Marcus P. Corrosion mechanisms in theory and practice

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Formation of Corrosion Products

The atmospheric corrosion rate is influenced by many parameters, one of the most

important being the formation and protective ability of the corrosion products

formed. The composition of the corrosion products depends on the participating

dissolved metal ions and the access to anions dissolved in the aqueous layer. The

eventual thickening of the film of corrosion products can be described in a

sequence of consecutive steps—dissolution–coordination–reprecipitation—where

the dissolution step is acid dependent, coordination is based on the HSAB principle,

and reprecipitation depends on the activities of the species involved.

Depending on the rate of crystallization and the rate of formation, the corrosion

products may be amorphous or crystalline. If the former is rate determining, one

expects amorphous phases to form. From colloid chemistry it is known that aging

or slow growth of amorphous phases may result in a transition from the amorphous

to the crystalline state, a process that may occur through slow transformation in

the solid state or through dissolution–reprecipitation processes [60]. This seems to

be in agreement with findings of a transition from amorphous to crystalline basic

nickel sulfates, the former being less corrosion resistant than the latter [61a].

Figure 5 is a schematic illustration of important processes occurring in or at the

solid phase.

Atmospheric Corrosion of Selected Metals

Access to new and more sensitive analytical techniques has resulted in substantial

progress in the characterization of corrosion products formed under both laboratory

and field exposure conditions. These techniques permit the determination of, e.g.,

thickness, chemical composition, and atomic structure of corrosion products formed

at both early and later stages of exposure. When combined with environmental data,

such as deposition rates of corrosion-stimulating atmospheric constituents, relative

humidity, temperature, and sunshine hours, the new techniques have resulted in a

more comprehensive understanding of the complex processes that govern atmospheric

corrosion. In a series of papers, Graedel has summarized the corrosion mechanisms

of zinc [62], aluminum [63], copper [18], iron and low-alloy steel [64], and silver

[19]. It is beyond the scope of this chapter to provide a detailed description of the

specific atmospheric corrosion behavior of each individual metal. Nevertheless, a

summary of atmospheric corrosion behavior of selected metals will be given in light

of the general processes that have been discussed. The summary is based on the

papers by Graedel, unless otherwise stated.

The atmospheric corrosion of zinc starts with the instant formation of a thin film

of zinc hydroxide, which seems to occur in different crystal structures, and the

subsequent formation of a protective layer of basic zinc carbonate, Zn

(CO

)

(OH)

The pH of the aqueous layer controls the stability of initial corrosion products and

results in the dissolution of Zn

. From the HSAB principle one expects Zn

classified as an intermediate acid, to coordinate with a number of different bases. In

accordance with this, the prolonged exposure of zinc can proceed along a variety of

different paths of reaction sequences depending on the actual deposition rates of

atmospheric constituents. Among these Cl

–

and SO

seem to be the most important.

Atmospheric Corrosion 543

In a relatively benign rural atmosphere, the basic zinc carbonate may continue to

grow slowly or may be followed by the formation of a protective basic zinc sulfate,

e.g., Zn

(OH)

· 4H

O. In a typical marine atmosphere, characterized by higher

amounts of deposited Cl

–

than of SO

, islands of a less protective basic zinc chloride,

(OH)

· H

O, are formed within days of exposure. These islands grow

laterally and coalesce. Within weeks of exposure, a more protective basic zinc

chlorosulfate, NaZn

Cl(OH)

· 6H

O, may be observed [65]. In a typical

urban environment, characterized by higher amounts of deposited SO

than of Cl

–

the basic zinc sulfate, Zn

(OH)

· 4H

O, is observed within, typically, weeks of

exposure. In highly polluted industrial environments, it is eventually followed by

another basic zinc chlorosulfate, Zn

(OH)

· 5H

O. Afeature common to most

zinc-containing corrosion products observed is their strong structural resemblance,

with sheets of Zn

in octahedral and tetrahedral coordination and with the main

difference between the structures being the content and bonding between the sheets

[66]. Ageneralized reaction sequence has been proposed that considers the evolution

of corrosion products on sheltered zinc in a variety of atmospheric environments,

including rural, marine, urban, and industrial [66]. Figure 6 displays schematically the

544Leygraf

Figure 5Schematic illustration of processes occurring in or at the solid phase.

reaction sequence in which sulfate deposition dominates over chloride deposition

(right part) and the reaction sequence in which chloride deposition dominates over

sulfate deposition (left part). Reported corrosion rates (in μm/year) of zinc in outdoor

atmospheres range from 0.2 to 0.3 (rural), from 0.5 to 8 (marine), and from 2 to 16

(urban, industrial).

