Electrochemical Characterization of Intermetallic Phases Common to Aluminum Alloys as a Function of Solution Temperature

Abstract

A practically important scenario in the corrosion of aluminum (Al) alloys is the variation in damage characteristics, i.e. mode, rate and morphology, as function of environmental temperature. In order to address this problem from a mechanistic point of view, this paper presents results for corrosion potentials, pitting potentials and electrochemical characteristics for intermetallic particles commonly present in high strength aluminum alloys, for tests conducted in 0.1M NaCl of varying temperature. Tests were conducted between 0 and 60 • C via the use of a micro-capillary electrochemical cell. The intermetallics investigated were Mg 2 Si, MgZn 2 , Al 7 Cu 2 Fe, Al 2 Cu, Al 2 CuMg and Al 3 Fe. Analysis of the results reveals that the electrochemical behavior of such compounds is heavily dependent on temperature, with ramifications for localized corrosion (and protection) In the most general sense, corrosion represents the adverse intersection between a material and the environment; while in the case of aluminum (Al) alloys, their prescription is widespread and covers many environmental exposure conditions. For a given material, the environment is responsible for the type and extent of corrosion that may result; warranting an investigation of the electrochemical characteristics as function of environment. [1][2][3][4][5][6] In an attempt to unambiguously characterize the mechanistic aspects of localized corrosion of heterogeneously structured high-strength Al-alloys, we have presented the electrochemical characteristics for a number of intermetallic particles and phases common to a large range of such alloys, [7][8][9][10] paying particular attention to the effect of chloride concentration 9 and the effect of solution pH.7 This approach has also recently been followed for investigation of the second phases that occur in magnesium alloys. 11It has been shown that both [Cl − ] and pH have a significant effect upon the corrosion morphology and rate of bulk Al-alloys, as demonstrated for AA7075-T651. 12 Of key relevance to the work herein, it was shown that the resultant alloy corrosion was able to be reconciled in terms of both morphology and rate, from knowledge of the individual electrochemical characteristics of the alloy constituents.13 Furthermore, it is of importance to note that the associated electrochemical kinetics upon alloy constituents is not trivial, in that kinetics may not be accurately predicted based upon composition or knowledge of corrosion potentials alone. However, polarization testing was able to reveal significant quantitative insights allowing a leap in understanding the origins of localized corrosion (and in some cases the absence of localized corrosion). Examples include; spontaneous passivity of Mg 2 Si at alkaline pH and the increase in the ability of Fe-containing intermetallics to sustain ORR as either pH or [Cl − ] increase, to name but a couple of important factors. 7,9 Similarly, such richness is not predicted by methods such as SKPFM, which has been previously used to predict loca...

The Effect of CeCl3as an Inhibitor on the Localized Corrosion of AA2024-T3 as a Function of Temperature

“…At higher temperatures when the OCP of S-phase climbs, production of hydrogen peroxide on S-phase could lead to the formation of Ce 4+ . Although the results of this work do not provide clear evidence on the role of Ce 4+ , clearly increases of pH will enhance the precipitation of both Ce(OH) 4 and Ce(OH) 3 . All this suggests that although the precipitation of Ce(OH) 4 could play a role in inhibition of the ORR on Al 2 Cu at all temperatures (and possibility on higher temperatures on S-phase), inhibition by Ce 3+ on the rest of the alloy will most likely arise from the precipitation of Ce(OH) 3 , which more strongly depends on local pH increase arising from the ORR.…”

“…Furthermore, the corrosion rates of secondary phase particles increase with increasing temperature. 3,32 The solubility product constants of REM hydroxides also decrease with increasing temperature. Therefore, for a given REM concentration, the minimum pH for precipitation is expected to decrease as temperature increases.…”

“…[3][4][5][6][7][8][9][10][11][12][13][14][15][16] Rare earth metal (REM) inhibitors have been studied extensively to characterize their ability to inhibit localized corrosion of aluminum alloys. [17][18][19] With the addition of 1000 ppm of CeCl 3 in NaCl solution at ambient temperatures, the corrosion rate of AA7075 was reduced by an order of magnitude. 20,21 Furthermore, the cathodic kinetics on AA7075 were suppressed by an order of magnitude after 10 minutes of immersion in NaCl plus 0.05 wt.% CeCl 3 , also at ambient temperatures.…”

“…[17][18][19] With the addition of 1000 ppm of CeCl 3 in NaCl solution at ambient temperatures, the corrosion rate of AA7075 was reduced by an order of magnitude. 20,21 Furthermore, the cathodic kinetics on AA7075 were suppressed by an order of magnitude after 10 minutes of immersion in NaCl plus 0.05 wt.% CeCl 3 , also at ambient temperatures. Other rare earth metal inhibitors such as YCl 3 , LaCl 3 , and PrCl 3 provided similar inhibition of anodic and cathodic kinetics.…”

A Summary of Corrosion Properties of Al-Rich Solid Solution and Secondary Phase Particles in Al Alloys

“…For particles containing both active and noble elements, such as Al 2 CuMg, the active alloying elements dissolve leaving behind remnants with noble elements such as Cu. This is commonly referred to as dealloying corrosion as shown in Figure 1 [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. Particles consisting primarily of noble elements generally possess a high corrosion potential, and act as a cathode to support the oxygen reduction reaction or hydrogen evolution.…”