CORROSION AND RUSTING. Many metals of indus trial importance undergo a chemical change when exposed to the action of air and water or salt solutions. This change often results in the production of spots of differently coloured material scat tered over the surface of the metal, e.g., brown rust on iron and steel, white specks on aluminium, and blue or green layers on cop per. The change not only spoils the appearance of the metal but sometimes penetrates so deeply into it that its mechanical prop erties are seriously impaired. Recent calculations of Sir Robert Hadfield suggest that the world-loss of iron and steel alone, due to corrosion, must be valued at many millions of pounds sterling every year.
Corrosion is essentially a process of oxidation. Metals are usually found in nature in an oxidized state as ores, and in order to fit them for industrial use they must be "reduced" from the ore, oxygen, water and other substances must be separated from the metal by chemical and physical processes. But when the metal has been so obtained it will tend to reassociate with oxygen and other substances whenever it comes into contact with them in suitable conditions : thus, iron tends to react with water and oxygen to form rust which is similar in composition to certain kinds of iron ore. The rusting of iron is a typical example of the process of corrosion whereby metals tend to return to an oxidized condition.
The process of oxidation was formerly thought to be usually a direct union between the metal, oxygen gas and sometimes traces of sulphur compounds occurring in the atmosphere. It is probable that this kind of action does take place in certain cases, such as atmospheric corrosion which can occur in the presence of mere traces or very thin films of moisture—the tarnishing of silver is an example. But it is not usually this kind of action which results in the formation of rust and the other conspicuous corro sion products mentioned above on metals immersed in liquids.
When a "base" metal is placed in water or in a salt solution, which can conduct electricity, it tends to dissolve as electrically charged metal atoms or "ions," displacing a proportionate number of charged hydrogen atoms which are constituents of water. With some metals, such as sodium, the hydrogen may appear as gas bubbles ; with other metals, such as iron and copper, little or no gas appears ; nevertheless the hydrogen is present as an invisible, probably atomic, film at the surface of the metal, and may com pletely cover it. Unless this film is removed the corrosion process is greatly retarded at this stage and no change may be visible; this may happen in the absence of oxygen; but oxygen has the property of uniting with hydrogen to form water, and so destroy ing the interfering film of hydrogen. Hence in the presence of air, which contains oxygen, a gas soluble in water, the process can continue, i.e., more hydrogen can be displaced by metal to replace that which has been oxidized. Thus the action of oxygen in cor rosion is usually indirect : it unites with hydrogen, the metal reacts with the water. This may be expressed in equations thus (neglecting electrical effects for the present) : It will be seen that the result of these reactions is the conver sion of the metal into an oxidized product, called an hydroxide. Metallic hydroxide is a typical product of corrosion, but it may undergo further changes, e.g., it may react with carbon dioxide from the atmosphere and finally form a complicated body; such a body is rust which usually contains ferrous and ferric hydroxide and carbonate and is practically insoluble in water. Some hydrox ides, such as those of calcium and sodium, are soluble in water, in which case little or no solid corrosion product will appear till the water becomes saturated after long periods of corrosion.
If corrosion occurs in a solution of a salt such as sodium chlo ride the reactions are a little more complicated and may be sum marized thus: The hydrogen will react with oxygen as before. It will be noticed that the corrosion product is a metallic hydroxide, as in the case of water, but formed indirectly, and that the sodium chloride is regenerated—not always completely for it may be partly carried out of solution as a complex compound such as a so-called "oxychloride." A remarkable feature of corrosion in water and salt solutions is its concentration at certain isolated spots or areas at which the metal is sometimes deeply pitted, the surrounding metal being scarcely attacked at all. This is due to the fact that such corro sion is mainly a "galvanic" or electrolytic action and is concen trated on areas representing the negative, anodic or zinc pole of a primary battery, such as is used for electric bells ; the surround ing, relatively slightly attacked, areas correspond to the positive or carbon pole; ordinary water and salt solutions, which are elec trolytes, and therefore conduct electricity, correspond to the sal ammoniac solution in the battery.
A primary battery operates largely because there is an electrical difference, a difference of potential, between the zinc and carbon when plunged in the same electrolyte. If the analogy be trust worthy, the conclusion is reached that there must be differences of electric potential between different parts of the same piece of metal when localized corrosion takes place. Such differences may be due to actual physical or chemical variations in the metal itself which are not readily discernible (such as local strains, segrega tion of impurities, or presence of second metals or "phases" such as carbon in cast iron), or more often to variations in the condi tions external to the metal, such as differences of oxygen or metal-ion concentration. Differences of oxygen concentration are particularly important and may be due to convection currents in the liquid, or to pores in the metal into which dissolved oxygen cannot easily penetrate; in all cases the metal forms a negative pole, and is corroded, where the oxygen concentration is least; where the oxygen concentration is greatest the hydrogen will be most rapidly removed according to equation (2) given above, which together with (4) states the typical reactions at a positive pole. Thus metals tend to be corroded most at crevices of all kinds.
The actual rate of corrosion in any particular case is determined by more than a dozen factors the interaction of which makes the subject exceedingly complicated. Some of these factors are tabulated below.
Factors which influence the Rate of Corrosion Relating to the metal Relating to external conditions I. Electrode potential. 8. Temperature.
2. State of aggregation. 9. Pressure of oxygen.
3. Presence of internal stresses. io. Rate of supply and distribu 4. Overpotential. tion of oxygen.
5. Nature and concentration of 11. Hydrogen-ion concentration.
metals in solid solution. 12. Nature and distribution of 6. Nature, amount and distri- corrosion products.
bution of second phases. 13. Conductivity of the liquid.
7. Chemical reactivity. 14. Metal-ion concentration. 15. Specific nature of the ions present.
For a detailed account of the way these factors operate, and for many other important aspects of corrosion, the reader should consult U. R. Evans, The Corrosion of Metals (1926) ; F. N. Speller, Corrosion: Causes and Prevention (1926), McGraw-Hill Book Co., New York. (G. D. B.)