized metal substrate in the presence of an aqueous phase. Recent in situ studies

by means of infrared reflection-absorption spectroscopy have provided evidence

that HSO

–

is coordinated with the hydroxylated metal oxide surface through a fast

ligand exchange mechanism, in which the surface OH

–

group is replaced by SO

2–

[38]; see Figure 1. Surface-bound H

released according to, e.g., reactions (8) and

(21), may easily move from one functional group to another or from one surface site

to another. In accordance with earlier proposed mechanisms for mineral dissolution

in aquatic environments [39], the SO

2–

-coordinated metal surface center may be

detached from the oxide lattice when surrounded by two or more H

-bonded

functional groups. This results in the release of H

ions, which can participate in the

detachment of another SO

2–

-coordinated metal surface center. The net effects represent

an acid-dependent dissolution rate (R

diss

) of the oxide that has been written as:

536 Leygraf

where [a

] is the activity of the protons, and C and n are constants [40]. When

further elaborated, Eq. (22) should include the influence of surface coordinated

ligands.

If S(IV) is oxidized to S(VI), this will result in the formation of H

and the

release of considerably more H

than if H

is involved. It follows that possible

reaction paths for the oxidation of S(IV) to S(VI) are of crucial importance to

atmospheric corrosion rates, and this has also been the subject of many investigations.

Examples of reaction paths have been summarized in the preceding section.

The acid-dependent dissolution rate of the initially formed oxide or oxyhydroxide

requires that all of the most important acidifying atmospheric pollutants be considered.

Besides SO

, these pollutants include CO

, NO

, and carbonyl compounds, such

as HCHO and CH

CHO, which can contribute to the total acidity in the aqueous film

by forming H

, HNO

, HCOOH, and CH

COOH.

At equilibrium between the water film and the atmosphere, the concentration

of dissolved atmospheric gases in the film is expressed according to Henry’s law:

where

[X] = concentration in aqueous phase

H(X) = Henry’s law constant for species X

p(X) = partial pressure of X in atmosphere

For many gases, reversible ionization reactions occur and the ordinary Henry’s

law constant, H, is replaced by an effective Henry’s law constant, H

. In the case of

this can be written as:

Less dissociation takes place if the pH is decreased, which also results in a decrease of

the effective Henry’s law constant and a further slowdown of the dissolution of SO

the aqueous layer [41].

The pH of the aqueous layer can be determined by formulating the general

requirement equation that electrical neutrality must be maintained within the layer.

Atmospheric Corrosion 537

Figure 1 Fundamental processes in the initial stages of SO

-induced atmospheric

corrosion, including coordination of HSO

–

with the hydroxylated metal oxide surface,

replacement of surface OH

–

group by SO

2–

and detachment of the SO

2–

-coordinated

metal surface center when surrounded by two or more H

-bonded functional groups.

This means that the sum of products of molar concentration and charge of cationic

species must equal the sum of products of molar concentration and charge of anionic

species. If we consider only the interaction of SO

with the aqueous layer, the pH is

determined by

538Leygraf

Figure 2Variation of pH of the aqueous layer as a function of p(SO

The calculated pH of the aqueous layer as a function of p(SO

), according to (24)

and (25), is shown in Figure 2. Amore exact estimate of the aqueous layer pH

must involve the contribution of other acidifying pollutants, such as CO

, NO

and HCHO, as well as the properties of the solid surface film, such as its coordination

properties and dissociation product. Acomplicating factor is the high ionic strength of

many species involved, especially if the aqueous film undergoes wetting and drying

cycles. If so, the concentrations of species are replaced by their activities. Henry’s

law constants for different gases are given in Table 3. It should be noted that

Henry’s law applies only if equilibrium is maintained between the gas in the

atmosphere and that in the aqueous phase. Consequently, it does not apply under

nonequilibrium conditions, caused by mass transport restrictions in the atmosphere,

in the aqueous layer, or across the gas–liquid interface.

Deposition of Pollutants

The incorporation of atmospheric species into the aqueous layer may occur

through either dry or wet deposition. In dry deposition there is no involvement of

any precipitation, whereas wet deposition requires, e.g., rain, dew, fog, or snow for

atmospheric pollutants to deposit. Indoors or in highly polluted areas close to

emission sources, dry deposition is considered to be dominating but the relative

importance of wet deposition may be difficult to establish because of the incidental

nature of precipitation. Controlled field studies combined with extensive laboratory

exposures have been undertaken within the National Acid Precipitation Assessment

Program (NAPAP) to explore the relative contribution of wet and dry deposition to

increased corrosion rates of a number of metals [45].

Auseful parameter is the dry deposition velocity, which is defined as the ratio

of deposition rate or surface flux per time unit of any gaseous compound and the

concentration of the same compound in the atmosphere [46]. The concept of dry

deposition velocity of SO

and its relevance to atmospheric corrosion rates is well

established [47]. By examining data based on both field and laboratory exposures.

it can be concluded that the factors controlling dry deposition fall into aerodynamic

processes and surface processes. Aerodynamic processes are connected with the

actual depletion of the gaseous constituent (e.g., SO

) in the atmospheric region

next to the aqueous phase and the ability of the system to mix new SO

into this

region. This ability depends on, for instance, the actual wind speed, type of wind

flow, and shape of sample. Surface processes, on the other hand, are connected with

the ability of the aqueous layer to accommodate SO

. This ability increases with the

thickness of the aqueous layer and, hence, with the relative humidity, the pH of the

solution (as discussed earlier), and the alkalinity of the solid surface.

