Outdoor and indoor atmospheric corrosion of non-ferrous metals

Outdoor and indoor atmospheric corrosion ofnon-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 aluminiumexposed on three test sites indoors and outdoors (coastal, urban-industrial and rural) underdi�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 environmentalparameters and main pollutants (SO2 and chlorides) on the atmospheric corrosion ofmetals. 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 metalsinvestigated; 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 themodel previously proposed for steel to non-ferrous metals. A classi®cation of theatmospheric corrosion aggressivity of the test sites based both on environmental data and

corrosion rate measurements was made according to ISO 9223. The corrosion aggressivityprognostic of this standard is not always in agreement with the results obtained in Cubanatmospheric conditions. 7 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Atmospheric corrosion; Indoor corrosion; Non-ferrous metals; Copper; Zinc; Aluminium

0010-938X/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S0010 -938X(99)00135 -3

Corrosion Science 42 (2000) 1123±1147

* Corresponding author. Tel.: +53-214-532.

E-mail address: [email protected] (A.R. Mendoza).

1. Introduction

As has already been emphasized by di�erent authors [2±7] indoor corrosionhas been less studied than outdoor corrosion mainly because of its lowcorrosion rate, and because the main contaminants are more signi®cant inoutdoor atmospheres. However, it has been reported that indoor corrosionseems to be markedly a�ected by a greater number of air pollutants than theoutdoor corrosion [3], specially when indoor sources of corrodents are present[8]. Many technological and scienti®c electronic applications, as well as someparts of our cultural heritage are showing serious problems due to indooratmospheric corrosion, even in the presence of traces of moisture andcontaminants.

Sulphur compounds and chloride ions are the most common and importantatmospheric corrosive agents, as has been reported by di�erent authors all overthe world [9±12]. However, there are other atmospheric pollutants whosemechanism of action on metals have been less studied and could be veryimportant in indoor corrosion. This has been either because they are rather non-corrosive due to their low reactivity, inasmuch as they cannot ionize on metalsurfaces, they cannot form aggressive electrolytes or depassivate metals, whichwould accelerate the anodic reaction of the corrosion process, such as mostorganic compounds [2,5] and CO [13], or because in spite of their extremeaggressiveness, such as Cl2, HCl, H2S and nitrogen containing compounds whoseatmospheric concentrations are too low. However, di�erent works concerning thein¯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 andiron show no SO2 + NO2 synergism [21], and that for steel is negligible [11,20]. Itwas reported that a thick layer of water on the metal surface seems to act as asink 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 synergistice�ects [21]. It has been showed by several authors that the SO2 + NO2 synergismon copper corrosion is only active at high relative humidity (90%) [23], and steelcorrosion at low relative humidity [15,18,20]. Kucera et al. [21] reported that thesynergistic e�ect of SO2 + O3 can be both stronger than SO2 + NO2, as forcopper, and weaker, as for nickel. In other works no synergistic e�ects ofsimultaneous interaction of SO2 and NO2 with either nickel or copper have beenobserved [22]. The dry deposition is in most cases dominating and SO2 exerts thestrongest corrosive e�ect [21]. The role of NO2 has not yet been clari®ed and itsstrong synergistic e�ect with SO2, shown for many materials in di�erentlaboratory studies, has not been observed in the ®eld exposure and may be due tothe strong correlation between SO2, NO2, and O3 concentrations [21].

The estimated minimum NO2 concentration inducing metal corrosion is believedto be 30 mg/m3 [13]. For a urban-industrial Cuban atmosphere it was obtaineddiary 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±11471124

in¯uence of this pollutant on the atmospheric corrosion in our conditions may benegligible.

The particulates could also act in a detrimental way [1,2,8,21,26] as well as toneutralize other acidic pollutants [21], specially if they are not washed away fromthe surface, as when the metal are exposed to sheltered conditions [19,24]. In arecent work, higher corrosion rates were obtained by our research group forsamples exposed horizontally indoors with regard to the verticals, where the dustdeposition was highly signi®cant. Particulate species in the atmosphere canaccelerate corrosion of metals in several ways, for example, by increasing theconductivity of the surface layer after dissolution of soluble ions from theparticulate [27]. It has been reported on the e�ect of particles of (NH4)2SO4 onthe corrosion of zinc [27], copper [28] and aluminium [29]. Ammonium andsulphate ions are the most abundant ions in ®ne dust particles commonly found inurban environments and may play a dominant role in the corrosion process[28,29]. Indoors, most of the surface degradation can be attributed to adsorbedsulphate aerosol particles [14,29]. The accumulation of inorganic ionic substancesis primarily due to particle deposition, and because of their high content and highindoor concentration, ®ne particles play a major role in the corrosion of electronicmaterials [29]. Organic molecules also may be supplied to metal surfaces by thedeposition of particles [2,8].

