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Procedia Engineering 00 (2009) 000–000

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ICM11

Cr2O3 Sealing of Anodized Aluminium Alloy by Heat Treatment

Junghoon Leea, Yonghwan Kima*, Heuiun Janga, Uoochang Jungb and Wonsub Chunga

aDepartment of Material Science and Engineering, Pusan National University, Republic of Korea bDongnam Technology Service Division, Korea Institute of Industrial Technology, Republic of Korea

Abstract

Anodizing is an attractive surface treatment process for aluminium alloys to improve hardness, corrosion resistance and wear resistance. Anodized film is composed of an inner thin barrier layer and an outer thick porous layer. By sealing process as a posttreatment of anodizing, a porous layer achieves only an improved mechanical and chemical performance also decorative colour. However, dramatically increased hardness could not be achieved by traditional sealing processes. In this study, the sealing of anodized aluminium alloy was carried out by repeating dipping in hexavalent chromium oxide solution and heat treatment. Using XRD and SEM, the variation of anodized layer before and after the sealing was analyzed. Electrochemical corrosion test was alsocarried out. The hardness and corrosion resistance were compared with traditional sealing methods, such as boiling water and nickel fluoride. By heat treatment after dipping in hexavalent chromium oxide solution, Cr2O3 was formed and it fills pores in the porous layer of anodized film. Thus, the hardness of anodized film was increased and moreover the corrosion resistance was improved.

© 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11

Keywords: Aluminium alloy; Anodizing; Sealing; Cr2O3

1. Introduction

Anodizing is an attractive surface treatment for aluminium alloys to provide enhanced performance, corrosion resistance and wear resistance. Aluminium oxide film built by anodizing is composed of inner thin compact layer and outer thick porous layer. Sealing is a post treatment of anodizing of aluminium to fill the outer thick porous layer [1, 2]. By sealing, enhanced corrosion resistance and decorative surface can be achieved. The most conventional sealing methods used in industrial field are boiling water, steam, dichromate, nickel acetate and cold nickel fluoride sealing and several researches were conducted [3-5]. Though the corrosion resistance is improved, conventional sealing methods are not enough to affect mechanical characteristics, such as hardness and wear resistance [6-7]. Until now, to enhance the mechanical characteristics of oxide film, hard ceramic particle or polymer emersion was dispersed during anodizing and electrolysis conditions, such as electrolyte composition, temperature and applied electric power were studied.

In this study, a new method was tried to improve mechanical characteristics and corrosion resistance. Sealing was conducted by repetitions of dipping in chromic acid and heat treatment. Microstructure of oxide film by this sealing

* Corresponding author. Tel.: +82-51-510-1446; fax: +82-51-510-3073. E-mail address: yhkim1@pusan.ac.kr.

1877–7058 © 2011 Published by Elsevier Ltd.doi:10.1016/j.proeng.2011.04.466

Physics Engineering 10 (2011) 2803–2808

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method was analyzed. Moreover, comparative evaluations, such as hardness, wear resistance and corrosion resistance, were conducted with boiling water and cold nickel fluoride sealing.

2. Experimental

5052 aluminium alloy, which was cut to a 30 × 100 mm size, was used as a substrate. The chemical composition was listed in table 1. Abrasion on #1500 sandpaper and an ultrasonic degreasing treatment in acetone were carried out as pretreatments. Activation was conducted for 1 minute in 0.5 M nitric acid after etching for 30 second in 80 oC2.5 M NaOH solution.

Table 1. The chemical composition of 5052 aluminium alloy (in wt. %)

Al Mg Cr Si Fe Cu Mn Zn

Bal. 2.5 0.25 0.2 0.35 0.08 0.09 0.09

Anodizing was carried out in 0 oC 15 wt.% sulfuric acid electrolyte and DC 50 mA·cm-2 was applied for 30 min. During anodizing, electrolyte was agitated by bubbled air. The new sealing method suggested in this study was conducted by repeating of dipping in 3 M H2CrO4(Chromic acid) electrolyte for 1 minute and heat treatment in 400 oC electric furnace for 30 minute. To evaluate corrosion resistance, wear resistance and hardness comparatively, boiling water and cold nickel fluoride sealing was selected as conventional sealing method and detail conditions were shown in table 2.

Table 2. Conditions of boiling water and cold nickel fluoride sealing

Composition Temperature Dipping duration

Boiling water Distilled water 100 oC 30 minute

Cold nickel fluoride 2.5 g/l NiF2 25 oC 30 minute

Microstructure after chromic acid heat treatment sealing(CHS) was analyzed using FE-SEM, EDS and XRD. Hardness was evaluated on cross section using micro Vickers hardness tester. Wear resistance was compared by ball on disk method and detail conditions were shown in table 3. Corrosion resistance also compared by potentiodynamic polarization test using potentiostat(Versastat4). Saturated calomel electrode and platinum mesh were used as reference electrode and counter electrode respectively.

Table 3. Conditions of ball on disk test

Track radius Linear speed Wear distance Load Counter body

10 mm 80 mm/s 100 m 10 N Alumina ball

Radius : 3.45 mm

3. Results and Discussion

Cross sections of anodized aluminium alloy and after CHS were observed using FE-SEM and EDS, and the results were shown in Fig. 1. The thickness of anodized oxide film was about 50 um. There was no noticeable thickness change after CHS. The anodized oxide film was composed of aluminium, oxygen and sulfur. After 3 times of CHS, chromium was detected at anywhere in the oxide film. This result means that hexavalent chromium ions were absorbed into the porous layer during dipping chromic acid solution.

