An Influence of Current density and Temperature about Anodic Oxidation Film Properties of Al Die-casting

Transcription

1 Vol.64 (Materials 2014), pp An Influence of Current density and Temperature about Anodic Oxidation Film Properties of Al Die-casting Sung Il Jeon, Won Sub Chung *1 Department of Materials Science and Engineering, Pusan National University, Busan , Korea * wschung1@pusan.ac.kr Abstract: The characteristic of anodized coating layer is highly influenced by its process parameter. Among them composition of current density and electrolyte temperature is one of the most important factors to determine the characteristic of coating layer on aluminium alloy. In this study, ADC12 aluminium alloy was treated by anodizing process changing current density and electrolyte temperature, and then the effect was compared. The morphology and chemical composition of the coating layer were characterized by SEM. The hardness of anodic film was measured by micro-vickers hardness tester. As a result, increasing with anodic oxidation current density, the oxidation layer was thicker and the morphology was bigger and denser on the surface. The hardness was highly increased when the electrolyte temperature was 20 in both electrolyte. Hardness was higher at mixed acid electrolyte than sulfuric acid electrolyte. Keywords: Anodizing, Al alloy, Die-casting, Current density, Electrolyte 1 Introduction Among aluminum alloy die castings, ADC12 is extensively used due to excellent mechanical properties as high castability, productivity, low density and contraction rate, and relatively high solidity,[1] but its easy corrosiveness causes low mechanical quality in many times. Therefore, it s highly required to make methods of improving corrosion resistance and securing reliability. [2][3] Currently, anodizing is examined as the most possible method to form protective film on aluminum alloy materials,[4][5] but it is reported that it s difficult to regularly form anodic oxidation film as growth speed of oxide film is different due to the different reactivity between Al and Si in aluminum alloy. The purpose of this study is to investigate differences of thickness, structure, corrosion resistance and hardness of anodic oxidation film following to electrolyte kinds, current density, and different temperature of electrolyte among the factors affecting the anodic oxidation film Corresponding author ISSN: ASTL Copyright 2014 SERSC

2 Vol.64 (Materials 2014) formation of ADC12 alloy die casting. 1.1 Purpose The purpose of this study is to analyze film thickness, film structure, corrosion resistance, and hardness differences due to the electrolyte kinds, current density, electrolyte temperature difference affecting anodic oxidation film formation of ADC12 die casting alloy. 1.2 Methods In this study, specimens were made by processing commercial ADC12 die casting parts to the size of 50 mm * 30 mm * 2t to fit with anodic oxidation film formation. Oxide films were made by using sulfuric acid(15 wt.% H2SO4) electrolyte and sulfuric acid/oxalic acid mixed electrolyte(15 wt.% H2SO wt.% (COOH)2 2H2O). 30 minutes of current power, 2, 3, 4 A/d m2 of current density, and 10, 20, 30 of electrolyte temperatures were applied. To identify the result of the experiment, film thickness was checked using an optical microscope, and crystal structure of film was checked using a scanning electron microscope (SEM), and Potentio-Dynamic test was arranged to evaluate film corrosion resistance. Film thickness was measured using Micro vickers hardness tester (Mitutoyo HM-124). 2 Results Fig. 1. Relationship between temp. and anodic oxidation thickness at 2, 3, 4 A/dm2 ; (a)15 wt.% H 2 SO 4 (b)15 wt.% H 2 SO wt.% (COOH) 2 2H 2 O Copyright 2014 SERSC 5

3 Vol.64 (Material 2014) 115 wt.% H 2 SO wt.% H 2 SO wt.% (COOH) 2 2H 2 O Fig. 2. SEM images of anodic oxide films(x 150,000) formed in electrolyte for 1, 2 ; (a) 2 A/dm min, (b) 2 A/dm min, (c) 4 A/dm min, (d) 4 A/dm min. Fig. 3. Potentio-dynamic polarization curves for anodic oxide films formed at ; (a) 15 wt.% H 2 SO 4 (b)15 wt.% H 2 SO 4 + (COOH) 2 2H 2 O Fig. 4. Relation between the hardness and ; (a) bath temperature & (b) Current density in 15wt. % H 2 SO 4 6 Copyright 2014 SERSC

4 Vol.64 (Materials 2014) Fig. 5. Relation between the hardness and ; (a) bath temperature & (b) Current density in 15wt. % H 2 SO wt. % (COOH) 2 2H 2 O 3 Conclusions In this study, differences of oxide film thickness, corrosion resistance, hardness and structure were analyzed after making anodic oxidation film of aluminum alloy material called ADC12 by different application of electrolyte kinds, current density, and temperature of electrolytes, and the summary and results are as below. 1 When current density was steady, film thickness was decreased with increased temperature of both of the electrolytes. In case of sulfuric acid/oxalic acid mixed electrolyte, thickness of anodic oxidation film was relatively thinner. Meanwhile, in case of regular electrolyte temperature, thickness of oxide film was increased when current density was elevated. 2 At the same electrolyte, surface of oxide film with higher current density was a bit denser. 3 The result of evaluating corrosion resistance through Potentio-Dynamic test, oxide films formed at sulfuric acid/oxalic acid electrolyte showed the highest corrosion potential and the lowest corrosion current density so to identify that the corrosion resistance is better than at sulfuric acid electrolyte. It was found that when each current density was steady, corrosion current density was decreased by increased temperature of electrolyte, while each temperature of electrolyte was steady, corrosion current density was also decreased when current density was increased. 4 When current density of electrolyte was steady, hardness of anodic oxidation film showed the highest value at 20 while the value was decreased when the temperature was increased to 30. Hardness of anodic oxidation film formed at sulfuric acid/oxalic acid electrolyte was higher than at sulfuric acid electrolyte. The above results showed that when anodic oxidation film was formed at sulfuric Copyright 2014 SERSC 7

5 Vol.64 (Material 2014) acid/oxalic acid electrolyte, ADC12 alloy die-casting material could get high corrosion resistance and hardness values though thickness of the films was relatively thinner. Meanwhile, among the properties of anodic oxidation film following to the difference of electrolyte temperature, the highest hardness value at 20 was exceptional and so it requires further studies to investigate the causes. We hope that conditions about optimized area for film formation can be found through further sustainable and repeated studies. References 1. Okayasu. M, Ohkura. Y, Takeuchi. S, Takasu. S, Ohfuji. H, Shiraishi. T, Materials Science and Engineering A, , (2012). 2. L. H. Hihara, R. M. Lantanision, International Materials Review, 39, 245 (1994). 3. R. L. Deuis, L. Green, and C. Subramanian, Journal of Materials Science letters, 16, 440 (1997). 4. M. Shahid, J 0/ Mater. Sci, 32, 3775 (1997). 5. P. P. TRZASKOMA, E. MCCAFFERTY and C. R. CROWE, J. Electrochem. Soc. 130, 9 (1983). 8 Copyright 2014 SERSC