Effect of NO 2 and / or SO 2 atmospheric contaminants and relative humidity on copper corrosion ^

A study has been made of the individual and combined roles of NO2 and SO2 atmospheric contaminants on corrosion and patina formation on copper in humid atmospheres. In most cases the combined effect of the two contaminants has been greater than the sum of their individual effects, although exception have been found with the mixture of800|ig/m^NO2 + 800 jig/m SOjXPS analysis has revealed important composition changes in the outermost layer of films formed on copper, depending on the nature of the atmospheric contaminant and humidity level. The presence of sulphates and sulphites has been clearly observed in exposure to atmospheres contaminated with SO2 at 50, 70 and 90 % RH. Nitrates and nitrites have been detected in exposure to NO2 at 50 and 70 %RH, but not at 90 % RH. A hydrogenated nitrogen compound has been detected with the mixture of NO2 and SO2 at 90 % RH .In this atmosphere, a certain inhibiting effect has been seen.


INTRODUCTION
SO2 is one of the corrosion contaminants with the greatest influence on the atmospheric corrosion of metals^ \ Less well known is the role played by NO2 in the atmosphere, this being a precursor of nitric acid and other nitrogenated compounds that participate in materials degradation processes^ \ The concentration of nitrogen oxides in the atmosphere can exceed even that of SO2 .In urban atmospheres it is easy to find NO2 levels of up to 120 |Llg/m^, with maximum values several times higher I Their effects on metallic corrosion seem, in general, to be of little importance, at least up to concentrations of some 500 |Ig/m^ ^''' ^^ """"^ ^^l Several papers have recently been published on the effect of NO2, acting individually or in combination with SO2, on the atmospheric corrosion of copper^ ' 9-11 and 1346] j^^ humidity Content of the air seems to condition the corrosivity of NO2 ^^''^^\ It even (•) Trabajo recibido el día 5 de mayo de 2003 y aceptado en su forma final el día 29 de julio de 2003.(*) Centro Nacional de Investigaciones Metalúrgicas (CENIM).Av.Gregorio del Amo 8,28040 Madrid.Spain.E-mail: sfeliu@cenim.Gsic.es(t) L Mariaca died in a car crash in Mexico on 12th November 1999.This paper is a small homage to the memory of Dra.Liboria Mariaca, a good friend and scientist, who generously dedicated the final years of her life to this work.Instituto de Investigaciones Eléctricas.Av.Reforma #113, Col. Palmira.Temixco, Mor 62490 Mexico.
seems possible that NO2 can have an inhibiting effect, for instance in the case of steel and tin at high RH conditions'^' ^^ ^^^ ^^l Some results on copper suggest that NO2, in certain mixtures, does not appreciably raise the corrosion rate.However, in other circumstances NO2 considerably increases the corrosion rate of this me tar .
The scarcity of conclusive results on the effect of NO2 on the atmospheric corrosion of copper justifies the need for further work'^^^ To this end, this contribution reports laboratory research into the individual and combined effects of NO2 and SO2 contaminants, and of the air humidity level, on corrosion data and on the constitution of patina layers on copper.
The work will consider contaminated atmospheres at 50, 70 and 90 %RH.The practical case of the formation of visible moisture layers on the metallic surface (e.g. when the dew point is reached or due to the precipitation of moisture in open air exposure), is excluded from the study.The present study is representative of behaviour in indoor atmospheres with humidity values below 100 % RH.In such atmospheres it is only possible for very thin moisture layers to form due to the adsorption of water molecules and capillary and chemical condensation phenomena'^^ ^"^l

Test conditions
It was decided to work with contaminant concentrations as higher as to give measurable degrees of attack in relatively short testing times, and yet not too much greater than those really found in the atmosphere in order to not modify substantially the mechanisms of attack.Thus a maximum contamination level of 800 )LLg/m^ was defined for both NO2 and SO2, which corresponds to peak concentrations in urban atmospheres at times of very high contamination.
A working temperature of 35 -C ± 1 "C was selected, this being a value that is easily controllable in the laboratory and similar to many surface temperatures in outdoor exposure conditions.In this way the obtainment of significant data in reasonable time periods was facilitated.
With regard to RH a value of 90 % ± 5 % was defined, typical of very humid atmospheres.Values of 50 and 70 % ± 5 % were also considered, being representative of the air humidity in many outdoor and indoor environments.
Copper was exposed in atmospheres with NO2 and SO2 contaminants, both individually and in combination.Similar tests were carried out in noncontaminated atmospheres for reference purposes.