The total corrosion e�ect during a period of time is determined by thetotal time of wetness and the composition of the water layer on the metallicsurface, 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 higherthan 08C, proposed by ISO 9223 [30], is often used for estimation of thetime of wetness. Although it may not be the actual time of wetness, becausewetness is in¯uenced by di�erent factors, it usually shows a good correlationwith corrosion data from ®eld tests under outdoor conditions, correspondingto the kinetically decisive time periods during which corrosion proceeds.However, under indoor conditions other criteria seem to be valid and havenot yet been fully clari®ed [24].

Several studies have attempted to develop, for di�erent purposes, modelsthat 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 corrosiondata.

In a previous publication [1], we reported the study of the in¯uence ofatmospheric pollutants and some meteorological parameters, as well as theirpossible interaction, on the atmospheric corrosion behaviour of carbon steelindoors and outdoors. A corrosion model that represents this in¯uence, besidesother 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 ontemperature. In the study presented here, we extend the previous work to thestudy of the atmospheric corrosion of copper, zinc and aluminium.

A.R. Mendoza, F. Corvo / Corrosion Science 42 (2000) 1123±1147 1125

2. Experimental method

Experiments were conducted in the same way as reported previously [1].Meteorological data and other atmospheric factors along with the corrosion rateof exposed samples were recorded in order to identify the e�ect on corrosion ofthe variation of the atmospheric factors under indoor and outdoor conditions indi�erent atmospheres. Pollution by chloride ions and SO2 was also measured.

The chloride deposition rate was determined at two-month periods (except inthe rural station under sheltered and ventilated sheds, conditions where it wasdetermined at 6 months, as well as the SO2 deposition rate, because of the lowconcentration of these contaminants there) using the dry plate method, consistingof 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 determinedanalytically at the end of the exposure period and the deposition rate is calculated.The SO2 deposition rate was determined using alkaline surfaces of porous ®lterplates saturated by a solution of sodium carbonate placed in the same shed usedfor chloride deposition rate determination [43]. The SO2 deposition rate includessulphur dioxide and other sulphur compounds, such as sulphate coming from thesea as aerosol (it should be taken into account that Cuba is an island wherealmost any place is a�ected by the airborne salinity). The pollutants measuredunder sheltered conditions are assumed to be the same as that of outdoorconditions, since they are always determined under a shed.

Data for temperature and relative humidity were processed using softwaredeveloped for this purpose in order to determine the characteristics of thetemperature-relative humidity complex [44]. Data for pollutants and corrosionwere 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 thesame site where the urban-industrial ones are evaluated (the urban-industrial islocated 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-continousmeasurements carried out), it is well known that the temperature and the relativehumidity keep practically constant all over the year, reaching values between 18±308C and 85±95%, respectively. A more detailed description of the test sites andexposure conditions, and their annual average environmental parameters and itsbehaviour, is given in a previous publication [1].

Table 3 shows the classi®cation of corrosivity for every test station based onenvironmental data (time of wetness and chloride and SO2 deposition rates)according to ISO 9223 [30].

Commercial pure copper, commercial pure zinc and aluminium alloy 2024samples were exposed under di�erent atmospheric conditions. Prior to exposurethe 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


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 desiccatorwith anhydrous CaCl2 for at least 24 h and then weighed. Since the ®rst corrosionevaluation was made after 6 months and, Cuba being a tropical island whoseatmosphere is very aggressive, it is considered that the in¯uence of surfacepreparation is almost negligible.

Then, duplicate samples of each metal were exposed to outdoor and indoorconditions in di�erent atmospheres (rural, urban-industrial and coastal). Inoutdoor conditions, the samples were exposed at 458 to the horizontal, facingsouth, 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 (inall the stations) conditions, they were exposed vertically. The corrosion rate wascalculated by weight loss at periods of 6, 12 and 18 months, according to ISO9226 [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 exposureperiod, the samples were immersed in the following solutions for the removal ofthe remainder of the corrosion products: aluminium in 35.5 ml of H3PO4 and 20 gof CrO3 per litre at a temperature of 808C; copper in 10% H2SO4 at ambienttemperature; zinc in 10% NH4Cl at temperature of 708C.

