In Situ Optical Emission Spectrometry during Porous Anodic Alumina Initiation and Growth in Phosphoric Acid

Abstract

We report on the use of in-situ optical emission spectrometry to monitor the Al 3+ loss rate during the anodic oxidation of aluminum in phosphoric acid. Three distinct stages were observed, the highest rate being measured during barrier layer growth. The evolution of the loss rate is markedly different from the evolution previously reported for sulfuric acid anodizing. We speculate that this may be related to the different pore morphologies obtained in these electrolytes. Our measurements also indicate that below 2 mA/cm 2 , field-assisted dissolution is the predominant contribution to Al 3+ loss, while direct cation ejection is predominant at higher current densities.Elucidating the mechanisms of initiation and growth of porous anodic oxide films has recently generated renewed research efforts. [1][2][3][4][5] Despite the widespread use of these films for corrosion protection and nanotechnology applications, several fundamental questions remain unanswered. For instance, the development of porous anodic oxide films can be conceptually described by the following sequence of four, possibly overlapping, stages. 6 The first stage (I) is the barrier layer growth stage, during which the oxide is non-porous. The second stage (II) is the pore initiation stage, during which incipient pores appear in the barrier layer. The third stage (III) is the pore widening stage, during which some of the incipient pores that appeared during pore initiation develop into their final morphology. The fourth stage (IV) is the steady-state pore growth stage during which the barrier layer thickness, the pore diameter and the pore spacing remain constant while the pore length increases with time. The fundamental reasons for the transitions between these stages are however not well understood. A necessary condition for pores to initiate in a dense barrier oxide film is that a sufficiently large fraction of the oxidized metal ends up in the electrolyte rather than in the anodic film. 7-10 For anodic alumina, concentrated acidic electrolytes result in a significant fraction of Al 3+ in the electrolyte, about 40%. 7 For anodic films on other valve metals such as Ti, Ta and Zr, using fluoride-containing electrolytes also results in a significant fraction of the metallic cations in the electrolyte and allows growing porous anodic oxides on these metals as well. Early studies on the growth of porous anodic alumina explained the loss of Al 3+ in the electrolyte and the formation of the porosity by considering an electric field-assisted (electro-)chemical oxide dissolution at the pore base, catalyzed by the acidity. 11-13 18 O tracer experiments revealed that no O 2− anions originally present in the anodic oxide film ended up in the electrolyte for anodic alumina formed in sulfuric acid at 10 V. 14, 15 The Al 3+ loss thus proceeds in these electrochemical conditions by direct ejection of Al 3+ cations from the anodic oxide into the electrolyte rather than by field-assisted dissolution. The direct ejection of Al 3+ in the electrolyte has recently been ...

Measurement of Stress Changes during Growth and Dissolution of Anodic Oxide Films on Aluminum

“…18,32 The latter value compares favorably with a directly measured efficiency of 0.33 during anodizing at the same conditions. 33 Based on the oxide thickness and measured force per width at 25 V, the apparent average stress in the oxide at the completion of anodizing was −58 MPa, in good agreement with prior measurements. 22 Indeed, close agreement with the deflectometry measurements of Van Overmeere et al was demonstrated over a wide range of current densities during anodizing in phosphoric acid.…”

Atomic Emission Spectroelectrochemistry: Real-Time Rate Measurements of Dissolution, Corrosion, and Passivation

“…The CCD detection systems are also less sensitive in the ultraviolet wavelength range. An ICP-AES system with CCD detection system and a quasiidentical electrochemical flow cell was used by Mercier, et al, to investigate the corrosion [13][14][15] and anodization [16][17] of Al, although acqusition was limited to the analysis of a single element, Al.…”

Factors Controlling Stress Generation during the Initial Growth of Porous Anodic Aluminum Oxide

“…Estimates of the anodizing efficiency, defined as the fraction of oxidized Al +3 ions contributing to film growth, were obtained from the slopes of the potential transients and measurements of the electric field at various current densities [19]. The calculated efficiencies increased from 32% to 52% with current density, the range typically found during anodizing in H 3 PO 4 and other acids [26][27][28][29]. Fig.…”