The 304 Stainless Steel (SS304) is severely affected by salt water corrosion due to its high surface wettability. By reducing its surface wettability, its corrosion can be reduced. To achieve this, topographical modification of the steel surface is an effective route. In this work, SS304 flat surfaces were topographically modified into microgrooves (ridge width 250 μm to 500 μm, groove width 200 μm, width ratio = ridge width/groove width >1). Wire cut electrical discharge machining was used to fabricate the microgrooves. Long-term wetting characteristics and long-term corrosion behaviour of flat surface and microgrooves were studied. The influence of the nature of wetting of the tested surfaces on their corrosion behaviour was examined. The sessile drop method and potentiodynamic polarization tests in sodium chloride (3.5 wt. % NaCl) solution (intermittent and continuous exposures for 168 h) were studied to characterize their wetting and corrosion behaviours, respectively. Topographical modification imparted long-term hydrophobicity and, as a consequence, long-term anticorrosion ability of the steel surface. Micropatterning reduced the corrosion rate by two orders of magnitude due to reduction in interfacial contact area with the corrosive fluid via composite wetting, i.e., solid–liquid–air interface. Microgrooves showed corrosion inhibition efficiency ³88%, upon long-term exposure to NaCl solution. By comparing the wetting and corrosion behaviours of the microgrooves with those of the previously studied microgrooves (ridge width/groove width <1), it was found that the surface roughness of their ridges strongly influences their wetting and corrosion properties.

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In this work, functionally graded lanthanum magnesium hexaluminate (LaMgAl11O19)/yttria-stabilised zirconia (YSZ) thermal barrier coating (FG-TBC), in as-sprayed and laser-glazed conditions, were investigated for their thermal shock resistance and thermal insulation properties. Results were compared with those of a dual-layered coating of LaMgAl11O19 and YSZ (DC-TBC). Thermal shock tests at 1100 °C revealed that the as-sprayed FG-TBC had improved thermal stability, i.e., higher cycle lifetime than the as-sprayed DC-TBC due to its gradient architecture, which minimised stress concentration across its thickness. In contrast, DC-TBC spalled at the interface due to the difference in the coefficient of thermal expansion between the LaMgAl11O19 and YSZ layers. Laser glazing improved cycle lifetimes of both the types of coatings. Microstructural changes, mainly the formation of segmentation cracks in the laser-glazed surfaces, provided strain tolerance during thermal cycles. Infrared rapid heating of the coatings up to 1000 °C showed that the laser-glazed FG-TBC had better thermal insulation capability, as interlamellar pores entrapped gas and constrained heat transfer across its thickness. From the investigation, it is inferred that (i) FG-TBC has better thermal shock resistance and thermal insulation capability than DC-TBC and (ii) laser glazing can significantly enhance the overall thermal performance of the coatings. Laser-glazed FG-TBC provides the best heat management, and has good potential for applications that require effective heat management, such as in gas turbines.

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