Influence of the anodizing temperature on the porosity and the mechanical properties of the porous anodic oxide film

https://doi.org/10.1016/j.surfcoat.2007.01.044Get rights and content

Abstract

The microhardness and fretting wear resistance of anodic oxide layers, produced on commercially pure aluminium by potentiostatic anodizing in sulphuric acid under conditions of controlled convection and heat transfer in a reactor with a wall-jet configuration, were evaluated as a function of the electrolyte temperatures in a wide range from 5 °C up to 55 °C. Additionally, information on the microstructure of the anodic films was acquired by FE-SEM analyses whereas image analysis of high-resolution surface images yielded quantitative information on the evolution of the surface porosity as a function of the electrolyte temperature. Hence measured mechanical properties were directly related to the corresponding microstructure. The microhardness of the anodic films progressively decreased with increasing electrolyte temperatures whereas the wear resistance remained constant for the lower considered temperatures from 5 °C to 25 °C, followed by a decreasing wear resistance with increasing electrolyte temperature from 25 °C onwards. Both mechanical properties displayed an important decrease when the electrolyte temperature was raised from 45 °C to 55 °C. FE-SEM analyses indicated the formation of porous oxides with initially equal pore diameters at the metal-oxide interface, though pore widening due to chemical dissolution of the oxide by the electrolyte led to films with cone-shaped pores. This phenomenon became more pronounced with increasing electrolyte temperature and towards the surface of the anodic layer. The deterioration of the microhardness with increasing electrolyte temperatures could mainly be attributed to the increase of the porosity in the outer region of the oxides since the rate of microhardness reduction is almost synchronous with the rate of porosity increase. In contrast, the variation of the wear resistance with increasing anodizing temperature indicates that the degradation of the wear resistance does not only depend on the oxide porosity and is also affected by other characteristics of the oxide.

Introduction

The surface properties of anodic alumina films (AAF) render anodized aluminium products suitable for a wide range of applications, including use in the packaging, architectural and aerospacial industries. In many of these applications a key role is played by the mechanical properties such as the hardness and wear resistance. Depending on the considered anodizing conditions these properties can be varied to a wide extent, hence the influence of the process conditions on the mechanical properties of the resulting AAF has received extensive attention in the past and information concerning this matter is available in literature.

Most studies have considered the (micro)hardness or wear resistance of porous AAF concern oxides formed under hard anodizing conditions. These conditions, which comprise the use of low electrolyte temperatures and often special electrolytes, are used to produce thick anodic oxides (typically thicker than 25 μm) with hardness and wear resistance as their primary characteristics [1]. Yet even in this field not much work has been conducted to relate the latter two properties to the electrolyte temperature [1]. According to Scott [2], who considered anodizing in a sulphuric acid electrolyte at a constant applied current density of 4 A/dm2, varying the electrolyte temperature in the range from − 5 °C up to 15 °C hardly influences the abrasion resistance of the resulting AAF. Another study by Koizumi et al. [3], dealing with galvanostatic anodizing at current densities from 1 to 8 A/dm2 in a mixed sulphuric acid–oxalic acid electrolyte at temperatures, varied between − 5 °C and 20 °C, report an almost constant wear resistance in the temperature range [− 5 °C to 5 °C], regardless of the applied current density. In the temperature range [5 °C to 20 °C] on the other hand, the authors observed a decreasing wear resistance with increasing electrolyte temperature, with the effect being more pronounced for the lower applied current densities. This latter observation should be handled with care, though, since the anodizing process was always performed for a duration of 45 min, regardless of the applied current density. Hence thicker oxide films were probably formed and evaluated for the higher current densities, whereas film thickness influences the wear resistance and hardness of the AAF [2], [4], [5], [6].

Anodizing to form thick oxide films in a sulphuric acid electrolyte at more elevated temperatures than those considered during hard anodizing often leads to the formation of anodic oxides with inferior hardness and wear resistance. Porous anodic oxides with a thickness of 25 μm or more, formed in a sulphuric acid electrolyte at temperatures between 15 °C and 30 °C (under different applied current densities from 1 up to 4 A/dm2) are reported to be characterised by a softer outer layer which reduces the transparency and wear resistance of the AAF [7], [8]. On the level of the microhardness a similar effect has been observed across a 25 μm thick AAF grown in a sulphuric acid electrolyte between 0 °C and 25 °C. Whereas at 0 °C an oxide with a uniform microhardness was formed, the microhardness of similar films grown at higher temperatures decreased from the metal base towards the oxide surface with this declining evolution becoming more pronounced with increasing temperature [9].

Together with the mechanical properties the process of oxide formation and the microstructure of the AAF are also influenced by variation of the electrolyte temperature. With increasing electrolyte temperature the aggressiveness of the electrolyte towards the oxide increases accordingly, hence enhancing the chemical dissolution of the AAF by the electrolyte [10], [11]. Under potentiostatic anodizing conditions this increased aggressiveness leads to higher steady state current densities, which imply a higher oxide formation rate [12]. Equal anodizing time and applied potential will induce the formation of thicker oxide layers as the electrolyte temperature is increased [12], [13], given that the maximum film thickness under the considered conditions is not reached [14].

