An instrument (Fig. 1 & 2), which is composed of several external plastic tubes, a circulating pump, a head tank, current-supplying anodes and corresponding inner metallic tubes as specimen, was developed to investigate the effect of cathodic current on the polarization of inner surface of metallic tubes for heat exchangers. Some test results on bare steel tubes and epoxy-coated steel tubes in artificial sea water were graphed as shown in Fig. 3-8 & 9, which told the promising performance of coated pipe, and some doubts on application of famous Michalke's Equation in the case of bare steel tube.

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The amount of sulfate absorbed by “artificial rust” and by natural rust separated from iron-copper alloys which had been exposed for one year has been determined in sodium sulfate and ferric sulfate solutions. The apparent permeation rate of sodium sulfate through pores of the artificial rust and the membrane potential difference across the rust layer in chloride solutions have also been determined. Calculation based on the amount of sulfate absorbed and the calculated pore volume of the rust shows that the sulfate within rust pores is highly concentrated. The ratio of the concentration of sodium sulfate in rust pores to that bulk solution in which the rust specimen is immersed is about 80 for the artificial rust consisting of crystalline magnetite while the ratio is about 250 for the amorphous artificial rust containing 5.1% copper. The presence of copper in rust retards permeation of sulfate through the rust. The determination of the membrane potential indicates that rust acts as an anion exchange membrane in dilute chloride solutions. The behavior of electrolyte in rust depends not only on the physical structure of the rust but also on its colloid-chemical characteristics.

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