Metals can react with oxygen in the atmosphere to form an oxide film on the surface. The iron oxide formed on ordinary carbon steel will continue to oxidize, causing the rust to continue to expand and forming holes in the later stage. This can use paint or oxidation-resistant metal plating to protect the surface of carbon steel, but this protective layer is just a thin film. If the protective layer is damaged, the steel below will begin to rust again. Whether the thick-walled steel pipe is corroded or not is related to the chromium content in the steel. When the chromium content in the steel reaches 12%, it is not easy to be corroded. In the atmosphere, a layer of passivated, dense chromium-rich oxide is formed on the surface of the stainless steel tube to protect the surface and prevent further re-oxidation. This oxide layer is very thin, and the natural luster of the steel surface can be seen through it, giving the stainless steel a special surface. If the chromium film is damaged, the chromium in the steel and the oxygen in the atmosphere will regenerate a passivation film to continue to play a protective role. In some special environments, stainless steel will also fail due to some local corrosion, but stainless steel, unlike carbon steel, will not fail due to uniform corrosion, so the corrosion allowance is meaningless for
stainless steel pipes.
What corrosion phenomena can occur in thick-walled steel pipes?
(1) Concentration cell corrosion: Corrosion cells composed of different electrode potentials due to the difference in the concentration of the corrosive near the electrode surface are called concentration corrosion cells. Underground large-diameter seamless steel pipes are common in corrosion batteries formed by different oxygen content in one place and another in the solution (or soil) resulting in different electrode potentials. The area with low oxygen concentration will become the anode area, and corrosion will accelerate. In fact, the corrosion mechanism of seam courtyard corrosion and the corrosion of concentration cell are the same, but the corrosion of concentration cell has more obvious anode and cathode areas.
(2) Pitting corrosion: occurs in a relatively local area of the metal surface, causing caves or pits and expanding to the inside, or even perforation. If the diameter of the hole is smaller than the depth of the hole, it is called pitting. If the diameter of the hole is larger than the depth of the hole, it is called pitting. In fact, there is no strict boundary between pitting and pitting. Pitting corrosion is one of the destructive and hidden forms of corrosion. It often makes the pressure large-caliber seamless steel pipe perforate and leak when the quality loss is still small.
(3) All corrosion: All corrosion is also called uniform corrosion, which is basically the same degree of corrosion that large-diameter seamless steel pipes produce on a larger area. For example, the inner wall surface of large-diameter
seamless steel pipes suffers all the corrosion of the medium, and the outer wall is exposed. (Or paint but all have failed) atmospheric corrosion, etc. The pressure large-caliber seamless steel pipe that suffered all the corrosion gradually became thinner in wall thickness and then was destroyed. However, the correspondingly uniform rot does not exist, and the thickness of the rupture is not the same. From an engineering point of view, total corrosion is not a threatening form of corrosion, because sufficient window size has been considered in the design, but uniform corrosion will cause the load-bearing capacity of large-diameter seamless steel pipes to decrease, which will lead to serious operating accidents.
(4) Crevice corrosion: Crevice corrosion occurs at or near the crevice. These gaps are formed due to the contact of the same or dissimilar metals, or the contact of metals and non-metallic materials. The extent of corrosion at the crevice is much greater than other areas on the metal surface. This corrosion is usually caused by the lack of oxygen in the crevice, the change of acidity in the crevice, and the accumulation of certain ions in the crevice. Crevice corrosion is a very common corrosion phenomenon. Crevice corrosion may occur in almost all metal materials. The silt, scale and impurities deposited on the flange connection surface, nut pressing surface, lap surface, weld pores, rust layer, and deposited on the metal surface will form crevices and cause crevice corrosion.
In industry, thick-walled steel pipes are mainly used to make some conveying pipes. Of course, it is widely used in the construction industry, such as stair handrails, window guards, railings, road partitions, etc. At present, many products are used Here comes the stainless steel pipes, such as bicycles, medical equipment, automobiles and so on. Thick-wall welded pipe products are not only rich in types, but also in terms of their processing technology and forming methods. In most cases, after forming, the next process is to expand the full-length diameter of the welded tube again to improve the quality of the welded tube. In other words, the diameter expansion process has become an important process in the quality of finished pipes in the production of thick-walled steel pipes.
Tips: ASTM A53 Grade B is more popular than other grades. API 5L Gr.B is an American standard material, A53 GR.B ERW refers to the electric resistance welded steel pipe of A53 GR.B; API 5L GR.B Welded refers to the material Welded steel pipe of API 5L GR.B.
These pipes can be bare pipes without any coating, or it may be Hot-Dipped or Zinc-Coated and manufactured by Welding or by a Seamless manufacturing process. In Oil and Gas, A53 grade pipes are used in the structural and non-critical applications.
