Editors’ Choice—Natural Convection Boundary Layer Thickness at Elevated Chloride Concentrations and Temperatures and the Effects on a Galvanic Couple
Ryan Katona, Jacob Carpenter, Andrew W. Knight, Rebecca Marshall, Brendan Nation, Eric John Schindelholz, Rebecca Schaller, Robert G. Kelly
Abstract
The natural convection boundary layer ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>δ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>n</mml:mi> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) and its influence on cathodic current in a galvanic couple under varying electrolytes as a function of concentration (1 − 5.3 M NaCl) and temperature (25 °C−45 °C) were understood. Polarization scans were obtained under quiescent conditions and at defined boundary layer thicknesses using a rotating disk electrode on platinum and stainless steel 304L (SS304L); these were combined to determine <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>δ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>n</mml:mi> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> <mml:mo>.</mml:mo> </mml:math> With increasing chloride concentration and temperature, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>δ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>n</mml:mi> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> </mml:math> decreased. Increased mass transport (Sherwood number) results in a decrease in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>δ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>n</mml:mi> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> </mml:math> providing a means to predict this important boundary. Using Finite Element Modeling, the cathodic current was calculated for an aluminum alloy/SS304L galvanic couple as a function of water layer ( WL ) thickness and cathode length. Electrolyte domains were delineated, describing (i) dominance of ohmic resistance over mass transport under thin WL , (ii) the transition from thin film to bulk conditions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>δ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>n</mml:mi> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> </mml:math> and (iii) dominance of mass transport under thick WL . With increasing chloride concentration, cathodic current decreased due to decreases in mass transport. With increasing temperature, increased cathodic current was related to increases in mass transport and solution conductivity. This study has implications for sample sizing and corrosion prediction under changing environments.