Concentration CellThe existence of anodic and cathodic siteson the surface of a piece of metal implies that differencesin electrical potential are found on the surface. These potential differences have a number of causes. One important mechanism is oxygen concentration cellcorrosion, in which the oxygen concentration in the electrolytevaries from place to place. An underground pipe that passes from clay to gravel will have a high oxygen concentration in the gravel region and almost no oxygen in the impermeableclay. The part of the pipe in contact with the clay becomes anodicand suffers damage.Corrosion Forms What is pitting corrosion1. Localized chemical or mechanical damage to the protective oxide film; water chemistry factors which can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations.2. Corrosion of a metal surface, confined to a point or small area, that takes the form of cavities.3. Ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss.4. An empirical relationship to predict the pitting resistance of austenitic and duplex stainless steels. What is galvanic corrosion?1. Galvanic corrosion (also called ' dissimilar metal corrosion' or wrongly 'electrolysis') refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte.2. A localized form of corrosion usually associated with a stagnant solution on the micro-environmental level.3. An acceleration in the rate of corrosion attack in metal due to the relative motion of a corrosive fluid and a metal surface.4. Corrosion damage induced under load and in the presence of repeated relative surface motion. What is pack rust?1. Pack rust is often used in relation to bridge inspection to describe built-up members of steel bridges which are already showing signs of rust packing between steel plates.2. Powdered rust particles relatively dense that are commonly used as additives to asphalt to improve the anti sliding properties of the pavement.3. Pack rust is a form a corrosion typical of steel components that develop a crevice into an open atmospheric environment.4. Rust settling as crud in boilers and other devices where the evaporation of water combined with normal corrosion of vessels leads to such accumulation of debris. What is dealloying?1. Dealloying is the selective leaching of one element from an alloy by corrosion processes.2. Dezincification of unstabilized brass, whereby a weakened, porous copper structure is produced.3. Corrosion attack usually related to the segregation of specific elements or the formation of a compound in the grain boundary regions.4. Graphitic corrosion of the porous graphite network when a gray cast iron is impregnated with insoluble corrosion products. What can cause stress corrosion cells? 1. General thinning.2. Regions within a metal subject to a high local stress.3. Cold work4. Grain boundaries.Getting to know the Corrosion DoctorsThe main mission of this popularWeb site is to improve the general awarenessof corrosion causes and solutions. Corrosion is often perceived as a cursewe have to endure helplesslyand it is a shock to many to learn that there are many ways to prevent and controlthis natural force. The Corrosion Doctors' educational focus is centered on a series of Information Modulesdesigned for training in corrosion science and engineering.What is Corrosion ?Corrosion is the destructiveattack of a material by reaction with its environment. The serious consequencesof the corrosion process have become a problem of worldwidesignificance. In addition to our everyday encounters with this form of degradation, corrosion causes plant shutdowns, waste of valuable resources, loss or contamination of product, reduction in efficiency, costly maintenance, and expensive overdesign. It can also jeopardize safety and inhibit technologicalprogress.Atmospheric CorrosionAtmospheric corrosion is an electrochemicalprocess, requiring the presence of an electrolyte. Thin film "invisible" electrolytes tend to form on metallic surfaces under atmospheric corrosion conditions, when a certain critical humiditylevel is reached. For iron, this level is around 60%, in unpolluted atmospheres. The critical humiditylevel is not a constant, it depends on the corroding material, the hygroscopic natureof corrosion products and surface depositsand the presence of atmospheric pollutants.

Galvanic Series in Flowing Seawater

Galvanic Series in Soil

The science of cathodic protection (CP) was born in 1824, when Sir Humphrey Davymade a presentation to the Royal Society of London: "The rapid decay of the copper sheeting on His Majesty'sships of war, and the uncertainty of the time of its duration, have long attracted the attention of those persons most concerned in the naval interest of the country. ... I entered into an experimental investigation upon copper. In pursuing this investigation, I have ascertained many facts ... to illustrate some obscure parts of electrochemicalscience... seem to offer important application." Davysucceeded in protecting copper against corrosion from seawater by the use of ironanodes. From that beginning, CP has grown to have many uses in marineand undergroundstructures, water storagetanks, gas pipelines, oil platformsupports, and many other facilities exposed to a corrosiveenvironment. Recently, it is proving to be an effective method for protecting reinforcingsteel from chloride-induced corrosion.