CHAPTER 2 (ii) CORROSION PRINCIPLES

1. CHAPTER 2 (ii) CORROSION PRINCIPLES Chapter Outlines 2.5 Electrochemical Aspects (cont`) 2.6 Environmental Effects 2.7 Metallurgical and Other Effects

2. 2.5 Electrochemical Aspects (cont`) POLARIZATION • The rate of an electrochemical reaction is limited by various physical and • chemical factors; Hence, an electrochemical reaction is said to be polarized or • retarded by these environment factors; • What is ‘POLARIZATION’ • The displacement of the electrode potentials from their equilibrium • values to a constant potential of some intermediate value and the • creation of a net current flow; • Polarization can be conveniently divided into two different types: • (i) Activation polarization • (ii) Concentration polarization

3. Polarization can be divided into two (2) different types; 1. Activation Polarization (AP) • AP refers to an electrochemical process/reactions that are controlled by • a slow step in a reaction sequence at the metal- electrolyte interface; • That is, there is critical activation energy needed to surmount the energy barrier associated with the slowest step; • This is easily illustrated by considering hydrogen-evolution reaction on zinc during corrosion in acid solution;

4. 2. Concentration Polarization (CP) • CP refers to an electrochemical process/reactions that are controlled by the diffusion of ions in the electrolyte; • This type of polarization is illustrated by considering the diffusion of hydrogen ions to a metal surface to form hydrogen gas by the cathodic reaction.

5. PASSIVITY • The passivation of a metal as it pertains to corrosion refers to the formation of a protective surface layer of reaction product that inhibits further reaction; • In other words, the passivation of metals refers to their loss of chemical reactivity in the presence of a particular environmental conditions; • There are two (2) main theories regarding the nature of the passive film: • The oxide-film theory- it is believed that the passive film is always a diffusion-barrier layer of reaction products (e.g., metal oxides or other compounds) that are separated the metal from its environment and slow down the reaction rate; • The adsorption theory- it is believed that passive metals are covered by chemisorbed films of oxygen; such a layer is supposed to displaced the normally adsorbed H₂O molecules and slow down the rate of anodic dissolution involving the hydration of metals ions; • The two theories have in common that a protective films forms on the metal surface to create the passive state, which results in increased corrosion resistance;

6. Active region- the behavior of this material is identical to that of a normal metal; • Passive region- if more oxidizing agent is added, the corrosion rate shows a sudden decrease, further increases in oxidizing agents produce little, if any, change in the corrosion rate of the material; • Transpassive region- at very high concentrations of oxidizers or in the presence of very powerful oxidizers the corrosion rate again increase with increasing oxidizer power;

7. 2.6 Environmental Effects • Effect of Oxygen and Oxidizers • Effect of Velocity • Effect of Temperature • Effect of Corrosive Concentration • Effect of Galvanic Coupling Please refer to Text Book - Fontana

8. 2.7 Metallurgical Aspects • Metallic Properties • Others (Economic Consideration, Inspection and Instrumentation) • Metallic Properties- grain boundaries are high-energy areas and are more active chemically; (e.g., pg 30-31); • Economic consideration- control of corrosion is primary an economic problem; • Inspection- proper inspection is must-particularly for critical components operating under hazardous conditions; • Instrumentation- the revolution brought about by the introduction of electronic instrumentation into electrochemistry;