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The effects of air pollution and climatic factors on atmospheric corrosion of marble under field exposure
Corrosion Science 47 (2005) 1023–1038 www.elsevier.com/locate/corsci
The effects of air pollution and climatic factors on atmospheric corrosion of marble under field exposure Tran Thi Ngoc Lan a,*, Rokuro Nishimura b, Yoshio Tsujino c, Yukihiro Satoh d, Nguyen Thi Phuong Thoa a, Masayuki Yokoi d, Yasuaki Maeda b a
Vietnam National University, 227 Nguyen Van Cu, HoChiMinh, Vietnam Osaka Prefecture University, 1-1 Gakuen-cho, Sakai 599-8531, Osaka, Japan c Osaka Prefectural Environmental Pollution Control Center, 1-3-62 Nakamichi, Higashinari-ku, Osaka 537-0025, Japan Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan b
d
Received 24 February 2004; accepted 11 June 2004 Available online 11 September 2004
Abstract The atmospheric corrosion of marble was evaluated in terms of SO2 concentration as air pollution and climatic factors such as rainfall, relative humidity, temperature and so on under the field exposure. Marble of calcite type (CaCO3) was exposed to outdoor atmospheric environment with and without a rain shelter at four test sites in the southern part of Vietnam for 3month, 1- and 2-year periods from July 2001 to September 2003. The thickness loss of marble was investigated gravimetrically. X-ray diffraction and X-ray fluorescent methods were applied to study corrosion products on marble. The corrosion product of marble was only gypsum (CaSO4 Æ 2H2O) and was washed out by rain under the unsheltered exposure condition. It was found that the most substantial factors influencing the corrosion of marble were rainfall, SO2 concentration in the air and relative humidity. Based on the results obtained, we
*
Corresponding author. E-mail address: [email protected] (T.T.N. Lan).
0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.06.013
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estimated the dose–response functions for the atmospheric corrosion of marble in the southern part of Vietnam. 2004 Elsevier Ltd. All rights reserved. Keywords: Calcareous stones; Passive sampler; X-ray diffraction (XRD); X-ray fluorescence (XRF); Ion chromatograph (IC)
1. Introduction The degradation of calcareous stones constituting historic buildings, monuments, sculptures concerns with not only natural weathering but also anthropological activity [1]. The discharge of air pollutants such as sulphur dioxide into the atmosphere leads to an acceleration of the atmospheric corrosion of materials including marble, which is one of the common materials of cultural assets [2]. Nowadays the preservation of cultural assets is an urgent issue. Therefore, a considerable attention has been paid to the corrosion of marble not only in Europe and America, but also in Asia where the atmospheric acidification becomes more and more serious especially in the developing countries like China, Thailand, Vietnam, etc. [3–8]. However, most of papers have reported the kinetic of marble corrosion under the laboratory exposure conditions that were far different from the field exposure conditions. The southern part of Vietnam has the tropical climate with a monthly average temperature of 298–304 K and an annual rainfall around 1500 mm. The preliminary survey showed that in big cities like HoChiMinh and Hanoi, the sulphur dioxide level in the air was 14–20 ppb [9]. The damage of materials due to atmospheric acidification and natural weathering brings the big losses to cultural heritages made of calcareous stones. However, up to now there has been no research concerning the corrosion behavior of any calcareous stone in Vietnam. This work was conducted to estimate the atmospheric corrosion of marble caused by dry and wet deposition processes under the field exposure condition. The effects of air pollutants and climatic factors on atmospheric corrosion of marble were elucidated in terms of thickness loss, and kinetic equations of marble corrosion under the sheltered and unsheltered exposure conditions were obtained.
2. Experiment 2.1. Four test sites in the southern part of Vietnam and climatic conditions The field exposure was taken out at four cities in the southern part of Vietnam representing different level of air pollution and climatic characteristics: HoChiMinh (HCMC: urban—industrial), Bien Hoa (urban), Vung Tau (urban—marine) and My Tho (rural), where the test sites were located in the field of meteorological stations. HoChiMinh is the biggest industrial and commercial center, hence the most polluted city in Vietnam. Bien Hoa is an urban site located about 30 km
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northeast of HoChiMinh City. My Tho is a rural area located about 70 km southwest of HoChiMinh, while Vung Tau is a marine town about 120 km southeast of HoChiMinh. My Tho and Vung Tau have no heavy industry and therefore they are a clean area. The climatic factors at four test sites such as temperature, relative humidity and number of rainy days were obtained from the meteorological stations. In the southern part of Vietnam, the temperature was relatively stable over the whole year. At four test sites, the average temperature for 1-year exposure was 299.8–301.0 K, while the average temperature for 3-month exposure was in the range of 298.8–302.8 K. A year in the southern part of Vietnam is divided into two seasons: dry (from the end of November to the end of May) and rainy (from the end of May to the end of November) seasons. The number of rainy days in the first year of exposure at four test sites is given in Table 1. Bien Hoa had more rain than other sites. The highest number of rainy days in a year at four test sites was in the period from July to September (the middle of rainy season). The absolutely dry period lasting from January to March had no rain at all. The average relative humidity for the 3-month and 1-year exposure periods was lowest at HCMC (67–78%), and highest at My Tho and Vung Tau (76–85%), as shown in Fig. 1. At four test sites, the
Table 1 The number of rainy days at four test sites: HoChiMinh (HCMC), My Tho, Vung Tau and Bien Hoa Period
Number of rainy days (day)
Jul/01–Sep/01 Oct/01–Dec/01 Jan/02–Mar/02 Apr/02–Jun/02 Jul/01–Jun/02
HCMC
My Tho
Vung Tau
Bien Hoa
45 40 0 36 121
53 29 0 29 111
54 24 0 29 107
68 44 0 40 152
RH (%)
90 85
Jul/01Sep/01
80
Oct/01Dec/01
75
Jan/02Mar/02
70
Apr/02Jun/02
65
Jun/01Jun/02
60 HCMC
My Tho
Vung Tau
Bien Hoa
Fig. 1. The 3-month and 1-year (solid line) average relative humidities at four test sites.
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average relative humidity for 3-month exposure was the lowest in the absolutely dry period, and the highest in period from July to September. From Table 1 and Fig. 1, at any test site, the average relative humidity for the 3-month period relates to the number of rainy days. However, this rule is not applicable to the wide area covering all four test sites. 2.2. Sampling method of SO2 and rain A passive sampling method was used to measure an amount of gaseous SO2 in the air [9]. Rain was collected in a polypropylene vessel with a mounted funnel holding a filter with a pore size of 0.8 lm, and pH of rain and rainfall was measured. Ion chro2 matography (IC) was used to analyze NO 3 and SO4 ions in rain that are considered to cause acid rain. The passive sampler and rain were collected to analyze once a month at four test sites from July/01 to September/03. 2.3. Material and exposure method The specimens used for exposure were freshly cut from white Italian marble consisting of calcite (CaCO3). Before an exposure to the atmosphere, the specimens with a size of 20 mm · 20 mm · 5 mm were washed in Millipore water, dried in a desiccator for 24 h and weighed with a precision of 0.01 mg. The duplicated specimens were exposed to the outdoor atmospheric environments at four test sites without a rain shelter (unsheltered specimens or condition) and with the shelter (sheltered specimens or condition). The exposure angle was 45 facing the south for the unsheltered specimens and 90 for the sheltered ones. The exposure was carried out for each 3month (July/01–September/03), 1-year (July/01–June/03) and 2-year (July/01–June/ 03) periods, where the specimens were denoted as the 3-month, 1- and 2-year sheltered (or unsheltered) specimens, respectively. 2.4. Analysis of the specimens A corrosion product formed on the specimens was identified by X-ray diffraction method using X-ray diffractometer (40 kV and 150 mA, Rigaku Co.) with Cu target. The scan was performed at a speed of 4.000/min, a scan step of 0.020 and a scan range from 5 to 85. The Rigaku RIX 3000 fluorescence analyzer was used to determine an amount of sulphur on the specimens. After being analyzed by XRD and XRF methods, the specimens were immersed and slightly shaken in a 20 ml of the Millipore water for one day to remove the corrosion products. The thickness loss was estimated gravimetrically from a mass loss before and after removing of the corrosion products as follows: T L ðcmÞ ¼ M L ðg=cm2 Þ=q ðg=cm3 Þ
ð1Þ
where TL is the thickness loss in cm and was converted to that in lm, ML the mass loss and q the specific density of marble assigned to 2.71 g/cm3. The thickness loss was estimated as the average one of the duplicated identical specimens.
