Everybody knows how widely steel and iron products are used. But almost in all applications regardless of the environment, these products are coated by some means in order to prevent corrosion and make possible lasting beauty. Among different types of coatings organic coatings comprising paints, lacquers and varnishes are most widely used. Organic coatings protect steel from corrosion by forming a corrosion-resistant barrier between steel and the corrosive environment, but corrosion protection offered by the barrier film is only as good as the corrosion resistance of the film itself. There is no protection at the discontinuities of the film. So in some cases organic coatings are reinforced by additional protective coatings.
Next widely used coatings are metallic coatings, which can be divided into two groups. Metals such as zinc, cadmium, aluminium and magnesium under most conditions are anodic to steel. The degree of protection depends on the corrosion resistance of the coating metal and its thickness. The second group includes metals cathodic to steel, such as lead, tin, nickel and chromium, which protect it as a corrosion barrier. The degree of protection depends on the corrosion resistance of the coating metal and its continuity.
Large tonnages of steel are also protected by ceramic coating commonly known as porcelain (or vitreous) enamel. It has exceptional corrosion and heat resistance and is available in a variety of colours.
Sizable amounts of steel are protected by lead-tin alloys and aluminium. Smaller quantities of & steel are protected by other special metals and alloys. Gold and tungsten, for example, could be good protective metals. Composite coatings are also important in protection of steel surfaces. These may contain more than two components.
Available Coating Methods. Because of natural instability of the steel surface, it is readily contaminated by oxides, rust, or oils used for rust prevention during transportation and storage; also by lubricants used in fabricating the steel products. To prepare the steel for satisfactory application of a protective coating, the surface must be freed of undesirable contaminants, the degree of cleanliness required depending on the characteristics of the protective coating to be applied.
Some paints and plastics can be applied as soon as the oils and lubricants are removed. With metallic coatings, the cleaning operation must remove all surface films to obtain adherent bonding to permit metal-to-metal-contact.
Acid pickling for removal of scale and oxides and alkali degreasing for removal of oil and lubricants are the most common processes to be used as surface cleaning methods.
In considering the methods for applying various coatings, it can be said that the most widely used organic coatings are applied by roller coating, spraying with or without electrostatic field and simple brushing.
Electrodeposition or electroplating are the most common methods for application of metallic coatings.
Dipping in molten metals by various processes is also widely used.
Temperatures obtained in plasma guns are contributing to the field of special sprayed coatings consisting of high-melting-point metals and ceramics.
Some limited quantities of steel are being protected by vapour plating and vacuumdeposition techniques.
In the coating of pipes they are often lowered into a hot black mixture based on coal tar. The film solidifies on cooling.
It is also possible to dissolve the film material in a volatile liquid and cover the metallic surface with either brush or spray. A solid film is left when the solvent has evaporated. Such films are thinner and less protective than those obtained from a hot mixture but they often adhere better; if it is decided to use a hot mixture, it may be good first to obtain a thin film by evaporative drying which will serve as a basis for the hot coating.
Many paints and lacquers based on plastics and resins dry by evaporation.In using these paints in nearly all cases a "plasticizer" is added, an organic compound having a higher boiling point than the solvent, and which retains in the film after the solvent has evaporated .Without a plasticizer, paints or lacquers based on plastics or synthetic resins would in many cases be too brittle.
Дополнительные слова и словосочетания:
1) regardless, adv, a – не взирая на, не считаясь с
2) coating, слой, n – покрытие, облицовка, грунтовка
3) roller coating – валиковое крашение
4) bonding, n – связь, соединение, сцепление
5) barrier film – запирающий слой
6) heat resistance – теплостойкость
7) portcelain (vitreous) enamel – фарфоровая (стекловидная) эмаль
8) plasma gun – плазменный нагнетатель (распылитель)
9) acid pickling – травление кислотой
10) alkali degreasing – обезжиривание щелочью
11) electroplating, n – гальванопокрытие
12) vapour plating – металлизация паром
13) "plasticizer" – пластификатор
IX. Укажите номера предложений, которые соответствуют содержанию текста.
1. The cleaned coke-oven gas is used for industrial heating in the sintering plant.
2. Some paints and plastics can be applied as soon as the oils and lubricants are removed.
3. The continuous mill is often called a Morgan mill, from the name oils inventor.
4. It has exceptional corrosion and heat resistance and is available in a variety of colours.
5. The increase in steelmaking capacity and concentration of production pose serious environmental problems.
X. Вставьте нужные по смыслу слова.
1. The degree of... depends on the . . . resistance of the coating . . . and its thickness.
a) production, shock, charge;
b) protection, corrosion, metal;
c) reduction, wear, alloy.
2. Metals such as ... ,cadmium, aluminium and . . . under most. . . are anodic to steel.
a) tin, gold, structures;
b) bronze, carbon, circumstances;
c) zinc, magnesium, conditions.
3. Dipping in ... metals by ... processes is also . . . used.
a) molten, various, widely;
b) solid, definite, often;
c) liquid, different, very seldom.
XI. Закончите следующие предложения в соответствии с содержанием текста.
1. Sizable amounts of steel are protected by ...
a) phosphorus-nickel alloys and lithium;
b) phosphorus-bearing materials and sulphur;
c) lead-tin alloys and aluminium.
2. Composite coatings are also important in ...
a) protection at the discontinuities of the film;
b) protection of steel surfaces;
c) rust prevention during transportation.
3. Next widely used coatings are metallic coatings, which . . .
a) are protected by other special metals;
b) must be freed of undesirable contaminants;
c) can be divided into two groups.
XII. Определите, о каких из перечисленных ниже фактах можно узнать из текста.
1. Современный металлургический завод – это очень большой и сложный комплекс, работающий в течение 24 часов.
2. Степень защиты зависит от сопротивления коррозии металлического покрытия и его целостности.
3. Твердая пленка остается на поверхности, когда растворитель испаряется.
4. Некоторая часть жидкой смолы идет на продажу, остальная подвергается дистилляции.
5. Гальванопластика и гальванопокрытие – наиболее общие методы применения металлопокрытия.
XIII. Основываясь на содержании текста, перечислите самые основные методы защиты стали от коррозии.
XIV. Составьте план к тексту.