Corrosion Principles and Types of Corrosion

In the previous chapter, we introduced the need for the study of corrosion, but we did not define corrosion. Though nearly everyone sees some very visible examples of corrosion in their daily lives, mostly in the form of brown rust deposited on steel and iron surfaces, there are several other forms of corrosion that are not recognized by the average person. This is because not everyone knows what corrosion is or how it manifests itself in different environments and on different materials. This introduction obviously leads us to the question; what is corrosion?

Because the corrosion phenomena is vast, there can be several responses to this question and that can lead to different definitions of corrosion. While they all may address a specific type of corrosion, none of them encompass all aspects and forms of corrosion. Thus, in an attempt to define corrosion we also discuss the principles that affect corrosion. One NACE publication defines corrosion as “The deterioration of a substance—usually a metal—or its properties because of reaction with its environment.”

From this NACE definition we understand that corrosion is the interaction between a material and the environment to which it is exposed. This leads us to accept the fact that understanding corrosion involves understanding both the material and its environment. Environments that cause corrosion could be atmospheric, the presence of certain liquids, high-temperatures, or being underground—which is similar to being in certain liquids. The other aspect of corrosion is the material; the corroding material could be metallic or nonmetallic; in our current scope, we focus on corrosion of metals. To understand corrosion it is essential that the metallurgical aspect of the material is fully understood, including the fundamentals of the structures and properties of engineering materials, metals, and nonmetals. To understand corrosion of a metal it is important to know the path taken to refine that metal from its natural form (ore) to its usable form, and subsequent processing and any heat treatment that may have been applied to the material to make it useful.

In common speech as well as in the NACE definition above, the word corrosion signifies deterioration of material, an element of natural degradation is implied. There is an element of correctness with this common understanding—there is degradation and nature is taking part in that degradation.

The above definition is a good description of the physical appearance of the action of corrosion. However, it does not address every aspect of the scientific explanation of corrosion. For example, it does not explain the flow of energy and thermodynamic reactions involved in the corrosion process. Although the thermodynamic reaction could be grouped with the environment aspect of the corrosion, it is, in itself, a very prominent aspect that needs independent study.

The following discussion complements the discussion we have had so far and addresses additional scientific explanations to bring about a better understanding of the term corrosion, one that is suitable for engineers.

Flow of Energy

Corrosion occurs in nearly all materials produced by nature or manmade materials; this includes metals and nonmetals such as certain plastics, ceramics, and concrete. In order to find the correct definition of the word corrosion, a question arises, why do metals corrode? A search for the answer leads us to the realm of thermodynamics.

Thermodynamics is the science of the flow of energy. This science explains a specific corrosion process and indicates if corrosion is possible in a given metal and environment. The flow of energy in the corrosion process is in the form of electrical energy. The rate of corrosion is similarly predicted by the kinetics. We discuss these topics in more detail in later sections of this book. As we know, with the exception of a few naturally occurring metals, most engineering materials are found in the form of ores, often metal oxides found in nature. A lot of energy is spent in the extraction process of these usable metals from their ores. Hematite (Fe2O3) is an ore of iron, and bauxite (Al2O3·H2O) is an ore of aluminum, there are some more complex ore like that of Nickel ore kupfer-nickel, smaltite ores are a combination of sulphur and arsenic which are roasted to form an oxide which is then reduced to the metal by hydrogen and purified by the Mond process to obtain nickel that engineers can use. Copper is found as pure metal; that is the reason copper is usually free from corrosion, but it is also extracted from various ores like Ruby ore (Cu2O), Copper Glance (Cu2S), or Pyrite (CuFeS2). It may be pointed out that copper obtained from nature and copper extracted from ores would display different potentials. As we can see, a lot of energy is put into the extraction of engineering metals. Some metals that are extracted as free metals from the earth and do not require additional energy to convert the natural form to make it usable are very low in corrosion galvanic energy. These metals are called Nobel metals. In the galvanic table, Table 2.1, metals are listed from the more negative potentials (Active) to more positive (Noble).

In the galvanic series table, a new term, “potential,” is introduced in relation to corrosion—it is one way of measuring the energy difference between two metals. Electrons flow from a higher energy state anode, which is a negative, to a low energy electrode, a cathode. The potential difference between two electrodes facilitates the flow of electrons. If a voltmeter of sufficient sensitivity is attached across the flow circuit, the potential difference between anode and cathode can be measured. The potential of each metal in reference to another is a unique number, these numbers by themselves do not establish a standard by which to measure and compare all possible potential differences and thus it is not of much practical use except in relation to the two metals. One unified scale is needed to compare and establish a universal reference for understanding the potential difference of various metals. For this purpose, a standard electrode is used; a reference electrode is so constructed that its potential is reproducible. There are a number of standard electrodes that are used as reference electrodes. Some of the common reference electrodes include Colomel, Copper-Copper Sulfate, and Silver-Silver Chloride and these, along with their potentials are listed in Table 2.2.