Preparation of specimens
Copper specimens of 99.95 % purity and dimensions 10 X 5 x 0.1 cm were used.Their surfaces were prepared by dry polishing on successively finer SiC abrasive papers down to grade 600, degreasing with acetone and washing with detergent, followed by ultrasonic stirring in ethanol for 15 min and drying in a hot air jet.The quality of degreasing was verified by observation of the continuity of a water film on the surface.
Once prepared, the specimens were stored for 24 h in a desiccator over a silica gel.Immediately before testing they were weighed in an analytical balance with sensitivity up to 10 (Ig.Given that the specimens had a surface area of 100 cm , the sensitivity per unit of surface area was 0.1 |Llg/cm . Five copper specimens were exposed for each exposure condition.Three were used to calculate the mass gain due to corrosion.The other two were used to characterize the corrosion product film formed (patina)^^^l

Exposure to contaminants and mass gain determinations
The specimens were placed in a testing cabinet equipped with a sensorized system of contaminant gas dosing and continuous RH and temperature control.The air flow rate was 1 m /h, and thus the total mass inside the cabinet was renewed 8 times every hour, in completely laminar regime conditions.It was decided to use mass gain rather than mass loss determinations, in order to keep to a minimum the operations involving data obtained by weighing in conditions close to the precision limit of the balance.

Characterization of patinas
Photoelectron spectra were recorded using a Fisons MT500 spectrometer equipped with a hemispherical electron analyzer (CLAM2) and a Mg/Al Ka x-ray dual source operated at 120 W. The samples were mechanically fixed on an XYZ manipulator placed in the analysis cabinet.The residual pressure in this ion-pumped analysis cabinet was maintained below 5 x 10'^ Pa during data acquisition.The spectra were collected for 20-90 min, depending on the peak intensities, at a pass energy of 20 eV which is typical of high-resolution conditions.The intensities were estimated after smoothing and substraction of the S-shaped background and fitting the experimental curve to a mix of Lorentzian and Gaussian lines of variable proportion.Although some sample charging was observed, accurate binding energies could be determined by referencing to the adventitious Cls peak at 285.0 eV.Atomic ratio percentages were computed from peak intensity ratios and reported atomic sensitivity factors^^ \ Carbon was not included in the computation of elemental composition because it was only attributable to surface contamination typical of all metallic systems not freshly annealed under vacuum.The high resolution O Is spectra acquired on the copper surfaces were broad and featureless and no attempt was made to computer fit these spectra.A computer curve synthesis procedure was used to separate the individual components of the Cu 2p3/2) S 2p and N Is high resolution spectra.
An attempt was made to calculate the thickness of the corrosion products layer on the copper surface using argon ion bombardment (AIB).A high oxygen content was detected on the copper surfaces even after bombardment times of 40 min (removed specimen thickness close to 80 A).The fact that oxygen does not cease to be present on the copper surface after long bombardment times, probably due to the non-uniform distribution of the corrosion products (in the form of islands, mainly at 70 and 50 %RH), made it impossible to obtain an approximate estimation of the patina thickness.

Humidity without contamination
In a non-contaminated reference atmosphere the copper specimens showed very slight mass gains after 7 d, less than 1 |Llg/cm .This value rose after 21 and 28 d of exposure to between 2-3 |Llg/cm .
According to the literature, at ambient temperature copper spontaneously becomes coated with a thin oxide film formed mainly of CU2O ^^^' ^^ ' ""^' ^ ^^l In a humid atmosphere countless local corrosion cells can act on the metallic surface coated by a moisture film which acts as electrolyte^ \ The reduction of oxygen from the air dissolved in this film to produce hydroxyl ions is the preferential cathodic process.Hydroxides, hydrated oxides and basic salts are therefore predictable products of atmospheric corrosion.
As was expected, XPS analysis of the nonexposed surface of the copper specimens used in this research showed the preferential presence of cuprous oxide CU2O i^^' ^^J as was also found in exposure in non-contaminated atmospheres with 50 and 70 % RH (Fig. 1).Curiously, exposure in the atmosphere with 90 % RH considerably modified this result.Thus, figure 1 shows that after 7 d of exposure the majority oxide had become cupric oxide CuO ^^^'^^^ at 90 % RH.Significant amounts of this oxide have also been detected on the surface of patinas obtained after exposing copper in the different contaminated atmospheres studied (Fig. 2).In general, the relationship between the detected amounts of cuprous and   cupric oxides seems to depend notably on the air humidity.In figure 1 it can be seen that as the RH of the non-contaminated atmosphere diminishes, so the CU2O content tends to rise, while at the same time the CuO content decreases.An increase in exposure time seems to lead to a partial transformation of CuO into Cu(OH)2 ^^^' ^^^ (Figs.2a and 2e).
It is important to highlight the clear tendency towards the formation of CuO at the highest of the humidity levels tested (90 % RH).At low RH values the thin film of condensed moisture (electrolyte) on the copper surface must be insufficient for efficient communication between the local anodes and cathodes and for the functioning of the corrosion microcells^^^^, though this situation could change at humidities close to air saturation.

