Anionic and Cationic Effects on the Crevice Corrosion Susceptibility of Alloy 22
- PDF / 413,918 Bytes
- 8 Pages / 595 x 841 pts Page_size
- 41 Downloads / 207 Views
Anionic and Cationic Effects on the Crevice Corrosion Susceptibility of Alloy 22 Ricardo M. Carranza1 and Raul B. Rebak2 1 Dep. Materiales, Comision Nacional de Energia Atomica Instituto Sabato, Universidad Nacional de San Martin / CNEA Av. Gral. Paz 1499, B1650KNA San Martin, Buenos Aires, ARGENTINA 2 GE Global Research, 1 Research Circle, CEB2505, Schenectady, NY 12309, USA ABSTRACT Alloy 22 is highly resistant to all forms of corrosion; however, it may suffer crevice corrosion in presence of chloride ions especially at temperatures higher than ambient and at anodic potentials. The susceptibility of Alloy 22 to suffer crevice corrosion is highly dependent on the type of electrolyte solution that is in contact with the alloy including variety of species in solution and their relative concentration. Laboratory research has shown that at a constant chloride concentration, the susceptibility of Alloy 22 to crevice corrosion is not influenced by the nature of the cations present in solution. On the other hand, that nature of the anions is highly influential. Of the anions present in ground water, only chloride is detrimental and the others are inhibitors or innocuous for crevice corrosion susceptibility. That is, the presence of the other anions counter balances the negative effect of chloride. The inhibition effect is explained and the likelihood that Alloy 22 would suffer crevice corrosion in contact with ground water is discussed.
INTRODUCTION Alloy 22 (N06022) belongs to the corrosion resistant family of Nickel-ChromiumMolybdenum (Ni-Cr-Mo) alloys. The required composition and mechanical properties of this family is described in ASTM B575 [1]. Alloy 22 has been especially designed to offer resistance to all forms of corrosion in a variety of environments, from highly acidic to highly alkaline. Ni offers protection against hot chloride cracking and against hot caustic conditions, Cr offers protection against oxidizing acids, and Mo offers protection against reducing acids [2]. Cr, Mo and W act synergistically to offer resistance to localized corrosion. Under certain tight occluded gap conditions, Alloy 22 may be susceptible to localized corrosion (mainly crevice corrosion) in chloride-containing solutions. The susceptibility to crevice corrosion increases as the chloride concentration, the temperature, and the potential increase [3-10]. The presence of nitrate (NO3-) and other oxyanions in the solution minimizes or eliminates the susceptibility of Alloy 22 to crevice corrosion [8-24]. The amount of nitrate or the value of the ratio RN = [NO3-]/[Cl-] has a strong effect on the susceptibility of Alloy 22 to crevice corrosion [16-24]. The higher the nitrate to chloride ratio, the stronger is the inhibition by nitrate. It has also been reported that other ions in solution will also inhibit the crevice corrosion by chloride ions. These ions include sulfate (SO42-) [3,20], carbonate (CO32-) [14-15] and fluoride (F-) [21].
ANIONIC AND CATIONIC COMPOSITION OF THE GROUND WATER Alloy 22 is a candidate material for the oute
Data Loading...
