Study Of Correlation Between Alloy690Thermal Treatment And Corrosion Resistance

Alloy690is civilian nuclear power plant steam generator （SG） heatexchange tube materials, The steam generator is a important equipment in anuclear power plant. China’s energy gap is large, which need to developnuclear power, and there are the great demand for this nickel-based alloys,however, China’s nuclear power plant steam generator heat exchange tubes areall dependent on imports to this day. To our strategic security, we developedthe alloy690.In this paper, we designed same heat treatment temperature and differentheat treatment time, same heat treatment time and different heat treatmenttemperature thermal treatment of alloy690to intergranular corrosion, stresscorrosion, pitting corrosion research programs, also studied the correlationbetween the heat treatment system and its corrosion resistance of alloy690,and discussed alloy690corrosion mechanism.Using double-loop electrochemical potentiokinetic reactivation （DL-EPR）, nitricacid boiling method, dynamic potential electrochemical impedancespectroscopy （DEIS）, we studied resistance to intergranular corrosion of alloy690, and observed the specimen morphology after the corrosion with opticalmicroscope （OM）, scanning electronmicroscopy （SEM）, conclusion is given,in the thermal treatment time and temperature range studied, with theextension of the thermal treatment time or temperature, alloy690resistance tointergranular corrosion is rapidly increasing, When the thermal treatment timeor thermal treatment temperature reaches a certain value, the resistance to intergranular corrosion can achieve the highest, and no longer increase withincreasing thermal treatment time or temperature. Screened of resistance tointergranular corrosion excellent alloy690heat treatment is solution treatmentat1090℃for2min, then at715℃for10h or at750℃for2h.Using the slow strain rate testing （SSRT）, researched resistance to stresscorrosion cracking of alloy690, we concluded that alloy690in roomtemperature air tensile was ductile fracture, while tensile specimens in boiling42%MgCl2＋2%PbCl2solution was ductile fracture, and tensile specimens inthe solution fracture was by the combined effect of many cracks, the final oneor a few cracks develop and result specimen fracture, the fracture and tensiledirection45°. There was a small decline of the tensile strength and yieldstrength to alloy690samples in solution and in the air compared, so thecorrelation between resistance to stress corrosion and heat treatment systemwas little.Using electrochemical method, we studied alloy690resistance to pittingcorrosion performance, and concluded that715℃thermal treatment above15h （15～50h） samples pitting resistance was better, and thermal treatment2hsamples pitting resistance as the raise of thermal treatment temperature （650℃～850℃） gradually increased, while thermal treatment10h samples pittingresistance by the thermal treatment temperature （650℃～850℃） had littleeffect.By transmission electron microscopy （TEM）, X-ray photoelectronspectroscopy （XPS） and electrochemical analyzer, the internal structure,composition, and surface passivation film composition, structure of the alloy690were studied. We concluded that carbides Cr₂₃C₆produced in alloy690grain boundaries, and with the thermal treatment temperature （650℃～850℃）,the carbide shape from elongated, continuous became thick, discontinuous,and near the grain boundary chromium depletion phenomenon was gradually disappearing. There were more twins and slip bands.By the Mott-Schottky curve and the XPS results obtained: the linearsection slope，which was of alloy690samples of different thermal treatment at50℃2mol/L H₂SO₄＋0.001mol/L KSCN solution of the Mott-Schottky curve,were0.08V and0.60V. There were double semiconductor films on the surfaceof the sample, which was more stable n-type semiconductor α-FeOOH whenlower than0.08V, and might be the instability of the n-type semiconductorγ-FeOOH when0.08V～0.60V, and p-type semiconductor Cr₍2O₃or CrO₂when over0.60V.With the increase of the thermal treatment time （0～50h）, the spacecharge layer capacitance C_SCof the passivation film became smaller, but itbecame larger when the thermal treatment time was more than20h, so surfacepassivation film of715℃thermal treatment10～15h alloy690was morecompact, better corrosion resistance.With the thermal treatment temperature （650℃～850℃） increased, thespace charge layer capacitance C_SCof the passivation film became smaller,surface passivation film became denser, corrosion resistance became better,and corrosion resistance was excellent when the thermal treatment reached715℃for10h or750℃for2h.