Corrosion is one of the main reasons for the damage and failure of buried pipelines. For the long-distance pipeline and centralized pipeline network of natural gas transmission, the corrosion of buried pipelines cannot be directly discovered and is inconvenient for maintenance. How to prevent corrosion damage is a key factor of pipeline safety engineering. According to the corrosion part, the corrosion of buried carbon steel pipe can be divided into internal corrosion and external corrosion.
The internal corrosion of pipeline is caused by corrosive chemical components in the transported medium. Different medium cause different corrosion factors. For example, natural gas is high in H2S, CO2, water content and dust, which may lead to perforation and burst accidents. The internal corrosion of pipeline is not only the result of multiple external factors, but also related to pipeline material and manufacturing method, as well as stress.
Soil erosion. Soil is essentially a porous gelatinous capillary with three phase states: solid, liquid and gas. The pores of the soil are filled with air and water. A certain amount of salt in the water makes the soil have ionic conductivity. The physical and chemical properties of the soil and the electrochemical inhomogeneity of the metal material satisfy the electrochemical corrosion conditions of buried pipelines, resulting in corrosion.
Stray current corrosion. Stray current is the current that corrodes and damages metal pipes outside of the protection system design for underground flow. Stray current corrosion includes DC stray current corrosion and ac stray current corrosion. The DC stray current mainly comes from the dc electrified railway, the DC electrolytic equipment grounding electrode, the anode ground bed in the cathodic protection system and so on. The stray current flow process forms two corrosion batteries established by the external potential difference. One is that the current flows out of the rail and into the steel pipe. The rail is the anode of the corrosion battery, and corrosion occurs. The other is the current flowing out of the pipe back to the rail, the pipe is the anode that corrodes the battery, corrodes, the rail is the cathode, does not corrode.
Casing corrosion. Through casing is widely used in long-distance transportation pipelines. The corrosion of the casing (especially the metal casing) through the pipe section is complicated and has a shielding effect on conventional cathodic protection. Casing corrosion can be divided into conventional corrosion and abnormal corrosion。
Erosion corrosion. Due to the erosion and cutting of the riverbed by the river, the underwater bare pipes are exposed in the river, resulting in erosion corrosion.
Internal corrosion detection
Internal inspection mainly includes the geometry of the inner wall of the pipe (such as ellipticity, bending, girth weld, corrosion reduction of wall thickness, etc.). Pipe c orrosion is usually characterized by thinning of the pipe wall and local pits and pitting. General pipeline corrosion detection is mainly to measure and analyze the changes in the term of pipe wall. The internal corrosion of complex fluid pipelines is closely related to the corrosion characteristics of the medium and the flow characteristics of the fluid.
External corrosion detection
Buried carbon steel pipeline external corrosion protection is generally composite layer made by insulating layer and cathode protection. Cathodic protection parameters can be from the damage of the pipeline protection layer, judge the pipeline corrosion. Developed on the basis of the principle of this method, the test parameters are mostly tube/potential measurement and measurement of tube current.
Now there are 6 main kinds of detection methods for outer cover: Pearson detection method, alternating current attenuation method, dc potential gradient method (DCVG), in-tube current and voltage method (also known as dc current and voltage method), variance-frequency selection method, and close interval potential method (CIPS).
Among them, the first three methods are mainly used to detect the damage point of the outer cover; The middle two methods are mainly used to detect the insulation resistance of the outer covering layer. The last method indirectly evaluates the condition of the outer covering layer by detecting the pipeline protection potential. These methods determine the condition of the outer coating by applying rated ac/dc power to the pipe, or by means of the negative protection of the pipe, and by detecting signal changes directly above the pipe.