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2.5. Regression analysis for the sheltered and unsheltered specimens The regression analysis was performed for the thickness losses of the specimens at four test sites after the 3-month, 1- and 2-year exposure under the sheltered and unsheltered exposure conditions. Taking into account a physicochemical character of the corrosion process of calcareous stones [10–12], the chosen independent variables were SO2 concentration (CSO2), total rainfall (TR), average relative humidity (RH), temperature, and pH and concentrations of SO2 4 and NO3 ions in rain. Moreover, the terms made up of products of two variables such as CSO2 · RH, TR · concentrations of rain components and so on were introduced for regression analysis as well.
3. Results 3.1. Environmental characteristics Fig. 2 shows the total rainfall (columns, left axis) of each 3-month exposure period in the first exposure year, and those (solid lines, right axis) of the 1-year (July/ 2001–June/2002) and 2-year (July/2001–June/2003) exposure periods at four test sites. The highest total rainfall of the 3-month exposure periods was observed in the first half of the rainy season, and the absolutely dry period lasting from January to March had no rain at all. Among four test sites, the total rainfall for the 1- and
1000
4000
900
3-months ra infall (mm)
800 3000 700 600
2500
500
2000
400
1500
300 1000 200
1-year and 2-year rainfall (mm)
3500 Jul/01Sep/01 Oct/01Dec/01 (none)Jan/02Mar/02 Apr/02Jun/02 Jul/01Jun/02 Jul/01Jun/03
500
100 0
0 HC MC
My Tho
Vung Tau
Bien Hoa
Fig. 2. Total 3-month (left axis), 1- and 2-year (right axis) rainfalls at four test sites.
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20.0
Oct/01-Dec/01
SO2 (ppb)
18.0
Jan/02-Mar/02
16.0
Apr/02-Jun/02
14.0
Jul/01-Jun/02
12.0
Jul/01-Jun/03
10.0 8.0 6.0 4.0 2.0 0.0 HC MC
My Tho
Vung Tau
Bien Hoa
Fig. 3. Average 3-month, 1- and 2-year SO2 concentrations at four test sites.
2-year exposure periods was the lowest at My Tho, and the highest at Bien Hoa. It appears that the average relative humidity (Fig. 1), the number of rainy days (Table 1) and the total rainfall of the exposure periods are not totally but partly correlated to each other. The average pHs of rain for the 3-month, 1- and 2-year exposure periods at four test sites were in the range of 4.3–6.2 with the main value of 4.8, but most of rain had pH of 4.5–5.5. The SO2 4 ions were the main anion in rain. The NO3 ions were very 2 2 low compared with SO4 ions. The average concentrations of SO4 ions in rain for the 3-month, 1- and 2-year exposure periods were 3.9–37.1 mg/l depending on the site and the exposure period, while those of NO 3 ions were in the range of 0.6–3.4 mg/l. The inversely proportional relationship between the total rainfall and average concentrations of these ions in rains for the 3-month exposure period was found, although this relationship was not true for 1- and 2-year exposure periods. Fig. 3 shows the average SO2 concentrations of the 3-month, 1- and 2-year exposure periods at four test sites. The average SO2 concentrations for all the exposure periods depended significantly on the test site, and increased in order of My Tho Vung Tau < Bien Hoa < HoChiMinh. The average SO2 concentration at HoChiMinh was around 17 ppb, while those at My Tho and Vung Tau were less than 6 ppb. The average SO2 concentration at Bien Hoa was almost half of that at HoChiMinh. The average SO2 concentrations at each site depended on 3-month exposure period, whereas their variation is insignificant in comparison with that among the sites. 3.2. Corrosion products of marble corrosion The XRD analysis was carried out for the sheltered and unsheltered specimens. Fig. 4 shows a representative result of the XRD analysis for the 3-month sheltered
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Fig. 4. XRD pattern of the 3-months sheltered specimen at HoChiMinh.
specimens at HoChiMinh, where the reflections of gypsum (CaSO4 Æ 2H2O) at d = 0.7622 nm, d = 0.4287 nm and d = 0.2880 nm are indicated by the symbols G and the other strong reflections correspond to those of calcite. At four test sites, gypsum was the only corrosion product found on all sheltered specimens of any exposure periods. On the other hand, the unsheltered specimens exposed for 3 months in the absolutely dry period had gypsum, but gypsum was found on only a few unsheltered specimens exposed in rainy periods regardless of the duration of exposure. 3.3. Relationship between XRF intensities of sulphur and XRD intensities The XRF analysis was performed to measure an amount of sulphur deposited on the sheltered and unsheltered specimens. The relationship between the intensity of the highest XRD reflection of gypsum at d = 0.7622 nm and the XRF intensity of sulphur on the sheltered and unsheltered specimens was shown in Fig. 5. It was in good
XRD intensity (cps)
5000 4000 3000 2000
y = 69.407x R2 = 0.9115
1000 0
0
20 40 60 XRF intensity (kcps)
80
Fig. 5. Relationship between intensities of the highest XRD reflections of gypsum at d = 0.7622 nm and XRF intensity of sulphur on the specimens at four test sites.
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Table 2 Intensity of X-ray fluorescence light (kcps) of sulphur on the specimens exposed at HoChiMinh
Season RH (%) Unsheltered exposure Sheltered exposure
Skyward side Groundward side
Jul/01– Sep/01
Oct/01– Dec/01
Jan/02– Mar/02
Apr/02– Jun/02
Jan/03– Mar/03
Jul/01– Jun/02
Rainy 78.4
Rainy–dry 74.1
Dry 67.0
Dry–rainy 71.7
Dry 67.5
Rainy–rainy 72.8
Intensity of X-ray fluorescence light (kcps) 0.2971 1.364 15.323 0.274 0.3332 3.159 12.712 1.715
17.483 15.684
0.574 8.454
11.4635
10.578
57.802
19.30
8.8751
16.263
linear function. Thus, the XRF intensity of sulphur was proportional to XRD intensity of gypsum. Consequently the intensity of XRF light of sulphur can be served as a measure for the amount of gypsum formed on the specimens. Table 2 shows the representative result of the XRF intensities of sulphur on the specimens with and without the shelter for the 3-month and 1-year exposure periods at HoChiMinh. From Table 2, the amount of sulphur deposited on the sheltered specimens was much higher than that on the unsheltered specimens except for the absolutely dry period. For the absolutely dry period, the XRF intensities of sulphur of the unsheltered specimens were higher than those of the other 3-month period, while those of the sheltered specimens showed the reverse result. In addition, for the unsheltered specimens exposed in period having rain, the amount of sulphur on the groundward side of the specimens was higher than that on the skyward side. Moreover, the amount of sulphur depended upon the seasonal characteristic of the exposure period, i.e. rain was at the beginning, or at the end of the exposure. The amount of sulphur on the unsheltered specimens exposed in the period ending in the dry season was higher than that on ones exposed in the period ending in the rainy season. The similar phenomena were observed at the other sites. 3.4. Thickness losses of the sheltered and unsheltered specimens The thickness losses of the sheltered specimens for 3-month, 1- and 2-year exposure periods at four exposure sites are shown in Fig. 6. The thickness losses for all exposure periods increased in the order of My Tho < Vung Tau < Bien Hoa < HoChiMinh. Moreover, the thickness loss at HoChiMinh was much higher than those at the other exposure sites. The site dependence of the thickness loss of the sheltered specimens corresponded to that of the average SO2 concentration in Fig. 3. On the other hand, with regard to the thickness loss of the specimens without the shelter, the corrosion behavior between the groundward side and skyward side was different, especially for the rainy season as shown in Table 2. Therefore, the thickness loss of the unsheltered specimens was obtained as an average value of both sides of the specimens. Fig. 7 shows the thickness losses of the unsheltered specimens for all
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Jul/01-Sep/01
2.50
Oct/01-Dec/01 Jan/02-Mar/02 Apr/02-Jun/02
thickness loss (µm)
2.00
Jul/01-Jun/02 Jul/01-Jun/03 3-month average
1.50
1.00
0.50
0.00 HCMC
My Tho
Vung Tau
Bien Hoa
4.00
16.00
3.50
14.00
3.00
12.00
2.50
10.00
2.00
8.00
1.50
6.00
1.00
4.00
0.50
2.00
0.00
Thickness loss-l and 2-year exposure (µm)
Thickness loss-3-month exposure (µm)
Fig. 6. Thickness losses of the 3-month, 1- and 2-year sheltered specimens at four test sites.
