Outdoor and indoor atmospheric corrosion of non-ferrous metals

Corrosion Science 42 (2000) 1123±1147

Outdoor and indoor atmospheric corrosion of non-ferrous metals Antonio R. Mendoza*, Francisco Corvo Corrosion Department, National Centre for Scienti®c Research Ave. 25 and 158, P.O. Box 6990, Cubanacan, Playa, Havana, Cuba Received 14 August 1998; accepted 13 October 1999

Abstract In the present paper, a study of the atmospheric corrosion of copper, zinc and aluminium exposed on three test sites indoors and outdoors (coastal, urban-industrial and rural) under di€erent exposure conditions up to 18 months is reported. Corrosion results are treated statistically and adjusted to a model previously proposed for steel [A.R. Mendoza, F. Corvo, Corrosion Science 41(1) (1999) 75±86.] based on the in¯uence of environmental parameters and main pollutants (SO2 and chlorides) on the atmospheric corrosion of metals. The interaction between the chloride deposition rate with the time of rainfall (outdoors) and with the time of wetness at temperature between 58C and 258C (indoors) were found to be the most signi®cant variables in¯uencing the corrosion of the three metals investigated; although other variables appeared to be important in the corrosion process depending on the metal nature. The results obtained con®rm and allow us to expand the model previously proposed for steel to non-ferrous metals. A classi®cation of the atmospheric corrosion aggressivity of the test sites based both on environmental data and corrosion rate measurements was made according to ISO 9223. The corrosion aggressivity prognostic of this standard is not always in agreement with the results obtained in Cuban atmospheric conditions. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Atmospheric corrosion; Indoor corrosion; Non-ferrous metals; Copper; Zinc; Aluminium

* Corresponding author. Tel.: +53-214-532. E-mail address: [email protected] (A.R. Mendoza). 0010-938X/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 9 9 ) 0 0 1 3 5 - 3

1124

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

1. Introduction As has already been emphasized by di€erent authors [2±7] indoor corrosion has been less studied than outdoor corrosion mainly because of its low corrosion rate, and because the main contaminants are more signi®cant in outdoor atmospheres. However, it has been reported that indoor corrosion seems to be markedly a€ected by a greater number of air pollutants than the outdoor corrosion [3], specially when indoor sources of corrodents are present [8]. Many technological and scienti®c electronic applications, as well as some parts of our cultural heritage are showing serious problems due to indoor atmospheric corrosion, even in the presence of traces of moisture and contaminants. Sulphur compounds and chloride ions are the most common and important atmospheric corrosive agents, as has been reported by di€erent authors all over the world [9±12]. However, there are other atmospheric pollutants whose mechanism of action on metals have been less studied and could be very important in indoor corrosion. This has been either because they are rather noncorrosive due to their low reactivity, inasmuch as they cannot ionize on metal surfaces, they cannot form aggressive electrolytes or depassivate metals, which would accelerate the anodic reaction of the corrosion process, such as most organic compounds [2,5] and CO [13], or because in spite of their extreme aggressiveness, such as Cl2, HCl, H2S and nitrogen containing compounds whose atmospheric concentrations are too low. However, di€erent works concerning the in¯uence of some of these pollutants on metallic corrosion have been reported [9,11,14±22]. It has been found, for example, that at high relative humidities, aluminium and iron show no SO2 + NO2 synergism [21], and that for steel is negligible [11,20]. It was reported that a thick layer of water on the metal surface seems to act as a sink for SO2, but as a barrier for NO2 [20]. For metals with a protecting oxide ®lm, NO2 may even act as an inhibitor; otherwise, there seems to be synergistic e€ects [21]. It has been showed by several authors that the SO2 + NO2 synergism on copper corrosion is only active at high relative humidity (90%) [23], and steel corrosion at low relative humidity [15,18,20]. Kucera et al. [21] reported that the synergistic e€ect of SO2 + O3 can be both stronger than SO2 + NO2, as for copper, and weaker, as for nickel. In other works no synergistic e€ects of simultaneous interaction of SO2 and NO2 with either nickel or copper have been observed [22]. The dry deposition is in most cases dominating and SO2 exerts the strongest corrosive e€ect [21]. The role of NO2 has not yet been clari®ed and its strong synergistic e€ect with SO2, shown for many materials in di€erent laboratory studies, has not been observed in the ®eld exposure and may be due to the strong correlation between SO2, NO2, and O3 concentrations [21]. The estimated minimum NO2 concentration inducing metal corrosion is believed to be 30 mg/m3 [13]. For a urban-industrial Cuban atmosphere it was obtained diary NO2 concentrations over 31 mg/m3 only in a 10% of the measurements [25]. Rural and coastal atmospheres do not report values over 30 mg/m3. Therefore, the

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

1125

in¯uence of this pollutant on the atmospheric corrosion in our conditions may be negligible. The particulates could also act in a detrimental way [1,2,8,21,26] as well as to neutralize other acidic pollutants [21], specially if they are not washed away from the surface, as when the metal are exposed to sheltered conditions [19,24]. In a recent work, higher corrosion rates were obtained by our research group for samples exposed horizontally indoors with regard to the verticals, where the dust deposition was highly signi®cant. Particulate species in the atmosphere can accelerate corrosion of metals in several ways, for example, by increasing the conductivity of the surface layer after dissolution of soluble ions from the particulate [27]. It has been reported on the e€ect of particles of (NH4)2SO4 on the corrosion of zinc [27], copper [28] and aluminium [29]. Ammonium and sulphate ions are the most abundant ions in ®ne dust particles commonly found in urban environments and may play a dominant role in the corrosion process [28,29]. Indoors, most of the surface degradation can be attributed to adsorbed sulphate aerosol particles [14,29]. The accumulation of inorganic ionic substances is primarily due to particle deposition, and because of their high content and high indoor concentration, ®ne particles play a major role in the corrosion of electronic materials [29]. Organic molecules also may be supplied to metal surfaces by the deposition of particles [2,8]. The total corrosion e€ect during a period of time is determined by the total time of wetness and the composition of the water layer on the metallic surface, as well as the duration of its action on the metal. These factors, together with the temperature, determine the corrosion rate [11,19,24,30±32]. The period when the relative humidity is over 80% at temperatures higher than 08C, proposed by ISO 9223 [30], is often used for estimation of the time of wetness. Although it may not be the actual time of wetness, because wetness is in¯uenced by di€erent factors, it usually shows a good correlation with corrosion data from ®eld tests under outdoor conditions, corresponding to the kinetically decisive time periods during which corrosion proceeds. However, under indoor conditions other criteria seem to be valid and have not yet been fully clari®ed [24]. Several studies have attempted to develop, for di€erent purposes, models that correlate some of the factors that in¯uence atmospheric corrosion (meteorological parameters, pollutants, etc.) with the performance of metals [9,10,33±41]. However, none of the models have included indoor corrosion data. In a previous publication [1], we reported the study of the in¯uence of atmospheric pollutants and some meteorological parameters, as well as their possible interaction, on the atmospheric corrosion behaviour of carbon steel indoors and outdoors. A corrosion model that represents this in¯uence, besides other considerations not reported or taken into account before, it was proposed. The corrosion model de®nes a new di€erence in time of wetness based on temperature. In the study presented here, we extend the previous work to the study of the atmospheric corrosion of copper, zinc and aluminium.

