Determination of atmospheric corrosion rate of metals in 12 Vietnamese areas - Pham Duy Nam

Vietnam Journal of Science and Technology 55 (5B) (2017) 272-278 DETERMINATION OF ATMOSPHERIC CORROSION RATE OF METALS IN 12 VIETNAMESE AREAS Pham Duy Nam, Nguyen Hong Du, Ha Huu Son * , Nguyen Hong Phong, Nguyen Van Vinh Institute of Tropical Durability/ Vietnam-Russia Tropical Centre, 63 Nguyen Van Huyen, Cau Giay, Ha Noi, Viet Nam * Email: hahuuson@gmail.com Received: 30 August 2017; Accepted for publication: 9 October 2017 ABSTRACT The corrosion of materials is a result of complex impact from many climatic factors such as temperature, humidity, air pollutant content in the air, rainfall etc. In addition, the corrosion rate of metals can be measured. Each climatic zone is characterized by its corrosion rate. This article presents the testing results to determine the corrosion rate of carbon steel, copper, aluminum and zinc in 12 districts characterizing different climate zones of Vietnam. Testing, evaluation, and classification of atmospheric corrosion were conducted in accordance with the standard ISO 9223. The results show that for all types of studied metals, their corrosion rates which are determined from climatic data are higher than the corrosion rates in reality, especially for carbon steel and aluminum. This difference is more visible in the rural areas. Keywords: atmospheric corrosion; corrosivity categories; carbon steel; metals and alloys. 1. INTRODUCTION Atmospheric corrosion is a complex process. This process is a part of the natural cycle in which metals tend to be moved to the most stable state (oxide or mineral) through chemical and electrochemical processes. In addition, atmospheric corrosions of different metals under different climatic conditions are not the same. In other words, the corrosion rates of different metals in certain climatic zones vary in different ways. The atmospheric corrosion rate depends on many factors such as temperature, relative humidity, rainfall, rainwater pH, concentration of major pollutants and contact time etc. [1]. This rate which can be measured is a representative parameter characterizing for each specific climatic condition. Therefore, determination of metals corrosion rates in atmospheric conditions (also known as "atmospheric corrosion") is necessary to provide an integrated assessment of the impact of environmental factors on material structures in general. Studies, evaluations and classification of atmospheric corrosion have been mentioned and standardized worldwide. The set of ISO standards from ISO 9223 to ISO 9226 presents methods for monitoring climatic factors. It also provides assessment and classification methods of Determination of atmospheric corrosion rate of metals in 12 Vietnamese areas 273 atmospheric corrosion in certain climatic regions [2-5]. The evaluation and classification results of atmospheric corrosion are an important scientific basis while making decisions in order to prevent corrosion caused by climatic factors. More importantly, these results have been also used in designing and selecting structural materials for buildings and in establishing technical norms to protect materials from corrosion. This paper presents the results of corrosion testing of carbon, copper, aluminum and zinc alloys in 12 regions representing different climatic regions of Vietnam. 