Other Types of Corrosion

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Direct Chemical Attack is also fundamentally electrochemical in nature. However, no current flow is detectable, nor are there any definite anodic or cathodic areas. The theoretical rate of a chemical attack can be affected by the formation of a protective film on the metal surface, through secondary reactions involving the products of corrosion, and the mechanical removal of protective films, such as by erosion, flexing of the metal surface, or by temperature changes.

Dry Oxidation and Tarnish result when the clean surfaces of metals are exposed to air or other gasses to form oxides or other compounds on the surface of the metals. Many of these films are invisible at room temperature, but at higher temperatures these films may reach considerable thickness. The rate of film growth is usually greater at higher temperatures. At higher temperatures, and more particularly under changes of temperature, the film may crack or spall to expose fresh metal to attack. Bending or stressing the metal can induce or increase the spalling. The presence of sulfur-bearing gasses may greatly increase the rate of attack. And the presence of moisture will accelerate attack, and complicate it by permitting electrolytic corrosion.

Atmospheric Corrosion combines electrochemical attack, direct chemical attack and oxidation. The many structures created by man which are subject to atmospheric corrosion, and the many specialized methods which have been developed to combat it, justify it for consideration as a basic type of corrosion. Temperature changes, alternate wetting and drying, and the washing action of the elements of weather modify the rates of film removal. Rural atmospheres differ from industrial, and industrial atmospheres differ from one another to such an extent that special tests and consideration must be given for the optimum selection of protective coatings and application methods.

Crevice Corrosion is the attack on the surface of a metal partly shielded from contact with the corroding fluid, usually by a non-metallic materials. Typically, this is a concentration cell effect, the shielded area being anodic.

Caustic Embrittlement is usually the result of steam escaping through a crevice, such as between boiler plates or pipe flanges. The escaping water, usually in a fissure of steam, becomes highly concentrate, and the increased alkalinity of the concentrated water causes failure by stress corrosion.

Stress Corrosion Cracking results when even a very small pit forms in a metal under stress. The concentrated stress may either deepen and extend the pit, or crack any protective film which might tend to form. Under continued exposure to the corrosive medium and stress, the crack extends by alternate corrosion and stress failure.

Hydrogen Embrittlement and hydrogen attack results when atomic hydrogen, contained in chemical and refinery processes or produced electrolytically by the process, penetrates onto the grain boundaries of steel producing microcracks, blistering and loss of ductility. The atomic hydrogen combines into molecules, cannot escape, resulting in blistering and laminations.

Pitting is caused by electrolytic corrosion and is the result of galvanic attack at the anodes of the metal surface. This can be seen where local failures in the film on stainless steel may be anodic to the remainder of the surface. This is more commonly seen on aluminum, and most commonly on steel in heterogeneous soil.

In Dezincification, one constituent of an alloy, such as the zinc in brass or bronze, may be selectively removed, leaving a porous replica of the original part. Often the whole alloy is initially dissolved, with one element redeposited in spongy form.

Graphitization is most often seen as the electrolytic corrosion of cast iron, and often takes a form very similar to the dezincification of brass. Iron is removed selectively, leaving a replica composed of carbon or graphite.

Cathodic Corrosion. Although the cathode of an electrolytic cell is not in itself corroded as a direct result of the electrolytic process, the cathode may be attacked by the reaction products formed by the process. A typical example is the corrosion of lead in the very alkaline environment produced at the cathode of cathodically protected lead cable.

Corrosion Fatigue occurs when a metal is subjected to alternating stress and relief in a corrosive environment. Metal failure occurs much more rapidly under the alternating stress than under either stress or relief alone. Continuous removal of protective films, and the repeated exposures of clean metal by small stress failures cause corrosion fatigue.

Erosion Corrosion, as the name implies, occurs when the corrosion products which would normally afford a protective film are scoured off by moving fluids, particularly if the fluids contain abrasive materials. The erosion will expose clean metal, and develop a physical pattern so obviously a result of erosion that the corrosive factor may not be recognized.

Cavitation or Impingement Attack is a process which is very similar to erosion. In cavitation, collapsing gas bubbles in regions of turbulence and local pressure fluctuations may activate serious corrosion. Condenser tubes and pump impellers are subject to this type of attack.

Fretting Corrosion. Metal surfaces in close physical contact, in a corrosive environment, and subjected to vibration, can accelerate corrosion attack by the continuous removal of protective films. Machine parts with small relative motion and high unit loads are subject to fretting corrosion.

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