Abstract
Because of the difficulty in assuring full-time electrical isolation of dissimilar metal piping, alternate methods of coping with the galvanic corrosion must be used. Use of short, electrically-isolated piping sections between the dissimilar metals can reduce galvanic corrosion by increasing the electrical resistance of the seawater path through which the galvanic current must flow. This objective of this project was to determine the magnitude and distribution of galvanic corrosion of 70/30 copper-nickel piping when coupled to alloy 625 piping, to determine the efficacy of various lengths of isolated separator pipes made of either alloy, and to determine the amount of stray current corrosion that could occur on the separator piping as a function of pipe material. The presence of separators of any length in 2-inch pipe lowered the amount of galvanic corrosion between copper-nickel and alloy 625, with a 50-60% reduction in metal loss using 3-ft (1-m) separators. A further reduction of another 30-50% was achieved by increasing separator length to 10-ft (3-m). The use of copper-nickel separators generally resulted in higher metal losses than the use of alloy 625 separators. The effect of the separator was maximum under low flow conditions, and alloy 625 separators are more effective than copper-nickel separators. Smaller diameter copper-nickel pipe experienced higher corrosion rates than pipe of equal diameter to the alloy 625 pipe to which they were coupled. Linearity of potential profile through the separators was an accurate indicator of whether stray current corrosion was taking place.