Table 1

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

Parameter 6 months 12 months 18 months

Rural Urban-industrial Rural Urban-industrial Rural Urban-industrial

Average T 25.0 25.9 24.3 25.2 24.5 25.4

Minimum T 9.8 15.4 6.8 11.4 6.8 11.4

Maximum T 33.8 33.4 33.8 33.7 34.0 33.7

Average minimum T 21.2 23.3 20.1 22.4 20.4 22.7

Average maximum T 30.2 29.1 29.5 28.6 29.7 28.7

Average RH 85 81 83 78 83 80

Minimum RH 46 44 29 42 29 42

Maximum RH 100 100 100 100 100 100

Average minimum RH 64 67 61 63 62 65

Average maximum RH 97 92 97 90 97 91

t80±100 2942 2425 5485 4273 8434 6917

t90±100 2109 888 3677 1496 5709 2526

t15±20 and RH>80% 299 60 830 267 1205 363

t20±25 and RH>80% 1855 1269 3126 2337 4766 3396

t25±30 and RH>80% 721 1096 1169 1664 2029 3153

Time of rainfall (h) 283 375 405 676 565 908

Millimeter of rainfall 887 620 1846 1167 2837 1782

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).


Table

2

Average,

maxim

um

andminim

um

chlorideandSO

2depositionrates(m

g/m

2d)

Parameter

6months

12months

18months

Rural

Urban-industrial

Coastal

Rural

Urban-industrial

Coastal

Rural

Urban-industrial

Coastal

Outdoor

Claverage

3.9

7.9

495.7

3.8

7.5

489.6

3.3

7.1

580.4

Clminim

um

2.0

2.6

36.0

2.0

2.6

7.5

1.1

2.6

7.5

Clmaxim

um

10.2

18.2

1333.6

10.2

18.2

1333.6

10.2

18.2

1333.6

SO

2average

10.5

39.4

34.6

10.8

34.5

30.1

9.1

29.6

24.5

SO

2minim

um

6.6

30.1

19.1

5.6

16.6

9.9

3.4

15.2

7.9

SO

2maxim

um

13.2

54.8

43.9

20.4

54.8

51.5

20.4

54.8

51.5

Ventilatedshed

Claverage

0.18

±10.67

0.27

±12.1

0.2

±11.59

Clminim

um

0.06

±3.2

0.06

±21.8

0.06

±3.2

Clmaxim

um

0.5

±21.8

0.5

±3.2

0.5

±21.8

SO

2average

3.07

±14.3

2.48

±13.5

2.77

±10.8

SO

2minim

um

1.64

±10.8

1.64

±5.2

1.64

±3.0

SO

2maxim

um

3.36

±16.7

3.36

±30.8

3.36

±30.8

ClosedSpace

SO

2average

4.46

±±

3.5

±±

2.82

±±

SO

2minim

um

2.77

±±

1.45

±±

1.45

±±

SO

2maxim

um

4.8

±±

4.8

±±

4.8

±±

Claverage

0.014

±±

0.014

±±

0.014

±±


Table

3

Corrosivityofthetest

stations(C

)andexposure

conditionsforaluminium,copper

andzincbasedontheclassi®cationofpollutionbyairbornesalinity(S

)

andsulphurcompounds(P

),timeofwetness�t�

accordingto

ISO

9223[30]

Metal

Rural

Coastal

Urban-industrial

Outdoor

Sheltered

Ventilatedshed

Closedspace

Outdoor

Sheltered

Ventilatedshed

Outdoor

Sheltered

S1P1t 4

S1P1t 4

S0P0t 4

S0P0t 4

S3P2t 4

S3P2t 4

S1P0t 4

S1P1t 4

S1P1t 4

Al

C3

C3

C3

C3

C5

C5

C3

C3

C3

Cu

C3

C3

C3

C3

C5

C5

C3

C3

C3

Zn

C3

C3

C3

C3

C5

C5

C3

C3

C3


2.1. In¯uence of environmental parameters on corrosion of metals: statisticalanalysis

The corrosion rate depends mainly on time of wetness and pollutants. However,if the di�erences in the corrosion process between outdoor and indoor conditionsare taken into account, the in¯uence of direct precipitation (rain) is important inoutdoor conditions and negligible indoors. The acceleration e�ect of pollutantscould change depending on wetness conditions of the surface, so the in¯uence ofthe rain time and quantity should be very important in determining changes incorrosion rate.

A model is proposed considering the in¯uence of the relationship between rainquantity/time and the interaction between pollutants and di�erent times ofwetness (Clÿt5±25, Clÿt25±35, SO2 t5±25 and SO2t25±35, Clÿ train, SO2train). Dataconcerning dew and fog are not reported, however, it is very well known that inCuban conditions, when the temperature is over 258C, dew and phase time ofwetness 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 andwhen it is higher (up to 358C). In this way, three di�erent times of wetness areconsidered: time due to rain, time including rain, dew and fog, and time whenevaporation of the electrolyte layer prevails (air temperature > 258C). It is a morequantitative step in studying the role of time of wetness.

