Study on the formation and corrosion protectiveness of rust layer on weathering steel in tropical atmosphere of Vietnam

MINISTRY OF EDUCATION VIETNAM ACADEMY OF AND TRAINING SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY ...*** HOANG LAM HONG STUDY ON THE FORMATION AND CORROSION PROTECTIVENESS OF RUST LAYER ON WEATHERING STEEL IN TROPICAL ATMOSPHERE OF VIETNAM Major: Metallography Code number: 9.44.01.29 ABSTRACT OF THESIS Hanoi – 2019 The Thesis finished at Graduate University of Science and Technology - Vietnam Academy of Science and Technology Supervisors: Assoc. Prof. Dr. Le Thi Hong Lien Dr. Pham Thy San Opponent 1: Assoc. Prof. Dr. Nguyen Xuan Hoan Opponent 2: Assoc. Prof. Dr. Nguyen Van Tu Opponent 3: Prof. Dr. Mai Thanh Tung The Thesis will be defended at the doctoral council which organizes by Graduate University of Science and Technology - Vietnam Academy of Science and Technology at .. h’ date ..2019 The Thesis can find out at: - The Library of Graduate University of Science and Technology - National Library of Vietnam THE LIST OF PUBLISHED PAPERS 1. Le Thi Hong Lien and Hoang Lam Hong, “Characteristics of corrosion product layer formed on weathering steel exposed to the tropical climate of Vietnam”, Material Science and Application, Vol. 4, 7A, 2013, pp. 6-16, USA. 2. Le Thi Hong Lien and Hoang Lam Hong, “Study on atmospheric corrosion of weathering steel in Vietnam”, Proceeding of JSCE Material and environments 2014. 3. Hoang Lam Hong, Le Thi Hong Lien and Pham Thi San, “Atmospheric corrosion of weathering steel in marine environment of Viet Nam”, Tạp chí Khoa học và công nghệ, tập 53-1B (2015). 4. Thy San P., Hong Lien L.T., Lam Hong H., Trung Hieu N., Thanh Nga N.T., “Establish mathematical models to predict corrosion of carbon steel and weathering steel in atmosphere of Viet Nam”, Tạp chí Khoa học và công nghệ, tập 53-1B (2015). 5. Le Thi Hong Lien and Hoang Lam Hong, “Corrosion behavior of weathering steel in atmosphere of Vietnam”, Proceeding of JSCE Material and environments 2015. 6. Le Thi Hong Lien, Hoang Lam Hong, Pham Thi San, “Corrosion behavior of weathering steel in tropical atmosphere of Vietnam”, International Journal of Engineering Research and Science (IJOER), Vol.2, Issue 11 (2016) 7. Le Thi Hong Lien, Hoang Lam Hong, Pham Thi San, Nguyen Trung Hieu and Nguyen Thi Thanh Nga, “Atmospheric corrosion of Carbon steel and Weathering steel – Relation of corrosion and environmental factors”, Proceeding of JSCE Material and environments 2016. PREFACE Weathering Steel (WS) is low alloy steel that has been using as un-painted steel in mild atmosphere due to forming protective layer of corrosion product (rust) on steel substrate, which can act as a protective barrier against the inward transportation of corrosive species. Using the WS on steel structures reduces the material cost, especially reduces maintenance cost of anti-corrosion whichis effectively applied WS to works of transportation, bridges, works of the century. WS has been used popular in temperate zones due to its corrosion resistance. However, the WS types have just stated using in Vietnam. The possibility to apply of the weathering steel in the humid tropical climate with heavy rain of our country has not been fully and systematically researched. So, in order to build the scientific bases of applying WS to construction works, highway bridges, works of the century, ... in Vietnam, and provide the necessary information for selecting and using effectively WS (Corten B) in each climatic region, the author selected the thesis titled: “Study on the formation and corrosion protectiveness of rust layer on weathering steel in tropical atmosphere of Vietnam”  The purpose of thesis:  Research on the influence of climatic and environmental parameters to the formation, structure and properties of corrosion products formed on the WS surface in different climatic regions of Vietnam.  