Marcus P. Corrosion mechanisms in theory and practice

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Mostly due to enhanced indoor absorption of gases and particulates and also

due to retardation and damping of outdoor variations by ventilating systems and air

filtration, comparisons between outdoor and indoor concentrations of the most

important gaseous corrosion stimulants in general reveal lower levels indoors than

outdoors (Table 1). Exceptions are NH

and HCHO, which usually exhibit higher

levels indoors than outdoors as the result of anthropogenic activity [17]. Another

important difference is the decreased levels of indoor atmospheric oxidants, many of

them photochemically produced as described earlier, and of Fe(III)- and Mn(II)-

containing catalysts for promoting the oxidation of S(IV) to S(VI). Accordingly, the

expected lifetime of S(IV) compounds is greater indoors than outdoors [17]. As

discussed earlier, not only the concentration of pollutants but also the air velocity

determines the dry deposition velocity of corrosion stimulants. With decreased indoor

air velocities follow significantly lower dry deposition velocities indoors than

outdoors.

Consistent with the differences in outdoor and indoor characteristics, the

corrosion rate of many metals is significantly lower indoors than outdoors. By

examining the corrosion rates of copper, silver, nickel, cobalt, and iron in eight indoor

locations it was found that all metals except silver exhibited much lower rates indoors

than outdoors [102]. Within the UN/ECE exposure program a detailed study was

made of the influence of sheltering nickel samples in a ventilated box, simulating

exposure in unheated storage conditions. It was concluded that the deposition of SO

outside and inside the ventilated box differed by (approximately) a factor of 10, this

difference being attributable to different airflow conditions, a factor of 2, and SO

concentrations, a factor of 5 [103].

As manifested by the ISO classification system, the outdoor corrosion behavior

of structural metals can often be adequately predicted by only three parameters:

concentration, deposition of Cl

–

, and time of wetness. With considerably

less influence of these parameters indoors, it is reasonable to assume that the

relative importance of other corrosion stimulants increases. Evidence for this can be

exemplified in different ways. A classification system from the Instrument Society

of America (ISA) for predicting corrosivity in indoor environments includes

measurements of SO

, NO

, H

S, and Cl

in addition to relative humidity and

temperature [104]. Corrosion products formed on copper show much larger

variations in chemical composition after indoor than after outdoor exposure [17].

Results from Fourier transform infrared reflection absorption spectroscopy analysis

of corrosion products on copper, zinc, nickel, and iron formed in various benign

corrosivity indoor environments exhibit large amounts of infrared bands from

carboxylate ions, such as formate, acetate, and oxalate [105]. This may be an

indication of a stronger influence of carboxylic acids or other organic species

under benign conditions than in more aggressive environments.

The deposition of particles on corroding surfaces is of significant importance

in indoor exposures [106,107]. This is clearly seen when a comparison is made

between the main sources of deposited SO

2–

and Cl

–

on aluminum and zinc

surfaces during outdoor and indoor exposure. Outdoors, the accumulated SO

2–

both metal surfaces mainly originates from gaseous SO

and from SO

2–

in pre-

cipitation. Indoors, surface-accumulated SO

2–

is derived mainly from particles.

Similarly, surface-accumulated Cl

–

has three approximately equal outdoor

Atmospheric Corrosion 553

sources—reactive gases, precipitation, and particles—and two approximately equal

indoor sources: reactive gases and particles [107,108]. It is evident that future

descriptions of the mechanisms governing indoor atmospheric corrosion processes

must involve deposition of particles as a major source of corrosion-stimulating

pollutants [109,110].

Lower corrosion rates indoors than outdoors will require the development of

more sensitive techniques for corrosivity evaluation of indoor environments and also

an extended classification of indoor corrosivities [111].

DISPERSION OF METALS

So far this chapter has treated various aspects of the impact of the atmospheric

environment on metals. The opposite situation, in which a corroding metal can have

detrimental influences on the surrounding environment, is now an important issue

because of the increased concern regarding the amount of heavy metals dispersed

from roofs, facades, and other outdoor structures. Whereas long-term corrosion rates

of metals, including copper and zinc, have been studied in many outdoor exposure

programs, the dispersion of the same metals and their potential impact on the

environment are less well understood. Risk assessments by, e.g., organizations and

legislators have assumed that the runoff rate of a metal from a structure is equal to its

corrosion rate. It has moreover been assumed that all metal dissolved and dispersed

is bioavailable and therefore toxic to the environment. Both assumptions represent

upper limits, and the risk assessments made therefrom are quite conservative.

A realistic risk assessment of the corrosion-induced dispersion of metals in the

environment needs answers to at least the following questions:

1. What is the ratio between the runoff rate of a metal and its corrosion rate?

2. How does the runoff rate vary with time?

3. How bioavailable is the metal?

Copper and zinc have become the most frequently studied metals from this

perspective. The copper corrosion rate in unsheltered outdoor environments is always

highest in the beginning and slowly decreases with exposure time. The reason is the

gradual formation of more corrosion-resistant corrosion products, including Cu

(cuprite), Cu

(OH)

•H

O (posjnakite), and Cu

(OH)

(brochantite). The

time for this sequence to happen varies considerably between different environments

but brochantite, the end product in most patina formation, is often found within 2

years of unsheltered exposure.

From the large changes in corrosion product composition during the first 2

years and the concomitant reductions in copper corrosion rate, one would suspect

that the runoff rate undergoes similar changes as a function of exposure period.