Jul/01Sep/01 Oct/01Dec/01 Jan/02Mar/02 Apr/02Jun/02 Jul/01Jun/02 Jul/01Jun/03
0.00 HCMC
My Tho
Vung Tau Bien Hoa
Fig. 7. Thickness losses of the 3-month (left axis), and 1- and 2-year (right axis) unsheltered specimens at four test sites.
exposure periods at four exposure sites, where the columns (left axis) represent the thickness losses for the 3-month exposure period and the solid lines (right axis) are the thickness losses for the 1- and 2-year exposure periods. The thickness losses for the 1- and 2-year exposure periods increased in the order of My Tho < Vung Tau < Bien Hoa HoChiMinh. With respect to the 3-month exposure period, the thickness losses at four exposure sites became low in the absolutely dry period and high in the rainy periods. In addition, from the comparison between the results
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in Figs. 6 and 7, the thickness loss was much higher for the unsheltered specimens than for the sheltered specimens for all exposure periods and sites. From Fig. 6, we obtained the relationships between the thickness loss and exposure period for the specimens with the shelter at four exposure sites as shown in Fig. 8. The relationships appeared to be an intermediate curve between a straight line and a parabolic curve through the origin of the coordinate and were expressed as T L ¼ At0:5–1:0
ðwith shelterÞ
ð2Þ
where TL is the thickness loss (lm), t the exposure period (year) and A the constant depending upon the exposure site. On the other hand, from Fig. 7, the relationships between the thickness loss and exposure period without the shelter at four exposure sites are shown in Fig. 9. These relationships were considered to be a straight line except the initial exposure period and were expressed as 2.5
HCMC
Thickness loss (µm)
My Tho Vung Tau
2
Bien Hoa
1.5 1 0.5 0 0
0.5
1
1.5
2
Duration of the exposure (year) Fig. 8. Thickness loss versus exposure time for the sheltered specimens at four test sites.
Thickness loss (µm)
15
10
HCMC
5
My Tho Vung Tau Bien Hoa
0 0
0.5
1 1.5 2 Duration of the exposure (year)
2.5
Fig. 9. Thickness loss versus exposure time for the unsheltered specimens at four test sites.
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T L ¼ Bt þ C
ðwithout shelterÞ
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ð3Þ
where B and C are the constant depending upon the exposure site. The linear relation between TL and t means that the corrosion rate is largely determined by supply of corrodants furnished to the surface and the corrosion products have less protective properties [13]. The parabolic relation implies that the corrosion rate is subjected to a diffusion of ions through the corrosion products.
4. Discussion 4.1. Effects of shelter, SO2 concentration and climatic factors From the comparison between Figs. 6 and 7, it was found that the thickness loss was much higher for the unsheltered specimens than for the sheltered ones for any exposure period at any test site. The shelter prevents the specimens to expose to rain and hence the direct effect of rain on the corrosion of the sheltered specimens would be neglected, while the unsheltered specimens are under the direct attack of rain. Therefore, it is reasonable to consider that the thickness losses of the unsheltered specimens are significantly influenced by rain. The effect of rain on the atmospheric corrosion of marble can be explained as described below. The corrosion product of the marble is only gypsum, which is highly soluble in water with a solubility product of 3.73 · 105 at 291 K [14]. Therefore, gypsum formed on the unsheltered specimens could be easily eroded by rain; that is, the washing effect of rain. This was supported that prior to the removal of the corrosion product, all the sheltered specimens as well as the 3-month unsheltered specimens exposed in the absolutely dry period increased in weight, while the other unsheltered specimens decreased in weight, although a part of gypsum remained on some of the unsheltered specimens. The other evidence of the washing effect of rain could be seen in Table 2 showing a significant amount of gypsum found on the sheltered specimens and on the unsheltered specimens exposed in the absolutely dry period compared to a tiny amount of gypsum on the unsheltered specimens exposed in the period ending in the dry season, and a negligible amount of gypsum on the unsheltered specimens exposed in the period ending in the rainy season. The difference in the amount of gypsum remained on skyward and groundward sides of the unsheltered specimens can be explained by a stronger washing effect of rain on the skyward side. On the other hand, the behavior of the thickness loss under the sheltered condition at four test sites would be clarified by the other factors, where the washing effect of rain is neglected. From Figs. 3 and 6, the variation in the thickness loss with shelter at four test sites for any duration of the exposure corresponds to that in the average SO2 concentration, but not the average relative humidity (Fig. 1). This means that the average SO2 concentration is a predominant factor for the marble corrosion. However, in the case of the absolutely dry period at HoChiMinh and Bien Hoa, the SO2 concentration was as high as those of the other 3-month periods (Fig. 3), but thickness loss in Fig. 6 tends to be the lowest compared to the other 3-month
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periods. To explain this, we need to consider the average relative humidity in Fig. 1, since the average relative humidity of the absolutely dry period at four test sites was the lowest in comparison to the other 3-month periods. This suggests that the average relative humidity affects the atmospheric corrosion of marble as well as the average SO2 concentration. It may be reasonable that the SO2 concentration and the average relative humidity would affect the corrosion of the unsheltered specimens, although their effects were not clearly observed because of the strong effect of rain. Furthermore, the thickness losses of the 3-month unsheltered specimens at any test site in Fig. 7 appear to correspond to the total rainfall of the 3-month exposure period in Fig. 2. Thus, the main factors affecting the atmospheric corrosion of marble may be concluded to be the SO2 concentration, relative humidity and rainfall in addition to the washing effect of rain. The temperature in the southern part of Vietnam was almost stable over the whole year. For this reason it was not able to elucidate its effect. Also, pH and the concen2 trations of NO 3 and SO4 ions in rain did not show any correspondence with the thickness loss of the specimens. This was in agreement with the results by Hutchinson and Johnson. They showed that free H+ concentration in acid rain at pH less than 4.5 was a critical factor in limestone degradation, while acid anions played minor roles [15–18]. At four test sites, most of rain had the pH value in the range from 4.5 to 5.5. As a result, pH and concentrations of the rain components were left out of consideration. 4.2. Dose–response functions for atmospheric corrosion of marble The regression analysis was carried out as described in the Section 2.4. The following equations were obtained for the relation between the thickness losses of the specimens and the environmental factors: T L ¼ ð0:00111RH C SO2 Þt0:784 ;
R2 ¼ 0:930 and n ¼ 48 ðwith shelterÞ
T L ¼ ð0:00233RH C SO2 Þt þ 0:00309T R ; n ¼ 48 ðwithout shelterÞ
ð4Þ
R2 ¼ 0:980 and ð5Þ
where TL is the thickness loss (lm) of the specimens, t the duration of the exposure (year), CSO2 the average SO2 concentration in the air (ppb), RH the average relative humidity (%), TR the total rainfall (mm) at the test site for each exposure period, R2 the correlation coefficients for observed versus predicted thickness loss for all test sites and exposure periods (3-months, 1- and 2-year), and n the number of data processed. As the correlation coefficients were large, the estimated equations were considered to be credible. The obtained dose–response functions showed that the substantial environmental factors influencing the corrosion of marble were TR, CSO2 and RH. Temperature and, 2 pH and concentrations of NO 3 and SO4 ions in rain were found to have an insignificant effect on the atmospheric corrosion of marble. Moreover, the dose–response functions showed that the RH and SO2 concentration affected marble corrosion as