1126

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

2. Experimental method Experiments were conducted in the same way as reported previously [1]. Meteorological data and other atmospheric factors along with the corrosion rate of exposed samples were recorded in order to identify the e€ect on corrosion of the variation of the atmospheric factors under indoor and outdoor conditions in di€erent atmospheres. Pollution by chloride ions and SO2 was also measured. The chloride deposition rate was determined at two-month periods (except in the rural station under sheltered and ventilated sheds, conditions where it was determined at 6 months, as well as the SO2 deposition rate, because of the low concentration of these contaminants there) using the dry plate method, consisting of the employment of a dry cotton fabric of known area exposed under a shed [42]. The amount of chloride deposition on the cotton fabric is determined analytically at the end of the exposure period and the deposition rate is calculated. The SO2 deposition rate was determined using alkaline surfaces of porous ®lter plates saturated by a solution of sodium carbonate placed in the same shed used for chloride deposition rate determination [43]. The SO2 deposition rate includes sulphur dioxide and other sulphur compounds, such as sulphate coming from the sea as aerosol (it should be taken into account that Cuba is an island where almost any place is a€ected by the airborne salinity). The pollutants measured under sheltered conditions are assumed to be the same as that of outdoor conditions, since they are always determined under a shed. Data for temperature and relative humidity were processed using software developed for this purpose in order to determine the characteristics of the temperature-relative humidity complex [44]. Data for pollutants and corrosion were processed using this software as well. Meteorological and pollutants data monitored are presented in Tables 1 and 2, respectively. The meteorological data of the coastal station are taken from the same site where the urban-industrial ones are evaluated (the urban-industrial is located at 500 m from Havana Bay and less than 3 km of the North shoreline). Although climatic parameters are not reported for closed space (non-continous measurements carried out), it is well known that the temperature and the relative humidity keep practically constant all over the year, reaching values between 18± 308C and 85±95%, respectively. A more detailed description of the test sites and exposure conditions, and their annual average environmental parameters and its behaviour, is given in a previous publication [1]. Table 3 shows the classi®cation of corrosivity for every test station based on environmental data (time of wetness and chloride and SO2 deposition rates) according to ISO 9223 [30]. Commercial pure copper, commercial pure zinc and aluminium alloy 2024 samples were exposed under di€erent atmospheric conditions. Prior to exposure the samples were treated super®cially. The surface of the zinc samples (150 mm  100 mm  1 mm) was polished on SiC paper to 360 mesh. According to ISO 8407 [45], aluminium (150 mm  100 mm  2 mm) and copper (150 mm  100 mm  1

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

1127

mm) samples were degreased and immersed in the following aqueous solutions: aluminium in 10% NaOH and then in 10% HNO3; and copper in 20% HNO3. Then they were thoroughly rinsed immediately, dried and kept in a desiccator with anhydrous CaCl2 for at least 24 h and then weighed. Since the ®rst corrosion evaluation was made after 6 months and, Cuba being a tropical island whose atmosphere is very aggressive, it is considered that the in¯uence of surface preparation is almost negligible. Then, duplicate samples of each metal were exposed to outdoor and indoor conditions in di€erent atmospheres (rural, urban-industrial and coastal). In outdoor conditions, the samples were exposed at 458 to the horizontal, facing south, as well as in the rural station under sheltered and closed space conditions. In the urban-industrial and coastal zones, under sheltered and ventilated shed (in all the stations) conditions, they were exposed vertically. The corrosion rate was calculated by weight loss at periods of 6, 12 and 18 months, according to ISO 9226 [46], considering the total a€ected area (faces to the sky and to the ground). In order to determine the metal loss of samples after each atmospheric exposure period, the samples were immersed in the following solutions for the removal of the remainder of the corrosion products: aluminium in 35.5 ml of H3PO4 and 20 g of CrO3 per litre at a temperature of 808C; copper in 10% H2SO4 at ambient temperature; zinc in 10% NH4Cl at temperature of 708C.

Table 1 Meteorological parameters (outdoors) in the test stations for each test perioda Parameter

6 months Rural

12 months

Urban-industrial Rural

Average T 25.0 25.9 Minimum T 9.8 15.4 Maximum T 33.8 33.4 Average minimum T 21.2 23.3 Average maximum T 30.2 29.1 Average RH 85 81 Minimum RH 46 44 Maximum RH 100 100 Average minimum RH 64 67 Average maximum RH 97 92 t80±100 2942 2425 2109 888 t90±100 t15±20 and RH > 80% 299 60 t20±25 and RH > 80% 1855 1269 t25±30 and RH > 80% 721 1096 Time of rainfall (h) 283 375 Millimeter of rainfall 887 620

24.3 6.8 33.8 20.1 29.5 83 29 100 61 97 5485 3677 830 3126 1169 405 1846

18 months

Urban-industrial Rural

Urban-industrial

25.2 11.4 33.7 22.4 28.6 78 42 100 63 90 4273 1496 267 2337 1664 676 1167

25.4 11.4 33.7 22.7 28.7 80 42 100 65 91 6917 2526 363 3396 3153 908 1782

24.5 6.8 34.0 20.4 29.7 83 29 100 62 97 8434 5709 1205 4766 2029 565 2837

a T = temperature, (8C); RH = relative humidity, (%); t90±100 , t80±100 , t15±20 , t20±25 , t25±30 = time of wetness at 80±100% and 90±100% RH, and at 15±208C, 20±258C and 25±308C, respectively, (h).

