Anticorrosion Protection

a complex of methods for protecting metals, alloys, and metallic products and structures from attack by corrosion. Anticorrosion protection must be provided for the protection of metal products at all stages of production and service from the design of the object and smelting of the metal to the transportation and storage of finished products, installation of metallic structures, and service of the product and structures. Corrosion losses account for some 12 percent of the quantity of metal smelted annually. Corrosion of metals leads not only to irreplaceable losses in metals but also to premature malfunctioning of expensive and crucial products and structures, impairment of production processes, and downtime of equipment. Corrosion is responsible for accidents and breakdowns in some cases.

The need to protect metals from corrosion arose with the appearance of the very first metallic products made from copper and iron. Hot-dip tinning, vegetable oils, and corrosion-resistant alloys (tin bronze, brass) were used even in ancient times to protect copper; polishing, blueing, and tinning were used to protect iron. The electrochemical method of anticorrosion protection, using sacrificial anodes (protectors), was discovered in the early 19th century. The possibility in principle of producing metallic coatings by the electrolytic method was established in the mid-19th century. Anticorrosion protection underwent more intensive development in the 20th century with the invention of stainless steels, new corrosion-resistant alloys, polymeric coatings, and so forth. The system of anticorrosion protection utilized is determined by the service conditions and by the mechanism underlying the corrosion (that is, electrochemical or chemical). All anticorrosion protection techniques can be divided into two basic groups depending on the mechanism operative: electrochemical methods, affecting the potential of the metal or its critical values; and mechanical methods, isolating the metal from the effects of the surroundings by means of a protective film or coating.

Among the basic methods of anticorrosion protection are alloying, heat treatment, inhibition of the environment, de-aeration of the medium, water pretreatment, protective coatings, and the creation of a microclimate and protective atmosphere.

Alloying. Alloying, in cases of electrochemical corrosion, brings the metal from an active state to a passive state, with the formation of a passive film exhibiting excellent protective qualities. For example, alloying of iron with chromium makes it possible to convert the iron to a stable passive state and to create a whole class of alloys known as stainless steels. Additional alloying of the stainless steels with molybdenum eliminates the proclivity to pitting corrosion in seawater. Alloying titanium with a slight amount of palladium drastically increases corrosion resistance in aggressive, weakly oxidative media. Alloying also protects steels and alloys from structural corrosion.

Heat treatment of metals. Heat treatment removes structural heterogeneity capable of promoting selective corrosion and relieves internal stresses in alloys, thereby eliminating their proclivity to intergranular and pitting corrosion as well as stress corrosion (for example, in austenitic stainless steels containing no titanium or niobium, in aluminum alloys, and in martensitic low-alloyed and stainless steels).

Inhibition of the medium. Corrosion inhibitors, which are introduced into the aggressive medium in small amounts and which set up an adsorptive film on the surface of the metal, thereby slowing down the electrode processes and altering the electrochemical parameters of the metals, are widely utilized in the fight to control corrosion.

Deaeration and water pretreatment. The presence of oxygen and aggressive anions, particularly chlorine ions, severely shortens the service life of power plants having crucial parts exposed to water because of the corrosion which brings about corrosion cracking in some cases. The stationary potential and the values of critical potentials and critical currents of the metals are altered as a result of deaeration and water pretreatment.

Protective coatings. Protective coatings are widely used in anticorrosion protection. They are divided into metallic (pure metals and their alloys) and nonmetallic coatings. Depending on the potential of the metal, the coatings can be either anodic or cathodic with respect to the metal to be protected. Anodic coatings diminish or totally eliminate corrosion of the parent metal in the pores of the coating—that is, they bring about electrochemical protection against corrosion because of the shift in potential—whereas cathodic coatings may intensify corrosion of the parent metal in the pores; they are nevertheless used frequently, since they enhance the physicomechan-ical properties of the metal such as wear resistance and hardness. However, cathodic coatings require greater thickness and, in some cases, additional protection against corrosion.

