Electrochemical Estimation of the Corrosion Rate of Magnesium/Aluminium Alloys

Abstract

The corrosion rate of AZ31, AZ80, and AZ91D magnesium/aluminium alloys immersed in 3.5 wt.% NaCl was determined comparing gravimetric and electrochemical measurements. The findings revealed that, for all investigated materials, a fraction of the metallic surface exposed to the corrosive medium did not reveal a normal electrochemical response to the applied signal. This may be associated with phenomena such as partial disintegration of specimens into fine metallic particles, electrochemical formation of ions, and/or anomalous chemical attack occurring simultaneously with the normal electrochemical corrosion attack. The abnormal electrochemical behaviour was more evident for lower amounts of aluminium in the bulk composition of the investigated materials. Thus, the electrochemical estimates of pure Mg and the AZ31 alloy were not reliable and tended to underestimate corrosion losses.

Revisiting the Electrochemical Impedance Spectroscopy of Magnesium with Online Inductively Coupled Plasma Atomic Emission Spectroscopy

“…positive Z' and positive Z''), is what is often called the inductive loop and is a characteristic of Mg that is observed in essentially all reported Nyquist plots for Mg and its alloys in NaCl electrolytes. [1][2][3][4][5][6][7] The Nyquist plot in Figure 3 includes the measured data, along with a fit employing the equivalent circuit proposed and validated by King and co-workers, [1] which includes a nested Randle's element and inductor. This fit has been included on the basis of validating the circuit proposed by King and coworkers, and the requisite inductor; however, the fit is inconsequential in the presence of AESEC data, the latter providing a definitive i Mg 2þ to compare the electrochemical response and actual dissolution.…”

“…[1][2][3][4][5][6] The approach commonly taken is to determine the charge transfer resistance (R t ), defined as the value of impedance (Z') when Z'' = 0 at intermediate frequencies. [6][7][8] The frequency at which Z'' = 0 is variable, and does not coincide with the low frequency impedance limit approaching DC conditions, which defines the polarisation resistance, R P (i.e. Z' as frequency!0).…”

“…[8] Criticism have been made of the electrochemical methods applied to Mg, relating to their ability to predict mass loss; [4] such criticism is based on the notion that chemical reactions cannot be monitored by electrochemical methods. [7] This has, however, been addressed by online inductively coupled plasma (ICP) methods, as first demonstrated by Ogle and coworkers [16][17] and since further developed [including inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS)]. [12,13,[18][19][20][21] Hence, given the urgent need for the unambiguous determination of corrosion rates for Mg, and the state of the current literature regarding Mg EIS, a definitive experiment coupling EIS with real time online atomic emission spectroelectrochemistry (AESEC) analysis is timely.…”

Assessing the Corrosion of Commercially Pure Magnesium and Commercial AZ31B by Electrochemical Impedance, Mass-Loss, Hydrogen Collection, and Inductively Coupled Plasma Optical Emission Spectrometry Solution Analysis

“…In addition, the Tafel extrapolation should be extracted from E-log(i) data at least 50 mV away from E corr in order to ensure that the Tafel slope is being analyzed over a linear Elog(i) region. [40][41] Quoted β a values were typically ≤150 mV/dec, while the β c values were ≥200 mV for commercially pure Mg. 27,30,[42][43][44][45] In the literature for AZ31, the β a and β c values are quoted ranging from 20 mV/ dec to 300 mV/dec. [45][46][47] For many of these measured Tafel slopes, the testing conditions are extremely varied and,…”

“…In much of the prior research utilizing the EIS-determined corrosion rate, the analysis relied on using R t , at intermediate frequencies in place of R p and subsequently omitting the significant inductive loop in the Mg Nyquist plot for many solutions and many Mg alloys, which results in a substantial underestimation of the corrosion rate. 43,45 Furthermore, on the basis of EIS determined R t (where R P is much less than R t ), underestimating the corrosion rate as determined from other methods (mass loss or hydrogen collection), authors have previously used this as a means for rationalizing the existence of Mg + . [43][44] It has been argued that use of Mg + in the calculation of corrosion rate by H 2 generation is justified because H 2 generated from the assumed Mg + reaction on the metal surface, as well as the proposed Mg + /Mg 2+ reaction in solution, are both collected.…”