Effect of SO2
In general, the presence of SO2 gas in the air tends to increase the attack of metals and of copper in particular^^^ ^^"^ 32-34] j]^gj-g ^j.^ discrepancies regarding the specific role of SO2 in this attack and in the composition of the patinas that are formedt^' ^^ ^^^ ^^\ The SO2 that reaches the copper surface is transformed into sulphurous and sulphuric acids, and there is a notable drop in the pH of the moisture film^^^l Basic sulphites and sulphates, particularly the latter, are often found in patinas after long exposure times^^^' ^^ ^' ^^ 37-39] jj^^ literature also notes a considerable presence of CU2O II ^ ^"^ 1^1 The basic copper sulphate Cu4(S04)(OH)6 is stable at pH > 4.0 ^^\ and for this reason the patinas can resist a certain acidity of the electrolyte.With an air humidity of 90 % RH, the colouring of the patinas formed on the copper specimens exposed in the atmosphere contaminated with 800 | LLg S02/m presented no significant changes compared with those exposed in the noncontaminated atmospheres.After 7 d the mass gain was 532 |Llg/cm^, which rose to 7.15 |Llg/cm^ after 28 d.Somewhat lower mass gains were found for this contaminant at 50 % RH (Fig. 3) XPS analysis of the patina surfaces reveals a transformation of the initial CU2O film into other species (Figs.2b and 2f), and that sulphur atoms come to form part of the patina composition (Fig. 4)-It is a singular fact that after 21 d of exposure the presence of CU2O is barely observed (Fig. 2f).After oxygen (mainly in the form of hydroxide, hydrated oxides and basic salts), the main component of the patina surface was copper in the form of CuO and, to a lesser extent, copper sulphates and sulphites (Fig. 2f ).After 21 d no increase was seen in the sulphur content on the patina surface, though there was a certain transformation of sulphite ions into sulphate ions.

Effect of NO2
NO2 gas is only slightly soluble inwater^^^^^^^^and its capture by the moisture film is a slow process^'^^l In the interaction of NO2 with water several   Binding Energy (eV)  reactions are possible, including the formation of nitrous and nitric acids^^ I The reduction of NO2 by CU2O also gives rise to nitrous acid and the formation of nitrites^^^l Thus it is logical to find nitrates and nitrites among the corrosion products^^^l In the tests carried out with an air humidity of 90 % RH and 800 pg/m^ of NO2, the colour of the patinas also showed no great changes compared with the patinas obtained in the noncontaminated atmosphere.XPS analysis of the patinas corresponding to atmospheres with and without contamination suggests a partial replacement of CuO by Cu(OH)2 (Figs. 2c and  2g).Unlike the behaviour in the atmosphere contaminated with SO2, in which sulphur compounds were identified in the patina composition, in the atmosphere contaminated with NO2 nitrogen did not apparently come to form part of the patina composition, at least this was not detected in the XPS analysis of the outermost surface of the patina (Fig. 5).The absence of nitrates and nitrites in the patina contrasts with the behaviour observed at lower RH levels (Fig. 5), and some results reported in the literaturetii^^^^^l According to figure 3, at 90 % RH the NO2 contaminant seems to be less corrosive than SO2 at 50 and 90 %RH; instead, the NO2 appears to be particularly corrosive at 50 % RH in terms of the individual action of these contaminants.

