Effect of Chloride on the Pitting Corrosion of Carbon Steel in Alkaline Solutions
Jie Qiu, Yakun Zhu, Yi Xu, Yanhui Li, Feixiong Mao, Angjian Wu, Digby D. Macdonald
Abstract
The effect of chloride concentration on the passivity breakdown of carbon steel in deaerated pH = 13.5 alkaline solution was investigated. The results show that chloride plays an important role for the pitting corrosion of carbon steel. When NaCl <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>≤</mml:mo> <mml:mspace width=".25em"/> <mml:mn>1</mml:mn> <mml:mspace width=".25em"/> <mml:mi mathvariant="normal">M</mml:mi> <mml:mo>,</mml:mo> </mml:math> the carbon steel is passive but for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1</mml:mn> <mml:mspace width=".25em"/> <mml:mi mathvariant="normal">M</mml:mi> <mml:mo><</mml:mo> </mml:math> NaCl < 2 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">M</mml:mi> <mml:mo>,</mml:mo> </mml:math> the carbon steel transfers from passive to passivity breakdown. When NaCl <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>≥</mml:mo> <mml:mspace width=".25em"/> <mml:mn>2</mml:mn> <mml:mspace width=".25em"/> <mml:mi mathvariant="normal">M</mml:mi> <mml:mo>,</mml:mo> </mml:math> passive film breakdown occurs and the passivity breakdown potential ( E b ) of carbon steel is linearly-dependent on the logarithm of the chloride activity in the alkaline solution. As predicted by the PDM, the near-normal distribution of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>b</mml:mi> </mml:mrow> </mml:msub> </mml:math> is well indicated by the near straight line in the cumulative distribution probability. The analysis in this study gives us a method to calculate the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>b</mml:mi> </mml:mrow> </mml:msub> </mml:math> of carbon steel in alkaline solution and its distribution for any given chloride concentration.