SVET Study of Galvanic Corrosion of Al/Mg₂Si Couple in Aqueous Solutions at Different pH

Abstract

The galvanic corrosion behavior between pure Al and synthesized bulk Mg 2 Si in 0.01 M NaCl solution at different pH values has been investigated by the scanning vibrating electrode technique (SVET). At pH 2, bulk Mg 2 Si actively dissolves and it acts as an anode. With the increase of exposure time, the anodic activity of Mg 2 Si decreases drastically and eventually anodic activity appears on Al surface. In alkaline solution, Mg 2 Si phase always acts as the cathode. The preferential dissolution of Mg and enrichment of Si found by energy dispersive X-ray spectroscopy are responsible for the observed anodic activity decay of Mg 2 Si in acidic solution. At pH 6, the galvanic corrosion current of Al/Mg 2 Si couple is much less than those at pH 2 and pH 13, and Mg 2 Si mainly undergoes self-dissolution. These results indicate that the dealloying of binary Mg 2 Si phase depends on the solution pH value and corrosion time, which subsequently have a great influence on the galvanic corrosion current of Al/Mg 2 Si couple. Al-Mg-Si alloys are widely used in automotive, aerospace and rail transportation industries because of their high stiffness, good formability, good weldability and high corrosion resistance.1-5 Due to the low solubility of the alloying additions, second phases (such as Mg 2 Si precipitates) are commonly formed in Al-Mg-Si alloys.6 These particles have different electrochemical activities from the alloy matrix, thus galvanic corrosion can occur between the particles and surrounding matrix, 7-10 which gives rise to the degradation of Al alloys in industrial applications.Potentiodynamic polarization tests carried out by Buchheit et al. 7,8 have shown that the corrosion potential of Mg 2 Si is much lower than the alloy matrix. The Volta potential of Mg 2 Si particle measured by scanning kelvin probe force microscopy (SKPFM) is about 100∼180 mV lower than the aluminum matrix. 10 These reports indicate that Mg 2 Si particle is initially anodic relative to the aluminum matrix.Selective dissolution of Mg has been found by many research groups during the corrosion of Mg 2 Si phases. 4,9,[11][12][13] For example, Jain et al. 11 found that Mg 2 Si particle can selectively dissolve in neutral solution. Wolka et al.12 measured the open circuit potential (OCP) of Mg 2 Si using a microcapillary electrochemical cell and stated that the increase of OCP after 1 h immersion in 0.01 M NaCl solution is due to the preferential dissolution of Mg. Mg 2 Si particles in AlMg-Si alloys have also been investigated by SKPFM in combination with atomic force microscopy (AFM) observation, 13 electrochemical micro-cell measurements combined with electron probe micro analysis (EPMA) and auger electron spectroscopy (AES).4 These techniques reveal that high reactivity of Mg leads to the dealloying of Mg 2 Si particles accompanied with the enrichment of Si, which finally makes the Mg 2 Si remnant exhibit cathodic activity. Although SKPFM can measure Volta potential, it cannot provide kinetics parameters relating to the galvanic corrosion ...

Volta Potential Evolution of Intermetallics in Aluminum Alloy Microstructure Under Thin Aqueous Adlayers: A combined DFT and Experimental Study

“…In dry atmosphere, the surface of metals is usually covered by a dense and protective (passive) metal-oxide layer which impedes corrosion reactions [67][68][69]. It has been reported that the surface of Mg 2 Si can form a passive oxide layer [70][71][72], which grows with time and results in an enhanced electrochemical nobility in both ambient air and corrosive environments [71]. The investigated sample was exposed to laboratory air for 2 months, allowing the formation of oxide/hydroxide on the surface (aging).…”

SVET and SIET Study of Galvanic Corrosion of Al/MgZn2in Aqueous Solutions at Different pH

“…These results are concordant with reports in the literature. 6,7,13,33 SVET maps reveal that severity galvanic coupling effect is well pronounced in very acidic or alkaline environment. MgZn 2 is anodic relative to Al at pH 2 but cathodic relative to Al at pH 12, while self-dissolution prevails under near neutral conditions with Al being almost in a passive state (Figs.…”

“…The SVET results provided a good spatial resolution which clearly demonstrates that it is possible to get complementary data supporting the physical interpretations of the galvanic corrosion processes based on the application of traditional electrochemical techniques. 13,15,19,20 …”

“…SVET and SIET have been used for several corrosion problems, such as the corrosion of aluminum, the galvanic activity at cut edges 17,18 zinc-steel corrosion, 19 intermetallic phases in alloys 13,20 and localized corrosion at inclusions.…”

“…13,15,20,22,23 A recent SVET study by Li et al 13 on the galvanic corrosion of Al/Mg 2 Si couple in aqueous solutions at different pH values revealed that the dealloying of binary Mg 2 Si occurs via preferential dissolution of Mg and enrichment of Si. This depends on the solution pH value and immersion time, which subsequently have a great influence on the galvanic corrosion behavior.…”