(1997) Environmental Effect on Materials-copper Zinc and Aluminium

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Corrosion Science, Vol.39, No. 9, pp. 1505-l 530, 1997

0 1997ElsevierScienceLtdPrinted in Great Britain.All rights reserved

001 938X/97 S17.00+0.00

PII: soo1~938x(97poo474

ENVIRONMENTAL EFFECTS ON MATERIALS: THE EFFECTOF THE AIR POLLUTANTS SOS, NOz, NO AND O3 ON THE

CORROSION OF COPPER, ZINC AND ALUMINIUM.A SHORT LITERATURE SURVEY AND RESULTS OF

LABORATORY EXPOSURES

S. OESCH and M. FALLER

Swiss Federal Laboratories for Materials Testing and Research (EMPA), CH-8600 Diibendorf, Switzerland

Abstreet-Laboratory exposures of copper, zinc and aluminium were carried out in humid air containing single air

pollutants (S02, N02, NO, 03) and laboratory air in order to investigate their role in atmospheric corrosion.

Realistic pollutant supply rates as normally encountered in outdoor exposures were chosen for the experiments.

Some experiments resulted in the formation of corrosion products with morphologies commonly formed during

outdoor exposure. The air pollutants sulphur dioxide, nitrogen dioxide and especially ozone play a crucial role in

the atmospheric degradation of the materials investigated. Ozone has the strongest effect on the corrosion ofcopper

followed by nitrogen dioxide and sulphur dioxide. Realistic corrosion products such as cuprite (CuzO), basic copper

nitrates and basic copper sulfates were identified in these experiments. The effect of sulphur dioxide exceeds by far

the effects of the other air pollutants in the corrosion of zinc. The experiments resulted in the formation of zinc

sulphates, which are water-soluble at the test conditions (relative humidity: 90%). Ozone plays a significant role inthe corrosion of aluminium, while the effect of sulphur and nitrogen dioxide was considerably smaller. The present

work summa&s recently discovered effects of the air pollutants mentioned and presents results of a comparative

study on the effects of single air pollutants on the corrosion of copper, zinc and aluminium under realistic test

conditions. The microstructures of the corroded surfaces were investigated by scanning electron (SEM/EDX) and

also partially by Auger electron microscopy (AES). Corrosion products were analysed using X-ray diffraction

(XRD) and ion chromatography (IC). Results from gravimetric evaluation are also reported. 0 1997 Elsevier

Science Ltd

Keywords: A. copper, A. zinc, A. aluminium, B. XRD, C. atmospheric corrosion.

INTRODUCTION

Many investigations have been performed in the current century in order to clarify the role

of environmental and climatic factors in the atmospheric corrosion of commonly used

building and construction metals as well as to simulate their corrosion behaviour. In the last

decades the improved analytical characterisation of the corrosion products on one hand and

the more complete consideration of environmental parameters on the other hand led to an

almost complete knowledge of the corrosion products, the structure of the corrosion layers

as well as the environmental parameters. The links between them, however, are not yet fully

understood since the processes occurring in the corrosion system, which is a multiphase

system, turned out to be rather complex, involving chemical reactions, equilibria and

transport phenomena in the gaseous, aqueous as well as the solid phase. They include

several corrosive species and corrosion products, which interact with each other and all of

Manuscript received 5 March 1996; in amended form 26 March 1997.

1505



1506 S. Oesch and M. Failer

them vary in amount and time. The electrochemical corrosion process itself, i.e. the metal

dissolution induced by a reduction reaction is only one but a decisive component among the

reactions occurring in the corrosion system.

For a long time the degradation of most metals in the atmosphere was attributed to the

deteriorative effect of sulphur dioxide and in special environments also to chlorides. Theyreceived thorough attention in atmospheric corrosion research and are the most extensively

studied pollutants. The environmental protection laws enacted about two decades ago in the

industrialized countries and their enforcement led to a notable decline in the sulphur dioxide

levels. Concentration levels of other air pollutants such as the nitrogen oxides and secondary

pollutants such as ozone, however, remained rather constant or even increased slightly. This

has increased the relative significance of these air pollutants in the atmospheric degradation

of materials.

THE EFFECTS OF AIR POLLUTANTS ON THE ATMOSPHERICDEGRADATION OF MATERIALS

The electrochemical nature of the corrosion process implies the presence of an

electrolyte provided by atmospheric precipitation or by adsorption of water molecules on

the surface of the corrosion layer. This electrolyte is obviously present during times of

precipitation and dew, i.e. if the temperature of the surface is lower than the dew point.

Adsorbed water layers with a sufficient thickness (i.e. some nm) for the corrosion process to

take place are, however, present much below the dew point. Thus the relative humidity also

plays a central role among the climatic factors, this, especially since the presence of corrosive

species, i.e. deliquescent corrosion products or salts, which attract water vapour and becomesoluble above a critical relative humidity, can lead to a sharp increase in the corrosion rate.

This critical relative humidity depends on the type of corrosion products or salts formed on

the surface and therefore on the type of metal and pollutants present in the system. The

electrolyte thus formed is a precondition not only for the electrochemical reaction to take

place but also for the pollutants to adsorp significantly faster, since they are transformed in

the electrolyte allowing further deposition. This leads to a more corrosive electrolyte.

Atmospheric pollutants are transported principally in two ways to the surface of the

metals, namely by dry and wet deposition processes. This leads to the fact that the

atmospheric constituents found on the metal surfaces are as abundant as in the atmosphere

itself. Among them sulfates, nitrates, nitrites, chlorides, carbonates, hydrogen ions,ammonium, metal ions, atmospheric particles and also organic compounds are commonly

found constituents of the electrolytes or corrosion layers and have been shown to have an

effect on the corrosion processes. They result either directly from the deposition processes or

from mainly aqueous phase reactions of the deposited atmospheric constituents. Sulphates,

nitrates and nitrites originate either directly from the wet deposition processes, from particle

deposition or from reactions of the gaseous air pollutants in the aqueous phase of the

electrolyte.

In the case of sulphur dioxide, dissociation equilibria and subsequent irreversible

homogenous or heterogenous catalysis by transition metal ions particularly ferric and

manganous ions, chlorides and to a lesser extent also cupric ions, partially also their

complexes and solid surfaces of oxides account for a major portion of the sulphate found on

the surfaces.’ -’ Its high water-solubility and the fast oxidation reactions are the reasons for

the comparatively high amounts of sulphate found on the surfaces. Other pathways for the



The effects of air pollutants on the corrosion of Cu, Zn and Al 1507

formation of sulphate are reactions with dissolved nitrogen oxides and their oxyacids.879 In

the presence of semiconducting corrosion products also photochemical reactions can

occur.“-‘3 Atmospheric oxidizing agents such as ozone and hydrogen peroxide, as well as

radicals such as hydroxy are also important reactants for the formation of sulphate.2*‘4 The

catalytic activity of particles, i.e. dust and flyash was mainly attributed to the already

mentioned activity of the dissolved metal ions, their complexes and oxides formed in the

electrolyte. l5

A consideration of the system of the nitrogen oxides and water shows16 that in the

equilibrium state a considerable concentration of nitrate and to a lesser extent nitrite is

present. However, the reactions, reversible in contrast to the formation of sulphate, leading

to the ionic species are slow under atmospheric conditions.8 This may not be the case for

higher nitrogen oxide concentrations encountered in laboratory experiments. Faster

pathways for the nitrate and nitrite formation are reactions involving ferrous ion,l7 ozone

and hydrogen peroxide’.” and in the presence of sulphur dioxide also the already mentionedreactions with the nitrogen oxides and their oxyacids. These mechanisms may be the main

reasons for the occurrence of considerable amounts of nitrites and nitrates found on the

surfaces of metallic materials’9-23 as well as building stones.24*25For metallic materials,

additional reduction reactions of the nitrogen species by the metals or corrosion products

have to be taken into account.

