New insights into the effect of chloride ions on structure and composition of rust layers

The effect of chloride ion concentration on the corrosion resistance of interstitial-free (IF) steel in a simulated tropical marine atmosphere has been studied. The corrosion kinetics of steel specimens subjected to varying NaCl concentrations exhibited an initial phase of elevated corrosion rates for specimens exposed to high chloride ion concentrations, followed by a stable phase regardless of chloride ion concentrations. When chloride ion concentrations is low, the majority of chloride ions are located in the outer rust layer, with the inner rust layer consisting mainly of a stable α-FeOOH phase. This particular composition led to the manifestation of a high protective nature of the rust layer, with elevated <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo fence="false" stretchy="false">|</mml:mo> <mml:mi>Z</mml:mi> <mml:mo fence="false" stretchy="false">|</mml:mo> </mml:math> 0.01Hz and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>R</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">ct</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> values. As the chloride ion concentration increased, the transformation of γ-FeOOH to α-FeOOH was suppressed due to the presence of chloride ions in the inner rust layer. Moreover, as the evaporation rate decreases with an increase in chloride ion concentration, the amount of water film adsorbed on the steel surface increases; this results in low protective nature of the rust layer with low <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo fence="false" stretchy="false">|</mml:mo> <mml:mi>Z</mml:mi> <mml:mo fence="false" stretchy="false">|</mml:mo> </mml:math> 0.01Hz and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>R</mml:mi> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">ct</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> values. Results show that the rust layer's nature is dependent on both the surface composition of the oxides and the Cl concentration.