Aluminum forms initially a few-nm-thick layer of aluminum oxide, γ-Al

which after prolonged exposure in humidified air is covered by aluminum

oxyhydroxide, γ-AlOOH, and subsequently by various hydrated aluminum oxides

and aluminum hydroxides. The stability of the compounds decreases with acidity and

results in the dissolution of Al

. The ability of aluminum to form various oxygen-

containing corrosion products is in agreement with the HSAB principle; this is also

the case with the general observation of frequent formation of basic or hydrated

aluminum sulfates and no detection of aluminum sulfides. The sulfates most

frequently found on aluminum are poorly soluble, amorphous, and highly protective.

As with other passivating metals, atmospheric corrosion rates of aluminum increase

readily in the presence of Cl

–

, resulting in local rather than uniform corrosion. Rates

of uniform corrosion (in μm/year) of aluminum outdoors are substantially lower than

for most other structural metals and are from 0.0 to 0.1 (rural), from 0.4 to 0.6

(marine), and ≈1 (urban).

In ambient air copper initially forms a film with a total thickness of around

3 nm. It seems to contain at least two layers, both of which contain copper as Cu

The inner layer close to the metal consists of Cu

O and the outer layer of bound

hydroxyl, water, and adventitious hydrocarbon [67]. Cu dissolves into the aqueous

layer as Cu

, which readily oxidizes to Cu

. From the presence of both Cu

(soft

acid) and Cu

(intermediate acid) a variety of corrosion products are expected.

Atmospheric Corrosion 545

Figure 6 Formation sequences for compounds in zinc corrosion products formed under

sheltered conditions dominated by chloride deposition (left sequence) and sulfate deposition

(right sequence). (From Ref. 66. Courtesy of the American Society for Testing and

Materials.)

With reduced sulfur compounds present in significant concentrations, copper mainly

forms Cu

S. Without reduced sulfur compounds, the corrosion products are frequently

complex mixtures of basic copper sulfates, chlorides, nitrates, and carbonates, with a

composition that depends on the actual deposition rates of the corresponding air

constituents. Abundant phases are Cu

(OH)

and Cu

(OH)

in urban areas

and Cu

Cl(OH)

and possibly basic sulfates in marine atmospheres. Other phases

observed are Cu

(OH)

· H

O, Cu

(OH)

, and Cu

(OH)

. Similar to that

for zinc, a generalized reaction sequence has been proposed for sheltered copper,

which integrates all existing knowledge of phases formed in copper patina during

exposure in atmospheric environments with varying degree of sulfur and chloride

pollution [67]. The phases observed bear structural resemblance and are mostly

layered, a common observation in basic salts of divalent cations such as Cu

and

. Representative corrosion rates (in μm/year) of copper exposed outdoors are

≈ 0.5 (rural), ≈ 1 (marine), 1–2 (urban), and <_ 2.5 (industrial).

Initial stages of atmospheric corrosion of iron involve the incorporation of

oxygen and water into a rich variety of different iron oxides or iron oxyhydroxides as

rust layers. These processes are preceded by initial dissolution of Fe

, which only

slowly oxidizes to Fe

. From this follows that corrosion products formed at earlier

stages of exposure contain iron in both valence states but only as Fe

at later stages.

A complex chemistry is anticipated with the participation of an intermediate acid

slowly transforming into a hard acid. The oxidation of Fe

in the aqueous layer may

involve several possible oxidants, such as the hydroxyl radical (OH·), the

hydroperoxyl radical (HO

·), and hydrogen peroxide (H

). Detailed discussions

of the interaction between SO

and iron have been presented elsewhere [68]. It results

in the formation of iron(II) sulfates, subsequent oxidation of Fe

, and concomitant

formation of iron(III) oxyhydroxide, the release of SO

2–

, the dissolution of more Fe

formation of new iron(II) sulfate, etc. [69]. This acid regeneration cycle continues until

basic iron(III) sulfate is formed as an end product. The complex interaction of

and iron is believed to take place in locally distributed sulfate “nests,”

546 Leygraf

Figure 7 Sketch of sulfate “nest” on steel. (From Ref. 6.)

which involve the assumed existence of semipermeable membranes of iron(III)

oxyhydroxide maintaining a high activity of species involved at localized parts of the

electrolyte (Fig. 7). Evidence of nitrate- or carbonate-containing corrosion products

is sparse, although coordination of these anions with Fe

is expected according to

the HSAB principle. There is abundant evidence of accelerated atmospheric corrosion

rates for iron caused by chlorides, which may result in the formation of basic

iron(II,III) chlorides and b-FeOOH. Atmospheric corrosion rates for iron are relatively

high and exceed those of other structural metals. They range (in μm/year) from 4 to

65 (rural), from 26 to 104 (marine), from 23 to 71 (urban), and from 26 to 175

(industrial).

