Welding can Induce Loss of Corrosion Resistance in Stainless Steel Tubes
Welding carried out on many stainless steels can result in a significant reduction in corrosion resistance , a significant consideration in the food and drink industry .
Stainless steels owe their resistance to corrosion to the formation of a very thin surface layer of chromium oxide . During welding at high temperatures in the presence of oxygen this film breaks down and presents sites for corrosion . Whilst the upper weld surface is protected by the torch gas , the underside is exposed to oxygen in the atmosphere .
Underside protection is thus essential and this can be achieved by providing inert gas coverage . To meet the need for total internal protection , called ‘ weld purging ’, dedicated equipment has evolved over the past 25 years .
Pipe and tube purging
Fig 1 Complex pipework in typical dairy processing plant
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 and easy to insert and remove .
Fig 2 ‘ Argweld ®’ inflatable pipe and tube purging concept . The product range , which includes QuickPurge ®’ and PurgElite ®’ designs is used globally for applications in the process industries .
The most effective solutions are based on the use of inflatable dams and fully integrated systems are now available covering pipe and tube diameters between 25 and 2400 mm . The recommended residual oxygen content for total protection is 20 parts per million These systems are quite capable of providing protection down to 10 parts per million .
Alternative purging systems are available to accommodate pipe diameters as small as 10mm .
Residual oxygen measurement instruments
Any effective weld purge process needs to be supported by monitoring with sensitive oxygen detecting equipment . The PurgEye ® instruments from Huntingdon Fusion Techniques , HFT ® read down to 10 ppm with extreme accuracy . These can be integrated easily into Argweld purge systems for continuous monitoring and control of oxygen levels .
• 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 .
Food and Drink TP-01-12-2020 ME
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