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96095 BINDING OF NITRIDED TYPE 410 STAINLESS STEEL VALVE STEMS CAUSED BY CORROSION PRODUCT BUILD-UP

Picture for 96095 BINDING OF NITRIDED TYPE 410 STAINLESS
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Product Number: 51300-96095-SG
ISBN: 96095 1996 CP
Author: Stephanie M. Coffin, Robert O. Hardies
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Nitrided type 410 stainless steel governor valve stems are used in safety-related reactor core isolation cooling (RCIC) and auxi Iiary feedwater (AF W) pumps at Boiling Water Reactors and Pressurized Water Reactors, respectively. Corrosion of the governor valve stems in the packing assembly area has been noted at at least seventeen nuclear power plants. Corrosion product build-up between the valve stem and packing assembly has resulted in binding of the valve stem at at least nine of these plants. Nitriding is known to degrade the corrosion resistance of stainless steels. This is due to the formation of chromium nitrides in the diffusion layer of the hardened case which leave the surrounding matrix depleted in chromium. The rate of corrosion is substantially affected by the presence or absence of a continuous, adherent compound layer. The compound layer is less susceptible to corrosion than the diffusion layer and acts to protect the underlying diffusion layer from being exposed to moisture. At Calvert Cliffs, the original nitrided stem had a continuous, adherent compound layer and a hardened case of approximately 60 microns in depth. This stem performed acceptably for years. In contrast, the replacement nitrided stems did not have a continuous, adherent compound layer and had a hardened case of approximately 10 microns in depth. These stems performed acceptably for three or four months. This difference in performance is attributed primarily to the absence of an adherent, continuous compound layer in the recently-supplied nitrided stems. In these stems, the corrosionsusceptible diffusion zone was exposed to moisture in the packing assembly area and general corrosion occurred. Since nitrided type410 stainless steel valve stems will corrode in the presence of moisture (although at substantially different rates), the replacement valve stems will be fabricated from Inconel 718. Keywords: stainless steel, valve stems, nitriding, corrosion, binding
Nitrided type 410 stainless steel governor valve stems are used in safety-related reactor core isolation cooling (RCIC) and auxi Iiary feedwater (AF W) pumps at Boiling Water Reactors and Pressurized Water Reactors, respectively. Corrosion of the governor valve stems in the packing assembly area has been noted at at least seventeen nuclear power plants. Corrosion product build-up between the valve stem and packing assembly has resulted in binding of the valve stem at at least nine of these plants. Nitriding is known to degrade the corrosion resistance of stainless steels. This is due to the formation of chromium nitrides in the diffusion layer of the hardened case which leave the surrounding matrix depleted in chromium. The rate of corrosion is substantially affected by the presence or absence of a continuous, adherent compound layer. The compound layer is less susceptible to corrosion than the diffusion layer and acts to protect the underlying diffusion layer from being exposed to moisture. At Calvert Cliffs, the original nitrided stem had a continuous, adherent compound layer and a hardened case of approximately 60 microns in depth. This stem performed acceptably for years. In contrast, the replacement nitrided stems did not have a continuous, adherent compound layer and had a hardened case of approximately 10 microns in depth. These stems performed acceptably for three or four months. This difference in performance is attributed primarily to the absence of an adherent, continuous compound layer in the recently-supplied nitrided stems. In these stems, the corrosionsusceptible diffusion zone was exposed to moisture in the packing assembly area and general corrosion occurred. Since nitrided type410 stainless steel valve stems will corrode in the presence of moisture (although at substantially different rates), the replacement valve stems will be fabricated from Inconel 718. Keywords: stainless steel, valve stems, nitriding, corrosion, binding
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