The adsorption and interaction of these corrosive species formed in the electrolyte lead

in a further stage to the actual corrosion process, i.e. the metal dissolution balanced by

reduction reactions and the formation of the corrosion products. In the presence of the

gaseous air pollutants discussed, not only the commonly considered oxygen reduction and

the hydrogen ion reduction reaction have to be taken into account but also reductionreactions with ozone (O(O)+ O(II)), the IV-valent sulphur species in combination with

oxygen (S(IV)-S(VI), O(O)+O(II), S(IV)+S(II)) and the nitrogen oxide species

(N(V)+N(III), N(IV)-+N(III), N(III)+N(II),...), all of them exhibiting high reduction

potentials.26

The corrosion products formed depend on the type and amount of corrosive species

present in the corrosion system. The physical and chemical properties of the corrosion

products and corrosion layers formed on the surface of a metal, their distribution as well as

their morphologies have an important impact on the further course of the corrosion, i.e.

whether a passivating corrosion layer can build up and whether a decline in corrosion rate

can therefore be observed. This is crucial in deciding whether or not a metal in its

unprotected nature is suitable for the use as a building and construction material under

outdoor exposure conditions.

opper

The outdoor corrosion of copper yields a passivating corrosion layer within a few days

or weeks, which turns in a further stage from reddish brown to dark brown or black, due to

the oxidation of copper and the formation of cupric and cuprous oxides, the latter being

thermodynamically more stable27 and also predominantly observed, as well as cuprous

hydroxides. 28329 his layer is generally considered as dense leading to a good protectionagainst further .corrosion. In the following corrosion stages, which can last up to 20 years,

depending on the environmental conditions, a greenish-blue patina with a layered structure

is formed. It mainly COnSkiS of the basic copper sulphate brochantite (Cu4S04(OH)&

but antlerite (Cu3S04(0H)4) and posnjakite (CU~SO~(OH)~.~H~O) as well as copper



1508 S. Oesch and M. Faber

sulfides’ 2,30are also commonly found constituents of the patina. The latter was found

especially at sites with comparative high sulphur dioxide pollution levels. In connection with

sulphur dioxide cathodic reduction reactions with S(W) have been shown to be

significant.2g*3’ In addition, copper chlorides, such as atacamite (Cu$l(OH)3) occur in

atmospheres containing high chloride concentrations, i.e. near the sea, as well as nitrates,such as gerhardite (CU~N~~(~H)~).‘*~*‘~~*~~~Carbonates, such as malachite

(C+CO3(OH)2) and organic copper salts occur more rarely and only in traces.‘2’34’35

The corrosion layers formed lead to a good protection of the metal from further

corrosion and are the reason for the wide use of copper as a building and construction

material as well as the good conservation of many cultural monuments. Corrosion rates

decrease drastically during the exposure to the environment. For samples recently exposed

(unsheltered) in Europe, in the first year of exposure typical corrosion rates of from 0.3 to

5 urn/y were observed, while subsequently steady-state corrosion rates of up to 1.5 urn/yresulted 20.23.32.36

These rates strongly depend on the climatic and environmental conditionsencountered.

From the corrosion product composition, it is obvious that sulphur dioxide and to a

minor extent the nitrogen oxides play a central role in the patina formation. The corrosion

accelerating effect of sulphur dioxide is well known. It was found that in its presence

corrosion starts to be significant as soon as the relative humidity is above 5G63°~.37-39 The

thickness of the electrolyte is then approximately 2-6 nm.40*4’ In multiphase systems,

however, the concept of critical humidity is of limited applicability without a precise

understanding of the corrosion products present, since no critical humidity in an absolute

sense can be found.33

Recent correlation analysis between mass loss and sulphur dioxide concentrations ofsamples exposed outdoors led only to poor correlation (correlation coefficient:

R <0.43).32*36 The additional consideration of other gaseous air pollutants and climatic

factors improved the correlation significantly R < 0.96).36Y42or a certain combination of

sulphur dioxide, wind speed and time of wetness a corrosion-stimulating effect was found,

while for nitrogen dioxide a corrosion-inhibiting effect resulted. However, since a

correlation between the nitrogen dioxide and the ozone concentration (N02= 63.7-

0.91 0,) exists, the negative effect of nitrogen dioxide can also be interpreted as a positive

effect of ozone or a combination of both.36 In laboratory exposures sulphur dioxide and

nitrogen dioxide resulted in comparable and small metal losses.46 At small sulphur dioxide

concentrations (50 ppb) the effect of sulphur dioxide was even found to be similar to the

effect of pure air.4s Combinations of sulphur dioxide and nitrogen dioxide, however,

resulted on one hand in a strong acceleration of the corrosion of copper at high relative

humidities (75-90%)44 and on the other hand in higher deposition rates of sulphur dioxide

compared to the experiments with sulphur dioxide alone.45’46Additionally, the evolution of

HN02(g)45 and N04’ was observed in experiments with nitrogen dioxide and to a greater

extent also with combinations of sulphur dioxide and nitrogen dioxide. The reactions of the

nitrogen oxides in the aqueous electrolyte lead to nitrates and nitritesI In the presence of

Cu2+ and nitrate, the copper nitrate CU(NO)~OH with a critical relative humidity of 5 1%

can be formed, while for nitrite no corresponding copper nitrite is known. These reactions

however cannot lead to the considerable amounts of HNOz(g) observed. Therefore copper

must participate in this reaction. Ericsson46 showed that reduction reactions of nitrogen

dioxide by cuprite or metallic copper take place. In combination with sulphur dioxide higher

sulphate as well as HN02(g) concentrations were found. This can be attributed to the




aqueous phase reaction of S(IV) with nitrogen dioxide mentioned above’*8 and the

reduction reactions by cuprite or copper.46 At a lower relative humidity (70%) no

synergism was found. The effect of the two gases on metal loss was found to be about 34

times smaller than at the higher humidity (95%).

In the presence of sulphur dioxide the corrosion products cuprite, basic coppersulphates, copper sulphites as well as hydrated copper sulphates (critical relative humidity:

97%) were observed, while in experiments with nitrogen dioxide cuprite and the basic

copper hydroxide gerhardite, Cu2(0H)sNOs were found. In water rinses nitrate dominates

over nitrite.&

Laboratory exposures with combinations of sulphur dioxide and ozone or nitrogen

dioxide (75% relative humidity) illustrated that the deposition rates of sulphur dioxde

increased in the presence of ozone more than in the presence of nitrogen dioxide.45,46 This

was mainly attributed to the oxidation of S(IV) by dissolved ozone or its reaction

products.2.‘4 However, Tidblad45 pointed out that an ozone effect other than that of the

sulphur dioxide oxidation is indicated. Cuprite, basic copper sulphates and copper sulphites

are also the main corrosion products identified here. The effect of ozone was also

investigated in the presence of hydrogensulphide and its corrosion accelerating effect was

attributed to the oxidation of hydrogensulphide to free sulphur and sulphate, thus

accelerating the corrosion.‘3Y38

Zinc

In the first step of corrosion the atmospheric corrosion of zinc leads to a thin surface

layer of zinc hydroxides. 35*48After a few months of exposure in medium polluted

atmospheres zinc carbonate, zinc hydroxide, hydrozincite (ZnS(OH)6(C03)J and zincoxide are formed, the latter being the thermodynamically most stable corrosion product in

