THE INFLUENCE OF ANODISING PARAMETERS ON THE CORROSION PERFORMANCE OF ANODISED COATINGS ON MAGNESIUM ALLOY AZ91D Zhiming Shi, Guangling Song, and Andrej Atrens (CRC for Cast Metals Manufacturing (CAST), Division of Materials, School of Engineering, The University of Queensland, Qld 4072, Australia) Abstract - Anodising current density and potential are the most important parameters for an anodisation technique. This paper presents results of some coatings formed on magnesium alloy AZ91D at various anodising current densities and potentials. The microstructures of the coatings were examined using SEM. The corrosion performance of the coatings was evaluated by salt spraying and salt immersion. The porosity of the coatings strongly depended on the anodising current density, and it had a significant influence on the corrosion resistance of the coating. This suggested that the corrosion performance of an anodised coating could be improved if a specially designed current waveform is applied during the anodising process. Key words: anodised coating, magnesium alloy, microstructure, corrosion resistance 1. INTRODUCTION Anodised coating is one of the most effective solutions to the corrosion problem of magnesium alloys 1. Various anodised coatings are being or have been developed, such as HAE, DOW17, Tagnite, ANOMAG, MGZ and Keronite. All these anodised coatings can be formed on magnesium alloys under certain anodising conditions of current density and potential. They can serve as a corrosion resistant and wear resistant layer or can provide a good base for top-paint. Anodising current density and potential are the most important anodising parameters for an anodization technique. The anodizing process of magnesium alloys proceeds with a strong sparking in an alkaline bath. The coating is formed in as a result of a chemical reaction between the magnesium alloy substrate, oxygen (from the decomposition of water), the electrolyte and other components of the anodizing bath 2. Because of the high applied voltage, the electrolyte near the substrate is locally heated up to a spark or a plasma temperature. At such a high temperature, complicated reactions such as chemical reactions (deposition of coating), electrochemical reactions (oxygen evolution and oxidation of magnesiu m alloys) and physical reactions (high - temperature deposition of the components in the bath) are usually involved in the anodisation process. The anodizing process can be controlled by through adjusting the anodizing potential or current density to produce a high quality coating. For example, Barton 2 anodised AZ91 magnesium alloy by increasing the potential slowly during the initial anodizing stage until sparks began and then adjusting the potential to maintain a constant sparking until the potential reached 90 V. The potential was kept constant at 90 V in the final stage as describe. Barton 3 also applied a high voltage (170-350 V) to anodise the magnesium alloys in the patent US5792335. Other researchers like Zozulin 4, Sharma 5, Khaselev 6 and etc. anodised magnesium alloys using a constant current density. It is a common sense that a high current density may result in a coarse and bad quality coating and a lower current density can affect the efficiency of production and produced a fine coating. The mechanism by which the current density or potential affects an anodization process has not been understood clearly. This paper aims to understand the influence of the anodising parameters on the corrosion performance of an anodised coating. 2. EXPERIMENTAL The samples were cut from an AZ91D ingot. They were polished up to 1200 grit emery paper and degreased using ethanol before being anodised. The AZ91D alloy contained 8.96% aluminium, 0.77% zinc, 0.23% manganese and <0.005% iron balanced with magnesium.
O 2 collector Current Limiter + - Bath VOLTAGE CURRENT IN Cooling water Sample OUT Cooling water Stirrer DC POWER SUPPLY AGITATOR Fig. 1 Schematic diagram of anodizing process. Anodised coatings on magnesium alloys were prepared using a controlled anodising current in a bath using a power supply whose current and voltage can be limited below a certain value. The schematic diagram of the anodizing process is shown in Fig. 1. The sample is the anode and a stainless steel mug containing bath solution is also used as a cathode. The bath was cooled by flowing tap water. The electrolyte was stirred by a magnetic stirrer to maintain an even temperature in the bath. The pre-treatment of the samples involved degreasing in a alkaline solution at 80-90 C. In the anodizing process, the current density was changed in different stages of the anodizing process. The current density was set in a range between 10 ma/cm 2 and 25 ma/cm 2. Five current control modes were used to anodise the AZ91D samples. For, the anodizing current density was 20 ma/cm 2 and the anodizing time was 10 minutes. For, the anodising current density was 15mA/cm 2 and the anodizing time was 20 minutes. For, the anodizing procedure was 20 ma/cm 2 for 7 minutes, 15 ma/cm 2 for 7 minutes and 10 ma/cm 2 for 7 minutes. For, the anodizing current density was 25 ma/cm 2 for 7 minutes, 20 ma/cm 2 for 4 minutes and 15 ma/cm 2 for 7 minutes. For, the anodizing current density was 20 ma/cm 2 for 5 minutes. Oxygen evolution from the specimen during the anodizing process was measured using a basic burette measurement of oxygen evolution was carried out to evaluate the efficiency of the anodization. The samples were washed in water and dried after anodizing process. The microstructure of AZ91D was shown in Fig. 2. The microstructure of anodised coatings was analysed using the Philips XL30 Scanning Electronic Microscopy (SEM). The composition of anodised coating on magnesium coatings was determined by X-ray photon spectroscopy (XPS). The corrosion performance of anodised coatings was evaluated by salt immersion testing in 5% NaCl solution.
