Most industries using stainless steel pipes do so because of their corrosion resistance.
The dairy and food sectors are major users since the end products must be contamination free. Pipe and tube joints inevitably contain crevices and any build-up of contaminants here present a potential problem. Large facilities manufactured by major producers such as Alfa Laval AB in Sweden can contain not metres but kilometres of pipes and joints.
Welded joints are common. Well made, they offer a smooth transition from one section to another, 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.
Fig 1 Complex pipework in typical dairy processing plant
Welds carried out on most metals without adequate inert gas coverage 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’.
Fig 1a The result of unprotected underbead in welded austenitic stainless steel
Fig 1b To ensure no discolouration occurs the oxygen content needs to be reduced to 20 ppm (0.002%).
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.
Pipe and tube purging
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.
Residual oxygen measurement instruments
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 manufactured by Huntingdon Fusion Techniques reads down to 10 ppm with extreme accuracy and has a display range from 1,000 to 10 ppm.
Fig 3 The Argweld® PurgEye® 600 Weld Purge Monitor®, has a USB connection and data logging capability allowing the operator ease of data transfer without the need for a computer connection.
The entire Argweld product range is supported by an extensive library of publications including Technical Notes, White Papers, Conference Proceedings and peer-reviewed International Articles. These are available on-line by application to Huntingdon Fusion Techniques Ltd.
- 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
Author: Michael Fletcher
Dr M J Fletcher is a qualified metallurgist with extensive experience in welding and non-destructive testing.
He works as an independent consultant, providing support to a wide range of manufacturing industry on a global basis.
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