By considering that the average corrosion rate is in¯uenced principally by thedeposition rates of chloride and SO2, and adding the e�ect of cleansing of themetallic surface, the following model is proposed:

C � a� bClÿt5±25 � cClÿt25±35 � dSO2t5±25 � eSO2t25±35 � fClÿtrain

� gSO2train � hmm=train,

where C is the weight loss in g/m2 during 6, 12 and 18 months of exposure; Clÿ isthe deposition rate of chloride ions in mg/m2d; SO2 is the deposition rate ofsulphur compounds in mg/m2d; train and mm are the time and millimetres ofrainfall, respectively; t5±25 and t25±35 mm are the time of wetness at temperaturesof 5±258C and 25-358C, respectively, when the RH is over 80%.

Corrosion data obtained for aluminium, copper and zinc exposed underoutdoor and indoor conditions were adjusted to above model.

For this purpose, a stepwise multiple regression analysis of the data was carriedout, removing the variables with insigni®cant contribution to the regressionequation until the best regression model was obtained. The statistical analysis wasperformed with commercial statistical software (STATISTICA Release 4.5). Thevariables 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 oneor more of the selected variables. In the cases where the constant "a" includedzero it was also removed. The present study included a separated treatment of


data obtained in outdoor conditions, with respect to ventilated shed conditions, aswell as a simultaneous treatment of both data sets (di�erent only in the corrosionvalues and the rainfall time).

3. Results and discussion

3.1. Copper corrosion results

3.1.1. Kinetics of the atmospheric corrosion of copperTable 4 reported the average corrosion values for copper, which corroded

uniformly. The standard deviations are included with the average values. Copperis corroded by acid or strongly alkaline solutions containing oxidizing agents, andpractically neutral or slightly alkaline solutions should passivate it [48].

As can be seen from Table 4, the copper corrosion rate decreases with exposuretime under outdoor conditions regardless of the atmosphere type. For the threestations, this decrease is approximately in the same ratio, and it may be ascribedto an improvement of the protective properties of the copper corrosion productsas it was obtained before for steel [1]. Under the other conditions it does not exista homogeneus behaviour, except for sheltered coastal conditions where thecorrosion increases with time.

The corrosion rate increase with exposure time of the sheltered coastal samplesmay be explained by the fact that in this condition the surface remains wet mostof the time mainly due to the absence of sunshine, and to the presence ofhygroscopic particulates coming from the sea. Thus, real time of wetness shouldbe greater than that calculated by ISO standard, and the occurrence ofcrystallisation 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 highestcopper rates, being about four times higher than in the urban-industrial and ruralatmospheres, and, in some cases, even much more higher than in the latterconditions.

Another detail to be noted in this table is that outdoor corrosion rate in therural station after 6 months, and under sheltered during the entire exposureperiod, is higher than in the urban-industrial atmosphere. Similar results were alsoobtained by Odnevall and Leygraf [49] under sheltered and under outdoorexposures, and by GoÂ mez [50] under outdoor conditions. Odnevall and Leygraf[49] stated that at the expense of posnjakite formation, cuprite formation is muchfaster in the rural than in the urban site, mainly due to the higher relativehumidity, despite lower concentrations of SO2 and NO2 in the rural site. Thecopper surface is always covered with cuprite [31,51±54], since it is generally thecorrosion 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 highercorrosion rate in the rural site during unsheltered and sheltered exposures, whichis observed after short and long exposures, and form evidence that initial


Table

4

Copper

corrosionrates(g/m

2a2

standard

deviation)

Exposure

time(m

onths)

Rural

Coastal

Urban-industrial

Outdoor

Sheltered

Ventilatedshed

Closedspace

Outdoor

Sheltered

Ventilatedshed

Outdoor

Sheltered

627.4

21.6

9.8

20.8

3.5

20.2

0.2

20.04

5.3

20.7

44.5

20.5

19.8

24.5

23.9

21.8

6.8

21.5

12

19.4

20.4

11.5

20.5

1.9

20.3

0.2

20.02

34.6

20.04

54.1

22.6

47.2

23.7

19.8

20.04

8.5

20.4

18

14.3

20.6

8.6

20.7

1.9

20.3

2.0

21.2

30.3

22.0

72.4

23.5

33.1

20.5

15.0

20.5

7.5

20.7


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. GoÂ mez [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 non-

oxidizing 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 urban-

industrial 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.


