Under extended low‐flow conditions, water can leach cementitious materials from cement–mortar lining, and significant increases in pH can result. Corrosion of cement–mortar lining—i.e., the leaching of lime from the cement matrix—can impair water quality by increasing pH, calcium, and alkalinity. Field tests at two utility sites evaluated the effects of water flow, lining method, seal coating, water quality, and cement–mortar composition. Water that remained stagnant in newly lined pipe sections for a relatively long time (one week) exhibited significant water quality effects—e.g., pH increases up to 12. Asphaltic seal coats significantly reduced cement–mortar corrosion in both tests. At one site, corrosion increased when the mortar contained Type II cement, a water‐reducing admixture, pozzolanic material, or a high proportion of sand. At the other site, none of these parameters significantly affected results. Design and operating measures to minimize stagnation can reduce corrosion effects.