Inhibition, corrosion types

There are many temporary protectives on the market and it would be impracticable to describe them individually. However, they may be classified according to the type of film formed, i.e. soft film, hard film and oil film the soft film may be further sub-divided into solvent-deposited thin film, hot-dip thick film, smearing and slushing types. All these types are removable with common petroleum solvents. There are also strippable types based on plastics (deposited by hot dipping or from solvents) or rubber latex (deposited from emulsions) these do not adhere to the metal surfaces and are removed by peeling. In addition there are volatile corrosion inhibitors (V.C.I.) consisting of substances, the vapour from which inhibits corrosion of ferrous metals. 

Both factors are sensitive to alloy composition, which can be adjusted to produce electrodes having an acceptable cycle life. In AB5 alloys the effects of Ce, Co, Mn, and A1 upon cycle life in commercial AB5 -type electrodes are correlated with lattice expansion and charge capacity. Ce was shown to inhibit corrosion even though lattice expansion increases. Co and A1 also inhibit corrosion. XAS results indicate that Ce and Co inhibit corrosion though surface passivation. 

Water-soluble corrosion inhibitors are necessary to prevent corrosion of the pipe walls, joints, pumps, and collection stations. An ampholytic, substituted imidazoline has been described for inhibiting corrosion in such systems [297]. This type of corrosion inhibitor is intended for continuous treatment. 

Ascorbic acid has been declared useful in intravaginal contraceptives in reducing sperm motility (899,900). The preservation of human blood by the addition of sodium ascorbate (901) has been found to have some merit. The usefulness of ascorbic acid in the treatment of industrial chemical toxicity (902,903), in the preservation of cut-blooms in water (904,905), in water treatment of the ferruginous type (906), in inhibiting corrosion (907), in treatment for the production of durable creases in cloth (908), and in brightened yarns with high light fastness (909) is mentioned. Adhesive compositions (910) and special cleansers (911, 912) are other potential applications. 

Typical basic pigments include basic lead carbonate, basic lead sulfate, red lead, and zinc oxide. The soaps formed when these materials Interact, for example, with linseed oil, are oxidized in the presence of water and oxygen to form mono- and dibasic straight chain Cy to Cg acids. Materials of this type (e.g., sodium and calcium azelate and pelargonate) are known to inhibit corrosion. Inhibition is associated with formation of complex ferric salts that reinforce the oxide film. Lead salts act at lower concentration than the sodium or calcium salts (3, 41). 

The consensus is that organic compounds inhibit corrosion by adsorbing at the metal/solu-tion interface. Three possible types of adsorption are associated with organic inhibitors n-bond orbital adsorption, electrostatic adsorption, and chemisorptions. A more simplistic view of the mechanism of corrosion inhibitors can be described as controlled precipitation of the inhibitor from its environment (water and hydrocarbons) onto metal surfaces. During the past decade, the primary improvements in inhibitor technology have been the refinement of formulations and the development of improved methods of applying inhibitors (Totlani and Athavale 2000 Farquhar et al. 1994). 

Vapor phase inhibitors (VPIs), also called volatile corrosion inhibitors (VCIs), are compounds that are transported in a closed system to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds such as morpholine or octadecylamine are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide (Boles et al. 2009). Compounds of this type inhibit corrosion by making the environment alkaline. In closed vapor spaces, such as shipping containers, volatile solids such as the nitrite, carbonate, and benzoate salts of dicyclohexylamine, cyclohexylamine, and hexamethyleneimine are used. 

It is useful to mention here more specifically some of the additions that inhibit corrosion. Liddiard et al. (1943) reported that 0.(X)5 mg/L of zinc salts can retard corrosive action of some waters, and this type of self-inhibition may often be important in practice. Sanyal et al. (1959), studying atmospheric attack under laboratory conditions with low SO2, observed that corrosion on steel is reduced in the presence of zinc not in contact with steel. Presumably, that is, zinc ions are transmitted to the steel surface. 

Experience has shown that wherever possible, only one type of alloy or family of nonmetaUics should be exposed in a given test apparatus. If several alloys or nonmetaUics are exposed in the same apparatus the corrosion products from one material may affect the corrosion rate on another material. For example, copper corrosion products can inhibit corrosion of stainless steel but accelerate corrosion of aluminum. 

The ability of aminated compounds to inhibit corrosion on metallic surfaces via adsorption phenomena has been already certified. Since operations taking place at interfaces are greatly affected by variations in surface tension, aminated surfactant molecules are expected to provide even better results. This has been the case, when self-assembled micellar or microemulsion systems are used as corrosion inhibitors. In that aspect, surfactants may be used as organic corrosion inhibitors, and act by forming a protective film onto surfaces which are exposed to corrosive media, like oxygen and saline or acidic solutions. When microemulsions are used, an oil film is also adsorbed onto the surface with the surfactants tails oriented towards it, in view of the usually positive character of the surface. In the petroleum industry, the oil itself may be the nonpolar component of such systems. Figure 15.10 is a schematic of these types of films. 

Corrosion Inhibitive A type of metal paint or primer that prevents rust by preventing moisture from reaching the metal. Zinc phosphate, barium metaborate and strontium chromate (all pigments) are common ingredients in corrosion-inhibilive coatings. These pigments absorb any moisture that enters the paint film. 

Brass alloys containing less than 15% zinc are resistant to dealloying. Alloying elements such as tin and arsenic inhibit this type of corrosion. 

Scanning electron micrograph of a bacterial biofilm of the type that is inhibiting corrosion of underlying stainless steel... 

At higher concentrations, e.g. 1000-10 000 ppm, the inhibition efficiency with alloys 2024 and 7075 is reduced and the inhibited corrosion rate appears to be dependent on the type of R cation. These results have not been completely explained (Hinton et al. 1984, 1985), but may be related to (a) the nature of the protective film or (b) the observation that the observed R oxide films are thicker and more defective when formed in solutions containing > 1000 ppm R cation. Also, the La and Ce films tended to be powdery and have poor adhesion. Defective films would not be expected to provide good corrosion inhibition. 

Detergent Additives. Diesel engine deposits ate most troublesome in the fuel dehvery system, ie, the fuel pump and both fuel side and combustion side of the injectors. Small clearances and high pressures mean that even small amounts of deposits have the potential to cause maldistribution and poor atomization in the combustion chamber. The same types of additives used in gasoline ate used in diesel fuel. Low molecular weight amines can also provide some corrosion inhibition as well as some color stabilization. Whereas detergents have been shown to be effective in certain tests, the benefit in widespread use is not fully agreed upon (77). 

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