Improvement of Corrosion Resistance of Magnesium by Anodizing in Alkaline Electrolytes
Anodization treatment of magnesium and its alloys is a useful surface treatment technique for forming protective coatings to improve their anti-corrosion properties, wear resistance and to enhance the adhesion capabilities of the surface. The anodic films formed on magnesium depend on the alloying constituents, anodizing process parameters and the electrolyte composition. To improve corrosion properties of anodic coatings on magnesium, environmentally-friendly, new aqueous alkaline electrolytes containing organic compounds, without toxic chromates and/or hazardous fluorides and phosphates, were investigated. The anodic film structure, component and surface morphology were analyzed using x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The anti-corrosion properties were investigated by electrochemical techniques. The results showed that the novel organic additive reduced the sparking voltage and provided a relatively smoother surface. Additionally, pores became considerably uniform in both size and distribution, due to the presence of the organic additive in the anodizing electrolyte. The anodic film produced with the organic additive exhibited the highest corrosion resistance for the AZ31B alloy.
Magnesium, the eighth most abundant metal on earth, has seen an increase in applications in a variety of sectors ranging from aerospace, military, defense, and automotive to commercial mobile phones, sporting goods and handheld tools, owing to its high strength-to-weight ratio, machinability, thermal conductivity and weldability.1-4 However, the poor corrosion resistance of magnesium and its alloys has limited applications in corrosive environments.4-7
Improving corrosion performance involves a variety of finishing processes, including oil applications, wax coating, anodizing, electroplating, conversion coating, painting, etc. Metal finishers are often faced with challenges to tailor a better combination of these processes to enhance the product corrosion performance, considering various factors ranging from specific applications to environmental conditions. Although development of new and high purity magnesium alloys and novel surface modification techniques such as ion implantation and laser annealing have mitigated the corrosion problem, magnesium alloys still suffer from performance issues, especially under aggressive corrosive environments. High chemical reactivity, complex alloy microstructures, hazardous pretreatment procedures and recycling problems to an extent have discouraged many applications of magnesium.8-10
Among the typical surface modification techniques, conversion coating, electroplating with alloys like zinc, nickel and chromium and anodizing processes such as DOW 17, HAE, and Tagnite have become widely popular for the surface treatment of magnesium alloys. However, these techniques either resulted in a good paint base surface or incorporated harmful fluorides, chromates and phosphates. Although methods involving CVD, PVD, and plasma spray techniques have shown promise, they have not found a widespread application owing to the high startup and maintenance costs.
A majority of the magnesium alloys are either sand-cast or die-cast and hence are prone to microstructural inhomogeneities and surface imperfections, such as porosity and impurities. Magnesium and its alloys are also prone to galvanic corrosion and alloy dissolution owing to the lesser nobility of magnesium. Design of a process routine that results in an effective barrier to corrosion as well as wear would be ideal. The anodization process has been a widely accepted solution for surface protection of light metals, especially aluminum. The anodic coatings formed are not only wear resistant but also offer a porous template for further treatments like sealing, dyeing and painting.
Anodization of an alloy surface can be conducted via current or voltage control. Under voltage control, current drops as the oxide layer grows whereas under constant current, voltage increases with time. During the anodization of magnesium alloys, for any combination of bath composition and temperature, there is a maximum voltage that can be supported before breakdown occurs. Once the voltage crosses the dielectric breakdown voltage of the film previously formed, a spark is initiated, which with time turns into localized micro-arcs. Thus, various factors, including the mechanical finishing as a pretreatment, surface activation processes, a combination of bath composition and temperature could affect the coating formation and subsequently the performance of anodic coatings.
Huber, et al.,11 reported an interesting relationship between applied voltage and anodizing characteristics. When anodized in an alkaline electrolyte, a grey magnesium hydroxide layer formation was observed up to 3.0 V. A thicker version of this layer resulted between 3.0 and 20 V and an intensive sparking was observed above 50 V. Verdier, et al.,12 observed a combination of three different stages during the anodization of AM60 magnesium alloy under constant current. Voltage monitoring revealed a transition of traditional anodizing into spark anodizing and into a full arc process. Along with spark formation and voltage variations, oxygen evolution is another important phenomenon associated with magnesium anodizing. After analyzing the anodic behavior of AZ91D in a silicate bath, Shi, et al.,13 stated that oxygen evolution is a result of the thermal decomposition of localized sparking.
