Abstract Glass fibre reinforced epoxy composite (GRE) pipe exhibits degrading rate of deterioration of mechanical properties like inter laminar shear strength (ILSS) and modulus after one year of sea water flow under fluctuating temperature. Glass transition temperatures (T g) of the treated pipe samples decrease compared to as-cured one. FTIR spectra of GRE samples reveal the mechanism of Tg variation. Depths of penetrations of various salt components as established through EDS analysis is found maximum for ‘Ca’ and minimum for ‘Na’. Cell culture test of the used sea water confirms ‘Na’ and ‘K’ are the favourable nutrients for growth of micro-organisms which lock these salts retarding their permeation to composite body. Deposition of ‘Na’ and ‘K’, as observed through XPS analysis, causes the rate of property deterioration of the pipe. The SEM fractographs of the treated pipe samples indicate the failure modes in agreement with the trends of mechanical properties. As per the present experimental findings, GRE composite can be chosen for piping applications for prolonged flow of sea water in desalination industries.