Corrosion Phenomena

Abstract

Corrosion mechanism and factors, which we have just analyzed, relate to corrosion phenomena defining by corrosion forms. There are both many metals (about 80) and a lot of environmental types (of course, more than 80!), and we might expect many different corrosion phenomena. All corrosion reactions can be divided according to their mechanism in two types: reactions in the presence of non-electrolytes, and in the presence of electrolytes (electrochemical mechanism). All corrosion phenomena can be also divided into two types: uniform (general) and non-uniform (localized) corrosion. Uniform corrosion is a process when all of a metal surface corrodes, or dissolves. Non-uniform corrosion is a process when only some definite part of a metal surface corrodes, i.e. anodic process concentrates on a small part of the metal's surface. A corrosion phenomenon is a process that occurs on the surface of or inside the metal. The corrosion form is a result of a corrosion phenomenon. It is possible to define the corrosion phenomenon according to the corrosion form. On the other hand, if we know the corrosion phenomenon type, we may expect what corrosion form, or deterioration type, will happen with a metal: uniform thickness loss, pits, cracks, blistering, etc. We have to keep in mind that many corrosion phenomena influence such important functions of metallic equipment as mechanical and electrical properties. All corrosion phenomena and corrosion forms are described by specific mechanisms. 2.1 Uniform, or General, Corrosion If we put a piece of zinc in a hydrochloric acid solution, or aluminum in a soda solution, we will observe uniform diminishing of the metal's thickness. If we put a piece of carbon steel in a furnace at high temperature, we will observe the formation of even iron oxides film on the steel surface. Uniform, or general, corrosion is a type of corrosion attack uniformly distributed over a metal surface (Figure 2.1). Here are some examples of uniform corrosion of metals and alloys. 53