It is common for weld beads of certain alloys to turn dull and appear white upon electropolishing. This phenomenon is called Weld Whitening and it occurs because the microstructure of the weld bead is different than that of the base metal. ASME’s standard for Bioprocessing Equipment (BPE) defines Weld Whitening as “a difference in appearance of grain structure between weld metal and base metal after electropolishing” [1]. Table SF-2.2-2 in the BPE standard lists Weld Whitening as an acceptable surface finish anomaly so long as the roughness average (Ra) of the whitened surface still conforms to the surface finish designation specified for the electropolished component. An example of an electropolished, welded 316L stainless steel surface with Weld Whitening is shown in Figure 1 below.
A weld can be categorized into three distinct zones, the zones exist because they experience different heat input during the welding process. The area with insufficient heat for any microstructural change is the unaffected base metal. This area can be considered identical in terms of mechanical properties and chemical resistance to the rest of the alloy far from the weld. Second is the heat affected zone (HAZ). The HAZ experiences enough heat to change the microstructure of the material, but not cause any melting. This zone will have a larger grain size than the unaffected base metal immediately after welding and generally, lower strength and hardness. The HAZ is typically characterized by heat tint, a discoloration of the metal surface outside of the weld bead. This heat tint does not exist or is significantly reduced when sufficient inert gas purging occurs during the welding process. The third zone is the fusion zone. This zone experiences the most heat during the welding process and fully liquifies. The microstructure of the fusion zone looks more like the microstructure of a casting of the same grade of stainless compared to the base metal or HAZ microstructure, characterized by large solidification dendrites.
When a weld turns white during electropolishing, it is because the solidification microstructure of the fusion zone is being revealed through electrolytic etching. This occurs because the microstructure of the fusion zone is different enough to those in the HAZ and base metal that it responds differently to the applied electropolishing parameters. Figure 2 From ASTM E1558 is a diagram of voltage vs current density of an electropolishing system using a mineral acid electrolyte such as phosphoric and/or sulfuric acid [2]. Electrolytic etching occurs in the near-linear region between points A and B on the curve while electropolishing occurs in the flat portion of the curve between points B and C, where current density does not respond to changes in applied voltage.
An anodic film develops on the metal surface in the B-C region that acts as an electrical resistor. This film facilitates the controlled dissolution of microscopic peaks during electropolishing, so the metal surface flattens to the level of the microscopic crevices. Dissolution is less controlled in the A-B region since no anodic film exists, and surfaces processed in this region will be etched. For a certain electrolyte and voltage combination, the solidification microstructure of the fusion zone may not support a stable anodic film on the surface while the microstructures of both the HAZ and base metal do, so the fusion zone is processed in region A-B and etched while the rest of the surface is processed in the B-C region and electropolished.
The etched surface of the fusion zone is microscopically rough enough to scatter light and appear white or dull, but not significantly rougher than the surrounding electropolished surface. The average of three roughness measurements taken on the whitened weld and base metal of the coupon imaged in Figure 1 are recorded below in Table 1.
| Location | Average Ra [µin] |
|---|---|
| Whitened Weld | 2.84 |
| Unaffected Base Metal | 1.96 |
As shown in Table 1, an electrolytically etched, whitened weld, may be slightly rougher than the unaffected base metal, however the resultant roughness should not cause the component to lose its compliance to a surface designation if the surface was polished comfortably within the Ra requirement. The smoothest surface finish designation for an electropolished surface prescribed in the ASME BPE standard is SF4 which limits a maximum Ra of 15 µin. The weld coupon measured in Table 1 is far smoother than the 15 Ra requirement, and because it is a smooth, cleanable surface, free of heat tint or other physical defects, the whitened weld is not considered a concern for service in a high purity environment.
References
- ASME, 2024, “Bioprocessing Equipment”, ASME BPE-2024
- ASTM Standard E1558-09, 2021, “Standard Guide for Electrolytic Polishing of Metallographic Specimens” ASTM International, West Conshohocken, PA, 2015, DOI: 10.1520/E1558-09R21, www.astm.org