Electrochemistry and corrosion science

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Forms of Corrosion
The forms of corrosion described here use the terminology in use at NASA-KSC. There are other equally valid methods of classifying corrosion, and no universally-accepted terminology is in use. Keep in mind that a given situation may lead to several forms of corrosion on the same piece of material.
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Form of Corrosion

Uniform Corrosion
This is also called general corrosion. The surface effect produced by most direct chemical attacks (e.g., as by an acid) is a uniform etching of the metal.

Galvanic Corrosion
Galvanic corrosion is an electrochemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path. It occurs when dissimilar metals are in contact.

Concentration Cell Corrosion
Concentration cell corrosion occurs when two or more areas of a metal surface are in contact with different concentrations of the same solution.

Pitting Corrosion
Pitting corrosion is localized corrosion that occurs at microscopic defects on a metal surface. The pits are often found underneath surface deposits caused by corrosion product accumulation.

Crevice Corrosion
Crevice or contact corrosion is the corrosion produced at the region of contact of metals with metals or metals with nonmetals. It may occur at washers, under barnacles, at sand grains, under applied protective films, and at pockets formed by threaded joints.

Filiform Corrosion
This type of corrosion occurs on painted or plated surfaces when moisture permeates the coating. Long branching filaments of corrosion product extend out from the original corrosion pit and cause degradation of the protective coating.

Intergranular Corrosion
Intergranular corrosion is an attack on or adjacent to the grain boundaries of a metal or alloy.

Stress Corrosion Cracking
Stress corrosion cracking (SCC) is caused by the simultaneous effects of tensile stress and a specific corrosive environment. Stresses may be due to applied loads, residual stresses from the manufacturing process, or a combination of both.

Corrosion Fatigue
Corrosion fatigue is a special case of stress corrosion caused by the combined effects of cyclic stress and corrosion. No metal is immune from some reduction of its resistance to cyclic stressing if the metal is in a corrosive environment.

Fretting Corrosion
The rapid corrosion that occurs at the interface between contacting, highly loaded metal surfaces when subjected to slight vibratory motions is known as fretting corrosion.
Erosion Corrosion
Erosion corrosion is the result of a combination of an aggressive chemical environment and high fluid-surface velocities.

Dealloying
Dealloying is a rare form of corrosion found in copper alloys, gray cast iron, and some other alloys. Dealloying occurs when the alloy loses the active component of the metal and retains the more corrosion resistant component in a porous "sponge" on the metal surface.

Hydrogen Damage
Hydrogen embrittlement is a problem with high-strength steels, titanium, and some other metals. Control is by eliminating hydrogen from the environment or by the use of resistant alloys.

Corrosion in Concrete
Concrete is a widely-used structural material that is frequently reinforced with carbon steel reinforcing rods, post-tensioning cable or prestressing wires. The steel is necessary to maintain the strength of the structure, but it is subject to corrosion.

Microbial Corrosion
Microbial corrosion (also called microbiologically -influenced corrosion or MIC) is corrosion that is caused by the presence and activities of microbes. This corrosion can take many forms and can be controlled by biocides or by conventional corrosion control methods.
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Cathodic Protection Basics
The basic principle of CP is simple. A metal dissolution is reduced through the application of a cathodic current. Cathodic protection is often applied to coated structures, with the coating providing the primary form of corrosion protection. The CP current requirements tend to be excessive for uncoated systems.
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The first application of CP dates back to 1824, long before its theoretical foundation was established. Cathodic protection has probably become the most widely used method for preventing the corrosion deterioration of metallic structures in contact with any forms of electrolytically conducting environments, i.e. environments containing enough ions to conduct electricity such as soils, seawater and basically all natural waters. Cathodic protection basically reduces the corrosion rate of a metallic structure by reducing its corrosion potential, bringing the metal closer to an immune state. The two main methods of achieving this goal are by either:
Using sacrificial anodes with a corrosion potential lower than the metal to be protected (see the seawater galvanic series)
Using an impressed current provided by an external current source

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