the Zn2+/H20-system.35 Due to the protective action of the corrosion layer formed, the

corrosion rate slows down somewhat.49 In moderately polluted atmospheres zinc hydroxy

sulphates such as Zn4S04(OH),j.nH20 and in marine sites zinc hydroxy chlorides, with a

comparatively low water-solubihty have been found as components of the corrosion

layers.52*53Nitrates and organic compounds are present only as traces.55 The stability of the

protecting layer, however, compared to the other investigated metals is restricted to a rather

small pH-range.59*60 The presence of high sulphur dioxide or chloride concentrations lead to

the dissolution of the protective layer and to the formation of corrosion products with a

good water-solubility. The protection qualities of the corrosion layer formed are therefore

rather low. This leads to the fact that practically no decline in the corrosion rate with

increasing exposure time was observed.5c52F54*6’Recent results from environments with

comparatively low sulphur dioxide pollution levels have shown that the decrease in

corrosion rate with time is rather small and therefore the protective properties of the

corrosion layer is also low even in low polluted atmospheres.36 The formation of water-

soluble zinc sulphates among other factors are responsible for the comparatively high

soluble sulphate and zinc ion concentrations” found in aqueous extract of the corrosion

products.

Corrosion rates during the first year of exposure lie typically in a range from 0.6 to

4.7 urn/y for samples recently exposed in Europe. The corrosion rates, however, do not slow

down significantly as seen by the steady-state corrosion rates, which lie in a range from 0.8 to

4.7 um/y.20’35,54

Until recently, sulphur dioxide was thought to be the main gaseous air pollutant in the



1510 S. Oesch and M Failer

atmospheric corrosion of zinc.56*57Y62 orrelation analysis from outdoor exposure

experiments resulted in statistically significant correlations between metal loss data and

sulphur dioxide concentrations (Rc0.70). This correlation became even higher when the

time of wetness and the wind speed were also taken into account (R<0.95).24@ The

additional consideration of nitrogen dioxide led mainly to negative coefficients, as already

observed with the copper samples.23*36*58 ere also, the inverse proportionality between the

nitrogen dioxide and ozone concentrations at the test sites has to be taken into account,

indicating a corrosion accelerating effect of ozone.

Laboratory exposure with realistic gas concentrations confirmed the corrosion

accelerating effect of sulphur dioxide. Haynie found a critical relative humidity of 49% in

the presence of sulphur dioxide.62 The deposition rates of sulphur dioxide are similar to

unalloyed steel or weathering steel samples and are rather high at high relative humidities44

in contrast to copper and aluminium samples.

Laboratory exposures conducted by Henriksen63 at high relative humidity (95%) withnitrogen dioxide as the single pollutant showed that its effect on weight gain is considerably

smaller compared to sulphur dioxide. Also for combinations of these gases practically the

same weight gain resulted as with sulphur dioxde alone. Johansson44,65 confirmed the

comparatively small effect of nitrogen dioxide but found a synergistic effect when combined

with sulphur dioxide at 95% as well as 90% relative humidity and found that small amounts

of nitrogen monoxide were formed. Svensson,66 however did not find nitrogen monoxide,

but HN02(g) and attributed this to a reduction of nitrogen dioxide by zinc, which was not

observed in the combined experiment. At 70% relative humidity no synergistic effect was

observed and the weight gains were considerably (factor 17) smaller. The apparent

contradiction between the results of Henriksen and Johansson for the combinedexperiment can be explained by the properties of the corrosion product formed

(ZnS04.7H20), which has a critical relative humidity of 89% (22°C). According to our

own experiments (see below) above the critical humidity, sulphur dioxide is absorbed and

oxidised quantitatively in the electrolyte. The uptake of water results in a rather thick

electrolyte, which drips off the samples. This, on one hand, and the lack of further sulphur

dioxide, which can be oxidised, on the other hand, could explain the differences

encountered.

Laboratory experiments with ozone are rare. However, Svensson66 conducted

experiments with ozone and combinations of sulphur dioxide and ozone. He found a

negligible effect of ozone in the absence of sulphur dioxide, while the combination with

sulphur dioxide resulted in a corrosion accelerating effect at 70% as well as 95% relative

humidity compared to the effect of sulphur dioxide alone. This is due to the aqueous phase

oxidation of S(IV) mentioned above. The oxidative effect of ozone was shown to be stronger

than that of nitrogen dioxide. Here also the experiment at the lower humidity resulted in

considerable smaller (up to factor 13) mass gains.

In the presence of sulphur dioxide and its combination with nitrogen dioxide, zinc

hydroxides, zinc carbonate, zinc oxide, zinc sulphates and basic zinc sulphates, as well as

water-soluble nitrates were found.52,59,65*66 hort time experiments in the presence of high

sulphur dioxide concentrations also led to the formation of zinc sulphites, which convertedto zinc sulphates in the later stages.67 The corrosion products in the presence of the single

pollutants nitrogen dioxide and ozone are not well known. However, nitrates and nitrites

were found in the water rinses of experiments with nitrogen dioxide.53*65,66 dney6s found

that nitrites dominate over nitrates and suggested that an electrochemical reduction of




nitrate to nitrite takes place. In contrast, Sydberger53 found that this reaction, here assumed

to be the reduction of dissolved nitrogen dioxide to nitrite, takes place only in the presence

of chlorides.

Aluminium

The corrosion resistance of aluminium against atmospheric degradation is mainly due to

a thin, compact, low electron-conducting corrosion layer, which. is rapidly formed and in

case of destruction reformed. The corrosion layer formed in the presence of water consists of

aluminium hydroxides and oxides and their hydrated compounds. Due to the instability of

this protection layer at pH < 4 and pH > 9, it can locally be destroyed by atmospheric

pollutant accumulations.34*26 In the presence of chlorides pitting was observed, while a

uniform, but locally enhanced corrosion attack was observed in the presence of sulphur

dioxide and particles.60369-72The occurrence of pitting in the presence of chlorides is

attributed to the formation of aluminium chlorides. They are less stable compared to thealuminium hydroxides or oxides and therefore react further thereby liberating chloride so

that the corrosion process can go on. In contrast, sulphate is captured and inactivated in

stable aluminium sulphates of mainly amorphous character. Due to the lower stability of

aluminium hydroxides and oxides, they can be transformed to aluminium sulphates.73 At

higher sulphate levels pitting was also observed in solutions.74 Moreover, samples exposed

to the atmosphere contain nitrates, ammonium and metal ions from airborne particles and

trace amounts of carbonates.‘9374Y75However, aluminium nitrates or carbonates were not

identified in field exposures and in addition, no naturally formed minerals are known. This

can be attributed to the good water-solubility of the nitrates and carbonates.

Corrosion rates of freely exposed samples decrease slightly with increasing exposuretime. Corrosion rates in a range from 0.04 to 0.7 pm/y for samples exposed for two years and

from 0.04 to 0.6 pm/y for those exposed for four years. These values were measured on

samples recently exposed in Europe.20,35354

Field exposures of aluminium resulted in a good corrosion resistance in sulphur dioxide

polluted atmospheres. However, localised corrosion attack was observed in most European

environments. Furthermore statistically significant correlations were found between the

sulphur dioxide deposition and the mass losses as well as the sulphur dioxide and the time of

wetness. 23*58*77*78heltered aluminium samples are attacked much more than freely exposed

samples (factor 2-2.5).58 This is in contrast to the other materials discussed and also to

unalloyed carbon and weathering steels.