2. EXPERIMENTAL 2.1. Sample and exposure method Four standard metals used for corrosion rate determination are low carbon steel, copper, aluminum and zinc. They were prepared in type of sample panels which have dimensions of 100 mm × 150 mm × 3 mm. The chemical composition of the metal is given as follows: - Low carbon steel: Fe 99,238; C 0,084; Mn 0,5214; Si 0,0239; and other - Copper: Cu 99,904; Zn < 0,003; Pb 0,0044; Fe < 0,008; P 0,0047; Sn 0,0015; and other. - Aluminum: Al 99,74; Si 0,14; Fe 0,12; Cu < 0,05; Mn < 0,05; Mg < 0,05; Zn < 0,05; Ti < 0,03; and other. - Zinc: Zn 98,75 %; Cd 0,09; Pb 0,08; Cu 0,11; Sn < 0,04, and other. First, the samples were abraded, polished by sandpaper, cleaned by acetone to remove oil. They then were dried in desiccators and weigh by electronic balance with equipment error 10 -4 g. Metal specimens then were put on the exposure rack and fixed at a 45 0 angle southerly direction. 2.2. Methods of Cl - and SO2 deposition determination Pollutants Cl - and SO2 in the atmosphere are collected according to ISO 9225:2012. As follows: - Alkaline surfaces used to collect SO2 with dimensions of 100 mm × 150 mm × 0,3 mm are held vertically on exposure rack and parallel to main wind direction in monitoring areas. After one month, the samples are collected to analyze chemical composition. Deposition rate of sulfur dioxide is expressed as [mg/(m 2 ·day)]. - The salt content in the atmosphere was determined by the Wet Candle method. The Wet Candle was exposed towards main wind direction or main exhaust source (e.g. towards the sea). After one month, solution in the bottle was analyzed and chloride deposition content was determined. The deposition rate of chloride is expressed as [mg/ (m 2 ·day)]. 3. RESULTS AND DISCUSSION 3.1. Results of monitoring environmental data The environmental data achieved in 12 regions is presented in Table 1. Pham Duy Nam, Nguyen Hong Du, Ha Huu Son, Nguyen Hong Phong, Nguyen Van Vinh 274 Table 1. Environmental data in 12 regions. Region Avg. Temper ature ( o C)* Avg. Relative Humidit y(%RH) * Time of Wetness Deposition rate of Cl - Deposition rate of SO2 Time (h) Classif ication Result mg/(m 2 .day) Classifi cation Result mg/(m 2 .day) Classifi cation Yen Bai City 23.3 87.8 5637 T5 17.40 S1 6.23 P1 Chuong My/Hanoi 24.3 80.9 5012 T4 22.28 S1 10.78 P1 Kien Xuong/Thai Binh 23.9 85.9 5996 T5 43.06 S1 6.41 P1 Island Bach Long Vy 23.5 86.0 6017 T5 62.14 S2 8.65 P1 Tho Xuan/Thanh Hoa 25.7 82.0 5331 T4 17.01 S1 7.02 P1 Son Tra island/Danang 27.0 79.9 5051 T4 60.15 S2 18.06 P1 Phu Cat/Binh Dinh 26.2 80.7 4395 T4 34.1 S1 5.45 P1 Hon Tre Island/ 25.0 82.5 - - 60.5 S2 5.58 P1 Pleiku/Gia Lai 23.0 79.3 5261 T4 15.53 S1 7.56 P1 Duc Trong/Lam Dong 24.0 78.5 5137 T4 13.11 S1 2.98 P0 Bien Hoa/Dong Nai 27.3 80.1 3975 T4 31.94 S1 7.99 P1 Ho Chi Minh City 27.78 79.5 3884 T4 24.29 S1 19.43 P1 Notice: The yearly average temperature and relative humidity were taken from Vietnam Center of Hydro-Meteorological Data. - About the level of chloride pollution in the air: On islands, peninsulas and in the areas close to the sea such as Bach Long Vy island, Hon Tre island, Son Tra peninsula, Kien Xuong/Thai Binh, the rates of chloride deposition are relatively high. The highest average rates corresponding to S2 level were found in the three tested location: Son Tra island (60.15 mg/m 2 .day), Bach Long Vy island (62.14 mg/m 2 .day) and Hon Tre island (60.50 mg/m 2 .day). These tested areas directly affected by marine climate, in which the wind brings a lot of chlorides contained in the air to islands and regions near the sea. So that the amounts of chloride deposition differ from month to month when the wind blows in different directions. In October, November and December, Bach Long Vi island saw the highest pinnacle of chloride deposition (from 74.17 mg/m 2 .day to 77.00 mg/m 2 .day in average). In the monitoring area at Kien Xuong/Thai Binh which is located about 5 km from the sea, value of chloride deposition is relatively