Effect of the mixture of NO2 and SO2
The literature is not clear about the effect of air humidity on copper corrosion when this metal is exposed to mixtures of NO2 and SO2 contaminants.Several researchers have mentioned a synergetic effect of NO2 and SO2 gases when there is a high  presence of both (^ 2000 |Llg/m^) and a high RH of the air (-90 %) í^' ^'^^' ^^' ^^ '"^ ^^l In these circumstances, the initial CU2O film dissolves in time and sulphur compounds appear on the copper surface, though oxygenated nitrogen compounds do not.In particular, mention has been made of the presence of Cu4(OH)6S04.2H2O ^^ ^^^ ^^l In addition to the typical reactions of NO2 and SO2 acting individually, it is possible to find the reaction SO2 + 2 NO2 + 2 H2O = 2 H^ + 804^' + 2 HNO2 which tends to create an electrolyte that is especially rich in sulphuric acid^ .Figure 6 compares the mass gains of copper in an atmosphere with the 90 %RH and contaminated with different mixtures of NO2 and SO2 with the sum of the mass gains with the same gases acting individually.In most cases the combined effect of the two contaminants has been greater than the sum of the individual effects (Figs.6b, c and d) in consonance with the aforementioned synergetic effect.Exceptionally, at the highest total concentration tested (800 |ag/m^ NO2 + 800 [i/nv' SO2), the mass gains have been approximately 20 % lower than the sum of the gains produced with NO2 and SO2 acting separately, which seems to indicate, in this particular case, a certain inhibiting effect of the mixture of NO2 + SO2.Similar behaviour is suggested by figure 3, for atmospheres contaminated with 800 | LLg/m^ NO2 and/or SO2 at 50 and 90 % RH, where gains due to the mixture of these gases are 15-22 % lower than the sum of individual gains.
The literature mentions the existence of an inhibiting effect of NO2 mixed with SO2 in the case of steel^^ ^^^ ^^^ and tin^^^^ in atmosphere with 95 and 90 % RH respectively.Other results in the literature also suggest a slight inhibition of the corrosion of copper exposed to mixtures of NO2 and SO2 gases for concentrations of 300-700 |Llg/m^ and 95 % RH ^^l However, for higher concentrations of these gases, of the order of 900-1100 |Llg/m^ ^^^\ 1300-1500 ¡iig/m^ ^^\ and 3000-5000 ¡ig/m^ ^^^, the synergetic effects are evident even at high humidities.All of this disparity of behaviours suggests a high sensitivity of the phenomena responsible for the formation of patinas on copper to the interaction of a series of factors whose action is insufficiently clarified.These factors may include the contaminant concentration, RH level, specimen type and exposed area, preparation and characteristics of the metallic surface, etc.It would probably be useful to carry out further research in this respect.XPS analysis of the surface of the patinas formed in the mixture of SO2 and NO2 did not reveal the presence of oxygenated nitrogen compounds (Fig. 7), which were also not seen with NO2 acting individually (Fig, 5).This behaviour seems to be peculiar to atmospheres with 90 % RH, and was not found with 70 and 50 % RH (Fig. 7), where the presence of nitrate and nitrite was detected.On the other hand, this analysis detected the presence of the ammonium ion (or another hydrogenated derivative of the nitrogen atom not differentiable by XPS analysis^"^^^), which did not appear with the individual action of NO2 (Fig. 5), nor in the patina formed at 50 %RH (Fig. 7).In theory, the formation of the ammonium ion is possible as a result of the reduction of an oxygenated nitrogen compound (e.g.nitric and nitrous acids, NO2, NO, etc.), provided that the acidity of the medium and the potential of the electrode are appropriate'^^'^^l For instance, the literature mentions the possibility of the reduction of nitrite to ammonia complexes through a cathodic reaction formation of ammonium from a mixture of nitric and sulphuric acids in contact with copper wool^'^^l However, it is surprising that the ammonium ion should only appear with the mixture of NO2 and SO2 at 90 % RH, and not in the other tested exposure conditions in which the NO2 contaminant is also present.Together with the greater thickness of the electrolyte film at 90 % RH another influencing factor may have been its higher acidity, since both of these factors tend to increase the effective potential difference that allows the functioning of microcells which reduce the nitrogenated species and promote copper dissolution.
XPS analysis has also revealed a comparatively important presence of the cuprous ion among the corrosion products that accompany the oxides in the patina (Figs.2d and 2h) on copper exposed to NO2+SO2 at 90 % RH, which is a singular fact considering the instability of Cu(I) salts; it would have been more normal to find the cupric ion, as occurs in all the other tested atmospheres.Both the ammonium ion (ammonium or another hydrogenated nitrogen compound) and the cuprous salts thus seem to be extraordinary products of exposure to the mixture of NO2 and S02-This result suggests the idea that the cuprous ion is linked to the hydrogenated nitrogen compound which stabilizes it through the formation of a complex ion.

