ABSTRACT
Pitting corrosion susceptibility of UNS N06600, UNS N06690 and UNS N08800 was studied in pure 1 M NaCl with and without Na2S2O3 additions. The alloys were tested in the as-received (AR), solution annealed (SA) and aged (SA + A) conditions. The pitting corrosion resistance of UNS N06600 was by far the lowest due to its higher Ni and lower Cr contents, being Ni the most detrimental and Cr the most beneficial alloying element for avoiding sulfur adsorption. The effect of thiosulfate additions to 1 M NaCl on the pitting corrosion resistance of the alloys depended on its concentration. Addition of 0.001 M Na2S2O3 was the most detrimental for UNS N06600, while 0.01 M Na2S2O3 was the most detrimental for the other alloys, as determined in potentiodynamic tests. Complete inhibition of pitting corrosion was achieved for UNS N06690 by addition of 1 M Na2S2O3. Generally, SA + A alloys showed higher pitting corrosion susceptibility and more metastable events than SA alloys. UNS N08800 and UNS N06690 tested in 0.001 M Na2S2O3 + 1 M NaCl showed chloride pitting corrosion at high potentials (pits with a lacy cover) and thiosulfate + chloride pitting corrosion at lower potentials (hemispherical pits), with a stable passivity range between those two pitting modes.
INTRODUCTION
Sulfate (SO42-) is a common impurity found in steam generator secondary water of nuclear power plants. Under reducing conditions, due to addition of hydrazine (N2H4) to secondary water, sulfate can be reduced to tetrathionate (S4O62-), thiosulfate (S2O32-) and sulfide (S2-) ions.1 Those are aggressive species that can promote stress corrosion cracking, pitting and crevice corrosion of passive alloys.2 In particular, thiosulfate promotes pitting corrosion in UNS N06600 (alloy 600)3-6, UNS N06690 (alloy 690)4,7 and UNS N08800 (alloy 800)3,4,8-10 currently used in steam generator tubes.
Thiosulfate promotes pitting corrosion by reducing on the metal surface and forming an adsorbed sulfur (S) or sulfide layer11, which prevents repassivation.2 The thermodynamics of thiosulfate reduction to yield sulfur layers (Sads) has been studied by Marcus and Protopopoff. These layers are stable in a broad region of pH and potential when deposited over iron (Fe), nickel (Ni) or chromium (Cr). For a pH of about 2, representative of acidification inside sulfate-thiosulfate pits12, and a bulk dissolved sulfur concentration of 10-4 mol/kg, Sads dominates in the potential range from -0.6 to 0.4 VSHE for a pure Fe substrate at 25°C.13 A similar behavior is exhibited for Ni and Cr, with Ni having the largest Sads stable zone. Sads stable region overlaps with the passive domains of Fe, Ni and Cr, thus explaining why it hinders passivation or repassivation.13,14