Corrosion, prevention

Corrosion Inhibitors. A water-soluble corrosion inhibitor reduces galvanic action by making the metal passive or by providing an insulating film on the anode, the cathode, or both. A very small amount of chromate, polyphosphate, or silicate added to water creates a water-soluble inhibitor. A slightly soluble inhibitor incorporated into the prime coat of paint may also have a considerable protective influence. Inhibitive pigments in paint primers are successful inhibitors except when they dissolve sufficiently to leave holes in the paint film. Most paint primers contain a partially soluble inhibitive pigment such as zinc chromate, which reacts with the steel 

Generally, electroplated coatings that are completely free of pores and other discontinuities are not commercially feasible. Pits eventually form at coating flaws, and the coating is penetrated. The resulting corrosion cell is shown in Fig. 12. The substrate exposed at the bottom of the resulting pit corrodes rapidly. A crater forms in the substrate, and because of the 

12 Crater formation in a steel substrate beneath a void in a noble metal coating, for example, passive chromium or copper. Corrosion proceeds under the noble metal, the edges of which collapse into the corrosion pit. 

13 Corrosion pit formation in a substrate beneath a void in a duplex noble metal coating. The top coating layer (M,) is cathodic to the coating underlayer (Mj), which is in turn cathodic to the substrate (Mj). As in Fig. 12, the coating tends to collapse into the pit 

Cathodic protection involves the reversal of electric current flow within the corrosion cell. Cathodic protection can reduce or eliminate corrosion by connecting a more active metal to a metal that must be 

Corrosion reactions may be minimized by essentially two means. Firstly, by covering the surfaces of metals with protective films and secondly, by exploiting inhibition processes. Steel, for example, may be protected by surface layers of chromium, nickel, zinc or tin. Cracks in a surface film of a more noble metal than the one being offered protection, can give rise to local cells in which the exposed base metal becomes an anode and the protective layer a cathode. Local corrosion then sets in. 

On the other hand, if the protective layer is of a less noble metal, the base material becomes cathodic in any region where there is a crack or fault, favouring metal deposition. This constitutes what has become known as cathodic protection and is employed extensively in the protection from corrosion of metals in marine use. 

Oxide layers formed by some metals in the atmosphere are most useful if they are physically tough and damage resistant. In any case, if damage should occur, the protective layer rapidly reforms. Thicker coatings may be obtained by anodizing, i.e. by anodically polarizing the material by electrolysis with some cathode in a suitable electrolyte. 

Water environments can also have a variety of compositions and corrosion characteristics. Freshwater normally contains dissolved oxygen as well as minerals, several of which account for hardness. Seawater contains approximately 3.5% salt (predominantly sodium chloride), as well as some minerals and organic matter. Seawater is generally more corrosive than freshwater, frequently producing pitting and crevice corrosion. Cast iron, steel, aluminum, copper, brass, and some stainless steels are generally suitable for freshwater use, whereas titanium, brass, some bronzes, copper-nickel alloys, and nickel-chromium-molybdenum alloys are highly corrosion resistant in seawater. 

Soils have a wide range of compositions and susceptibilities to corrosion. Compositional variables include moisture, oxygen, salt content, alkalinity, and acidity, as well as the presence of various forms of bacteria. Cast iron and plain carbon steels, both with and without protective surface coatings, are economical for undergroimd structures. 

Because there are so many acids, bases, and organic solvents, no attempt is made to discuss these solutions in this text. Good references are available that treat these topics in detail. 

Some corrosion prevention methods were treated relative to the eight forms of corrosion however, only the measures specific to each of the various corrosion types were discussed. Now, some more general techniques are presented these include material selection, environmental alteration, design, coatings, and cathodic protection. 

Perhaps the most common and easiest way of preventing corrosion is through the judicious selection of materials once the corrosion environment has been characterized. Standard corrosion references are helpful in this respect. Here, cost may be a significant factor. It is not always economically feasible to employ the material that provides the optimum corrosion resistance sometimes, either another alloy and/or some other measure must be used. 

Since the invention of metal technology several thousand years ago, the prevention of metal corrosion has been a problem of fundamental practical importance. To try to solve corrosion problem, several methods can be adopted, depending on environment aggressiveness, corrosion heaviness and materials costs. In this section, some of the most diffuse protection methods are described, in order of efficacy and impact on corroding systems (environments, type and aspect of materials). 

Modification of environment Since corrosion is caused by chemical interactions between metal and ionic species in the surrounding environment, removing metal from, or changing, the type of environment the metal deterioration can be reduced. For example, the contact of metal with rain or seawater can be limited or the content of sulphur, chloride or oxygen can be reduced. 

Passivation. Passivation refers to a material becoming passive. In dry air and at room temperature some non-noble metals form a thin primary oxide film of different properties, structure and thickness on the surface. If this oxide film is of low solubility, compact and free of pores, then the corrosion rate can be reduced by some orders of magnitude. This phenomenon is called passivation and the oxide film is called passive film. For some metals, like chromium, the passive film is very thin (some nanometres) and can become conducting. For another group of metals, like titanium and aluminium, the oxide layer can get thicker (micrometers) and blocks further electrode processes. 

