A process for coating the exterior surfaces of a pipeline with a polymer that is cross-linkable under exposure to water, including a) coating the exterior surface of the pipeline with at least one polymer that is cross-linkable under exposure to water, where the employed cross-linkable polymer includes alkoxy silane grafted HDPE; and b) cross-linking of the cross-linkable polymer by exposure to water at elevated temperatures so as to form a cross-linked polymer layer having a degree of cross-linking of from 30% to 80% is reached.