THE VARIOUS TYPES OF CORROSION There are many types and causes of corrosion and dependingon a particular scientist's or corrosion engineer's tendencies tolook upon the subject more broadly or narrowly there is no clearand exact total for the number of corrosion types. Most corrosionexperts however can agree on the basic varieties of corrosionlisted below.

ATMOSPHERIC: Atmospheric corrosion and its severity isessentially determined by four variables: 1. air pollution (both man made and natural such as volcanicgases)2. airborne salt spray or droplets3. temperature4. and moisture. The presence of industrial pollutants in the polar ice capsdemonstrates that even the most remote corners of the Earth arenot immune to its effects. Nitrogen and sulfur compounds canform acids when discharged or acidic material may be releasedto the atmosphere directly by some industries accelerating thecorrosion process on metal these substances encounter. Ofcourse urban areas and those areas in close proximity to ordownwind from "smokestack type" industries and power plantsare subject to the most corrosive effects of airborne pollution. Salt spray and airborne saltwater droplets introduce chloride ionsto metal surfaces with a corresponding deleterious effect on themetal. But one does not have to live on the beach to experiencethese corrosive effects as it is estimated that airborne sea spraysaltwater micro droplets can carry as much as 7 miles inlandfrom the coast. The presence of moisture is an absolutenecessity for most corrosion processes and when combined withelevated temperatures and salt or pollutants further enhances

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the atmospheric corrosion process. Atmospheric corrosion is ubiquitous and is responsible for moremetal damage (both on a cost accounting basis and an actualquantitative basis) then any other form of environmentalcorrosion. After all, everything on the Earth's surface is exposedto the atmosphere be it vehicles, buildings, bridges, etc.

GALVANIC: When dissimilar metals are connected in thepresence of an electrolyte a galvanic corrosion reaction occurs.Both the presence of an electrolyte solution and a bi-metalliccoupling is required for this type of corrosion to occur. Electrolytes are electrically conductive solutions such as seawater or road salt spray which contain chloride or rain, mist ordew containing sulfur or nitrogen compounds. The most commonsources of chloride contamination are marine and coastalenvironments as well as winter road salting spray. Sulfur andnitrogen compounds are more ubiquitous being the product of airpollution and industrial fallout. When two dissimilar metals are connected electrically through anelectrolyte or salt bridge one metal will become anodic withreference to the other and the potential difference will create acorrosion current causing the more anodic metal to corrode.

UNIFORM/GENERAL: When corrosion occursuniformly over a wide area of the metal surface producing ageneral thinning of the metal leading to eventual failure. The rateof general corrosion is usually relatively predictable.

PITTING: Pitting corrosion is quite often evident on metalsurfaces where no uniform corrosion is present and is usuallyhighly localized. Pitting usually proceeds fairly rapidly and isaccelerated by the presence of chlorides and is particularlycommon at the base of breaks in coatings. Pitting corrosiontakes advantage of the different metallurgical phases present onthe surface of most common modern alloys. Generally it isconsidered to be the product of localized anodic dissolutionwhere the anodic portion of the corrosion cell is dwarfed by thelarger cathodic portion.

STRESS CORROSION CRACKING : Stresscorrosion is the product of tensile stress (including residual

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stress remaining after fabrication) and localized corrosion whichcombine to produce a brittle cracking of metal under certainconditions. Examples of environments which enhance stresscorrosion are high pH amine solutions for most common steelsand chloride bearing solutions for most stainless steels as wellas certain aluminum alloys.

CREVICE CORROSION: Crevice corrosion occurs insheltered, localized areas such as crevices, joints, bolted andthreaded parts and under existing corrosion deposits. It is theresult of concentration of salts, acids and moisture which resultsin the formation of an occluded corrosion cell in such shelteredareas. A small anode is created in the crevice with the remainderof the body acting as a large cathode so corrosion at the creviceis highly accelerated as well as concentrated.

HIGH TEMPERATURE: Under very high temperaturemetals need only the presence of oxygen or other oxidizinggases to corrode. This type of corrosion is referred to as hightemperature oxidation, scaling or tarnishing and requires neithermoisture nor dissolved electrolytes (salts, acids) to proceed. Again there are many types of corrosion but these listed aboverepresent some of the most common and destructive forms aswell as some of the most unique.

See Also;

A Recipe for Rust

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