In addition to the structural metals already discussed, results have been reported

from studies of the atmospheric corrosion behavior of other metals, many of which

are used as materials in electronic or electric equipment.

The atmospheric corrosion of nickel is similar to that of zinc. Nickel exists

solely as Ni

and instantaneously forms nickel hydroxide and, subsequently,

NiSO

· 6H

O, has been observed [61b]. After prolonged exposure, an amorphous

basic nickel sulfate is formed, frequently mixed with small amounts of carbonate,

with less protective ability. This phase can crystallize and form another basic nickel

sulfate, with higher stability and protective ability [61a]. No evidence of other anions

in the corrosion products has been found so far. The corrosion rates of nickel are

comparable to those of copper [70].

Silver exhibits a corrosion behavior which hardly resembles that of any of the

other metals described. Its unique behavior is to a large extent governed by the

existence of Ag

upon dissolution of silver into the aqueous layer. Ag

S is the most

abundant component of the corrosion products formed. AgCl can form in

environments with high chloride content, whereas no oxides, nitrates, sulfates, or

carbonates have been reported in connection with atmospheric exposure. Silver

exhibits corrosion rates comparable to those of aluminum, lower than those of zinc,

and much lower than those of iron.

Tin forms both SnO

and SnO as dominating corrosion products in a variety of

environments, resulting in a relatively high protective ability. Laboratory exposures

involving mixtures of SO

, NO

, and H

S suggest that tin is relatively unaffected by

these pollutants [71]. Accordingly, efforts to correlate corrosion effects of tin in

natural atmospheric environments with concentrations of SO

and NO

have so far

been unsuccessful [72]. Tin exhibits corrosion rates comparable to those of silver [72].

ATMOSPHERIC CORROSION OUTDOORS

Early field exposure programs with selected metals were implemented in the United

States and the United Kingdom in the beginning of the 20th century. The environments

were classified as “rural,” “urban,” “marine,” and “industrial,” and it was soon

recognized that corrosion rates vary considerably between different types of

environment. Over the decades a large number of outdoor corrosion test programs

have been implemented with the general aim of identifying and, possibly, quantifying

the most important environmental parameters involved in atmospheric

corrosion processes. The large body of data available has provided evidence that

corrosion rates under outdoor exposure conditions are strongly influenced by SO

Atmospheric Corrosion 547

and Cl

–

as well as by climatic factors (humidity and temperature). Despite this, the

general goal of predicting the performance of a given metal in a given environment

is far from being attained [73]. The reason is that many other factors influence

corrosion rates: initial exposure conditions, sample mass and orientation, extent of

sheltering, wind velocity, the nature of corrosion products formed, and pollutants that

are not measured. The difficulties in making accurate predictions of corrosivity can

be exemplified by the variations in corrosion rates between steel and zinc in a world-

wide ASTM Site Calibration at 45 outdoor locations [74]. The mass loss between

different sites for both steel and zinc after 2 years of exposure varied two orders of

magnitude or more. Moreover, the ranking of test sites with respect to mass loss of

zinc was considerably different from that of iron. The ratio of steel to zinc mass loss

varied from around 10 to more than 350. Significant variations in mass loss between

different exposure periods were also observed for both metals. Similar experience has

been gained from several other exposure programs [75,76].

According to Barton [2], a generalized description of corrosion rate versus time

includes an induction period, a period for establishing stationary conditions, and a

stationary period; see Figure 8. These periods can be associated with the present

description of the aqueous layer and formation of corrosion products. During the

induction period, the metal is covered with a spontaneously formed oxide and

the aqueous layer, which affords a certain protection depending on the metal and the

aggressiveness of the atmosphere. The transition period involves the transformation

from the oxide layer into a fully developed layer of corrosion products via local

formation and coalescence of corrosion products. The stationary period, finally, is

characterized by full coverage of the surface by corrosion products, eventually reaching

constant properties with respect to chemical composition and atomic structure, and

stationary corrosion rates. The two initial periods are shorter the more aggressive the

exposure conditions are. For steel they typically last for years in very benign (indoor)

atmospheres but only a few months in highly polluted industrial areas [2].

