SOURCES OF STRAY CURRENTS

12.2 SOURCES OF STRAY CURRENTS

Sources of dc stray currents are commonly electric railways, grounded electric dc power lines, electric welding machines, cathodic protection systems, and electro- plating plants. Sources of ac stray currents are usually grounded ac power lines or currents induced in a pipeline by parallel power lines. An example of dc stray current from an electric street railway system in which the steel rails are used for current return to the generating station is shown in Fig. 12.1 . Because of poor

Figure 12.1. Stray - current corrosion of a buried pipe.

SOURCES OF STR AY CURRENTS

bonding between rails, combined with poor insulation of rails to the earth, some of the return current enters the soil and fi nds a low - resistance path, such as a buried gas or water main. The owner of a household water service pipe at A benefi ts by cathodic protection and experiences no corrosion diffi culty; but owner

B , to the contrary, is harassed by corrosion failures because the service pipe of his house is anodic with respect to the rails. If B coats the pipe, which is an understandable layman ’ s reaction to any corrosion problem, matters are made worse because all stray currents now leave the pipe at defects in the insulating coating at high - current densities, accelerating penetration of the pipe. The basic rule: Never coat the anode.

Street railways have now in large part been replaced by other forms of transportation, but the problems of stray - current corrosion originating from metropolitan railway transit systems continue [6] . Also, cathodically protected structures requiring high currents, when located in the neighborhood of an unpro- tected pipeline, can produce damage similar to that by the railway illustrated in Fig. 12.1 .

Another example of stray - current corrosion is illustrated in Fig. 12.2 . A welding motor generator located on shore with grounded dc lines to a ship under repair can cause serious damage to the hull of the ship by current returning in part from the welding electrodes through the ship and through the water to the shore installation. In this case, it is better to place the generator on board ship and bring ac power leads to the generator, since ac currents leaking to ground cause less stray - current damage.

Current fl owing long a water pipe (e.g., used as an electric ground) usually causes no damage inside the pipe because of the high electrical conductivity of steel or copper compared to water; for example, since the resistance of any con- ductor per unit length equals ρ / A , where ρ is the resistivity and A is the cross -

sectional area, then the ratio of current carried by a metal pipe compared to that carried by the water it contains is equal to ρ W A M ρ / M A W , where the subscripts W and M refer to water and metal, respectively. For iron, ρ M

= 10 −5 Ω - cm; and for a potable water, ρ W may be 10 4 Ω - cm. Assuming that the cross - sectional area of water is 10 times that of the steel pipe, it is seen that if current through the pipe is 1 A, only about 10 −8 A fl ows through the water. This small current leaving the

Figure 12.2. Stray - current damage to a ship by a welding generator.

244 STR AY- CURRENT CORROSION

Figure 12.3. Effect of current fl owing along a buried pipeline on corrosion near insulated couplings.

T A B L E 12.1. Weight Loss of Metals by Stray - Current

Corrosion Metal

Equivalent Weight

Weight of Metal

Corroded per Ampere - Year

pipe and entering the water causes negligible corrosion. If seawater is transported instead, with ρ W

Ω - cm, the ratio of currents is 2 × 10 5 , indicating that, even

in this case, most of the current is carried by the metallic pipe and there is very little stray - current corrosion on the inner surface. However, where such currents leave the pipe and enter the soil, stray - current corrosion of the outer surface may

be appreciable. If insulating joints are installed in the above - mentioned pipe in order to reduce stray - current pickup, corrosion is now focused on the water side of the joint where any current that persists leaves the pipe to enter the water. Or, if a high - resistance joint exists between two sections of a buried pipe, corrosion may

be more pronounced on the side where current enters the soil (Fig. 12.3 ).