Directly buried steel pipes are buried in the soil, and electrochemical corrosion is an unavoidable problem. Cathodic protection is a commonly used protection method, but cathodic protection for insulated pipes is much more complicated than for ordinary pipes.
Cathodic protection of ordinary steel pipelines is relatively simple-coating an anti-corrosion layer on the outer wall of the pipeline and then applying cathodic protection current. But the structure of the insulation pipe is different: the outside of the steel pipe is a polyurethane insulation layer, and the outside of the insulation layer is a polyethylene outer protective pipe. The insulation layer and the outer protective tube are both insulating materials, forming an insulation structure of "anti-corrosion layer-insulation layer-protective layer". Although this structure is good for heat preservation, it brings troubles to cathodic protection - the cathodic protection current is blocked by the insulation layer and cannot reach the surface of the steel pipe. This is the so-called "primary shielding" effect.
There are two main ways to solve this problem. One is to spirally wrap the sacrificial anode (such as aluminum alloy sacrificial anode strip) to the surface of the bare steel pipe before prefabricating the insulation layer, weld the anode core to the pipe body at certain intervals, and then perform insulation foaming. In this way, the sacrificial anode is wrapped inside the insulation layer, directly protecting the steel pipe. The other is to establish a dedicated cathodic protection current channel through minimally invasive methods for existing pipelines that have been buried underground and introduce the protective current into the insulation layer.
Insulation issues also need to be paid attention to when designing. Good insulation should be maintained between the working steel pipe and the outer protective pipe, and components such as sliding brackets must also meet insulation requirements. In this way, a uniform polarization potential can be obtained with the minimum protection current.
The design and construction of cathodic protection should comply with the "Technical Specifications for Cathodic Protection of Buried Steel Pipelines" GB/T 21448-2017. The cathodic protection of insulated pipes is more complicated than that of ordinary pipes. If it is not considered properly during the design and construction stages, it will be difficult to remedy it later.