Inhibitive effect
The aforementioned inhibitive effect of the tested mixture of NO2 and SO2 is a peculiar behaviour that suggests a possible relationship with chemical passivation^'^'^'^^^l As is known, this phenomenon is possible when a sufficiently positive potential is established on the cathodic areas to draw the potential (in positive direction) from the anodic areas on the same metallic surface until passivation conditions are reached.For this it is necessary, in turn, to reach a critical anodic current density.In the case in hand, the anodic process would be the formation of copper oxides and the principal cathodic process would be the reduction of oxygen mainly from the air.
It is well known that the presence of some dissolved species in the electrolyte reduce the magnitude of the critical current and/or the Flade potential, facilitating the passivation phenomena^"^"^ I In the case considered here, it is suggested that this could be a species derived from NO2, perhaps the nitrite ion, bearing in mind the known predisposition of this ion to act as inhibitor^"^^ ^^'^^ "^^l Its effect would probably be exerted through an adsorption process rather than incorporation in the passivating film, since XPS analysis has not revealed the presence of oxygenated nitrogen compounds.The aforementioned sulphate, sulphite and ammonium salts, detected with the mixture of NO2 and SO2, could have formed in the phase prior to passivation.
At low RH the electrolyte films are very thin and the small amounts of water tend not to be evently distributed but to coalesce to clusters, particularly if hygroscopic particles are present on the metallic surface^^^' ^^ ^"^ ^^\ The lack of continuity of the electrolyte layer should not prevent the functioning of local microcells.However, their activity area will now be confined to immediate vicinity of the anode-cathode junctions, giving rise to an intense localized attack at some points, as shown in figure 8, for a contaminated atmosphere at 50 % humidity.

CONCLUSIONS
-A laboratory study has been made of the individual and combined effect of NO2 and SO2 contaminants on the atmospheric corrosion of copper at 50, 70 and 90 % RH. -XPS analysis has revealed the presence of copper sulphate and sulphite in the patinas formed in atmospheres contaminated with SO2 at the three levels of humidity studied.-Copper nitrate and to a lesser extent copper nitrite have been detected in exposure in atmospheres contaminated with NO2 at 50 and 70 % HR.These compounds did not appear at 90 % RH, but only changes in the proportion of cuprous and cupric oxides were revealed.-In the patinas formed in the mixture of SO2 and NO2 at 90 %RH sulphur compounds were

Figure 3 .
Figure 3. Copper mass gain with exposure time in atmospheres polluted with 800 fjg.m'^SO2 and 800 pg.m"^ NO2, individually or combined, at 90 and 50 % RH.Humidity levels and pollutants are indicated in the figure.

Figure 4 .
Figure 4. S2p high resolution XPS spectra obtained on the outer surface of copper specimens after 21 d of exposure in atmospheres with 800 pg.m'^ SO2 at 50, 70 and 90 % RH.

Figure 5 .
Figure 5. NTs high resolution XPS spectra obtained on the outer surface of copper specimens after 21 d of exposure in atmospheres contaminated with 800 fjg.m"^NO2 at 50, 70 and 90 % RH.

Figure 7 .
Figure 7. Nls high resolution XPS spectra obtained on the outer surface of copper specimens after 21 d of exposure in atmospheres polluted with the mixture of 800 jjg.m'^NO2 + 800 fjg.m"^NO2 at 50, 70 and 90 % RH.

Figure 8 .
Figure 8. Point of localised attack formed on a copper specimen after 21 d of exposure in an atmosphere polluted with the mixture of 800 pg.m'^ NO2 + 800 pg.m"^ NO2 at 50 % RH.Ó5X.
detected but oxygenated nitrogen compounds [15^ were not.This behaviour was not found at 50 and 70 % RH where the presence of nitrate and [16 nitrite was detected.At 90 % RH, the XPS analysis seems to indicate the presence of [17 ammonium ion, which did not appear at 50 and 70 % RH.-The combined effect of NO2 and SO2 on the [18 formation of the patina on copper (mass gain) has been greater than the sum of the effects of [19 the two contaminants acting separately.