Electrochemical Behavior and Its Relation to Composition and Sensitization of NiCrMo Alloys in ASTM G-28 Solution

Nickel-chromium-molybdenum alloys are usually tested in a ferric sulfate-sulfuric acid solution according to ASTM(1) G-28 to ensure that intergranular attack caused by sensitization of grain boundaries will not occur. An attempt was made in this case to show how the application of electrochemical techniques allow a better understanding of the test. Current density (CD) potential curves were established for various NiCrMo alloys with up to 22% chromium, 13 to 16% molybdenum, up to 3% tungsten, and up to 3% iron in 50% sulfuric acid, heated close to the boiling point and compared to composition and microstructure of the alloys. In addition, potentiostatic measurements in 50% sulfuric acid and at free corrosion potential in ferric sulfate + 50% sulfuric acid were performed. Electrochemical testing can provide more reliable test conditions than the ASTM immersion test, and shorter testing times might be possible. Whereas, reliable testing of Alloy C-276 for irregularities requires lower sulfuric acid concentrations than 50%, the Alloy C-22 type of materials exhibits a sufficient degree of passivity. The time-temperature-sensitization (TTS) diagram indicates sensitization after an anneal of 1 h in the 750- to 800-degree range. Sensitization makes passivation more difficult and increases the passive current primarily because of enlargement of the corroding surface. Tungsten inert gas (TIG) welds show a more noble behavior than the base metal. Substituting molybdenum for tungsten in the 22% chromium-bearing alloys results in essentially the same electrochemical and corrosion behavior in the test solution and in the 50% sulfuric acid.