Nitric Acid Immersion Corrosion of Shotcrete Lining: Diffusion Law of Hydrogen Ions (H+) and Nitrite Ions (NO3-), and the Corrosion Mechanism
WANG Jiabin1, NIU Ditao2,3
1 School of Civil & Architecture Engineering, Xi’an Technological University, Xi’an 710021 2 College of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055 3 National Key Laboratory of Green Building in West China, Xi’an University of Architecture and Technology, Xi’an 710055
Abstract: The commonly used single-layer shotcrete linings in long highway tunnels under general atmospheric environment usually suffer nitric acid corrosion, and as consequences, the durability and service lifetime are prone to deterioration. This paper presents the experimental study and the mechanism analysis over the diffusion of hydrogen ions (H+) and nitrate ions (NO3-) in the pore solution of shotcrete lining experiencing immersion corrosion in nitric acid. The nitric acid (with pH values of 1, 2 and 3) immersion tests upon a series of specimens of ordinary shotcrete, steel-fiber-reinforced shotcrete and cast concrete were conducted. The changes in physical and mechanical properties of the specimens during the entire 180 d immersion process were measured, and the H+ and NO3- concentration gradients along vertical (depth) direction for the pore solution within the three kinds of concrete specimens were investigated systematically by varying the nitric acid pH value and immersion duration. Moreover, the mineral composition determination and microstructure observation for the corrosion products were also carried out, and the diffusion laws of H+ and NO3- in the pore solution were analyzed. As was expected, both of the H+ and NO3- concentrations of pore solution within shotcrete specimens were found to be lower than those for cast concrete specimens, confirming a higher resistance against nitric acid corrosion for shotcrete. The steel fiber addition has inconspicuous effect for the durability improvement of shotcrete under relatively high-concentration nitrate corrosive circumstances, as nearly no difference could be identified for H+/NO3- concentrations between ordinary shotcrete specimens and steel-fiber-reinforced shotcrete specimens, both of which were immersed in pH≤2 nitric acid. It was concluded according to the corrosion mechanism analysis that the corrosion process of shotcrete in nitric acid includes three steps: reaction of hydration products (with the acid), decomposition of calcium silicate hydrate gel, and corrosion of ettringite and aggregate. Therefore, the pore solution within shotcrete displayed an H+ content change tendency of rapid increase (early stage) and subsequent slower increase (late stage), while on the other hand, a NO3- content change tendency of always fast increase, as the corrosion getting processed.
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2019, Vol. 33 