Laboratory tests with sulphur dioxide, sulphate and chloride solutions confirmed the

corrosion-stimulating effect of these species in atmospheric degradation.79-81 In contrast to

the other materials discussed, few laboratory exposures with realistic test atmospheres have

been carried out to investigate the effect of the other gaseous air pollutants on the corrosion

of aluminium. Experiments of Johannson and Syderberger8* using sulphur dioxide and

nitrogen dioxide as well as a combination of both showed small effects of the single gases

and a synergistic effect of the gas combination.

Goal of t he laborat ory exposuresThe atmospheric corrosion of materials is rather complex, as shown above. Past research

on the corrosion of copper, zinc and aluminium has mainly concentrated on the effect of one

single air pollutant or on sulphur dioxide and its combinations with other air pollutants.

One goal of the following laboratory experiments is to compare the effects of single air




pollutants on materials. This cannot be extracted from literature data, since experimental

conditions and materials and their preparation differ from study to study. Another goal is to

give quantitative information on the effects of the less frequently investigated air pollutants

nitrogen dioxide and nitrogen monoxide as well as ozone, which were neglected in corrosion

research up to now. The data presented give additional information about the time

dependence of the corrosion loss and the micro structure of the corrosion layers and

corrosion products formed. These are important since they indicate the corrosion

mechanisms and protection qualities of the corrosion layers formed.

The experiments help to clarify the effects of the air pollutants on materials in natural

atmospheres. Due to the complexity and variability of the atmospheric parameters and the

corrosion reactions which can take place, this would almost be impossible in outdoor

exposure programmes.

EXPERIMENTAL METHOD

Labor at ory exposure s.vst em and exposure condit i ons

The samples were subjected to a laboratory exposure system, which has been described

previously.26 Experiments were carried out using the gas concentrations and gas supply

rates given in Table 1. Air pollutant concentrations of 10 f 0.2 ppm were chosen resulting in

realistic gas supply rates, except for sulphur dioxide. Since the sulphur dioxide levels (1.5-

10.4 pg/m3-0.4-2.7 ppb) at the outdoor exposure sites in Switzerland were considerably

lower compared to those of the other air pollutants considered, additional experiments with

0.5 ppm were carried out yielding realistic sulphur dioxide supply rates. The samples were

orientated 45” towards the flow direction. For all experiments, the temperature was25 + 0.5”C and the relative humidity 90 f 1%.

The process gases are taken from cylinders (SO2 = lo%, NOz = lOO%, NO = 10%)

Table I, Ambient air pollutant concentrations and corresponding gas supply rates (presentation rates) of some

typical environments of Switzerland [23,36] are compared with the laboratory exposure conditions. The

presentation rate is calculated taking an average wind speed of 2 ms-’ (horizontal) for outdoor exposure and

0.004msC’ (vertical) for the laboratory exposure. For the calculation of the actual presentation rates the

orientation (45” towards the flow) of the samples has to be taken into account

Test-r

Pollutant

type of site

Outdoor exposure

Pollutant Pollutant

concentration supply rate

Wm31 Wm2sl

Laboratory exposure

Pollutant Pollutant

concentration supply rate

[pg/m31 @g/m2sl

SO2Rural

Suburban

Urban

NO2

Whole range

NO

Whole range

03

Whole range

1 o 2 2 to 4 4 (1.5 ppb) 0.016

2to 10 4 to 20 1309 (0.5 ppm) 5.24

lot025 20 to 50 26180 (10 ppm) 104.7

5 to 50 10 to 100 18800(10ppm) 75.2

0.5 to 85 I to 170 12200 (10 ppm) 48.8

20 to 75 40 to 150 19200 (10 ppm) 76.8



The effects of air pollutants on the corrosion of Cu, Zn and Al 5 3

except for ozone, which is generated from clean and dry air by high voltage. The ozone

generator (model: Sander 50) produced a significant amount of nitrogen dioxide. For the

experiment with 10 ppm ozone, the average nitrogen dioxide concentration in the test

chamber was 150 ppb. The gases pass through mass flow controllers and are mixed with the

conditioned laboratory air prior to entering the test chamber.

The gas concentrations in the laboratory air used as diluent have an effect on the

experiment as will be shown below. Average concentrations of the gases in the laboratory air

during the experiments were as follows: SO1 = 0.6kO.4 ppb, NO2 = 10 + 3 ppb and

03= 16+6ppb.

M aterials and preparati on

Plates 5 x 10 cm in size and 1 or 2 mm in thickness were used. Commercially available

grades of copper (Cu >99.85, desox.), zinc (Zn >99.995) and aluminium (Al >99.5,

Fe ~0.40) were employed. The copper and zinc samples were treated with aqueouspickling solutions and thoroughly cleaned in de-ionized water prior to exposure. After

exposure the corrosion products were removed by the same pickling solutions. The

following aqueous pickling solutions were used: 5% HCl (Cu) and 10% chromic acid (Zn).

Aluminium samples were used as delivered after degreasing.

Inv esti gati on met hods

The evaluation is based on gravimetric measurements of the samples. Material loss and

the mass of the corrosion products retained on the surface were determined by weighing the

test specimens before exposure, after exposure and after removal of the corrosion products

using the pickling solutions mentioned above. The corroded surfaces were investigated byelectron microscopy (SEM, JEOL JSM 6300 F). Chemical compositions of the corrosion

products were determined by EDX at an acceleration voltage of 20 kV. The ZAF correction

was used to calculate the composition based on K,-intensities and internal standards. X-ray

diffraction in the conventional 0-28 set-up (XRD, Siemens D 500, CuK,) was used to

identify the crystal structure of the corrosion products. In the case of aluminium Auger

electron spectroscopic (AES) measurements were carried out. For the determination of the

water-soluble anions, the plates were treated with 50 ml de-ionized (Mini-Q) water in an

ultrasonic bath for 10 min. The anions were determined by ion chromatography (Shimadzu

HIC 6A, anion column: Metrohm Supersep). These solutions were also used to determine

pH and electrical conductivity.

EXPERIMENTAL RESULTS AND DISCUSSION

Copper

The results of the gravimetric evaluation are given in Fig. 1. Laboratory air and nitrogen

monoxide have practically no effect on the corrosion of copper. The material losses, were

lower than 30 nm in four weeks exposure and the corresponding weight gains lower than

80 mg/m’. In contrast, sulphur dioxide, nitrogen dioxide and especially ozone led to

considerable material losses. For up to one week exposure, the effects of these gases arealmost equivalent. Later, sulphur dioxide and then also nitrogen dioxide led to a

considerable decline in the corrosion rate and therefore to a passivating effect, while the

corrosion in ozone progressed uninfluenced. The weight gains (Fig. 2) show a rather

different evolution, especially concerning sulphur dioxide. The weight gains of the samples



1.514 S. Oesch and M. Failer

2. 0

1. 5

- z

3

a, o 1. 0

3

&

I

0. 5

0. 0C.a

10ppmOq 10ppmN 2

LilOppmS02

A lOppmN0

0 Laboratory air

Exposure time [wseksl

Fig. 1. Material loss of the copper samples exposed to air containing different air pollutants (SOZ,

NOz, O,, NO, laboratory air) at 25°C and 90% relative humidity. Error bars = standard deviation of

three samples.

J

0.0 1 0 2.0 3.0 4.0

Exposure time [weeks]

Fig. 2. Weight gain of the copper samples exposed to air containing different air pollutants (SOs,

N02, Os, NO, laboratory air; for legend see Fig. I at 25°C and 90% relative humidity. Error

bars = standard deviation of three samples.