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the interacting factor and the washing effect of rain for the unsheltered specimens was reflected in the second term of the right side in Eq. (5). In addition, the estimated equations showed a good correspondence to the equations obtained experimentally, Eqs. (2) and (3). This would confirm that the thickness loss without the shelter is proportional to the exposure period, while the relation between the thickness loss and exposure time with the shelter is the intermediate one between TL / t0.5 (parabolic) and TL / t (linear). 4.3. An implication on the atmospheric corrosion of marble For the formation of gypsum as the corrosion product, SO2 g in the atmosphere needs to be oxidized to SO2 4 in a water layer on the specimens by oxidizers such as O2 and O3 (ozone) [19], SO2 g ) SO2 aq ) SO2 4
ð6Þ
where SO2 aq is the SO2 absorbed in the water layer. Then, the transformation of calcite to gypsum takes place [20], 2 CaCO3 þ SO2 4 þ 2H2 O ) CaSO4 2H2 O þ CO3
ð7Þ
where H2O stands for water in the water layer on the specimens. A part of gypsum thus formed simultaneously dissolves away in the water layer with its successive formation. Specifically, the water layer including the dissolving ions (Ca2+ and SO2 4 ) of gypsum would be easily flowed from the specimens surface by rain, by which the formation of gypsum is enhanced. This is the washing effect of rain which leads to the significant difference in thickness loss of the specimens between with and without the shelter. In this case, the corrosion rate is subjected to the supply of SO2 aq and hence the relationship between thickness loss (corresponding to corrosion rate) and exposure period becomes linear, where the water layer exists always on the surface during raining. In addition, the degree of the washing effect of rain can be considered to depend upon the strength of rain (heavy or weak) and the time during raining. Hence, the total rainfall would be reasonable to be a predominant factor rather than the number of the rainy days to elucidate the degree of the washing effect of rain. The effects of the other factors on the corrosion of marble except the washing effect of rain would be basically the same for the specimens with and without the shelter. From Eqs. (6) and (7), SO2 g and H2O are the important species for the atmospheric corrosion of marble. As the increase in SO2 g concentration leads to that of SO2 4 concentration in the water layer, the corrosion of marble increases with increasing SO2 g concentration. This is supported from the fact that the thickness loss with the shelter at four exposure sites (Fig. 6) is associated to the SO2 concentration (Fig. 3). On the other hand, with regard to H2O, this is complicated, because H2O in this case corresponds to water in the water layer (including water adsorbing on the specimens), whose existing time (corresponding to wetness time) and thickness are associated with the relative humidity, the number of rainy days and rainfall. The
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number of rainy days and rainfall should be connected to the relative humidity as well as the degree of the washing effect of rain, since the relative humidity becomes higher during raining than without raining. It is considered that as the number of rainy days and rainfall increase, the time during raining would become long and hence a high relative humidity would be held. This implies that the existing time of the water layer on the specimens becomes longer and its thickness becomes thicker during raining than those without raining. However, even if the wetness time becomes large, the corrosion of marble would depend upon the SO2 concentration in the air. Therefore, the interrelationship between the relative humidity and SO2 concentration in Eqs. (4) and (5) becomes important for the atmospheric corrosion of marble. Thus, it is concluded that the thickness loss would be predominantly subjected to the combinations between the existing time of the water layer, its thickness and SO2 concentration. In the case of the absolutely dry period with no rain (total rainfall = 0 and the number of the rainy days = 0), the temperatures at four exposure sites are not so different and the average relative humidity (Fig. 1) is not correlated to the thickness losses in Figs. 6 and 7. Therefore, we need to consider the difference in temperature between day and night times; that is, the formation of dew which becomes an important factor for the corrosion of marble with and without the shelter. Assuming that the formation condition of the dew would be almost identical at each exposure site, only the SO2 concentration is different at each exposure site. This is the reason why the thickness loss was detected during the absolutely dry period and would lead to the difference in thickness loss for the absolutely dry period at each exposure site as shown in Figs. 6 and 7.
5. Conclusions (1) The corrosion product on marble was gypsum (CaSO4 Æ 2H2O), which was easily flowed from the specimen surface by rain. (2) Rain strongly affected the corrosion of marble and led to the significant difference in the thickness loss of marble under the sheltered and unsheltered conditions. (3) The atmospheric corrosion of marble with and without the shelter was mainly affected by the three factors: rainfall, SO2 concentration in the air and relative humidity. (4) The linear relationship between the thickness loss and exposure time for the unsheltered specimens was obtained, while the sheltered specimens showed an intermediate relationship between the linear and parabolic ones; T L ¼ At0:5–1:0
ðwith shelterÞ
T L ¼ Bt þ C
ðwithout shelterÞ
where TL is the thickness loss (lm), t the exposure period (year) and, A, B and C are the constants depending upon the exposure site.
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(5) The following dose response functions were obtained as follows; T L ¼ ð0:00111RH C SO2 Þt0:784 ðwith shelterÞ T L ¼ ð0:00233RH C SO2 Þt þ 0:00309T R ðwithout shelterÞ where TL is the thickness loss of marble (lm), TR the total rainfall (mm), t the exposure period, CSO2 the average SO2 concentration (ppb), and RH the average relative humidity (%). The SO2 concentration in the air and relative humidity showed a synergetic effect on corrosion of marble.
Acknowledgements This research is supported by The Japan Society for Promotion of Science. We highly appreciate the help from Osaka City Institute of Public Health and Environmental Science, Osaka Prefecture Environmental Pollution Control Center, Technology Research Institute of Osaka Prefecture, College of Engineering—Osaka Prefecture University, Vietnam National University and Meteorological Center of South Vietnam.
References [1] M. Franzini, International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts 33 (3) (1996) 105A. [2] T. Komeiji, Corrosion Engineering 41 (1992) 161. [3] M. Warashina, M. Tanaka, Y. Tsujino, T. Mizoguchi, S. Hatakeyama, Y. Maeda, Water, Air, and Soil Pollution 130 (2001) 1505. [4] E. Alonso, L. Martı´nez, Building and Environment 38 (2003) 861. [5] N.A. Katsanos, F. De Santis, A. Cordoba, F. Roubani-Kalantzopoulou, D. Pasella, Journal of Hazardous Materials 64 (1) (1999) 21. [6] G. Lamenti, P. Tiano, L. Tomaselli, Journal of Applied Phycology 12 (3/5) (2000) 427. [7] Y. Maeda, J. Morioka, Y. Tsujino, Y. Satoh, X. Zhang, T. Mizoguchi, S. Hatakeyama, Water, Air, and Soil Pollution 130 (2001) 141. [8] M.C. Metallo, A.A. Poli, M. Diana, F. Persia, M.C. Cirillo, The Science of the Total Environment 171 (1) (1995) 163. [9] T.T.N. Lan, R. Nishimura, Y. Tsujino, K. Imamura, M. Warashina, N.T. Hoang, Y. Maeda, Analytical Science 20 (1) (2004) 213. [10] C. Leygraf, T.E. Graedel, Atmospheric Corrosion, Wiley-InterScience, A John Wiley & Sons Inc., 2000, p. 261. [11] T.E. Graedel, Journal of the Electrochemical Society 147 (2000) 1006. [12] T.E. Graedel, Corrosion Science 38 (1996) 2153. [13] p. 103 in Ref. [10]. [14] I.U. Lurie, The Handbook of Analytical Chemistry, Chemistry, Moscow, 1971, p. 95. [15] A.J. Hutchinson, J.B. Jonson, G.E. Thomson, G.C. Wood, P.W. Sage, M.J. Cooke, Corrosion Science 34 (11) (1993) 1881. [16] J.B. Johnson, M. Montgomery, G.E. Thompson, S.J. Haneef, G.C. Wood, Corrosion 34 (3) (1993) 510. [17] S.J. Haneef, J.B. Jonson, M. Jones, G.E. Thomson, G.C. Wood, S.A. Azzaz, Corrosion 34 (3) (1993) 497.
