Stress Corrosion Cracking of Stainless Steel and Nickel Alloys at Controlled Potentials in 10% Caustic Soda Solutions at 550°F

Abstract

Potentiodynamic polarization curves were obtained in deaerated 10% NaOH solution at 550~ for nickel, Inconel 600,1 Incoloy 800,1 and Type 304 stainless steel. The curves for the first three materials exhibited narrow current density peaks at the same value of potential, indicating that these peaks were caused by nickel corrosion or by the oxidation of a lower oxide of nickel. Multiple stress corrosion cracking tests were performed on spring-loaded bent-beam specimens in deaerated 10% sodium hydroxide solution at 550~ at controlled potentials and at open-circuit conditions. A well-defined critical potential range was observed for the stress corrosion cracking of Inconel 600 specimens. Cracking of Incoloy 800 specimens at controlled potentials was difficult to reproduce. However, cracking could be reproduced under opencircuit conditions with a nitrogen cover gas.Type 304 stainless steel was originally selected for the fabrication of heat-exchanger tubing for pressurized water reactors in order to reduce corrosion to a minimum. However, stress corrosion cracks can be formed in this alloy as a result of accidental entry of chlorides into the boiler water. Accordingly, it has now been generally replaced by the nickel alloys, Inco]oy 800 and Inconel 60:0, which are very resistant to chloride attack. They are, nevertheless, susceptible to stress corrosion cracking in solutions of strong alkaliesThe heat flux through the steam generator tube can provide a concentrating mechanism for impurities in the boiler water. In regions where the water circulation is restricted, and particularly under conditions of high heat transfer, a steam blanket can form at the tube surface. The thermal insulating effects of the steam then permit the surface to be superheated with respect to the boiler water. This superheat allows impurities to be concentrated by factors which have been estimated to exceed 104 so that a few parts per million of free NaOH can build up locally to several weight per cent. This is particularly likely to occur in the hot leg of the U-tube steam generator where the temperature differential between the primary and secondary coolants is greatest (1).Several investigations of the stress corrosion behavior of stainless steels and nickel alloys in caustic soda solutions at high temperatures have been reported (2-4) but without control of the specimen potential. However, Mazille and Uhlig (5) and Parkins (6) have pointed out the importance of specimen potential on both the occurrence and the rate of caustic stress corrosion cracking. The purpose of this paper is to demonstrate the importance of electrical potential on the stress corrosion cracking behavior of Type 30,4 stainless steel and the nickel alloys, Inconel 600 * Electrochemical Society Active Member. 1 Registered trademarks of the International Nickel Company. and Incoloy 800, in deaerated 10% by weight NaOH solution at 550~ An attempt is also made to determine the specimen potentials for maximum rates of cracking for each material. The results reported here are a...

Effect of Silicon Compounds on Stress Corrosion Cracking of Alloy 600 in Caustic Solutions

“…Effect of the applied potential on the susceptibility of alloy 600 to SCC, observed in short tests by other authors, [14][15][16] seemed to disappear when the testing time was sufficiently long, in spite of the different characteristics regarding the composition and thickness of the oxide layers formed. Surfaces of the specimens tested at E corr were covered by a layer of pure nickel and not by a layer of oxide, as was to be expected given the presence of cracking.…”

“…In SCC tests, stressed specimens are used, manufactured from as-received steam generator tubes. [15][16] In the present tests, stressed C-ring specimens were used for polarization curves and for SCC tests, since certain differences were observed in the curves obtained from stressed and nonstressed specimens. Furthermore, this methodology ensured the potentials applied during the corrosion tests were those selected from the polarization curves.…”

Effect of Chemical Composition, Thermal Treatment, and Caustic Concentration on the SCC Behavior of Alloy 800

“…2 Caustic stress corrosion cracking (SCC) has been experienced mainly by alloy 600; accordingly, numerous studies have been performed on this subject. Less attention has been paid to alloy 800 because the corrosion performance in service has been satisfactory up to now.…”

On the Use of NaOH Solution to Simulate the Crevice Conditions of a Nuclear Steam Generator

“…[ 9 , 10 , 13 , 14 , 15 , 16 ]. Accordingly, the following solutions have been widely used: solutions containing sodium hydroxide (NaOH) [ 17 , 18 , 19 , 20 , 21 , 22 ], sodium chloride (NaCl) [ 23 , 24 , 25 ], sulfate (SO 4 2− ), sulfite (SO 3 2− ), tetrathionate (S 4 O 6 2− ) and thiosulfate (S 2 O 3 2− ) [ 26 , 27 , 28 , 29 ], lead oxide (PbO) [ 30 , 31 , 32 ], and silicone compounds (SiO 2 , Na 2 SiO 3 ) [ 33 , 34 , 35 ]. Among them, NaOH solution has been frequently used to investigate the performance of SG tubing under impurity-concentrated crevice conditions.…”