high reaching an average of 43.06 mg/m 2 .day. The data of chloride deposition in these areas follows the general rule of salt diffusion in the air and is consistent with the data in the published articles [6, 7]. The chloride deposition rates in other areas such as Yen Bai, Tho Xuan, Chuong My, Ho Chi Minh City, Pleiku, Binh Dinh, Bien Hoa are generally low, they are corresponded to S1 level. - About the level of SO2 pollution in the atmosphere: Over a one-year testing period, the atmosphere in most monitored areas was not significantly affected by SO2 pollution, except Ho Chi Minh City. According to the classification in ISO 9223:1992, these regions basically meet the standards of clean atmosphere, ie Po level (countryside atmosphere). However, according to new version ISO 9223:2012 which more rigorously defines the level of SO2 pollution in the atmosphere, the monitoring areas are in the P1 level. Only Duc Trong/Lam Dong can be considered as Po level. Determination of atmospheric corrosion rate of metals in 12 Vietnamese areas 275 - About the time of wetness (TOW): The highest average time of wetness of 6764 hours was found in Yen Bai, the lowest was 3884 hours in Ho Chi Minh City. So the TOW in most areas is more than 50 % total hours of the year, especially in the Northern Vietnam. Additionally, in areas from Da Nang to the South, TOW gradually decreased to lower 50 %. However, TOW in Vietnam that mostly corresponds to level of T4 and T5 is much higher than other countries in the world. This is a very favorable condition for atmospheric corrosion to occur at high speeds, especially in atmosphere zone polluted by salt, in urban or industrial areas. 3.2. Results of corrosion rates determination of standard metal samples Four standard metals (carbon steel, copper, aluminum and zinc) were exposed in 12 monitoring areas in order to determine the corrosion rates. After one year exposing, the corrosion products were removed from the samples and the corrosion rates were conducted according to [4]. Results are shown in Table 2 below. Table 2. Corrosion rates of standard metals after one year exposing. Region Corrosion rate of standard metals rcorr Fe rcorr Zn rcorr Cu rcorr Al μm/year C μm/year C μm/year C g/m2. year C Yen Bai City 20.79 C2 1.20 C3 0.74 C3 0.7 C3 Chuong My/Hanoi 33.31 C3 1.97 C3 0.81 C3 2.1 C4 Kien Xuong/Thai Binh 39.36 C3 2.35 C4 1.18 C3 0.6 C2 Island Bach Long Vy/ Ha Long 753.04 CX 3.18 C4 5.29 C5 7.4 C5 Tho Xuan/Thanh Hoa 22.20 C2 1.27 C3 1.06 C3 3.9 C4 Son Tra island/ Danang 83.35 C5 2.54 C4 2.28 C4 4.5 C4 Phu Cat/Binh Dinh 28.80 C3 3.54 C4 1.51 C4 2.5 C4 Hon Tre Island/ 51.02 C4 4.62 C5 4.00 C5 0.77 C3 Pleiku/Gia Lai 12.00 C2 0.92 C3 1.24 C3 0.8 C3 Duc Trong/Lam Dong 9.64 C2 1.90 C3 1.33 C4 0.3 C1 Bien Hoa/Dong Nai 25.88 C3 2.03 C3 1.82 C4 1.7 C3 Ho Chi Minh City 26.13 C3 1.62 C3 1.85 C4 0.1 C1 First of all, this is the most reliable data for evaluating atmospheric corrosion in these areas. Because it reflects actual corrosion on these metals and has even shown the effect of thin protective layer formed by oxides on surfaces of the metal plates. The testing results show that continental regions of Viet Nam saw moderate or high corrosion levels in all four standard metals. Rural climatic areas that have not been affected by industrialization and urbanization such as Duc Trong/Lam Dong have lower corrosion levels (C2). In contrast, the tropical marine climatic areas such as Bach Long Vy Island saw an extremely high corrosion level (CX) for carbon steel and very high levels (C5) for the remaining metals. In other coastal locations such as Hon Tre island, Son Tra peninsula the atmospheric corrosion rates are a little