Research on the mechanism of forming protective rust layer on WS and corrosion resistance of WS in tropical climate of Vietnam.  The research object: Corten B steel.  Methods for research:  In order to research the ability of forming protective rust layer on WS in the Vietnam climate, WS was exposed to three different climate zones: (1) Northern (Hanoi) with four seasons, the temperature in the year fluctuates sharply, high humidity, long time of wetness and cold winter; (2) The humid marine atmosphere at Dong Hoi with seasonal temperature changes and high content of airborne sea salt; (3) Dry marine atmosphere at Phan Rang with low rainfall and high temperature in the year. The impacts of climatic and environmental parameters on atmospheric corrosion of WS were studied using the samples exposed at 15 difference climatic zones in Vietnam: Son La, Yen Bai, Tam Dao, Cua Ong, Con Vanh, Hanoi, Dong Hoi, Quang Ngai, Pleiku, Phan Rang, Bien Hoa, Ho Chi Minh City, Can Tho, Rach Gia and Ca Mau. 1  The corrosion rate is determined by mass lost method. The parameters of the environment are collected and determined simultaneously in the same test period.  The physical methods (SEM-EDX, X-ray diffraction, Raman scattering, optical microscopy) are used to study the morphology, structure, chemical composition and phase composition of the rust formed on WS.  The electrochemical methods (polarization curves, electrochemical impedance spectroscopy EIS) were applied to study the corrosion protectiveness able of rust that formed on WS.  The scientific and practical values of the thesis:  This is the first time the study of atmospheric corrosion of WS in Vietnamese tropical climate was implemented systematically. The effect of atmospheric conditions on corrosion dynamics and the formation of protective rust layer on WS is discussed. Some critical values of using unpainted WS in Vietnamese climate are initially mentioned in the thesis.  The research results can be referred as the scientific bases for the application of WS in Vietnam, and provide the necessary information for effective selection and us of WS types in each climatic zone.  The research results will contribute scientific valuable data to the world’s database of WS in humid tropics, especially in Asia.  The composition of thesis: the thesis including 113 pages, 17 tables and 94 figures was separated to 4 chapter:  Chapter 1: The overview of atmospheric corrosion of WS.  Chapter 2: Experience and methods of research  Chapter 3: The characteristic of WS corrosion in the Vietnamese climate.  Chapter 4: The formation and protectiveness of corrosion product formed on WS in climate of Vietnam  Conclusion  List of published papers and references. Chapter 1. THE OVERVIEW OF ATMOSPHERIC CORROSION OF WS  Introduce the history of WS development.  The overview of effect of atmospheric environmental factors (temperature, humidity, rainfall and atmospheric pollutions) and alloying elements to WS corrosion resistance.  Summarize the characteristics of corrosion product (CP) formed on WS (phase composition and structure of rust) 2  The mechanisms of forming and growing corrosion protectiveness rust layer on WS were discussed.  Some critical values of atmospheric environment factors for using un- coated WS in some atmosphere in the world were shown.  Introduce the characteristics of climate of Vietnam.  