A Study On Corrosion And Inhibition Of 316L Stainless Steel In The Chilling Section Of Coal Gasifier And Q/235 In NH₄Cl Waste Liquid

In coal gasification and refining,large quantities of aqueous effluents were produced.These effluents were processed through the system of piping and equipment.The presence of aggressive ions e.g.Cl^-,and high temperatures cause corrosion in the steel piping and equipments.Stainless steels have very good resistance against corrosion in general;therefore,its utilization in industry is on the rise.One of the major applications of stainless steels is for the storage or processing of liquid effluents of different industries.Similarly,carbon steels are also used widely in refining industry as process piping and in liquid handling equipment.In addition,in order to reduce corrosion,the industry also needs to use inorganic and organic corrosion inhibitors to suppress corrosion.In this study,The effect of sulfate(SO₄^2-)ions on pitting corrosion of stainless steel-316?SS-316?and the inhibition mechanisms of meta-silicate ions(SiO₃^2-)above critical pitting temperature?CPT?in the chilling section of a coal gasifier are studied in the simulated conditions(100 mgL^-1–Cl^-,?10 mgL^-1—NO₃^-,300 mgL^-1–SO₄^2-and 300mgL^-1–HCO₃^-).Potentiodynamic and static polarization techniques were employed to estimate the CPT of SS-316 in the simulated electrolyte.Electrochemical experiments in conjunction with SEM studies demonstrated that sulfate ions prevented the metastable pits from initial passivation and afterwards facilitated pit growth by modifying the pit morphology.SiO₃^2-ions protected the metal surface from chloride ions by absorbing and afterwards inhibited the pit growth and suppressed the metal dissolution by changing the pit solution chemistry.The effects of phosphate ions(PO₄^2-)on the pitting corrosion of stainless steel 316L were studied by electrochemical and thermo-chemical methods.DC polarization measurements showed that the pitting could occur spontaneously above 72 ^oC on SS-316L in chloride containing simulated coal gasification waste water?CGW?.Adding phosphate ions above the critical pitting temperature?CPT?in the simulated CGW will delay the oxygen reduction reactions,reduce the pitting frequency and significantly hinder the pitting growth.The EIS studies show that PO₄^3-ions absorb and modify the passive film through interaction with hydrated Fe species,thereby enhancing the pitting resistance of SS-316L.XRD analysis of SS-316L surface exposed in simulated CGW containing PO₄^3-ions at 170 ^oC further confirmed the existence of iron phosphate in the surface corrosion layer.Corrosion behavior of steel Q-235 was studied in 7.5%NH₄Cl solution without/with various concentration of newly synthesized 2-?2-heptadecyl-4,5-dihydro-1H-imidazol-1-yl?ethanol?HDIE?inhibitor by potentiodynamic polarization,electrochemical impedance spectroscopy and weight loss measurements in 298-338K temperature range.Results showed that inhibition efficiency above 90%is achievable at298K with 0.5 mML^-1 inhibitor concentration.Adsorption studies revealed that HDIE follows Langmuir model and its adsorption mechanism is mixed-type with predominant effect on anodic half cell.It was seen from EDS characterization of corroded steel surfaces that the HDIE retards the corrosion by impeding the chloride attack.Quantum chemical calculations demonstrated that the imidazoline ring and hetero-atoms are active sites.Moreover,an inhibition mechanism was proposed by correlating quantum chemical calculations and experimental corrosion studies.