lower than Bach Long island. However, they remain high compared to continental regions. These results demonstrate an extreme severity of tropical climate on the islands of Viet Nam. Pham Duy Nam, Nguyen Hong Du, Ha Huu Son, Nguyen Hong Phong, Nguyen Van Vinh 276 3.3. Classification of atmospheric corrosion rates according to environmental data According to ISO 9223, atmospheric corrosion rate is also calculated through environmental factors such as temperature, humidity, level of SO2 and Cl - pollution (P and S) and time of wetness ( ). Corrosion rates of the four standard metals measured according to the environmental data are given by the following formulas [5]: + For carbon steel: 0,52 0,62 or 1,77 exp(0,020 ) 0,102 exp(0,033 0,040 )c r d St dr P RH f S RH T 0,150 ( 10)Stf T when 10T o C; in other cases 0,054 ( 10)Stf T + For zinc: 0,44 0,57 or 0,0129 exp(0,046 ) 0,0175 exp(0,008 0,085 )c r d Zn dr P RH f S RH T 0,038 ( 10)Znf T when 10T o C; in other cases 0,071 ( 10)Znf T + For copper: 0,26 0,27 or 0,0053 exp(0,059 ) 0,01025 exp(0,036 0,049 )c r d Cu dr P RH f S RH T 0,126 ( 10)Cuf T when 10T o C; in other cases 0,080 ( 10)Cuf T + For aluminum: 0,73 0,60 or 0,0042 exp(0,025 ) 0,0018 exp(0,020 0,094 )c r d Al dr P RH f S RH T 0,009 ( 10)Alf T when 10T o C; in other cases 0,043 ( 10)Alf T where: rcorr: atmospheric corrosion rate after one year exposing, measured in μm/year; T: Average temperature per year, measured in o C; Table 3. Atmospheric corrosion rates of standard metals according to environmental data measured in one year. Region Atmospheric corrosion rates rcorr Fe rcorr Zn rcorr Cu rcorr Al μm/year C μm/year C μm/year C g/m2. year C Yen Bai City 40.53 C3 1.94 C3 2.16 C4 1.61 C2 Chuong My/Hanoi 40.86 C3 2.10 C3 1.81 C4 1.81 C2 Kien Xuong/Thai Binh 58.79 C4 2.83 C4 2.46 C4 2.66 C3 Island Bach Long Vy 72.35 C4 3.37 C4 2.69 C4 3.21 C3 Tho Xuan/Thanh Hoa 35.50 C3 1.94 C3 1.80 C4 1.72 C2 Son Tra island/ Danang 68.50 C4 3.92 C4 2.38 C4 3.86 C3 Phu Cat/Binh Dinh 46.16 C3 2.67 C4 2.02 C4 2.53 C3 Hon Tre Island/ 63.72 C3 3.36 C4 2.38 C5 3.27 C3 Pleiku/Gia Lai 31.46 C2 1.59 C3 1.49 C2 1.27 C2 Duc Trong/Lam Dong 24.57 C2 1.38 C3 1.36 C3 1.14 C2 Bien Hoa/Dong Nai 46.77 C3 2.81 C4 2.04 C4 2.69 C3 Ho Chi Minh City 46.41 C3 2.68 C4 1.96 C4 2.54 C3 Determination of atmospheric corrosion rate of metals in 12 Vietnamese areas 277 RH - Average relative humidity per year, measured in %; Pd - Average SO2 deposition rate per year, measured in mg/m 2 .d; Sd - Average Cl - deposition rate per year, measured in mg/(m 2 .d). Atmospheric corrosion rates according to environmental data measured in one year have been calculated and classified as shown in Table 3. Looking at the table above, it can be seen that coastal areas, especially Bach Long Vy island saw very high corrosion rates (C4, C5). However, the corrosion rates of metals calculated according to environmental data tends to be lower than these rates of metals naturally exposed after one year. The reason may be that when the salt steam settles on the metal surface, it strongly combined with moisture, so that a layer of electrolysis solution is created and it accelerates the corrosion process of metal. Condensation on the metal surface occurs even when the relative humidity is lower than 80 %. In other words, the actual time of wetness will be greater than the TOW calculated according to ISO 9223: 2012. Consequently, the corrosion rates of metals measured by environmental data are lower than the actual rates (Table 4). Table 4. Comparison of atmospheric corrosion rates of metals calculated by two methods. Method Carbon steel μm/year Zinc μm/year Copper μm/year Aluminum g/m 2 . year Bach Long Vy