548 Leygraf

Figure 8 Schematic division of atmospheric corrosion into different periods, namely an

induction period (A), a period for establishing stationary conditions (B to C), and a stationary

period (from C). (From Ref. 2. Courtesy of John Wiley & Sons.)

Atmospheric Corrosion 549

A frequent observation during the two initial periods is the marked influence of

initial exposure conditions on the subsequent corrosion rate [77,78]. Wet

conditions—caused by rainfall or high relative humidity—are more severe than dry

conditions during the first days of exposure; they result in higher corrosion rates

when the exposure time extends to several months. In this respect zinc seems to be

more sensitive than steel. These observations are, at least partly, explained by the

formation of corrosion products possessing different protective abilities. Especially

on zinc, a diversity of structurally related corrosion products can be formed, the

nature of which depends on initial exposure conditions [66]. An additional cause may

be the seasonal dependence of the concentrations of H

and O

[62].

During the third period of exposure, characterized by stationary corrosion rates,

the interpretation of corrosion rates in terms of environmental data is more easily

accomplished than during the two initial periods with varying corrosion rates. In

reality, atmospheric corrosion is only quasi-stationary and regarded as a series of

processes, when the surface is wet enough to allow rapid corrosion rates, and

interrupted by periods of dryer conditions with negligible corrosion rates. An

important concept here is the so-called time of wetness based on devices that

monitor the actual time during which the surface is wet or the critical relative

humidity exceeded [79]. In many cases, time of wetness estimates have been based

on measurements of temperature and relative humidity. A common definition of time

of wetness is the time during which, simultaneously, the temperature is above 0°C

and the relative humidity ≥80%. However, this definition should not be taken too

literally because the actual time when the surface is wet enough for rapid corrosion

to occur depends on many other parameters, such as type, mass, orientation, and

degree of sheltering of the metal; hygroscopic properties of corrosion products

and of surface contaminants; type and level of pollutants; air velocity; and solar

flux. An alternative way to estimate the time of wetness is by means of electrochemical

cells (Fig. 9), which consist of thin electrically separated metal electrodes and involve

the detection of a current or an electromotive force when the thickness of the aqueous

layer is sufficient to accelerate atmospheric corrosion [79–82]. In this case, too, the

result will depend on many parameters, such as cell design, formation of

corrosion products on metal electrodes, and criteria for defining time of wetness

based on current or electromotive force responses.

Figure 9 An electrochemical cell for determining time of wetness. (A) Material containing

the embedded electrodes. (B) Noble metal electrodes (e.g., Pt or Cu). (C) Base metal

electrodes (e.g., Fe or Zn). (From Ref. 2. Courtesy of John Wiley & Sons.)

550 Leygraf

It is also possible to measure quantitatively the amount of water on metal

surfaces under in situ conditions by means of the quartz crystal microbalance [83].

This is illustrated in Figure 10, where the water content quotient on a gold surface,

given as the mass of water in the aqueous adlayer divided by the mass of other

deposited species on gold, is plotted as a function of relative humidity during

consecutive 24-h cycles in an outdoor environment. Each cycle is represented by one

increase (during nighttime) and one decrease (during daytime) in water mass and

relative humidity, respectively. From the figure it is evident that significant amounts

of water are present at relative humidity far below 80%. Analysis of the data shows

that the actual time when the surface is wetted is significantly longer than the time

when the temperature is above 0°C and the relative humidity ≥ 80%. Hence, this

definition of time of wetness may be erroneous [83]. Despite these uncertainties in

definition and experimental accuracy, the time of wetness exhibits a surprisingly

good correlation with outdoor corrosion rates of steel in many types of environ-

ment [84].

