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Effect of Thiosulfate on the Pitting Corrosion of Nickel Base Alloys in Chloride Solutions

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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.

Product Number: 51317--9036-SG
ISBN: 9036 2017 CP
Author: Mariano Kappes
Publication Date: 2017
Industry: Science of Corrosion
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Nickel base alloys 600 690 and 800 are used in steam generator tubing of nuclear reactors. These alloys can suffer pitting corrosion problems in chloride solutions in the presence of thiosulfate in a similar manner to what was studied in more detail in stainless steels. Thiosulfate may be present in the environment to which these alloys are exposed in service as it is a product of the reaction of sulfate impurities with the hydrazine used in the steam generator secondary water chemical treatment.In this paper the pitting corrosion susceptibility of alloys 600 690 and 800 in deaerated 0.1 M and 1 M chloride solutions at room temperature was studied. The effect of thiosulfate additions was studied in molar ratios of thiosulfate:chloride between 10^-3 and 1. Pitting potential (EP) and repassivation potential (ERP) were measured by cyclic potentiodynamic tests. The pitting potential measured potentiodynamically was a function of the thiosulfate:chloride ratios. The pitting potential exhibited a minimum at a certain thiosulfate:chloride ratio. The ratio corresponding to this minimum was a function of the alloy type and chloride concentration. In general alloy 800 was the most resistant in thiosulfate-0.1M chloride solutions while alloy 690 was the most resistant in thiosulfate-1M chloride solutions. The pitting potentials measured potentiodynamically and potentiostatically were compared.. Results were analyzed with the available models of thiosulfate effect in pitting of stainless steels. The microstructure of the alloys was characterized by optical and scanning electron microscopy. It was found that it consisted in twinned equiaxed grains with discontinuous dispersion of nanometric Cr carbides at grain boundaries. Also micrometric faceted Ti rich particles were observed at grain boundaries and within grains.

Key words: UNS N06600, UNS N06690, UNS N08800, localized corrosion, thiosulfate, chloride

Nickel base alloys 600 690 and 800 are used in steam generator tubing of nuclear reactors. These alloys can suffer pitting corrosion problems in chloride solutions in the presence of thiosulfate in a similar manner to what was studied in more detail in stainless steels. Thiosulfate may be present in the environment to which these alloys are exposed in service as it is a product of the reaction of sulfate impurities with the hydrazine used in the steam generator secondary water chemical treatment.In this paper the pitting corrosion susceptibility of alloys 600 690 and 800 in deaerated 0.1 M and 1 M chloride solutions at room temperature was studied. The effect of thiosulfate additions was studied in molar ratios of thiosulfate:chloride between 10^-3 and 1. Pitting potential (EP) and repassivation potential (ERP) were measured by cyclic potentiodynamic tests. The pitting potential measured potentiodynamically was a function of the thiosulfate:chloride ratios. The pitting potential exhibited a minimum at a certain thiosulfate:chloride ratio. The ratio corresponding to this minimum was a function of the alloy type and chloride concentration. In general alloy 800 was the most resistant in thiosulfate-0.1M chloride solutions while alloy 690 was the most resistant in thiosulfate-1M chloride solutions. The pitting potentials measured potentiodynamically and potentiostatically were compared.. Results were analyzed with the available models of thiosulfate effect in pitting of stainless steels. The microstructure of the alloys was characterized by optical and scanning electron microscopy. It was found that it consisted in twinned equiaxed grains with discontinuous dispersion of nanometric Cr carbides at grain boundaries. Also micrometric faceted Ti rich particles were observed at grain boundaries and within grains.

Key words: UNS N06600, UNS N06690, UNS N08800, localized corrosion, thiosulfate, chloride

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