Читайте также:
|
When contacting a gaseous or liquid environment, essentially all metals undergo a corrosion destruction. Corrosion is a spontaneous destruction of metal, caused by a chemical or electrochemical heterogeneous reaction proceeding on the surface of the metal. Corrosion may be chemical or electrochemical, uniform or concentrated.
Chemical corrosion is characterized by the formation of a chemical compound at an interaction between the metal and a corrosive environment without giving rise to electric current. The formation of scale at a contact of metal with air at high temperatures may serve as the most typical example of chemical corrosion. When steel is in contact with air, a thin oxide film is formed on the steel surface. As the temperature and duration of heating increase, the oxide film thickness increases as well.
Electrochemical corrosion is a dissolution of metal in electrolytes with an appearance of electric current. As known, various acids and their aqueous solutions, alkalis and their solutions, solutions of salts in water (including sea water) can serve as electrolytes. The rate of electrochemical corrosion of metals depends on the nature of electrolyte solution (acid, alkali, solution of gases), the concentration and temperature of the solution, and also on the electrochemical properties of the metal.
Several types of corrosion are distinguished: an interctystalline corrosion, a corrosion at boundary layers of grains; a structural corrosion, a predominant dissolution of one of phases of a heterophase alloy; a point corrosion, a corrosion predominantly at localized spots (points) of the metal surface with its propagation inside the metal.
The intercrystalline corrosion is the most dangerous type of the corrosion destruction. Developing at the boundaries of grains, it propagates into the bulk of metal. A metal affected by the intercrystallyne corrosion fractures even under insignificant loads. The intercrystalline corrosion affects as a rule those metals which have a single-phase structure.
The structural corrosion is observed in welded joints of two-phase stainless steels. Subjected to the predominant dissolution may be either the ferritic or the austenitic phase, which depends on the quantitative ratio of these structural components in the steel or in the weld, on their chemical composition and also on the nature of the corrosive environment.
The point corrosion can develop both in the welded joint and in the base metal which has not been affected by the thermal cycle of welding.
Exercises
1. Learn the following words and word-combinations:
gaseous environment -газове середовище
heterogeneous reaction -гетерогенна реакція
oxide film -оксидна плівка
dissolution -розчинення
heterophase alloy -гетерофазний сплав
propagate -поширюватися
the ferritic or austenitic phase -феритна або аустенітна фаза
quantitative ratio -кількісний коефіцієнт
Дата добавления: 2015-11-16; просмотров: 65 | Нарушение авторских прав
| <== предыдущая страница | | | следующая страница ==> |
| Fill in the gaps with a proper article. | | | ELECTRIC WELDING OF SOFT TISSUES IN SURGERY |