ABSTRACT

In this investigation, the effect of 0.195wt% Bismuth addition on the electrochemical properties of Al-4.22wt%Zn based sacrificial anode in artificial seawater has been evaluated. Al-Zn based sacrificial anodes are widely used for the protection of steel structures against corrosion in marine environment. The electrochemical behavior of Al-4.22wt%Zn-0.195wt%Bi anode and Carbon Steel samples in examined through Metallography, Weight Loss, Open Circuit Potentials (OCP), Electrochemical Impedance Spectroscopy (EIS) and Linear Polarization Resistance (LPR) and the results were analysed. The microstructure of Al-anode showed α-Al matrix, needle like β-AlFeSi intermetallic whereas spherical Bi-particles were uniformly distributed, that promotes uniform dissolution process. OCP of Al-anode and carbon steel shows a potential difference of ∼ 400 mV vs. Ag/AgCl implies low voltage sacrificial anode. No inductive loop appeared in EIS curves at low frequency region representing that pitting corrosion of Al-5%Zn is suppressed by Bismuth addition. Resistance of carbon steel reduces continuously, indicating increased corrosion rate with increase in immersion time, moreover, polarization resistance Al-anode is higher than carbon steel, exhibiting ease in anodic dissolution process.

INTRODUCTION

Steel is a common material for the construction of large infrastructures. It is a main constituent used for building of, offshore drilling platforms, steel cast dock, pipeline in seabed, coastal bridges and ship hulls1. Corrosion of offshore structures is a serious matter in terms of degradation and deterioration of these structures in a corrosive electrolyte such as seawater, which could lead to fatigue cracks, brittle failure and unstable failure2.

Cathodic protection of steel structures is cheap and most commonly used technique as compared to the use of corrosion resistant Nickel base alloy to mitigate corrosion in aggressive marine environments3. Two types of cathodic protection techniques could be applied to metallic bodies, one is Impressed Current Cathodic Protection (ICCP) and the other is Sacrificial Anode Cathodic Protection (SACP). In ICCP current required to mitigate corrosion is provided from an outer source of DC current. The negative terminal of the DC source is connected to the structure which makes it cathode and positive terminal is attached to a noble element such as Mixed Metal Oxides (MMO) or platinized titanium anode etc. It is suitable for large structures with notable corrosion and where expected structure's life is > 25 years 4, 5. In SACP galvanic anodes, which are electrochemically more active in nature (having more negative reduction potential) are attached directly to the structure, providing it the required current for the protection against corrosion. This method is applied where budget is limited and predicted structure's life is around 10 years 6, 7.

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