Passivation of Surfaces within Aluminum Etch Tunnels

2000

Self Cite

The potential dependence of the metal dissolution current density of aluminum in etch tunnels was measured, using pulsed increases of current during galvanostatic etching experiments. Etching was carried out in 1 N HCl solution at 65ЊC, and was followed by analysis of the topographic development of the dissolving surfaces using scanning electron microscopy. By manipulation of the current waveform, the dissolving area and the applied current were varied independently. Parallel experiments with current interruptions were used to help identify the area of the dissolving surfaces during the anodic pulse, which were shown to be submicron size patches on the tunnel tip surfaces. After correction for solution-phase potential drops, the current was found to obey an exponential Tafel dependence on potential. Measurements of the dissolved depth vs. time show that the rate of cathodic hydrogen evolution is significantly increased during the anodic pulse, by comparison with that at the more negative potentials during constant current etching. Good agreement of the present current-potential relation was obtained with that derived from the measurements of pit currents in thin films by Frankel et al.

supporting

confidence: 82%

Section: Methodsmentioning

confidence: 99%

Metal Dissolution Kinetics in Aluminum Etch Tunnels

Tak

Sinha

2000

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“…The potential at the entrances of tunnels growing from the surface of such pits would be decreased by an amount equivalent to the pit's ohmic drop. Approximating the pit as a hemisphere, the ohmic drop can be roughly estimated as 3i sp r sp /, 4 where i sp and r sp are the surface pit current density and radius. Taking i sp ϭ 5 A/cm 2 , r sp ϭ 5 m, and ϭ 0.5 (⍀ cm͒ Ϫ1 , 4 a representative ohmic drop is found to be 15 mV, a value which according to Fig.…”

Section: Resultsmentioning

confidence: 99%

“…[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.…”

mentioning

confidence: 99%

A Mathematical Model for the Growth of Aluminum Etch Tunnels

2001

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A simulation of the growth of pits on aluminum during anodic etching in hot chloride solutions was developed. The simulation is based on equations for mass transport and for the potential-controlled removal of chloride ions from the dissolving surface. The latter process initiates oxide passivation. Etch pits are found to transform into tunnels which at first maintain parallel sidewalls and then begin to taper. The predicted tunnel shapes agree quantitatively with those measured experimentally. Tunnel formation is possible only when the potential at the tunnel entrance during etching is within 20-30 mV of the repassivation potential; as a result, the size of the dissolving surface is nearly constant during pit growth. In the tapered-width regime of tunnel growth, the AlCl 3 concentration at the end of the tunnel is near saturation, despite the absence of precipitation from the model equations. The model shows that this condition derives from the low conductivity of the concentrated solution, coupled with the sensitivity of the rate of surface chloride removal to changes in the potential at the dissolving surface.

“…The boundary condition far from the pit is the bulk concentration C6 C(r = , t) = C6 [5] while that at the hemisphere surface is given by the current density i(t) passing this surface = at) i(t)t 3FD [6] Since the dissolution current density, i, on the active surface inside the pit is always constant, the current density on the hemisphere, i(t), is found by multiplying i by the actively dissolving area in the pit and dividing by the hemisphere area…”

Section: Infroductionmentioning

confidence: 99%

A Mathematical Model for the Initiation of Aluminum Etch Tunnels

Zhou

1998

Self Cite

A mathematical simulation is presented which predicts the spontaneous shape evolution of cubic etch pits on aluminum as they develop into etch tunnels during anodic etching in chloride solutions. The simulation is based on a model for oxide passivation, according to which the rate of oxide film coverage increases as the potential at the dissolving surface is made more negative than the critical repassivation potential, which depends on the local chloride ion concentration. Mass transport calculations are used to predict the electrolyte concentration and potential in the pit, which in turn determine the rate of oxide advance and hence the shape change of the pit. The model predicts the pit-tunnel transformation, as well as width expansion of tunnels near their mouths. The occurrence of these features is independent of the choice of the passivation rate constant, an adjustable parameter in the simulation. Tunnel width oscillations were found at relatively low values of the rate constant. In the model, the pit-tunnel transformation is produced by rapid pit sidewall passivation, which is due to the relatively slower increase of the pit electrolyte concentration relative to the ohmic drop, during the pit's initial growth. A fully quantitative comparison of the model and experiment is possible with independent experimental information on passivation kinetics. A mathematical simulation is presented which predicts the spontaneous shape evolution of cubic etch pits on aluminum as they develop into etch tunnels during anodic etching in chloride solutions. The simulation is based on a model for oxide passivation, according to which the rate of oxide film coverage increases as the potential at the dissolving surface is made more negative than the critical repassivation potential, which depends on the local chloride ion concentration. Mass transport calculations are used to predict the electrolyte concentration and potential in the pit, which in turn determine the rate of oxide advance and hence the shape change of the pit. The model predicts the pit-tunnel transformation, as well as width expansion of tunnels near their mouths. The occurrence of these features is independent of the choice of the passivation rate constant, an adjustable parameter in the simulation. Tunnel width oscillations were found at relatively low values of the rate constant. In the model, the pit-tunnel transformation is produced by rapid pit sidewall passivation, which is due to the relatively slower increase of the pit electrolyte concentration relative to the ohmic drop, during the pit's initial growth. A fully quantitative comparison of the model and experiment is possible with independent experimental information on passivation kinetics.