Stray-current electrolysis

Stray-current electrolysis occurring as a result of the application of cathodic protection to a nearby immersed or buried structure is known as cathodic-protection interaction and is described in Section 10.6. 

Stray-current corrosion, or stray-current electrolysis, is different fixim natural corrosion because it is caused by an externally induced electrical current (alternating, ac, or direct current, dc) and is basically independent of such environmental factors as oxygen concentration or pH. Environmental factors can enhance other corrosion mechanisms invcdved in the total corrosion process, but the stray-current corrosion portion of the mechanism is unaffected. 

Stray current corrosion is an electrolytic degradation of a metal caused by unintentional electrical currents. Bad grounds are the most prevalent causes. The corrosion is actually a typical electrolysis reaction. 

Investigating the presence of stray currents to prevent or explain corrosion problems is not a new field in corrosion engineering. In fact, as mentioned in App. A, such activities were carried out by probably the first corrosion engineers in North America when the American Committee on Electrolysis was established at the turn of the twentieth century to combat the serious effects of railcar stray currents to underground metal structures (Figs. 7.4 and 7.5). 

Among pioneers in studying the effects of corrosion was the American Committee on Electrolysis, which noted in 1921 that its preliminary report had been published in October, 1916. This committee, composed of representatives of the American Institute of Electrical Engineers, American Electric Railway Association, American Railway Engineering Association, National Bureau of Standards, and others, concerned itself with the then serious problem of stray current damage to underground metal structures, especially the protection of communication cable from electrified street and interurban railways. 

Stray currents are currents flowing in the electrolyte from external sources, not directly associated with the cathodic protection system. Any metallic structure, for example, a pipeline, buried in soil represents a low-resistance current path and is therefore fundamentally vulnerable to the effects of stray currents. Stray current tends to enter a buried structure in a certain location and leave it in another. It is where the current leaves the structure that severe corrosion can be expected. Corrosion damage induced by stray current effects has also been referred to as electrolysis or interference. For the study and understanding of stray current effects it is important to bear in mind that current flow in a system will not only be restricted to the lowest-resistance path but will be distributed between paths of varying resistance, as predicted by elementary circuit theory. Naturally, the current levels will tend to be highest in the paths of least resistance. 

Badly conceived relations between individual materials of a complex can ruin even the best design. Thus it is imperative that aU intermaterial relationships are properly appreciated and evaluated before any final decision in design is taken, whether these are caused by direct contact between dissimilar metals or induced by changes in polarity, transfer of electrolysis through a medium, carrying metallic particles in the stream, the adverse influence of stray currents, or by any other negative effect arising from the near proximity of materials (e.g. chemical, thermal, or radiation) selected to form the required unit. 

In precision work great care must be taken to eliminate effects due to electrolysis and those due to variation in temperature. Controlling the temperature is a particularly difficult problem because of the heating effect of the current. Water of extreme purity (conductivity water) must be used, since stray impurities in the water can produce sensible variations in the value of the conductivity of the solution. The contribution of water itself to the conductivity must be subtracted from the measured value for the solution. 

Wyer, Samuel, Digest of Publications of Bureau of Standards on Electrolysis of Underground Structures caused by the Disintegrating Action of Stray Electric Current from Electric Railways, National Bureau of Standards, Washington, DC, 1918. 

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