It has long been known that metals can undergo changes in which they lose their characteristic properties; in most cases, the metal ceases to be an element, and becomes a compound. Iron exposed to damp air is changed to a brownish hydrated oxide, known as rust. Iron placed in salt water with access of air also produces rust, and the change is known to be connected with electric currents flowing through the liquid. If heated in air, iron becomes covered with a dark scale, consisting, usually, of more than one oxide. If placed in dilute sulpheric acid, it passes into solution with evolution of hydrogen gas, and the solution on evaporation and cooling deposits crystals of ferrous sulphate.
The name "corrosion" describes these and similar chemical and electrochemical changes, in which a metal passes from the elementary to the combined state. However, the term is today sometimes used where a solid metal passes into solution in another (liquid or molten) metal - although this is not generally regarded as a chemical change; thus a steel vessel used for containing molten zinc slowly loses thickness the iron being dissolved by the zinc, the vessel is then said to undergo corrosion. We may also say that "corrosion" is the destruction of metal or alloy by chemical change, electro-chemical change or physical dissolution. This excludes mechanical processes which destroy the value of a metallic article by grinding it away as metallic dust. Such purely mechanical damage is called "erosion".
Conjoint Action. Often corrosion and erosion operate simultaneously and the damage produced is frequently far greater than the damage caused when they proceed one at a time. This disastrous combination of destructive agents is called "conjoint action". Certain types of conjoint action deserve special mention.
"Stress-corrosion cracking" denotes the cracking caused when a steady tensile stress acts on a metal in a corrosive environment, whereas cracking caused by alternating or cycling stresses in a corrosive environment is called "corrosion fatigue". Sometimes the cracking, whether due to stress-corrosion or corrosion fatigue, passes along grain-boundaries, and is then known as "intergranular corrosion", sometimes it passes across the grains, and is then called "transcrystalline-corrosion".
Oxidation. The conversion of the surface portions of metal to oxide on heating in air or oxygen is generally termed "oxidation". Oxidation is regarded as a type of corrosion.
Other Special Types of Corrosion. Generally corrosion starts on the surface of a metallic specimen. If a large portion of the surface is affected, it is said to be "general corrosion"; if only small areas, it is called "localized corrosion"; if confined to small points, so that definite holes are produced, we speak of "pitting". If corrosion produces grooves following grain-boundaries, or perhaps (on a much larger scale) following the water-line on a partly immersed metallic plate, we may speak of "grooving"; grooves may also be developed along the line, where two dissimilar metals meet, or in zones running parallel to a weld. If in rolled or extruded metals corrosion extends along certain planes parallel to the surface, it is called "layer corrosion"; under atmospheric conditions, the voluminous corrosion-product formed along these planes may lever the intervening layers apart, so that the material divides into flakes; this is known as "foliation".
Etching. The metallographist uses certain corrosive reagents to develop the structure of polished micro-sections. The reagents may show up the grains by producing crystal-facets, so that some grains appear dark and some bright when viewed under the microscope or it may attack the grain-boundaries, developing them as dark lines; or again it may attack and colour certain constituents of an alloy, which can thus be identified. Although these are all examples of corrosion the metallographist rarely uses the word; he speaks, generally of "etching" his sections.
Other scientists are actively engaged in corrosion work without employing the term. Metal-physicists treat metallic surfaces with certain acid mixtures, and count the pits which appear along grain-boundaries or on slip-planes, believing that they reveal to him the number of dislocations present. Other physicists study oxidation or film-growth, using the theory of lattice defects to interpret their results. But each of them is studying a corrosive process, and his work may be contributing much to our knowledge of the subject.
Дополнительные слова и словосочетания:
1) conjoint action – общее действие
2) stress-corrosion cracking - – коррозионное растрескивание под напряжением
3) corrosion fatigue – коррозионная усталость
4) intergranular corrosion – межкристаллитная коррозия
5) transcrystalline corrosion – транскристаллитная коррозия
6) pitting – точечная коррозия
7) grooving – коррозия бороздками
8) layer corrosion – послойная коррозия
9) foliation – расслоение
VI. Прочтите текст и дайте краткое содержание текста в 4-x предложениях.
VII. Установите, какие из приведенных утверждений, соответствуют содержанию текста.
1. Generally corrosion starts on the surface of a metallic specimen.
2. Certain types of conjoint action deserve special mention.
3. Iron for commercial use is obtained by extraction from its ores.
4. The study of crystals has made it possible to produce alloys such as steel with certain desired properties.
5. The discovery of the electron and of radioactivity had prepared physicists for a revision of accepted ideas about matter.
VIII. Переведите данные предложения с русского языка на английский.
1. Металлы могут подвергаться коррозии.
2. Железо, помещенное в соленую воду, также ржавеет.
3. Cчитают, что названия «коррозия» относится к тем изменениям, при которых металл из элемента превращается в соединение.
4. Этот термин, по-видимому, не включает механическое разрушение металла, называемое эрозией.
5. Иногда коррозия и эрозия происходят одновременно, принося гораздо больший ущерб, чем когда они обе происходят раздельно.
IX. Вставьте нужные по смыслу слова.
1. It has long been known that metals can undergo … in which they lose their … properties.
a) rusting, desirable;
b) melting, alloying;
c) changes, characteristic.
2. Iron exposed to … is changed to a brownish hydrated oxide, known as …
a) oxide, thickness;
b) damp air, rust
c) oxide-film, oxygen atoms.
3. The … uses certain corrosive reagents to develop the structure of …
a) chemists, oxidation or film-growth;
b) engineers, steel surfaces from corrosion;
c) metallographist, polished micro-sections.
X. Закончите следующие предложения в соответствии с содержанием текста.
1. If heated in air, iron becomes covered with a dark scale, consisting, usually, of …
a) thick organic coating;
b) chemical composition of substances;
c) more than one oxide.
2. Other physicists study oxidation or film-growth, using the theory of lattice defects to …
a) exchange the electrons;
b) measure temperature in industry;
c) interpret their results.
3. Other scientists are actively engaged in corrosion work …
a) without employing the term;
b) for the admission of blast;
c) produce quality castings.
XI. Выберите из текста предложения, в которых характеризуются основные виды коррозии.
XII. Составьте план к тексту на русском языке.