TWI Frequently asked questions
by Alan Smith
The effect of loss of thickness due to corrosion at welds can be considered using existing procedures (see
Further information, below: 'How should I assess the structural integrity of corroded regions?'). However, relatively little research has been done to confirm how conservative these procedures are for welds. Brittle fracture is particularly important at welds because corrosion can expose crack-like flaws such as lack of sidewall fusion, thereby increasing the risk of failure.
There is also concern that the locally high stresses associated with irregular corrosion damage could induce cleavage at locally brittle zones. To prevent this, the fracture toughness properties of the weld and heat affected zone need to be sufficient to avoid the risk of brittle fracture. It is therefore recommended that the methods used for assessing corrosion, described in Annex G of BS7910 [1] , are not used with flash welds, low frequency electrical resistance welds, or welds with particularly coarse microstructures such as electron beam welding, unless adequate toughness can be demonstrated. It is also recommended that the corroded welds should be thoroughly inspected to avoid the presence of flaws to ensure the component is 'fit for purpose'.
Note Annex G of BS7910 stipulates the procedure is not valid for:
| i) | Materials manufactured prior to 1927 |
| ii) | Any material that has been shown to have a full-scale fracture initiation transition temperature above the operating temperature |
| iii) | Material of thickness greater than 12.7mm unless the full-scale initiation transition temperature is below the operating temperature |
| iv) | Defects in mechanical joints fabricated, formed or cast fittings |
| v) | Lap welded or furnace butt welded pipe |
| vi) | Semi-killed steel |
| vii) | Sharp defects combining corrosion and cracks |
| viii) | Fabrication defects in welds |
The provisions are largely to remove the risk of brittle fracture. If brittle fracture cannot be excluded, then it is conservative to assess the corroded region using the fracture assessment procedures of BS7910, assuming that the locally thinned region is equivalent to a planar defect.
Other than the risk of brittle fracture, the effect of mismatch between the weld and line-pipe strength should be considered. Pipeline Research Council International (PRCI) are currently sponsoring a major research programme into the evaluation of structural integrity of corroded regions at welds; this is due to start 2001/2002.
References
- How should I assess the structural integrity of corroded regions?
FAQ: How can I assess corrosion damage at nozzles in steel pressure vessels?
