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A VALUABLE METAL

In 1789 while analyzing the mineral zircon Martin Heinrich Klaproth, the German chemist and member of the Berlin Academy of Sciences, discovered a new element which he called zirconium. Owing to its beautiful colours ranging between golden, orange and pink this mineral was considered a precious stone as early as the epoch of Alexander the Great.

It was only at the beginning of the last century that scientists were able to extract zirconium free from impurities and to study the properties of this metal.

Pure zirconium resembles steel but it is stronger and very ductile. One of its most remarkable properties is its resistance to many aggressive media. In anti-corrosion properties it is even superior to such resistant metals as niobium and titanium. While stainless steel immersed in five-per-cent hydrochloric acid loses about 2.6 millimetres a year at a temperature of 60C and titanium about one millimetre, zirconium's loss is only one-thousandth of that. Zirconium's resistance to alkalies is even greater.

Thanks to its remarkable corrosion resistance zirconium has found an application in neurosurgery, this extremely delicate field of medicine. Zirconium alloys are good for surgical instruments, and sometimes zirconium filaments are used for stitches during brain operations.

After scientists had noticed that introduction of zirconium improved many properties of steel it was acknowledged as a valuable alloying element. In this sphere its uses are varied: it increases the hardness and strength of steel, improves its machinability, hardenability, weldability and fluidity, crushes the sulphides contained in it and makes its structure finegrained.

Zirconium also produces good results when alloyed with many nonferrous metals. An addition of zirconium to copper greatly increases its strength, leaving its electrical conductivity almost unaffected.

At present big deposits are being developed in the United States, Australia, Brazil, India and West Africa. Often coastal sands prove to be excellent zirconium ore. In Australia zirconium placers stretch for nearly 150 kilometres along the ocean coast. Russia also possesses considerable zirconium reserves.

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6) CORROSION OF METALS AND ALLOYS

Corrosion has a negative influence on the national economy of all countries. It can be compared to a natural disaster. In the industrialized countries direct and indirect losses from corrosion and the cost of protective measures make up about 10 per cent of the national income annually. Atmospheric corrosion known in everyday terms as rust was the least dangerous form. If 50 years ago due to the activity of atmospheric corrosion rust claimed around 1.5 - 1.8 per cent of the world steel output, today, as a result of the pollution of the atmosphere, the corrosion rate has become higher, and according to calculations made by experts, total losses from corrosion amount up to 20,000 tonnes per each million tonnes of metal (i.e. 2%). The corrosion of metals provides a major source of failure of metallic structures.

Corrosion may be defined as a gradual destruction of metals in the presence of aggressive media (environments). Air, water, especially sea water, different acids and alkalies, soil are considered to be aggressive media and cause corrosion.

The rate of corrosion depends on the following three factors: 1) the percentage of

aggressive substances in the engineering material; 2) the percentage of aggressive substances in the surrounding medium. Such substances as moisture, oxygen, hydrogen, sulphur and others cause corrosion of metals and alloys; 3) the temperature of the surrounding medium - the higher the temperature, the higher the rate of corrosion.

There are different types of corrosion. Some of them are as follows:

1) uniform corrosion in which the whole surface of the metal is subjected to the action of corrosion with equal intensiveness;

2) localized corrosion in which only certain areas of the surface corrode;

3) selective corrosion in which separate structural parts of an alloy are attacked;

4) intercrystalline corrosion in which destruction occurs along the grain boundaries of the metal or alloy;,;

5) galvanic corrosion may be a problem when dissimilar metals are placed in contact with each other and are located in a corrosive environment. This problem should be carefully considered while designing and selecting materials. For example, being connected to steel brass may cause rapid corrosion of the steel, in the case of aluminium - steel rapid corrosion of aluminium may be caused.,ff.

From the mentioned above it is clear that corrosion involves great difficulties. There is an intensive growth of metals and alloys application in those industries in which machines are subjected to the aggressive action of the industrial environment - for instance, the chemical, petrochemical, pulp-and-paper and others. A nationwide programme of combating corrosion should be developed in each country.

The programme includes the following principal corrosion protection methods:

1) developing technologies for alloying metals to obtain chemically inactive alloys;

2) forming oxide films which protect metals against further corrosion;

3) developing technologies to protect metals with the help of different coatings.

These methods include:

a. electroplating, i.e. applying protective metallic coatings, for example, nickel, chromium, tin on other metals;

b. applying coatings of paint or lacquer;

c. applying polymer and powder coatings;

4) electrochemical protection method, i.e. cathodic and anodic protection.

These measures are expected to result in a reduction of metal losses which in turn will lead to saving national riches.

 

 





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