Research On Corrosion Mechanism Of X80 Steel In Acidic Red Clay Soil

X80 pipeline steel was applied to the second West-East pipeline project massively as a high strength pipeline. Soil corrosion of pipeline steel is the key problem that service life of pipeline must face. Currently, the research on soil corrosion of pipeline steel mainly focus on stress corrosion of Pipeline steel in soil and effects of soil environmental factors on soil corrosion of pipeline steel. However, the researchs that corrosion mechanism of Pipeline in red clay soil is studied from the perspective of the cathodic process are seldom reported. Many researchers agree that hydrogen evolution corrosion of X80 pipeline steel in red clay soil occurs, but there is no direct proof that hydrogen evolution corrosion of X80 pipeline steel in red clay soil occurs. In addition, there is no conclusion that corrosion cathodic process of X80 steel is exactly the main oxygen depolarized, or hydrogen depolarization.In this paper, thermodynamics of hydrogen evolution corrosion and oxygen consuming corrosion for X80 pipeline steel in simulated red soil solution was analyzed. In order to collect hydrogen, a gas collection device was designed. And the cathodic corrosion process of X80 steel was researched. Effects of water content on the corrosion behavior of X80 steel in red soil also were studied simultaneously.The results show that the pH value and the dissolved oxygen content of the simulated red soil solution determine that X80 steel is of hydrogen evolution corrosion thermodynamic conditions or oxygen consuming corrosion thermodynamic conditions. When the pH value of the simulated red soil solution is 3.0, hydrogen evolution corrosion and oxygen consuming corrosion of X80 pipeline steel are of thermodynamic conditions. When the pH value of the simulated red soil solution is 5.0 and DO ≤ 4.45 ppm, hydrogen evolution corrosion and oxygen consuming corrosion of X80 pipeline steel are of thermodynamic conditions. When the pH value of the simulated red soil solution is 7.0, hydrogen evolution corrosion of X80 pipeline steel is not of thermodynamic conditions.The corrosion cathodic process of X80 steel in simulated acidic red soil solution is composed of hydrogen depolarization and oxygen depolarization. In this case, pH value of the simulated red soil solution and DO concentration determine that the cathodic process of X80 steel corrosion in simulated acidic red soil solution is the oxygen depolarized based, or hydrogen depolarization based.When the pH value of the simulated red soil solution is 3.0, 3.5, 4.0, 4.5 and 5.0, with time increasing, the oxygen content of the simulated red soil solution decreased from the state of nature to approximately 1.0ppm, the amount of hydrogen evolution gradually increase, the hydrogen evolution rate reaches the maximum at the time of 1h; The average corrosion rate of X80 steel( iFe), the average rate of oxygen consumption(2Oi) and the average rate of hydrogen evolution(2Hi) are reduced. When corrosion time is not more than 16 hours, iFe is equal to the sum of 2Oi and2 Hi.When the pH value of the simulated red soil solution is 3.0 or 3.5 and content of DO is not more than 7.0ppm, 2Hi is more than2 Oi and the cathodic process is the hydrogen depolarized based. When the pH value is 5.0, 2Hi is less than2 Oi and the cathodic process is the oxygen depolarized based. When the pH is 4.0, with the corrosion time increasing from 0 h to 8 h, DO concentration decreased from 7.0ppm to 2.0ppm, the amount of hydrogen evolution is low, 2Hi is less than2 Oi and the cathodic process is the oxygen depolarized based; with the corrosion time increasing from 8 h to 24 h, DO concentration decreased from 2.0ppm to 0.5ppm, the amount of hydrogen evolution sharp increase, 2Hi is greater than2 Oi and the cathodic process is the hydrogen depolarized based.When water content(WC) of red clay soil is low(WC<34%), the moisture in the air of red clay soil pores easily reach saturated humidity and steam condenses into droplets or thin film, the result is that X80 pipeline steel occurs localized corrosion. As water content increases, the corrosion mode gradually translated from localized corrosion into General corrosion. When the water content is not less than 34%, the corrosion mode for X80 steel is general corrosion.