Possible locations of corrosion points in buried steel pipes and anti-corrosion measures

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Possible locations of corrosion points on buried steel pipes and anti-corrosion measures 1. Locations: In buried steel pipe systems, the areas where the pipe-ground potential is relatively negative are the anodic zones, and corrosion occurs there first. 1. Areas that may cause galvanic corrosion: joints between different materials, and regions with varying surface conditions (such as coatings, rust layers, coatings on bare metal, oxide scales on the base metal) ； Iron-copper connections: iron accelerates corrosion; galvanized iron sheets: zinc accelerates corrosion; connections between old and new pipes: the new pipe accelerates corrosion; damaged areas on coated pipes: exposed iron accelerates corrosion ； 2. Areas on the metal surface subjected to different stresses: such as scratches, defects, bends, threads, weld areas, etc. Stress on the metal surface in areas subject to mechanical stress, such as bends, scratches, defects, and threads, can accelerate corrosion ； Thermal stress: Corrosion is likely to accelerate at the weld area ； 3. Differences caused by the external environment: oxygen concentration, salt concentration, temperature differences, pH value differences, bacteria, stray currents, etc. The external factor of corrosion is the environment, and the external factor comes into play through internal factors. If external factors are absent, corrosion will not occur. The corrosivity of soils with different compositions varies greatly depending on the geomorphic environment. At the boundaries between clay, marshes, silt, slag, cinder, and adjacent sandy soils, calcareous materials, gravel, etc ； Points across rivers, railways, and highways ； Wearing cement and concrete walls ； At the surface-ground junction, near the waterline ； Different salt concentrations (soils with a salt content of around 3 suffer the most severe corrosion) ； Acidic soils cause more corrosion than neutral and alkaline soils ； Higher temperatures cause more severe corrosion than lower temperatures; soils containing bacteria, such as sludge and humus-rich soil, suffer more severe corrosion ； Pipeline corrosion is severe due to interference from stray currents, among other issues. Buried oil and gas pipelines can extend for thousands of kilometers, passing through various types of soil, rivers, and lakes. Due to the variability of the soil, freezing and thawing in winter and summer, changes in groundwater levels, plant roots penetrating the coating, microorganisms, stray currents, and other complex conditions associated with burial, these pipelines are exposed to a challenging corrosion environment. Corrosion rates vary greatly across different regions in our country; the minimum rate is 0.004 mm/a, while the maximum can reach 1 mm/a or even higher. In the presence of stray currents or microorganisms, a 7-mm-thick steel pipe can be perforated within a few months. II. Measures 1. Petroleum asphalt anti-corrosion coatings are among the earlier types of anti-corrosion coatings; they perform well in most dry areas. Epoxy coal tar asphalt and coal tar varnishes offer good resistance to bacterial corrosion as well as to penetration by plant roots. Their application techniques are well-developed, making them the most widely used and in the largest quantities, with excellent results achieved. 2. Polyethylene adhesive tape offers good corrosion resistance, is easy to apply, is inexpensive, and its quality is easy to control; there are many successful application examples in China. Sintered epoxy powder coatings possess excellent adhesion, corrosion resistance, and good temperature tolerance. Its excellent resistance to cathodic stripping and coating shielding properties enable it to work well in conjunction with cathodic protection, and it has been widely used in China in recent years. The main performance parameters of domestically produced epoxy powders are as follows: the curing time at 230°C is 1.5 min or less; at temperatures of 230°C or lower, it is 30 s. The quality of the coating meets the requirements of China’s standard SY/T 0315 \"Technical Standards for Sintered Epoxy Powder Coatings on Steel Pipes\", and is generally on par with similar foreign standards. 3. The coating of polyethylene and polyethylene foam jackets is a technology developed in China in the 1980s; as the adhesive and jacket materials have been continuously improved, their overall performance has also improved. In the 1990s, three-layer PE production lines were introduced. The three-layer PE coating acts as an anti-corrosion layer; it is formed by combining epoxy powder, a copolymer binder, and polyethylene together, thereby creating a strong bond with the steel pipe and resulting in an excellent anti-corrosion layer. This coating overcomes the problem of poor adhesion associated with two-layer PE hot-melt adhesives, as well as the brittleness of single-layer epoxy powder coatings, making it an ideal coating solution. The introduction of this technology has promoted the advancement of pipeline anti-corrosion technology in our country and facilitated the development of anti-corrosion materials there. 4. Epoxy powder, copolymer primer, and outer polyethylene coating have been domestically produced, with quality meeting the standards of ***, Germany, and Italy ； China’s standard SY/T4013, \"Technical Standard for Polyethylene Coatings on Buried Steel Pipelines,\" specifies that the coating should have an environmental stress cracking resistance value of F50 ≥ 1000 hours, and an elongation at break of ≥ 600%; its resistance to thermal aging also meets the requirements of German standards. It also utilizes domestic radiation-crosslinked heat-shrinkable sealing tape technology, making it a world-class anti-corrosion project.