Microcapillary Polarization of Friction Stir Welds in AZ31B Magnesium Alloy

Introduction

Magnesium alloys can be successfully joined by friction stir welding (FSW) to achieve high mechanical integrity of the weld. Preliminary work done on the corrosion performance of friction stir spot welds (FSSW) in AZ31B by James et al.¹ identified that the weld region was more cathodic than the bulk material, resulting in the accelerated corrosion of the region adjacent to the weld. Most of the research on the corrosion of FSW joints focuses on spot welds; little information is available on the corrosion behaviour of FSW seam welds.

The present work investigates the individual corrosion potential/current of various weld zones in FSW, and compares it to information obtained from FSSW.

Experimental Methods

All samples were welded using a flat shoulder tool with thread matching that of a standard M4 metric thread. The welds were made in two 1.6 mm AZ31B overlapping sheets.

FSSW samples were produced using a tool rotational speed of 3000 rpm and 4 seconds dwell time. To produce the FSW samples, a tool rotational speed of 1400 rpm and transverse speed of 180 mm/min were used.

Microcapillary polarization was performed by methods modified from literature². A microcapillary with an inner diameter of 40 μm was filled with the 0.1M NaClO₄ electrolyte, and both the platinum mesh electrode along with the 3M KCl Ag/AgCl reference electrode were placed inside the capillary cell. The sample was polarized from a potential that is rounded to the nearest 50 mV below the recorded open cell potential to a potential that is 100 mV nobler than E_corr. The corroded area was examined after the experiment through SEM imaging and Image J software.

Results and Discussions

Three distinct microstructural regions of the weld were analyzed in this investigation: stir zone (SZ), thermo-mechanically affected zone (TMAZ), and base metal (BM). The average microcapillary polarization results are summarized in figure 1. Each zone was tested at least three times, and the corrosion potential and current were found through Tafel extrapolation of individual graphs, the results are given in figure 1b. Measurements from bulk polarization of AZ31B in 0.1M NaClO₄ are also included. The results obtained from bulk and microcapillary polarizations of the BM are statistically the same. The results show a potential gradient between the noble SZ and the active BM, confirming previous findings by James et al.¹ who examined the corrosion potential distribution on the cross sectional surface of a FSSW joint using the scanning reference electrode technique (SRET).Additionally, the reported SRET results showed that the TMAZ was noble to the BM, but failed to explain why a noble region (TMAZ) corroded faster than the adjacent active region (BM).

In this investigation, little difference in the corrosion potential (E_Corr) and current (I_Corr) is observed between the TMAZ and the BM regions. The results suggest the formation of the macrogalvanic cell between the noble SZ and the active TMAZ is the primary cause in the accelerated corrosion attack at the interface between the two regions, hence explaining optical observations of localized pitting in the TMAZ region.

Conclusions and Future Work

This work investigated the corrosion potential and current of individual microstructural regions of a FSSW joint. The accelerated corrosion attack of the TMAZ was explained by the formation of a macrogalvanic cell between the active TMAZ and the noble SZ. Further work examines the corrosion performance of the same microstructural regions on a FSW joint, and compares it to the information obtained in this investigation.

 References

1. A. James, T. H. North, S. J. Thorpe, ECS Trans. 41(25) 193 (2012)
2. T. Suter, H. Bohni, Electrochim. Acta, 47, 191 (2001)