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[18] D. Sikiotis, P. Kirkitsos, International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts 33 (1996) 243A. [19] p. 261 in Ref. [10]. [20] p. 208 in Ref. [10].
The effects of air pollution and climatic factors on atmospheric corrosion of marble under field exposure Tran Thi Ngoc Lan a,*, Rokuro Nishimura b, Yoshio Tsujino c, Yukihiro Satoh d, Nguyen Thi Phuong Thoa a, Masayuki Yokoi d, Yasuaki Maeda b a
Vietnam National University, 227 Nguyen Van Cu, HoChiMinh, Vietnam Osaka Prefecture University, 1-1 Gakuen-cho, Sakai 599-8531, Osaka, Japan c Osaka Prefectural Environmental Pollution Control Center, 1-3-62 Nakamichi, Higashinari-ku, Osaka 537-0025, Japan Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan b
d
Received 24 February 2004; accepted 11 June 2004 Available online 11 September 2004
Abstract The atmospheric corrosion of marble was evaluated in terms of SO2 concentration as air pollution and climatic factors such as rainfall, relative humidity, temperature and so on under the field exposure. Marble of calcite type (CaCO3) was exposed to outdoor atmospheric environment with and without a rain shelter at four test sites in the southern part of Vietnam for 3month, 1- and 2-year periods from July 2001 to September 2003. The thickness loss of marble was investigated gravimetrically. X-ray diffraction and X-ray fluorescent methods were applied to study corrosion products on marble. The corrosion product of marble was only gypsum (CaSO4 Æ 2H2O) and was washed out by rain under the unsheltered exposure condition. It was found that the most substantial factors influencing the corrosion of marble were rainfall, SO2 concentration in the air and relative humidity. Based on the results obtained, we
*
Corresponding author. E-mail address: [email protected] (T.T.N. Lan).
0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.06.013
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estimated the dose–response functions for the atmospheric corrosion of marble in the southern part of Vietnam. 2004 Elsevier Ltd. All rights reserved. Keywords: Calcareous stones; Passive sampler; X-ray diffraction (XRD); X-ray fluorescence (XRF); Ion chromatograph (IC)
1. Introduction The degradation of calcareous stones constituting historic buildings, monuments, sculptures concerns with not only natural weathering but also anthropological activity [1]. The discharge of air pollutants such as sulphur dioxide into the atmosphere leads to an acceleration of the atmospheric corrosion of materials including marble, which is one of the common materials of cultural assets [2]. Nowadays the preservation of cultural assets is an urgent issue. Therefore, a considerable attention has been paid to the corrosion of marble not only in Europe and America, but also in Asia where the atmospheric acidification becomes more and more serious especially in the developing countries like China, Thailand, Vietnam, etc. [3–8]. However, most of papers have reported the kinetic of marble corrosion under the laboratory exposure conditions that were far different from the field exposure conditions. The southern part of Vietnam has the tropical climate with a monthly average temperature of 298–304 K and an annual rainfall around 1500 mm. The preliminary survey showed that in big cities like HoChiMinh and Hanoi, the sulphur dioxide level in the air was 14–20 ppb [9]. The damage of materials due to atmospheric acidification and natural weathering brings the big losses to cultural heritages made of calcareous stones. However, up to now there has been no research concerning the corrosion behavior of any calcareous stone in Vietnam. This work was conducted to estimate the atmospheric corrosion of marble caused by dry and wet deposition processes under the field exposure condition. The effects of air pollutants and climatic factors on atmospheric corrosion of marble were elucidated in terms of thickness loss, and kinetic equations of marble corrosion under the sheltered and unsheltered exposure conditions were obtained.
2. Experiment 2.1. Four test sites in the southern part of Vietnam and climatic conditions The field exposure was taken out at four cities in the southern part of Vietnam representing different level of air pollution and climatic characteristics: HoChiMinh (HCMC: urban—industrial), Bien Hoa (urban), Vung Tau (urban—marine) and My Tho (rural), where the test sites were located in the field of meteorological stations. HoChiMinh is the biggest industrial and commercial center, hence the most polluted city in Vietnam. Bien Hoa is an urban site located about 30 km
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northeast of HoChiMinh City. My Tho is a rural area located about 70 km southwest of HoChiMinh, while Vung Tau is a marine town about 120 km southeast of HoChiMinh. My Tho and Vung Tau have no heavy industry and therefore they are a clean area. The climatic factors at four test sites such as temperature, relative humidity and number of rainy days were obtained from the meteorological stations. In the southern part of Vietnam, the temperature was relatively stable over the whole year. At four test sites, the average temperature for 1-year exposure was 299.8–301.0 K, while the average temperature for 3-month exposure was in the range of 298.8–302.8 K. A year in the southern part of Vietnam is divided into two seasons: dry (from the end of November to the end of May) and rainy (from the end of May to the end of November) seasons. The number of rainy days in the first year of exposure at four test sites is given in Table 1. Bien Hoa had more rain than other sites. The highest number of rainy days in a year at four test sites was in the period from July to September (the middle of rainy season). The absolutely dry period lasting from January to March had no rain at all. The average relative humidity for the 3-month and 1-year exposure periods was lowest at HCMC (67–78%), and highest at My Tho and Vung Tau (76–85%), as shown in Fig. 1. At four test sites, the
Table 1 The number of rainy days at four test sites: HoChiMinh (HCMC), My Tho, Vung Tau and Bien Hoa Period
Number of rainy days (day)
Jul/01–Sep/01 Oct/01–Dec/01 Jan/02–Mar/02 Apr/02–Jun/02 Jul/01–Jun/02
HCMC
My Tho
Vung Tau
Bien Hoa
45 40 0 36 121
53 29 0 29 111
54 24 0 29 107
68 44 0 40 152
RH (%)
90 85
Jul/01Sep/01
80
Oct/01Dec/01
75
Jan/02Mar/02
70
Apr/02Jun/02
65
Jun/01Jun/02
60 HCMC
My Tho
Vung Tau
Bien Hoa
Fig. 1. The 3-month and 1-year (solid line) average relative humidities at four test sites.
