The relation of soil characteristics and chemical constituents of soil solutions to the self corrosion of underground lead cable
The experiments made in this study covered the investigation of the “self corrosion” of antimony and tin alloy lead cable sheath that was effected by various soil characteristics and by various classes of soil solutions. The study was made entirely with laboratory specimens and solutions and while it was difficult to attain the desired results, yet results of sufficiently distinctive character were secured in most cases to permit of an intelligent comparison and interpretation.
In general, the principal cause of soil corrosion of underground lead cable are the presence of organic matter and poor drainage. When organic matter decomposes the resulting organic solids, mainly acetic, attack the cable sheath with resulting corrosion products. Moisture, up to a certain point, is very detrimental, not only in aiding electrolysis, but by causing the formation of hydroxides. However, the presence of a great amount of water seems to retard corrosive action by making the approach of oxygen difficult. The presence of oxidizing agents, such as the nitrates, also enhances corrosion.
The nature of soil corrosion on cable sheath is usually that of a crater-like pitting of the surface. In these pits are found the corrosive products, probably lead salts in the form of carbonate or sulphate. The amount of corrosion varies over different parts of the surface, due to the non-uniform distribution of the agent causing the corrosion.
Just how much of this corrosion is effected by local galvanic action is indeterminable from this study, but the American Committee on Electrolysis holds this as a very important factor.
Alkalis, as well as acids, have detrimental corrosive effects on lead cable. This was shown very decisively in this study, and for these reasons, it is thought a bad policy to place non-protected cable in the vicinity of calcareous substances, such as concrete. In some cases where acids are probably present in drainage waters, limestone placed near the cable may have a neutralizing effect, but to what extent this principle should be practiced is a matter for conjecture.
It is believed that the chief corrosive effect of salts is due to their aid to galvanic action. However, in many cases it is evident that they also effect corrosion products by chemical union with the cable sheath itself.
From this study it is evident that the whole matter of the corrosion of lead in the soil is very complicated. Not only does the popular amphoteric character of this metal enter into the problem, but also the physical character of the metal structure itself. These factors, combined with the complexities of the chemical content of the ground solutions, make the whole question of the soil corrosion of lead a matter for further and more intense study.
In conclusion, the author wishes to express his appreciation to Mr. D. S. Hilborn, Electrolysis Engineer of the Bell Telephone Company of Pennsylvania, and Professor F. O. Anderegg of the Chemistry Department of the Purdue University, for their advice and directions in the conduction of these experiments.