3.1.2. Relation between the copper corrosion rate and the meteorological parametersand pollutants

Fitting the proposed model for copper corrosion data and environmentalvariables gave the following relationships:

Outdoor

C � 6:13 2 3:44� �0:97 2 0:09�Clÿtrain � �0:29 2

0:08�SO2t5±25 � 0:2420:09 mm=train

R � 0:98; R2 � 0:95; n � 9

Indoor

C � �0:88 2 0:06�Clÿt5±25 � �0:17 2 0:06�SO2t5±25

R � 0:99; R2 � 0:97; n � 9

Outdoor and indoor

C � ��1:03 2 0:06�Clÿ � �0:24 2 0:06�SO2

�t5±25 ÿ �0:46 2

0:05� Clÿtrain � �0:19 2 0:05�mm=train

R � 0:98; R2 � 0:94; n � 18

According to these equations, the increase of the chloride ion concentrationenhances the copper corrosion, both in outdoors and indoors. In outdooratmospheres the time of rainfall also provokes an increase in the corrosion. Ascan also be seen, the increase of the variable represented by the relation betweenthe amount of rainwater and the time of rainfall, which describes the rate ofwashing, promotes higher corrosion. This e�ect is probably due to the fact thatCu corrosion products are more di�cult to be washed away from the surface, andthe 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 oflocalised corrosion at di�erent points of the surface as it generaly occurs inaluminium 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 thelatter, the higher is the humidity the higher is the corrosion rate.

The constant ``a'' includes zero (occur with a negative sign), in the equationobtained for indoor exposure, as well as for outdoor-indoor; that is why it wasnot taken into account for the regression analysis.

The equation obtained by the treatment of outdoor and indoor data together


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 ofthe chloride deposition rate and time of rainfall variables.

3.2. Zinc corrosion results

3.2.1. Kinetics of the atmospheric corrosion of zincThe same as copper, general corrosion is the predominating attack type for

zinc. The exposed Zn surfaces are characterized by the loss of the shinyappearance and the white patina formation on them, which was more noticeableon the samples exposed in the coastal station.

The average corrosion rates for the exposed Zn samples and the standarddeviations are reported in Table 5. As can be seen from this table, the Zncorrosion decreases with time for most of the test conditions, so that the Zncorrosion 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 ratiofor each exposure condition, regardless of the atmosphere type. However, it doesnot happen the same for similar exposure conditions.

It should be noted that Zn corrosion rate after 6 months of exposure in theoutdoor rural site is higher than that in the urban-industrial site.

The sheltered and outdoor coastal samples showed 5±11 times higher corrosionrates compared with those of the rural and urban-industrial test sites. It should beremembered that copper showed a similar behaviour, even under shelteredconditions.

According to Table 5, the di�erence among the corrosion rates obtained in thetest sites is more noticeable during the ®rst 6 months of exposure. However, after12 months the corrosion rate under shelter in the coastal station is about 10 timeshigher than that in the urban-industrial site. It should be realized that thecorrosion rate after 6 and 12 months of exposure under outdoor and shelteredconditions, respectively, in the rural atmosphere is higher than that in the urban-industrial test site. It is well known that the atmospheric corrosion of zinc isstrongly in¯uenced by the presence or absence of moisture [3,14], which could bethe 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 5that outdoor corrosion rate is always higher than the corrosion rates obtained inthe other exposure conditions. Most of the zinc compounds (mainly sulphurcompounds) are soluble in water [24,55,61], so for exposure under unshelteredconditions, 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 metalfrom further attack. This may be a reason for the higher corrosion rates outdoorsthan under sheltered conditions.

On the other hand, Lobnig et al. [27] found that the presence of zinchydroxycarbonate is responsible for the corrosion of zinc, not only at and abovethe critical relative humidity of (NH4)2SO4, but also below it. They state that this


Table

5

Averagezinccorrosionrates(g/m

2a2

standard

deviation)

Exposure

time(m

onths)

Rural

Coastal

Urban-industrial

Outdoor

Sheltered

Ventilatedshed

Closedspace

Outdoor

Sheltered

Ventilatedshed

Outdoor

Sheltered

619.1

21.3

7.6

22.3

6.4

21.4

3.7

20.3

123.3

228

40.3

21.7

15.8

20.2

14.9

21.6

7.9

20.1

12

9.6

20.4

7.4

20.5

3.8

20.3

3.8

20.01

79.1

211

71.8

21.8

11.6

20.9

11.0

21.2

6.7

20.0

18

8.3

20.01

5.5

20.1

3.1

20.2

3.7

20.6

67.0

22.5

62.8

25.4

8.5

20.4

9.8

20.9

6.0

21.1


compound is probably what makes the reaction between zinc and (NH4)2SO4

particles possible at 60±65% relative humidity. These results were in contrast tothe corrosion behaviour of copper [28] and aluminium [29], which react with theparticles only at or above the critical relative humidity (75% at 373 K and 81% at300 K). They conclude that (NH4)2SO4 particles in the atmosphere may beresponsible for the hydroxysulphates found as ®nal corrosion products afterextended ®eld exposures of zinc [27]. However, it has been reported that for zincexposed outdoors, the SO2 incorporation and the precipitation provide thegreatest portion of the incorporated sulphate, with airborne particles being anegligible source [14]. Zinc is highly sensitive to dissolved SO2, which is present inrainwater in moderate concentrations as well as to formaldehyde and formic acid[8].