Outdoor Cl average Cl minimum Cl maximum SO2 average SO2 minimum SO2 maximum Ventilated shed Cl average Cl minimum Cl maximum SO2 average SO2 minimum SO2 maximum Closed Space SO2 average SO2 minimum SO2 maximum Cl average

Parameter

7.9 2.6 18.2 39.4 30.1 54.8 ± ± ± ± ± ± ± ± ± ±

3.9 2.0 10.2 10.5 6.6 13.2 0.18 0.06 0.5 3.07 1.64 3.36 4.46 2.77 4.8 0.014

± ± ± ±

10.67 3.2 21.8 14.3 10.8 16.7

495.7 36.0 1333.6 34.6 19.1 43.9

3.5 1.45 4.8 0.014

0.27 0.06 0.5 2.48 1.64 3.36

3.8 2.0 10.2 10.8 5.6 20.4

Rural

Coastal

Rural

Urban-industrial

12 months

6 months

Table 2 Average, maximum and minimum chloride and SO2 deposition rates (mg/m2d)

± ± ± ±

± ± ± ± ± ±

7.5 2.6 18.2 34.5 16.6 54.8

Urban-industrial

± ± ± ±

12.1 21.8 3.2 13.5 5.2 30.8

489.6 7.5 1333.6 30.1 9.9 51.5

Coastal

2.82 1.45 4.8 0.014

0.2 0.06 0.5 2.77 1.64 3.36

3.3 1.1 10.2 9.1 3.4 20.4

Rural

18 months

± ± ± ±

± ± ± ± ± ±

7.1 2.6 18.2 29.6 15.2 54.8

Urban-industrial

± ± ± ±

11.59 3.2 21.8 10.8 3.0 30.8

580.4 7.5 1333.6 24.5 7.9 51.5

Coastal

1128 A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

Al Cu Zn

Metal

S0 P0 t4 C3 C3 C3

S3 P2 t4 C5 C5 C5

S1 P1 t4 C3 C3 C3

S1 P1 t4 C3 C3 C3 S0 P0 t4 C3 C3 C3

Outdoor

Closed space

Sheltered

Outdoor

Ventilated shed

Coastal

Rural

S3 P2 t4 C5 C5 C5

Sheltered

S1 P0 t4 C3 C3 C3

Ventilated shed

S1 P1 t4 C3 C3 C3

Outdoor

S1 P1 t4 C3 C3 C3

Sheltered

Urban-industrial

Table 3 Corrosivity of the test stations (C ) and exposure conditions for aluminium, copper and zinc based on the classi®cation of pollution by airborne salinity (S ) and sulphur compounds (P ), time of wetness …t† according to ISO 9223 [30]

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147 1129

1130

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

2.1. In¯uence of environmental parameters on corrosion of metals: statistical analysis The corrosion rate depends mainly on time of wetness and pollutants. However, if the di€erences in the corrosion process between outdoor and indoor conditions are taken into account, the in¯uence of direct precipitation (rain) is important in outdoor conditions and negligible indoors. The acceleration e€ect of pollutants could change depending on wetness conditions of the surface, so the in¯uence of the rain time and quantity should be very important in determining changes in corrosion rate. A model is proposed considering the in¯uence of the relationship between rain quantity/time and the interaction between pollutants and di€erent times of wetness (Clÿt5±25 , Clÿt25±35 , SO2 t5±25 and SO2t25±35 , Clÿ train, SO2train). Data concerning dew and fog are not reported, however, it is very well known that in Cuban conditions, when the temperature is over 258C, dew and phase time of wetness practically does not exist [47]. So, the usual time of wetness (RH 80± 100%) was divided in two parts: when the temperature is lower than 258C and when it is higher (up to 358C). In this way, three di€erent times of wetness are considered: time due to rain, time including rain, dew and fog, and time when evaporation of the electrolyte layer prevails (air temperature > 258C). It is a more quantitative step in studying the role of time of wetness. By considering that the average corrosion rate is in¯uenced principally by the deposition rates of chloride and SO2, and adding the e€ect of cleansing of the metallic surface, the following model is proposed: C ˆ a ‡ bClÿ t5±25 ‡ cClÿ t25±35 ‡ dSO2 t5±25 ‡ eSO2 t25±35 ‡ fClÿ train ‡ gSO2 train ‡ hmm=train , where C is the weight loss in g/m2 during 6, 12 and 18 months of exposure; Clÿ is the deposition rate of chloride ions in mg/m2d; SO2 is the deposition rate of sulphur compounds in mg/m2d; train and mm are the time and millimetres of rainfall, respectively; t5±25 and t25±35 mm are the time of wetness at temperatures of 5±258C and 25-358C, respectively, when the RH is over 80%. Corrosion data obtained for aluminium, copper and zinc exposed under outdoor and indoor conditions were adjusted to above model. For this purpose, a stepwise multiple regression analysis of the data was carried out, removing the variables with insigni®cant contribution to the regression equation until the best regression model was obtained. The statistical analysis was performed with commercial statistical software (STATISTICA Release 4.5). The variables showing a signi®cant contribution to the model were: Clÿ t5±25 , Clÿ t25±35 , SO2 t5±25 , SO2 t25±35 , Clÿ train and mm/train. Only the variable, SO2 train, was removed from the model since its e€ect could be included (or masked) by one or more of the selected variables. In the cases where the constant "a" included zero it was also removed. The present study included a separated treatment of

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

1131

data obtained in outdoor conditions, with respect to ventilated shed conditions, as well as a simultaneous treatment of both data sets (di€erent only in the corrosion values and the rainfall time). 3. Results and discussion 3.1. Copper corrosion results 3.1.1. Kinetics of the atmospheric corrosion of copper Table 4 reported the average corrosion values for copper, which corroded uniformly. The standard deviations are included with the average values. Copper is corroded by acid or strongly alkaline solutions containing oxidizing agents, and practically neutral or slightly alkaline solutions should passivate it [48]. As can be seen from Table 4, the copper corrosion rate decreases with exposure time under outdoor conditions regardless of the atmosphere type. For the three stations, this decrease is approximately in the same ratio, and it may be ascribed to an improvement of the protective properties of the copper corrosion products as it was obtained before for steel [1]. Under the other conditions it does not exist a homogeneus behaviour, except for sheltered coastal conditions where the corrosion increases with time. The corrosion rate increase with exposure time of the sheltered coastal samples may be explained by the fact that in this condition the surface remains wet most of the time mainly due to the absence of sunshine, and to the presence of hygroscopic particulates coming from the sea. Thus, real time of wetness should be greater than that calculated by ISO standard, and the occurrence of crystallisation process of the present phases in the patina is more dicult. From Table 4 it may also be seen that the coastal site showed the highest copper rates, being about four times higher than in the urban-industrial and rural atmospheres, and, in some cases, even much more higher than in the latter conditions. Another detail to be noted in this table is that outdoor corrosion rate in the rural station after 6 months, and under sheltered during the entire exposure period, is higher than in the urban-industrial atmosphere. Similar results were also obtained by Odnevall and Leygraf [49] under sheltered and under outdoor exposures, and by GoÂmez [50] under outdoor conditions. Odnevall and Leygraf [49] stated that at the expense of posnjakite formation, cuprite formation is much faster in the rural than in the urban site, mainly due to the higher relative humidity, despite lower concentrations of SO2 and NO2 in the rural site. The copper surface is always covered with cuprite [31,51±54], since it is generally the corrosion product ®rst formed [24,49,54,55] regardless of the exposure conditions, even after 3 years of exposure [54]. The fast formation rate of cuprite may well explain the signi®cantly higher corrosion rate in the rural site during unsheltered and sheltered exposures, which is observed after short and long exposures, and form evidence that initial

