PERFORMANCE OF DUCTILE IRON PIPES AND ISSUES ASSOCIATED WITH THEIR DEGRADATION

摘要

Whilst PE sleeving is by far the cheapest corrosion protection method available, opinion is divided on its corrosion mitigation effectiveness. Field experience has shown that the use of PE sleeving has a beneficial effect in reducing corrosion of DI pipe, especially when used in conjunction with sand backfill. However, corrosion damage has been found to occur under both damaged and undamaged PE sleeving in aggressive environments and the level of this damage may become an issue as thinner walled DI pipes come into common use. This damage raises a number of issues: For example whether the theory is correct, that oxygen in the entrapped water between the sleeve and pipe will be consumed by cathodic reactions and corrosion will halt after initial rusting has occurred. Field studies indicate that entrapped moisture can lead to premature failure. Furthermore, the oxygen depletion theory may not be valid in variable groundwater conditions or those situations when the cathode in the corrosion cell is external to the water main. A second issue is the susceptibility of PE sleeving to damage during installation. Both Canadian and Australian field studies indicate that corrosion pitting can be preferentially located under cuts and tears in sleeve and it has been confirmed that it is almost impossible to lay DI pipes without damaging the PE sleeving. However, the long term effects of this damage to the PE sleeving is currently inconclusive because in the most detailed Australian study where PE sleeved DI pipe was laid in aggressive soils for 10 years, the expected through-wall corrosion (due to the high cathodic/anodic ratio in the corrosion cell) was not observed. This is in contrast to United States and Canadian observations, in which accelerated corrosion under both undamaged sleeving and also at tears in the sleeve, has lead to full penetration of the pipe wall and premature failure. However, in one particular Canadian case the authority may have exacerbated their corrosion problems by joint bonding and using un-insulated service connections. A third issue is the shielding effect of PE sleeving on cathodic protection. Whilst some industry experts in Australia offer the opinion that this does not make economic sense, others are investigating the effect in greater detail. However, it should be noted that PE sleeving does appear to have benefits in reducing current consumption because whilst electrical current requirements for cathodic protection in PE-sleeved pipe were higher than those for well-coated steel pipe, they were still lower than those for un-protected DI pipe.