It has been reported that the anodic oxide coating may have a high porosity of up to 40%.14 As a result, the corrosion resistance of an oxide coating on magnesium alloy decreases with increasing porosity. Therefore, it is critical to reduce the coating porosity in order to modify the anti-corrosion performance. It is also important to develop an anodizing electrolyte, which does not contain any hazardous compounds like chromates, fluorides and phosphates, due to the environmental restrictions.
In this study, the anodizing process in an environmentally-friendly alkaline silicate-based electrolyte with and without a novel organic additive was investigated. The anodization process, anodic film morphology, structure and composition were examined. Electrochemical testing techniques were used to study the effect of the organic ingredient into the anti-corrosion properties of the produced anodic coatings.
The formation of anodic films with the AZ31B magnesium alloy in alkaline silicate-based electrolytes was investigated. Magnesium samples (4 × 4 in.) were ground down using 150-grit SiC paper so that the surface was flat. Samples were degreased with acetone prior to anodization. The anodizing cell consisted of an agitated bath with two steel cathodes and anodization was conducted under constant current with a high voltage rectifier (70 A, 150 V) and the electrolyte temperature was maintained at 15 ± 5°C. Current and voltage transients were recorded by a commercially available AD-converter and a computer (JobPro). Anodizing was performed in concentrated alkaline solution containing silicates and an organic additive. The effect of the organic component on the anodic film was investigated by varying its concentration in the electrolyte.
Surface morphology of anodic films was observed using a Hitachi S-4700 cold-field emission Scanning Electron Microscope (SEM/EDS). The crystalline phase composition of the anodized samples was examined using a room temperature x-ray diffractometer (Philips PW 2273) with CuKα radiation.
The resultant anodic coating thickness was measured in accordance with ASTM B244 (Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and of Other Nonconductive Coatings on Nonmagnetic Basis Metals with Eddy-Current Instruments) using a pre-calibrated eddy current instrument. On each sample eighteen data points were obtained at different locations, and the average value was taken as the anodic film thickness.
The average surface roughness (Ra) of the anodic films was measured by a Surfometer, Series 400 (Precision Devices, Inc.). Potentiodynamic polarization measurements were carried out using a Gamry potentiostat and Gamry Echem Analyst software. A three-electrode configuration was employed: the anodized magnesium substrate as the working electrode, with a surface area of 3.0 cm2; a carbon rod as a counter electrode; and a saturated calomel electrode (SCE) as the reference electrode.
Results and Discussion
Characterization of the anodizing process
The effects of constant voltage control and constant current density control on the growth and formation of anodic coatings have been studied extensively. It has been shown that constant current density process has many advantages over traditional constant voltage controlled anodizing processes.15
It is virtually impossible for constant voltage control to produce the anodic coatings with consistent quality because of its inherent problems. In general, the growth rate of the anodic coating is found to be stable under constant current control, while the applied voltage is allowed to float, responding to the variations of physical, chemical and electrical factors such as surface area, contact resistance, solution resistance, etc., which frequently change from run to run, shift to shift, and day to day. Therefore, anodizing under the constant current was adopted in order to control and produce repeatable results.
The anodizing behavior of magnesium alloys depends on the combined effects of voltage variation and simultaneous sparking action. Anodic film growth is influenced by the relative concentrations of the chemical constituents of the electrolyte, current density or voltage levels, anodizing time, etc. It is well known that the electrolyte chemistry has a major influence on the anodizing process. The experiments were performed in an alkaline bath containing silicates at a fixed concentration and an organic component at varying concentrations.
Voltage-time response. The voltage transients for AZ31B magnesium alloy anodized in solutions with and without the organic additive are shown in Fig. 1. In both cases, the voltage curve shows three distinct regions based on the different rates of increasing voltage and the range of voltage oscillation:
- (Region I) a steep voltage rise up to 62 V at the beginning of the process
- (Region II) a slower rise with characteristic oscillations up to 90 V and
- (Region III) a constant voltage thereafter.