Effects Of Some Factors In Water Chemistry On Stress Corrosion Charackin Of Advanced Dissimilar Metal Weld SA508-52M-316L Used For Nuclear Power Plants

The connection between the low alloy steel pressure vessel nozzle and stainless steelsafe end of main piping in a pressurized water reactor （PWR） nuclear power plant isdissimilar metal weld. During operation, failure at this part caused by stress corrosioncracking （SCC） in high temperature high pressure water environments can be a potentialproblem affecting safe operation of the plant. In this work, effects of several factors in waterchemistry on the SCC behavior of main parts of an advanced dissimilar metal weldSA508-52M-316L used in AP1000as third generation nuclear power plant were investigatedby means of slow strain rate testing （SSRT） and electrode potential monitor/control. Thewater chemistry factors were chloride-sulfate impurities contamination and electrodepotential varied in the range from-780to+400mV（vs.SHE）, which simulated theelectrochemical conditions of the weld in the service environments from ideal waterchemistry to bad water chemistry with significant contamination of oxygen/chloride/sulfate.Following research works were carried out and main conclusions were obtained.1) The SCC behaviors of dissimilar metal weld SA508-52M in simulated PWR primarywater （plus the impurities） environments at290℃were investigated. The results showed thatthe electrode potential which is closely related to the engineering parameters of the dissolvedoxygen and flow rate had an important effect on SCC. In simulated PWR primary waterenvironment without impurities comtamination, the specimens always failed in the bulk zoneof the Ni-based weld metal in ductile appearances when tested in the potential range from-720to+200mV（vs.SHE）. When electorde potential was raised to+300mV and+400mVwhich corresponded to oxygen-contaminated water chemistry, the weld exhibited brittlefracture by significant SCC. The area around the SA508-52M interface was the weakest placewhere transgranular SCC happened both along the interface and in SA508heat-affected zone,intergranular SCC occurred in the Ni-based weld metal close to the interface. This means thata critical potential for SCC, ESCC, exists between+200～+300mV（vs.SHE）. Contamination ofthe primary water with chloride and/or sulfate increased the SCC susceptibility by decreasingESCC, with details as follows: when adding10mg/L Cl-into this environment, ESCCwasdecreased into the range of+100⁺200mV（vs.SHE）; when adding10mg/L SO₄^2-in thisenvironment, ESCCwas in the range of-100⁺100mV（vs.SHE）; when adding10mg/L SO₄^2-and10mg/L Cl-together into the environment, ESCCwas in the range of-200⁺100mV（vs.SHE）. When tested at some electrode potentials which were within200mV lower thanESCC, the specimens fractured in the bulk zone of the Ni-based weld metal with ductileappearances, but some signs like tiny SCC were observed around SA508close to the interface. The SCC signs increased with the rising of electrode potential and decrease ofstrain rate.2) The SCC behaviors of weld52M-316L in the same high temperature environmentsat290℃were studied. The SCC susceptibility of52M-316L did not show simple regularitywith the rise of electrode potential. The Nickel-based weld metal had a better resistance toSCC than the316L stainless steel. Adding impurities of Cl-and SO₄^2-increased thesuscptibity of the weld to SCC. For52M-316L, Cl-was more harmful than SO₄^2-.3) As the low alloy steel part of the SA508-52M-316L was not big enough, a block ofChinese homemade low alloy steel SA-508Ⅲ with similar chemical composition especiallylow sulfur was used to study the SCC behavior in the same high temperature waterenvironments at290℃. In three different high temperature water environments, the SCCsuscptibity increased with the rising of electrode potential. Adding Cl-and SO₄^2-into thewater increased the SCC suscptibility by decreasing ESCC. The SCC cracks were mainlynucleated at inclusions and propagated in fan-shaped quasi-cleavage transgranular mode.