The effects of air pollutants on the corrosion of Cu. Zn and Al 1515

exposed to sulphur dioxide show a much smaller decrease in corrosion compared to the

material losses and are comparable to those of the samples exposed to ozone. This is due to a

further uptake of sulphur dioxide and subsequent formation of corrosion products

containing sulphate while the attack of the base metal was already insignificant, as will be

shown below.

The experiment with 0.5 ppm sulphur dioxide led to surprisingly small weight gains,

namely only slightly higher than those found with 10 ppm nitrogen monoxide and

laboratory air. Similar results were also obtained in other laboratory experiments.45*46

The results from ion chromatographic analysis of the water-soluble anions are given in

Table 2. The highest concentration of corrosive species was found on the samples exposed to

sulphur dioxide. This was followed by nitrogen dioxide and ozone, while very small

concentrations were found on the samples exposed to nitrogen monoxide and laboratory

air. Although the corrosion rate of the samples exposed to sulphur dioxide decreased nearly

Table 2. Water soluble anions [ug/lOO cm*], pH and conductivity x [uS/crn] of aqueous extracts of the corroded

copper and zinc specimens* after different exposure times t [weeks]. Neither nitrites nor sulphites were detected

500 ppb SO2 10 ppb SO*

t : sopMaterial [weeks] PH [u&m] [u;m2] $%I~] pH I@yml hUm21 bzi21

cu I 7.4 9.6 155 <5 5.6 44 850 <5

cu 2 6.6 10.4 160 <5 5.6 103 2000 <5

cu 4 6.3 17.8 305 <5 5.7 126 2450 <5

10 ppb NO2 10 ppb NO

t x so:-

Material [weeks] pH W/cm1 [mldm21

NO?

Wdd PH:

C&4 fr$t:21NO:

hsldm21

cu 1 6.2 9.2 6 80 6.1 2.34 7 <5

cu 2 6.2 7.1 12 110 6.1 2.52 29 <S

cu 4 6.1 5.9 4 72 6.0 1.71 8 <5

Zn 1 6.6 36.0 22 820 6.2 2.82 17 <5

Zn 2 6.5 54.3 12 1400 6.1 2.34 15 <5Zn 4 6.7 65.9 13 1530 6.0 2.22 25 <5

lOppb03 Laboratory air

t x so:- NO, so*-

Material [weeks] PH W/c4 Wdm21 Wdm2 PH WLI bg/~~21 $21~1

cu 1 6.1 7.3 45 110 6.8 5.14 15 15

cu 2 6.6 14.1 16 220 6.8 4.61 I5 <5

cu 4 6.5 14.1 10 230 6.7 5.83 IS 15

Zn 1 6.2 17.7 16 400 6.5 4.62 5 15

Zn 2 6.4 49.3 19 1230 6.8 4.90 15 <5

Zn 4 6.6 81.5 20 2180 6.9 4.99 10 (5

*Liquid zinc sulphates were formed and dropped off the samples in the experiments with S02. Therefore the

sulphate content was not determined but can be estimated from the mass loss.




to zero after about one week exposure, the uptake of sulphate still remained at a high level.

The uptake of sulphur dioxide (water-soluble sulphate) however, is rather small in relation

to the supplied amount. After four weeks exposure an uptake of 0.46% for the 0.5 ppm

experiment and 0.18% for the 10 ppm experiment resulted. The experiment with nitrogen

dioxide led to considerably lower nitrate concentrations compared to the sulphateconcentrations found in the experiment with sulphur dioxide where no nitrites were

detected. This reflects on one hand the low water-solubility of the nitrogen oxides (NOz:

0.01 mol/(l atm), NO: 1.9 x 10e3 mol/(l atm)) compared to sulphur dioxide (1.24 mol/

(1 atm)) and on the other hand the slow reaction kinetics of the nitrogen oxides in the

aqueous solution of the electrolyte. The exposure to ozone led to the formation of a

considerable amount of water-soluble nitrites. The reason for this was previously

discusseds3 and is on one hand due to the formation of HN03 and N205 in the gas phase

by reactions of nitrogen oxides and ozone (NO2 + 03-+NOs + 02, NO3 + NOz+N205,

Nz05 + HzO-+2HN03) and the subsequent adsorption of HNOs in the electrolyte (Henryconstant: 2.1 10’ mol/(l atm)) and on the other hand due to aqueous phase reactions of

ozone, namely NO*- +O,+NOs- + OZ. Since the high gas phase concentrations lead to

much faster reaction kinetics than observed in natural environments, it must be mentioned

that the formed concentrations of HN03 and Nz05 are not realistic. At the sample positions

a calculation of the reaction kinetics (reaction time: 2 min) yielded concentrations of

approximately 6 ppb HN03 and 43 ppb N205. Taking into account the sample orientation

and the flow conditions (see Table 1) an amount of 130 ug HN03/week is expected to be

deposited on the sample (surface area: 50 cm*) accounting for 100% of the water-soluble

nitrate found on the samples (compare with Table 2). For the experiment with 10 ppm

nitrogen dioxide a HN03 concentration of 1.4 ppb and a Nz05 concentration of 13 ppbresulted.

In the experiments with nitrogen dioxide, nitrogen monoxide, ozone and laboratory air,

the deposition of sulphur dioxide originating from the laboratory air (used as diluent) was

small and the amounts were comparable. This shows that the aqueous phase oxidations of

sulphur dioxide to sulphate in the presence of ozone and also the nitrogen oxides are not

relevant under the experimental conditions employed.

X-ray diffraction analysis of the samples exposed to sulphur dioxide revealed the basic

copper sulfate antlerite (CU+SO~(OH)~) which was also often found in outdoor exposures.

After four weeks exposure in the 10 ppm atmosphere chalcanthite (CuS04.5H20)) was

probably also formed. For the samples exposed to 0.5 ppm sulphur dioxide the investigation

of the micro structure of the corrosion products by light- and scanning electron microscope

(SEM/EDX) revealed some local attacks consisting of corrosion products containing

sulphur. Corrosion spreads out from these local areas (Fig. 3) and attacks the base metal

further, yielding a thin corroded surface layer. The main portion of the surface, however,

remained unattacked, as could already be concluded from the small weight gains. The extent

of corrosion of the samples exposed to 10 ppm sulphur dioxide was considerably higher but

the nature of corrosion was the same as observed at the lower concentration, The attack of

the surface layer started at many local points in connection with corrosion products

containing sulphur and then spread out from these areas forming a thin corroded surface

layer (Fig. 4). After one week exposure, between 5 and 10% of the surface was visible

attacked; after four weeks exposure almost the whole surface was attacked (g&90%). EDX-

measurements yielded Cu/S-ratios of 2.9kO.4 and O/Cu-ratios of 2.3kO.35 for the

uniformly attacked areas. These are quite close to the theoretical values of antlerite (Cu/




S= 3,O/Cu = 2.67). The ratio m of the mass of the corrosion products divided by the metal

loss (Table 3) yielded a value of 1.74 after four weeks exposure, which is also quite cl1se to

the Iheoretical value of antlerite m = 1.86). The lower values for the shorter exposure times

and for the experiment with 0.5 ppm sulphur dioxide suggest the presence of unde,tected

corr ,osion products such as Cu20 m = 1.13), CuO m = 1.25) or copper hydroxides, wit h low

ratic3s m. The values m increase continuously during the experiment, even if the cot-r osion

rate was insignificant. This is due to further uptake of sulphur dioxide and the subse quent

t i lOurnFig. 3. Microstructure (SEM/SEI) of the corrosion products of the copper samples after 4 weeks

exposure in 0.5 ppm SO* at 25°C and 90% relative humidity. Presentation rate see Table 1.

