Electrochemical Corrosion Behavior of Heat Treated HVOF Coatings on ASTM SA213-T22 steel

Herein we report the electrochemical corrosion behavior of pre and post heat-treated composite coatings of NiCrMoFeCoAl-30%SiO2 and NiCrMoFeCoAl-30%Cr2O3 on ASTM SA213-T22 boiler tube steel by high velocity oxygen fuel (HVOF) spraying technique. The samples were subjected to hot molten salt (Na2SO4–60%V2O5) corrosion environment in a tubular furnace at 7000C under thermocyclic conditions. The microscopic, structural and electrochemical investigations of post-heat treated specimens reveal NiCrMoFeCoAl-30%Cr2O3 composite HVOF coating exhibits a superior corrosion resistance compared to NiCrMoFeCoAl-30%SiO2 composite coating and bare ASTM SA213-T22 steel boiler tube steel in neutral electrolyte. The room temperature potentiodynamic and impedance investigations of heat-treated samples suggest high interfacial charge transfer resistance for HVOF coatings over a wide anodic potential window. This could be ascribed to the protective nature of the chromium oxide containing coatings on high temperature treatment. AC impedance analysis reveals NiCrMoFeCoAl-30%Cr2O3 coating exhibits very high resistive behaviour with very high charge transfer resistance, in the order of 106 Ohm higher than the NiCrMoFeCoAl-30%SiO2 coating and uncoated ASTM SA213-T22 steel boiler tube steel. Furthermore, the high temperature induced formation of metal chromates/chromites along with the presence of Cr2O3 provides good resistance towards corrosion.

Corrosion Behavior of Thermal Seamless Carbon Steel Boiler Pipes

The current study was absorbed on corrosion of ASTM A106 grade B -02 seamless carbon steel boiler pipes. Beyond corrosion experiments in corrosive medium with varying pH values, the weight lost in addition to corrosion rate (m.p.y) values were computed. The weight loss of boiler tube specimens exposed to corrosive liquid was shown to rise as the exposure period of the specimens increased. The results of the microstructure imaging showed that a de-carburized film of 240 µm thickness was shaped on the fireside of the pipe boiler, with ferrite and a few phases of pearlite. On the water lateral side, it was revealed that boiler pipe failure begins with small rust particles that expand to greater sizes and form scales that are displaced from the boiler pipe's surface. On the surfaces of the boiler pipe water side, several pits with crevice corrosion were observed. The corrosion amounts were discovered to decrease when the specimens' exposure time to corrosive environments and hydrogen ion concentration contents increased (pH). The findings of mechanical characteristic values such as hardness, yield strength, and tensile strength revealed that the waterside had higher values than the fireside, while the middle of the pipe had reasonable values. The findings also demonstrated that at low pH values, a tiny size of rust was created on the boiler tube specimen surface. However, at high pH values of corrosive medium, big sizes of corrosion rust were observed on the specimen surfaces.

Hot corrosion behaviour of CNT-reinforced ZrO2-Y2O3 composite coatings on boiler tube steel at 900°C

Purpose In Indian thermal power plants, the main cause of boiler tube failure is the presence of molten sulphates and vanadates, which deteriorate the tube material at high temperatures. To combat the hot corrosion failure of metals, thermal spray technology is adopted. This study aims to investigate and study the effect of hot corrosion behaviour of carbon nanotube (CNT)-reinforced ZrO2-Y2O3 composite coatings on T-91 boiler tube steel in a molten salt environment at 900 °C for 50 cycles. Design/methodology/approach A plasma spray technique was used for development of the coatings. The samples were exposed to hot corrosion in a silicon tube furnace at 900 °C for 50 cycles. After testing, the test coupons were analysed by X-ray diffraction, scanning electron microscopy/energy dispersive spectroscopy and cross-sectional analysis techniques to aid understanding the kinetics of the corrosion reaction. Findings CNT-based reinforced coatings showed lower weight gain along with the formation of protective oxide scales during the experimentation. Improvement in protection against hot corrosion was observed with increase in CNT content in the coating matrix. Originality/value It is pertinent to mention here that the high temperature behaviour of CNT-reinforced ZrO2-Y2O3 composite on T-91 steel at 900°C temperature in molten salt environment has never been studied. Thus, the present research was conducted to provide useful results for the application of CNT-reinforced composite coatings at elevated temperature.

A Failure Mechanism Study of Boiler Water-Wall Tube using Numerical and Experimental Analysis

The aim of this study is to investigate the boiler tube failure mechanism using finite element analysis and experimental Analysis. The numerical software used is ANSYS steady-state thermal and static structural analysis. The independent variable for the analysis is temperature from flue gas and wall thickness of boiler tube. The scope of the study is to determine the temperature distribution, von mises stress, deformation and safety factor by implemented design of experiment. The material of boiler tube sketch in two dimensional for simulation is SA210 Grade C. this is standard requirements for the boiler made of seamless medium-carbon steel with superheater tubes according to ASTM. From the result and discussion, the potential of failure mechanism will determine upon independent variables. In addition, hoop stress or circumference stress will help to support the statement of failure of boiler tube.

Failure investigation into the boiler tubes of power plant

Failure of tubes is a very critical problem in boilers. The current study describes the detailed investigation for failure of boiler tubes in the power plants. Different characterization techniques like; metallography/microscopy, hardness, chemical analysis, SEM/EDS were utilized to find out the actual cause of failure. The tubes were failed within one year of its service. It was concluded that the brittle fracture (rupture) of tubes was occurred due to over-heating caused the formation of thermally insulating thick iron oxide scale on the inner surface of the boiler tube. Evidence of hydrogen embrittlement of the boiler tube in the rupture region was also found. Insights of this investigation might be helpful for the prevention of this type of failures in future.