6 12 18

Exposure time (months) Closed space

27.421.6 19.420.4 14.320.6

9.820.8 11.520.5 8.620.7

3.520.2 1.920.3 1.920.3

0.220.04 0.220.02 2.021.2

5.320.7 34.620.04 30.322.0

Outdoor

Ventilated shed

Outdoor

Sheltered

Coastal

Rural

Table 4 Copper corrosion rates (g/m2 a2standard deviation)

44.520.5 54.122.6 72.423.5

Sheltered

19.824.5 47.223.7 33.120.5

Ventilated shed

23.921.8 19.820.04 15.020.5

Outdoor

6.821.5 8.520.4 7.520.7

Sheltered

Urban-industrial

1132 A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

1133

weathering conditions have a signi®cant in¯uence on atmospheric corrosion rates of copper. Galdo et al. [56] reported cuprite as the predominating phase for a Cuban atmosphere with similar characteristic to our rural site. On the other hand, it is known that the copper corrosion is extremely dependant on the relative humidity [52]. High relative humidities and temperatures enhance the copper corrosion in rural atmospheres [54]. One could not discard the possibility that an environmental variable not measured in this work is accelerating the copper corrosion rate under the above conditions. For example, Vilche et al. [57] show that synergistic e€ects detected for sulphur dioxide and ozone can explain the unexpectedly high corrosion rates of copper found at rural sites, which are characterized by high ozone concentrations. It has also been reported that ammonia increases the corrosion rates of copper [58]. In the presence of ammonium salts the domain of passivation in the diagram of Pourbaix almost disappears, producing vigorous corrosion of copper, even in the absence of oxidizing agents [8,48]. It has been reported small amounts of ammonium ions present on copper samples, which are due to deposited ammonium sulphate particles [2,28]. For Cuban rural atmospheres there have been reported higher ammonium concentrations than for urban-industrial atmospheres [25]. The statement of Odnevall and Leygraf [49], may also explain the higher corrosion rates under ventilated shed than outdoor conditions in the coastal station after 12 months of exposure. GoÂmez [50] obtained similar results, although not only for coastal atmospheres but also for the urban ones. It should be realized that the aerosol containing chloride ions cannot easily get into the indoor space of the ventilated sheds. It is supposed that under sheltered conditions the copper corrosion rates are enhanced also by the chloride ions present in the airborne salt coming from the sea. The possible presence of carbonic acid also prevents the formation of a protective ®lm of oxide [48]. It is not attacked by nonoxidizing acids. In addition to the larger time of wetness and the pH increase, another possible explanation to this behaviour may be the in¯uence of organic compounds and/or of ammonium originated by the degradation of organic matter or metabolites. Despite there being no evidence of the presence of micro-organisms it is known that they accelerate the corrosion rate of copper [31]. Copper is also degraded by reduced sulphur gases and formaldehyde, and it is also sensitive to formic acid [8]. The rural and the urban-industrial sites showed a di€erent behaviour to that observed in the coastal one. The outdoor corrosion rates in the rural and urbanindustrial test sites are always higher than indoors, decreasing with the degree of sheltering. The most marked di€erences are observed in the rural station, where the corrosion rates in the open air can reach values from 7.5 to 10 times with respect to those obtained in ventilated shed, and much more higher (150 times) than those in closed space.

1134

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

3.1.2. Relation between the copper corrosion rate and the meteorological parameters and pollutants Fitting the proposed model for copper corrosion data and environmental variables gave the following relationships: Outdoor C ˆ 6:13 2 3:44 ‡ …0:97 2 0:09†Clÿ train ‡ …0:29 2 0:08†SO2 t5±25 ‡ 0:2420:09 mm=train R ˆ 0:98;

R2 ˆ 0:95;

nˆ9

Indoor C ˆ …0:88 2 0:06†Clÿ t5±25 ‡ …0:17 2 0:06†SO2 t5±25 R ˆ 0:99;

R2 ˆ 0:97;

nˆ9

Outdoor and indoor   C ˆ …1:03 2 0:06†Clÿ ‡ …0:24 2 0:06†SO2 t5±25 ÿ …0:46 2 0:05† Clÿ train ‡ …0:19 2 0:05†mm=train R ˆ 0:98;

R2 ˆ 0:94;

n ˆ 18

According to these equations, the increase of the chloride ion concentration enhances the copper corrosion, both in outdoors and indoors. In outdoor atmospheres the time of rainfall also provokes an increase in the corrosion. As can also be seen, the increase of the variable represented by the relation between the amount of rainwater and the time of rainfall, which describes the rate of washing, promotes higher corrosion. This e€ect is probably due to the fact that Cu corrosion products are more dicult to be washed away from the surface, and the sorption of moisture (water) by them is higher than by Al corrosion products. As a more noble metal, copper is much less susceptible to the development of localised corrosion at di€erent points of the surface as it generaly occurs in aluminium due to its passive conditions. As it was seen above, the copper corrosion rates in the rural atmosphere were, in general, higher than those in the urban-industrial station. According to the latter, the higher is the humidity the higher is the corrosion rate. The constant ``a'' includes zero (occur with a negative sign), in the equation obtained for indoor exposure, as well as for outdoor-indoor; that is why it was not taken into account for the regression analysis. The equation obtained by the treatment of outdoor and indoor data together

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

1135

shows some aspects mentioned above. The copper corrosion rate is, in some cases, higher on sheltered than on outdoor surfaces, which explains the change in sign of the chloride deposition rate and time of rainfall variables. 3.2. Zinc corrosion results 3.2.1. Kinetics of the atmospheric corrosion of zinc The same as copper, general corrosion is the predominating attack type for zinc. The exposed Zn surfaces are characterized by the loss of the shiny appearance and the white patina formation on them, which was more noticeable on the samples exposed in the coastal station. The average corrosion rates for the exposed Zn samples and the standard deviations are reported in Table 5. As can be seen from this table, the Zn corrosion decreases with time for most of the test conditions, so that the Zn corrosion does not show a linear behaviour as has been stated for many scientists [14,38,59,60], at least during the test period in this work. As illustrated in Table 5, the corrosion rate varies with time in the same ratio for each exposure condition, regardless of the atmosphere type. However, it does not happen the same for similar exposure conditions. It should be noted that Zn corrosion rate after 6 months of exposure in the outdoor rural site is higher than that in the urban-industrial site. The sheltered and outdoor coastal samples showed 5±11 times higher corrosion rates compared with those of the rural and urban-industrial test sites. It should be remembered that copper showed a similar behaviour, even under sheltered conditions. According to Table 5, the di€erence among the corrosion rates obtained in the test sites is more noticeable during the ®rst 6 months of exposure. However, after 12 months the corrosion rate under shelter in the coastal station is about 10 times higher than that in the urban-industrial site. It should be realized that the corrosion rate after 6 and 12 months of exposure under outdoor and sheltered conditions, respectively, in the rural atmosphere is higher than that in the urbanindustrial test site. It is well known that the atmospheric corrosion of zinc is strongly in¯uenced by the presence or absence of moisture [3,14], which could be the reason for the higher corrosion rate in the rural station. Regarding the in¯uence of the degree of sheltering, it can be seen in Table 5 that outdoor corrosion rate is always higher than the corrosion rates obtained in the other exposure conditions. Most of the zinc compounds (mainly sulphur compounds) are soluble in water [24,55,61], so for exposure under unsheltered conditions, they will dissolve and also, to some degree, be washed o€ during rain [24,62]. Therefore, the corrosion products will give no protection to the metal from further attack. This may be a reason for the higher corrosion rates outdoors than under sheltered conditions. On the other hand, Lobnig et al. [27] found that the presence of zinc hydroxycarbonate is responsible for the corrosion of zinc, not only at and above the critical relative humidity of (NH4)2SO4, but also below it. They state that this

