W. Dietzel

General and localized corrosion of magnesium alloys: A critical review

Magnesium (Mg) alloys as well as experimental alloys are emerging as light structural materials for current, new, and innovative applications. This paper describes the influence of the alloying elements and the different casting processes on the microstructure and performance of these alloys and corrosion. It gives a comprehensible approach for the resistance of these alloys to general, localized and metallurgically influenced corrosion, which are the main challenges for their use. Exposure to humid air with ∼65% relative humidity during 4 days gives 100–150 nm thickness. The film is amorphous and has an oxidation rate less than 0.01 µm/y. The pH values between 8.5 and 11.5 correspond to a relatively protective oxide or hydroxide film; however above 11.5 a passive stable layer is observed. The poor corrosion resistance of many Mg alloys can be due to the internal galvanic corrosion caused by second phases or impurities. Agitation or any other means of destroying or preventing the formation of a protective film leads to increasing corrosion kinetics. The pH changes during pitting corrosion can come from two different reduction reactions: reduction of dissolved oxygen (O) and that of hydrogen (H) ions. Filiform corrosion was observed in the uncoated AZ31, while general corrosion mainly occurred in some deposition coated alloys. Crevice corrosion can probably be initiated due to the hydrolysis reaction. Exfoliation can be considered as a type of intergranular attack, and this is observed in unalloyed Mg above a critical chloride concentration.

Testing of general and localized corrosion of magnesium alloys: A critical review

The degradation of materials generally occurs via corrosion, fatigue, and wear. Once a magnesium (Mg) alloy is chosen for a certain application, corrosion testing is generally required as a function of the expected service environment, the type of corrosion expected in service, and the type of surface protection, depending on the material and its use in the intended surface. In the absence of appropriate standards for the testing of magnesium alloys, a brief summary of the various procedures of accelerated electrochemical and corrosion testing of Mg alloys that have been adopted by different schools is given, accompanied by some critical comments for future work. Hydroxide, hydroxide-chloride, and corrosive water formulated according to American Society for Testing Materials (ASTM) standard 1384-96 are considered to evaluate general corrosion, localized corrosion, and corrosion influenced by metallurgical parameters. The influence of agitation, oxygenation, pH, and temperature are discussed. Surface cleaning, superficial microstructure, and surface preparation for testing are discussed. Appropriate electrochemical methods that can be applied to this relatively new and electrochemically active structural material are described. Corrosion potential measurements, polarization, impedance, noise electrochemistry, and surface reference electrode technique are recommended as valuable methods for evaluating the resistance of existing or experimental alloys to these types of corrosion. Corrosion kinetics and varying properties of the solution at the alloy/solution interface are examined. A critical description of the relevance and importance of these methods to corrosion testing of Mg alloys is given.

Microstructure and corrosion behavior of Mg-Sn-Ca alloys after extrusion

Abstract Mg-Sn-Ca alloys promise a reasonable corrosion resistance in combination with good creep resistance, likely due to the presence of Ca 2− x Mg x Sn and other phases. The selected alloys with 3% Sn and Ca in the range of 1%–2% have been extruded in order to achieve more homogeneous microstructure compared with the as-cast alloys. Optical microscopy(OM) and X-ray diffraction(XRD) techniques were used to study the microstructure and phases of these alloys. The corrosion behavior of these alloys was investigated by means of salt spray test and potentio-dynamic measurements. The results obtained on the alloys Mg-3Sn (T3), Mg-3Sn-1Ca (TX31), and Mg-3Sn-2Ca (TX32) indicate the presence of the same phases in as-cast and after extrusion, namely Mg 2 Sn, Ca 2− x Mg x Sn, and Ca 2− x Mg x Sn/Mg 2 Ca, respectively. However, due to the occurrence of extensive recrystallization in the extrusion process, the grain size has significantly reduced after extrusion. The reduction leads to the improvement of the corrosion resistance after extrusion which is then comparable with the commercial alloy AZ91D.

Microstructure and corrosion behavior of shielded metal arc-welded dissimilar joints comprising duplex stainless steel and low alloy steel

This work describes the results of an investigation on a dissimilar weld joint comprising a boiler-grade low alloy steel and duplex stainless steel (DSS). Welds produced by shielded metal arc-welding with two different electrodes (an austenitic and a duplex grade) were examined for their microstructural features and properties. The welds were found to have overmatching mechanical properties. Although the general corrosion resistance of the weld metals was good, their pitting resistance was found to be inferior when compared with the DSS base material.

