Technique for a Scanning Electron Microscope Study of Etched Aluminum

Abstract

The light microscope has long been used to reveal the surface morphology of aluminum foil etched for electrolytic capacitors (1), but its effectiveness has been p a r t l y limited by insufficient depth of focus at high magnifications. The electron microscope with its greater depth of focus and higher resolution has been used to reveal considerable detail in configurations like those produced by tunnel corrosion in a 2S aluminum alloy (2) and tunnel electroetching in aluminum foil of 99.8% purity (3), but the anodic oxide replica samples for this technique have to be relatively transparent to electrons.The scanning electron microscope (SEM), on the other hand, has neither of these limitations. It has a depth of focus generally 50-100 times that of the optical microscope and samples m a y be thick and opaque to electrons. Nonconducting samples, however, require the addition of a thin conducting layer such as vapordeposited carbon or gold. In fact, gold-coated anodic oxide replicas are found to be excellent SEM samples for studying the morphology of etched aluminum. Illustrations have been reported for lightly etched foil (4, 5). The present communication gives a brief description of the preparation of SEM samples for studying both lightly and heavily etched foil.Lightly etched foil and final oxide replicas were obtained in the following way. A specimen of annealed 99.99% A1 foil was given an oxide-removing treatment in a NaOH bath, a 5-sec electroetch at 10 A/in. 2 in a hot NaC1 electrolyte, and a thorough wash in distilled water. An anodic film of about 600A thickness was then formed in an aqueous solution of ammonium pentaborate. Small sheared pieces were placed in a b r o m i n e -m e t h a n o l solution to dissolve the aluminum matrix and thus free the oxide replicas. Each replica was mounted with the o x i d e -m e t a l interface up. Vapor deposition of about 500A of gold in the normal direction completed the preparation of the SEM samples.

Pitting Behavior of Aluminum Foil during Alternating Current Etching in Hydrochloric Acid Containing Sulfate Ions

“…2,[22][23][24][27][28][29][30] The cubic pit formed during dc etching propagates along the tip of the pit while leaving the four sidewalls passive, thus resulting in tunnel pit. Pulsed reductions of the anodic etching current of several milliseconds duration bring about passivation of the active pit tip, while tunnel growth continues after the resumption of the current.…”

Metal Dissolution Kinetics in Aluminum Etch Tunnels

“…The morphology of the internal surfaces of etch tunnels was observed using scanning electron microscopy (SEM, JEOL JSM 840) with the help of an oxide replication technique. 21 Replicas of the etched surfaces were fabricated by first forming 80-90 nm thick anodic oxide films on the etched surfaces, and then dissolving the metal in a bromine/methanol solution. The anodic film left by this procedure serves as a replica of the etched metal surface.…”

Evolution of Microscopic Surface Topography during Passivation of Aluminum

“…For SEM, oxide replicas of etched aluminum surfaces were fabricated by a standard procedure, in which anodic oxide films were first formed on the etched surfaces, and then the aluminum metal was dissolved in a bromine/methanol solution. 9 The forming voltage of the anodic films was 50 to 75 V, corresponding to film thicknesses of 60 to 85 nm. The replicas were sputter coated with gold prior to SEM observation.…”