Because of the many parameters capable of influencing atmospheric corrosion

rates, exposure programs have placed increased emphasis on standardized test

procedures. The International Organization for Standardization (ISO) has

formulated several corrosion testing standards [85–88] and also implemented a

Figure 10 The water content quotient on gold, given as the mass of water in the aqueous

adlayer divided by the mass of other deposits, as a function of relative humidity (RH) during

14 consecutive days of outdoor exposure in an urban environment. (From Ref. 83. Courtesy

of The Electrochemical Society.)

worldwide atmospheric exposure program known as ISOCORRAG [89]. This

program aims at generating a basis for the procedures used in the ISO classification

system. The ISO classification system presents two different approaches to assessing

the corrosivity of any environment. One approach is based on the exposure of

standard specimens of aluminum, steel, copper, and zinc for 1 year and determining

a measured corrosivity class from mass loss data. The other approach is based on SO

concentration, deposition of Cl

–

, and time of wetness at the site, which results in an

estimated corrosivity class.Having determined a corrosivity class with one of these

methods, it is possible to estimate the extent of corrosion damage to aluminum, steel,

copper, and zinc for either short-term or long-term exposures. The ISO classification

system provides adequate data for many practical purposes, including prediction of

long-term corrosion behavior in different environments and the need for protective

coatings. However, experience has shown that certain observations need further

clarification. These observations include the frequent difference in corrosion rate

between the top side and bottom side of flat standard specimens and between

flat standard specimens and open helix standard specimens [90]. They also include

the possible role of pollutants other than those measured. Future work within

ISOCORRAG and other exposure programs should shed more light on these and

other not yet fully elucidated issues.

With the increasing concern about acidifying pollutants and their influence on

atmospheric corrosion rates, scientists’interest has been focused on NO

as an

additional gaseous corrosion stimulant. Whereas the SO

concentration has shown a

significant decline over the past decades in many urban and industrial areas,

estimated emission of NO

has shown a continuous increase in the same type of

environment [91]. Studies in laboratories using synthetic air have provided

unambiguous evidence of increased corrosion rates when NO

is added to air

containing SO

(Fig. 11).

The synergistic effects of SO

and NO

interactions are based on observations

of several metals: copper [92], nickel [93], steel [94], and zinc [95]. With the general

aim of performing a quantitative evaluation of the effect of sulfur pollutants in

combination with NO

, other pollutants, and climatic parameters, an international

exposure program within the UN Economic Commission for Europe (UN/ECE) was

implemented in 1987, including 39 test sites in 12 European countries, the United

States, and Canada [96]. The program is based on exposure of structural metals, stone

materials, paint coatings, and electric contact materials at test sites where measure-

ments of environmental parameters are already in progress. Judging from analysis of

results after 8 years of exposure, the influence of SO

on the corrosion rate of, e.g.,

carbon steel, weathering steel, zinc, cast bronze, and nickel is significant. No influence

of NO

has so far been observed on any of the materials investigated [97]. Hence, the

important role of NO

in laboratory exposures is not visible in the present field

exposures—most likeky as a result of catalysts or oxidizers (e.g., O

) in field

exposures that aid in promoting the oxidation of S(IV) to S(VI) and hide the effect of

. Future work should explore more precisely the reason for this discrepancy.

The conclusions from the UN/ECE program can be compared with those from

another exposure program performed at three test sites in Southern California and

characterized by very low levels (< l0 ppb) of SO

. Under these conditions, SO

Atmospheric Corrosion 551

shown to be of little consequence, whereas NO

, which is photochemically

converted to HNO

, is of significant importance [98].

There is ample evidence that synergistic effects caused by interacting

atmospheric constituents are significant in laboratory exposures. It remains to be

proved to what extent they contribute during field exposures. In addition to SO

and

, the interaction between SO

and O

[95] and between H

S and Cl

[99] is

expected to be important.

ATMOSPHERIC CORROSION INDOORS

Whereas the accumulated knowledge of outdoor exposures has extended over many

decades of corrosion tests, the history of indoor corrosion tests is short and mainly

goes back to the growing interest in corrosion effects on electronic materials during

the last two decades or so [100,101]. There are obvious differences in outdoor and

indoor exposure conditions and, consequently, expected differences between outdoor

and indoor corrosion behavior, some of which are summarized here.

The aqueous layer formed under outdoor exposure conditions is strongly

influenced by seasonal and diurnal variations in humidity and by precipitation, dew,

snow, or fog. Indoors, on the other hand, the aqueous layer is often governed by

relatively constant humidity conditions. This means that there are practically no

wet-dry cycles indoors and that the influence of indoor humidity can hardly be

described by introducing a time of wetness factor.

552 Leygraf

Figure 11 Mass changes of nickel as a function of exposure time in humid (75% relative

humidity) flowing air. The filled arrow indicates introduction of 250 ppb SO

and/or 300 ppb

, and the open arrow indicates termination of the supply of gaseous pollutants and of high

humidity in the flowing air. (From Ref. 93. Courtesy of The Electrochemical Society.)