This review focusses on the application of physiological conditions for the mechanistic understanding of magnesium degradation. Despite the undisputed relevance of simplified laboratory setups for alloy screening purposes, realistic and predictive in vitro setups are needed. Due to the complexity of these systems, the review gives an overview about technical measures, defines some caveats and can be used as a guideline for the establishment of harmonized laboratory approaches.

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Many reports are available on degradation rates and corrosion product characterization of magnesium alloys for implant application. Typically, the data are obtained ex situ, as post factum analysis of occurred degradation processes or in bulk electrolyte that does not fully reflect concentration gradients at the interface and in the diffusion layer. Meanwhile, these local conditions are essential for tissue‐implant compatibility. Only a limited number of studies employ the techniques that visualize ongoing degradation and are localized enough to observe the changes in the electrolyte layer directly adjacent to corroding magnesium. Here, local pH in Hank's solution is studied in operando, with potentiometric micro‐probes (d = 2 µm) located 10–50 µm above the surface of four Mg alloys potentially relevant for implant applications. A significant difference in local pH is observed for Mg in simple Hank's solution (near surface pH 9.9–10.5) or Hank's solution modified with 2.5 × 10−3 m Ca2+ (pH 7.8–8.5). The corresponding pH values are established during the first seconds of immersion. These findings indicate different degradation kinetics in electrolytes with or without Ca2+. The degradation rate of Mg alloys decreases by almost two times in Ca2+ containing Hank's solution. Calcium‐phosphate/carbonate protective layer stabilizes the surface pH below 8.5 controlling Mg degradation.

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of the original manuscript:Matykina, E.; Arrabal, R.; Pardo, A.; Mohedano, M.; Mingo, B.; Rodriguez, I.; Gonzalez, J.: Energy-efficient PEO process of aluminium alloys AbstractThe influence of pre-anodizing in sulphuric and phosphoric acids on energy efficiency of voltage-controlled PEO process of three commercial wrought and cast aluminium alloys (AA1050, AA6082 and A356) has been investigated. The precursor anodic porous films enable up to 57% energy savings during PEO and produce ~35-40% increase of the coating microhardness compared with direct PEO. Total specific energy consumption values of 2.5-2.7 kW h m -2 µm -1 and 3.1-3.8 kW h m -2 µm -1 were achieved using phosphoric and sulphuric acid-formed precursors, respectively.

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Mg-xGd alloys show potential to be used for degradable implants. As rare earth containing alloys, they are also of special interest for wrought products. All applications from medical to engineering uses require a low and controlled degradation or corrosion rate without pitting. Impurities from fabrication or machining, like Fe inclusions, encourage pitting, which inhibits uniform material degradation. This work investigates a suitable etching method to remove surface contamination and to understand the influence of etching on surface morphology. Acetic acid (HAc) etching as chemical surface treatment has been used to remove contamination from the surface. Extruded Mg-xGd (x = 2, 5 and 10) discs were etched with 250 g/L HAc solution in a volume of 5 mL or 10 mL for different times. The microstructure in the near surface region was characterized. Surface characterization was done by SEM, EDS, interferometry, and ToF-SIMS (time-of-flight secondary ion mass spectrometry) analysis. Different etching kinetics were observed due to microstructure and the volume of etching solution. Gd rich particles and higher etching temperatures due to smaller etchant volumes promote the formation of pits. Removal of 2–9 µm of material from the surface was sufficient to remove surface Fe contamination and to result in a plain surface morphology.

Although certified magnesium-based implants are launched some years ago, the not well-defined Mg degradation mechanism under physiological conditions makes it difficult to standardize its use as a degradable biomaterial for a wide range of implant applications. Among other variables influencing the Mg degradation mechanism, monitoring the pH in the corrosive solution and, especially, at the corroding interface is important due to its direct relation with the formation and stability of the degradation products layer. The interface pH (pH at the Mg/solution interface) developed on Mg-2Ag and E11 alloys are studied in situ during immersion under dynamic conditions (1.5 mL min -1 ) in HBSS with and without the physiological amount of Ca 2+ cations (2.5 × 10 -3 m). The results show that the precipitation/dissolution of amorphous phosphate-containing phases, that can be associated with apatitic calcium-phosphates Ca 10-x (PO 4 ) 6-x (HPO 4 or CO 3 ) x (OH or ½ CO 3 ) 2-x with 0 ≤ x ≤ 2 (Ap-CaP), promoted in the presence of Ca 2+ generates an effective local pH buffering system at the surface. Thus, high alkalinization is prevented, and the interface pH is stabilized in the range of 7.6 to 8.5.