Phenomenological models for describing the process of straining of bitumen-mastic insulation used for protection of the natural gas pipelines from electrochemical corrosion is proposed. The viscoelastic characteristics of insulation at different temperatures and after aging are determined. It is shown that the viscosity and stiffness coefficients determined on the basis of the accepted rheological model of the Maxwell viscoelastic body can be used to assess the degree of degradation of insulation.