Corrosion failure analysis of galvanized steel pipes in a closed water cooling system

Abstract

The present study is focused on the corrosion failure analysis of a water cooling system serving a polyethylene terephtalate (PET) preform molding plant. Piping mainly located both after the water pretreatment and in the injection molding machines was affected by an extensive corrosion phenomenon and several leakages of coolant occurred, involving the risk of loss of functionality of the whole plant. The main problem detected in closed cooling circuits was a marked modification of the water chemistry and a great amount of metal ions leading to recurring fouling of the inner surfaces of molds and cooling channels. Analysis of water flowing in the closed cooling circuit operating at 6 °C revealed a very high amount of metals in the flowing water and in the sediment (zinc and iron). Moreover the water pH was slightly acidic. Data were compared with water from supply well. Preliminary electrochemical characterization was carried out on galvanized steel pipe, in order to evaluate the aggressiveness of water both from the cooling circuits and from the supply wells. The results of laboratory analysis together with the scrutiny of the available technical documentation showed that the damage of the plant was the consequence of an inadequate plant management, mainly for what concerns materials selection and water chemical treatment.