The dry deposition velocity (V

) can be expressed as the inverse of the sum of

two resistances, namely the aerodynamic resistance (R

)and the surface resistance

Atmospheric Corrosion539

Table 3Characteristics of Selected Gaseous Air Constituents

The equilibrium solution concentration and deposition rate values were based on concentrations

from Table 1 and using geometric mean values for the intervals. 1.8 (–2) means 1.8 ˘ 10

–2

Henry’s law constant, from Ref. 19.

Ref. 42.

Ref. 19.

Ref. 43.

Ref. 44.

Under most exposure conditions the dry deposition velocity will be the combined

effect of both resistances [46]. At well-stirred and highly turbulent airflow conditions,

however, R

» 0 and the dry deposition velocity solely depends on surface

processes. Ideal absorbers of SO

are alkaline surfaces such as lead peroxide or

triethanolamine, for which R

» 0. The dry deposition velocity in this case is

determined mainly by aerodynamic processes. Typical ranges for dry deposition

velocities onto various materials under outdoor and indoor conditions are summarized

in Table 3.

Acombined experimental and theoretical approach to mass transport limitations

in atmospheric corrosion has been performed by Volpe [48,49] to obtain dry

deposition velocities and probabilities of H

S, NO

, and O

reacting with Ag. This

approach clearly shows the effect of airflow velocity on the corrosion rate and the

significance of hydrodynamic and concentration boundary layers next to the metal

surface, characterized by deviating tangential airflow velocity and concentration

of air constituents; see Figure 3. An application of boundary layer theory to buildings

and other objects has been reported aimed at postulating the economic assessment of

corrosion damage [50].

An immediate consequence of dry deposition velocities is the kinetic constraints

to obtain equilibrium between a gas in the atmosphere and the same gas in the

aqueous layer. Especially in outdoor exposure conditions, characterized by

wet-dry cycles, it is anticipated that the actual concentration of most corrosion-

stimulating gases under many conditions may be far from equilibrium.

Nevertheless, thermodynamic considerations have been most useful for predicting

the formation of different corrosion end products and their stability domains

[51,52]. Ageneral and useful observation made by Graedel is the similarity

between corrosion products found after prolonged exposure of metals and minerals

formed by natural processes and containing the same metals (see, e.g., Ref. 53).

Figure 4 is a schematic illustration of important processes occurring in or at the

aqueous layer.

540Leygraf

Figure 3Influence of airflow velocity on atmospheric corrosion rate of Cu in a

laboratory mixed-gas environment. (From Ref. 49. Courtesy of The Electrochemical Society.)

THE SOLID PHASE

Acid-Dependent Dissolution

As discussed earlier, initial electrochemical reactions and repeated wet-dry cycles

may result in precipitation of thin films of metal hydroxide, oxyhydroxide, or

oxide. The precipitation processes of these compounds are complex and pass

through the colloid state before solid films are formed [54]. These films form on

the metal surface normally by fast nucleation and spreading and by slower thickening,

once the film has completely covered the surface. If the film is stable, it thickens

through a high-field ion conduction mechanism; otherwise the film may thicken

through a dissolution-reprecipitation mechanism [55]. One approach to predicting

the ability to form a protective oxygen-containing film is by considering the heat

of the adsorption of oxygen on the metal. Compilations of such data [56] suggest

strong metal-oxygen bond formation and highly protective layers in various

atmospheres in the case of, e.g., Ti, Al, or Cr, but only weak metal-oxygen bonds

Atmospheric Corrosion 541

Figure 4 Schematic illustration of processes occurring in or at the aqueous layer.

and practically nonexistent protective oxygen-containing films in the case of, e.g.,

Ag or Zn.

As a consequence of exposure to the aqueous layer, the oxygen-containing film

can dissolve—the corrosion rate being normally controlled by the rate of dissolution

of the film formed [55]. As mentioned in the preceding section, the dissolution rate

of many oxides and other minerals is acid dependent and can be written in the form:

542 Leygraf

Metal-Anion Coordination Based on the Lewis Acid–Base Concept

During prolonged exposure a variety of different corrosion products may form, the

exact nature of which largely depends on the ability of the dissolving metal ion to

coordinate with other ions in the aqueous layer. A general treatment of classification

of participating ions in coordination processes can be based on the Lewis acid–base

concept. So far this theory has found only few applications in corrosion science [57].

It turns out to be most useful in rationalizing the specific behavior of different

metals during atmospheric corrosion and therefore merits a short introduction.

Upon interaction of two species, a pair of electrons from one species can

be used to form a covalent bond to the other one. The species that donates the

electron pair is called a Lewis base, and the species that accepts the electron pair

is called a Lewis acid. Acids or bases with valence electrons that are tightly held

and not easily distorted are hard acids or bases. Acids or bases with valence elec-

trons that are easily polarized or removed are soft. According to the hard and soft

acid-base (HSAB) principle, hard acids are preferably coordinated with hard

bases and soft acids are preferably coordinated with soft bases [58]. The hardness

of an element usually increases with its oxidation state. Selected Lewis acids

and bases and their classification into hard, intermediate, and soft acids and bases

are listed in Table 4 [59].

In full agreement with experience from atmospheric corrosion, Table 4 suggests

that hard acids such as Cr

and Ti

form oxygen-containing films, whereas soft

acids such as Cu

and Ag

coordinate with reduced sulfur compounds. Intermediate

acids, such as Fe

, Cu

, and Zn

, are expected to coordinate with a broader range

of bases.

Source: From Ref. 59.