Corrosive Characteristics of Alloy 690 Investigated by Electrochemical Analysis

Abstract

Pitting corrosion is one of the major damage forms for Alloy 690, which is one of the usual used materials in SG tubing. It is very important to understand corrosion behavior deeply to ensure the structural integrity. In this paper, the electrochemical experiments are used to analyze the corrosion characters of Alloy 690. The results show that the corrosion rate is nearly to 0.00948 mm/a at 25 ºC, 0.01 mol·L -1 NaCl+0 mg·L -1 O 2 . The repair of the pitting is easy when the oxygen content is relatively high. Chloride ions in the solution have a certain impact on the repair potential and great influence on the pitting initiation potential. The pitting initiation potentials change from 0.64 V to 0.50 V then to 0.37 V (referring to SCE) when the content of Chloride ions increases from 0 mol/L to 0.001 mol/L to 0.01 mol/L. The competition between repair and breakdown will lead to the result of the peaks current. The repair of oxide film becomes more difficult under high potential. The difference value between pitting initiation potential and repair potential are -44.1 mV，0.0 mV，186.0 mV and 245.2 mV, respectively, when C Cl-equal to 0 mol·L -1 , 0.001 mol·L -1 , C Cl-=0.005 mol·L -1 and C Cl-=0.05 mol·L -1 . The above conclusions are important for recognizing the corrosion mechanism of Alloy 690.