Concerning the influence of the electrolyte temperature on the microstructure of the oxide film formed under potentiostatic conditions, some authors report an increasing electrolyte temperature to induce a declining barrier layer nm/V-ratio, and hence a decreasing barrier layer thickness and pore diameter [15]. More recent studies on the other hand indicate the barrier layer thickness to be independent of the electrolyte temperature and to be determined by the applied potential [13], [16]. Generally accepted is the concept of the enhanced chemical dissolution of the oxide by the electrolyte at elevated temperatures leading to conically shaped pores with an increasing pore diameter towards the surface of the AAF [15], [17]. Accordingly the dissolution of material from the pore walls increases the porosity of the porous oxide film [13], [17].

Despite the information available in literature on the influence of the anodizing temperature on the mechanical properties and on the microstructure of porous AAF, studies which consider both aspects and which link the measured mechanical properties with the observed microstructure are rare.

In this study the influence of the electrolyte temperature on the microhardness and on the wear resistance of porous AAF grown in a sulphuric acid electrolyte has been evaluated over a wide temperature range. In contrast to common anodizing experiments in a stirred electrochemical cell, anodizing was performed under conditions of controlled convection and heat transfer in a reactor with a wall-jet configuration. In addition to the control of the convective heat transfer, the evolution of the local electrode temperature during anodizing was measured at different radial positions on the backside of the aluminium anodes. The latter features enabled the verification of whether regular anodizing behaviour, e.g. without the occurrence of local phenomena such as “burning”, was encountered [12], [18]. The microstructure of the formed anodic films has been observed by plane and cross-sectional FE-SEM analyses, whereas image analysis performed on high-resolution images of the surface yielded quantitative information on the evolution of the surface porosity as a function of the electrolyte temperature. Hence measured mechanical properties of the porous oxide films have been directly related to and explained by the observed corresponding microstructure.

Section snippets

Experimental

For the evaluation of the microhardness and for FE-SEM analysis disk shaped AA1050 (99.5% Al sheet 0.3 mm) samples of 55 mm diameter were used with the diameter of the active surface of the working electrode (WE) being 40 mm. Prior to anodizing samples were alkaline etched in 60 g/l NaOH solution at 40 °C for 60s, followed by a desmutting treatment consisting out immersion in a 1:1 concentrated HNO3:H2O solution at room temperature during 60 s. The samples used for the evaluation of the wear

Microhardness measurements

The observed evolution of the microhardness of the anodic oxide layers as a function of the electrolyte temperature is depicted in Fig. 2. With increasing electrolyte temperature a decreasing microhardness, evaluated at the surface of the oxide, was recorded. The results display a linear decrease in the temperature range [15 °C to 45 °C], with the microhardness values of the aluminium electrodes anodized at 5 °C and 55 °C slightly deviating from this tendency. The former electrodes possessed a

Influence of electrolyte temperature on microstructure

All anodizing experiments, performed at the different electrolyte temperatures, were executed under potentiostatic conditions during which an equal potential difference of 17.0 V was applied. During steady state anodizing the pore diameter of the porous anodic oxide (as well as barrier layer thickness and cell diameter) is known to be directly related to the applied potential [15], hence porous oxides with similar pore diameters should have been formed. Near the pore base indeed a constant pore

Conclusions

In this study the microhardness and wear resistance, as well as the microstructure of porous anodic oxides, produced by anodizing of 99.5% pure aluminium electrodes under controlled conditions while varying the electrolyte temperature in the wide range from 5 °C to 55 °C, were evaluated.

The microhardness of the anodized electrodes progressively decreased with increasing electrolyte temperatures. The wear resistance of the anodic oxide films, evaluated by means of fretting wear tests, remained

Acknowledgements

The authors acknowledge the support from the Instituut voor de aanmoediging van innovatie door Wetenschap & Technologie in Vlaanderen (IWT, contract nr. SBO 040092). M. Peeters (KULeuven, dept. MTM) is greatly acknowledged for the performed wear tests.

References (25)

  • L.E. Fratila-Apachitei et al.

    Surf. Coat. Technol.

    (2003)
  • J.W. Diggle et al.

    Electrochim. Acta

    (1970)
  • Y.-C. Kim et al.

    J. Electroanal. Chem.

    (1999)
  • F. Debuyck et al.

    Mater. Chem. Phys.

    (1993)
  • W.J. Albery et al.

    J. Electroanal. Chem.

    (1983)
  • P.G., Sheasby, R., Pinner, The Surface Treatment and Finishing of Aluminium and its Alloys, 6th Edition, ASM...
  • B.A. Scott

    Trans. Inst. Met. Finish.

    (1965)
  • P.G. Sheasby, R. Pinner, The Surface Treatment and Finishing of Aluminium and its Alloys, 6th Edition, ASM...
  • K. Okubo

    Met. Finish.

    (1983)
  • A.P. Gruar et al.

    Trans. Inst. Met. Finish.

    (1985)
  • J. Herenguel et al.

    Rev. Met.

    (1949)
  • R.W. Thomas

    Trans. Inst. Met. Finish.

    (1981)
  • Cited by (0)

    View full text