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Corrosion studies under dynamic conditions Dey, Walter Ross

Abstract

Laboratory tests on steel pipe corrosion subjected to varying velocities of aerated Vancouver city water have been made. The water was circulated through seven vertical sections each containing from eight to ten specimens of standard ¾ inch steel pipe. The water was removed at a constant rate so that the volume of water in the apparatus was renewed from one to two times daily. Exposure times varied from 5 to 25 days with temperatures of 90°F and 130°F. The amount of corrosion was evaluated by determining the weight loss. The solution potential of some designated corroding specimens was measured against a silver-silver chloride reference electrode throughout the time of exposure. No changes in the pH from that of the inlet water occurred and the iron ion content fluctuated corresponding to the fluctuation in inlet water. At 130°F the amount of corrosion, for 10 day tests, was greatest at 1.5 fps velocity, but for tests of greater duration, was greatest at .75 fps velocity. This reversion did not take place at 90°F where the amount of corrosion was greatest at 1.5 fps for all tests. At both temperatures the lowest amount of corrosion occurred at the highest velocity level irrespective of the test duration within the limits investigated. Amounts of corrosion were in all cases higher at 130°F as expected from previous work where maximum temperature effects occurred at about 180°F. The corrosion product consisted mainly of a black fine-crystalline matter (assumed to be magnetite) at the surface of the specimen, with a porous reddish precipitate (assumed to be ferric hydroxide) over the magnetite. The total amount of magnetite increased with increasing temperature for similar conditions of flow and time of exposure. The corrosion mechanism is considered in the light of available oxygen supply, potential change, the effect of velocity, cathodic depolarization and variables involved in the mass transfer of the oxygen. The correlation for the latter with velocity is compared with results from an oxygen-pick up investigation of corrosion. The results of this investigation are qualitatively consistent with previous knowledge of oxygen as a passivator in such systems.

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