SCC in high temperature pure water

In 1960 H. Coriou and his colleagues first announced that Inconel 600 alloy produces intergranular stress corrosion in degassed high-temperature pure water. The authenticity of the Inconel 600 alloy has been questioned for more than 20 years, and eventually the water in the nuclear power plant is pressurized with this corrosion. The continuous occurrence of water in the generator’s running primary circuit was eliminated and the Inconel 600 alloy was identified in the high-temperature intercrystalline SCC. At present, consensus has been reached on the intergranular stress corrosion susceptibility of solid solution strengthened alloy (600) and precipitation strengthened alloy (X-750 and 718) in high temperature water.
For the intergranular stress corrosion problem of 600 alloys, extensive and in-depth research has been conducted on the material factors that cause this problem, focusing on both the alloy composition and the organizational structure.
In the Ni-Fe-Cr balance. The low-nickel high-chromium alloys Incoloy 800 and Inconel 690 alloys are insensitive to grain boundary type stress corrosion generated in alloy 600 of high purity water, demonstrating the effectiveness of Cr in the alloy against IGSCC, combined with some test results indicating that at the hydrogenation of 360 °C Hydrogen-containing H3BO, LiOH solution and high-purity water of 300°C and [O], the SCC resistance of the alloy increases with the increase of the chromium content in the alloy, see Figure 2-53 and Figure 2-42, respectively. Chinese studies have pointed out that in iron-nickel-based alloys, 25% Cr makes the SCC resistance of the alloy mutated to an optimal level. This result is the most basic data for the development of 00Cr25Ni35AITi stress corrosion-resistant alloys.
Alloy microstructures related to heat treatment conditions have a significant effect on the stress corrosion behavior of the alloy. In-conel 600 tubing has a fine grain structure with a large amount of intracrystalline carbide precipitates at annealing temperatures below 950°C. There are no carbide precipitates on the grain boundaries and this structure exhibits high sensitivity to intergranular stress corrosion in high temperature water. High-temperature annealing increases grain size, reduces intragranular precipitation, and induces preferential precipitation of carbides at the grain boundaries. This microstructure, despite being not immune to scc, significantly increases resistance to intergranular stress corrosion. The solution treated alloys were aged at 700 degrees Celsius for several hours. Discontinuous Cr7C3 precipitates appeared at the grain boundary of the alloy. This structure made the alloy more resistant to intergranular stress corrosion. The extent to which this treatment is effective depends on the solution annealing temperature, and a reasonable solution treatment should be above 1000 degrees to ensure that sufficient carbon is dissolved in the solid solution. The 700 °C aging treatment should be long enough (over 15 hours) to repair or partially repair the chromium-depleted zone formed along the grain boundary. This heat treatment system eventually evolved into the desensitization treatment that Inconel 600 and Inconel 690 alloy must perform. Technological measures.
So far, 00Cr25Ni35AITi and Inconel690 alloys with excellent stress corrosion resistance in high temperature and high pressure water have become the preferred materials for the heat transfer tubes of PWR nuclear power plant evaporators. The common feature of both materials is a chromium content of more than 25%.
In addition to solid-solution strengthening alloys for the evaporator heat transfer tubes of pressurized water reactor power plants, high-strength materials are used for the high-strength nails and bolts used in the fuel assembly fixed springs and high-strength nails and bolts for reactor cores, which have high strength and are suitable for high temperatures and pressures. Precipitation-strengthened nickel-base alloys with stress corrosion resistance in water have become the main choices. Inconel X-750 (OCr15 Ni70Ti3AINb) and Inconel 718 (OCr19Ni52M03Nb5AITi) have been applied in practical projects. The former has a wide range of applications and a large amount of application. There are many corrosion accidents caused by lanthanum. According to in-depth studies, the stress corrosion resistance of the alloy can be improved by increasing the chromium content of the alloy (Fig. 2-54), and the stress corrosion resistance of the alloy subjected to a single aging treatment can be improved. Better than double aging treatment.