Few things to be noted particularly about polymer Insulators , first it's wrong to say polymer is not used in costal areas and due to high salinate , it's about porcelain it damages its surface and require frequent coating of silicon whereas polymer Insulators made of silicon rubber and proved highly effective in costal areas infact lots of coastal areas replacing old porcelain disc insulators to polymer Insulators Second fact that polymer is rarely used in to transmission lines you will be surprised to know that in this decade 2011-2020 80% EHV lines comprising 400 kV and above used polymer Insulators , India's largest transmission utility power grid used polymer Insulators in all its transmission lines in this decade . About life of polymer insulator yes it's less than porcelain but cost wise about 40% cheaper than porcelain
