Using jet-in-slit apparatus, erosion-corrosion-proof test of 10 kinds of copper base alloys was conducted. Since a 1% copper chloride (II), CuCl2, aqueous solution was adopted for the test liquid, corrosion rate of the test specimens was accelerated over 200 times higher than the rate of these materials in the field (as used for the potable water valves). Nevertheless, the ranking order of durability for the materials based on the test results agreed well with the ranking based on the experience of engineers who manufactured the water valves. Thus, owing to the materials comparison test, the reliability of jet-in-slit test methodology was confirmed. Further, a jet-in-slit apparatus was improved for conducting tests under the conditions of high temperature and high pressure which simulated the boiler feed water. With this jet-in-slit, corrosion tests were carried out on carbon steel and low alloy steel under various environmental conditions such as temperature, pH, and oxygen concentration as well as fluid flow conditions such as ordinary flow and reverse flow. Based on the test results the generation mechanism of erosion-corrosion on carbon steel in the water at elevated temperatures was clarified, which has made it possible to predict the erosion-corrosion damage on carbon steel under any environmental and fluid flow condition. Lastly, the mutually contradicting intentions of laboratory corrosion testing methodology were discussed: one is to obtain the test result in the shortest testing duration and another is to simulate most closely the corrosion mechanism of materials in the field.