6 12 18

Exposure time (months) Closed space

19.121.3 9.620.4 8.320.01

7.622.3 7.420.5 5.520.1

6.421.4 3.820.3 3.120.2

3.720.3 3.820.01 3.720.6

123.3228 79.1211 67.022.5

Outdoor

Ventilated shed

Outdoor

Sheltered

Coastal

Rural

Table 5 Average zinc corrosion rates (g/m2 a2standard deviation)

40.321.7 71.821.8 62.825.4

Sheltered

15.820.2 11.620.9 8.520.4

Ventilated shed

14.921.6 11.021.2 9.820.9

Outdoor

7.920.1 6.720.0 6.021.1

Sheltered

Urban-industrial

1136 A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

1137

compound is probably what makes the reaction between zinc and (NH4)2SO4 particles possible at 60±65% relative humidity. These results were in contrast to the corrosion behaviour of copper [28] and aluminium [29], which react with the particles only at or above the critical relative humidity (75% at 373 K and 81% at 300 K). They conclude that (NH4)2SO4 particles in the atmosphere may be responsible for the hydroxysulphates found as ®nal corrosion products after extended ®eld exposures of zinc [27]. However, it has been reported that for zinc exposed outdoors, the SO2 incorporation and the precipitation provide the greatest portion of the incorporated sulphate, with airborne particles being a negligible source [14]. Zinc is highly sensitive to dissolved SO2, which is present in rainwater in moderate concentrations as well as to formaldehyde and formic acid [8]. 3.2.2. Relation between the zinc corrosion rate and the meteorological parameters and pollutants The statistical treatment of the zinc corrosion data in order to ®t the proposed model for the in¯uence of environmental data gives rise to the following relationships: Outdoor C ˆ 12:22 2 2:98 ‡ …0:98 2 0:07†Clÿ train R ˆ 0:98;

R2 ˆ 0:96;

nˆ9

Indoor

  C ˆ …1:98 2 0:33†Clÿ ‡ …0:13 2 0:04†SO2 t5±25 ÿ …1:09 2 0:33†Clÿ t25±35 R ˆ 0:99;

R2 ˆ 0:989;

nˆ9

Outdoor and indoor

 C ˆ 6:28 2 2:82 ‡ …1:86 2 0:44†t5±25 ÿ …0:96 2 0:44†t25±35 ‡ …0:12 2  0:07†train Clÿ ‡ …0:06 2 0:05† mm=train R ˆ 0:98;

R2 ˆ 0:95;

n ˆ 18

These equations are similar to the ones obtained for copper, although other variables occur. According to them the higher the chloride deposition rate and the time of rainfall, the higher the zinc corrosion rate outdoors. The e€ect of the chloride deposition rate is the same under sheltered conditions, as well as the time of wetness at a temperature of 5±258C. As can be seen from these equations the time of wetness at a temperature of 25±358C provokes a decrease in the zinc

1138

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

corrosion. For indoor conditions the intercept is not taken into account for the same reason of copper. In the simultaneous treatment of indoor and outdoor data, it was obtained that the time of rainfall and the chloride deposition rate provoke an increase on the corrosion rate, di€erent to the results obtained for copper. The latter is due to the fact that the zinc corrosion rate outdoors is higher than obtained under sheltered conditions. The presence of the time of rainfall in the equation seems to be related to the fact that the meteorological variables predominate (over pollutants), which is explained by a higher corrosion in the rural than in the urban-industrial station after 6 months outdoor and 12 months under sheltered conditions. This was also obtained in the copper results. 3.3. Aluminium corrosion results 3.3.1. Kinetics of the atmospheric corrosion of aluminium The presence of chloride ions in the atmosphere is the main cause of aluminium pitting corrosion. Chlorides are capable of breaking the passive ®lm formed on the surface. This is very noticeable in the coastal station where exists the highest level of chlorides compared with the other two test sites. The latter explains the higher number of pits on the surface of the samples exposed in the coastal station. In general, the groundward side of the samples showed higher pitting density, the pits being larger than those formed on the skyward side. This behaviour could be due to a lower in¯uence of the cleansing e€ect of rain, and it has been reported by other authors [54]. Under a ventilated shed the side of the samples facing the sea was the one with lower pits but of larger size, even more than that of the samples exposed outdoors. It could be explained by the absence of rain. Washing of the surface by rain reduces pitting corrosion, since it removes the corrosion stimulators. Pits are therefore always found to be deeper and more densely distributed on samples exposed to free air but protected from rain than those exposed to the open air [63]. The samples exposed outdoors in the urban-industrial and rural atmospheres were tarnished after 12 and 18 months, respectively. The average corrosion rates for aluminium exposed under di€erent test conditions are given in Table 6, as well as the standard deviations. According to this table the samples exposed in the coastal station show the highest corrosion rates; even under ventilated shed the corrosion rate is higher in this station than that in the rural test site for all exposure conditions. It is, in general, more than four times higher than that in the rural and urban-industrial stations, and in outdoor atmospheres it can reach values more than 10 times higher with respect to the rural station. It can also be seen that this corrosion rate relation among the three stations diminishes with time, being in the coastal station more noticeable. The outdoor corrosion rate after 18 months of exposure in the rural and urbanindustrial stations is something lower than that obtained after 6 months, and even a little more in the coastal station.

