Formation of the Layered Etch Films on AA1050 Aluminum Plates Etched in Nitric Acid Using Alternating Currents

Abstract

The structure of the etch film on aluminum plates etched in nitric acid was systematically studied by varying the ratio of positive to negative charges provided by a rectangular current waveform. When etched using a symmetrical square current, the plates were dotted with the etch pits inside which a layered etch film formed via the stacking of dark-contrast vs. light-contrast layers. The former was presumably aluminum hydroxide, while the latter was the channels that were filled with the electrolyte during etching. The thickness of the aluminum hydroxide layer increased with increasing the charge passed during the cathodic half-cycle. Hydrogen discharge during the cathodic half-cycle led to the formation of the hydroxide layer. Hydrogen evolution at the interface formed microvoids and might induce defects in the hydroxide layer. Based on dissolution-passivation of the same pit, a possible mechanism was proposed to explain the formation of the layered etch film on aluminum etched in nitric acid using the alternating current with emphasis on the effects of hydrogen discharge.

Theoretical and experimental studies on the corrosion inhibition potentials of some purines for aluminum in 0.1M HCl

“…In such solutions the surface film is insoluble but may be locally attacked by aggressive anions, primarily chlorides. The effect of Cl − ions (which can be generated by hydrolysis of HCl) on the corrosion of aluminum and its alloys has been the subject of several studies [3–5] . The cost of replacing metals due to corrosion is often exorbitant and economically unbearable.…”

Effect of Additives on Smut-Layer Formation and Pitting during Aluminum Etching in Hydrochloric Acid

“…Extensive research aimed at understanding the electrograining processes has been carried out; etch films 57,58 and their resulting morphology have been investigated by varying numerous parameters like frequency, 15,17,32 applied current density, 18,28 waveform, 59,60 and electrolyte concentration, primarily through the addition of aluminum nitrates 60,61 but also of boric acid 62 and other chemicals. 63 The microstructure in particular has been subject to investigation to determine the influence of the microstructure and the role of second-phase particles on the final grained surface.…”

“…63 The microstructure in particular has been subject to investigation to determine the influence of the microstructure and the role of second-phase particles on the final grained surface. 60,64,65 Far less research, however, has focused on the buffering effects during electrograining 66 and on how the link between smut-layer formation and the final morphology of the underlying aluminum surface can be exploited through the inclusion of additives within the electrograining bath to produce improved grain-surface morphologies.…”

Importance of Etch Film Formation During AC Controlled Pitting of Aluminium

“…Thus at high current we can obtain less but bigger pits. Lin and Chiu, (2005) showed that the variation of the duration time of the anodic, t a to cathodic, t c half cycle changes the thickness of the smut. Smut is decreased by increasing the t a /t c ratio while the average size of the individual hemispherical pits increases.…”