Atmospheric corrosion monitoring and modeling of a low alloy steel under an electrolyte film

1. Atmospheric corrosion monitoring and modeling of a low alloy steel under an electrolyte film in cyclic wet–dry condition Prof. Dr. Junhua Dong Environmental Corrosion Center Institute of Metal Research, Chinese Academy of Sciences Shenyang, China Sept. 26, 2014

2. Introduction The aim of corrosion study is to know: • Thermodynamics: boundary condition • Kinetics: reaction way • Corrosion rate: how fast? Need monitoring www.mti-global.org

3. Middle stage: The deposition of gaseous phase, The chemical changes in the liquid phase, and the dissolution of steel. Final stage: The aggregation, growth, and thickening of the corrosion product. Initial stage: The adsorption and absorption of water Introduction Atmospheric corrosion evolution of Low Alloy Steel (LAS) The initial stage and the middle stage experienced very short time, even within 1 wet/dry cycle. The final stage occupied most of the service time. www.mti-global.org

4. Introduction Atmospheric corrosion occurs under thin electrolyte film with Wet-dry cyclic change Humidity and temperature codetermine the electrolyte film exists or not and the time of wetness. Moreover, temperature increase promotes corrosion rate Atmospheric pollutants such as SO2, Cl-, etc.impacts on the corrosion mechanism. The rust can also make effects on the corrosion process Alloy elements make effects on the corrosion process by improving the rust structure and composition Fe（M）= Fe2+sol）+ 2e（M） 4Fe(OH)2 +O2 + 2H2O = 4Fe(OH)3 8FeOOH + Fe2+ + 2e = 3Fe3O4+ 4H2O • Industrial (SO2): - or - FeOOH • Coastal (Cl-):  -FeOOH • Industrial – coastal (SO2+ Cl-) correlate with each content 1. Weathering steel: Cu, P, Cr, Ni, Mo, Si,  Cu: Fe3O4; Ni: Fe3O4,- FeOOH; Cr: - FeOOH; Mn-Cu: Fe3O4,  -FeOOH Misawa, T., Asami, K., Hashimoto, K., and Shimodaira, S., Corr. Sci., vol. 14, pp. 131-149, 1974. Kamimura, T., Hara, S., Miyuki, H., Yamashita, M., and Uchida, H., Corr. Sci., vol. 48, pp. 2799-2812, 2006.

5. Wet/dry cyclic test • 25C, 60 (RH), 12hrs • Solution: 0.005% - 5% NaCl • Wet/dry cyclic corrosion test • 25C, 60 (RH), 12hrs • Solution: 0.005% - 5% NaCl Introduction Atmospheric corrosion evaluation of Low Alloy Steel (LAS) • Traditional data sampling in exposure site:0.5, 1, 2, 4, 8, 16, , in year • CyclicWet/dry corrosion test in laboratory: wet/dry cycle number

6. No data sampling within a year or 6 months. • monotonic increasing function for corrosion loss but decreasing function for corrosion rate • Data sampling start from the first wet/dry cycle. • increasing function for corrosion loss. • first increase then decrease functions for corrosion rate Introduction www.mti-global.org

7. Atmospheric corrosion monitoring Characteristics of Atmospheric Corrosion • Atmospheric corrosion of steel involves • Electrolyte properties : Air pollutants , e.g. SO2 and Cl- • Cycle of wetting and drying , TOW : Temperature and Humidity • The rust evolution • For Weathering steel , CR initially increases , but when the protective rust is formed , CR decrease. • Therefore, the rust evolution plays the important role to determine the remaining life time of this steel.

8. Atmospheric corrosion monitoring Atmospheric environment classification -Categorized by the nature and pollutions content, the corrosive atmosphere can be divided into Rural, Urban, coastal, industrial, and coastal-industrial atmosphere. Most severe corrosive atmospheres • Industrial atmosphere: In this atmosphere , sulfur dioxide plays the major role in atmospheric corrosion process. • 2. Coastal atmosphere : Chlorides from the sea play an important role. • 3. Coastal – Industrial atmosphere • -The expansion of the industry along the coastline. • -In this atmosphere, the combined effect of sulfur dioxide and chloride plays the decisive role