6 12 18

Exposure time (months) Closed space

0.320.04 0.320.02 0.220.01

1.120.14 1.220.14 0.820.11

0.220.03 0.320.01 0.220.00

0.420.08 0.320.10 0.320.04

4.320.2 3.220.03 2.220.4

Outdoor

Ventilated shed

Outdoor

Sheltered

Coastal

Rural

Table 6 Average aluminium corrosion rates (g/m2 a2standard deviation)

4.820.1 4.020.4 3.620.2

Sheltered

2.220.1 1.620.3 1.420.1

Ventilated shed

0.720.05 0.620.05 0.420.00

Outdoor

1.120.04 1.720.13 1.820.1

Sheltered

Urban-industrial

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147 1139

1140

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

It is noteworthy that for the three stations the corrosion rate under sheltered conditions is higher than outdoors. This may be due to the e€ect of rain on outdoor aluminium corrosion, by washing away the atmospheric pollutants deposited on the surface. In the rural station the corrosion rate changes the most from indoor to outdoor conditions. However, it should be noted that the other indoor conditions show lower corrosion rates than those observed outdoors. Under sheltered conditions the deposition of pollutants is higher than in ventilated shed and closed space. The corrosion rate of sheltered samples can reach values about three times higher than those obtained outdoors, speci®cally in rural and urban-industrial atmospheres. This relation increases with the exposure time in the coastal and urban-industrial atmospheres. In polluted environments under sheltered conditions dust particles often also play an important role in accelerating corrosion since they are not washed away. There the corrosion rate, being low at ®rst, may after some time accelerate rapidly, eventually attaining a steady-state value signi®cantly higher than that in open air exposure [24]. GoÂmez [50] also obtained higher indoor corrosion rates than outdoors. It should be noted that only in the urban-industrial station, and under sheltered conditions, the corrosion rate increases with exposure time. Dust may accelerate corrosion by absorbing moisture and sulphur compounds from the atmosphere, thus for long periods producing an acid medium on the surface and, under such conditions the protective alumina coating is not stable. In rain sheltered positions dust and other pollutants may collect and accelerate corrosion by disturbing the formation of a protective oxide coating [24]. 3.3.2. Relation between the aluminium corrosion rate and the meteorological parameters and pollutants Fitting the corrosion data for aluminium exposed under outdoor and indoor conditions to the proposed model gave the following relationships Outdoor C ˆ 0:46 2 0:16 ‡ …0:96 2 0:12†Clÿ train R ˆ 0:96;

R2 ˆ 0:90;

nˆ9

Indoor C ˆ …0:80 2 0:07†Clÿ t5±25 ‡ …0:37 2 0:07†SO2 t5±25 R ˆ 0:99;

R2 ˆ 0:96;

nˆ9

Outdoor and indoor

   C ˆ 0:55 2 0:36 ‡ …1:81 2 0:66†Clÿ ‡ …0:18 2 0:09†SO2 t5±25 ÿ …0:24 2  0:11†train ‡ …0:89 2 0:65†t25±35 Clÿ ÿ …0:22 2 0:09† mm=train

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

R ˆ 0:96;

R2 ˆ 0:89;

1141

n ˆ 18

where R is the coecient of multiple correlation; R 2 is the coecient of multiple determination; n is the number of data. According to the equation obtained for outdoor conditions the variable represented by the interaction between the chloride deposition rate and the rainfall time is the one which has the higher in¯uence on the aluminium corrosion. Thus, an increase in the value of this variable provokes an increase in the corrosion rate. It must be noted that chloride ions are the main cause of the passive ®lm breakdown, while the electrolyte ®lm with the high thickness is formed during rainfall (phase layers). However, the regression coecient a€ecting the interaction is lower than the corresponding to the relation obtained including data outdoors and indoors. It would mean that the interaction Clÿ train could represent the in¯uence of other variables not monitored or reported. However, under sheltered conditions the aluminium corrosion is mainly in¯uenced by the interaction between the time of wetness at a temperature of 5± 258C, the chloride ions and the sulphur compounds deposition rate, respectively, having a higher in¯uence on the former interaction. It should be noted that this case does not include the intercept either, as in the indoor equations for copper and zinc. As can be seen from the equation when both data are analysed together, it would appear that the same variables are obtained as in the previous equations; however, in this case the interaction between the chloride deposition rate and time of rain has an inverse e€ect to that obtained under outdoor conditions. This may be related to the fact that the corrosion rate under sheltered conditions is, in general, higher than the outdoor corrosion rate. The variable Clÿ train becomes zero indoors, so that the di€erence between indoors and outdoors is given by this relation. Other variables that entered into this equation are the interaction between the time of wetness at temperature of 25±358C and the sulphur compounds deposition rate and, between the amount of rain and the time. The higher in¯uence corresponds to the interaction Clÿt5±25 : The moderate e€ect of the latter may be due to a predominant washing e€ect of rain that removes the soluble pollutants deposited on the surface. 3.4. Comparison of the corrosion results obtained for the three metals As can be seen from the results, the corrosion rates of the aluminium samples are the lowest and the least attacked by general corrosion, followed by zinc samples and ®nally by copper. As is very well known, copper is the most noble and resistant of the three metals studied. The fact that it shows the higher corrosion rate could be due to some other factors for a relatively short time of exposure. The following di€erences on this behaviour were detected: in the rural station under ventilated shed and closed space, the corrosion rate of zinc is higher than that of copper in the entire exposure period. The same was obtained in the

1142

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

coastal station but outdoors, as well as in the urban-industrial and coastal stations under sheltered conditions after 6 and 18 months of exposure, respectively. The corrosion rate of steel [1] exposed in the rural station under sheltered conditions is higher than for the non-ferrous metals studied, even when they are exposed under the most aggressive conditions. While their corrosion rate decreases with time in the coastal station outdoors, the steel corrosion rate increases to corrode completely. The severity of attack in various atmospheres decreases, as for carbon steel [1], in the order coastal- industrial-rural for the three metals studied, in general. However, higher copper and zinc corrosion rates in the rural atmosphere than in the industrial one were obtained. The latter could be due to a predominant e€ect of the relative humidity over copper and zinc. A synergistic e€ect between ozone and SO2 over copper under these conditions has been reported [57]. Higher aluminium corrosion rates under sheltered conditions than those outdoors are observed at all the test sites, while for copper only at the coastal station. The former may be because of the di€erences in the rinsing action of rain, which removes the pollutants on the outdoor surfaces and diminishes corrosion; the latter might be due to the formation of a hygroscopic corrosion products layer and the presence of pollutants capable of increasing the electrolyte ®lm pH. For zinc exposed outdoors, the corrosion rates are the highest compared with all other exposure conditions. It could mean that rain and humidity are the most important factors in determining Zn corrosion rate for a given station. As mentioned before, the zinc corrosion rate is strongly in¯uenced by the presence or absence of moisture [3,14]. Cramer and McDonald [26] noted that the copper corrosion products have a substantially lower anity for sulphur dioxide than the zinc corrosion products. They also found that dry deposition of sulphur dioxide has the greatest e€ect on the zinc corrosion products; and wet deposition of hydrogen ions has the greatest e€ect on the copper corrosion products. These authors showed that zinc corrosion ®lms continue to grow well beyond one year of exposure and copper corrosion ®lms grow for much longer times. They also found that, compared with non corroded surfaces, moisture is more readily available on corroded zinc and copper surfaces from capillary condensation and from the hydrophilic nature of their corrosion products. The corrosion rates for the metals tested, in general, tend to decrease as the exposure time increases. Only in a few cases the behaviour was inverse; for example, the aluminium corrosion in industrial atmosphere under sheltered conditions tend to increase, as well as the corrosion rate of zinc samples in the coastal station under similar conditions. In the regression equations obtained for outdoor conditions, the main variable in¯uencing the corrosion of the three metals is that represented by the interaction between the chloride deposition rate and the time of rainfall. It means that the main component of time of wetness in¯uencing on corrosion is the time of rainfall. Only in the copper case appeared other variables (SO2t5±25 and mm/train). For indoors, the equations for the three metals show the interaction between the

