Corrosion control barriers

Alkenylsuccinic anhydrides made from several linear alpha olefins are used in paper sizing, detergents, and other uses. Sulfosuccinic acid esters serve as surface active agents. Alkyd resins (qv) are used as surface coatings. Chlorendric anhydride [115-27-5] is used as a flame resistant component (see Flame retardants). Tetrahydrophthalic acid [88-98-2] and hexahydrophthalic anhydride [85-42-7] have specialty resin appHcations. Gas barrier films made by grafting maleic anhydride to polypropylene [25085-53-4] film are used in food packaging (qv). Poly(maleic anhydride) [24937-72-2] is used as a scale preventer and corrosion inhibitor (see Corrosion and corrosion control). Maleic anhydride forms copolymers with ethylene glycol methyl vinyl ethers which are partially esterified for biomedical and pharmaceutical uses (189) (see Pharmaceuticals). 

The present chapter begins with a brief overview of metallic corrosion and mechanisms of corrosion control. Methods of evaluating polymer performance and electrochemical characterization techniques are discussed. Barrier and adhesion aspects of corrosion control are reviewed, and some critical issues needing further study are outlined. 

Corrosion can be controlled by Isolation of the metal from the corrosive environment by suppression of the anodic dissolution of metal and by suppression of the corresponding cathodic reaction. Isolation of corrosion prone metals from corrosive environments is probably the most general mechanism of the corrosion protection afforded by paint films, sealers, and similar polymer-based materials. Effective isolation requires that polymeric materials have good barrier properties and remain adherent in the presence of water and the products of metallic corrosion. Barrier properties and adhesion aspects of corrosion control are discussed in detail in subsequent sections. 

Parylenes can be used for contamination control, that is, securing small particles to prevent them from damaging a surface in a sealed unit barrier coaling coaling for corrosion control and as dry lubricants. 

A paint consists of (i) vehicle (ii) pigment and (iii) additives or fillers. A vehicle renders the fluidity to the paint and upon drying leaves a film. A pigment suspended in the vehicle offers the corrosion protection. Additives or fillers enable the dry coating to withstand the environment. Corrosion control by barrier coatings may be envisaged as shown in Figure 1.64. 

The polymeric materials are used in (i) barrier applications (ii) self-supporting structures (tanks, piping, valves and pumps) (ii) column internals, seals, gaskets, adhesives and caulkings. An overview of polymeric materials used in corrosion control applications is given in Figure 4.8. 

Corrosion Control. Surfaces that become wetted by a lubricant and its additives are typically much less prone to corrosive damage from water, acids, bacteria, and other similar corrosion agents. Additives can neutralize acids as well as form a barrier film, which repels water and other chemically aggressive contaminants. 

There are two principal ways in which materials of construction may be protected from aggressive conditions in liquids and include corrosion inhibitors and controlled scaling. In general the protection arises from the imposition of an electrical barrier on the metal likely to suffer corrosion, so that the electrical circuit is broken or the flow of ions and electrons restricted. Some systems have "natural" corrosion control. A thin oxide of the metal or the constituents of an alloy, in fact the products of corrosion, act as an effective electrical barrier provided that a... 

The terminology anodic and cathodic inhibitors is based on these functions. Anodic protection prevents or limits electron flow to the cathode area. Cathodic inhibitors generally reduce the corrosion rate by forming a barrier at the cathode thereby restricting the hydrogen ion or oxygen transport to the cathode surface. Tables 14.6 and 14.7 provide some information on common corrosion inhibitors. Specific corrosion control requirements are usually based on blends of two or more of the listed chemicals perhaps, in addition to chemicals to control scale formation and biological activity. 

Paint and galvanized steel (steel coated with zinc) can be used together to provide superior corrosion control than either method used alone. This technique, known as a duplex system, ccmsists of painting steel that has been hot-dip galvanized after fabrication [12], The paint serves as a barrier protecting the galvanized steel from the atmosphere. It slows down the rate at which zinc is consumed and therefore extends the life of the galvanized steel. 

The problem with ordinary carbon steel is that the oxide formed does not stick very tightly to the metal surface. The essence of corrosion control then is to manipulate process parameters so that a protective barrier of products of corrosion does adhere to the surface of carbon steel vessels and piping. 

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