Corrosion of Metals and Their Alloys

The cost of corrosion to US industries and the American public is currently estimated at $170 billion per year. Although corrosion is only nature`s method of recycling, or of returning a metal to its lowest energy form, it is an insidious enemy that destroys our cars, our plumbing, our buildings, our bridges, our engines, and our factories.

Corrosion can often be predictable or it can be totally unpredictable and catastrophic, such as the hydrogen embrittlement or stress corrosion of critical structural members and pressure vessels in the aerospace and chemical processing industries.

Corrosion involves the interaction (reaction) between a metal or alloy and its environment. Corrosion is affected by the properties of both the metal or alloy and the environment. In this discussion, only the environmental variables will be addressed, the more important of which include:

• pH (acidity)
• Oxidizing power (potential)
• Temperature (heat transfer)
• Velocity (fluid flow)
• Concentration (solution constituents)

The concept of pH is complex. It is related to, but not synonymous with, hydrogen concentration or amount of acid.

While corrosion obeys well-known laws of electrochemistry and thermodynamics, many variables that influence the behavior of a metal in its environment can result in accelerated corrosion or failure in one case and complete protection in another, similar case.

Avoiding detrimental corrosion requires the interdisciplinary approach of the designer, the metallurgist, and the chemist. Sooner or later, nearly everyone in these fields will be faced with major corrosion issues. It is necessary to learn to recognize the forms of corrosion and the parameters that must be controlled to avoid or mitigate corrosion.

The theory of corrosion from the thermodynamic and kinetic points of view covers the principles of electrochemistry, diffusion, and dissolution as they apply to aqueous corrosion and high-temperature corrosion in salts, liquid metals, and gases.

We can face the various forms of corrosion, and we must know how to recognize them, as well as the driving conditions or parameters that influence each form of the corrosion, for it is the control of these parameters which can minimize or eliminate corrosion.

All corrosion processes show some common features. Thermodynamic principles can be applied to determine which processes can occur and how strong the tendency is for the changes to take place. Kinetic laws then describe the rates of the reactions. There are, however, substantial differences in the fundamentals of corrosion in such environments as aqueous solutions, non-aqueous liquids, and gases.

Corrosion and Environment

Corrosion in aqueous solutions. Although atmospheric air is the most common environment, aqueous solutions, including natural waters, atmospheric moisture, and rain, as well as man-made solutions, are the environments most frequently associated with corrosion problems.

Because of the ionic conductivity of the environment, corrosion is due to electrochemical reactions and is strongly effected by such factors as the electrode potential and acidity of the solution.

Corrosion of metals in aqueous environments. This type of corrosion is almost always electrochemical in nature. It occurs when two or more electrochemical reactions take place on a metal surface. As a result, some of the elements of the metal or alloy change from a metallic state into a non-metallic state.

The products of corrosion may be dissolved species or solid corrosion products. In either case, the energy of the system is lowered as the metal converts to a lower-energy form. Rusting of steel is the best-known example of a conversion of the metal (iron) into a nonmetallic corrosion product (rust). The change in the energy of the system is the driving force for the corrosion process and is a subject of thermodynamics.

Thermodynamics examines and quantifies the tendency for corrosion and its partial processes to occur. It does not predict if the changes actually will occur and at what rate. Thermodynamics can predict, however, under what conditions the metal is stable and corrosion cannot occur.

Corrosion in Molten Salts and Liquid Metals. These are more specific but important areas of corrosion in liquid environments. Both have been strongly associated with the nuclear industry, for which much of the research has been performed, but there are numerous non-nuclear applications as well.

Corrosion in Gases. In gaseous corrosion, the environment is nonconductive, and the ionic processes are restricted to the surface of the metal and the corrosion product layers.

Because the reaction rates of industrial metals with common gases are low at room temperature, gaseous corrosion, generically called oxidation, is usually an industrial problem only at high temperatures when diffusion processes are dominant.

Forms of Corrosion

Over the years, corrosion scientists and engineers have recognized that corrosion manifests itself in forms that have certain similarities and therefore can be categorized into specific groups. However, many of these forms are not unique but involve mechanisms that have over lapping characteristics that may influence or control initiation or propagation of a specific type of corrosion.

The most familiar and often used categorization of corrosion is: uniform attack, crevice corrosion, pitting, intergranular corrosion, selective leaching, erosion corrosion, stress corrosion, and hydrogen damage. This classification of corrosion was based on visual characteristics of the morphology of attack.

Other prominent corrosion authors have avoided a classification format and have simply discussed the classical types of corrosion (for example, pitting and crevice corrosion) as they relate to specific metals and alloys.

Forms of corrosion are:

1. General corrosion
   • Atmospheric corrosion
   • Galvanic corrosion
   • Stray-current corrosion
   • General biological corrosion
   • Molten salt corrosion
   • Corrosion in liquid metals

2. High-temperature corrosion
   • Oxidation
   • Sulfidation
   • Carburization
   • Other forms(a)

3. Localized corrosion
   • Filiform corrosion
   • Crevice corrosion
   • Pitting corrosion
   • Localized biological corrosion

4. Metallurgically influenced corrosion
   • Intergranular corrosion
   • Dealloying corrosion

5. Mechanically assisted degradation
   • Erosion corrosion
   • Fretting corrosion
   • Cavitation and water drop impingement
   • Corrosion fatigue

6. Environmentally induced cracking
   • Stress-corrosion cracking
   • Hydrogen damage
   • Liquid metal embrittlement
   • Solid metal induced embrittlement

General Corrosion. General corrosion is defined as corrosive attack dominated by uniform thinning. Although high-temperature attack in gaseous environments, liquid metals, and molten salts may manifest itself as various forms of corrosion, such as stress-corrosion cracking and de-alloying, high-temperature attack has been incorporated under the term "General Corrosion" because it is often dominated by uniform thinning.

Localized Corrosion. The forms of corrosion under this category need no explanation, even though other forms could be placed in this category. It should be noted, however, that localized biological corrosion often causes or accelerates pitting or crevice corrosion.

Metallurgically influenced corrosion was so classified as a result of the significant role that metallurgy plays in these forms of attack. It is well understood that metallurgy is important in all forms of corrosion, but this classification is meant to emphasize its role in these specific forms of attack.

Mechanically assisted degradation groups those forms of corrosion that contain a mechanical component, such as velocity, abrasion, and hydrodynamics, that has a significant effect on the corrosion behavior. Corrosion fatigue was included in this category because of the dynamic stress state; however, it could easily be categorized as a form of environmentally induced cracking.

The environmentally induced cracking follows the current trend in the literature of combining forms of cracking that are produced by corrosion in the presence of stress.