1993 年 42 巻 479 号 p. 912-916
Pitting corrosion of carbon steel is one of the most serious problems in cooling water systems, which shortens the life of equipments and sometimes induces unscheduled shutdowns. In order to solve these problems, pitting corrosion and its countermeasures were studied focusing on suspended solids in cooling water.
Pit formation at initial stage was investigated by a rotating disk method. Open pores and precipitation of streamlined rust around pits were observed. Increase of pits and enlargement of tubercles were observed with addition of suspended solids. Deposition of solids were typically observed behind the pores within the streamline. This is explained that dissolved iron ions from pits act as a coagulant, so that suspended solids coagulate by iron ions and deposit. After all, suspended solids accelerate pit formation and enlarge tubercles by corrosion products.
The influence of suspended solids on pit formation and its growth was confirmed by recirculating loop test. With addition of suspended solids, pit number and pitting depth increased with time, as expected, under the condition where less pits occurred without suspended solids. This indicates the additional countermeasures besides corrosion inhibitors are necessary to inhibit pitting corrosion in cooling water systems. Suspended solids in cooling water are the mixture of inorganic materials and microorganisms, and they are covered with sticky filamentous materials produced by microorganisms. Therefore sterilization by biocides and filtration were expected to inhibit deposition of suspended solids and pitting corrosion. These countermeasures were found effective to inhibit not only deposition of suspended solids on the surface of carbon steel but also under deposit corrosion.
Finally the effectiveness of practical methods were confirmed by pilot cooling tower test. The results showed that combination of continuous chlorination and filtration was more effective than conventional slug dose of chlorine or respective method itself. Moreover, the use of chlorine stabilizer was most effective to inhibit pitting corrosion.