3.2.2. Relation between the zinc corrosion rate and the meteorological parametersand pollutants

The statistical treatment of the zinc corrosion data in order to ®t the proposedmodel for the in¯uence of environmental data gives rise to the followingrelationships:

Outdoor

C � 12:22 2 2:98� �0:98 2 0:07�Clÿtrain

R � 0:98; R2 � 0:96; n � 9

Indoor

C � ��1:98 2 0:33�Clÿ � �0:13 2 0:04�SO2

�t5±25 ÿ �1:09 2 0:33�Clÿt25±35

R � 0:99; R2 � 0:989; n � 9

Outdoor and indoor

C � 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 � 0:98; R2 � 0:95; n � 18

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


corrosion. For indoor conditions the intercept is not taken into account for thesame reason of copper.

In the simultaneous treatment of indoor and outdoor data, it was obtained thatthe time of rainfall and the chloride deposition rate provoke an increase on thecorrosion rate, di�erent to the results obtained for copper. The latter is due to thefact that the zinc corrosion rate outdoors is higher than obtained under shelteredconditions.

The presence of the time of rainfall in the equation seems to be related to thefact that the meteorological variables predominate (over pollutants), which isexplained by a higher corrosion in the rural than in the urban-industrial stationafter 6 months outdoor and 12 months under sheltered conditions. This was alsoobtained in the copper results.

3.3. Aluminium corrosion results

3.3.1. Kinetics of the atmospheric corrosion of aluminiumThe presence of chloride ions in the atmosphere is the main cause of aluminium

pitting corrosion. Chlorides are capable of breaking the passive ®lm formed onthe surface. This is very noticeable in the coastal station where exists the highestlevel of chlorides compared with the other two test sites. The latter explains thehigher 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, thepits being larger than those formed on the skyward side. This behaviour could bedue to a lower in¯uence of the cleansing e�ect of rain, and it has been reported byother authors [54]. Under a ventilated shed the side of the samples facing the seawas the one with lower pits but of larger size, even more than that of the samplesexposed outdoors. It could be explained by the absence of rain. Washing of thesurface by rain reduces pitting corrosion, since it removes the corrosionstimulators. Pits are therefore always found to be deeper and more denselydistributed on samples exposed to free air but protected from rain than thoseexposed to the open air [63].

The samples exposed outdoors in the urban-industrial and rural atmosphereswere tarnished after 12 and 18 months, respectively.

The average corrosion rates for aluminium exposed under di�erent testconditions are given in Table 6, as well as the standard deviations. According tothis table the samples exposed in the coastal station show the highest corrosionrates; even under ventilated shed the corrosion rate is higher in this station thanthat in the rural test site for all exposure conditions. It is, in general, more thanfour times higher than that in the rural and urban-industrial stations, and inoutdoor atmospheres it can reach values more than 10 times higher with respectto the rural station. It can also be seen that this corrosion rate relation among thethree stations diminishes with time, being in the coastal station more noticeable.The outdoor corrosion rate after 18 months of exposure in the rural and urban-industrial stations is something lower than that obtained after 6 months, and evena little more in the coastal station.


Table

6

Averagealuminium

corrosionrates(g/m

2a2

standard

deviation)

Exposure

time(m

onths)

