Ionic Liquid Treatments for Enhanced Corrosion Resistance of Magnesium‐Based Substrates

Abstract

SONG INTRODUCTIONMagnesium (Mg) is the eighth most abundant element on earth, possessing several advantageous properties including a high strength to weight ratio and one of the lightest metals with a density that is only two-thirds that of aluminum and one-fourth that of iron at 1.74 g cm À3 . Additional properties that contribute to magnesium's versatility in the automotive, electronic, and aerospace industries include a high thermal conductivity, high dimensional stability, good electromagnetic shielding characteristics, high damping characteristics, good machinability, and easy recyclability [1]. These properties make the utilization of magnesium of particular interest to the automotive and aerospace industries where the weight reduction provides a simple means to achieve higher fuel efficiencies without sacrificing structural strength.Despite its many valuable properties, magnesium remains a very reactive element, prone to a number of undesirable properties, including poor corrosion and wear resistance, poor creep resistance, and high chemical reactivity, which have limited its wider industrial use. As such, automobiles currently possess few magnesium cast parts, averaging only a few pounds per car. Pure magnesium corrodes rapidly in humid atmospheric and/or aqueous environments where anions such as Cl À , Br À , I À , and SO À x promote local and generalized corrosion [2][3][4][5][6][7][8][9][10][11][12]. Compounds like alcohols, ethers, and phenols also attack magnesium [13]. Although alloying magnesium with elements such as manganese, aluminum, zinc, zirconium, and rare earths [14,15] can improve its properties, magnesium and its alloys continue to be extremely susceptible to galvanic corrosion, which can cause severe pitting in the metal, resulting in decreased mechanical stability and an unattractive appearance of the surface. Although uniform attack [16][17][18] is generally the most prevalent form of electrochemical corrosion, occurring with equal intensity over the entire surface of the metal, in the case of magnesium, especially pure magnesium, the partially protective surface film makes localized galvanic [19][20][21][22][23][24] and pitting corrosion [25-35] more common and worrisome, largely due to the difficulty in their detection and suppression. Galvanic corrosion occurs when two metals/alloys having differing compositions, and thus standard electrode potentials, are electrically coupled in the presence of an electrolyte. This coupling results in the formation of a galvanic cell, which preferentially corrodes the more reactive, or electronegative, of the two metals. Since magnesium has a highly negative standard electrode potential (E 0 ¼À2.37 V) there are few metal couples with which serious and rapid corrosion does not occur. Pitting corrosion, defined by the formation of small pits on the surface of a metal/alloy, usually stem from galvanic corrosion of a surface defect and progresses similar to crevice corrosion, where stagnant liquid in a crevice begins oxidizing the metal and/or its pass...

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