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

1143

chloride and SO2 deposition rates and the time of wetness at temperature between 58C and 258C is the most signi®cant. The constant ``a'' includes zero in all the equations obtained for indoor exposure. Also, we have the variables Clÿ t5±25 and SO2 t5±25 for zinc and SO2 t25±35 for copper. When outdoor and indoor corrosion data are analysed together the chloride deposition rate and the time of wetness at temperature between 5 and 258C were found to be among the most signi®cant variables in¯uencing the corrosion of the three metals investigated; the SO2 deposition rate and the amount of rainwater/time of rainfall of the aluminium and copper corrosion; the time of rainfall of aluminium and zinc corrosion. Also, the time of wetness at temperatures between 25 and 358C appears as an important variable for aluminium and zinc, but with a negative e€ect on corrosion. It con®rms that this latter time represents the evaporation of the electrolytic ®lm. In the case of copper the evaporation of the ®lm could be stopped by the characteristics of the corrosion products layer. The sheltered and outdoor coastal zinc samples showed 5±11 times higher corrosion rates compared with those of the rural and urban-industrial test sites. It should be remembered that aluminium and copper showed a similar behaviour, even under sheltered conditions. The linearity in the atmospheric corrosion behaviour of galvanized steel does not occur as is commonly reported (galvanized steel behaves as does zinc in the atmosphere) [8]. It apparently requires that solid acidic contaminants be deposited on the metal surface [59]. It is extremely dicult for a protective ®lm to form on galvanized steel in a coastal atmosphere, but once formed, the ®lm is able to very e€ectively withstand any detrimental interaction with the environment [59]. Atmospheric corrosion rates in a coastal environment accordingly become progressively lower with increasing time of exposure [59]. Table 7 shows the classi®cations of atmospheric corrosivity according to the corrosion rates obtained for each metal exposed under di€erent test conditions. As can be seen from this table, taking into account Table 3, about 59% of the cases do not agree with the established in ISO 9223 [30]. The corrosion rates are in general one order higher or lower than those predicted by this standard. However, the copper corrosion rates obtained are very di€erent from those predicted by the standard, overestimating or underestimating one class number, but in three cases exceed the upper limit given in this standard. Dean [64] obtained a ful®lment of this standard in 60% of the cases examined.

4. Conclusions 1. The results obtained con®rm and allow expansion of the model previously proposed for steel to copper, zinc and aluminium. The importance of a more quantitative step in de®ning time of wetness taking into account the in¯uence of temperature and rain is also con®rmed. 2. The main di€erences obtained in ®tting the model proposed for non-ferrous

b

a

Al Cu Zn

C2a C1a C2a

C4 a C5 C5a,b

C3 C3 C3

C2a C4a C3

C2a C2a C2a

Outdoor

Closed space

Sheltered

Outdoor

Ventilated shed

Coastal

Rural

Corrosion rate values are not in the range given in ISO 9223. Corrosion rates exceed the upper limit value given in ISO 9223.

Metal

Table 7 Classi®cation of atmospheric corrosivity (C ) according to the corrosion rates for each metal

C4 a C5a,b C5a,b

Sheltered

C3 C5a C3

Ventilated shed

C3 C4 a C3

Outdoor

C3 C3 C2 a

Sheltered

Urban-industrial

1144 A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

3.

4.

5.

6.

1145

metals regarding steel is the role of time of rain, more signi®cant than time of wetness at temperature 5±258C in its interaction with chloride deposition rate under outdoor conditions. It could be due to the fact that chloride and sulphate compounds of non-ferrous metals are less soluble than in the case of steel. According to the adjusted model the interaction between the chloride deposition rate with the time of rainfall (outdoors) and with the time of wetness at temperatures between 5 and 258C (indoors) were found to be the most signi®cant variables in¯uencing the corrosion of the three metals investigated, as well as the interaction between the sulphur compounds deposition rate and the time of wetness at temperature between 5 and 258C (indoors). However, other variables appeared to be important in the corrosion process depending on the metal nature. Higher corrosion rate values than predicted are obtained for zinc and copper in coastal sites, and lower for aluminium. The classi®cation of corrosivity of the test stations based on environmental data, according to ISO 9223, is not in agreement with that based on the corrosion rates in about 40% of the cases. The highest corrosion rate values are shown, in general, for the samples exposed outdoors. However, the aluminium specimens showed the highest corrosion in ventilated shed in all the test sites, as well as the copper ones, but only in the coastal station. The zinc and copper corrosion rates in rural conditions are, in some cases, higher than those obtained in urban-industrial ones. This might be due to a speci®c sensibility of zinc to the relative humidity and to a possible synergistic e€ect already reported between ozone and SO2 on copper corrosion under these conditions. In spite of its noble nature, copper shows the highest corrosion rates between non-ferrous metals in the majority of the exposure conditions; only in a few cases zinc show higher corrosion rates. It could perhaps be due to a relatively short time of exposure. Lower corrosion rates for copper should be expected for longer time periods. Only for sheltered copper and aluminium samples exposed in coastal and urban-industrial conditions, respectively, the corrosion rate increases with time.

Acknowledgements We are grateful to Ms. Y. Leon and Ms. J. Perez for compilation of meteorological and pollution data.

