Environmental oxidation processes in yellow gas pipe: implications for electrowelding

Abstract

The thermal, hydrolytic and photochemical oxidation of yellow gas pipe material (125 mm) has been investigated by using Fourier transform infra-red spectroscopy (FTIR), differential scanning calorimetry (DSC), UV absorption spectrophotometry, density measurements and parallel cleavage tests. The results indicate that oxidation of the pipe occurs predominantly on the outer surface and to a lesser extent on the bore, often with little or no change in the central layers. FTIR analysis of microtomed sections of the pipe supports the DSC analysis (oxidation induction time at 200 °C, OIT) and indicates leaching and consumption of the polymer additives at the outer and bore surfaces of the pipe; the effect being more prevalent for the antioxidants than the UV stabiliser (Cyasorb UV 531). Oxidation profiles at 80 °C in water, as measured by FTIR, indicate an unusual hydrolytic oxidation process resulting in extraction of the additives and pigment and formation of a substantial amount of oxidation products. This occurs only at the pipe outer surface to a depth of 0.5 mm, forming a clear transparent layer. These oxidation analyses are consistent with density profile changes through the pipe wall from the outer surface. Both water and UV exposure are seen to exhibit variable effects on pipe stability, with temperature playing an intimate role in terms of their severity. The effects of these oxidation processes on the subsequent electro-welding process for pipe jointing is examined by parallel cleavage tests. Both water and UV exhibit long-term effects in inducing brittle failure at weld joints as does the presence of surface dust, thus indicating the necessity of surface scraping before electrowelding in service applications.