Evolution of Microscopic Surface Topography during Passivation of Aluminum

Abstract

The time evolution of microscopic topography on corroding aluminum surfaces during oxide film passivation was characterized. Passivation was studied after galvanostatic etching in 1N at 65°C, in both aluminum etch tunnels (by scanning electron microscopy) and micron-size cubic etch pits (by atomic force microscopy).Step reductions of applied current initiated passivation. At times of 1 to 300 ms after current steps, the corroding surface was microscopically heterogeneous, consisting of a number of small corroding patches 0.1 to 1 μm in width, which were surrounded by passive surface. As some patches grew by dissolution, others were passivated, until eventually only one patch remained on the pit or tunnel surface. The topography of the corroding surface was controlled by the potential: the surface dissolved uniformly at the repassivation potential , while partial passivation to produce patches occurred at potentials more cathodic than . Patches were unstable at potentials below and would ultimately passivate. Topographic evolution during passivation was very similar for pits as for tunnels, except that the time scale of the process is much longer for tunnels than for pits (200-300 ms vs. 11-20 ms). The difference of time scales was due to the different corroding surface areas of pits and tunnels. Patches are probably defined by a surface layer which inhibits passivation. KeywordsZoology and Genetics, corrosion, passivation, oxidation, surface morphology, electrochemistry, dissolving Disciplines Chemical Engineering Comments This article is from ABSTRACTThe time evolution of microscopic topography on corroding aluminum surfaces during oxide film passivation was characterized. Passivation was studied after galvanostatic etching in IN HCI at 65~ in both aluminum etch tunnels (by scanning electron microscopy) and micron-size cubic etch pits (by atomic force microscopy).Step reductions of applied current initiated passivation. At times of 1 to 300 ms after current steps, the corroding surface was microscopically heterogeneous, consisting of a number of small corroding patches 0.1 to i pure in width, which were surrounded by passive surface. As some patches grew by dissolution, others were passivated, until eventually only one patch remained on the pit or tunnel surface. The topography of the corroding surface was controlled by the potential: the surface dissolved uniformly at the repassivation potential Ea, while partial passivation to produce patches occurred at potentials more cathodic than E~. Patches were unstable at potentials below ER and would ultimately passivate. Topographic evolution during passivation was very similar for pits as for tunnels, except that the time scale of the process is much longer for tunnels than for pits (200-300 ms vs. 11-20 ms). The difference of time scales was due to the different corroding surface areas of pits and tunnels. Patches are probably defined by a surface layer which inhibits passivation.

Metal Dissolution Kinetics in Aluminum Etch Tunnels

“…7, but with no anodic pulse, the tip surfaces appeared flat in the SEM, and patch structures could be viewed only with atomic force microscopy (AFM). 16 Thus, the anodic current pulses in the present experiments stimulate a significantly enhanced metal dissolution rate on the patch areas formed by passivation at the time of the current step reduction. Figure 8 shows potential transients measured during the anodic current pulses following current reductions.…”

“…On the other hand, the current reductions from i a1 to i a2 cause only a fraction of the dissolving tip surface area to be covered with oxide, while the remainder continues to dissolve. 16,20 Thus, anodic pulses after current interruptions will include only phenomena specific to oxidecovered surfaces, such as nucleation of new pits, oxide film growth, and metal dissolution through the oxide layer. Anodic pulses after current reductions, in addition to these current sources, also include continuing dissolution from the portion of the tip surface which was not covered with oxide at the time of the current reduction.…”

“…16 In the former experiments, the potential is equal to E R , while in the latter ones it is more negative than E R . Both types of measurement yield a constant dissolution rate of 2.1 m/s (equivalent to a current density of 6.1 A/cm 2 ) for 1 N HCl etchant at 65ЊC, the conditions of this work.…”

Kinetic Model for Aluminum Dissolution in Corrosion Pits

“…19 The step current reduction to i a2 passivated a fraction 1 Ϫ (i a2 /i a1 ) of the dissolving tunnel tip surface, but did not alter the dissolution current density. 14 The step increase to i a3 forced a higher current through the unpassivated portion of the tunnel tip. The current density on the active portion of the tip was then i d0 (i a3 /i a2 ), where i d0 is 6.1 A/cm 2 , the equivalent current density of the 2.1 m/s dissolution velocity.…”

A Mathematical Model for the Growth of Aluminum Etch Tunnels

“…[4][5][6][7] In these experiments, the step or ramp causes the oxide film to cover part of the tunnel tip. In step experiments, passivation occurred in a time of order 1 ms, comparable to the characteristic time of potential decreases accompanying the step, but much smaller than the time for concentration changes to occur.…”