Cathodic Protection in Dallas-Fort Worth

Clay soils and ductile iron

The soils we have tested in the area are mostly clays. Along a new 24-inch water line in McKinney, resistivity at pipe depth averaged 954 ohm-cm with a low of 515, matching the silty clay the soil survey maps there, and we classed it as highly corrosive to ductile iron. At the Dallas wastewater plant the layers at pipe depth measured 1,500 to 3,500 ohm-cm in clays and loamy sand, which on its own is moderately corrosive.

The McKinney line was designed for ductile iron in polyethylene encasement, with every bell and spigot joint bonded, insulating joints where it meets other piping, and magnesium anodes, and it was commissioned in 2020.

City standards and bid options

The three Fort Worth designs were prepared for City of Fort Worth water projects and follow the city standard details for test stations, magnesium anode stations, permanent reference electrodes and bonding. Because the pipe material was still open at bid time, each drawing set carries a test station schedule for every option. On one 24-inch line, ductile iron needs 16 anodes and concrete cylinder pipe needs 40.

Heat, small systems and start-up

Temperature changes the picture. The hot water at the Dallas plant runs at 180 °F, and heat speeds corrosion, so we expected rates three to four times those of the same soil at normal temperature and specified 48 magnesium anodes in twelve groundbeds for a 30-year life.

Small systems need the same attention. The buried gas piping at the Denton power plant is only about 400 feet long, and we have surveyed it every January since 2024. Its instant off readings have drifted 125 to 185 mV less negative over two years while still meeting the criterion, which is the kind of trend an annual survey is there to catch.

Start-up visits find what drawings cannot. In McKinney the anode stations had been built with 7 anodes instead of 12, and the flange kit at the tie-in to an existing main was not insulating. The contractor fixed both, and the final readings met the -850 mV criterion. In Dallas, flange kits missing their washers on one side were shorting the new pipe to building steel.