Local film damage corrosion

The problem with relying solely on anodic area corrosion inhibition is the risk of local film damage, which concentrates the corrosion current flow and permits a highly active anodic cell to be developed and causing accelerated corrosion to take place. This in turn leads to severe metal wastage, often in the form of deep pitting. 

Fig. 8.4. Siniulatioii of corrosion onset with periodic boundary conditions. (Top) Siiajishuts showing the local film damage at the indicated time moments. Blue corresponds to low, orange to high film damage. (Middle) Space/tiine diagram along the line marked with ab. (Bottom) Red line Acciimutatcd total number of pitting sites. Blue line Total current.

In typical cause-and-effect mode, where chlorides penetrate the deposit or where a localized overconcentration of hydroxyl ions occurs, the magnetite film is disrupted and particular forms of very damaging corrosion occurs. In addition, where localized heat flux exceeds design limits within a boiler and may be accompanied by departure from nucleate boiling (DNB) conditions, overheating and metal failure may also occur. 

Erosion corrosion is associated with a flow-induced mechanical removal of the protective surface film that results in subsequent corrosion rate increases via either electrochemical or chemical processes. It is often accepted that a critical fluid velocity must be exceeded for a given material. The mechanical damage by the impacting fluid imposes disruptive shear stresses or pressure variations on the material surface and/or the protective surface film. Erosion corrosion may be enhanced by particles (solids or gas bubbles) and impacted by multi-phase flows [29]. Increased flow stream velocities and increases of particle size, sharpness, density, and concentration increase the erosion corrosion rate. Increases in fluid viscosity, density, target material hardness, and/or pipe diameter tend to decrease the corrosion rate. The morphology of surfaces affected by erosion corrosion may be in the form of shallow pits or horseshoes or other local phenomena related to the flow direction. 

Copper-containing lead alloys undergo less corrosion in sulfuric acid or sulfate solutions than pure lead or other lead alloys. The uniformly dispersed copper particles give rise to local cells in which lead forms the anode and copper forms the cathode. Through this anodic corrosion of the lead, an insoluble film of lead sulfate forms on the surface of the lead, passivating it and preventing further corrosion. The film, if damaged, rapidly reforms. 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

Vaporous cavitation can remove protective films, such as oxides, from metals and so initiate corrosion . In addition, the very high local pressures and temperatures associated with the final stage of cavity collapse can induce chemical reactions that would not normally occur. Thus certain additives are damaged by cavitation and their decomposition products can be corrosive. 

Electrochemical and nonelectrochemical ways to protect metals against corrosion can be distinguished. The nonelectrochemical ways include dense protective films that isolate the metal against effects of the medium and may be paint, polymer, bitumen, enamel, and the like. It is a general shortcoming of these coatings that when they are damaged mechanically, they lose their protective action, and local corrosion activity arises. 

Chemically, the film is a hydrated form of aluminum oxide. The corrosion resistance of aluminum depends upon this protective oxide film, which is stable in aqueous media when the pH is between about 4.0 and 8.5. The oxide film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface film is rapidly repaired. The conditions that promote corrosion of aluminum and its alloys, therefore, must be those that continuously abrade the film mechanically or promote conditions that locally degrade the protective oxide film and minimize the availability of oxygen to rebuild it. The acidity or alkalinity of the environment significantly affects the corrosion behavior of aluminum alloys. At lower and higher pH, aluminum is more likely to corrode. 

Erosion corrosion results in an increased rate of corrosion attack attributable to the velocity of a corrodent over the exposed surface. The movement of the corrodent can be associated with mechanical wear. The increased corrosion is usually related to the removal or damage of a protective surface film. The mechanism is usually identified by localized corrosion, which exhibits a pattern that follows the flow of the corrodent. 

Fretting corrosion is a specialized form of erosion corrosion where two metal surfaces are in contact and experience very slight relative motion that causes damage to one or both surfaces. Again, in the presence of a corrodent, the movement causes mechanical damage of the protective film, leading to localized corrosion. 

Rupture of Organic Protective Films This condition differs from other causes of localized corrosion since these protective films are nonconductors and, as such, do not support the cathodic reaction. After the rupture in the coating, corrosion may progress under the coating by crevice corrosion mechanisms, resulting in further damage. 

The resistance of many metals and alloys to corrosion depends critically upon the presence of a thin (10-1000 A [168]) passive surface film [169]. In "aggressive environments, this film may become damaged locally via several processes, e.g. surface stress effects (either flow-induced [170,171] or as a result of anion adsorption [168]), the impingement of small particles on the surface [169], spontaneous depassivation [169]. Retention of the protective film by the metal only results if repassivation of the unprotected area is feasible compared with pit growth. 

Slug flow is the dominant flow regime in multiphase systems. Flow visualization has shown that bubbles distort and elongate in the vicinity of a pipe wall in a manner similar to collapsing bubbles. The corrosion rate increases because of a thinning of the mass transfer and corrosion product layers, as well as because of localized damage of the corrosion product film. 

Figure 7.58 pFl-potential regions in which mild steel is liable to SCC in different environments. Note that there is a strong tendency to SCC in the regions where a protecting film is unstable (i.e. if the film is damaged locally, corrosion can... 

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