island Actual 753.04 CX 3.18 C4 5.29 C5 7.40 C5 Theoretical 72.35 C4 3.37 C4 2.69 C4 3.21 C3 Son Tra /Da Nang Actual 83.35 C5 2.54 C4 2.28 C4 4.50 C4 Theoretical 68.50 C4 3.92 C4 2.38 C4 3.86 C3 Hon Tre island/Nha Trang Actual 51.02 C4 4.62 C5 4.00 C5 0.77 C3 Theoretical 63.72 C3 3.36 C4 2.38 C5 3.27 C3 An opposite figure was seen in relatively clean atmospheric zones with low levels of SO2 and Cl - pollution (rural areas). The actual corrosion rates of carbon steel and aluminum are lower than the corrosion rates calculated based on the environmental data (Table 5). Table 5. Comparison of atmospheric corrosion rates of metals calculated by two methods in non-polluted atmospheric zones. Method Carbon steel μm/year Zinc μm/year Copper μm/year Aluminum g/m 2 . year Yen Bai Actual 20.79 C2 1.20 C3 0.74 C3 0.70 C3 Theoretical 40.53 C3 1.91 C3 2.16 C4 1.61 C2 Pleiku/Gia Lai Actual 12.00 C2 0.92 C3 1.24 C3 0.80 C3 Theoretical 31.46 C2 1.59 C3 1.49 C2 1.27 C2 DucTrong/LamDong Actual 9.64 C2 1.90 C3 1.33 C4 0.30 C1 Theoretical 24.57 C2 1.38 C3 1.36 C3 1.14 C2 The first reason is probably that the calculation method of corrosion rate based on environmental data does not take into its account the protection ability of corrosion products formed on metal surfaces. The thicker layer of corrosion products is, the less corrosion agents can penetrate into the metal through this layer. Therefore, the theoretical corrosion rate is higher than actual. Copper and zinc had thinner corrosion layers than steel so that they can be more Pham Duy Nam, Nguyen Hong Du, Ha Huu Son, Nguyen Hong Phong, Nguyen Van Vinh 278 impacted by corrosion agents than steel. For aluminum, the oxide formed on the sample surface is quite stable and it protects aluminum metal inside very well. The second reason might be due to a weaker interaction between Cl - and SO2 with humidity, so the atmospheric corrosion rates in reality was reduced. On the other hand, the less active corrosion agents affect on metal, the more passive corrosion agents appear on metal surface, consequently, the protection of metals increases when they exposed in nature. In other climatic zones, testing results show similarity in corrosion rates determined by both methods. The results also demonstrate that, standard ISO 9223 permits a reliable assessment and classification of atmospheric corrosion rates based on environmental data only in areas with normal atmospheric conditions. In extreme atmospheric regions (clean or polluted), this classification has great errors. 4. CONCLUSIONS Most areas in the mainland of Vietnam has moderate or high corrosion rates for all four standard metals. In marine and coastal atmospheric regions, levels of corrosion are higher. It is particularly high on offshore islands. For instance, a CX corrosion level (extreme level) was found on Bach Long Vy island for steel samples. In areas with strong corrosion agents (atmospheric zones polluted by Cl- and SO2) as well as in relatively clean atmospheric zones, corrosion rates of metals estimated according to environmental data had some noticeable errors compared to actual corrosion rates. And these rates vary in two different way. For instance, in areas with strong corrosion factors, the calculated corrosion levels are less than actual ones while the clean areas saw an opposition figure. REFERENCES 1. Tidblad J., Mikhailov A. A., Kucera V. - Model for prediction of time of wetness on the basis of annual average data on relative humidity and temperature, Zashchita Metallov 36 (2000) 533-540. 2. ISO 9223:2012, Corrosion of metals and alloys, Corrosivity of atmospheres. Classification. 3. 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