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.
MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF 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 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 Le Thi Hong Lien and Hoang Lam Hong, “Study on atmospheric corrosion of weathering steel in Vietnam”, Proceeding of JSCE Material and environments 2014 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 cơng nghệ, tập 53-1B (2015) 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 công nghệ, tập 53-1B (2015) Le Thi Hong Lien and Hoang Lam Hong, “Corrosion behavior of weathering steel in atmosphere of Vietnam”, Proceeding of JSCE Material and environments 2015 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) 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 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 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 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) The mechanisms of forming and growing corrosion protectiveness rust layer on WS were discussed Some critical values of atmospheric environment factors for using uncoated 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 EXPERIMENTAL AND METHODS OF RESEARCH 2.1 Material Tab 2.1 The chemical composition of materials, mass % WS CS C 0.111 0.135 V Mn 1.06 1.35 Si Ni 0.236 0.1860 0.341 0.0597 Cr 0.528 0.024 Cu 0.3200 0.0616 Mo 0.048 0.048 Ti Al W Co Fe < < WS 0.0158 < 0.05 < 0.005 97.4 < 0.005 < CS 0.005 0.0243 < 0.05 < 0.005 97.9 0.005B type 0.005 WS of Corten and CS with chemical compositions shown in tab.2.1 were used for study The samples with size 100x75x2mm were exposed at 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, 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 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 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, 20102013) Time of wetness TOW Total Cl-, SO2, Time Kind of o RH, Site T, C rainfall TOW, of w , % mg/(m mg/(m2 atm % (*) mm h/y dryness d/w (**) 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 After 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 12 Exposed time, month 0 12 Exposed time, month Fig.3.3 The variable of ML of WS by exposure time in urban of HN Fig.3.4 The variable of ML of WS by exposure time in marine of DH 160 120 80 40 0 12 Exposed time, month Fig.3.5 The variable of ML of WS by exposure time in marine of PR Fig.3.6 The ML of CS by exposure time at test sites 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 Dong Hoi> Phan Rang It is possible that the wetting time in Hanoi (52.7%) and Dong Hoi (65.1%) are longer than those in Phan Rang (42.4%), which accelerates the formation of -FeOOH phase and transformation into intermediate amorphous phases before conversion into αFeOOH However, the d/w (0.5) in Dong Hoi was much smaller than in Hanoi (0.9), so the α-FeOOH phase was not as much supported as in Hanoi In Hanoi and Dong Hoi, α/> 0.3, so the CR decreases when this ratio increases - fig.4.26, however, in Phan Rang, these relationship is no clear It is because the weather in Phan Rang is too dry, the transition rate of lepidocrocite to goethite very slow, thus the amount of goethite is not enough to contribute to the protection of rust Tab.4.4 The relationship of Lepidocrocite, amorphous phases and w, d/w ratio with the exposed time Period Site Hanoi 12 Dong Hoi months Phan Rang Hanoi 24 Dong Hoi months Phan Rang Hanoi 36 Dong Hoi months Phan Rang -FeOOH, % 31,84 32,84 33,61 29,74 36,95 68,68 24,32 30,87 50,44 Amorphous, % 54,53 55,09 55,65 49,16 35,51 11,53 54,71 46,38 30,34 w, % 49,4 64,9 42,8 51,3 66,9 42,9 52,7 65,1 41,4 d/w 1,02 0,5 1,34 0,95 0,5 1,33 0,9 0,5 1,41 Fig.4.25 The α/ ratio of CP formed after years exposed 20 Fig.4.26 The relationship between / ratio with CR of WS 4.2.3 The distribution of Cu and Cr elements in the CP The results of the above analysis showed that the compact internal layer which is rich of stable α-FeOOH phase was formed on the WS surface Meanwhile, under the same test conditions, such layer has not