Effect of Grain Size on the Corrosion Resistance of Friction Stir Welded Mg Alloy AZ31B Joints

Abstract

The objective of this study was to clarify the effect of grain size on the corrosion rate across friction stir welded AZ31B joints when exposed in aqueous NaCl solutions. Grain size variations were achieved by preparing welded joints using AZ31B in different tempers. Single potentiodynamic polarization measurements made on the isolated base metal and stir zone in 0.6 M NaCl solution revealed no significant difference between the anodic kinetics, but revealed an apparent difference between the cathodic kinetics. The attempt to correlate extracted corrosion current density (i corr ) values to grain size (d −0.5 ) revealed that the slope coefficient was insignificantly different from zero, meaning that no meaningful correlation exists (albeit during the corrosion propagation stage). Scanning vibrating electrode technique (SVET) measurements across an intact AZ31B-O welded joint revealed that grain size had a more complex effect on the anodic and cathodic kinetics of the filiform-like corrosion exhibited in 0.86 M NaCl solution. Grain size had an effect on filament initiation as it consistently occurred on the surface of the base alloy (coarser-grained structure). In contrast, grain size had no effect on filament propagation as the anodic and cathodic kinetics were unaffected when moving from the base metal (coarser-grained surface) across the stir zone (finer-grained surface).

Effect of grain size and residual stress on the corrosion resistance of friction stir spot welded AZ31B joints

“…The base metal comprised a mix of fine and coarse grained α‐Mg (2.3–4.5 μm) and evidence of deformation twinning. Similar observations have been reported by other investigators . The stir zone was comprised of fine, dynamically recrystallized grains having an average grain size of 5.0 ± 0.6 μm in close agreement with previous observations by Chowdhury et al (their average grain size was 5.4 ± 2.2 μm in friction stir processed AZ31 ).…”

“…Many attempts have been made to relate changes in corrosion resistance properties in completed joints with variations in the grain size in different weld regions . It has been suggested, for example, that the stir zone region exhibits improved corrosion resistance properties, because of the fine‐grained microstructure resulting from dynamic recrystallization .…”

“…The improved corrosion resistance of fine‐grained microstructure was related to its ability to react more rapidly with atmospheric CO 2 forming a protective surface film, see Cristóbal et al . In direct contrast, it has been suggested that the grain size variation across the weld regions have no influence on the corrosion behavior of completed joints .…”

“…A major limitation in previous investigations examining the electrochemical behavior of different weld regions has been the inability to adequately isolate one microstructural region from another. Prior investigations examining the corrosion resistance of different weld regions have involved selective masking using electroplater's adhesive tape, and physical extraction of the areas of interest . In this connection, many publications fail to mention their electrode preparation methodology .…”

Localized Corrosion Behavior of AZ31B Magnesium Alloy with an Electrodeposited Poly(3,4-Ethylenedioxythiophene) Coating

“…It has been reported that the Mg passivity breakdown can be identified by local maximum in the E corr transients. 34,43 The E corr Figure 6. Corresponding images of the scan area are also provided alongside the SVET plots.…”