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average relative humidity for 3-month exposure was the lowest in the absolutely dry period, and the highest in period from July to September. From Table 1 and Fig. 1, at any test site, the average relative humidity for the 3-month period relates to the number of rainy days. However, this rule is not applicable to the wide area covering all four test sites. 2.2. Sampling method of SO2 and rain A passive sampling method was used to measure an amount of gaseous SO2 in the air [9]. Rain was collected in a polypropylene vessel with a mounted funnel holding a filter with a pore size of 0.8 lm, and pH of rain and rainfall was measured. Ion chro2 matography (IC) was used to analyze NO 3 and SO4 ions in rain that are considered to cause acid rain. The passive sampler and rain were collected to analyze once a month at four test sites from July/01 to September/03. 2.3. Material and exposure method The specimens used for exposure were freshly cut from white Italian marble consisting of calcite (CaCO3). Before an exposure to the atmosphere, the specimens with a size of 20 mm · 20 mm · 5 mm were washed in Millipore water, dried in a desiccator for 24 h and weighed with a precision of 0.01 mg. The duplicated specimens were exposed to the outdoor atmospheric environments at four test sites without a rain shelter (unsheltered specimens or condition) and with the shelter (sheltered specimens or condition). The exposure angle was 45 facing the south for the unsheltered specimens and 90 for the sheltered ones. The exposure was carried out for each 3month (July/01–September/03), 1-year (July/01–June/03) and 2-year (July/01–June/ 03) periods, where the specimens were denoted as the 3-month, 1- and 2-year sheltered (or unsheltered) specimens, respectively. 2.4. Analysis of the specimens A corrosion product formed on the specimens was identified by X-ray diffraction method using X-ray diffractometer (40 kV and 150 mA, Rigaku Co.) with Cu target. The scan was performed at a speed of 4.000/min, a scan step of 0.020 and a scan range from 5 to 85. The Rigaku RIX 3000 fluorescence analyzer was used to determine an amount of sulphur on the specimens. After being analyzed by XRD and XRF methods, the specimens were immersed and slightly shaken in a 20 ml of the Millipore water for one day to remove the corrosion products. The thickness loss was estimated gravimetrically from a mass loss before and after removing of the corrosion products as follows: T L ðcmÞ ¼ M L ðg=cm2 Þ=q ðg=cm3 Þ
ð1Þ
where TL is the thickness loss in cm and was converted to that in lm, ML the mass loss and q the specific density of marble assigned to 2.71 g/cm3. The thickness loss was estimated as the average one of the duplicated identical specimens.
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2.5. Regression analysis for the sheltered and unsheltered specimens The regression analysis was performed for the thickness losses of the specimens at four test sites after the 3-month, 1- and 2-year exposure under the sheltered and unsheltered exposure conditions. Taking into account a physicochemical character of the corrosion process of calcareous stones [10–12], the chosen independent variables were SO2 concentration (CSO2), total rainfall (TR), average relative humidity (RH), temperature, and pH and concentrations of SO2 4 and NO3 ions in rain. Moreover, the terms made up of products of two variables such as CSO2 · RH, TR · concentrations of rain components and so on were introduced for regression analysis as well.
3. Results 3.1. Environmental characteristics Fig. 2 shows the total rainfall (columns, left axis) of each 3-month exposure period in the first exposure year, and those (solid lines, right axis) of the 1-year (July/ 2001–June/2002) and 2-year (July/2001–June/2003) exposure periods at four test sites. The highest total rainfall of the 3-month exposure periods was observed in the first half of the rainy season, and the absolutely dry period lasting from January to March had no rain at all. Among four test sites, the total rainfall for the 1- and
1000
4000
900
3-months ra infall (mm)
800 3000 700 600
2500
500
2000
400
1500
300 1000 200
1-year and 2-year rainfall (mm)
3500 Jul/01Sep/01 Oct/01Dec/01 (none)Jan/02Mar/02 Apr/02Jun/02 Jul/01Jun/02 Jul/01Jun/03
500
100 0
0 HC MC
My Tho
Vung Tau
Bien Hoa
Fig. 2. Total 3-month (left axis), 1- and 2-year (right axis) rainfalls at four test sites.
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20.0
Oct/01-Dec/01
SO2 (ppb)
18.0
Jan/02-Mar/02
16.0
Apr/02-Jun/02
14.0
Jul/01-Jun/02
12.0
Jul/01-Jun/03
10.0 8.0 6.0 4.0 2.0 0.0 HC MC
My Tho
Vung Tau
Bien Hoa
Fig. 3. Average 3-month, 1- and 2-year SO2 concentrations at four test sites.
2-year exposure periods was the lowest at My Tho, and the highest at Bien Hoa. It appears that the average relative humidity (Fig. 1), the number of rainy days (Table 1) and the total rainfall of the exposure periods are not totally but partly correlated to each other. The average pHs of rain for the 3-month, 1- and 2-year exposure periods at four test sites were in the range of 4.3–6.2 with the main value of 4.8, but most of rain had pH of 4.5–5.5. The SO2 4 ions were the main anion in rain. The NO3 ions were very 2 2 low compared with SO4 ions. The average concentrations of SO4 ions in rain for the 3-month, 1- and 2-year exposure periods were 3.9–37.1 mg/l depending on the site and the exposure period, while those of NO 3 ions were in the range of 0.6–3.4 mg/l. The inversely proportional relationship between the total rainfall and average concentrations of these ions in rains for the 3-month exposure period was found, although this relationship was not true for 1- and 2-year exposure periods. Fig. 3 shows the average SO2 concentrations of the 3-month, 1- and 2-year exposure periods at four test sites. The average SO2 concentrations for all the exposure periods depended significantly on the test site, and increased in order of My Tho Vung Tau < Bien Hoa < HoChiMinh. The average SO2 concentration at HoChiMinh was around 17 ppb, while those at My Tho and Vung Tau were less than 6 ppb. The average SO2 concentration at Bien Hoa was almost half of that at HoChiMinh. The average SO2 concentrations at each site depended on 3-month exposure period, whereas their variation is insignificant in comparison with that among the sites. 3.2. Corrosion products of marble corrosion The XRD analysis was carried out for the sheltered and unsheltered specimens. Fig. 4 shows a representative result of the XRD analysis for the 3-month sheltered
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Fig. 4. XRD pattern of the 3-months sheltered specimen at HoChiMinh.
specimens at HoChiMinh, where the reflections of gypsum (CaSO4 Æ 2H2O) at d = 0.7622 nm, d = 0.4287 nm and d = 0.2880 nm are indicated by the symbols G and the other strong reflections correspond to those of calcite. At four test sites, gypsum was the only corrosion product found on all sheltered specimens of any exposure periods. On the other hand, the unsheltered specimens exposed for 3 months in the absolutely dry period had gypsum, but gypsum was found on only a few unsheltered specimens exposed in rainy periods regardless of the duration of exposure. 3.3. Relationship between XRF intensities of sulphur and XRD intensities The XRF analysis was performed to measure an amount of sulphur deposited on the sheltered and unsheltered specimens. The relationship between the intensity of the highest XRD reflection of gypsum at d = 0.7622 nm and the XRF intensity of sulphur on the sheltered and unsheltered specimens was shown in Fig. 5. It was in good
XRD intensity (cps)
5000 4000 3000 2000
y = 69.407x R2 = 0.9115
1000 0
0
20 40 60 XRF intensity (kcps)
80
Fig. 5. Relationship between intensities of the highest XRD reflections of gypsum at d = 0.7622 nm and XRF intensity of sulphur on the specimens at four test sites.
