Welded joints are common. Well made, they offer a smooth transition from one section to another, are high strength and are cosmetically attractive. However, the welding process itself can lead to significant loss of corrosion resistance in the joint area and a reduction in mechanical properties unless precautions are taken to prevent oxidation.
Welds carried out on most metals without adequate inert gas coverage will oxidise. The effect is even noticeable with many stainless steels. To some, the discolouration due to oxidation is an inconvenient feature that can be removed after welding, but this may be difficult and costly, especially if access is restricted. Unfortunately, any oxidation can result directly in a reduction in corrosion resistance and in some cases loss of mechanical strength. This is significant in dairy and food pipe applications where stainless steels are employed principally for their corrosion resistance and mechanical properties.
It will come as a surprise to many that oxygen contents as low as 50 ppm (0.005%) in the protective gas can produce discolouration or ‘heat tint’. See Figures 1 and 2.
The mechanism of corrosion
Stainless steels owe their resistance to corrosion to the formation of a very thin (10-5 mm), transparent surface layer of chromium oxide. This provides a passive film that acts as a barrier to penetration by an invasive environment. When heated to a high temperature in the presence of oxygen this film increases in thickness until it becomes visible – the colour becomes darker with increasing film thickness.
At a critical film thickness the film becomes unstable and begins to break down. The fractured zones created offer sites for localised corrosion. Protection is thus essential and this is achieved by surrounding the joint with an inert gas such as argon or helium. The gas shield associated with a GTAW torch will protect the upper surface of the joint but the inside of pipes and tubes needs special attention. To meet the need for total internal protection, called weld purging, dedicated equipment has evolved over the past 25 years.
Residual oxygen measurement instruments
Systems for weld root protection are based on sealing the inside of a pipe on either side of the weld zone then displacing air with an inert gas. The seals must be reliable and leak tight, effective and easy to insert and remove.
Any effective weld purge process needs to be supported by suitable oxygen detecting equipment. Weld purge monitors have now been developed to meet the need for reliable, robust and sensitive measurements.
For reactive and refractory alloy welding these must be capable of measuring oxygen levels down to 10 ppm. As an example, the PurgEye® 600 instrument (Figure 3) reads down to 10 ppm with extreme accuracy and has a display range from 1,000 to 10 ppm.
- Microbiologically influenced corrosion of stainless steel, 2nd symposium on orbital welding in high purity industries, La Baule, France
- Effects of purge gas purity and Chelant passivation on the corrosion resistance of orbitally welded 316L stainless steel tubing, Pharmaceutical Engineering. Vol 17 Nos 1 & 2 1997
- Considerations for Orbital Welding of Corrosion Resistant Materials to the ASME Bioprocessing Equipment Standard, Stainless Steel America conference 2008
- Heat Tint Poses Corrosion Hazard in Stainless Steel, Welding Journal December 2014
- ASM International. Corrosion in Weldments. 2006
- Technical notes and white papers, Huntingdon Fusion Techniques