1OpmFig. 4. Microstructure (SEM/SEI) of the corrosion products of the copper samples after one week

exposure in 10 ppm SO* at 25°C and 90% relative humidity. Presentation rate see Table I,



1518 S. Oesch and M. taller

Fable 3. Ratio m of the corrosion product mass divided by the metal loss for the copper samples exposed to

different atmospheres at 25°C and 90% relative humidity. (Standard deviation of three measurements)

CU

Exp. time

01

LOppm

NO2

LOppm

so2

10 ppm

so>0.5 ppm

NO *’

LOppm

Laboratory

air*’

I week I .23 (0.01) 1.31 (0.01) 1.420.05) I .29 (0.05) 1.430.25) I .36 (0.25)

2 weeks I 22 (0.01) 1.31 (0.01) 1.630.01) I .33 (0.04) I .49 (0.06) I.l0(0.11)

4 weeks 1.220.02) 1.320.04) I .74 (0.02) 1.390.01) I .3 1 (0.15) I 30 (0.19)

*’ Large error, since very small amount of corrosion products (see text)

formation of antlerite and probably chalcanthite. Hence, the oxidation of copper

(Cu-+Cu’ ’ + 2e-) does not take place any further in the later stage of the corrosion.

Other reactions such as the oxidation of cuprite (Cu’ +Cu2+) and the subsequentformation of copper sulphates may occur. This is supported by the thermodynamic

instability of CulO (CuzO + 2H ’ +Cu2 ’ t Cu” H,O; AC‘ = -25.6 kJ/mol) and the

thermodynamic stability of the basic copper sulphates in the neutral pH region.”

For both the samples exposed to nitrogen dioxide and to ozone and for all exposure

times, the main corrosion product was found to be cuprite (CuzO). This was followed by

comparatively small amounts of the basic copper nitrate gerhardite (CU~(OH)~NO~). For

samples exposed to humid nitrogen dioxide these corrosion products were also identified by

Simonx4 using XPS. The investigation of the micro structure of the corrosion layer revealed

a large amount of cuprite crystals. The size of the plate-like crystals formed in nitrogen

dioxide (Fig. 5) was much smaller than those formed in ozone (Fig. 6). The morphology ofthe corrosion products suggests that a dissolution and precipitation mechanism was

involved. In contrast to the samples exposed to sulphur dioxide, the corrosion attack was

H 1 PmFig. 5. Microstructure (SEM SEl) of the corrosion products of the copper samples after 4 v+eeks

exposure in IO ppm NO1 at 25°C and 90% relatrve humidity. Presentation rate see Table I.




1 PmFig. 6. Microstructure (SEM/SEI) of the corrosion products of the copper samples after 4 aweeks

exposure in 10 ppm O3 at 25°C and 90% relative humidity. Presentation rate see Table I.

uniform in both atmospheres. Additionally, the ratios m (Table 3) are small and

independent of exposure time. Consequently, the composition of the corrosion layer is

independent of exposure time and consists mainly of a corrosion product with a low ratio m

(cuprite, m = 1.15 and probably CuO, m = 1 2545) and small amounts of a corrosion productwith a high ratio m (gerhardite, m = 1.89). Considering the similar micro structures of the

corrosion layers it is not clear why a passivation occurred in the presence of nitrogen dioxide

and not in the presence of ozone.

Zinc

The results of the gravimetric evaluation are given in Fig. 7. The effect of sulphur dioxide

by far exceeds those of the other air pollutants. Practically no effect was found in laboratory

air and nitrogen monoxide (weight gain < 80 mg/m2 in 4 weeks) and only comparatively

small effects of nitrogen dioxide (weight gain: 640 mg/(4 weeks m*)) and ozone (weight gain:

1750 mg/(4 weeks m*)). However, the weight gains of zinc in nitrogen dioxide and ozone are

comparable to those of copper (NO*: 1280 mg/(4 weeks m2), 0s: 2950 mg/(4 weeks m2)). In

contrast to copper, 0.5 ppm sulphur dioxide had a much stronger effect on the corrosion of

zinc (Fig. 8).

In all experiments with sulphur dioxide, liquid corrosion products were formed. These

dripped off the samples during the experiments. Therefore the ratios m could not be

calculated. After taking the samples out of the exposure chamber, i.e. lowering the relative

humidity, zinc sulphates crystallised out of the aqueous electrolyte. In the experiment with

0.5 ppm sulphur dioxide the main corrosion product was gunningite, ZnS04.H20 followed

by ZnS04.6H20, while in the experiment with 10 ppm sulphur dioxide only the formercorrosion product was detected. The gunningite crystals which formed on the samples

exposed to 10 ppm sulphur dioxide are shown in Fig. 9.

In the presence of nitrogen dioxide a uniform corrosion attack was observed. White

corrosion products were formed all over the sample surface. According to the XRD analysis




_1 .o 2.0 3.0 4.0

Exposure time [weeks

Fig. 7. Material loss of the zinc samples exposed to air containing different air pollutants (SO2,

NO*, 03, NO, laboratory air) at 25°C and 90% relative humidity. Error bars = standard deviation of

three samples.

;L.0 1 .o 2.0 3.0 4.0

Exposure time [weeks]

Fig. 8. Material loss of the zinc samples exposed to air containing different amounts of SO2 at 25°C

and 90% relative humidity. Error bars = standard deviation of three samples.




IOpm

Fig. 9. Microstructure (SEM/SEI) of the corrosion products of the zinc samples after one week

exposure in 10 ppm SO2 at 25°C and 90% relative humidity. Presentation rate see Table 1.

the basic zinc nitrates ZnNOj(OH).H20 and Zn5(N03)2(OH)8.2H20 were formed (Fig . 10).

The corrosion layer formed is not able to protect the metal from further corrosion as ahready

indicated by the linear increase of the metal loss with time, since the corrosion products werepowdery and easily removable from the surface. The ratio m increased from 1.84 f 0.06 after

one week exposure, to 2.03 f 0.04 after two weeks and to 2.08 f 0.02 after four weeks . The

1pmFig. IO. Microstructure (SEM/SEI) of the corrosion products of the zinc samples after 4 weeks

exposure in 10 ppm NO2 at 25°C and 90% relative humidity. Presentation rate see Table 1.



1522 S. Oesch and M. Faller

composition of the corrosion layer is therefore a mixture of ZnN0s(0H).H20 with m = 2.43

and Zns(NOs)z(OH)s.2HzO with m = 1.91. The lower value after one week exposure

indicates the presence of other corrosion products such as Zn(OH), m= 1.52) or zinc

hydroxide carbonates with low m values (hydrozincite, m = 1.68).

The attack of ozone resulted macroscopically in the same, uniform corrosion consisting

of white corrosion products. The main corrosion product detected by XRD was

Zn(NO&(OH)se2H20 m = 1.89) followed by ZnNOs(OH)-Hz0 m = 2.48) for all

exposure times. Similar to the samples exposed to nitrogen dioxide, but to a greater

extent, crystals formed on the surface of zinc. The ratio m increased from 1.44 &- .23 (one

week) to 1.82 _+0.07 (two weeks) and to 1.9 1 I):0.01 after four weeks exposure, indicating the

presence of hydroxides or carbonates in addition to the detected nitrates.

After removal of the corrosion products a uniform corrosion attack was observed for all

zinc samples.

The analysis of the water-soluble compounds is given in Table 2. Because of corrosionproduct losses in the experiments with sulphur dioxide, the anion concentrations were not

determined. However, under the assumption that only the corrosion products gunningite

and ZnS04.6H20 with a good water-solubility were formed, an estimation of the total

sulphate content yields a sulphate concentration of 1.08 x lo6 ug/dm2 for the samples

exposed during four weeks to 10 ppm sulphur dioxide and 7.7 x lo4 ug/dm2 for 0.5 ppm.