In order to determine the structure of absorbed chromium, XRD analysis was carried out and the result was shown in Fig. 2. Before CHS, there are any anodized oxide peaks, with the exception of aluminium peaks. It was

Junghoon Lee et al. / Physics Engineering 10 (2011) 2803–2808 2805 Author name / Procedia Engineering 00 (2011) 000–000 3

reported that anodized oxide film was amorphous structure, for this reason that is a typical XRD pattern of anodized aluminium [8]. However, Cr2O3 peaks were observed after CHS in the XRD pattern. It seem that the chromium presents in the anodized oxide layer as Cr2O3. This trivalent chromium oxide(Cr2O3) was originated from heat reduction reaction of absorbed hexavalent chromium ions (formula 1) [9].

4CrO42- + 4H+ 2Cr2O3 + 4O2 + 2H2O (1)

The microstructure of anodized oxide film before and after CHS observed using FE-SEM was shown in Fig. 3. Before CHS, typical anodized oxide structure was observed, hexagonal cell and center pore. The diameter of center pore was about 30 nm. After 3 times of CHS, center pore was not found at the center of hexagonal cell or diameter of center pore was greatly decreased. It could be assumed that Cr2O3 fills the center pore in the porous layer.

Fig. 1. Cross-sectional view of (a)as anodized 5052 and (b)after CHS 3 times. EDS spectra and chemical composition at the site of (c)A and (d)B.

Fig. 2. The XRD patterns of (a)as anodized 5052 and (b)after CHS 3 times.

* A * B

(a) (b)

(c) (d)

Resin Resin

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Chromium concentration and micro hardness with respect to CHS repeating times were shown in Fig. 4. The hardness of anodized oxide film was about 400 Hv. After 1 time of CHS, the hardness was increased to about 600 Hv. With repeating CHS, the hardness was increased to 680 Hv; 2 times and 750 Hv; 3times. In case of 4 times CHS, the hardness was similar to 3 times. Chromium concentration in oxide film was increased with CHS repeating times. Moreover, chromium concentrations of CHS 3 times and 4 times were similar. From these results, it is assumed that the hardness is increased with increasing the chromium concentration in oxide film. The chromium is present in oxide film as Cr2O3. Therefore, as pores are filled with Cr2O3, the hardness of anodized oxide film is increased. Furthermore, 3 times of CHS was the most effective to enhance the hardness.

Fig. 3. FE-SEM images of porous oxide film of (a)as anodized 5052 and (b)after CHS 3 times.

Fig. 4. (a) The hardness and (b) chromium concentration of oxide film with respect to the repeating times.

The hardness of anodized oxide film were compared with three sealing methods and the results were shown in Fig. 5. There was no considerable change in hardness with boiling water and cold nickel fluoride. However, after 3 times of CHS, the hardness was considerably increased to 750 Hv. The pores of anodized oxide film were filled with boehmite after hot boiling water sealing and with boehmite, nickel hydroxide and aluminium fluoride after cold nickel fluoride sealing. However, after CHS, the pores were filled with Cr2O3, which is comparatively harder than boehmite, nickel hydroxide and aluminium fluoride [7]. Therefore, it seems that anodized oxide film after CHS shows considerable enhanced hardness.

The wear resistance of anodized oxide film with three types of sealing methods was compared and the results are shown in Fig. 6. In case of without sealing, volumetric wear loss was higher than other cases. In case of CHS, the volumetric wear loss was the lowest. These decreases wear loss seems to due to the enhanced hardness of oxide film.

(a) (b)

(a) (b)

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Fig. 5. The hardness of anodized oxide film with various sealing. Fig. 6. Volumetric wear rate of anodized oxide film with various sealing.

An improvement of corrosion resistance is assumed to be a main purpose of sealing. Therefore, corrosion resistance was evaluated by potentiodynamic polarization measurement in 3.5 wt.% NaCl solution and the result was shown in Fig. 7. As anodizing, corrosion potential was increased to mV and current density at the anodic potential was also decreased. After three types of sealing, corrosion current density and current density at the anodic potential were lower than as anodized one. These results are in good agreement with other researches which focused on corrosion resistance of anodized aluminium alloys and its sealing [3-5]. In case of boiling water sealing, corrosion potential was decreased than as anodized one. As cold nickel fluoride sealing, corrosion potential was similar to as anodized one. Although CHS showed lower corrosion current density than cold nickel fluoride sealing, corrosion potential was the highest. Nevertheless CHS is not seem to be the most effective sealing to improve corrosion resistance, it is certain that improved corrosion resistance is achieved by CHS.

Fig.7. Potentiodynamic polarization curves of anodized aluminium alloy 5052 sealed by difference methods.

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4. Summary and conclusions

Pores in anodized oxide film are filled with hard ceramic Cr2O3 by heat treatment after dipping in chromic acid solution as sealing and most effective repeating time is three. Cr2O3 in pores is originated from heat reduction of absorbed CrO3. The hardness of anodized oxide film was considerably increased and wear resistance was improved by CHS than other conventional sealing. The corrosion resistance was also improved by CHS.

Acknowledgements

This work was supported by a grant-in-aid for the National Core Research Center Program from the Ministry of Education Science & Technology and the Korea Science & Engineering Foundation (No. R15-2006-022-01001-0).

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