Assessing the Corrosion of Multi-Phase Mg-Al Alloys with High Al Content by Electrochemical Impedance, Mass Loss, Hydrogen Collection, and Inductively Coupled Plasma Optical Emission Spectrometry Solution Analysis

Abstract

The corrosion behavior for several die-cast Mg-Al alloys (AM50, AM50, and AZ91) was compared to commercial purity Mg and AZ31B-H24 utilizing simultaneous measurement of electrochemical impedance spectroscopy (EIS), hydrogen gas collection over a 24 h immersion period, gravimetric mass loss, and inductively coupled plasma optical emission spectrometry (ICP-OES) solution analysis of the total Mg concentration released. Tests were conducted in three electrolytes, unbuffered 0.6 M NaCl, 0.1 M tris(hydroxymethyl)aminomethane (TRIS), and 0.6 M NaCl buffered with TRIS to a pH of 7. EIS derived polarization resistance was monitored periodically, as determined from EIS circuit modeling using data collected to 0.001 Hz, and considering the pseudo-inductive low-frequency impedance time constant. EIS derived corrosion rates and oxidation charge density were similar to charge density determined from cumulative mass loss, ICP-OES solution analysis, and the volume of hydrogen collected for the die-cast AM50, AM60, and AZ91, as well as for Mg and AZ31B determined previously. The variation in the cathodic hydrogen evolution reaction kinetics for the die-cast alloys were also determined over 0, 3, 12, and 24 h immersion periods and compared to commercial purity Mg and AZ31B-H24. The global corrosion rate decreased with increasing Al content, even though Al wt% above the solubility limit (2 wt% at room temperature) resulted in increasing volume fractions of the Al 8 Mn 5 (Fe), Al 2 Mn 3 , and Al 3 Fe intermetallic particles. Each of the alloys contained varying volume fractions of primary α, β-phase (Mg 17 Al 12), and eutectic α+β depending on Al content and processing. Al in the solid solution α-Mg phase decreased the overall net anodic reaction rate for the Mg 2+ half-cell reaction. The Mg 17 Al 12 phase was reasoned to not function as a strong cathode as deduced from cathodic E-log(i) studies. Moreover, the extent of anodically-induced cathodic activation was speculated to decrease with increasing Al content, which was a factor in determining overall corrosion rate and accumulated damage. However, corrosion damage depth as determined from a pitting factor analysis increased with Al content.

Initial Stages of AZ31B Magnesium Alloy Degradation in Ringer’s Solution: Interpretation of EIS, Mass Loss, Hydrogen Evolution Data and Scanning Electron Microscopy Observations

“…Good agreement between corrosion rates of bulk magnesium and several alloys calculated from charge transfer resistance values, R t , obtained from impedance diagrams and atomic absorption or gravimetric measurements have been observed by Makar and Kruger [45] and Pebere et al [50]. Recently, King et al [34] and Bland et al [46,47] have obtained excellent correlation between the values of the corrosion rate determined by weight loss, hydrogen gas collection and EIS measurements, extrapolating the inductive loop of the impedance diagrams to zero frequency, defined as the polarization resistance (R p ), rather than R t . Most of the studies are conducted at the room temperature, using rather simple electrolytes, for example, NaCl, Na 2 B 4 O 7 and Na 2 CO 3 , Na 2 SO 4 , ammonium/carbonate solutions.…”

“…The nondestructive character of the impedance technique allows derivation of in situ changes in Mg-based implant material degradation, which in clinical applications is a rather long-term dynamic process [3,4,38]. Some research effort has been focused on the relationship between EIS parameters and independently-measured corrosion rate data, such as average of corrosion mass loss [34,[45][46][47][48][49], solution analysis by atomic absorption spectroscopy [50], or hydrogen gas volume evolution [34,36,[46][47][48]51]. Good agreement between corrosion rates of bulk magnesium and several alloys calculated from charge transfer resistance values, R t , obtained from impedance diagrams and atomic absorption or gravimetric measurements have been observed by Makar and Kruger [45] and Pebere et al [50].…”

The role of Al 2 Ca and Al 2 (Sm,Ca,La) particles in the microstructures and electrochemical discharge performance of as‐extruded Mg‐3wt.%Al‐1wt.%Zn‐based alloys for primary Mg‐air batteries

“…Figure 6D shows the equivalent circuits for the investigated alloys. The circuit for AZ31 and AZ31‐Ca alloys contains the solution resistance ( R s ), charge transfer resistance ( R t ), constant phase element for the double layer capacitance ( CPE dl ), and the inductor for the desorption of intermediates ( L ) . Table lists the fitted results of each parameter in the circuit.…”

“…The circuit for AZ31 and AZ31-Ca alloys contains the solution resistance (R s ), charge transfer resistance (R t ), constant phase element for the double layer capacitance (CPE dl ), and the inductor for the desorption of intermediates (L). 51 Table 3 lists the fitted results of each parameter in the circuit. The AZ31 alloy with Al 2 Ca particles has much larger R t value than the original, indicating a better corrosion resistance.…”

Corrosion Performance of Friction Stir Linear Lap Welded AM60B Joints

“…Well-developed corrosion of multi-phase Mg-Al (AMxx and AZxx) alloys in aqueous chloride-containing solutions is strongly affected by the heterogeneous nature of the alloy microstructure; including the size, shape and distribution of secondary intermetallic phases. [8][9][10][11][12][13][14][15] Arguably, the Al distribution within the microstructure is the most important factor in controlling corrosion as regions deficient in Al corrode at significantly higher rates. [16][17][18] The multi-phase microstructure is particularly susceptible to micro-galvanic corrosion with the more noble Alrich phases serving as local cathodes.…”