From Plant Engineer's Reference Book, Second Edition
Michael J Schofield,
BSc, MSc, PhD, MIM, CEng
Independent Consultant

33.1 Corrosion Basics

33.1.1 Definitions of Corrosion

Corrosion is generally taken to be the wastage of a metal by the action of corrosive agents. However, a wider definition is the degradation of a material through contact with its environment. Thus corrosion can include non-metallic materials such as concrete and plastics and mechanisms such as cracking in addition to wastage (i.e. loss of material). This chapter is primarily concerned with metallic corrosion, through a variety of mechanisms.

In essence, the corrosion of metals is an electron transfer reaction. An uncharged metal atom loses one or more electrons and becomes a charged metal ion:

In an ionizing solvent the metal ion initially goes into solution but may then undergo a secondary reaction, combining with other ions present in the environment to form an insoluble molecular species such as rust or aluminium oxide. In high-temperature oxidation the metal ion becomes part of the lattice of the oxide formed.

33.1.2 Electrochemical Corrosion

The most important mechanism involved in the corrosion of metal is electrochemical dissolution. This is the basis of general metal loss, pitting corrosion, microbiologically induced corrosion and some aspects of stress corrosion cracking. Corrosion in aqueous systems and other circumstances where an electrolyte is present is generally electrochemical in nature. Other mechanisms operate in the absence of electrolyte, and some are discussed in Section 33.1.4.

Figure 33.1 depicts a metal such as iron, steel or zinc immersed in electrolyte such as sodium...

Products & Services
Corrosion Protection Services
Corrosion protection services mitigate environmental or chemical attack through the application or installation of coatings, liners, cathodic protection, sacrificial anodes, and corrosion inhibitors.
Chemical Water Treatment Equipment
Chemical water treatment equipment mitigates the formation of scale, calcium carbonate deposits, in process water systems such as cooling towers, boilers and heat exchangers by controlled chemical addition and system monitoring.
Surface and Corrosion Protection Products
Surface protection products and corrosion protection products are used to provide protection to industrial surfaces particularly in the marine, transportation, chemical process, pulp and paper, and petroleum industries.
Anodes are consumables used in a variety of electrochemical processes such as corrosion protection (sacrificial anodes) and electroplating (plating anodes).
Rust and Corrosion Inhibiting Fluids
Rust preventives and corrosion inhibitors are lubricants, greases, oils or fluid additives that form a protective film or barrier to prevent corrosion or rust formation.

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