Metallic coatings can also be distinguished according to the method of obtaining them. Galvanic coatings, chemical methods of precipitating metals by reduction from aqueous solutions of salts, and the hot method of applying coatings of zinc, tin, and aluminum melts are widely used, particularly in machine-building. The hot method is resorted to principally in metallurgy on high-productivity automated production lines for hot galvanizing, tinning, and aluminizing. A similar technique is the thermodiffusion surface alloying of steels by plating the steels with chromium, aluminum, silicon, or zinc, with the object of enhancing the refractory behavior and corrosion resistance of the steels in corrosive environments (such as diffusion metallization, aluminizing, and siliconizing). Nitriding is also included among the thermodiffusion coating processes. Electroplating from molten salt baths is also common, thereby combining cathodic precipitation of metals with thermodiffusion coating processes and making it possible to obtain coatings with excellent protective and adhesive properties. Plating—a thermomechani-cal method of applying thin layers of corrosion-resistant metal—is also widely used. Metallized coatings are very convenient in the fabrication and finishing of oversize components and structures. Plasma spraying and precipitation from the vapor phase are used to apply coatings of refractory metals. Vacuum metallization of products by condensing metal vapor in a vacuum onto the metallic surface to be protected is in use. Layers of aluminum, cadmium, and other metals can be precipitated in varying thicknesses by such a method.

Inorganic coatings consisting of oxide, phosphate, chromate, fluoride, or other complex inorganic compounds are also utilized in anticorrosion protection. Inorganic coatings are applied by chemical and electrolytic techniques (oxidizing, phosphatizing, passivation, and anodizing). These methods are also utilized to enhance the protective properties of galvanic coatings. Inorganic coatings produced by the hot method include enameling, which is widely used in the fabrication of household appliances and for protection of metals from vapor corrosion at high temperatures. Nonmetallic and combined oxide-metallic coatings are applied by the method of electrophoresis (electrophoretic coatings). Protective lubricants are used where close tolerances and tight fits are mandatory, where coatings cannot be applied, and to provide additional protection; however, they are effective only if they are renewed periodically.

Electrochemical methods of corrosion prevention are used in the case of seagoing vessels, underground structures, and hydraulic-engineering structures, as well as for chemical equipment operating with corrosive and electrically conducting media. Cathodic or anodic polarization from an extraneous current source, or the connection of sacrificial anodes to the structure to be protected, is helpful in shifting the potential of the metal to values at which corrosion is either terminated completely or severely inhibited.

Various nonmetallic coatings are also in widespread use in corrosion protection service; these include paints and varnishes, plastic, and rubber. Paint coats exhibit excellent protective properties, are inexpensive, and can be renewed while the equipment is in service. Plastic coatings of polyethylene, polyisobutylene, fluoroplastic, nylon, polyvinyl chloride, and other plastics having high resistance to attack by water, acid, or alkali are more and more widely used. Many plastics are now in use as lining materials for chemical equipment and plating baths (polyvinyl chloride laminate, faolit, and so on). Sealing polymeric compounds and resins are utilized in the protection of radio and electronics components. Rubber base coatings (rubber linings) provide effective protection against attack by acids and other reagents.

Underground structures such as piping are protected against corrosion by bitumens and asphalts, as well as polymeric strips and enamels; they are protected against stray currents by means of drainage, which draws the currents away from the structure.

Long-term corrosion proofing is used for prolonged storage and transportation of metallic products and spare parts. Protective atmospheres are used in heat treatment and hot processing of readily oxidizable metals in order to protect them from vapor corrosion (for example, welding of metals in argon or nitrogen).

Proper design plays a major role in protecting structures against corrosive attack. Corrosion-minimizing design can be helpful in eliminating sites on structures which are particularly vulnerable to corrosion (slats, clearances, dead pockets), in averting unfavorable contacts between unlike metals that may enhance corrosion, in insulating such sites of contact and in eliminating sudden impacts of the surroundings on the structure, and so on.