These concentrations by far exceed those of the anions formed in the other experiments and

correspond to sulphur dioxide uptake ratios of 115% for the 0.5 ppm experiment and 81%

for the 10 ppm experiment. The calculated uptake of more than 100% of the supplied

sulphur dioxide can either be attributed to the presence of other corrosion products, i.e.

corrosion products without sulphate, or to the uptake of sulphur dioxide from gas volumesnot considered passing right next to the samples.

In agreement with the material losses, the anion concentrations of the other experiments

were rather small. The uptake of nitrogen dioxide, however, was considerably higher than

for the copper samples and reached 0.18% of the supplied gas after four weeks exposure

from which 3.7 to 9% can be attributed to the adsorption of HNOs formed in the gas phase.

For the experiment with 10 ppm ozone 18-33% can originate from this pathway, while the

rest would then be explained by the aqueous phase oxidation by ozone.

The sulphate concentrations found in the experiments without sulphur dioxide are also

small and comparable here, hence the aqueous phase oxidation of sulphur dioxide by ozone

or nitrogen dioxides are not relevant here either.

Aluminium

The material losses after four weeks exposure are given in Fig. 11. Ozone led to the

highest material loss followed by sulphur dioxide, nitrogen dioxide, laboratory air and

nitrogen monoxide.

The results from aqueous extracts analysis are given in Table 4. The pH-values of the

samples exposed to ozone were lowest, while the other samples followed in the same order as

observed for the material losses. This suggests a strong influence of the cathodic hydrogen

ion reaction (2H+ + 2e- +H2). The highest amount of corrosive species was also found on

the samples exposed to ozone. It consisted only of nitrates, as was the case for the samples

exposed to nitrogen dioxide.

For the samples exposed to 10 ppm sulphur dioxide XRD analysis yielded the

corrosion product Als(SO&(OH)s.9HzO and probably also Alz(SO4)3*16H2O. For the




2.5

0”EL

Environment

Fig. 11. Material loss of the aluminium samples after four weeks exposure in air containingdifferent air pollutants (SO*, NOz, 0s. NO, laboratory air) at 25°C and 90% relative humidity.

Table 4. Water soluble anions [ug/lOO cm2], pH and conductivity x [IS/

cm] of aqueous extracts of the corroded aluminium specimens after four

weeks exposure. Neither nitrites nor sulphites were detected

Environment PH &cm]

so:-[Wdm21

NO

Wdm21

10ppm03

10 SO2pm10 NO2pm

10ppmNO

laboratory air

0.5 ppm SO2

4.1 202 <5 5100

4.5 74.4 1620 <54.8 99.3 <5 2520

5.9 4.4 96 <5

5.9 5.0 105 <5

6.2 8.6 160 <5

other samples no crystalline corrosion products were detected. The exposure to sulphur

dioxide led to a locally enhanced attack of the surface layer. Corrosion was initiated at

local areas, usually at defects of the protecting layer, and then proceeded further, attacking

a thin surface layer around them. This resulted in platelets, which broke off from the basemetal (Fig. 12). Similar micro structures were also found in field exposures.36 AES

measurements on the surface of the areas where the surface layer was already broken off

showed that Al0 was still present in the metallic state. This showed that the natural

protection layer formed is very thin. Depth profiles in these areas showed that the



1524 S. Oesch and M. t.aller

Fig. 17. Microstructure (SEM;SEI) of’ the corrosion products of the aluminium samples after

2 weeks exposure in 10 ppm SO2 at 25 C and 90% relative humidity. Presentation rate see Table I.

thickness of the layer is in a range from 1 to 2 nm. After four weeks exposure further

corrosion led to efflorescences. most probably at the place where the local attack started.

They consisted of crystals. EDX measurements (excluding H) yielded considerable sulphur

concentrations, for both the uniformly corroded areas and the crystals of theefflorescences. For the crystals a composition of 11. I k 0.9 at.% Al, 81.6 + 2 at.% 0 and

7.3 + 1.2% S was measured. This coincides with the composition of A~~(SO&(OH)~.~HIO

(1 I .l at.% Al, 81.5 at.% 0 and 7.4 at.% S). For the uniformly corroded areas (thicker

platelets) the average composition amounted to 28 at.% Al, 4.3 at.% S and 67.7 at.% 0.

This suggests that these platelets as well contain hydrated aluminium sulphates. AES

measurements showed that in the corroded surface layer aluminium was, as expected, in

the oxidation state III. Depth profiles of the upper 50 nm showed rather constant Al and 0

concentrations through the layer, while the S concentration increased gradually from

about 2 at.94 to values of 7-8 at.%. This indicates that the diffusion of sulphate with the

higher ionic radius is the rate-determining step in the formation of the uniformly corroded

surface areas. i.e. A13(S0&(OH)5.9H20.

The exposure to nitrogen dioxide led to a uniform corrosion mechanism with many

hemispherical corrosion products where locally enhanced corrosion took place. In contrast

to the samples exposed to sulphur dioxide no platelets of the corroded surface layer were

formed. EDX analysis (low sensitivity for N) showed the presence of Al and 0 in the

hemispherical as well as the uniformly corroded areas. Additionally, Al/O ratios of

0.62 f 0.14 were measured, indicating the presence of Al203 and/or aluminium hydroxides

(AIOOH). AES measurements showed that the nitrogen concentration is low, i.e. below the

detection limit of about I at. %.

The samples exposed to ozone also exhibited a uniform corrosion attack with

many circular areas where locally enhanced corrosion took place (Fig. 13). EDX

measurements led to similar conclusions as observed on the samples exposed to nitrogen

dioxide.




H IOvmFig. 13. Microstructure (SEM/SEI) of the corrosion products of the aluminium samples after

4 weeks exposure in IO ppm 03 at 25°C and 90% relative humidity. Presentation rate see Table I.

CONCLUSIONS

The study gives an overview of the effect of air pollutants on the atmospheric corrosionof copper, zinc and aluminium. Moreover, quantitative data as well as corrosion products

and corrosion mechanisms in the presence of the individual air pollutants sulphur dioxide,

nitrogen dioxide, nitrogen monoxide and ozone from laboratory exposures are reported

under realistic test conditions. This allows a comparison of the effect of the air pollutants on

various materials. It also helps to understand the role of the air pollutants in the much more

complex, natural environment and as well to choose the right material for the a special

environment.

Ozone was recognised to be a potential corrosion accelerator in corrosion research. Up

to now its action was mainly attributed to oxidise H$, S(IV) - and nitrogen species. The

results presented here, i.e. the laboratory exposures of copper and aluminium exposed toozone, appear to be the first to demonstrate that ozone can enhance the corrosion processes

substantially on its own. The high corrosion rate observed can be attributed to the

electrochemical reduction reactions of ozone (0s + 2H + + 2e- + Hz0 + 02, E” = 2.08

- 0.06 pH + 0.03 log (po,/po,}); 0s + 6H + + 6e- -+ 3H20, E” = 1.5 - 0.06 pH +

0.098 log (PO,}) or one of its reaction products, i.e. the hydroxy radical, which is balanced by

the metal dissolution.