Influence of circumferential notch and fatigue crack on the mechanical integrity of biodegradable magnesium‐based alloy in simulated body fluid

Applications of magnesium alloys as biodegradable orthopaedic implants are critically dependent on the mechanical integrity of the implant during service. In this study, the mechanical integrity of an AZ91 magnesium alloy was studied using a constant extension rate tensile (CERT) method. The samples in two different geometries that is, circumferentially notched (CN), and circumferentially notched and fatigue cracked (CNFC), were tested in air and in simulated body fluid (SBF). The test results show that the mechanical integrity of the AZ91 magnesium alloy decreased substantially (∼50%) in both the CN and CNFC samples exposed to SBF. Fracture surface analysis revealed secondary cracks suggesting stress corrosion cracking susceptibility of the alloy in SBF.

Characterisation of tribological and corrosion behaviour of plasma electrolytic oxidation coated AM50 magnesium alloy

Abstract A cast AM50 magnesium alloy was plasma electrolytic oxidation treated in a silicate based electrolyte using a direct current power source. Plasma electrolytic oxidation coatings produced at four different visible discharge conditions were characterised for their microstructural features, composition, tribological behaviour and corrosion resistance. The chemical composition, thickness and the roughness of the coating were found to be influenced by the processing voltage and thick coatings produced at relatively higher voltage levels provided a better wear and corrosion resistance.

Development of decorative and corrosion resistant plasma electrolytic oxidation coatings on AM50 magnesium alloy

Abstract Plasma electrolytic oxidation of AM50 magnesium alloy was performed in alkaline phosphate electrolyte with and without the addition of titania sol. The coatings produced in the phosphate electrolyte were constituted with MgO and Mg3(PO4)2. The coatings obtained in the phosphate electrolyte with the addition of titania sol were blue in colour and contained additionally the TiO2 and Mg2TiO4 phases. The phosphate PEO coating provided a two order of magnitude improvement in corrosion resistance to the AM50 magnesium alloy as was shown by the potentiodynamic polarisation measurements. With differences in the physical appearance, microstructural morphology and phase composition, the coatings produced in titania sol containing electrolytes offered an improved corrosion resistance compared to the mere phosphate PEO coating.

Enhanced corrosion protection of AZ31 magnesium alloy by duplex plasma electrolytic oxidation and polymer coatings

Abstract The corrosion behaviour of an AZ31 magnesium alloy coated with plasma electrolytic oxidation (PEO), polymer and a combination of them was assessed by electrochemical impedance spectroscopy (EIS) in 0·1M NaCl solution. The polymer coating was applied on a just cleaned surface (without any special preparation) and also on PEO treated magnesium substrate. While the polymer coated and the mere PEO treated magnesium alloy specimens lasted less than 50 h in the EIS tests, the specimens with the duplex coating (PEO+polymer) successfully resisted 1000 h in the EIS tests without showing any degradation. The improved adhesion of the polymer coating in presence of the PEO layer and the effective sealing of the pores in the PEO coating with the polymer were responsible for the enhanced corrosion resistance. The synergistic beneficial effect of the duplex coatings on the corrosion behaviour was also witnessed in the salt spray tests.

Microstructure evolution and tensile properties of friction-stir-welded AM50 magnesium alloy

Friction stir welding (FSW) technique was utilized to weld cast AM50 magnesium alloy plates. The microstructures in the base metal (BM) and the weld joint were observed by optical microscopy. The mechanical properties were investigated by using hardness measurement and tensile test, and the fractographs were observed by scanning electron microscopy. The results show that the microstructure of the base material was characterized by bulk primary a phase, a-matrix and intermetallic compound, β (or Mg17Al12), and the weld nugget exhibiting recrystallized microstructure consists α-matrix and β phase. The grain size in the weld is smaller than that in the base metal. The hardness of the weld joint is improved but the tensile strength and yield strength, as well as the elongation to failure of the base material decline. The fracture of BM has a rougher surface with more dimples, which is a characteristic of the ductile fracture, whereas the fracture on the nugget reveals a quasi-cleavage feature. The ultimate tensile strength and yield strength of the FSWedAM50 are 86.2% and 94.0% of those of the base metal, respectively.

Corrosion of Magnesium and its Alloys

Magnesium alloys generally have a fairly good corrosion resistance, but as the most electrochemically active engineering materials, they require corrosion protective measures if they are used in aggressive environments and/or in contact with other materials. Another large concern is stress corrosion cracking. The combination of mechanical stresses and corrosive attack can promote the initiation and growth of cracks which may result in abrupt failure without preceding plastic deformation. After a basic introduction to magnesium and magnesium processing, the major corrosion problems and their mechanisms are discussed in this chapter. This is followed by a summary of corrosion prevention strategies and examples of typical applications.