1 Inspection results The appearance of the steel pipe is covered with thick brown rust, loose, shedding, surface wrinkling, and more defects, and the rust layer has been received in the first draft: 199907-01 received revised draft: 1999020 obvious The hole, some parts have been perforated, the maximum pit diameter is 5 corrosion speed up to 2.5mm / * the degree of damage is more serious.
A metallographic micrograph of a section of a non-corroded perforation (hereinafter referred to as non-corroded) and a corrosion perforated (hereinafter referred to as corroded) steel pipe after chemical impregnation treatment is shown. Observations show that the microstructure of 20 steel is ferrite and pearlite. The ferrite structure is a network of polyhedral grains. The bright part of the photo is ferrite, and the black part is pearlite.
Comparing (a) and 1 (b), it can be clearly seen that the uncorroded steel tube has finer grain and uniform structure, while the corroded steel tube has coarsened grains and a decarburized layer appears on the inner wall of the tube. Observed by the microscope and compared with the standard grain size of GB4335-84, it was found that the difference in microstructure after heat treatment of the corroded and uncorroded steel tubes was caused by the difference in heat treatment conditions. It can be considered that they are not the same batch material.
The results of electronic energy spectrum analysis (EDX) on the surface of uncorroded and corroded steel pipes are shown. It can be seen from the figure that the surface layer of the uncorroded steel pipe is mainly composed of Fe, and the surface of the corroded steel pipe is composed of elements such as Mg, Ca and Fe. This is due to the deposition of Mg and Ca salts in the seawater on the inner wall surface of the tube. It is worth noting that the surface of the corroded steel pipe was found to have C1 elements. This shows that in the seawater, the C1 element participates in the corrosion process of the cooling device steel pipe.
The results of X-ray diffraction analysis of the rust layer on the surface of the uncorroded and corroded steel pipe are listed. It can be seen from the figure that the rust layer on the surface of the uncorroded steel pipe is mainly composed of Fe23 and Fe34, and the surface of the corrosion-resistant steel pipe is FeCl3, CaO and MgO in addition to Fe23 and Fe34, and the peaks of diffraction intensity of FeCl3 and cao are large, and the result is The above electron spectroscopy analysis is basically consistent.
In order to verify the difference in corrosion resistance between corroded and uncorroded materials, the anodic polarization curve () was tested in seawater. The self-corrosion potential of the uncorroded sample is -0.559V and the corrosion sample is 1 from the curve. It can be seen that the 20 steels are all in the active dissolution state of the anode, and the anode current density ratio of the corrosion sample increases with the anodic polarization. The current density of the uncorroded sample is sharply large.
2 Corrosion analysis 20 steel is a high-quality carbon structural steel. The steel contains lower S and P impurities, and its quality is higher than that of ordinary carbon steel. It is one of the important materials for offshore engineering structure. Since the room temperature structure of 20 steel is ferrite and pearlite and the pearlite structure is less and there are many ferrites, it is a material with good plasticity and low strength and hardness. In order to improve the mechanical properties, annealing and normalizing treatment are generally required. The purpose of heat treatment is 111: () to improve hardness, strength and improve cutting ability; (2) to eliminate internal stress caused during processing; (3) to improve surface wear resistance, corrosion resistance, etc.; after normalizing, quenching The treatment has less tendency to crack and deform than other tissues.
Through normalizing treatment, the grain can be refined, and the internal structure and properties of the metal can be improved to meet different application requirements. Generally, the pearlite fraction can improve the corrosion resistance by improving the MM property. Conversely, the presence of coarse pearlite in the steel reduces the strength and corrosion resistance of the steel and increases the brittleness of the steel. The results of metallographic analysis and electrochemical experiments show that the uncorroded 20 steel has fine grain structure and uniform structure, and the corrosion potential is positive in seawater and the active current density of the anode is small, which improves the corrosion resistance. This supports the above viewpoint.
The excessively high heat treatment temperature indicates that the grains of the steel become coarse, and the mechanical properties, particularly the ductility and toughness, of the steel are remarkably lowered. At the same time, in the high temperature heating process of steel, the steel is decarburized due to the oxidizing atmosphere in the furnace. At high temperatures, the carbon in the surface layer combines with the oxidation in the furnace gas to form a carbon monoxide gas that escapes the surface and reduces the carbon content of the steel surface. If the excessively thick decarburized layer remains on the surface of the workpiece, the strength, hardness, wear resistance, fatigue strength and corrosion resistance of the steel are severely lowered.
In summary, 20 steel is a multi-phase structure and the properties are not uniform. Uneven phase structure is prone to electrochemical corrosion in seawater. 121. It is reported that the corrosion potential of ferrite in the metallographic structure is relatively negative, and the pearlite is the anode, thus causing local selective corrosion in seawater. Become a source of pitting corrosion. In most cases, pitting corrosion originates from thermodynamically unstable grain boundaries or phase boundaries and is on the ferrite phase side. Therefore, it can be considered that the steel microcrystals are coarsened or local defects such as decarburization layer, and the active anions such as cr are easily etched and concentrated at the place where they are in contact with seawater, thereby causing local anode active dissolution in the pit, which is to promote The main cause of corrosion of 20 steel. The results confirm this view. For the above reasons, once the etched holes are formed, the metal in the etched holes is dissolved |41, ie: Fe*Fe2+2e, so that the etched holes will be continuously added, in order to maintain electrical neutrality, The concentration of cr in the well increased. Further, since the metal ions in the etched holes are high and hydrolyzed: this causes the H+ concentration in the pores to rise, the pH value to decrease, the pores to be acidified, and the activated dissolved state. At the same time, cr continues to migrate into the pores. This allows the pitting corrosion to progress in an autocatalytic process, thereby promoting the rapid development of corrosion damage.
3 Conclusions and measures 1 In the seawater, 20 steel pipes are electrochemically corroded, that is, the ferrite structure with negative potential is active and dissolved, resulting in local pitting damage.
2 Metallographic analysis showed that the corrosion-producing steel tube grains were coarsened and a decarburized layer appeared on the inner wall of the tube. It can be concluded that the steel pipes that produce corrosion perforations and that do not have corrosion perforations are not the same batch material.
3 The reason why the steel pipe is corroded and perforated is because the normalizing heat treatment temperature is too high, which causes the grain coarsening and decarburization, resulting in a decrease in corrosion resistance.
4 requires that the actual production must pass a reasonable heat treatment system to avoid grain coarsening and decarburization, and fundamentally eliminate the hidden dangers of corrosion.
5 Many studies have shown that 151, in addition to the use of steel pipes suitable for heat treatment specifications, the use of electrochemical cathodic protection can not only improve corrosion resistance, reduce costs, but also increase production efficiency and extend service life.
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