유/무기 하이브리드 용액으로 코팅한 스테인리스강의 내식특성

Abstract

This paper was carried out study for the mechanical properties, corrosion resistance, pitting corrosion resistance and fracture toughness due to sensitization, and was identifed the damage cause of a damaged heat exchanger tubes. Stainless steel (STS310S, STS316L and STS347H) using as a heat exchanger material had artificially degradation at a temperature range of 420~720 ℃. The first was evaluated mechanical properties. Nominal stress is slightly increased at 480 and 540 ℃ of STS310S, 600 and 660 ℃ of STS347H, STS316L was not almost changed. Nominal yield strength is decreased with increase of the heat treatment temperature on STS310S and STS316L, and STS347H was increased by about 12% at 660 and 720 ℃. The Cr amount of STS310S increased at 600 and 660 ℃ and decreased at 720 ℃. STS316L and STS347H was almost constant regardless of heat treatment temperature. However, Nb of STS347H is increased up to 540 ℃ while decreased from 600 ℃. The true tensile strength was similar to the base material, regardless of the heat treatment temperature in the three kinds of materials. However, STS316L was some decline at 480~660 ℃ and slightly increased at 720 ℃. STS347H was increased slightly at 660 and 720 ℃. On the other hand, true yield strength was the same tendency as the nominal yield stress. STS310S appeared the dimple to 600 ℃, and showed a brittle fracture at 660 and 720 ℃. STS316L shows a dimple at the base material and 480 ℃, was mixed a dimple and brittle fracture at 540~720 ℃. STS347H was observed the dimple, and dimples exhibited the tendency to decrease according to increasing of the heat treatment temperature. Vickers hardness showed larger according to increasing of the heat treatment temperature. The tensile strength of STS310S was increased in proportion with the Vickers hardness, and after that decreased. Vickers hardness of STS316L was increased, and tensile strength was almost equally. Vickers hardness of the STS347H was increased in proportion to the tensile strength. The second was evaluated the corrosion resistance by salt spray. OIBD-1 coating solution did not corrosion regardless of the heat treatment temperature at STS310S and STS316L, but was slightly corroded at 660 and 720 ℃ of STS347H. On the other hand, OIBD-2 coating solution was occurred the corrosion at the three kinds of materials. In the bending test, the coating layer by the two kinds of coating solution did not occur the peeling at all. Therefore, the attachment ability of coating solution was very excellent. In the boiling test, the coating layer of three types of the specimen were not occurred phenomenon such as wrinkles, swelling, cracking and peeling in a boiling water for 1 hour. But, all the surface were changed by rubbing with fingers. OIBD-1 and OIBD-2 coating solution were satisfied the hardness of 3H and 4H, respectively. The third was carried out an immersion test during 240 hours in acidicic solution and alkaline solution using non-coating specimen, OIBD-1 coating specimen and OIBD-2 coating specimen. The three types of non-heat treated specimens did not cause pitting regardless of whether or not the coating. In acidicic solution, non-coating specimen and OIBD-2 coating specimen was occurs pitting, but OIBD-1 coating specimen did not occur at all. In alkaline solution, all specimen did not cause pitting at all. OIBD-2 coating specimens were less occurrence of pitting than the non-coating specimens. In acidicic solution, STS347H was occurred the most pitting, and STS316L and STS310S were similar. We determined that pitting was inhibited by organic components and Si included in the organic/inorganic hybrid coating solution. The fourth was evaluated fracture toughness. The fracture toughness of the 3 kinds of materials is slightly higher than the base material at 480 ℃, thereafter decreased. The width of variation of fracture toughness of STS316L was smaller than STS310S and STS347H. The fracture toughness of STS347H is higher than that of STS310S and STS316L due to the microstructure and the reinforcement effect for high-temperature strength by adding of stabilizing element, Nb. The last was evaluated the cause for the damaged heat exchanger tube, STS310S. STS310S tube has excellent the heat transfer ability, and is widely used in heat transfer tube material for heat exchange devices. The inside of tube flow the mixtures of gas and water, and the outside flows the hot flame. In this environment, tubes were embrittlement, and were occurred the leakage. Cracks were propagated from inside to outside, and brittle fracture.