The effect of anodization temperature and tartaric acid concentration on the morphology and corrosion resistance of the anodic film formed on AA2099-T8 alloy in tartaric-sulfuric acid was investigated. It was found that the dissolution of the anodic film during anodizing led to increased pore size, rod-shaped cavities and grain boundary grooves in the anodic films. The rod-shaped cavities and grain boundary grooves are associated with selective dissolution of the anodic film formed from fine T 1 (Al 2 CuLi) phase precipitates due to the difference in the reactivity of the films formed from different phases. The increased porosity due to dissolution degraded the corrosion resistance of the anodic film. In the temperature range of 22-47 • C, with 0.53 M tartaric acid addition, anodizing at 42 • C provided the best corrosion performance and a relatively high anodizing efficiency; in the tartaric acid concentration range of 0-0.9 M, at 37 • C, anodizing in electrolytes containing 0.7 and 0.9 M tartaric acid provided good corrosion resistance with little decrease of anodizing efficiency. The corrosive medium did not penetrate the anodic film uniformly but preferentially at local sites, resulting in localized corrosion of the anodized alloy. Aluminum and its alloys are protected from corrosion by an airformed oxide film of several nanometers thickness. However, the protective ability of the air-formed oxide film is limited due to its small thickness. In order to obtain more reliable and durable protection for aluminum and its alloys, a relatively thick oxide film has been pursued. As a case in point, aerospace aluminum alloys are anodized in acidic electrolyte to produce a relatively thick anodic oxide film of a few micrometers, providing the alloys with reasonable corrosion resistance and/or suitable surface for painting and adhesive bonding. Chromic acid anodizing (CAA) used to be the most widely employed anodizing process in aerospace industry.1-3 However, the high toxicity associated with Cr (VI) has restricted the application of CAA. Since 1990s, there have been many efforts to find alternative anodizing process to replace CAA in aerospace industry. [4][5][6] Sulfuric acid anodizing (SAA) is a commonly used, environmentally friendly anodizing process. However, a relatively thicker anodic film is needed for SAA than for CAA if the same corrosion resistance is required. 3On the other hand, increased film thickness will sacrifice fatigue resistance of the anodized components, 3,7-9 not mentioning cost increase. In 1990, Wong et al.3 invented a boric-sulfuric acid anodizing (BSA) process, which modifies the traditional SAA process by reducing the concentration of sulfuric acid and adding boric acid. It is now recognized that the performance of the anodic film produced by BSA is similar to that of the anodic film produced by CAA. 5,10 At the begin of the twenty first century, a new anodizing process, called tartaricsulfuric acid anodizing (TSA), was patented by Alenia Aeronautica S.P.A.11 to compete with BSA. Since th...
T1 (Al2CuLi) phase precipitates, the main strengthening precipitates in third generation aluminum-copper-lithium (Al-Cu-Li) alloys, play a critical role in determining the corrosion behavior of these alloys. Herein, the T1 precipitates, sufficiently large to be visualized by scanning electron microscopy, were intentionally grown in a commercial Al-Cu-Li alloy through a high temperature annealing process. The corrosion and anodizing behavior of the alloy associated with individual T1 precipitate plates was subsequently investigated. It was observed that corrosion initiated instantaneously on T1 precipitate plates when the alloy was exposed to laboratory air. When immersed in NaCl solution, T1 precipitate plates corroded through a dealloying process and then, drove anodic dissolution of the adjacent aluminum alloy matrix by forming copper-rich nanoparticles at the sites of dealloyed T1 precipitates. The T1 phase precipitates were anodized relatively faster than the aluminum matrix in tartaric-sulfuric acid solution under a constant voltage of 14 V. The anodic film formed from T1 precipitates was dissolved quickly by the anodizing electrolyte during anodizing at relatively higher temperatures, resulting in cavities of sizes similar to those of T1 precipitate plates.
The electrochemical corrosion behaviour of biomedical Ti-25Nb-3Mo-3Zr-2Sn (TLM) alloy was investigated in various simulated body fluids at 37¡0?5uC utilising potentiodynamic polarisation and current-time curves. The Ti-6Al-4V (TC4) alloy was also investigated to make a comparison. The different simulated body fluids comprised of 0?9%NaCl saline, Hank's and Ringer's solution were employed. The effect of heat treatment on the electrochemical behaviour of the TLM alloy was also considered. It was discovered that all the test specimens were passivated once immersed into the simulated body fluids. It was also found that the TLM alloy has poorer corrosion resistance in Hank's solution, due to the chemical composition of the Hank's. After different heat treated, the TLM alloy had different phases and microstructure, and the corrosion behaviour of the TLM alloy was different. In this study, after the heat treatment of 760uC/1 h/ACz550uC/6 h/AC, the TLM alloy had better corrosion resistance. Owing to the corrosion resistance of the TLM alloy was influenced by numerous factors, such as microstructure and the chemical composition of electrolyte, the corrosion behaviour of the TLM alloy is complex. By comparing with the corrosion behaviour of the TC4 alloy, the TLM alloy has poorer corrosion resistant than the TC4 alloy under the same conditions. But the current-time curves of the TLM alloy were more stable than these of the TC4 alloy with further experiments, because of the more passivation film on the surface of the TLM alloy.
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