International Journal of Scientific & Engineering Research, Volume 5, Issue 6, June-2014 1329 ISSN 2229-5518 IJSER © 2014 http://www.ijser.org Galvanic Corrosion of a Mild Steel Bolt In A Magnesium Alloy (AZ91D) Plate Simulation Using Comsol Multiphysics A.T Olanipekun, A.E Faola, K. E. Oluwabunmi, T.L Oladosu Abstract— The research work focuses on the simulation of galvanic corrosion prediction of a mild steel bolt in a magnesium plate (AZ91D) immersed in 0.5 M (sodium chloride) as an electrolyte. The simulation has a model made in two dimensions using axial symmetry with electrolyte domain of radius 120mm and height 25mm. The electrolyte conductivity is set to 7.95s/m. Three different disc radii are investigated: 8mm, 12mm, and 25mm. An anodic Tafel expression is used to describe the electrode kinetics on the steel disc. The line graph of electrode currents for the three different disc radii and a revolved surface plot of the electrolyte potential for a disc radius of 25 mm shows slow kinetics due to large anode area, therefore the local current density of the anode reaction in the vicinity of the steel disc increase significantly when the disc radius increase, which predicts lesser corrosion susceptibility for the magnesium alloy plates. This model demonstrates the application of COMSOL model development and simulation to galvanic corrosion simulation. Index Terms— corrosion, current density, electrode, electrolyte, Modeling, simulation ——————————  —————————— 1 INTRODUCTION The model was developed through reference to the modeling techniques developed in the COMSOL corrosion module guide [1]. Galvanic corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. The less noble ore more reactive metal in the particular environment will experience [2]. Magnesium to steel is a very bad combination, with magnesium being the metal that would take all the attack due to their wider distance apart in the galvanic series, other than this the relative sizes of the anode and cathode also control galvanic corrosion, if the anode is small with respect to the cathode, the attack on the anode will be great. If the situation is reversed, the attack will be low [3]. Magnesium is the most active engineering metal in the galvanic series, so it should not be coupled with other metals unless the intent is to use the magnesium as a sacrificial anode. The use of high purity AZ91D has reduced the propensity for salt corrosion, but coatings sometimes are still necessary to prevent corrosion[4]. ———————————————— • Olanipekun Ayorinde Tayo, National Agency for science and Engineering infrastructure (prototype Engineering Development Ilesa), Nigeria. 07061541108. E-mail: olanipekunayo2010@yahoo.com • Faola Aderonke Esther, National Agency for science and Engineering infrastructure (prototype Engineering Development Ilesa), Nigeria. 07061368352. E-mail: kemrons2000@yahoo.com • Oluwabunmi Kayode Ebenezer, National Agency for science and Engineering infrastructure (prototype Engineering Development Ilesa), Nigeria. 07061541108. E-mail: kayoxemak@yahoo.com • Oladosu Temidayo Lekan, National Agency for science and Engineering infrastructure (prototype Engineering Development Ilesa), Nigeria. 07061541108. E-mail: temidayooladosu@gmail.com 2 METHODS 2.1 Materials A mild steel bolt used to tighten two magnesium plate (AZ91D) is considered as the model that finds application when mounting an Mg alloy component using steel fasteners 2.2 The Physics The following equations are the governing equation for the simulation, Tafel equation The Tafel equation is an equation in electrochemical kinetics relating the rate of an electrochemical reaction to the overpotential [5]. On a single electrode the Tafel equation can be stated as ∆ =  ×  �   0 � (1 ) ∆ is the overpotential, V  is the so-called "Tafel slope", V  is the current density, A/m 2 and  0 is the so-called "exchange current density", A/m 2 . Where an electrochemical reaction occurs in two half reactions on separate electrodes, the Tafel equation is applied to each electrode separately. The Tafel equation assumes that the reverse reaction rate is negligible compared to the forward reaction rate. The Tafel equation is applicable to the region where the values of polarization are high. Electrode potential IJSER