Rural

Coastal

Urban-industrial

Outdoor

Sheltered

Ventilatedshed

Closedspace

Outdoor

Sheltered

Ventilatedshed

Outdoor

Sheltered

60.3

20.04

1.1

20.14

0.2

20.03

0.4

20.08

4.3

20.2

4.8

20.1

2.2

20.1

0.7

20.05

1.1

20.04

12

0.3

20.02

1.2

20.14

0.3

20.01

0.3

20.10

3.2

20.03

4.0

20.4

1.6

20.3

0.6

20.05

1.7

20.13

18

0.2

20.01

0.8

20.11

0.2

20.00

0.3

20.04

2.2

20.4

3.6

20.2

1.4

20.1

0.4

20.00

1.8

20.1


It is noteworthy that for the three stations the corrosion rate under shelteredconditions is higher than outdoors. This may be due to the e�ect of rain onoutdoor aluminium corrosion, by washing away the atmospheric pollutantsdeposited on the surface. In the rural station the corrosion rate changes the mostfrom indoor to outdoor conditions. However, it should be noted that the otherindoor conditions show lower corrosion rates than those observed outdoors.Under sheltered conditions the deposition of pollutants is higher than in ventilatedshed and closed space. The corrosion rate of sheltered samples can reach valuesabout three times higher than those obtained outdoors, speci®cally in rural andurban-industrial atmospheres. This relation increases with the exposure time in thecoastal and urban-industrial atmospheres. In polluted environments undersheltered conditions dust particles often also play an important role in acceleratingcorrosion since they are not washed away. There the corrosion rate, being low at®rst, may after some time accelerate rapidly, eventually attaining a steady-statevalue signi®cantly higher than that in open air exposure [24]. GoÂ mez [50] alsoobtained higher indoor corrosion rates than outdoors.

It should be noted that only in the urban-industrial station, and under shelteredconditions, the corrosion rate increases with exposure time. Dust may acceleratecorrosion by absorbing moisture and sulphur compounds from the atmosphere,thus for long periods producing an acid medium on the surface and, under suchconditions the protective alumina coating is not stable. In rain sheltered positionsdust and other pollutants may collect and accelerate corrosion by disturbing theformation of a protective oxide coating [24].

3.3.2. Relation between the aluminium corrosion rate and the meteorologicalparameters and pollutants

Fitting the corrosion data for aluminium exposed under outdoor and indoorconditions to the proposed model gave the following relationships

Outdoor

C � 0:46 2 0:16� �0:96 2 0:12�Clÿtrain

R � 0:96; R2 � 0:90; n � 9

Indoor

C � �0:80 2 0:07�Clÿt5±25 � �0:37 2 0:07�SO2t5±25

R � 0:99; R2 � 0:96; n � 9

Outdoor and indoor

C � 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


R � 0:96; R2 � 0:89; n � 18

where R is the coe�cient of multiple correlation; R 2 is the coe�cient of multipledetermination; n is the number of data.

According to the equation obtained for outdoor conditions the variablerepresented by the interaction between the chloride deposition rate and the rainfalltime 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 ®lmbreakdown, while the electrolyte ®lm with the high thickness is formed duringrainfall (phase layers). However, the regression coe�cient a�ecting the interactionis lower than the corresponding to the relation obtained including data outdoorsand indoors. It would mean that the interaction Clÿ train could represent thein¯uence of other variables not monitored or reported.

However, under sheltered conditions the aluminium corrosion is mainlyin¯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 thiscase does not include the intercept either, as in the indoor equations for copperand zinc.

As can be seen from the equation when both data are analysed together, itwould appear that the same variables are obtained as in the previous equations;however, in this case the interaction between the chloride deposition rate and timeof rain has an inverse e�ect to that obtained under outdoor conditions. This maybe related to the fact that the corrosion rate under sheltered conditions is, ingeneral, higher than the outdoor corrosion rate. The variable Clÿ train becomeszero indoors, so that the di�erence between indoors and outdoors is given by thisrelation.

Other variables that entered into this equation are the interaction between thetime of wetness at temperature of 25±358C and the sulphur compounds depositionrate and, between the amount of rain and the time. The higher in¯uencecorresponds to the interaction Clÿt5±25:

The moderate e�ect of the latter may be due to a predominant washing e�ect ofrain 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 samplesare the lowest and the least attacked by general corrosion, followed by zincsamples and ®nally by copper. As is very well known, copper is the most nobleand resistant of the three metals studied. The fact that it shows the highercorrosion rate could be due to some other factors for a relatively short time ofexposure. The following di�erences on this behaviour were detected: in the ruralstation under ventilated shed and closed space, the corrosion rate of zinc is higherthan that of copper in the entire exposure period. The same was obtained in the


coastal station but outdoors, as well as in the urban-industrial and coastal stationsunder sheltered conditions after 6 and 18 months of exposure, respectively. Thecorrosion rate of steel [1] exposed in the rural station under sheltered conditions ishigher than for the non-ferrous metals studied, even when they are exposed underthe most aggressive conditions. While their corrosion rate decreases with time inthe coastal station outdoors, the steel corrosion rate increases to corrodecompletely.

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 inthe industrial one were obtained. The latter could be due to a predominant e�ectof the relative humidity over copper and zinc. A synergistic e�ect between ozoneand SO2 over copper under these conditions has been reported [57].

