Characterization of Filiform Corrosion of Mg–3Zn Mg Alloy

Abstract

The filiform corrosion of Mg-3Zn Mg alloy in various corrosion environments was investigated by scanning electron microscopy (SEM) observations, scanning vibrating electrode technique (SVET) and electrochemical measurements. The results indicate that corrosion filaments can cross grain boundaries and fine precipitation phases to develop continually. The propagation of filiform corrosion is greatly associated with the existence of Cl − in the solutions. A small amount addition of F − or SO 4 2− to the NaCl solution can delay the initiation of filiform corrosion, while abundant F − or SO 4 2− addition can change the corrosion forms. The micro-anodes and micro-cathodes transfer dynamically with the growth of corrosion filaments. Mg-3Zn alloy exhibits a filiform corrosion mode at the applied anodic potentials but keeps intact at the applied cathodic potentials. © The Author Mg and its alloys show high strength/mass ratio and low density, which attract great interest in the aeronautical and automotive fields.1,2 However, their poor corrosion resistance limits the widespread applications. Mg alloys are susceptible to localized corrosion. Among the different localized corrosion forms, filiform corrosion as one of common localized corrosion can damage the surface oxide films of Mg alloys, leading to an exposed surface in active state. 4 If the filiform corrosion of Mg alloys is restrained, their life time will be prolonged dramatically. Thus, it is very necessary to clarify the filiform corrosion mechanism of Mg alloys.However, the classical filiform corrosion mechanism of traditional metallic materials cannot well explain the filiform corrosion of Mg alloys. The classical filiform corrosion mechanism believes that corrosion filaments consist of acidic filament heads and alkaline filament tails, and the different oxygen concentrations at the filament heads and tails are the driving force for the growth of corrosion filaments. That is to say, the classical filiform mechanism defines the oxygen reduction reaction at the cathodic regions as the most significant factor in the growth of corrosion filaments.5 However, the classical filiform corrosion mechanism is not available for Mg alloys because hydrogen evolution reaction instead of oxygen reduction reaction occurs at the cathodic regions as proved by Baril et al. 6 and Ambat et al. 7 Also, a great number of experimental results indicate that the propagation of corrosion filaments on the surface of Mg alloys is insensitive to the existence of oxygen. In view of the ambiguous knowledge about the filiform corrosion mechanism of Mg alloys, a lot of investigations have been carried out to clarity this case, and some regular results have already been obtained. All these studies mainly focus on the effects of microstructures 9-11 and corrosive environments 11-14 on the filiform corrosion of Mg alloys. Lindstrom 12 et al. disclosed the effects of CO 2 and the roles of the noble inclusions in the filiform corrosion of pure Mg in the relative humidity air. Schmutz 15 confirmed tha...

Microstructure and Corrosion of Cast Magnesium Alloy ZK60 in NaCl Solution

“…The second phase in the grain boundaries and the Zr-rich region within the grains have an essential influence on the origination and propagation of the corrosion filaments on the cast ZK60. These filiform-like corrosion characteristics of the cast ZK60 were quite different from those of Mg-3Zn and Mg-8Li alloys reported previously [ 65 , 71 ], in which the corrosion filaments seem not to be affected by the second phases and can originate from the center of the grains and spread to various directions. These differences should be ascribed to their different microstructures, especially the quantity and size of the second phase and the distribution of alloying elements in these alloys.…”

“…Only Mg, O, and Cl elements were found in the corrosion product layer, which should result from Mg(OH) 2 and Cl − in the test solution. Moreover, a large number of Cl − through the whole corrosion product layer suggests that Cl − could penetrate the product layer easily and may become enriched in the bottom of the corrosion pit to propagate the pit corrosion [ 71 ]. The second-phase particles in the corrosion pit imply that they may be related to the formation of corrosion pits, which is discussed later.…”

Characterization and Corrosion Properties of Fluoride Conversion Coating Prepared on AZ31 Magnesium Alloy

“…The morphology of the corroded area took the form of filiform-like corrosion. This type of corrosion attack is characteristic of the proliferation of the corrosion under the coating or under the created layer of corrosion products in the case of untreated AZ31 magnesium alloy [47]. The corrosion products covered the whole surface after 168 h of sample immersion in SBF solution, Figure 9.…”

Effect of Area Ratio on the Galvanic Corrosion of AZX611 Magnesium Alloy/A6N01 Aluminum Alloy Joint

“…During the test, the sample surface was observed to determine the hydrogen gas generated on the entire surface of the cathode, whereas filiform-type corrosion was observed at the anode with hydrogen gas generated at the tip of the filament. Wang et al 15) evaluated the filiform corrosion of a MgZn alloy in a 2.5-mM NaCl solution by SVET and reported that the tip of the filament is the anode. After 24 h, a region exhibiting white or its opposite color was generated on both anode and cathode, suggesting that the polarity involved in galvanic corrosion has changed in this section.…”