Results from measurements on new or naturally aged copper have shown,

however, that the copper runoff rate is relatively constant during the investigated

2 year periods. Values in the range between 1.0 and 1.5 g/m

per year have been

found in relatively benign urban environments such as Stockholm. These values

increase with intensity and acidity of precipitation [112,113]. From a decreasing

copper corrosion rate with time, measured as the total copper mass loss, and a

554 Leygraf

relatively constant copper runoff rate follows that the quotient between copper runoff

and copper mass loss initially is low and increases with time. Taking Stockholm as

an example, the quotient was 7% after 1 month and 22% after 2 years [112].

As with copper, zinc runoff rates in atmospheric environments have turned out

to remain relatively constant over exposure periods of a few years. Measured zinc

runoff rates ranged from around 3 to 5 g/m

per year and could be correlated with the

concentration at sites with comparable annual amounts of precipitation [114].

For most sites the quotient between zinc runoff rate and zinc corrosion rate varied

between 50 and 60% during the first 2 years of exposure.

In all, evidence has been provided for both copper and zinc that suggests that

the runoff rate is determined more strongly by precipitation parameters (e.g., amount,

intensity, and pH) than by corrosion product parameters (e.g., chemical composition,

age, and morphology). This is in contrast to corrosion rates of the same metals, which

are also determined by corrosion product properties. Runoff rates are in general

significantly lower than corrosion rates. These values gradually get closer until the

corrosion product eventually reaches a constant thickness, at which point the runoff

rate equals the corrosion rate. This may take a few years or more on zinc and a few

decades or so on copper.

The runoff rates found for copper and zinc show only the total release of metal

from an outdoor construction without considering the chemical speciation of the

metal or its bioavailable fraction, i.e., the part of the metal that can interact with

living organisms and cause toxic effects. The most bioavailable, and therefore most

toxic, form is usually the free, hydrated, cation. Hence, in the case of copper or zinc,

Cu(H

and Zn(H

are among the most bioavailable species. Immediately

after leaving the copper surface, the fraction of free, hydrated, cupric ion in the

copper runoff is very high, in the range from 60 to 90% depending on the conditions

of the actual precipitation event. During its way to the biosphere, copper will react

with, e.g., organic matter and the fraction of bioavailable copper will become

successively smaller. When copper eventually enters agricultural soil only about

2% is considered to be bioavailable and only 1% of all copper in natural water

systems [115].

CONCLUDING REMARKS

Atmospheric corrosion of metals comprises a broad range of electrochemical,

chemical, and other processes in the interfacial domain, ranging from the atmos-

pheric region over the thin aqueous layer to the solid metal region. Of utmost

importance is the thin aqueous layer, which forms the medium for electrochemical

reactions and accommodates gaseous, particulate, or other pollutants from the

atmospheric region. Anode rather than cathode reactions are normally rate limiting

and may be facilitated by acidifying pollutants, such as SO

. The subsequent reaction

sequences in the aqueous layer are governed mainly by kinetic constraints, including

the concept of dry deposition velocity of pollutants and the nucleation, spreading, and

coalescence of corrosion products. Atmospheric corrosion rates are strongly

influenced by the formation and properties of corrosion products, where the formation

Atmospheric Corrosion 555

process can be described in a sequence of consecutive steps: dissolution–

coordination-reprecipitation. The dissolution step is usually acid dependent;

possible coordination steps can be predicted by the HSAB principle, and the

reprecipitation depends on the activities of species involved.

Extensive outdoor exposure programs have provided evidence that corrosion

rates can be interpreted in terms of deposition of primarily SO

and Cl

–

and time of

wetness. The history of indoor exposure programs is much shorter. Atmospheric

corrosion indoors is, in general, influenced by more constant humidity conditions and

lower levels of SO

and Cl

–

, from which follows an increased relative importance of

other corrosion stimulants including organic gaseous species and particulate

pollutants.

Despite all progress made in understanding individual processes, there is a need

to develop more comprehensive models in the study of atmospheric corrosion. The

first computional model developed in atmospheric corrosion [116] considers six

distinct regimes, namely the gas phase (G), the gas-liquid interface (I), the liquid

phase (L), the deposition layer (D), the electrodic regime (E), and the solid phase (S),

and is designated GILDES. Chemical reactions within the regimes can occur to

change the constituents of the regimes. Transport of chemical species of interest

between the regimes is also considered together with condensation and dissolution

processes and deposition and transport through the reaction products. For the six

regimes, mathematical formulations have been specified to describe the transitions

and transformations that occur. The conceptual framework used in the GILDES

formulation is truly multidisciplinary and requires the incorporation of many

scientific fields: gas phase—atmospheric chemistry; interface layer—mass transport

engineering and interface science; liquid phase—freshwater, marine, and brine

chemistry; deposition layer—colloid chemistry, surface science, and mineralogy;

electrodic regime—electrochemistry; and solid phase—solid-state chemistry.

GILDES model studies have so far included the initial SO

-induced atmospheric

corrosion of copper [117] and of nickel [118]. Comparison with experimental data

suggests that the model has been able to capture some of the most important processes.

Atmospheric corrosion involves a series of processes with periods of

high corrosion rates interrupted by periods of negligible corrosion rate. For a

deeper understanding of atmospheric corrosion, there is a need for more

sophisticated techniques for measuring instant corrosion rates coupled with

monitoring the deposition of the most important corrosion stimulants.

Activities in these and other areas are presently being carried out, and it is

anticipated that atmospheric corrosion will continue to develop into a multi-

disciplinary scientific field.

ACKNOWLEDGMENTS

The author wishes to express his gratitude to Dr. J. Tidblad for compilation of

environmental data and to Prof. E. Mattsson, Dr. I. Odnevall Wallinder, Dr. D.

Persson, and Dr. J. Tidblad, all from the Royal Institute of Technology or Swedish

Corrosion Institute in Stockholm, Sweden, for most valuable review comments. Mr.

B. Edin, Stockholm, is acknowledged for linguistic examination of this chapter.

556 Leygraf

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