Corrosion Thermodynamics

Module Four of CCE 281 Corrosion: Impact, Principles, and Practical Solutions

Lesson Objectives

•  Examine the relation between energy and surface potential

•  Discuss standard potentials in relation to the stability of chemical species

•  Describe the main reference electrodes used to measure corrosion potential

•  Explain the features and functions of E-pH (Pourbaix) diagrams

•  Describe the stability of water, iron, and aluminum from a thermodynamic point of view

Table of Contents

•  Introduction

•  Free Energy of a corrosion reaction

•  Standard electrode potentials

•  Nernst equation

•  Reference Half-Cells (Electrodes)

  •  Conversion between reference half-cells electrodes

  •  Silver/silver chloride reference electrode

  •  Copper/copper sulfate reference electrode

•  Measuring the corrosion potential

•  Measuring pH

  •  pH glass electrodes

  •  pH antimony electrode

•  Potential-pH Diagram

  •  Aluminum E-pH (Pourbaix) Diagram

  •  E-pH diagram of water

  •  E-pH diagram of metals

  •  Iron E-pH (Pourbaix) Diagram

Introduction

One can use thermodynamics, e.g. Pourbaix or E-pH diagrams, to evaluate the theoretical activity of a given metal or alloy in a corrosion situation provided the chemical make-up of the environment is known. But for practical situations, it is important to realize that the environment is a variable that can change with time and conditions. It is also important to realize that the environment that actually affects a metal corresponds to the micro-environmental conditions this metal really 'sees', i.e. the local environment at the surface of the metal. (reference)

It is indeed the reactivity of this local environment that will determine the real corrosion damage. Thus, an experiment that would investigate only the nominal environmental condition without consideration for local effects such as flow, pH cells, deposits, and galvanic effects is useless for lifetime prediction.

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