Shu-Lin Bai

Surface Characterization and Erosion–Corrosion Behavior of Q235 Steel in Dynamic Flow

The study aims at investigating the surface evolution and erosion–corrosion behavior of Q235 steel during erosion–corrosion process in various dynamic flows. For the purpose, true flow fields with the average flow velocities of 0.4 and 0.8xa0m/s and impact angles of 0°, 30° and 90° to the sample surface were successfully measured by particle image velocimetry. The topography of erosion–corrosion surface was observed by laser scanning confocal microscopy. The evolution of localized corrosion pattern is found to be determined by impact angle, i.e., round or elliptical corrosion pit corresponds to impact angle of 90° and ribbon-like corrosion pit corresponds to 0°. The deeper corrosion pits were observed at impact angle of 30° than those at the other two impact angles owing to combined effects of shear and normal stresses. Electrochemical impedance spectroscopy of samples shows smaller radiuses of capacitive loops at velocity of 0.8xa0m/s than those at 0.4xa0m/s. Equivalent circuit analysis implies unstable surface state of sample in dynamic flow. Above results indicate that the flow velocity and impact angle play the key role in the erosion–corrosion behavior of Q235 steel.

Study on Cavitation Erosion–Corrosion Behavior of Mild Steel under Synergistic Vibration Generated by Ultrasonic Excitation

In order to simulate the real condition of a ship sailing in seawater, a cavitation apparatus of ultrasonic excitation was set up with variable temperature field and self-vibration of a Q235 steel sample. The sample self-vibration in a cavitation environment was firstly measured and its erosion–corrosion behaviors at ultrasonic generator output powers of 40, 70, and 100 W were fully investigated. It is found that the displacement fluctuation of sample is enhanced with increasing output power. The temperature rise by cavitation bubble collapse and sample vibration is proved by the higher temperature of the sample than that of the corrosive solution. The surface morphologies of samples in cavitation erosion–corrosion tests show more extensive and worse local corrosion than that in the static case. The maximum depths of erosion–corrosion pits are 39.5, 31.8, and 24.6 μm, corresponding to output powers of 100, 70, and 40 W, respectively, which are higher than that in the static case. The content of Fe in the corrosive solution increases as corrosion time lasts and output power increases, revealing the progressive corrosion of the steel sample. The collapse of cavitation bubbles, sample vibration, as well as electrochemical reaction enhanced by temperature rise are considered to be the main reasons for worse cavitation erosion–corrosion of the sample in an ultrasonic excitation environment.

Corrosion behavior of Hastelloy C22 coating produced by laser cladding in static and cavitation acid solution

Abstract The Hastelloy C22 coatings on Q235 steel substrate were produced by high power diode laser cladding technique. Their corrosion behaviors in static and cavitation hydrochloric, sulfuric and nitric acid solutions were investigated. The electrochemical results show that corrosion resistance of coatings in static acid solutions is higher than that in cavitation ones. In each case, coating corrosion resistance in descending order is in nitric, sulfuric and hydrochloric acid solutions. Obvious erosion-corrosion morphology and serious intercrystalline corrosion of coating are noticed in cavitation hydrochloric acid solution. This is mainly ascribed to the aggressive ions in hydrochloric acid solution and mechanical effect from cavitation bubbles collapse. While coating after corrosion test in cavitation nitric acid solution shows nearly unchanged surface morphology. The results indicate that the associated action of cavitation and property of acid solution determines the corrosion development of coating. Hastelloy C22 coating exhibits better corrosion resistance in oxidizing acid solution for the stable formation of dense oxide film on the surface.

Microstructure and mechanical properties of interface between laser cladded Hastelloy coating and steel substrate

Abstract In order to improve the corrosion resistance of carbon steel, Hastelloy coatings were prepared on E235 steel substrate by a high power diode laser with laser scanning speeds of 6 and 12 mm/s, respectively. The interface between the coating and substrate was firstly exposed by dissolving off the substrate. Its microstructure, composition and mechanical properties were systemically studied. Special “edges” along the grain boundary were found at coating/substrate interface. These “edges” consisted of intergranular corrosion area and real grain boundary. The interface of coating mainly displayed austenite structure ascribed to the rapid solidification as well as the dilution of Ni during preparation. Additionally, Hastelloy coating and its interface prepared at the speed of 12 mm/s showed higher hardness than that prepared at the speed of 6 mm/s. Grain boundaries had higher friction coefficient than grains at both coating/substrate interfaces. Moreover, the interface at higher laser scanning speed exhibited smaller grains, lower dilution rates of Ni and Fe as well as a better tribological property.

Effects of Tungsten and Laser Re-Melting on the Properties of Hastelloy C22 Coating

Hastelloy C22 coatings with (H1) and without tungsten (H2, H3) were prepared on Q235 steel substrate by laser cladding. Coating H3 was also re-melted by the same laser process. The effects of tungsten and laser re-melting process on coating properties were studied in terms of the microstructures, mechanical properties and corrosion resistance. The results show that coating H1 exhibits higher micro-hardness and better corrosion resistance than that of coatings H2 and H3. It is mainly ascribed to the solution strengthening and stable chemical property of tungsten added in coating H1. Coating H3 after laser re-melting displays more compact microstructure than coating H2, which would be conducive to form dense oxide layer on the coating surface. Thus more positive corrosion potential of −0.415 V and lower corrosion current density of 1.149 ×10-6 A/cm2 of coating H3 indicate better corrosion resistance than coating H2.