Fig. 2 Metallographic image of AZ91D ingot 3. RESULTS AND DISCCUSION 3.1 Effect of current density on the efficiency Fig. 3 (a) showed the effect of the controlled anodising current density on the anodising potential for AZ91D ingot in an anodising electrolyte developed in CAST corrosion laboratory. The potential increased more quickly in the initial stage when the applied current density was higher. For example, when the initial current density was changed from 15 to 25 ma/cm 2 (curves, and in Fig. 3 (a)), it took a shorter time for the anodizing potential to climb up to 300 volts. The current density influenced the initial stage of the anodization. The anodizing reaction in the initial stage consisted of oxidation of magnesium and deposition of the components from the bath. Almost all the current applied was used for the coating formation on AZ91D. The anodisation potential is approximately proportional to the coating thickness. So the initial growth rate of the anodising potential strongly depended on the increase of the thickness of the coating which was determined by the current. The increase of potential in the initial stage normally corresponded to the formation of an anodised film on the AZ91D surface. Fig. 3 (b) present the oxygen evolution in the anodising process and this is shown in more detail in Fig. 4 which also shows the rate of oxygen evolution. The evolved oxygen is a side reaction during magnesium anodisation, which consumes the anodising current. The higher the evolution rate of oxygen, the lower the efficiency of the anodization. Fig. 3 (b) shows that the volume of oxygen increased more sharply at 20 ma/cm 2 () than at 15 ma/cm 2 (). This means that in the same anodizing stage, the efficiency of anodization was dependent on the current density. In the initial stage, the effect of the current density on oxygen evolution was small because the oxygen volume was small. In the second stage, the evolution of oxygen was varied significantly with current density (see curves, and in Fig. 3(b)), the corresponding efficiency of anodization varied in different stages of the anodization process. 3.2 Corrosion performance of anodised coatings The corrosion performance of the anodised coatings was measured by salt immersion. The corrosion of these anodised coatings in 5% NaCl solution was mainly initialised from specks, progressed to pitting corrosion and finally became localised or filiform corrosion. The corrosion resistance of the above anodised coatings decreased in the following order: > > > >.
Potential (Volts) 400 300 200 100 Volume of O 2 (ml/cm 2 ) 2.5 2.0 1.5 1.0 0.5 0 0 2 4 6 8 10 12 14 16 18 20 22 24 Time (min) 0.0 0 4 8 12 16 20 24 Time (min) (a) (b) Fig. 3 The potential curves and oxygen evolution curves during anodising processes. in which : 20 ma/cm 2 (10 min), : 15 ma/cm 2 (20 min), : 20 ma/cm 2 (7 min)-15 ma/cm 2 (7 min)-10 ma/cm 2 (7 min), : 25 ma/cm 2 (7 min)-20 ma/cm 2 (4 min)-15 ma/cm 2 (7 min), : 20 ma/cm 2 (5 min). 3.3 The role of anodisation current The oxygen evolution during the anodizing process and corrosion performance of the anodised coatings depended on the current control mode. The effect of anodising current density on the anodising process varied obviously in different stages. In the first stage, the higher the current density the higher the growth rate of the initial film. When the potential increased to about 310 volts, then if the current density was still high, a very strong localised plasma sparking process would be initiated. It caused local breakdown of the film, resulting in a bad quality anodised coating. Based on the corrosion performance and surface quality, the optimum waveform of current density can be chosen. The optimum waveform of current density can improve the microstructure, surface appearance and the final quality of the coating. In process, the anodised coating formed very quickly before the potential reached 310 volts. The anodised coating was very coarse because of the vigorous sparking. In process, the current density was decreased in the later stages. The better corrosion performance suggested that the quality of the anodised coating was better than that from the process. For the process, the coating was formed very quickly in the first stage with higher current density, but the current density in the second stage was still very high and caused strong sparking. The thickness of the anodised coating changed with the anodising current density and time. The deposition of the anodised coating was proportional to the quantity of electricity (product of current and time) if all the current is used for anodisation. According to the electrical charge consumed in the anodising process, the order of thickness of these coating should be (340 ma.min/cm 2 ) > (315 ma.min/cm 2 ) > (300 ma.min/cm 2 ) > (200 ma.min/cm 2 ) > (100 ma.min/cm 2 ). The real thickness of the coatings was listed in Table 1, which is similar to the order predicted by the consumed electrical charge. The thicker the coating, the higher its block effects. So the thickness can also affect the corrosion resistance of the coating. Thickness ( m) Table 1Thickness of the anodised coatings. 17 19-20 18-21 20-23 10-11
