Corrosion Engineering_Chapter 5_Polarization Methods Danny A Jones

Corrosion Engineering ME 4953/ME 5013

Spring 2015

Chapter 5 Polarization Methods to Measure Corrosion Rate

Introduction

3/25/2015 Corrosion Engineering, Spring 2015 2

Corrosion rate can be determined by weight loss measurements or electrochemical polarization methods. Weight loss measurements monitor the dissolution

rate as a function of area and time of exposure. These are time consuming.

Electrochemical polarization methods are based on the principle: Amount of metal dissolution Amount of current passing

through the solution

Corrosion rate Current density

Electrochemical Polarization Methods

Polarization is defined as the deviation of potential from equilibrium potential because of net charge flow (current flow).

Polarization methods are nondestructive and may be repeated numerous times to measure consecutive corrosion rates on the same electrode.

Electrochemical Polarization Methods

Tafel extrapolation

Polarization resistance

Tafel Extrapolation

Tafel Extrapolation

Method is applicable only to systems containing one reduction reaction.

Tafel region must extend over a current range of at least one order of magnitude.

Not achievable because of interference from concentration polarization and other extraneous effects.

Applied-Current Linear Polarization method

The applied current density is a linear function of the electrode potential.

Advantages of linear-polarization technique

Polarization Resistance Method

Galvanostatic Method

Polarization resistance curve Potential-time strip-chart trace during polarization resistance measurement on zinc coating on steel in 0.1N NaCl at room temperature.

Electrochemical Impedance Spectroscopy Method

Electrochemical Impedance Spectroscopy Method - Principles

Electrochemical Impedance Spectroscopy Method - Equations

Electrochemical Impedance Spectroscopy Method

Electrochemical Impedance Spectroscopy Method

Electrochemical Impedance Spectroscopy Method- Data

Electrochemical Impedance Spectroscopy Method - Data

Electrochemical Impedance Spectroscopy Method - Data

Summary

EIS has the capability to give valid measurements of polarization resistance rate corrected for ohmic interferences from solution resistance.