Low Potential Pitting Corrosion of Ni-Cr-Fe Alloys in Chloride Plus Thiosulfate Solutions: Determination of Potential and Concentration Boundaries

Low potential pitting corrosion (LPPC) of Alloys 690 and 800 (UNS N06690 and N08800) was studied in neutral solutions, containing chloride ions from 0.1 M to 1 M and thiosulfate ions from 5 × 10−5 M to 10−3 M. LPPC occurred close to the corrosion potential (–0.25 VAg/AgCl) by a synergic effect of the chloride and thiosulfate ions. The threshold concentrations of aggressive species for LPPC occurrence were identified by potentiostatic tests with mechanical scratching of the surface, which yielded more conservative estimations compared to other electrochemical tests. The ranges of potential and concentration of chloride and thiosulfate where LPPC occurred and where it merged with the conventional chloride pitting were determined. The lowest threshold concentrations were measured in Alloy 800, with a lower Cr content than Alloy 690. Thermally-aged Alloys 800 and 690 had lower resistance to LPPC than the corresponding solution-annealed material. In 1 M chloride-based solutions, for aged Alloy 800, LPPC occurred at a thiosulfate concentration as low as 5 × 10−5 M; in 10−3 M thiosulfate-based solutions, the chloride threshold was 0.1 M. For thermally-aged alloys, pits propagated intergranulary in some experiments. In solutions with chloride and thiosulfate concentrations close to the threshold required for pitting, repassivation of LPPC was observed after a sustained period of pit growth, originally initiated by scratching the surface. Such a repassivation has not been reported before in the literature in chloride and thiosulfate solutions. Two possible explanations were presented for this phenomenon.

Optimization of Effective Parameters during Electric Discharge Machining of Incoloy 800

Experimental work is totally focused on the inclusive work material Incoloy 800 because of its low cost and effective thermal properties The aim of this proposed work is to find out the optimum condition using RSM technique under the influence of effective parameters which includes MRR, TWR, Surface Roughness so that process parameters such as pulse on time, current, voltage can be optimized while working with Incoloy 800 as a work material. Incoloy 800 has inevitable properties in heat application and machining because of its nickel, chromium and iron content. It also comes under the category of super alloy having various important application in aerospace industries. Chromium present in the alloy helps in heat resistance. Internal oxidation during machining is resisted by iron present in the alloy. The nickel content present in Incoloy 800 maintains ductile and austenitic structure. Therefore, alloy 800 can be machined easily as per desired shape. Because of its moderate strength, low cost and good resistance to oxidation and carburization at elevated temperatures, alloy 800 and its grades are center of attraction for alloy manufacturing companies