Summary the study’s results of WS atmospheric corrosion in Vietnam Chapter 2. EXPERIMENTAL AND METHODS OF RESEARCH 2.1. Material Tab. 2.1. The chemical composition of materials, mass % C Mn Si Ni Cr Cu Mo WS 0.111 1.06 0.236 0.1860 0.528 0.3200 0.048 CS 0.135 1.35 0.341 0.0597 0.024 0.0616 0.048 V Ti Al W Co Fe WS < < 0.0158 < 0.05 < 0.005 97.4 CS 0.005< 0.005< 0.0243 < 0.05 < 0.005 97.9 WS of Corten0.005 B type0.005 and CS with chemical compositions shown in tab.2.1 were used for study. The samples with size 100x75x2mm were exposed at 3 test site: Hanoi (HN), Dong Hoi (DH) and Phan Rang (PR). 2.2. Methods of research 2.2.1. Exposed test The samples were exposed to atmosphere following ISO 8565 standard [118]. The corrosion product (rust) was removed according to the standard ISO 8407 [119]. The corrosion rate (CR) were determined by mass loss of tested samples (ISO 9226 [120]). The exposure period at HN, DH and PR was divided into two kinds: long term period (1, 3, 6, 12, 24 and 36 months) and short term period (1, 3, 5, 7 and 14 days). The 12-month exposure were repeated three times at 15 different climatic zones in Vietnam. The environmental factors are collected at the same time of exposure: temperature, humidity, rainfall and atmospheric pollutions. 2.2.2. Study on structure and characteristic of the corrosion product by physical methods  The morphology and chemical conposition of corrosion product (CP) was investigated by Scaning Electron Microscope - SEM.  The chemical compound of CP was detected by X-ray diffraction and Raman spectroscopy. 3  The cross-section structure of CP was investigated using Optical Microscope. 2.2.3. Study on corrosion protective property by electrochemical methods  EIS method  Polarization curves method 2.2.4. Chemical methods for atmospheric impurities analysis Chapter 3. THE CHARACTERISTIC OF WS CORROSION IN THE VIETNAMESE CLIMATE 3.1. The corrosion mass lost of WS During the periods less than 6-12 months, the corrosion mass losses (ML) of WS and CS are similar because the rust is porous and uncover entire of surface. Tab. 3.1: The environmental parameter of the exposed test (annual value, 2010- 2013) Time of wetness TOW - Total Time Cl , SO2, Kind of o RH, 2 2 Site T, C rainfall TOW, of w , % mg/(m mg/(m atm. % d/w (*) mm h/y dryness (**) .day) .day) h/y HN urban 24,2 1606 78,6 4615 4145 0,898 52,7 3,16 1,835 DH marine 24,9 2445 83,1 5705 3055 0,535 65,1 17,55 0,575 PR marine 26,7 1130 76,8 3719 5041 1,35 42,4 8,77 0,87 *d/w: ratio of drying time /TOW **w: ratio of TOW/total exposure Fig. 3.1. ML of WS and CS at the exposed test sites 4 After 6 months (in marine sites) or 12 months (in urban sites), ML of WS becomes lower than that of CS. The difference in ML between the WS and CS at the test sites in Hanoi and Dong Hoi where the climate is humid, - is greater than that in the dry climate of Phan Rang. Fig.3.2. ML and CR of WS and CS at the exposed test sites The ML and CR of WS at the test sites show that the corrosion is most severe at Dong Hoi site, at Hanoi is lower and it is lowest at Phan Rang site. 3.2. Kinetics of WS atmospheric corrosion 300 250 200 200 150 100 100 50 0 0 3 6 9 12 0 Exposed time, month 0 3 6 9 12 Exposed time, month Fig.3.3. The variable of ML of WS by Fig.3.4. The variable of ML of WS by exposure time in urban of HN exposure time in marine of DH 160 120 80 40 0 0 3 6 9 12 Exposed time, month Fig.3.5. The variable of ML of WS by Fig.3.6. The ML of CS by exposure exposure time in marine of PR. time at test sites 5 The variation of ML depending exposure time was built from the experimental and fitted using Excel software, the results show that:  The variation of ML with the exposure time conforms to the power law with coefficient of n < 1, it shows the evidence of the existence and gradual improvement of protective layer of CP on the WS. The n coefficient of equation at Dong Hoi > at Phan Rang > at Ha Noi which shows that the time to form protective CP at Dong Hoi is longer than that of two other sites.  