Corrosion Protection of Friction Stir Spot Welds Produced in an AZ31B Magnesium Alloy

Introduction

Magnesium alloys can be successfully joined by friction stir spot welding (FSSW) to achieve high mechanical integrity of the weld. Previous work done on AZ31 FSSW joints has shown that the welding process reduced their corrosion resistance, and resulted in a localized attack around the weldment^1-2. The welding process resulted in the dissolution of noble β-Mg₁₇Al₁₂ and Al-Mn intermetallics³, eliminating harmful microgalvanic couples between the particles and the α-Mg matrix in the stir zone. The localization of the corrosion attack resulted from the newly formed noble stir zone coupled with the active base metal.

If FSSW technology is to be used in an industrial application, the challenges associated with the corrosion behaviour of the weld will need to be addressed. Plasma electrolytic oxidation (PEO) coatings have been shown as a promising technology to improve the corrosion resistance of friction stir welds in AZ31⁴. It is important to understand the effect of process parameters on the microstructure of the coating in order to optimize their properties.

The present work explores possible post welding treatments that minimize the negative effect of welding on the corrosion resistance of magnesium alloys. The paper assesses the feasibility of these treatments through mass loss testing supplemented with scanning reference electrode technique (SRET), microcapillary polarization (MCP), and scanning electron microscopy.

Experimental Methods

All samples were spot welded using a flat shoulder tool from 1.6 mm thick AZ31B magnesium sheets.

PEO coatings were produced using a sodium silicate basic electrolyte with a current density of 10 mA/cm²and oxidation time between 15 and 30 minutes.

SRET experimental procedures are described elsewhere². The reference electrode chosen for this investigation was a 10 µm diameter platinum wire encapsulated in a glass tube. The measurements were performed in 0.086 M NaCl electrolyte solution.

Microcapillary polarization was performed in 0.1M NaClO₄ electrolyte, using a capillary with an inner diameter of 40 μm. Further details can be found elsewhere¹.

Mass loss testing was conducted by submerging samples in NaCl solutions of varying concentrations for a duration between 120 and 480 hours. The samples were imaged and analyzed to produce their respective corrosion rates.

Results and Discussions

PEO coating of FSSW joints yielded uniform coating across the weld and base metal for all coating parameters studied. Increases in processing time and polyethylene glycol (PEG) concentration in the bath electrolyte resulted in a smoother and thicker coating formed on the surface of the weld, as seen from the micrographs in Figure 1. The coating thickness increased from 6.6 µm for the coating processed for 15 minutes to 12.3 µm for the coating processed for 30 minutes. The roughness of the coatings was measured by white light profilometry. The roughness was reduced from 6.3 µm to 5.8 µm with increased processing time and PEG concentration in the electrolyte.

Figure 1shows that application of PEO coating to FSSW joints increased the corrosion resistance by at least a factor of two for all processing parameters investigated. The results indicate that the corrosion resistance of the joint is proportional to the coating thickness, and inversely proportional to its roughness.

Conclusions and Future Work

The results suggest that PEO coating can be successfully applied to FSSW joints made in AZ31 to improve their corrosion resistance. Process parameters were shown to affect the microstructure of the coating and its subsequent corrosion resistance. Further work will examine the breakdown mechanism of PEO coatings through a successive series of potential maps using SRET. Other post welding treatments to improve the corrosion resistance of the joint will be evaluated as well.

References

1. Y. Savguira, T.H. North, S.J. Thorpe, Mater. Corros. 65 (2014): p.1055.
2. Y. Savguira, W. H. Liu, D.J. Miklas, T.H. North, S.J. Thorpe, Corrosion 70 (2014): p.858.
3. A. D. Sudholz, N.T. Kirkland, R.G. Buchheit, N. Birbilis, Electrochem. Solid-State Lett.14, 2 (2011): p. C5.
4. T. Chen, W. Xue, Y. Li, X. Liu, J. Du, Mater. Chem. Phys. 144, 3 (2014): p. 462.