Localized corrosion protective coatings

Hematite is found in large quantities in the vicinity of Malaga in Spain (Spanish red) and near the Persian Gulf (Persian red). The Spanish reds have a brown undertone. Their water-soluble salt content is very low and their Fe203 content often exceeds 90 %. The Persian reds have a pure hue, but their water-soluble salt content is disadvantageous for some applications. Other natural hematite deposits are of only local importance. A special variety occurs in the form of platelets and is extracted in large quantities in Karnten (Austria). This micaceous iron oxide, is mainly used in corrosion protection coatings. 

Corrosion susceptibility in aqueous media is assessed on the basis of the rating numbers [3, 14], which are different from those of soils. An increased likelihood of corrosion is in general found only in the splash zone. Particularly severe local corrosion can occur in tidal regions, due to the intensive cathodic action of rust components [23, 24]. Since cathodic protection cannot be effective in such areas, the only possibility for corrosion protection measures in the splash zone is increased thickness of protective coatings (see Chapter 16). In contrast to their behavior in soils, horizontal cells have practically no significance. 

Coatings of more noble metals than the substrate metal (e.g., Cu on Fe) are only protective when there are no pores. In other cases severe local corrosion occurs due to cell formation (bimetallic corrosion). Cathodic protection is theoretically possible. This protection combination is not very efficient since the coating usually consumes more protection current than the uncoated steel. 

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. 

Overcoats are an integral part of thin-film disk structures. Their primary role is to provide wear protection. The most common overlayers, such as Rh, plasma-polymerized coatings, SiOz, and carbon, are all chemically stable if they were fully to cover the disk surface, they would provide good corrosion resistance. The thinness of the overcoats and the roughness of the surface preclude perfect coverage and open up the path for localized corrosion at the sites where the magnetic layer is exposed to the environment. 

Protective coatings of paint or plastic resins are a familiar means of corrosion. control by cutting off access of water, air, and electrolytes to the structure. The coating must, however, be complete and remain intact to be protective. Local penetration of the coating will generally create an... 

Locus of failure studies 75 80) on metal/epoxy joints that had been exposed to water indicate that corrosion of the metal substrate does not occur until after interfacial failure has occurred. This suggests that corrosion itself does not play a primary role in the loss of adhesion strength mechanism of metal/epoxy joints, but rather is a post-failure phenomenon. However, for the case of metal/epoxy protective coating systems, Leidheiser and coworkers 88-91 -92) and Dickie and coworkers 5 87-89-90> have proposed that localized corrosion processes are part of an important delamination mechanism. 

Coatings and localized corrosion. This type of corrosion can occur where protective coatings are applied over metal and where there is a break in the coating so that the large coated area acts as a cathode, and the small defective area as the anode.26... 

Perform nondestructive testing, inspection and maintenance programs to avoid SCC precursors, such as a concentration of stresses by localized corrosion. In case of protective coatings, routine maintenance is essential since scratches could create favorable sites for initiation of SCC. 

We have learned to contain corrosive media much more successfully in the last 20 to 30 years. This can be attributed to the many-fold increase in knowhow in metallurgy, in protective coatings and in the field of plastics. The need for overall protection of the structure or building has evolved into a more localized protection and is needless to say, more sophisticated. 

Localized Corrosion Induced by Rupture of Otherwise Protective Coatings... 

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. 

One of the reasons for local corrosion at the metal-polymer interface is sorption of electrolytes by polymers and permeability of the polymer barrier towards electrolytes. Sorption of electrolytes (acid solutions, bases and salts) leads to essential variation in the service characteristics of the protecting polymer coatings and anticorrosion packaging films under mechanical loads. These variations under mechanical loads, especially in seals and friction joints, are much deeper and can affect mechanisms of contact interactions. 

Blistering by volume expansion due to swelling Organic coatings for corrosion protection typically absorb water up to 3 or 4% if this absorption occurs locally, bKsters may form because of accumulation of water. This mechanism is however unimportant when compared with other mechanisms. 

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