Exploring the Effects of Intermetallic Particle Size and Spacing on the Corrosion of Mg-Al Alloys Using Model Electrodes

Abstract

The effect of the area fraction, size and distribution of model cathodic, intermetallic particles (IMPs) in an anodic Mg matrix on corrosion was investigated. Model Mg-Al electrodes were developed to study IMP effects in isolation from other metallurgical effects, with particles simulated by Al electrodes embedded in a Mg matrix. Arrays of model Mg-Al electrodes were constructed using high purity Al as a surrogate for Al-rich IMPs and flush mounted in commercial purity Mg. The area fraction, size and spacing of these electrodes each altered the corrosion rate and cathodic reaction kinetics assessed after a 24 and 48 hour immersion period at the open circuit potential. Corrosion rate increased with increasing area fraction of Al electrodes but decreased with increasing electrode spacing given a fixed area fraction. The affected zone around electrodes and at the Al/Mg interface was explored to ascertain its impact on the resultant global corrosion rate and kinetics. The effect of local pH at the Al electrode on the prospects for Al corrosion and chemical redeposition were also explored. Magnesium (Mg) alloys continue to be of growing interest due to their good balance of specific properties (i.e. properties relative to weight).1-3 However, due to the inherently negative electrochemical potential of Mg and its alloys, 4,5 Mg-alloys are highly reactive compared to other engineering metals. Mg-alloys are susceptible to several forms of localized corrosion, whilst also highly prone to macro-as well as micro-galvanic corrosion. Due to the low solid solubility of most alloying elements in Mg 6 and particularly low solubility limits for most transition elements; secondary phases readily form during most types of material processing, including casting 7,8 and welding, 9-15 which can adversely alter the corrosion performance.There are many secondary phases, or intermetallic particles (IMPs), which are particularly common in Mg alloys. Each IMP 16 has its own unique dissolution or reduction kinetics, dependent on its composition, size and dispersion within the material. For example, in the Mg-Al alloys which contain Mn (such as AZ31, AZ91 and AM50), Al-Mn IMPs that are rich in Al such as Al 4 Mn, Al 6 Mn, display relatively low rates of cathodic kinetics in comparison to other IMPs that are rich in Mn such as Al 8 Mn 5 ; the latter displaying relatively rapid cathodic kinetics. 1,7,17 Similarly, the so called Al-Mn-Fe IMPs, such as Al 8 (Mn,Fe) 5 function as highly potent cathodic sites in Mg, although the Mn has been shown to prevent some of the detrimental galvanic effects of Fe impurities by incorporating the Fe into the Al-Mn IMPs. 1,6,16 Furthermore, Mg-Zn IMPs have approximately the same open circuit potential (OCP) as many Mg alloys. However, the cathodic kinetics of this IMP are more rapid than Mg, 16 attributed to the presence of Zn. Mg-Zn IMPs are cathodic to the α-Mg and, therefore, tend to cause localized corrosion through micro-galvanic coupling which is often manifest at the Mg matrix/IMP interface. Some ...

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

“…Because of these differences in the electrochemical 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 6,11,70,80 and augmented by the experimental data of this study (denoted by *). Information on the cathodic kinetics for these curves contained in Table 6.…”

“…These solidification boundaries and IMPs, which form during processing, [5][6][7][8][9] have unique electrochemical characteristics and can function as active cathodes during corrosion. 6,[10][11] The resultant microstructure of Mg-Al, die-cast alloys is well known. [12][13][14][15][16][17][18][19] The microstructure of these alloys, containing above 3 wt% Al, contain some amount of the β-phase (Mg 17 Al 12 ) as well as other Al-Mn and Al-Mn-Fe IMPs.…”

“…R 2 is the charge transfer resistance (the sum of R 1 and R 2 is often called R CT in other work). 11,40,51 Last, the inductor L 1 is taken to represent relaxation of the coverage of adsorbed intermediates in corroding areas of the samples surface but might also represent more complex effects such as darkening during corrosion. [52][53] Before electrochemical testing, the electrolyte was pre-saturated with H 2 as it has been shown that H 2 gas is extremely soluble in aqueous environments.…”

“…Al surface enrichment and incorporation in oxides could lower rates. [10][11]30,79 The time dependency and interplay between these various phenomena could change trends in short-term test versus long-term test results as the surface, electrolyte, and relative rates of reaction become altered with time. In a direct comparison of the die-cast alloys, using four parallel techniques, it is shown that an increase in the Al content increases the global corrosion resistance for die-cast alloys (Figure 16, Tables 5 and 6), confirmed or independently corroborated for the four methods used here.…”

Corrosion Performance of Friction Stir Linear Lap Welded AM60B Joints

“…[16][17][18] The multi-phase microstructure is particularly susceptible to micro-galvanic corrosion with the more noble Alrich phases serving as local cathodes. [19][20][21] Al-rich phases also can locally alter the surface film and, thus, regulate the local cathodic activity as Al enrichment occurs. [16][17][18]22 However, the ability of the sub-surface Al distribution and any associated Al enrichment layer to alter the surface film to avoid cathodic activation has not been investigated in a unified context.…”