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Microcapillary polarization procedures were optimized to examine the corrosion characteristics of the various microstructural zones developed during friction stir spot welds made in AZ31 magnesium alloy. Tungsten carbide tracing confirmed the location and shape of the stir zone in the weldment. The stir zone was determined to be the noble region while the base metal and the thermo‐mechanically affected zone were found to be active. The thermo‐mechanically affected zone was found to be most susceptible to localized corrosion attack due to its proximity to the noble stir zone, and the subsequent formation of a macrogalvanic cell. Corrosion rates of individual weld regions were measured.

Localized corrosion at the weld periphery of AZ31B friction stir spot welds has been associated with the formation of a galvanic cell between the stir zone material and the adjacent base metal. However, this explanation did not take into account the effects of residual stress and grain growth in material adjacent to the stir zone region. In this study, the influence of microstructural changes on the electrochemical characteristics of FSSW joints is investigated using a combination of microcapillary polarization, stress relief heat‐treatment and micro‐hardness testing. It is confirmed that residual stress has no significant effect on the corrosion resistance properties of completed joints. Also, based on a detailed examination of the electrochemical characteristics of materials in the stir zone, TMAZ and HAZ regions, grain growth had negligible influence on the corrosion resistance properties.

The influence of welding flash on the corrosion resistance of friction stir spot welded (FSSW) AZ31B was examined by mass loss testing complimented with the scanning reference electrode and microcapillary polarization techniques. The microstructure of the flash was characterized by optical microscopy and the use of tungsten carbide tracer and correlated with the corrosion morphology of the joints. It was observed that the flash increased the corrosion rate of the welds, and its removal can reduce the corrosion rate by 20%. The increase in susceptibility for corrosion was explained by examining the electrochemical characteristics of the flash, and in particular to show the presence of a second stir zone (SZ) region in the flash. The electrochemical properties of the flash were correlated to second phase particle dissolution using a detailed microscopy analysis.The coupling of the two regions resulted in the formation of a local galvanic cell in the flash, leading to accelerated corrosion. Increases in dwell time and/or rotational speed of the tool during FSSW resulted in the formation of a larger SZ region in the flash, and produced a greater cathode to anode ratio. Materials and Corrosion 2017, 68, No. 4 Effect of welding flash on the corrosion of FSSW Figure 6. Influence of welding flash and exposure time on the location and morphology of corrosive attack. Important weld surfaces are labeled with letters that are consisted with the schematic in Fig. 1b. Abottom of pin hole surface, Cshoulder penetration surface, Fupper sheet surface, Iwelding flash www.matcorr.com

Corrosion of friction stir welds made in AZ31B magnesium alloy has been investigated using the microcapillary polarization technique. Various weld regions were examined in both spot and seam welds. The stir zone showed a higher reduction potential than the thermo-mechanically affected zone (TMAZ) and the base metal, due to the dissolution of β-Mg17Al12 particles. The absence of second phase particles resulted in elimination of harmful microgalvanic cells between the β-Mg17Al12 particles and the α-Mg matrix, and an increased concentration of aluminum in solid solution. The noble stir zone formed a macro galvanic cell with the base metal, which resulted in accelerated corrosion of the TMAZ. No significant variation in the electrochemical response of various weld zones was seen between seam and spot welds.

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Corrosion of dissimilar friction stir welds (FSW) made in AZ31/AZ80 magnesium alloys was investigated using the scanning reference electrode technique (SRET), and microcapillary polarization technique, complemented by optical and SEM/EDX microscopy. The corrosion rate of the base metals along with the welded specimen was estimated by mass loss testing. The stir zone material in both alloys showed a higher corrosion potential than the base metal due to the partial dissolution of β-Mg 17 Al 12 and Al-Mn particles. The basic corrosion mechanism in dissimilar welds was determined to be different from that of a similar joint. The corrosion behavior of the dissimilar FSW joint was governed by the galvanic coupling of the two alloys, and not by the microstructural evolution occurring during the welding process. The corrosion behavior of the joint was governed by the galvanic coupling between the α-Mg matrix in AZ31 and the Al-rich intermetallics in AZ80. The welded specimens exhibited the highest corrosion rate, while AZ80 was the most corrosion resistant material. Magnesium alloys can be successfully joined by friction stir welding (FSW) to achieve excellent mechanical integrity in completed welds. 1,2 One challenge arising from the utilization of this joining method is an increased susceptibility to localized corrosion in the material immediately adjacent to the welded joint. 3-6 Previous microcapillary polarization and scanning reference electrode technique (SRET) studies have revealed that the stir zones in AZ31B spot and seam welds are more cathodic than the bulk material. [3][4][5][6] Due to the difference in electrochemical potentials, a macrogalvanic cell is formed causing pitting corrosion in the thermo-mechanically affected zone (TMAZ) and the heat affected zone (HAZ) regions, which are located between the noble stir zone and the active base metal. 5 The differences in the electrochemical behavior of different regions in completed joints have been related to their microstructural evolution that occurred during the welding operation. The stir zone microstructure comprised a dynamically recrystallized matrix which was nominally devoid of intermetallics. The increase in the corrosion potential of the stir zone has been attributed to the dissolution of β-Mg 17 Al 12 and Al-Mn particles via two mechanisms: 4,5 1. Dissolution of intermetallics, which increases the concentration of Al in the α-Mg matrix, resulting in a higher corrosion potential of this region, 7 and 2. The absence of the intermetallics, which eliminates the microgalvanic coupling between the intermetallic particles (IMPs) and the α-Mg matrix, and reduces the susceptibility of the welded joint to localized pitting corrosion. [8][9][10][11] Many industrial applications require joining of dissimilar alloys, and therefore it is essential to understand the influence of the welding process on the corrosion behavior of dissimilar joints. It is currently unknown whether the corrosion behavior of dissimilar welds is governed by the microstructural changes o...

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