A series of tests were carried out to obtain the effective data for protection of evaporators in operation. In each test, zinc anodes of various types and test pieces of iron and copper were fixed in brine inside an evaporator. The brine (28-35°Bé) contained a large amount of MgCl
2 as described in the 1st report and was for the most period kept boiling at 110°C. After the test, the weight losses of anodes and test pieces were measured. Assuming the current efficiency of zinc as 90%, the protective current produced could be estimated. Moreover, protecting degrees were calculated from the weight losses of test pieces.
Through the thirty runs of test, the following results and conclusion were obtained:
(1) Cast anodes of high-purity zinc are most suitable for use in high-temperature brine, while some zinc alloys containing aluminum show a tendency to crack during operation.
(2) Coated anodes, whose side and back surfaces are covered with iron sheet, are far superior to the ordinary bare anode. Because, during long-term use, the coated anodes supplied fairly constant current and, therefore, the excessive consumption of zinc can be prevented.
(3) The cathodic current needed to protect iron surface of evaporators is about 0.2A/m
2 and the average anodic current density of zinc is about 20mA/cm
2 with coated-type and 7-8mA/cm
2 with bare anodes.
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