formed on CP of CS This indicate that Cu and Cr alloying elements are needed to promote the formation of protectiveness rust layer The line scan results of rust cross-section showed that: At all test sites, Cu and Cr are concentrated in the rust with distance of about 20 ÷ 40 m (Hanoi and Phan Rang) or 60 m (Dong Hoi) far from WS substrate, this distance is corresponding to the thickness of the internal rust layer, this proves the richness of Cu and Cr elements in the inner layer of CP The concentration of Cr element in the rust layer very closed to the steel substrate demonstrates the ability to form a Cr-FG compound closed to the steel surface; the thickness of Cr-FG rust increases over time Fig.4.31 The distribution of Cu and Cr in the rust formed on WS after months exposure 21 Fig.4.32 The distribution of Cu and Cr in the rust formed on WS after 24 month- exposure 4.2.4 The corrosion protectiveness of rust formed on WS The protective rust layer enriched with α-FeOOH, Cr-FG and Cu and Cr alloying elements was formed on the surface of WS The formation of corrosion protectiveness rust has increased the corrosion resistance of WS, which was shown by the power laws with the exponents n 0.5) Stage – changing wet to dry stage – water evaporates and the thickness of water film reduces: WS is corroded continuosly according to reactions 4.1, 4.2 and 4.3, the transformation of amorphous to -FeOOH occurs simultaneously In the early stage, the -FeOOH phase was formed as soon as w reduces, the ratio d/w 0,5 ÷ promotes this transformation During formating -FeOOH, Cr atom replaces some Fe atoms to form Cr-FG compound very close substrate, the thickness of Cr-FG layer increases over time Stage – drying stage: The corrosion is temporarily stopped, the evaporation of water from water film and CP volume reduces size of rust, the rust layer becomes more compact When a certain thickness is reached, the rust layer can be cracked when it is too thick The development of rust: Stage 4: Repeat the above stages: rust is formed to fill the pores and cracks Stage 5: after years exposure approximately, the corrosion process reaches a steady state corresponding to the CR of about 9.41 m/year (Hanoi), 14.29 m/year (Dong Hoi) and 7.78 m/year (Phan Rang) Stage 6: stable rust is formed after 13 ÷ 19 years approximately in dry atmosphere (d/w > 1) and inland wet atmosphere (d/w = 0.5-1), the CR anual average is m/y Mechanism of formation and development of protective rust layer on WS are shown on fig.4.40 26 Fe(OH)2 (,)FeOOH , Fe2O3 -FeOOH amorphous Fe2+Cu2+ Cr3+ Fe C Cr u Steel base amorphous -FeOOH Cr3 Cu+2 Fe2 + + Fe C Cr Steelu base Stage 1: weting stage – corrosion occurs immediately (approx second/minute/hour) CP: Fe(OH)2, Fe2O3, -FeOOH, -FeOOH, amorphous FeOx(OH)3-2x, Cu2O, CuSO4, Cr2O3 Stage 2: the water on CP layer evaporates (change from wetting to drying stage) - WS is corroded continuosly according to reactions 4.1, 4.2 and 4.3 - Transformation of amorphous to -FeOOH (after 1-7 exposed days) Stage 3: dry stage Steel base The corrosion is stopped, the size of rust reduces; the rust layer can be cracked when it is too thick Fe2+ Cu2+ Fe Cu Cr Steel base Fe2+ Fe Cu2+ Cr3+ Cu Cr Steel base Fe2+ Fe Stage 4: the development of rust Cr3+ Shorter than year: the inner layer is discontinuous, CP layer has cracks; from to years: the inner layer is continuous; new CP formed to fill the pores and cracks; the outer layer is peeled off Stage 5: steady state (longer than years) The rust is stable, less cracks, the corrosion process reaches to steady state Stage 6: stable rust – the CR anual average is m/y Cu2+ Cr3+ Cu Cr Steel base Lepidocrocite -FeOOH Forming after 13 ÷ 19 years approximately in dry atmosphere (d/w > 1) and wet atmosphere inland (d/w = 0.5 - 1) Goethite α-FeOOH and amorphous are rich Cu and Cr crack Fig.4.40 Mechanism of formation and development of protectiveness rust layer on WS in conditions of humid tropical climate in Vietnam CONCLUSION Corrosion kinetics of WS over time follows power law with exponent n