0.20 Volume of O 2 0.8 Rate of O 2 Volume of O 2 (ml/cm 2 ) 0.15 0.10 0.05 0.6 0.4 0.2 Rate of O 2 (ml/min.cm 2 ) 0.00 0.0-0.05 0 2 4 6 8 10 12 14 16 18 20 22 24 Time (min) Fig. 4The rate of oxygen evolution during anodising process of AZ91D ingot (I d =15mA/cm 2, 20minutes, as same as above ). 3.4 Microstructure and composition of anodised coatings The composition of the anodised coating mainly consisted of magnesium, silicon, and oxygen. The alloying elements aluminium and zinc were not found in the coating by XPS. The surface appearance of the anodised coatings formed under different anodising current modes was shown in Fig. 5. The rate of the anodizing reaction was controlled by the anodising current density. It seems that the rate of the anodization reaction is associated with the microstructure. This is because the coating formation process is a film formation, breakdown and depositing process 3,4. The porous characteristics determine the properties of the anodised coating. By comparing the microstructures of anodised coatings formed under and in Fig. 5, it was found that the pores of the anodised coating were generally smaller than 1 m in diameter and those of coating were near 2 m. The results showed that in the first stage before the potential reached 310 volts, the anodised coating was very compact with tiny pores, because only tiny and even sparking was observed in this stage. For the coating, the microstructure was coarser because of the more vigorous sparking during the longer anodising time. The microstructure of coatings and were also compared in Fig. 5. The current density for the coating was 20 ma/cm 2 for 7 minutes, 15 ma/cm 2 for 7 minutes then 10 ma/cm 2 for 7 minutes. For coating, the current density was 25 ma/cm 2 (7 minutes), 20 ma/cm 2 (4 minutes) then 15 ma/cm 2 (9 minutes). The pores of coating were smaller than those of coating, and many pores in both coatings were sealed during the anodizing process. The EDAX analysis also showed that the compositions of the coatings on localised sparking spot and even sparking areas were similar. Smaller pores and sealed pores of the anodised coating are beneficial to the corrosion resistance of the coating. The sealing is attributed to the lower current density applied in the final anodisation stage. For the coating, the initial current density was 15 ma/cm 2 for 20 minutes. The pore size of coating was smaller than that of. The above analyses suggest that the microstructures of anodised coatings were significantly affected by the waveform of the anodising current density. In the first stage, the anodised coating formed on the surface was very compact and thin even for a high current density. In the second stage, the coating formed on the surface by deposition and breakdown process through plasma sparking which is important in thickening the coating. In the final stage when the potential was very high, the smaller current density was used to seal and repair the coarse pores produced in the second stage. A high current density in the third stage was not required.
Fig. 5 The SEM images of the anodised coating on AZ91D ingot. in which : 20 ma/cm 2 (10 min), : 15 ma/cm 2 (20 min), : 20 ma/cm 2 (7 min)-15 ma/cm 2 (7 min)-10 ma/cm 2 (7 min), : 25 ma/cm 2 (7 min)-20 ma/cm 2 (4 min)-15 ma/cm 2 (7 min), : 20 ma/cm 2 (5 min). 4.CONCLUSION The thickness of anodised coatings depended on the current density and anodizing time. The thickness increased with the current density and anodizing time. The effect of the anodizing current density on the microstructure of the anodised coating varied with the anodizing stage. In the first stage, the anodised coating was compact and thin. In the later stage of the anodizing process, the anodised coatings became coarser at a higher current density. The corrosion performance of the anodised coatings was influenced by both the microstructure and the thickness. The corrosion performance of anodised coating was related with the control anodizing parameters. In order to improve the corrosion performance of anodised coating, an optimised operation anodising current waveform should be applied.
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