ADS 18950020 18" x 20' N-12 Solid Dual Wall Pipe. The Dual Wall N-12 pipe, with its smooth interior wall and corrugated exterior, offers exceptional hydraulics and strength. Built using application-specific high-density polyethylene, Dual Wall N-12 corrugated pipe also provides superior corrosion and abrasion resistance. This makes it an excellent alternative to reinforced concrete (RCP) and corrugated metal pipe (CMP).