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Table 2 Intensity of X-ray fluorescence light (kcps) of sulphur on the specimens exposed at HoChiMinh
Season RH (%) Unsheltered exposure Sheltered exposure
Skyward side Groundward side
Jul/01– Sep/01
Oct/01– Dec/01
Jan/02– Mar/02
Apr/02– Jun/02
Jan/03– Mar/03
Jul/01– Jun/02
Rainy 78.4
Rainy–dry 74.1
Dry 67.0
Dry–rainy 71.7
Dry 67.5
Rainy–rainy 72.8
Intensity of X-ray fluorescence light (kcps) 0.2971 1.364 15.323 0.274 0.3332 3.159 12.712 1.715
17.483 15.684
0.574 8.454
11.4635
10.578
57.802
19.30
8.8751
16.263
linear function. Thus, the XRF intensity of sulphur was proportional to XRD intensity of gypsum. Consequently the intensity of XRF light of sulphur can be served as a measure for the amount of gypsum formed on the specimens. Table 2 shows the representative result of the XRF intensities of sulphur on the specimens with and without the shelter for the 3-month and 1-year exposure periods at HoChiMinh. From Table 2, the amount of sulphur deposited on the sheltered specimens was much higher than that on the unsheltered specimens except for the absolutely dry period. For the absolutely dry period, the XRF intensities of sulphur of the unsheltered specimens were higher than those of the other 3-month period, while those of the sheltered specimens showed the reverse result. In addition, for the unsheltered specimens exposed in period having rain, the amount of sulphur on the groundward side of the specimens was higher than that on the skyward side. Moreover, the amount of sulphur depended upon the seasonal characteristic of the exposure period, i.e. rain was at the beginning, or at the end of the exposure. The amount of sulphur on the unsheltered specimens exposed in the period ending in the dry season was higher than that on ones exposed in the period ending in the rainy season. The similar phenomena were observed at the other sites. 3.4. Thickness losses of the sheltered and unsheltered specimens The thickness losses of the sheltered specimens for 3-month, 1- and 2-year exposure periods at four exposure sites are shown in Fig. 6. The thickness losses for all exposure periods increased in the order of My Tho < Vung Tau < Bien Hoa < HoChiMinh. Moreover, the thickness loss at HoChiMinh was much higher than those at the other exposure sites. The site dependence of the thickness loss of the sheltered specimens corresponded to that of the average SO2 concentration in Fig. 3. On the other hand, with regard to the thickness loss of the specimens without the shelter, the corrosion behavior between the groundward side and skyward side was different, especially for the rainy season as shown in Table 2. Therefore, the thickness loss of the unsheltered specimens was obtained as an average value of both sides of the specimens. Fig. 7 shows the thickness losses of the unsheltered specimens for all
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1031
Jul/01-Sep/01
2.50
Oct/01-Dec/01 Jan/02-Mar/02 Apr/02-Jun/02
thickness loss (µm)
2.00
Jul/01-Jun/02 Jul/01-Jun/03 3-month average
1.50
1.00
0.50
0.00 HCMC
My Tho
Vung Tau
Bien Hoa
4.00
16.00
3.50
14.00
3.00
12.00
2.50
10.00
2.00
8.00
1.50
6.00
1.00
4.00
0.50
2.00
0.00
Thickness loss-l and 2-year exposure (µm)
Thickness loss-3-month exposure (µm)
Fig. 6. Thickness losses of the 3-month, 1- and 2-year sheltered specimens at four test sites.
Jul/01Sep/01 Oct/01Dec/01 Jan/02Mar/02 Apr/02Jun/02 Jul/01Jun/02 Jul/01Jun/03
0.00 HCMC
My Tho
Vung Tau Bien Hoa
Fig. 7. Thickness losses of the 3-month (left axis), and 1- and 2-year (right axis) unsheltered specimens at four test sites.
exposure periods at four exposure sites, where the columns (left axis) represent the thickness losses for the 3-month exposure period and the solid lines (right axis) are the thickness losses for the 1- and 2-year exposure periods. The thickness losses for the 1- and 2-year exposure periods increased in the order of My Tho < Vung Tau < Bien Hoa HoChiMinh. With respect to the 3-month exposure period, the thickness losses at four exposure sites became low in the absolutely dry period and high in the rainy periods. In addition, from the comparison between the results
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in Figs. 6 and 7, the thickness loss was much higher for the unsheltered specimens than for the sheltered specimens for all exposure periods and sites. From Fig. 6, we obtained the relationships between the thickness loss and exposure period for the specimens with the shelter at four exposure sites as shown in Fig. 8. The relationships appeared to be an intermediate curve between a straight line and a parabolic curve through the origin of the coordinate and were expressed as T L ¼ At0:5–1:0
ðwith shelterÞ
ð2Þ
where TL is the thickness loss (lm), t the exposure period (year) and A the constant depending upon the exposure site. On the other hand, from Fig. 7, the relationships between the thickness loss and exposure period without the shelter at four exposure sites are shown in Fig. 9. These relationships were considered to be a straight line except the initial exposure period and were expressed as 2.5
HCMC
Thickness loss (µm)
My Tho Vung Tau
2
Bien Hoa
1.5 1 0.5 0 0
0.5
1
1.5
2
Duration of the exposure (year) Fig. 8. Thickness loss versus exposure time for the sheltered specimens at four test sites.
Thickness loss (µm)
15
10
HCMC
5
My Tho Vung Tau Bien Hoa
0 0
0.5
1 1.5 2 Duration of the exposure (year)
2.5
Fig. 9. Thickness loss versus exposure time for the unsheltered specimens at four test sites.
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T L ¼ Bt þ C
ðwithout shelterÞ
1033
ð3Þ
where B and C are the constant depending upon the exposure site. The linear relation between TL and t means that the corrosion rate is largely determined by supply of corrodants furnished to the surface and the corrosion products have less protective properties [13]. The parabolic relation implies that the corrosion rate is subjected to a diffusion of ions through the corrosion products.
4. Discussion 4.1. Effects of shelter, SO2 concentration and climatic factors From the comparison between Figs. 6 and 7, it was found that the thickness loss was much higher for the unsheltered specimens than for the sheltered ones for any exposure period at any test site. The shelter prevents the specimens to expose to rain and hence the direct effect of rain on the corrosion of the sheltered specimens would be neglected, while the unsheltered specimens are under the direct attack of rain. Therefore, it is reasonable to consider that the thickness losses of the unsheltered specimens are significantly influenced by rain. The effect of rain on the atmospheric corrosion of marble can be explained as described below. The corrosion product of the marble is only gypsum, which is highly soluble in water with a solubility product of 3.73 · 105 at 291 K [14]. Therefore, gypsum formed on the unsheltered specimens could be easily eroded by rain; that is, the washing effect of rain. This was supported that prior to the removal of the corrosion product, all the sheltered specimens as well as the 3-month unsheltered specimens exposed in the absolutely dry period increased in weight, while the other unsheltered specimens decreased in weight, although a part of gypsum remained on some of the unsheltered specimens. The other evidence of the washing effect of rain could be seen in Table 2 showing a significant amount of gypsum found on the sheltered specimens and on the unsheltered specimens exposed in the absolutely dry period compared to a tiny amount of gypsum on the unsheltered specimens exposed in the period ending in the dry season, and a negligible amount of gypsum on the unsheltered specimens exposed in the period ending in the rainy season. The difference in the amount of gypsum remained on skyward and groundward sides of the unsheltered specimens can be explained by a stronger washing effect of rain on the skyward side. On the other hand, the behavior of the thickness loss under the sheltered condition at four test sites would be clarified by the other factors, where the washing effect of rain is neglected. From Figs. 3 and 6, the variation in the thickness loss with shelter at four test sites for any duration of the exposure corresponds to that in the average SO2 concentration, but not the average relative humidity (Fig. 1). This means that the average SO2 concentration is a predominant factor for the marble corrosion. However, in the case of the absolutely dry period at HoChiMinh and Bien Hoa, the SO2 concentration was as high as those of the other 3-month periods (Fig. 3), but thickness loss in Fig. 6 tends to be the lowest compared to the other 3-month
