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CORROSION
Copper and copper-base alloys possess certain physical and chemical properties that make these metals highly corrosion resistant, a fact of unusual significance in the development of our industrial economy.
It is not our purpose to name the multitude of uses for which copper-base alloys are suitable and superior to other commercially obtainable metals, although some mention of the broader applications will be made from time to time. For the moment, however, our objective is to delve into some of the technical aspects that occasionally arise in the study of corrosion problems.
The reason it is frequently difficult to satisfy an inquiry, as to whether or not copper or its alloys can successfully withstand a corroding medium, is because of the very large number of factors which enter into the actual corrosion.
There are at least twelve factors affecting the service life of the metal under industrial conditions. These are:
1. Time of Exposure —
Obviously, the longer time of contact between the corrodent and the metal, the more chances for corrosion to take place. Designers may take this into account, so that a known corrosion rate may offer useful life in spite of eventual failure if left in service long enough.
2. Temperature —
There is a general rule in any chemical reaction that as the temperature increases the rate of reaction also increases, Frequently a rise of a relatively few degrees can greatly accelerate corrosion.
3. Concentration of Corroding Medium —
It is especially essential to have some idea as to the amount of the corroding agent present, since the corrosion rate will change with concentration, and not always as might be expected. A typical example is commercial concentrated sulphuric acid, which may be shipped in steel tank cars, but which is extremely corrosive to steel as a 10% acid solution.
4. Oxygen —
Oxygen is a cause of corrosion in itself, as well
as being an accelerator of corrosion for other substances. As an example, sulphuric acid will not attack copper when oxygen is absent but, since this condition rarely occurs in industry, sulphuric acid can be considered an active corrodent. On the other hand, in the absence of oxygen, it will attack stainless steel.
5. Movements —
The velocity with which the corroding medium passes by the metal surfaces greatly affects the rate at which corrosion may proceed. A typical example is Condenser Tube, where an increase in velocity may greatly increase the rate at which corrosion takes place.
6. Purity —
Industrially speaking, it is impossible to be sure that substances are pure, and the presence of very small amounts of foreign materials can radically change the corroding rate. As examples, the presence of oxygen is largely responsible for the corrosiveness of many substances which are otherwise innocuous or the addition of small amounts of carbon dioxide to ordinary water produces serious corrosion on copper and finally, the addition of moisture to sulphur dioxide makes it corrosive to copper.
7. Corroding Agents —
Obviously, it is necessary to know what corroding agent is in contact with the metal. Whether or not experience may be available to give a proper answer to expected life, will depend upon the type of corrodent.
8. Static Stress —
Static stress, which is due to the loads imposed upon metal by water pressure, steam pressure, or due to structural design, can affect corrosion damages, as it may induce stress corrosion cracking among other things.
9. Alternating Stress —
Alternating stresses are those which typically are found in springs which are vibrating back and forth. The presence or absence of such alternations of stress are part of the background in determining expected service life.
10. Galvanic Action (Electrolysis) —
This will be discussed in greater detail later.
11. Metal involved —
As will be discussed later, it makes a great deal of difference as to whether the metal is inherently resistant to corrosion because of its position in the electromotive series or, whether it is capable of producing oxides or other insoluble compounds which will either retard or completely stop the corrosion. On the other hand, formation of highly soluble corrosion products usually means accelerated corrosion rates.

The type of crystal structure may also be important. Selective corrosion of certain phases can take place such as in the “beta” brasses or the “sigma” phase in stainless steel.
12. Type of Attack — Some corroding agents typically produce pitting instead of uniform corrosion. Obviously, under such circumstances, expected service life might be considerably shorter than if overall corrosion can take place.


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Busby Metals, Inc | 55 Davids Drive | Hauppauge, NY 11788

Busby Metals, Inc. is a stocking distributor of brass, bronze, copper, beryllium copper & aluminum bus conductors. Inventory includes rod, bar, sheet, strip, plate, tube, pipe, wire, castings & forgings. specialty alloys available from stock include: aluminum, naval, tobin, commercial, manganese, phosphor & silicon bronzes; beryllium, tellurium, leaded, zirconium, chromium, ETP & oxygen free coppers; free cutting brass; nickel silver; copper nickel. International specifications. Custom shapes. Metric sizes. Metal processing performed. Accredited ISO 9000 and MIL-I-45208A quality system. Chemical & physical test reports available. Technical support provided.