| Water chemistry of a pressurized water reactor (PWR) plantsecondary loop is required to be high in pH and low in dissolved oxygen,in order to protect steam generators (SG). However, under this waterchemistry condition, the carbon steel piping of secondary loop is prone toflow accelerated corrosion (FAC), a corrosion mechanism in which anormally protective oxide layer on a metal surface dissolves in fastflowing water, and underlying metal corrodes to recreate the oxide, andthus the metal loss continues. FAC always occurs in secondary piping ofa PWR nuclear power plant, main steam lines, drain lines and feed waterlines, where saturated steam, two-phase or single liquid phase flow rate ishigh.The purpose of this study is to obtain FAC thinning rate of carbonsteel materials with different concentration of Cr, and under staturatedsteam flow velocity of from 40 to 60m/s. A SA106B with no Cr(SA106B-0Cr), a SA106B with about 0.2Cr (SA106B-0.2Cr) and aSA335-P11 were tested. Two kinds of test specimens are used, tubespecimens with diameter ofΦ2.0 to 2.4mm and disk specimen withdiameterΦ9mm. Experimental temperature is kept constatnt at 273℃,which corresponds to staturated pressure of about 5.8MPa. The mass flowrate is about 20kg/h, which corresponds to the maximum flow velocity of60m/s in a 2.0mm inner diameter specimen. The heating power is about20kW. The pH value is adjusted by ammonia and kept in the range of9.5~10.0. High purity Ar gas is used as the oxygen scavenger, anddissolved oxygen of feed water to test loop is kept below 5ppb. FACthinning rate was evaluated by weight loss of the specimens. Surface morphology of specimen surfaces were characterized by scanningelectron microscopy (SEM) after each tests. At the same time, AnsysCFD & CFX was used to simulate FAC of disk specimens, tubespecimens and straight pipe with orifice.Experiment and simulation results show that, P11 disk specimenshowed a lower thinning rate than SA106B specimens. SEM observationshowed that the partial surface oxide exists on P11 specimens, while nooxide remained on SA106B specimens. Corrosion of pearlite is fasterthan the surrounding ferrite phases in the specimens. Pitting was found onall specimens due to the possibility of invasion of Cl- ions in feed water.Pitting is more prone to occur on SA106B specimens than P11 specimen.FAC phonemon is clearly observed at the entrance of P11 tubularspecimens, where turbulence causes high relative velocity betweenstaturated steam fluid and specimen inner wall. FAC thinning turnsweaker in the middle of the tube specimen, and neglectable near to theexit. Temperature and dryness of fluid greatly affect FAC thinning rate.According to the calculation and A corroding to experimental andsimulation results, a critical dryness of staturated steam to cause theoccurrence of FAC on P11 steel is proposed to be about 92%.Under high-velocity fluid of saturated steam, the corrosion rate of P11steel decreases with the decrease of flow velocity. The highestwall thinning rate is estimated to be less than 0.16mm per year insaturated steam at the temperature of 273℃, and flow velocity of 60m/s.Effects of pipe inner diameter, orifice inner diameter and fluidvelocity on FAC thinning rate of downstream pipe were studied usingAnsys CFD & CFX. The results show that with the decrease of orificeinner diameter, the location of shear force peak moves further from theorifice. Maximum shear force and FAC thinning dangerous zone isestimated to be located between 1 to 2 pipe inner diameter downstream oforifice, where special attention needs to be aroused in refueling outageinspection. The FAC thinning rate of disk specimen is greatly affected bythe impact angle between high-velocity fluid and disk specimen surface.Under 90°impact angle, shear force peaks form a ring pattern around theimpact center of fluid, and the surface morphology of disk specimens after testing is in good agreement with simulation results. |
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