1146

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

References [1] A.R. Mendoza, F. Corvo, Corrosion Science 41 (1) (1999) 75±86. [2] D. Persson, C. Leygraf, Journal of the Electrochemical Society 142 (5) (1995) 1468±1477. [3] E. Johansson, et al., Comparison of di€erent methods for assessment of corrosivity in indoor environments, in: 13th International Corrosion Congress, Melbourne, Australia, November, 1996. [4] I. Odnevall, C. Leygraf, Corrosion Science 36 (9) (1994) 1551±1567. [5] Y. Fukuda, et al., Journal of the Electrochemical Society 138 (5) (1991) 1238±1243. [6] J. Vlckova, D. Knotkova, CORROSION 95, Paper No. 230, NACE, 1995. [7] D. Knotkova, J. Vlckova, K. Kreislova, Regional and microclimatic pollution e€ects on atmospheric corrosion in Prague and Europe, Environmental E€ects, MP/June 1995. [8] T.E. Graedel, R. McGill, Environmental science and technology 20 (1986) 1093±1100. [9] S. Feliu, M. Morcillo, S. Feliu Jr, Corrosion Science 34 (3) (1993) 403±414. [10] S. Feliu, M. Morcillo, S. Feliu Jr, Corrosion Science 34 (3) (1993) 415±422. [11] C. Arroyave, F.A. Lopez, M. Morcillo, Corrosion Science 37 (11) (1995) 1751±1761. [12] F. Corvo, I. Leon, Rev Iberoamericana de CorrosioÂn y ProteccioÂn 19(5) (1988). [13] C. Arroyave, M. Morcillo, Corrosion Science 37 (2) (1995) 293±305. [14] T.E. Graedel, Journal of the Electrochemical Society 136 (4) (1989) 193C±203C. [15] R. Baboian, CORROSION 91, Paper No. 371, NACE, 1991. [16] J.E. Svensson, L.G. Johansson, Corrosion Science 34 (5) (1993) 721±740. [17] G.W. Walter, Corrosion Science 32 (12) (1991) 1353±1376. [18] T.E. Graedel, R.P. Frankenthal, Journal of the Electrochemical Society 137(8) (1990). [19] H. Guttman, Atmospheric, weather factors in corrosion testing, in: Atmospheric Corrosion, Wiley, New York, 1982. [20] C. Arroyave, M. Morcillo, A climatic chamber study of the role of nox on the atmospheric corrosion of steel, in: 13th International Corrosion Congress, Melbourne, Australia, November, 1996. [21] V. Kucera, et al., Dose-response relations from the un ece project as a tool for air pollution abatement strategies, in: 13th International Corrosion Congress, Melbourne, Australia, November, 1996. [22] J. Tidblad, C. Leygraf, V. Kucera, Journal of the Electrochemical Society 138 (12) (1991) 3592± 3598. [23] P. Eriksson, et al., Journal of the Electrochemical Society 140 (1) (1993) 53±59. [24] V. Kucera, E. Mattsson, Atmospheric corrosion, in: F. Mansfeld (Ed.), Corrosion Mechanisms, Marcel Dekker, New York, 1987, pp. 211±284. [25] O. Cuesta, Ph.D. Thesis, 1993. [26] S.D. Cramer, L.G. McDonald, ASTM STP 1000, 1990, pp. 241±259. [27] R.E. Lobnig, et al., Journal of the Electrochemical Society 143 (5) (1996) 1539±1546. [28] R.E. Lobnig, et al., Journal of the Electrochemical Society 141 (11) (1994) 2935±2941. [29] R.E. Lobnig, et al., Journal of the Electrochemical Society 143 (4) (1996) 1175±1182. [30] ISO 9223:1992, Corrosion of metals and alloys Ð corrosivity of atmospheres Ð classi®cation. [31] Morcillo, M., Feliu, S. Mapas de EspanÄa de corrosividad atmosfeÂrica, Programa CYTED, 1993, p. 3,142. [32] P.J. Sereda, ASTM STP 558, 1974, pp. 7±22. [33] F. Corvo, N. Betancourt, A.R. Mendoza, Corrosion Science 37 (12) (1995) 1889±1901. [34] S.D. Cramer et al., Paper No. 5, Corrosion 95, 1995. [35] J.J. Gmez, et al., in: Proceedings del II Encuentro NACE Regin Iberoamericana, Septiembre, 1996. [36] S.D. Cramer, et al., Cubic model for describing the atmospheric corrosion of structural metals, in: 13th International Corrosion Congress, Melbourne, Australia, November, 1996. [37] S. Cole, et al., Implications of studies of surface chemistry to the development of a methodology for life prediction of metallic components, in: 13th International Corrosion Congress, Melbourne, Australia, November, 1996.

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147

1147

[38] M.E.M. Almeida, M.G.S. Ferreira, CorrosaÄo AtmosfeÂrica 60 (1997) 341±344. [39] M.E. Komp, CORROSION 87, Paper No. 423, NACE, 1987. [40] A.A. Bragard, H.H. Bonnarens, Prediction at long terms of the atmospheric corrosion of structural steels from short-term experimental data, C.R.M. (Belgium), No. 57, 15, 1980. [41] C.R. Shastry et al., ASTM STP 965, 1988, pp. 5±15. [42] NC-12-01-09: Mtodo de determinacin de cloruros de la atmsfera. [43] ISO 9225:1992, Corrosion of metals and alloys Ð corrosivity of atmospheres Ð measurement of pollution. [44] F. Corvo, E. Alvarez, Rev Iberoamericana de CorrosioÂn y ProteccioÂn, XXI(5) (1990). [45] ISO 8407:1991, Corrosion of metals and alloys Ð removal of corrosion products from corrosion test specimens. [46] ISO 9226:1992, Corrosion of metals and alloys Ð corrosivity of atmospheres Ð determination of corrosion rate of standard specimens for the evaluation of corrosivity. [47] F. Corvo, Ph.D. Thesis, CNIC, 1980. [48] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd English ed., NACE, 1974. [49] D. Odnevall, Leygraf, The atmospheric corrosion of copper Ð a multianalytical approach, in: 13th International Corrosion Congress, Melbourne, Australia, November, 1996. [50] J.J. Gomez, Ph.D. Thesis, 1999. [51] D. Persson, C. Leygraf, Journal of the Electrochemical Society 140 (5) (1993) 1256. [52] UN/ECE International co-operative programme on e€ects on materials, including historic and cultural monuments, Report No. 11, June 1993. [53] A. Atrens, et al., Composition and structure of copper patinas, in: 13th International Corrosion Congress, Melbourne Australia, November, 1996. [54] M. Morcillo, et al. (Eds.), CorrosioÂn y proteccioÂn de metales en las atmsferas de IberoaeÂmeÂrica. Parte I. Ð Proyecto MICAT, XV.1/CYTED, 1998, pp. 547±573. [55] M. Morcillo, et al., Atmospheric corrosion in Ibero-America, in: 13th International Corrosion Congress, Melbourne, Australia, November, 1996. [56] A. Galdo, et al., AnaÂlisis de fases de patinas de cobre formadas en el clima tropical humedo de Cuba, in: Proceedings of Quimindustria '90. I Simposio Internacional de CorrosioÂn y Tropicalizacin, Havana, May 9±12, 1990. [57] J.R. Vilche, et al., Corrosion Science 39 (4) (1997) 655±679. [58] P.R. Mehta, Salt Research and Industry 5 (1) (1979) 55±59. [59] R.A. Legault, Atmospheric corrosion of galvanized steel in: Willian Ailon (Ed.),. O€print from Atmospheric Corrosion., 1982, pp. 607±613. [60] J.B. Mohler, Pollution Engineering, October 1972. [61] I. Dehri, et al., Corrosion Science 36 (12) (1994) 2181±2191. [62] I. Odnevall, Characterization of corrosion products formed on rain sheltered aluzink tm and aluminium in a rural and an urban atmosphere, in: 13th International Corrosion Congress, Melbourne, Australia, November, 1996. [63] K. Barton, Protection against atmospheric corrosion. Theories and methods, 1973, p. 268. [64] S.W. Dean, The USA contribution to the ISO CORRAG program, in: 13th international corrosion congress, Melbourne, Australia, November, 1996.