How the Parameters of Pulse Electrolysis Determine the Structure of Chromium Oxide Composites

The present work was devoted to a study of the influence of pulse electrolysis parameters on the structure of chromium oxide composites. The morphology and elemental composition of the composites formed on stainless steel AISI 304 were investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The electrochemical properties and defects of the composites were studied by impedance spectroscopy. It has been established that varying the duration of pulses and pauses during electrolysis significantly affects the crystal size, the presence of cracks, and the uniformity of element distribution on the surface of the composites. Specifically, it was found that composites formed with pulse and pause durations of 1 second and 0.25 seconds, respectively, exhibit a crystalline structure with average crystal sizes of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$104 \pm 11 ~\text{nm}$</tex> and are free of surface cracks. Increasing the pause duration to 0.5 seconds leads to the formation of cracks on the composite surface with sizes of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$220 \pm 10 ~\text{nm}$</tex> and an increase in crystal size to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$149 \pm 11 ~\text{nm}$</tex>. Composites with pulse and pause durations of 0.5 seconds display a globular structure and an island-like formation mechanism, with a network of cracks <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$800 \pm 50 ~\text{nm}$</tex> wide on the surface. The study also revealed that changes in current density amplitude during electrolysis do not affect the crystalline structure of composites formed with a pulse duration of 1 second and a pause of 0.25 seconds. However, decreasing the current density to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$30 ~\mathrm{A} / \mathbf{d m}^{2}$</tex> results in composites with wide and deep cracks. Conversely, composites formed at a current density of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$90 ~\mathrm{A} / \text{dm}^{2}$</tex> are free of cracks, while those formed at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$130 ~\mathrm{A} / \text{dm}^{2}$</tex> exhibit a crack network on the surface. These findings indicate that the structural and electrochemical properties of chromium oxide composites can be precisely controlled by adjusting the parameters of pulse electrolysis.