Modified Streicher and 288 C (550 F) electrochemical caustic stress corrosion tests were performed on Alloy 600 to determine the relationship between acid intergranular attack susceptibility and caustic stress corrosion cracking (SCC) susceptibility. Mill annealed and solution annealed materials with and without a subsequent 621 C (1150 F) heat treatment (simulated stress relief) were evaluated. Susceptibility to attack in the Streicher test was greatest for material that had received a 621 C (1150 F) heat treatment, whereas this heat treatment caused the same material to be least susceptible to cracking in the electrochemical tests. The conclusions drawn from these results are: (1) stress relieving Alloy 600 does improve its resistance to caustic SCC, and (2) resistance of Alloy 600 to acid intergranular attack does not imply resistance of Alloy 600 to caustic SCC. Therefore, the results demonstrate the need for selecting corrosion qualification tests which are relevant to service conditions.
The polarization resistance method was used to assess the corrosion rate and to study the oxidation kinetics of carbon steel in a fused nitrate-sulfate-chloride salt mixture at 316°C (600°F) and in an alkali iron trisulfate salt at 677°C (1250°F). The same method was also applied to estimate the corrosion rate of carbon steel through the solidified frozen alkali iron trisulfate salt.
Equilibrium potential/pO2− diagrams were constructed for iron, nitrate, and nitrate-reaction products to aid in the interpretation of corrosion data for the nitrate-rich salt and to determine the various chemical reactions causing the corrosion.
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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