Effect of hydrazine and formaldehyde on the corrosion of SS 304L in hot nitric acid environment

Electrochemical and immersion tests were carried out for SS 304L at 80 °C in 2 M nitric acid containing hydrazine and formaldehyde. The addition of hydrazine resulted in the higher anodic current and no appreciable change in weight loss. For formaldehyde, the reaction with nitric acid resulted in copious evolution of NO<SUB>2</SUB> gas and the subsequent dissolution of the gas in the solution. The open circuit potential (OCP) shifted to anodic direction by 300 mV. The OCP and polarization behaviour are controlled by the solution chemistry rather than the material surface

Engineering approach to corrosion management

Comprehensive engineering approach to corrosion management is a rational and cost-effective way to combat corrosion in industrial plants, and also to predict the remnant life of the components. The paper discusses how such an approach helps in the understanding of corrosion processes and their mechanisms which, in turn, aids in the selection of appropriate materials and fabrication processes, good design, management of residual stresses and environment and, thereby, helps in the control of corrosion. The importance of corrosion monitoring using NDE techniques in industries to assess the corrosion damage and the remnant life of the component has been described. The paper deals with the importance of accurate predictive models for remnant life estimation based on quantitative fitness-for-service assessment methodologies, and utility of laboratory experimental simulation techniques for solving the corrosion problems

MIC failure of cupronickel condenser tube in fresh water application

The failure of 90/10 cupronickel heat exchanger tubes in hydrogen coolers were seen after few years in use. The tubes had river water flowing at a velocity of 2.5 m/s. The shell side was exposed to hydrogen at a pressure of 3.5 kg/cm<SUP>2</SUP>. The inlet and outlet temperature of the cooling water of the cooler are 37 °C and 47 °C, respectively. The failures were pin hole type with a thick layer of muck deposit on the inner side of the tube. The metallographic studies carried out for the tube showed a well annealed microstructure. The region near the pit showed attack along the grain boundaries with dislodging of the grains. The electron probe micro analysis (EPMA) studies revealed the denickelification near the pits. The energy dispersive spectroscopy (EDS) analysis of the pit showed the presence of copper chloride and also the variation in the copper and nickel concentration with respect to the unaffected regions of the tube. Microbial presence was detected in the water as well as the muck deposits in the tubes. The pin hole failure of the tube was a result of the combined action of the chloride as well as the deposits in the tube which gave a conducive environment for the microbial activity to flourish which has caused the propagation of the pit through intergranular corrosion

Effect of structural defects, surface irregularities, and quenched-in defects on corrosion of Zr-based metallic glasses

X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) revealed a fully amorphous structure for as-cast bulk metallic glass (BMG) and melt-spun ribbons of Zr<SUB>52</SUB>Ti<SUB>6</SUB>Al<SUB>10</SUB>Cu<SUB>18</SUB>Ni<SUB>14</SUB> prepared by copper mold casting and the melt-spinning technique, respectively. For melt-spun ribbons, the wheel-side surface contained a higher concentration of quenched-in defects (air pockets), whereas surface irregularities were noticed in the air side. Fluctuation microscopy indicated more medium-range order for the as-cast structure of BMG than for the melt-spun ribbon, whereas in the BMG, the medium-range order was located more in the central region than in the periphery. Macro- and microcell electrochemical experiments of the BMG along the cross section and macrocell experiments of each side of the ribbons in dilute acidic chloride environments indicated that the corrosion resistance of the BMG decreased with the increase in the medium-range order; the medium-range order, however, was less deleterious than either the surface irregularities or the quenched-in defects from the viewpoint of corrosion. A surface film formed on the metallic glasses in a dilute acidic chloride environment mainly consisted of oxide of zirconium, in which zirconium was present in the Zr<SUP>+3</SUP>, Zr<SUP>+2</SUP>, and Zr<SUP>+1 </SUP>state in the BMG and in the air-side and wheel-side surfaces, respectively

Electrochemical characterization of oxide film formed at high temperature on Alloy 690

High temperature passivation studies on Alloy 690 were carried out in lithiated water at 250 °C, 275 °C and 300 °C for 72 h. The passive films were characterized by glow discharge-quadrupole mass spectroscopy (GD-QMS) for compositional variation across the depth and micro laser Raman spectroscopy for oxide composition on the surface. The defect density in the oxide films was established from the Mott-Schottky analysis using electrochemical impedance spectroscopy. Electrochemical experiments at room temperature in chloride medium revealed best passivity behaviour by the oxide film formed at 300 °C for 72 h. The electrochemical studies were correlated to the chromium (and oxygen) content of the oxide films. Autoclaving at 300 °C resulted in the best passive film formation on Alloy 690 in lithiated water