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...
Constant pull rate tests were conducted on tensile specimens of Inconel Alloy 600, Incoloy Alloy 800, and Type 304 stainless steel in deaerated 10% NaOH solution at 288 C (550 F) with a cover gas of 5% H2 in N2. The pull rate used for most experiments was 3.3 x 10−6 cm/s, which corresponds to an initial strain rate of 3 x 10−6 s−1. The electrical potential of the specimens was controlled by a potentiostat using a nickel wire as a hydrogen reference electrode. Under open circuit conditions, Type 304 stainless steel specimens cracked rapidly, but Alloys 600 and 800 specimens exhibited only ductile fracture. However, cracks readily formed in Alloy 800 specimens at potentials in the +50 to +300 mV range and in Alloy 600 specimens at potentials in the +150 to +250 mV range. Scanning electron microscope (SEM) photographs of some of the cracked specimen surfaces showed the transition from ductile to brittle fracture as a consequence of changes in the specimen's electrical potential. Longitudinal metallographic cross sections also revealed the grain structure and the mode of cracking. They showed that the cracks were intergranular in Alloy 600 and Type 304 stainless steel specimens, and were transgranular in Alloy 800 specimens.
This paper describes slow strain-rate test equipment, operable at elevated temperatures and pressures, that includes electrochemical potential control capability. Applications in caustic stress corrosion cracking (SCC) studies of both nuclear steam generator and fossil boiler materials are presented. Electrochemical potential regions for stress corrosion cracking of Inconel alloy 600, Incoloy alloy 800, and Type 304 stainless steel are compared to results obtained using constant load specimens. Tests with titanium stabilized Alloy 800 (Sanicro 30) tubular specimens also demonstrate the effect of potential on cracking mode. A comparison of SCC test results, using both long term exposure tests and short term straining electrode tests, is made between a mild steel and its weld metal. In these examples, favorable comparisons are obtained with tests using conventional methods, proving the value and usefulness of the slow straining device in accelerating SCC studies and in defining more accurately conditions under which SCC can occur.
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