Study On The Corrosion And Inhibition Mechanism Of Low Pressure Steam Turbine In Phase Transition Zone

Safety and economic operation of steam turibine is directly related to the safety and economic benefits of the entire power plants Steam turbines have experienced decades of corrosion,most outage hours are due to the corrosion of in low pressure steam turbine phase transition zone(PTZ),PTZ in low pressure steam turbine is susceptible to corrosion due to corrosive impurities concentrated in the early condensates and form a strong corrosive environment This paper analyzed the corrosion mechansim of corrosion impurities on the corrosion of low pressure steam turbine PTZ by studied the effect of Cl-,SO42-and acetic acid on the 2Crl2MoV steel corrosion behavior in early condensates and the effect of SO42-and acetic acid on conjunction with chloride on the pitting corrosion of 2Cr12MoV steel in early condensates,which provided guidance for assessing and control of corrosion in low pressure steam turbine PTZ.The inhibition effect and mechanism of octadecylamine and ethanolamine on the corrosion of low pressure steam turbine PTZ stuied to provided guidance for assessing the feasibility of shutdown protection using octadecylamine at high temperatures and the effect.of ethanolamine control of corrosion in low pressure steam turbine PTZ.The main results as below:(1)The Cl-ions act as a catalyst role in the 2Cr12MoV steel corrosion process,and destry passive film.The corrosion rate of 2Cr12MoV steel increased with the concentrtion of Cl-ions increasing in early condensates.When the concentration of Cl-ions higher than 5ppm,2Cr12MoV steel changed form uniform corrosion to pitting corrosion due to combined effect of Cl-accelerate hydrolysis of metal ions inducing local acidification and local corrosion of 2Cr12MoV steel under passive film.(2)SO42-ions just accelerate uniform corrosion of 2Cr12MoV steel and without cauing pitting corrosion in early condensates,SO42-accelerate corrosion due to SO42-involved an intermediate dissolution stage.SO42-ions supperssed pits initiated by corrpetitive absorption with Cl-ions and pits growth by forming tow solubility salt with metal ions inside the pits and covered the pit mouths.With an increase in SO2-ions concentration,the effect of suppressed pits initiated increased and pits growth reduced.(3)HAc accelerates corrosion of2Cr12MoV steel and promotes pits initiated and growth of 2Cr12MoV steel in the early condensates containiong Cl-ions.The mechanism of HAc accelerates 2Cr12MoV steel corrosion is HAc dissociates and provides an additional source of free hydrogen ions near the steel surface increases cathodic reaction and the mechanism of HAc promotes pits initiated and grow are HAc increases corrosion rate and decreases pitting corrosion resistance of 2Cr12MoV steel in early condensates to promotes pits initiated and reduces pits grow resistance by increases the solubility of metal ions to reduce corrosion products inside the pitsand covered the pit mouths to promotes pits grow.(4)Octadecylamine form a protective film on the surface of 2Cr12MoV steel inhibited HAc promotes corrosion.When the concentration of HAc is lower than 10mg/L,the effect of HAc on accelerates corrosion and promotes pits initiated and growth of 2Cr12MoV steel in the early condensates containiong Cl-ions can absolutely inhibited by octadecylamine which added in the shutdown protection process.We speculate form the results of tests that the low molecular organic acids form octadecylamine decomposition has no corrosion harm for PTZ of lo w pressure steam turbine because of octadecylamine inhibited corrosion,and using octadecylamine for shutdown protection at high temperatures is feasible.Four times octadecylamine for shutdown protection at high temperatures(the reheat steam temperature was 542??559?)carried out at three units in one power plant found no signs of corrosion of low pressure steam turbine in PTZ,which proved the accurracy of test results.(5)The key for ethanolamine inhibited the effect of HAc on accelerates corrosion of 2Cr12MoV steel and promotes pits initiated and growth of 2Cr12MoV steel in the early condensates containiong Cl-ions is control the pH of early condensates.Ethanolamine used as alkalizing agent is useful for control the corrosion of low pressure steam turbine in PTZ.