Chemical properties

The chemical properties of metals and alloys determine the ability of these materials to resist attack by moisture, air, acids, and other corrodents. The chemical decomposition of materials under the action of corrodents is known as corrosion.

Corrosion can be chemical and electrochemical. Chemical corrosion occurs in a medium that does not conduct electric current, for example, in dry gases and gasolene. This results in the oxidation of metal parts, such as exhaust valves of internal combustion engines. Electrochemical (galvanic) corrosion is caused by the action of electrolytes on metals, i.e. current-conducting media, such as moisture, air, acids, and alkalis. This corrosion occurs as a result of a conductive coupling formed through the electrolyte between dissimilar metallic elements. One of the elements becomes a cathode and the other an anode. The result is that a galvanic current begins to flow between the cathode and the anode causing the latter to decompose. Metals and alloys differ in the value of the electrolytic potential. If two dissimilar metals are brought in contact, the metal with a lower electrolytic potential will begin to decompose. For example, aluminum that comes in contact with copper will decompose because it has a lower potential.

Corrosion shows up as rusty spots on steel and iron, and also as green oxide films on copper and black oxide films on brass.

By the character of decomposition of a metal corrosion can be general, local and intercrystalline. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate-conversion, can increase the corrosion resistance of a material. However, some corrosion mechanisms are less visible and less predictable.

To protect metals against corrosion, metal surfaces are given the coats of metals that are resistant to corrosion, the coats of varnishes, paints and enamels, and also the coats of oxide films produced by blueing or phosphotizing. This type of corrosion control is widely used in machine-tool industry and in many other fields. However, modern industry requires more and more metals and alloys capable of working in corrosive media and in the conditions of high temperatures and speeds. Therefore, to protect metals against corrosion special methods of heat treatment are used, and to protect metals against corrosion cracking alloying elements are added.

Corrosion inhibition is an effective protection. Corrosion inhibitors are organic and inorganic substances, such as aldehides, phosphates, amines and sulphonic acids. The inhibitors added to gases, acids, solutions and petroleum products slow down the destructive processes of corrosion. Inhibitors are especially useful for the protection of oil and gas pipelines.

1. Find in the text English equivalents for the words and word-combinations given below.

Іржаві плями, захист від корозії, газопровід, отже, в результаті, руйнівні процеси, під дією, вихлопні клапани, неорганічні речовини, в умовах високих температур, проводити електричний струм, особливо корисний, окислення металевих частин, верстатобудування, очевидний, передбачуваний.

1. Answer the following questions:

1. What is corrosion?

2. Why does chemical corrosion take place?

3. What is electrochemical corrosion?

4. When does it take place?

5. Do metals and alloys have different electrolytic potential?

6. How does corrosion show itself on the surface of metals?

7. How can metals be protected from corrosion?

8. What do we call the substances slowing down the processes of corrosion?