9. Atmospheric corrosion monitoring Effect of electrolyte film thickness on Atmospheric Corrosion Prof. Tomashov’s model Corrosion Monitoring in the this thin layer provides us with direct information about the transient change in the corrosion behavior. www.mti-global.org

10. Atmospheric corrosion monitoring • In 1990 , Stratmann et al Technique : Polarization technique The too high solution resistance during the evaporation process can cause error. • 1995 , Tsuru et al Technique : Needle probe + Electrochemical Impedance Spectrography (EIS ) Electrode : Chip shaped bi-electrodeThe Sensitivity is 10 um. - 2007 , Dong et al Technique : thickness measurement + EIS Electrode : Chip shaped bi-electrode Almost the end of evaporation process , Electrolyte tends to accumulate along the electrodes. Electrolyte thickness measurement , Dong et al Needle probe techniques Chip shaped bi-electrode

11. Atmospheric corrosion monitoring In 2009 , Dong et al Technique : Electrolyte thickness measurement + EIS Electrode : Comb shaped bi-electrode , aiming to improve accuracy of data. The Sensitivity is 1 um. - All of works are valuable. • These works did not study the rust evolution of steel in different polluted atmospheres and the effect of temperature on the corrosion kinetics of steel. • Besides , all of works were carried out at the low humid conditions. Comb shaped bi-electrode www.mti-global.org

12. Atmospheric corrosion monitoring A. Material : Weathering steel was used as the electrode material B. The simulated typical atmospheric corrosion electrolytes

13. Atmospheric corrosion monitoring • To study the effect of the electrolyte thickness on the atmospheric corrosion behaviors taking place on the substrate of weathering steel under the simulated atmospheres. • To obtain the corrosion kinetics of this weathering steel as a function of the cyclic wet – dry corrosion test (CCT) under the simulated atmosphere. • To study the effect of temperature on the atmospheric corrosion of this weathering steel. www.mti-global.org

14. Atmospheric corrosion monitoring C. Electrode preparation *During EIS measurement, one part with the area of 0.5 cm2 worked as working electrode, and the other part worked as reference electrode and auxiliary electrode as indicated in Fig. d.

15. Atmospheric corrosion monitoring D. Schematic Diagram of the apparatus of the experiment The analytic balance with accuracy d = 0.1 mg. Temperature and humidity were maintained with desired Temperature ± 1 oC and 60 ± 2 % RH The humidity control system contains water-glycerol mixture, the ratio of which was prepared according to ASTM D 5032 www.mti-global.org

16. Electrolyte 1st CCT Drying Water Other CCTs Drying Electrolyte Evaporation process. X Electrolyte Steel Steel Atmospheric corrosion monitoring E. Cyclic wet-dry Corrosion Test ( CCT ) F. The thickness of electrolyte film (X ) : where  and S are the density and surface area of the electrolyte film

17. Atmospheric corrosion monitoring G.Electrochemical Parameter Monitoring the electrical double layer ( edl) solution Metal HF current Cdl LF Rp−1 = ( ZLF - ZHF ) -1 Rrust = ZHF Rsol Rp

18. Atmospheric corrosion monitoring H.Temperature conditions Atmospheric corrosion monitoring study The influence of temperature

19. Trapezoidal rule N 1 b a Atmospheric corrosion monitoring The mathematical approach

20. Atmospheric corrosion monitoring Thickness measurement EIS results Substrate ( 1st CCT) • - Electrolyte decreases linearly. • Concentration-build up during the evaporation process. • Accelerating corrosion process. • Minimum corrosion resistance appears at 13 um, maximum corrosion rate.

21. Atmospheric corrosion monitoring Rusted Steel ( 56 th CCT ) EIS results Thickness measurement • TOW becomes longer due to the formation of the thick rust layers. • The corrosion process is also accelerated. • The frequency characteristics shifts to • the lower frequency side, indicating the • increased capacitance of the substrate.

22. Atmospheric corrosion monitoring - Evolution in the charge transfer resistance • During Evaporation Process, • In all CCTs, CR gradually increases with evaporation of electrolyte . • B. During CCT, • Initial Stage : CR increase • 2. Barrier effect = the corrosion effect , corrosion reaches a max • 3. Second Stage : • Barrier effect > the corrosion • effect , corrosion decreases • 4. Finally , Stationary corrosion • Process.

