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51316-7824-Hot Corrosion of Nickel-Chromium Alloys in a Molten Eutectic Salt Environment

Picture for Hot Corrosion of Nickel-Chromium Alloys in a Molten Eutectic Salt Environment
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Product Number: 51316-7824-SG
ISBN: 7824 2016 CP
Author: Vilupanur Ravi
Publication Date: 2016
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Aircraft engines turbines and industrial machinery operating at high temperatures near marine environments are prone to accelerated degradation of their components due to corrosion when their surfaces are coated in a thin film of fused salt (“hot corrosion”). Typical salts formed under these conditions are sodium sulfate from fuels and sodium chloride from the marine environment. In general the addition of chromium to alloys is known to increase their corrosion resistance especially in mitigating attacks from molten sulfates. However studies on the systematic increases in chromium contents on the resistance of binary nickel-chromium alloys to molten salts are sparse. We report on the corrosion kinetics of Type II hot corrosion using Ni-Cr alloys in Na2SO4 - 30.8 wt% NaCl eutectic environments at 700° and 800°C using a novel experimental arrangement.A salt coat method was developed to create a thin salt film necessary for Type II hot corrosion attack. Samples of Ni-Cr alloys ranging from 2.5 – 10 wt% Cr were coated with 8 mg/cm2 of the Na2SO4 / NaCl eutectic and tested under controlled environmental conditions with a reservoir of excess melt to stabilize the salt films. Tests were run from 25 – 100 h and the corrosion attack was characterized by X-ray diffraction analysis optical and electrical microscopy. XRD patterns show that with the exception of pure nickel and Ni-2.5 wt% Cr chromium oxide is present in all other Ni-Cr alloy compositions studied at 25 h at 700°C. The amount of chromium oxide was found to increase for test coupons tested at 800°C relative to those at 700°C. DC electrochemical techniques were used to determine the corrosion rates of these model alloys with intent of validating the reliability of high temperature electrochemistry as an accelerated test for hot corrosion. A three electrode arrangement using platinum as a reference electrode was developed to conduct the electrochemical measurements. This validation was confirmed using destructive metallography of the electrochemically tested specimens and those tested using the salt coat method.
Aircraft engines turbines and industrial machinery operating at high temperatures near marine environments are prone to accelerated degradation of their components due to corrosion when their surfaces are coated in a thin film of fused salt (“hot corrosion”). Typical salts formed under these conditions are sodium sulfate from fuels and sodium chloride from the marine environment. In general the addition of chromium to alloys is known to increase their corrosion resistance especially in mitigating attacks from molten sulfates. However studies on the systematic increases in chromium contents on the resistance of binary nickel-chromium alloys to molten salts are sparse. We report on the corrosion kinetics of Type II hot corrosion using Ni-Cr alloys in Na2SO4 - 30.8 wt% NaCl eutectic environments at 700° and 800°C using a novel experimental arrangement.A salt coat method was developed to create a thin salt film necessary for Type II hot corrosion attack. Samples of Ni-Cr alloys ranging from 2.5 – 10 wt% Cr were coated with 8 mg/cm2 of the Na2SO4 / NaCl eutectic and tested under controlled environmental conditions with a reservoir of excess melt to stabilize the salt films. Tests were run from 25 – 100 h and the corrosion attack was characterized by X-ray diffraction analysis optical and electrical microscopy. XRD patterns show that with the exception of pure nickel and Ni-2.5 wt% Cr chromium oxide is present in all other Ni-Cr alloy compositions studied at 25 h at 700°C. The amount of chromium oxide was found to increase for test coupons tested at 800°C relative to those at 700°C. DC electrochemical techniques were used to determine the corrosion rates of these model alloys with intent of validating the reliability of high temperature electrochemistry as an accelerated test for hot corrosion. A three electrode arrangement using platinum as a reference electrode was developed to conduct the electrochemical measurements. This validation was confirmed using destructive metallography of the electrochemically tested specimens and those tested using the salt coat method.
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