If each molecule passing over the sample is absorbed in the electrolyte and reacts further

according to the first reduction reaction with ozone a metal loss of 4.9 pm/week should be

observed in the case of copper, considering the unhindered dissolution reaction of

Cu+Cu’+ + 2e-. The lower values observed (0.5 pm/week) indicate that these idealised

conditions obviously do not hold. The reasons for this may be diverse, such as an incomplete

absorption of ozone, the occurrence of other reactions of ozone in the aqueous phase or low

reaction rates of the electrochemical reduction or dissolution reactions. This holds not only




for copper, but also for other materials. In the case of aluminium the metal loss in presence

of ozone is comparable to that of copper For aluminium the idealised calculation indicates

a metal loss of 4.6 pm/week if the dissolution reaction A1+A13+ + 3e- is considered. The

observed loss was 0.6 urn/week. For zinc the reactions seem to be more hindered since a

metal loss of 6.2 urn/week, using Zn+Zn’+ + 2e- is calculated and a metal loss of 0.07 urn/week was observed. A similar measurement for unalloyed carbon steel and weathering steel

samples recently exposed to ozone (10 ppm)83 yielded a metal loss of about 0.85 urn/week,

while a metal loss of 4.9 urn/week should result according to the idealised calculation. This

indicates the importance of ozone in atmospheric degradation.

Ozone had the strongest influence on the corrosion of copper followed by nitrogen

dioxide, sulphur dioxide, nitrogen monoxide and laboratory air. Corrosion mechanisms

and the properties of the corrosion layers formed depend significantly on the air pollutants

present. Ozone and nitrogen dioxide led to a uniform corrosion attack, while in the presence

of sulphur dioxide a locally enhanced corrosion attack was observed. The exposure to

nitrogen dioxide and sulphur dioxide, in contrast to the exposure to ozone, led to a decline in

the corrosion rate and therefore to a passivating effect. The reason for the occurrence of a

passivating effect in the presence of nitrogen dioxide and for the absence of this effect in

presence of ozone, although the main corrosion product found was the same, namely cuprite

is still under investigation. It may be due to the strong oxidative power of ozone which does

not allow a passivating layer to be formed.

The experiments with sulphur dioxide (10 ppm) resulted in the formation of the

corrosion product antlerite, which was also often found in field exposures. The experiment

with the lower sulphur dioxide concentration (0.5 ppm), however, resulted only in a weak

surface attack and the metal losses were only slightly higher than for the samples exposed tonitrogen dioxide (10 ppm) and laboratory air. This fact and the absence of copper oxides,

which are first formed in natural environments, indicate that the simulation of atmospheric

corrosion needs more time and that the exposure to sulphur dioxide alone can not yield

satisfactory results. More parameters have to be incorporated in test procedures such as the

air pollutants nitrogen dioxide and ozone. These would lead to the desired copper oxides.

The air pollutant concentrations, however, should then be lowered, since synergistic effects

will occur. The pollutant supply rates will thereby soon reach ambient values. Thus realistic

environmental testing including an acceleration will be achieved, which is a basic goal of

laboratory tests.

In the corrosion of zinc, sulphur dioxide is by far the most aggressive air pollutant,followed by ozone and nitrogen dioxide, while for nitrogen monoxide and laboratory air no

significant effects were found. None of the experiments resulted in a significant decline in

corrosion rate with increasing exposure time and therefore no protective layers were

formed. This is in good agreement with results from outdoor exposures, where mainly

constant corrosion rates occurred in the case of a high concentration level of pollutants. For

all exposures uniform corrosion attack was observed.

In the presence of sulphur dioxide hydrated zinc sulphates with a good water-solubility

and a critical relative humidity of 89% were formed. According to an estimation,

quantitative (100%) uptake of sulphur dioxide occurred. High uptake ratios were also

observed earlier for unalloyed steel and weathering steel samples.

For the samples exposed to nitrogen dioxide and ozone basic hydrated zinc nitrates were

found by XRD. They were only loosely adhered to the base metal, readily removable and

therefore not able to protect the metal from further corrosion.



The effectsof air pollutants n the corrosion of Cu, Zn and Al 1521

The corrosion of aluminium is strongly influenced by ozone, followed by sulphur

dioxide and nitrogen dioxide. The exposure to ozone, sulphur dioxide and nitrogen dioxide

led to a locally enhanced attack. For the samples exposed to sulphur dioxide this resulted

in a thin surface layer consisting, according to XRD- and EDX-measurements, of

A13(S04)2(OH)5.9H20 and aluminium oxide or hydroxides. This layer flaked off the base

metal due to an increase in volume during the transformation from aluminium oxides or

hydroxides to the mentioned sulphate. Similar corrosion products and mechanisms are

also observed outdoors.23*36 The exposure to ozone and nitrogen dioxide resulted in

aluminium oxides or hydroxides. No nitrogen could be detected by AES (detection limit:

1 at.%).

Among the air pollutants sulphur dioxide, nitrogen dioxide and nitrogen monoxide,

sulphur dioxide exhibited by far the highest deposition rate for all materials investigated. It

ranges from an uptake ratio of 100% for zinc to comparatively small uptake ratios in the

percentage range for copper and aluminium. The uptake rate of nitrogen dioxide is muchsmaller than the values found for sulphur dioxide, while for nitrogen monoxide no

deposition was detected.

The deposition rate of ozone is not easily detectable in thin electrolytes, but under the

assumption of the calculations above is also rather high. In the case of copper an uptake

ratio of > lo%, for aluminium of > 13% and for zinc of > 1.1% results if only that portion

of ozone is considered which is cathodically reduced.

These deposition rates or uptake ratios refer to exposures to single air pollutants. Gas

combinations can strongly influence the deposition rates as pointed out in the beginning.

However, if quantitative absorption occurs already with the single gas no further

enhancement will be observed as is the case for sulphur dioxide and zinc where an uptakeof 100% already resulted.

The results obtained in laboratory exposures are very useful to explain effects found in

real outdoor environments.

In the case of copper exposed outdoors, correlation analysis yielded corrosion

accelerating effects of sulphur dioxide. It was not clear whether nitrogen dioxide has a

corrosion inhibiting or ozone a corrosion stimulating effect. The findings of the laboratory

exposures strongly support the latter possibility. Furthermore, high corrosion rates found

for copper at sites with a comparatively low primary pollutant level, i.e. sites with medium to

low sulphur dioxide and low nitrogen dioxide levels and consequently high ozone levels, can

clearly be attributed to the strong influence of ozone. The main corrosion product found in

short time exposures at these sites was cuprite, which is also in agreement with the

laboratory exposures.

For zinc samples exposed outdoors, taking into account the sulphur dioxide level and

climatic parameters already led to very high correlation coefficients (R~0.95). This

indicates only minor significance of the other air pollutants. The results of the laboratory

experiments confirm this. Moreover, field exposures did not result in a significant decline in

corrosion rate with increasing exposures time and high corrosion product losses were

observed. The laboratory exposures also confirm this, since either water-soluble or easily

from the zinc surface removable corrosion products were formed. The sulphur dioxideuptake ratio of 100% at the lower pollutant level (0.5 ppm), i.e. at the realistic gas supply

rate, indicates that the S(IV)-oxidations mechanisms of ozone or the nitrogen species cannot

increase the amount of sulphate deposited on the surface. This, however, does not mean that

these reactions do not contribute to the sulphate deposited.



1528 S. Oesch and M. Faller

Acknowledgemenrs-The authors would like to thank M. Romer for XRD measurements and helpful discussions,

R. Figi for IC measurements and P. Richner for proof-reading.

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Series Vol.3, ed. J.G. Calvert. Boston, 1984, p. 63.

3. M.R. Hoffmann, Envi r. Sci. Technol . 14, 1061 1980).

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(1997) Environmental Effect on Materials-copper Zinc and Aluminium

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