Higher aluminium corrosion rates under sheltered conditions than thoseoutdoors are observed at all the test sites, while for copper only at the coastalstation. 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 layerand the presence of pollutants capable of increasing the electrolyte ®lm pH. Forzinc exposed outdoors, the corrosion rates are the highest compared with all otherexposure conditions. It could mean that rain and humidity are the most importantfactors 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 ofmoisture [3,14].

Cramer and McDonald [26] noted that the copper corrosion products have asubstantially 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 onthe zinc corrosion products; and wet deposition of hydrogen ions has the greateste�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 noncorroded surfaces, moisture is more readily available on corroded zinc and coppersurfaces from capillary condensation and from the hydrophilic nature of theircorrosion products.

The corrosion rates for the metals tested, in general, tend to decrease as theexposure time increases. Only in a few cases the behaviour was inverse; forexample, the aluminium corrosion in industrial atmosphere under shelteredconditions tend to increase, as well as the corrosion rate of zinc samples in thecoastal station under similar conditions.

In the regression equations obtained for outdoor conditions, the main variablein¯uencing the corrosion of the three metals is that represented by the interactionbetween the chloride deposition rate and the time of rainfall. It means that themain component of time of wetness in¯uencing on corrosion is the time ofrainfall. 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


chloride and SO2 deposition rates and the time of wetness at temperature between58C and 258C is the most signi®cant. The constant ``a'' includes zero in all theequations obtained for indoor exposure. Also, we have the variables Clÿ t5±25 andSO2 t5±25 for zinc and SO2 t25±35 for copper. When outdoor and indoor corrosiondata are analysed together the chloride deposition rate and the time of wetness attemperature between 5 and 258C were found to be among the most signi®cantvariables in¯uencing the corrosion of the three metals investigated; the SO2

deposition rate and the amount of rainwater/time of rainfall of the aluminium andcopper corrosion; the time of rainfall of aluminium and zinc corrosion. Also, thetime of wetness at temperatures between 25 and 358C appears as an importantvariable for aluminium and zinc, but with a negative e�ect on corrosion. Itcon®rms that this latter time represents the evaporation of the electrolytic ®lm. Inthe case of copper the evaporation of the ®lm could be stopped by thecharacteristics of the corrosion products layer.

The sheltered and outdoor coastal zinc samples showed 5±11 times highercorrosion rates compared with those of the rural and urban-industrial test sites. Itshould be remembered that aluminium and copper showed a similar behaviour,even under sheltered conditions.

The linearity in the atmospheric corrosion behaviour of galvanized steel doesnot occur as is commonly reported (galvanized steel behaves as does zinc in theatmosphere) [8]. It apparently requires that solid acidic contaminants be depositedon the metal surface [59]. It is extremely di�cult for a protective ®lm to form ongalvanized steel in a coastal atmosphere, but once formed, the ®lm is able to verye�ectively withstand any detrimental interaction with the environment [59].Atmospheric corrosion rates in a coastal environment accordingly becomeprogressively lower with increasing time of exposure [59].

Table 7 shows the classi®cations of atmospheric corrosivity according to thecorrosion rates obtained for each metal exposed under di�erent test conditions. Ascan be seen from this table, taking into account Table 3, about 59% of the casesdo not agree with the established in ISO 9223 [30]. The corrosion rates are ingeneral 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 thestandard, overestimating or underestimating one class number, but in three casesexceed the upper limit given in this standard. Dean [64] obtained a ful®lment ofthis standard in 60% of the cases examined.

4. Conclusions

1. The results obtained con®rm and allow expansion of the model previouslyproposed for steel to copper, zinc and aluminium. The importance of a morequantitative step in de®ning time of wetness taking into account the in¯uenceof temperature and rain is also con®rmed.

2. The main di�erences obtained in ®tting the model proposed for non-ferrous


Table

7

Classi®cationofatm

ospheric

corrosivity(C

)accordingto

thecorrosionratesforeach

metal

Metal

Rural

Coastal

Urban-industrial

Outdoor

Sheltered

Ventilatedshed

Closedspace

Outdoor

Sheltered

Ventilatedshed

Outdoor

Sheltered

Al

C2a

C3

C2a

C2a

C4a

C4a

C3

C3

C3

Cu

C4a

C3

C2a

C1a

C5

C5a,b

C5a

C4a

C3

Zn

C3

C3

C2a

C2a

C5a,b

C5a,b

C3

C3

C2a

aCorrosionrate

values

are

notin

therangegiven

inISO

9223.

bCorrosionratesexceed

theupper

limitvaluegiven

inISO

9223.


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 rateunder 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.

3. According to the adjusted model the interaction between the chloridedeposition 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.

4. 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 inagreement 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.5. 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.6. 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 expectedfor 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.


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Outdoor and indoor atmospheric corrosion of non-ferrous metals

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