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periods. To explain this, we need to consider the average relative humidity in Fig. 1, since the average relative humidity of the absolutely dry period at four test sites was the lowest in comparison to the other 3-month periods. This suggests that the average relative humidity affects the atmospheric corrosion of marble as well as the average SO2 concentration. It may be reasonable that the SO2 concentration and the average relative humidity would affect the corrosion of the unsheltered specimens, although their effects were not clearly observed because of the strong effect of rain. Furthermore, the thickness losses of the 3-month unsheltered specimens at any test site in Fig. 7 appear to correspond to the total rainfall of the 3-month exposure period in Fig. 2. Thus, the main factors affecting the atmospheric corrosion of marble may be concluded to be the SO2 concentration, relative humidity and rainfall in addition to the washing effect of rain. The temperature in the southern part of Vietnam was almost stable over the whole year. For this reason it was not able to elucidate its effect. Also, pH and the concen2 trations of NO 3 and SO4 ions in rain did not show any correspondence with the thickness loss of the specimens. This was in agreement with the results by Hutchinson and Johnson. They showed that free H+ concentration in acid rain at pH less than 4.5 was a critical factor in limestone degradation, while acid anions played minor roles [15–18]. At four test sites, most of rain had the pH value in the range from 4.5 to 5.5. As a result, pH and concentrations of the rain components were left out of consideration. 4.2. Dose–response functions for atmospheric corrosion of marble The regression analysis was carried out as described in the Section 2.4. The following equations were obtained for the relation between the thickness losses of the specimens and the environmental factors: T L ¼ ð0:00111RH C SO2 Þt0:784 ;
R2 ¼ 0:930 and n ¼ 48 ðwith shelterÞ
T L ¼ ð0:00233RH C SO2 Þt þ 0:00309T R ; n ¼ 48 ðwithout shelterÞ
ð4Þ
R2 ¼ 0:980 and ð5Þ
where TL is the thickness loss (lm) of the specimens, t the duration of the exposure (year), CSO2 the average SO2 concentration in the air (ppb), RH the average relative humidity (%), TR the total rainfall (mm) at the test site for each exposure period, R2 the correlation coefficients for observed versus predicted thickness loss for all test sites and exposure periods (3-months, 1- and 2-year), and n the number of data processed. As the correlation coefficients were large, the estimated equations were considered to be credible. The obtained dose–response functions showed that the substantial environmental factors influencing the corrosion of marble were TR, CSO2 and RH. Temperature and, 2 pH and concentrations of NO 3 and SO4 ions in rain were found to have an insignificant effect on the atmospheric corrosion of marble. Moreover, the dose–response functions showed that the RH and SO2 concentration affected marble corrosion as
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the interacting factor and the washing effect of rain for the unsheltered specimens was reflected in the second term of the right side in Eq. (5). In addition, the estimated equations showed a good correspondence to the equations obtained experimentally, Eqs. (2) and (3). This would confirm that the thickness loss without the shelter is proportional to the exposure period, while the relation between the thickness loss and exposure time with the shelter is the intermediate one between TL / t0.5 (parabolic) and TL / t (linear). 4.3. An implication on the atmospheric corrosion of marble For the formation of gypsum as the corrosion product, SO2 g in the atmosphere needs to be oxidized to SO2 4 in a water layer on the specimens by oxidizers such as O2 and O3 (ozone) [19], SO2 g ) SO2 aq ) SO2 4
ð6Þ
where SO2 aq is the SO2 absorbed in the water layer. Then, the transformation of calcite to gypsum takes place [20], 2 CaCO3 þ SO2 4 þ 2H2 O ) CaSO4 2H2 O þ CO3
ð7Þ
where H2O stands for water in the water layer on the specimens. A part of gypsum thus formed simultaneously dissolves away in the water layer with its successive formation. Specifically, the water layer including the dissolving ions (Ca2+ and SO2 4 ) of gypsum would be easily flowed from the specimens surface by rain, by which the formation of gypsum is enhanced. This is the washing effect of rain which leads to the significant difference in thickness loss of the specimens between with and without the shelter. In this case, the corrosion rate is subjected to the supply of SO2 aq and hence the relationship between thickness loss (corresponding to corrosion rate) and exposure period becomes linear, where the water layer exists always on the surface during raining. In addition, the degree of the washing effect of rain can be considered to depend upon the strength of rain (heavy or weak) and the time during raining. Hence, the total rainfall would be reasonable to be a predominant factor rather than the number of the rainy days to elucidate the degree of the washing effect of rain. The effects of the other factors on the corrosion of marble except the washing effect of rain would be basically the same for the specimens with and without the shelter. From Eqs. (6) and (7), SO2 g and H2O are the important species for the atmospheric corrosion of marble. As the increase in SO2 g concentration leads to that of SO2 4 concentration in the water layer, the corrosion of marble increases with increasing SO2 g concentration. This is supported from the fact that the thickness loss with the shelter at four exposure sites (Fig. 6) is associated to the SO2 concentration (Fig. 3). On the other hand, with regard to H2O, this is complicated, because H2O in this case corresponds to water in the water layer (including water adsorbing on the specimens), whose existing time (corresponding to wetness time) and thickness are associated with the relative humidity, the number of rainy days and rainfall. The
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number of rainy days and rainfall should be connected to the relative humidity as well as the degree of the washing effect of rain, since the relative humidity becomes higher during raining than without raining. It is considered that as the number of rainy days and rainfall increase, the time during raining would become long and hence a high relative humidity would be held. This implies that the existing time of the water layer on the specimens becomes longer and its thickness becomes thicker during raining than those without raining. However, even if the wetness time becomes large, the corrosion of marble would depend upon the SO2 concentration in the air. Therefore, the interrelationship between the relative humidity and SO2 concentration in Eqs. (4) and (5) becomes important for the atmospheric corrosion of marble. Thus, it is concluded that the thickness loss would be predominantly subjected to the combinations between the existing time of the water layer, its thickness and SO2 concentration. In the case of the absolutely dry period with no rain (total rainfall = 0 and the number of the rainy days = 0), the temperatures at four exposure sites are not so different and the average relative humidity (Fig. 1) is not correlated to the thickness losses in Figs. 6 and 7. Therefore, we need to consider the difference in temperature between day and night times; that is, the formation of dew which becomes an important factor for the corrosion of marble with and without the shelter. Assuming that the formation condition of the dew would be almost identical at each exposure site, only the SO2 concentration is different at each exposure site. This is the reason why the thickness loss was detected during the absolutely dry period and would lead to the difference in thickness loss for the absolutely dry period at each exposure site as shown in Figs. 6 and 7.
5. Conclusions (1) The corrosion product on marble was gypsum (CaSO4 Æ 2H2O), which was easily flowed from the specimen surface by rain. (2) Rain strongly affected the corrosion of marble and led to the significant difference in the thickness loss of marble under the sheltered and unsheltered conditions. (3) The atmospheric corrosion of marble with and without the shelter was mainly affected by the three factors: rainfall, SO2 concentration in the air and relative humidity. (4) The linear relationship between the thickness loss and exposure time for the unsheltered specimens was obtained, while the sheltered specimens showed an intermediate relationship between the linear and parabolic ones; T L ¼ At0:5–1:0
ðwith shelterÞ
T L ¼ Bt þ C
ðwithout shelterÞ
where TL is the thickness loss (lm), t the exposure period (year) and, A, B and C are the constants depending upon the exposure site.
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(5) The following dose response functions were obtained as follows; T L ¼ ð0:00111RH C SO2 Þt0:784 ðwith shelterÞ T L ¼ ð0:00233RH C SO2 Þt þ 0:00309T R ðwithout shelterÞ where TL is the thickness loss of marble (lm), TR the total rainfall (mm), t the exposure period, CSO2 the average SO2 concentration (ppb), and RH the average relative humidity (%). The SO2 concentration in the air and relative humidity showed a synergetic effect on corrosion of marble.
Acknowledgements This research is supported by The Japan Society for Promotion of Science. We highly appreciate the help from Osaka City Institute of Public Health and Environmental Science, Osaka Prefecture Environmental Pollution Control Center, Technology Research Institute of Osaka Prefecture, College of Engineering—Osaka Prefecture University, Vietnam National University and Meteorological Center of South Vietnam.
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