During exposure time < 6-12 months, because either the CP have not covered fully WS substrate, or the structure of CP was porous, t CP doesn’t express protectiveness and the variation of ML with exposure time conform to linear law. When the exposure time >12 months the CP fully covered WS surface and the dense rust layer was formed at very close WS substrate, this restrained corrosion and the ML variation with exposure timeconforms to power law.  A and n coefficients in kinetic equations of WS are lower than that in kinetic equations of CS. It shows that the ML of WS and CS are more and more diffirent over exposure time andthe corrosion protectiveness of WS in atmosphere of Vietnam is better than CS clearly. Tab.3.2. The kinetic equations of WS at test sites Test site Equation A n R2 Hanoi M = 182,31*t0,5363 182,31 0,5363 0,981 Dong Hoi M = 236,67*t0,6172 236,67 0,6172 0,974 Phan Rang M = 142,46*t0,5653 142,46 0,5653 0,982 Tab.3.3. The kinetic equations of CS at test sites (calculated from fig.3.6) Test site Equation A n R2 Hanoi M = 210,02*t0,660 210,02 0,660 0,993 Dong Hoi M = 340,67*t0,711 340,67 0,711 0,960 Phan Rang M = 156,66*t0,693 156,66 0,693 0,978 The ML of WS for 20 year- exposure in atmosphere of Vietnam is calculated from the kinetic equations of test site. According to [31,110], the corrosion process reaches steady state when the first one year CR increases <10%. Accordingly, the process of atmospheric corrosion of WS in Vietnam will reach steady state after 7 years. Following the guidelines of US Steel Association for 6 using un-coated WS (the ML after 20 years is 120 μm [45]), it is possible to use bare WS in Hanoi and Phan Rang atmosphere. Tab.3.4. The ML of WS that is calculated from the kinetic equation for 20 years Hanoi Dong Hoi Phan Rang Time Increment Increment Increment , ML, ML, ML, ML, ML, ML, of ML, % of ML, % of ML, % year g/m2 m g/m2 m g/m2 m (*) (*) (*) 5 432,2 54,99 12,71 639,1 81,31 14,77 353,8 45,02 13,44 6 476,6 60,63 10,27 715,2 90,99 11,91 392,3 49,91 10,86 7 517,6 65,86 8,62 786,6 100,1 9,98 428,0 54,45 9,11 8 556,1 70,75 7,42 854,1 108,7 8,59 461,5 58,72 7,84 9 592,3 75,36 6,52 918,5 116,9 7,54 493,3 62,76 6,88 12 691,1 87,93 4,78 1097 139,6 5,52 580,4 73,85 5,04 13 721,5 91,79 4,39 1152 146,6 5,06 607,3 77,26 4,63 14 750,7 95,51 4,05 1206 153,5 4,68 633,3 80,57 4,28 18 859,0 109,3 3,11 1409 179,3 3,59 729,9 92,87 3,28 19 884,1 112,5 2,94 1457 185,3 3,39 752,6 95,75 3,10 20 909,0 115,6 2,79 1504 191,3 3,22 774,7 98,57 2,94 3.3. The role and influence of environmental parameters on atmospheric corrosion of WS For studying the influence of environmental parameters on atmospheric corrosion of WS, the test samples were exposed for one year which were repeated three times at different time in the year (February, May and November) at 15 test sites (Son La, Yen Bai, Tam Dao, Cua Ong, Con Vanh, Hanoi, Dong Hoi, Quang Ngai, Pleiku, Phan Rang, Bien Hoa, Ho Chi Minh City, Can Tho, Rach Gia and Ca Mau). The seasonal impacts on atmospheric corrosion of WS were studied on samples exposed every month in year at the test sites of Hanoi, Dong Hoi and Phan Rang. 3.3.1. The effect of temperature  T < 200C: The ML of WS increases with the increasing of air temperature because air temperature accelerates corrosion reactions. 