Abstract
Exposure and electrochemical tests have been done with UNS NI0276 (Alloy C-276) and UNS N06625 (Alloy 625) in NaCl solutions, 0.1% - saturation, at 25-200 C (77-392 F) to establish T-Cl- diagrams for the localized corrosion of these high performance nickel alloys and to compare the observed corrosion with that predicted on the basis of their electrochemical behaviour. The exposure tests were done in aerated solutions with specimens fitted with artificial crevices formed between the metal and a slotted PTFE washer. The electrochemical tests comprised pitting scans in deaerated solutions and the following of the corrosion potentials during some of the exposure tests.
Alloy C-276 was more resistant to attack than Alloy 625. It appears that there is a critical temperature for each metal below which localized corrosion will not occur regardless of the NaCl concentration. Thus alloy C-276 was not attacked at temperatures < 150 C (302 F) and Alloy 625 was not attacked at < 125 C (257F). The existance of such critical temperatures would have important implications for the use of these alloys under heat transfer conditions where NaCl concentration may occur, including their use as container materials for nuclear waste disposal which is the basis of the present study.
The localized corrosion is considered to be a special form of pitting attack which occurs preferentially in crevices and was observed under conditions where the pitting scans showed large classical hysteresis loops with passivation breakdown at potentials which decreased to more active values at elevated temperatures. Such potentials were attained during the exposure tests.
The resutls are compared with those for some titanium alloys which are an alternative material for use in high temperature chloride solutions.
