Intelligent Coatings for Corrosion Control

Atul Tiwari,Lloyd Hihara,James Rawlins

• Publisher: Butterworth-Heinemann
• Year: 2014
• ISBN:
  9780124115347
• Topic: Naturwissenschaften
• Subtopic: Industrielle & technische Chemie

Intelligent Coatings for Corrosion Control covers the most current and comprehensive information on the emerging field of intelligent coatings. The book begins with a fundamental discussion of corrosion and corrosion protection through coatings, setting the stage for deeper discussion of the various types of smart coatings currently in use and in development, outlining their methods of synthesis and characterization, and their applications in a variety of corrosion settings. Further chapters provide insight into the ongoing research, current trends, and technical challenges in this rapidly progressing field.

• Reviews fundamentals of corrosion and coatings for corrosion control before delving into a discussion of intelligent coatings—useful for researchers and grad students new to the subject
• Covers the most current developments in intelligent coatings for corrosion control as presented by top researchers in the field
• Includes many examples of current and potential applications of smart coatings to a variety of corrosion problems

Chapter 1

Electrochemical Aspects of Corrosion-Control Coatings

L.H. Hihara Hawaii Corrosion Laboratory, Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii, USA

Abstract

Corrosion of metals is an electrochemical process and is affected by moisture content, the chemical environment, and the electrochemical state of the metal. How the properties of a coating or coating system affect the above parameters governs the effectiveness of the coating in suppressing corrosion. Fundamental electrochemical aspects of corrosion are discussed, as well how coating properties (i.e., barrier characteristics and electrical resistivity, chemical and electrochemical) affect the corrosion behavior of the substrate metal. Barrier protection, corrosion inhibition, and cathodic protection as they relate to coatings are discussed.

Keywords

Corrosion protection

Coatings

Barrier

Corrosion inhibition

Cathodic protection

1.1 Introduction

In nature, almost all metals are found in their thermodynamically stable states which are ores that are comprised primarily of oxides, sulfides, and halides.¹ Energy must be expended to extract the elemental metals from the ores. Hence, as soon as the elemental metals are extracted from their ores, they have a propensity to revert back to their thermodynamically stable compounds. In most cases, metals will form oxides when exposed to moisture. If the oxide is porous or does not have good adhesion to the substrate metal, the metal will actively corrode. If the oxide forms a compact impervious layer and has good adhesion, the metal will passivate, resulting in excellent corrosion resistance. In environments that contain aggressive ions, however, the passive film can breakdown, resulting in localized corrosion and very high corrosion rates. Hence, coatings are very frequently needed to suppress corrosion in metals that do not naturally form protective passive films or for aggressive environments that can break down passivity.

1.2 Corrosion

For a metal to corrode by aqueous corrosion, water molecules must be present. The metal, however, need not be fully immersed because water can condense on a surface due to temperature fluctuations, even if the relative humidity is lower than 100% (e.g., condensation on cold surface on a warm, humid day), or due to hygroscopic impurities such as airborne salts.²

1.2.1 Thermodynamics

Aqueous corrosion is an electrochemical process involving anodic (or oxidation) and cathodic (or reduction) reactions. Dissolution of a metal M, which is an anodic reaction, is represented by the half-cell reaction

$\text{M}\to {\text{M}}^{n+}+n{\text{e}}^{-}$

(1.1)

The electrons of the anodic reaction must be consumed by a cathodic reaction for corrosion to proceed. Two predominant cathodic reactions in aqueous corrosion are oxygen reduction (Equations 1.2 and 1.3) and hydrogen evolution (Equations 1.4 and 1.5), respectively. Their half-cell reactions are represented as follows:

${\text{O}}_2+4{\text{e}}^{-}+4{\text{H}}^{+}\to 2{\text{H}}_2\text{O}\left(\text{in}\text{acidic}\text{solutions}\right)$

(1.2)

${\text{O}}_2+4{\text{e}}^{-}+2{\text{H}}_2\text{O}\to 4{\text{OH}}^{-}\left(\text{in}\text{basic}\text{solutions}\right)$

(1.3)

$2{\text{H}}^{+}+2{\text{e}}^{-}\to {\text{H}}_2\left(\text{in}\text{acidic}\text{solutions}\right)$

(1.4)

$2{\text{H}}_2\text{O}+2{\text{e}}^{-}\to {\text{H}}_2+2{\text{OH}}^{-}\left(\text{in}\text{basic}\text{solutions}\right)$

(1.5)

Oxygen reduction can only occur in aerated solutions, which contain dissolved oxygen molecules. Hydrogen evolution can occur ...