7  T > 200C: The ML of WS decreases with the increasing of air temperature because TOW and solubility of oxygen as well as other gases into water thin film on WS surface are decreased when air temperature increases. Fig.3.7. The effect of temperature to the ML of WS. 3.3.2. The effect of wet-dry cycle The ratio of drying time and wetting ness time: d/w = (t-TOW)/TOW, here t is exposure time. - d/w 67%), the surface of WS is too wet.; When the drying time is longer, the evaporation of water only makes oxygen dissolved easily into water film and makes the concentration of electrolyte layer increases, accordingly, the ML increases with d/w. - 0.5 < d/w < 1: the ML decreases when d/w increase at non-marine test sites because the drying time is long enough in order to dry the WS surface. - d/w > 1: the drying time is too long which makes the ML decreased when d/w increases . d/w < 0,5 0,5 < d/w < 1 d/w > 1 Fig.3.8. The relationship between ML and d/w 3.3.3. The effect of rainfall total 8 Fig.3.9. The relationship between ML and rainfall total  At the inland test sites: the ML increases with the increasing of rainfall total either because the thickness of water film increases, or because rain water is kept in the porous rust layer which prolongs the time of wetness on WS surface.  At the coastal test sites: the ML increases with the increasing of rainfall total because the air pollutions (including ion Cl-) are washed away by rain and it makes corrosion reduced. 3.3.4. The effect of airborne salinity (Cl-) The airborne salinity accelerates the ML of WS in the atmospheric regions where the Cl- concentration is higher than 5 mg/m2.day. When Cl-< 5 mg/m2.day it doesn’t expresses the corrosion accelerated role. Fig.3.10. The relationship between ML and airborne salinity 3.3.5. The effect of SO2 deposition rate The ML of WS shows the trend of increasing when SO2 concentration in atmospheric increases, however this relationship is not apparent. 3.3.6. The effect of season 3.3.6.1. Hanoi test site: Hanoi has four seasons: high temperature and rainfall in the summer, low temperature and little sunshine in the winter, therefore, the environment shows the different effects to corrosion of WS depending the exposure duration in the 9 year. The monthly of ML shows that the TOW dominates atmospheric corrosion of WS in Hanoi: the ML increases in Spring when w is highest and reduces in Summer when w is lowest. c) d) Fig.3.12. The relationship between ML and: (c) – ratio ư and (d) – ratio d/w in Hanoi 3.3.6.2. Dong Hoi test site: c) d) Fig.3.14. The relationship between ML and: c) – ratio ư and d) – ratio d/w in Dong Hoi Fig.3.15. The relationship between ML and airborne salinity in Dong Hoi Dong Hoi is effected by two main directions of monsoons: the south-west monsoon blows from inland to the sea (in the end of April to middle of September) and the north-east monsoon blows from sea to inland, thus the climate in Dong Hoi depends on monsoons. The monthly ML shows that the TOW (w and d/w) and airborne salinity dominates atmospheric corrosion of WS in Dong Hoi. 10 Chapter 4. THE FORMATION AND PROTECTIVENESS OF CORROSION PRODUCT FORMED ON WS IN CLIMATE OF VIETNAM The results presented in the chapter 3 showed that after 36 months of testing, the ML of CS and WS are in accordance with the power law with n exponents <1, the CR decreases time by time. This proves that the rust on the steel surface has the effect of protecting the steel substrate, thus it inhibits the corrosion process. Therefore, WS has shown the corrosion resistance is better than that of CS. The durability of rust depends on the structure, properties as well as the process of forming the protective layer on the steel surface. Thus, in Chapter 4, the mechanism of formation and development of corrosion products on WS was discussed in order to elucidate the protective possibility of rust formed on WS in the atmosphere of Vietnam. 4.1. The formation of corrosion product on WS substrate in early stage of exposure In order to study the formation of CP on WS substrate, the samples were exposed at Hanoi, Dong Hoi and Phan Rang for the short periods: 1, 3, 7, and 14 days. The variation of environmental parameters over time are shown on the fig.4.1 and 4.2. Fig.4.1. The variation of temperature and humidity at the