Pitting corrosion of magnesium (Mg) alloys is greatly associated with their microstructure, especially second phases. The second phases in traditional Mg alloys such as AZ91 are electrochemically nobler than Mg matrix, while the second phases in Rare earth (RE) Mg alloy GW93 are more active than Mg matrix. As a result, the pitting corrosion mechanism of Mg alloy GW93 is different from the traditional ones. This paper aims to clarify the pitting corrosion mechanism of Mg alloy GW93 through the studies of Volta potential by Scanning Kelvin Probe Force Microscopy (SKPFM), corrosion morphology by Scanning Electron Microscope (SEM), and corrosion resistance by electrochemical tests. Results reveal that the pitting corrosion of GW93 includes three stages, first, dissolution of the second phases, followed by corrosion of Mg matrix adjacent to the dissolved second phases, and finally, propagation of corrosion pits along the depth direction of the dissolved second phases. Anodic second phases and enrichment of Cl− in the thick corrosion product films dominate the propagation of pitting corrosion.

It is well known that second phases act as micro-cathodes in the corrosion of traditional Mg alloys. However, the effect of second phases on the corrosion behavior of Mg-rare earth (RE) alloys is ambiguous in view of the second phases consisting of Mg and more active RE elements. The role of second phases in the corrosion of cast EW75 (Mg–5Y–7Gd–1Nd–0.5Zr) was studied by scanning electron microscopy (SEM) observations, Scanning Kelvin Probe Force Microscopy (SKPFM) analysis, immersion and electrochemical tests. It is found that the second phases in EW75 are more active than Mg matrix and preferentially dissolved at the initial corrosion stage. It indicates that the second phases act as micro-anodes, which are greatly different from the role of second phases in traditional Mg alloys.

•A hydrotalcite film has been formed on Al-free Mg alloys by in situ growth method.•The influence of alloying elements on the composition of the films is discussed.•The role of microstructure in the formation of hydrotalcite film is illustrated.The influence of alloying elements and microstructure of Mg substrates on the formation of hydrotalcite film has been investigated. It is found that the two-step process is also available for the pure Mg and other alloys after modification. A small amount of Zn does not impact the composition of the hydrotalcite film much; whereas the highly active rare earth (RE) affects the constituents of the precursor film as well as the final film on WE54 alloy significantly. The microstructure impacts the initial nucleation and the film morphology depending on the size and chemical activity of the intermetallic particles.

•The PEO films formed in the solutions with and without KF were compared.•KF makes a main contribution to the in-situ sealing pores of PEO film.•KF can change the initial film formation process and enhance the film growth rate.The influence of potassium fluoride (KF) on the in-situ sealing pores of plasma electrolytic oxidation (PEO) film on AM50 Mg alloy has been investigated using a stereo–zoom optical microscope, scanning electron microscope (SEM) and energy dispersive X–ray spectroscope (EDS). The films formed in the solutions with and without KF were compared. The sketch map for the formation process of in-situ sealing pores was established. The in-situ sealing pore PEO film can be obtained in the solution with KF, while the PEO film with open pores is obtained in the solution without KF. It indicates that the addition of KF plays a key role in the formation of in-situ sealing pores. Moreover, KF can change the initial film and promote the film growth rate.

•A novel self-sealing pore MAO film is developed on AM60 Mg alloy.•Fluorides are beneficial in improving the compactness of initial passive film.•Different melting points of film constituents are key factor for pore self-sealing.•High titanium oxide content greatly enhances the compactness of MAO film.Ceramic films are formed on AM60 magnesium alloy by micro-arc oxidation (MAO) using a new fluorotitanate electrolyte system. Compared to the films obtained with traditional electrolytes, the film has the characteristics of self-sealing pores and different chemical compositions. To investigate the film's growth mechanism, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) were used to characterize the phase structure and chemical composition. Mott–Schottky (M–S) curve measurements were used to study the electrical properties of the passive layer and the influence of the fluorine during initial film deposition. The NaF content in the new electrolyte plays an important role in improving the compactness of the initial film. By increasing oxidation voltages in the MAO process, an increase of titanium oxides in the film is produced. Different melting points of film constituents and high titanium oxide content in the MAO film are key factors for forming self-sealing pores.

•There are compact surface films formed on the Mg–2Zn and Mg–5Zn alloys in NaCl.•The degradation behavior of the corrosion product films is studied.•The protection ability of the film on Mg–2Zn is superior to that on Mg–5Zn.•Chemical composition and microstructure greatly affect the corrosion product films.The corrosion product films formed on the surface of Mg–2Zn and Mg–5Zn alloys in NaCl solution were investigated by electrochemical measurement, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. It is found that a compact corrosion product film is formed in the initial stage of immersion, and then the film gradually degrades due to dissolution reaction. The product film formed on Mg–2Zn alloy presents better protection property than that on Mg–5Zn alloy, which can be attributed to the different chemical composition and microstructure of the both alloys.

Hydrogen evolution reaction is inevitable during the corrosion of Mg alloys. The effect of hydrogen on the corrosion behavior of the Mg–2Zn and Mg–5Zn alloys is investigated by charging hydrogen treatment. The surface morphologies of the samples after charging hydrogen were observed using a scanning electron microscopy (SEM) and the corrosion resistance was evaluated by polarization curves. It is found that there are oxide films formed on the surface of the charged hydrogen samples. The low hydrogen evolution rate is helpful to improve the corrosion resistance of Mg alloys, while the high hydrogen evolution rate can increases the defects in the films and further deteriorates their protection ability. Also, the charging hydrogen effect is greatly associated with the microstructure of Mg substrate.

A novel biodegradable composite coating is prepared to reduce the biodegradation rate of Mg–3Zn alloy. The Mg–3Zn substrate is first immersed into 0.02 mol L− 1 nicotinic acid (NA) solution, named as vitamin B3, to obtain a pretreatment film, and then the electrodeposition of calcium phosphate coating with ultrasonic agitation is carried out on the NA pretreatment film to obtain a NA/calcium phosphate composite coating. Surface morphology is observed by scanning electron microscopy (SEM). Chemical composition is determined by X-ray diffraction (XRD) and EDX. Protection property of the coatings is evaluated by electrochemical tests. The biodegradable behavior is investigated by immersion tests. The results indicate that a thin but compact bottom layer can be obtained by NA pretreatment. The electrodeposition calcium phosphate coating consists of many flake particles and ultrasonic agitation can greatly improve the compactness of the coating. The composite coating is biodegradable and can reduce the biodegradation rate of Mg alloys in stimulated body fluid (SBF) for twenty times. The biodegradation process of the composite coating can be attributed to the gradual dissolution of the flake particles into chippings.Highlights► NA/calcium phosphate composite coating is prepared to protect Mg–3Zn alloy implant. ► Nicotinic acid (vitamin B3) is available to obtain a protective bottom film. ► Ultrasonic agitation greatly improves the compactness of calcium phosphate coating. ► The composite coating can reduce the biodegradation rate of Mg–3Zn twenty times. ► The composite coating is biodegraded by the dissolution of flakes into chippings.