23. Atmospheric corrosion monitoring - Evolution in the rust resistance • During Evaporation Process, • Rrust gradually decreases with the evaporation of the electrolyte and then abruptly increases. • B. During CCT, • Initial Stage : • The rust layer is non-protective. • As the corrosion process proceeds ,the rust layer becomes compact and adhesive to the steel substrate. • Hence, the improved barrier effect of the rust layer can be attributed to an increase in the CCT number.

24. Atmospheric corrosion monitoring The influence of the rust and temperature on TOW Substrate Substrate Rusted Rusted

25. Atmospheric corrosion monitoring The effect of temperature on substrate

26. Atmospheric corrosion monitoring The effect of temperature on rusted steel

27. Atmospheric corrosion monitoring The effect of temperature on the whole EIS data

28. Atmospheric corrosion monitoring The effect of temperature on the corrosion rate and corrosion mass loss • Corrosion process of steel in all condition showed 2 stages • Transition point ( TP) indicates the formation of the protective rust • 30 C : The earliest formation of the protective rust. • 15 C : Rusting rate of 15 C is too slow to form the protective rust. • Increasing T accelerates the formation of the protective rust and enhance the protectiveness of the rust

29. Atmospheric corrosion monitoring The effect of temperature on the corrosion rate and corrosion mass loss Under the higher temperature (30 and 25 C) Corrosion process is composed of two stages. - Steel shows the higher corrosion mass loss - The later appearance of the transition point • Under the lower temperature (20 and 15 C) • Corrosion process is composed of four stages • - Steel shows the lower corrosion mass loss • Decreasing T can prolong TOW, • leading to • 1.the longer period for corrosion • 2.an increase in oxygen solubility in • an electrolyte layer www.mti-global.org

30. Atmospheric corrosion monitoring The effect of temperature on the corrosion rate and corrosion mass loss • Corrosion process of steel in all condition showed 2 stages • - TP represents the formation of the protective rust. • The higher temperature can lead to the earlier formation of the protective rust. • Increasing T accelerates the formation of the protective rust and enhance the protectiveness of the rust. www.mti-global.org

31. Atmospheric corrosion monitoring The effect of TOW and temperature on corrosion mass loss Under temp and TOW Too low Temp • The higher the exposure temperature is, the earlier the transition point appears. • The lower exposure temperature brings simultaneously with the longer TOW. • In suitable conditions, T and TOW will not act in the opposite way , • But both jointly accelerate the rusting process of steel

32. Atmospheric corrosion monitoring Industrial atmosphere Coastal-Industrial atmosphere Coastal atmosphere

33. Conclusions • This research provided the systematic study of weathering steel • under Simulated coastal atmospheres, industrial atmospheres, Coastal-industrial atmospheres, The role of temperature on corrosion kinetics, and Mathematical approach based on the numerical integration method. • During the evaporation process of the electrolyte • - The peak of phase angle moves to the higher frequency side. • This is due to the increase in Cl concentration • - The peak of phase angle moves to the lower frequency side • because of the increase in sulphate concentration • 2. During the evaporation process of the electrolyte, • The corrosion rate increases due to • - an increase in the concentration of chloride or sulphate, • - an enhancement of oxygen diffusion • which are influenced by the electrolyte thickness reduction. Graphic Goes Here

34. Conclusions 3. During wet-dry corrosion process , In the initial stage : the corrosion rate increases. When the protective effect of the rust layer is in balance with the corrosion effect, the corrosion rate reaches a maximum. Then, as the rust layer grows in thickness and compactness , the corrosion rate decrease. 4.The application of the numerical method is successfully demonstrated in the study. This method would be an alternative to the traditional corrosion assessment and it would be useful for the development of the simulated indoor atmospheric corrosion tests. 5.The results on the influence of the temperature indicated that -The corrosion process of steel for all temperature conditions was composed of more than one corrosion stages. -An increase in temperature accelerated the formation of the protective rust and improved the protectiveness of the rust. -Under the appropriate weathering conditions, TOW and temperature did not act in the opposite way, but both jointly enhanced the rusting process of steel. www.mti-global.og

35. Thank you for your attention!