Corrosion Review of Anode Well Vents on a Flood Gate CP System, Upper Texas Coast (2021)

In July 2021 we reviewed corrosion on the anode well vents of the cathodic protection system that protects two flood gates at a coastal industrial facility on the upper Texas coast. Client details are confidential.

What maintenance found

During routine maintenance, facility personnel reported corrosion on the vent piping at the anode well heads. The damage varied from well to well and was worst at two wells on the north gate. The system has nine anode wells across the two gates, each holding five anodes, and the commissioning records showed all nine set to similar outputs of 12 to 16 A.

A field change at installation

Anode wells are vented so the gases the anodes produce in normal operation can escape. At installation, a field change added a stainless steel float valve to each vent to keep floodwater out, and because the valve was heavy, the top of each 1-inch vent, designed in PVC, was replaced with rigid metal conduit. The photographs showed corrosion mostly on the inside of those metal parts. That pattern fits attack by chlorine gas, which the anodes produce where chlorides are present at anode depth. Some of the gas was also finding its way into the conduit for the positive header cable and damaging metal conduit further along.

The fix

We recommended returning the vents to the original design, built only of PVC. PVC resists the gases from the anodes, and when chlorine is present at high concentration it damages metal over time, whatever the grade or alloy. Water in a vent pipe does not stop the gases from venting, so the float valves are not needed. The header cable conduit should be sealed at the nearest PVC to metal transition with closed-cell foam or another approved sealant, to keep the gas out of the metal sections.

Questions from the project team

The team asked why two wells showed more damage. Gas production can vary with chloride concentration, anode current density, the physical layout and exposure. Water seen moving up a vent would suggest the anode column reaches a higher-pressure zone, which does not affect how the CP system works but helps carry gas out at the well head. They also asked whether those wells would wear out early. Anode consumption depends on the current each well discharges, not on how much gas it vents, and at the set outputs the anodes are rated well beyond the system 25-year design life. Finally, drain holes at ground level in each well head keep the epoxy-sealed cable splices from sitting under water for long periods, and water that enters through them in a storm drains back out.

CP consulting

The systems we design, test and troubleshoot are described on our page for cathodic protection.