This paper reviews various aspects of anodizing of magnesium alloys, such as the basics, processes, properties and applications. It systematically summarises the existing fundamental studies and technical developments of anodizing of magnesium alloys, and concludes that new anodizing processes based on electrolytic plasma anodizing that convert the surface of a magnesium alloy into a hard ceramic coating in an electrolytic bath using high energy electric discharges can offer improved wear and corrosion resistance. These new anodized coatings are often claimed to perform better than the traditional ones obtained through older anodizing processes, such as DOW17 or HAE. The new anodizing techniques are chromate free and hence environment friendly. It is expected that more cost-effective, environment-friendly and non-toxic anodizing techniques will be developed and applied to magnesium alloy components in the future.

Anodizing Treatments for Magnesium Alloys and Their Effect on Corrosion Resistance in Various Environments

Hydrogen generation through photoelectrochemical (PEC) water splitting using solar light as an energy resource is believed to be a clean and efficient way to overcome the global energy and environmental problems. Extensive research effort has been focused on n-type metal oxide semiconductors as photoanodes, whereas studies of p-type metal oxide semiconductors as photocathodes where hydrogen is generated are scarce. In this paper, highly efficient and stable copper oxide composite photocathode materials were successfully fabricated by a facile two-step electrochemical strategy, which consists of electrodeposition of a Cu film on an ITO glass substrate followed by anodization of the Cu film under a suitable current density and then calcination to form a Cu2O/CuO composite. The synthesized Cu2O/CuO composite was composed of a thin layer of Cu2O with a thin film of CuO on its top as a protecting coating. The rational control of chemical composition and crystalline orientation of the composite materials was easily achieved by varying the electrochemical parameters, including electrodeposition potential and anodization current density, to achieve an enhanced PEC performance. The best photocathode material among all materials prepared was the Cu2O/CuO composite with Cu2O in (220) orientation, which showed a highly stable photocurrent of −1.54 mA cm−2 at a potential of 0 V vs reversible hydrogen electrode at a mild pH under illumination of AM 1.5G. This photocurrent density was more than 2 times that generated by the bare Cu2O electrode (−0.65 mAcm−2) and the stability was considerably enhanced to 74.4% from 30.1% on the bare Cu2O electrode. The results of this study showed that the top layer of CuO in the Cu2O/CuO composite not only minimized the Cu2O photocorrosion but also served as a recombination inhibitor for the photogenerated electrons and holes from Cu2O, which collectively explained much enhanced stability and PEC activity of the Cu2O/CuO composite. Thus, the electrochemical strategy proposed in this study for the synthesis of the Cu2O/CuO composite opens a new way to use copper oxides as photocathode materials in PEC cells for a highly stable and effective water splitting.

Highly stable copper oxide composite as an effective photocathode for water splitting via a facile electrochemical synthesis strategy

Conducting polymers (CPs) such as polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh) are used for the corrosion protection of metals and metal alloys. Several groups have reported diverse views about the corrosion protection by CPs and hence various mechanisms have been suggested to explain anticorrosion properties of CPs. These include anodic protection, controlled inhibitor release as well as barrier protection mechanisms. Different approaches have been developed for the use of CPs in protective coatings (dopants, composites, blends). A judicious choice of synthesis parameters leads to an improvement in the anticorrosion properties of the coatings prepared by CPs for metals and their alloys. This article is prepared as a review of the application of CPs for corrosion protection of metal alloys.

Conducting polymers for corrosion protection: a review

Electrochemical corrosion properties of CeO2-containing coatings on AZ31 magnesium alloys prepared by plasma electrolytic oxidation

Corrosion performance of conducting polymer coatings applied on mild steel

Corrosion protection of polypyrrole electrodeposited on AZ91 magnesium alloys in alkaline solutions

Microstructure and properties of magnesium alloy processed by a new severe plastic deformation method

Corrosion behavior of pulse-plated Zn–Ni alloy coatings on AZ91 magnesium alloy in alkaline solutions

Zn–Ni alloy coatings are plated on AZ91 magnesium alloy using a pulse potential after an appropriate pretreatment. The current density, frequency, t on / t off and time have effects on thickness, morphology, Ni content, microhardness and anticorrosion of Zn–Ni coatings. Corrosion can reach above 200 h in salt spray test according to ASTM B117; the corrosion behavior of coatings is interfacial corrosion of particles and results in particle degradation as observed by scanning electron microscopy. Weight loss method and polarization curves are also used to investigate anticorrosion properties. Bonding strength is above 14.8 MPa in adhesive tensile tests.

Zn–Ni alloy coatings pulse-plated on magnesium alloy

This paper reports the application of ultrasonic power on the anodizing of magnesium alloys. The influence of ultrasonic power at a constant frequency of 25 kHz and two various acoustic power values, 280 and 400 W, on the structure and composition of anodizing coatings formed on magnesium alloy AZ31 was investigated. Results clearly show that ultrasound enhances the growth rate of anodic coatings and plays a very important role in the formation of coating structure and the distribution of coating composition. The anodic coatings consisted of only one layer if the ultrasound field was not applied in the solution used in this work. The anodic coatings consist of two layers when the ultrasonic field was applied and the acoustic power value increases to 400 W at a constant frequency of 25 kHz. The inner layer is compact and enriched in aluminum and fluorine, moreover, its thickness is uniform. In contrast, the contents of aluminum and fluorine in the external layer are very low and its thickness is non-uniform, some areas are thick, some thin. It was noted that there are some obvious fracture sites in the inner layer and external layer although the total thickness of coating is thick. An optimal acoustic power value should exist under the ultrasound frequency of 25 kHz to avoid the layer fracture and obtain thick coatings. The anodic coatings are composed of two phases, MgO and Al2O3, no matter whether the ultrasound is applied or not.

Zhai Chunquan

Papers

Corrosion protection of polypyrrole electrodeposited on AZ91 magnesium alloys in alkaline solutions

Microstructure and properties of magnesium alloy processed by a new severe plastic deformation method

Corrosion behavior of pulse-plated Zn–Ni alloy coatings on AZ91 magnesium alloy in alkaline solutions

Zn–Ni alloy coatings pulse-plated on magnesium alloy

Influence of ultrasonic power on the structure and composition of anodizing coatings formed on Mg alloys