多段フラッシュ海水淡水化装置の伝熱管材料に関する研究

Abstract

The application of ferrous materials to heat exchanger tubes in an MSF desalination plant have attracted serious attention, because of their low price and availability. In this national project, the ferrous materials mainly examined were low alloy steels, but stainless steels and coated steels were also tested. The test results of these ferrous materials were as follows. (1) Low alloy steels: In the loop test 17 kinds of low alloy steels were tested, the majority of which were low chromium steels. The corrosion rates of these steels were reduced with increasing content of chromium, but above 5% of chromium the tendency of local corrosion increased. Under the brine condition corresponding to a heat recovery section, 2Cr to 3Cr steels exhibited good corrosion resistance. In the field test, 2Cr to 3Cr steel tubes selected by the loop test were mounted in each stage of the heat recovery section. Contrary to expectation, in the field test the corrosion rates increased more than several times over those in the loop test. From the results of these tests 3Cr-1Al, 3Cr-1Ni and 2Cr-0.1Mo-1.2Mn-0.26Cu steels were selected. The increase of the corrosion rates of these steel tubes in the field test was inferred as follows: deposition of iron rich sludge from circulating brine in the steel tubes (produced by the corrosion of steel shells in contact with the brine) might have an accelerating effect on corrosion of the tubes by the Schikorr reaction. This suggested that continuous removal of iron rich sludge from the steel tubes by sponge-ball cleaning might improve the corrosion resistance of the steel tubes. (2) Stainless steels: In the loop test stainless steels indicated more or less the tendency of local corrosion, especially at the crevice formed by Teflon holders. In the field test, test tubes of stainless steels selected from the loop test were placed in the sections of the brine heater and the heat rejection (where corrosion troubles of copper alloy tubes have been liable to occur). As the results, several stainless steels superior to 316 steel were found. These were three 25 chromium dual phase steels, two high nickel austenite steels and one 25 chromium ferrite steel. However some open type micro pits were observed under sludge in all of these high grade stainless steel tubes. This pitting-like local corrosion may be a form of crevice corrosion caused by the deposition of sludge. Therefore, the continuous removal of sludge by sponge-ball cleaning or the development of more resistant stainless steels is desired. (3) Coated steels: In the loop and the field test some metallic coatings (chromized, aluminized, etc.) and some organic coatings (phenol resin or epoxy resin) were tested. The defect in chromized steel was pitting corrosion originated in pinholes, and that in aluminized steel was the dissolution of aluminum layer which was accelerated by the increased temperature of sea water. Corrosion resistance of the organic coatings was excellent, but because of the extremely low thermal conductivity, coating thickness and the adherence of sludge, the overall heat transfer coefficients were very small. Because of these disadvantages, it seems that the use of coated steel tubes is still not practical.