Online Monitoring of Infrared Pyrometer Captured Thermal Signature for Prediction of Iron Dilution and its Effect on Corrosion Properties of Laser Deposited Inconel 625 Coating on SS 304L

Abstract

Abstract The current study deals with understanding the implication of iron (Fe) dilution from underlying stainless steel (SS) substrate (SS304L) on corrosion properties of laser deposited Inconel 625 through insitu/online monitoring of thermal signatures captured by infrared (IR) pyrometer. Online thermal signature monitoring helps to detect any changes in temperature during deposition. The thermal signature showed an early initiation of solidification at higher temperatures with the increase in laser input power from 600 W to 1200 W, thus indicating the possibility of an early nucleation. Higher fluence led to increased penetration depth and meltpool life coupled with a greater extent of Marangoni convection, thus resulting in higher % Fe dilution into the coating. This increase in Fe dilution from the SS 304L substrate might have caused early nucleation due to the formation of higher melting point iron carbides in the deposited tracks. The elemental dispersive X-ray spectroscopy (EDS) and X-Ray diffraction (XRD) data reveal an increment in % Fe dilution with the increase in laser fluence. This increase in Fe dilution might have an impact on the corrosion property of deposited Inconel 625 on SS 304L. Electrochemical corrosion test was performed in a simulated seawater medium to understand the implications of iron dilution on the corrosion property of deposited Inconel 625. Potentiodynamic polarization (PDP) test showed lower Ecorr values for samples deposited under 1200 W as compared to 600 W. Passivation at lower currents was seen for deposit conditions of 600 W and 800 W compared to 1200 W. Nyquist plot obtained from Electrochemical Impedence Spectroscopy (EIS) test showed lower radius of curve at 1200 W laser power compared to 600 W. A decrease in corrosion resistance under higher laser power was also evident from the Bode impedance and phase plots. A decrease in corrosion resistance property with the increase in Fe dilution at higher laser fluence was thus evident from PDP and EIS analysis results. Therefore, an increase in solidification shelf temperature detected through IR pyrometer due to Fe dilution could be seen as an indication of a decrease in the corrosion property of the deposit and thus might be utilized for predicting the corrosion behavior directly from insitu thermal signals and also be helpful in finding the optimum input processing conditions for laser cladding.