internal corrosion
Internal corrosion of pipes is caused by corrosive chemicals in the transport medium. Different mediums lead to different corrosion factors. For example, natural gas is high in H2S, CO2, water content and dust, which can cause perforation and explosion accidents. Internal corrosion of pipelines is not only the result of multiple external factors, but is also related to pipeline materials and manufacturing methods, as well as stress.
external corrosion
Soil erosion. Soil is basically a porous gel-like capillary with three phases: solid, liquid and gas. The pores of the soil are filled with air and water. A certain amount of salt in the water gives the soil its ionic conductivity. The soil and chemical properties of metal materials and the electrochemical inhomogeneity of metal materials meet the electrochemical corrosion conditions of buried pipelines, leading to corrosion.
Stray current corrosion. Stray current is the current that corrodes and damages metal pipes outside of the design of underground flow protection systems. Stray current corrosion includes DC stray current corrosion and AC stray current corrosion. DC stray current mainly comes from DC electrified railways, DC electrolysis equipment ground electrodes, anode ground beds in cathodic protection systems, etc. The stray current flow process forms two corrosion cells established by the external potential difference. One is that the current flows out of the track and into the steel pipe. The rails are the anode of the corrosion battery, where corrosion occurs. The other is that the current flowing out of the pipe returns to the guide rail. The pipe is the anode of the corrosion battery and corrodes, and the guide rail is the cathode and does not corrode.
Casing corrosion. Through shells are widely used in long-distance transport pipelines. Corrosion of casings (especially metal casings) through pipe sections is complex and has a shielding effect on traditional cathodic protection. Casing corrosion can be divided into routine corrosion and abnormal corrosion.
Erosion corrosion. Due to erosion and cutting of the river bed, exposed pipes underwater are exposed in the river, causing corrosive corrosion.
Corrosion test of directly buried steel pipes
Internal corrosion detection
Internal inspection mainly includes the geometry of the inner wall of the pipe (such as oval, bend, perimeter welding, wall thickness corrosion, etc.). Tube C fireplaces are characterized by thinning of the tube walls and local pitting and alteration. General pipeline corrosion detection mainly measures and analyzes changes in pipe wall items. The internal corrosion of complex fluid pipelines is closely related to the corrosion characteristics of the culture medium and the flow characteristics of the fluid.
External corrosion detection
External corrosion protection for buried carbon steel pipelines is usually through a composite layer of insulation and cathodic protection. Cathodic protection parameters can be derived from damage to the pipeline protective layer to determine pipeline corrosion. Developed on the basis of the principles of this method, the test parameters are mostly potential measurements and measurements of tubes/currents.
There are now 6 cover detection methods: Pearson detection method, AC attenuation method, direct current potential gradient method (DCVG), tube current and voltage method (also called DC current and voltage method), variance-frequency selection method, and shutdown Interval Potential Method (CIPS).
Among them, the first three methods are mainly used to detect the damage points of the outer cover; the middle two methods are mainly used to detect the insulation resistance of the outer covering. The last method is to indirectly evaluate the status of the outer protective layer by detecting the pipeline protection potential. These methods determine the condition of the exterior coating by applying rated AC/DC power to the pipe, or by negatively protecting the pipe, and by detecting signal changes above the pipe.