Moreover, some materials form spontaneously their passive layer, whereas other are passivated under special oxidising conditions. A special example is iron in contact with concentrated nitric acid. Although iron dissolves in non-oxidising acids, it is stable in nitric acid and shows 

Corrosion inhibitors. They are chemicals that react with the metal surface or the environmental gases causing corrosion, thereby interrupting the chemical reactions that cause corrosion. Inhibitors can work by adsorbing themselves on the metal surface, reducing the rate of either metal dissolution (anodic inhibition) or the rate of the counter reaction, e.g. oxygen 

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Geiiings P J 1976 introduotion to Corrosion Prevention and Controi for Engineers Nijgh-Woiters-Noordhoff Universitaire Uitgevers Rotterdam... 

Munger C G 1984 Corrosion Prevention by Protective Coatings (Houston, TX National Association of Corrosion Engineers)... 

These fibers may find use in controUed release of dmgs, bactericides, and corrosion prevention chemicals (103). Fibers with different active groups have been made for sorption of chemicals. These fibers are designed to replace granular sorbents for air purification, for example, in air filtration masks (104). 

One of the first attempts to produce polyurethane was from the reaction of an intermediate polyol of 1,3- and l,4-bis(hydroxyhexa uoroisopropyl)benzene m- and -12F-diols) by reaction with epichlorohydrin. This polyol was subsequentiy allowed to react with a commercial triisocyanate, resulting in a tough, cross-linked polyurethane (129,135,139). ASTM and military specification tests on these polyurethanes for weather resistance, corrosion prevention, bUster resistance, and ease of cleaning showed them to compare quite favorably with standard resin formulations. 

Corrosion. Anticorrosion measures have become standard ia pipeline desiga, coastmctioa, and maintenance ia the oil and gas iadustries the principal measures are appHcation of corrosion-preventive coatings and cathodic protection for exterior protection and chemical additives for iaterior protectioa. Pipe for pipelines may be bought with a variety of coatiags, such as tar, fiber glass, felt and heavy paper, epoxy, polyethylene, etc, either pre-apphed or coated and wrapped on the job with special machines as the pipe is lowered iato the treach. An electric detector is used to determine if a coatiag gap (hoHday) exists bare spots are coated before the pipe is laid (see Corrosion and corrosion control). 

Replacing one carbon atom of naphthalene with an a2omethene linkage creates the isomeric heterocycles 1- and 2-a2anaphthalene. Better known by their trivial names quinoline [91-22-5] (1) and isoquinoline [119-65-3] (2), these compounds have been the subject of extensive investigation since their extraction from coal tar in the nineteenth century. The variety of studies cover fields as diverse as molecular orbital theory and corrosion prevention. There is also a vast patent Hterature. The best assurance of continuing interest is the frequency with which quinoline and isoquinoline stmctures occur in alkaloids (qv) and pharmaceuticals (qv), for example, quinine [130-95-0] and morphine [57-27-2] (see Alkaloids). 

Corrosion prevention (liners, coatings, cathodic protection)... 

In most cylindrical carbon—zinc cells, the zinc anode also serves as the container for the cell. The zinc can is made by drawing or extmsion. Mercury [7439-97-6J has traditionally been incorporated in the cell to improve the corrosion resistance of the anode, but the industry is in the process of removing this material because of environmental concerns. Corrosion prevention is especially important in cylindrical cells because of the tendency toward pitting of the zinc can which leads to perforation and electrolyte leakage. Other cell types, such as flat cells, do not suffer as much from this problem. 

Arnold, J.R., In Proc. Automotive and Corrosion Prevention Conference. Society of Automotive Engineers, Warrendale, PA, 1986, p. 21. 

One of the reasons why it is important to remove suspended solids in water is that the particles can act as a source of food and housing for bacteria. Not only does this make microbiological control much harder but, high bacteria levels increase the fouling of distribution lines and especially heat transfer equipment that receive processed waters (for example, in one s household hot water heater). The removal of suspended contaminants enables chemical treatments to be at their primary jobs of scale and corrosion prevention and microbial control. 

In the above discussion only the drillstem was discussed. However, when designing other systems in drilling operation, such as drilling fluid systems and casing design to name a few, the engineer must also consider corrosion prevention. 

Temporary corrosion preventives are products designed for the short-term protection of metal surfaces. They are easily removable, if necessary, by petroleum solvents or by other means such as wiping or alkaline stripping. Some products for use in internal machine parts are miscible and compatible with the eventual service lubricant, and do not, therefore, need to be removed. 

Hence, the hot-dip compounds, or greases smeared cold, are better for assemblies with non-metallic parts masked if necessary. Solvent-containing protectives therefore find greater application in the protection of simple parts or components. The available means of application, the nature of any additional packaging and the economics and scale of the protective treatment are further factors that influence the choice of type of temporary corrosion preventive. 

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