Zou Jiqiang 1 Zhang Lihua 2
(1·Dalian West Pacific Petrochemical Co., Ltd., Dalian 116001, China; 2. Dalian Petrochemical Co., Ltd., Dalian Petrochemical Company, Dalian 116030, China)
Abstract: Corrosion failure leakage of TS-212 crude benzene reaction heat exchanger in ethylene plant was carried out. X-ray diffraction, energy spectrum analysis and scanning electron microscopy were used to comprehensively analyze the failure of heat exchanger tube bundle, and pointed out TS. The main cause of corrosion in -212 is due to the CO2+H2O corrosion system, and the presence of oxygen in the system exacerbates this corrosion. Secondly, the CO2 corrosion rate is affected by many factors such as CO2 partial pressure, flow rate, temperature, protective film and solution composition, especially the erosion and impact caused by cavitation corrosion of the shell side also exacerbates corrosion. And proposed a series of prevention measures.
Key words: crude benzene reaction heat exchanger CO2+H2O corrosion turbulent corrosion scale corrosion copper alloy electroless coating
CLC number: TQ050·9 Document code: A Article ID: 1007-015X(2009)04-0060
1 Equipment overview
Since the crude styrene heat exchanger of 1Mt/a ethylene plant of a large petrochemical enterprise of PetroChina entered into June 2008, corrosion leakage has occurred frequently. The heat exchanger life of this part is generally more than 10 years. In order to find out the causes of frequent leakage in the near future, Therefore, a comprehensive failure detection analysis is performed on the discarded tube bundle.
Equipment specifications: 1638 mm × 5072 mm
Material: 10 carbon steel.
Process parameters are shown in Table 1.
Process: The TS-212 tube bundle was exchanged from the MS-202 oil phase (crude styrene) through TT-239 and TT-262 into the AS-202 column, and the overhead material distillate was condensed in TS-212. Visual inspection: TS-212 equipment was found to have severe shell corrosion after opening, and the sheet-like hard scale attached to the tube wall was about 1 mm thick. When the water side of the tube was observed, it was found that a small amount of slime was deposited inside the tube bundle, and the slime was removed to check the corrosion.
2 Detection and analysis
2·1 macro inspection
The outer wall of the TS-212 tube bundle is distributed with coke. The scale attached to the tube wall is hard, but it is loose. Careful observation shows that there are corrosion spots, pits and large-area ulcers on the surface of the tube. Internal observation revealed that there was slime deposit, and the surface of the deposited scale was slightly corroded, and no etch pits and ulcer-like corrosion morphology were observed.
2·2 microscopic analysis
2·2·1 component analysis
The results of pipe bundle material composition analysis are shown in Table 2.
Analysis by SPECTRO quantitative spectrometer showed that the material of the tube was of carbon steel No. 10, and the composition was within the normal range.
2·2·2 internal inspection analysis
(1) X-ray diffraction detection analysis of the inner wall of the tube bundle. Component analysis results:
Fe, Fe3O4, Fe2O3, Cu6·26Sn5 and FeO, of which Fe2O3 and Fe, Fe3O4 accounted for the vast majority, no harmful medium components such as sulfides and chlorides were found, and sulfur was not directly involved in corrosion.
As for the analysis of the detected Cu6·26Sn5 component, it belongs to the composition of the rehydrator Navy copper or brass. The corrosion of the copper alloy equipment associated with the system should be carefully investigated for the presence or absence of rehydrator leakage.
(2) Analysis of the energy spectrum of TS-212 tube. The results of the energy spectrum analysis of TS-212 tube are shown in Table 3.
The energy spectrum of the inner wall of the TS-212 tube showed that the anion Cl- accounted for 2.77%. It was found that the three main indicators of Fe, Cl and O accounted for nearly 100% of the corrosion products or sediments. Detection analysis believes that:
a corrosion product and scale of the inner wall of the tube are mainly iron oxides, followed by carbonates and zinc salts;
b No distribution of sulfur and sulfate components was found. The Zn salt is most likely to come from water treatment agents, and does not rule out the corrosion of brass and naval copper equipment adjacent to the associated rehydrator.
In this way, it is judged that the corrosion of the inner wall of the TS-212 tube bundle is mainly caused by circulating water dissolved oxygen corrosion, and the system is not subjected to secondary corrosion by the corrosive corrosive medium.
(4) Scanning electron microscopy analysis of TS-212 tube. It is observed that there are many sediments on the inner wall surface, and there is not much scouring corrosion morphology, and there are pit deposits in the corrosion products and sediments. In short, there are many scales on the inner wall of the tube, and the circulating water flow rate is not high. Conditions for causing corrosion under scale.
It is comprehensively judged that the inner wall of the TS-212 heat exchanger tube bundle is slightly corroded, and the corrosion caused by the inner wall is not the main factor causing corrosion leakage of the tube.
2·3 TS-212 tube bundle external detection analysis
(1) X-ray diffraction analysis of the outer wall of the tube bundle. It can be seen by X-ray diffraction analysis that the surface of the outer wall of TS-212 mainly contains Fe, Fe3O4, Fe2O3, Cu6·26Sn5 and MgO.
Judging from the shell component analysis, Fe3O4 and Fe2O3 are dissolved oxygen corrosion products of normal water system, and the presence of MgO is inherent to circulating water, which should be caused by the intrusion of Mg2+ contained in the leaking water of the pipe into the shell side. As for the analysis of the detected Cu6·26Sn5 component, it belongs to the composition of the rehydrator Navy copper or brass. The corrosion of the copper alloy equipment associated with the system should be carefully investigated for the presence or absence of rehydrator leakage.
(2) Scanning electron microscopy analysis of the outer wall of TS-212 tube bundle. Scanning electron microscopy analysis of the outer wall of TS-212 tube bundle shows that the distribution of the surface of TS-212 outer wall is extremely uneven, showing a phenomenon of “step shapeâ€, and the local scouring phenomenon is not obvious, showing local ulcer-like corrosion characteristics.
(3) Analysis of the outer wall energy spectrum of TS-212 tube. The results of the energy spectrum analysis of the outer wall of TS-212 tube are shown in Table 4.
It was found that the scale of the outer wall of the tube mainly contains three elements of C, O and Fe, and the corrosion products are still dominated by iron oxide, which is consistent with X-ray diffraction analysis.
No harmful elements such as sulfur and chlorine were found in the corrosion products, indicating that the tube bundle corrosion is not related to sulfur and Cl-, and there is no corrosion system or synergistic corrosion of H2S + H2O and HCl + H2O, which is related to X-ray diffraction. The results of the analysis are consistent.
3 Corrosion mechanism
From the above analysis of the material composition of the inner and outer walls of the pipe, the composition and morphology of the scale, shell corrosion is the main cause of corrosion of TS212. The medium in the pipe process is circulating water, and the scale and sediment mainly contain iron oxide and zinc salt. The secondary cause of corrosion failure of TS-212 is the scale corrosion caused by the circulating water in the tube bundle.
Shell-side corrosion is a typical CO2+H2O corrosion system, which is the main cause of corrosion in TS212, and the presence of O2 in the system exacerbates this corrosion.
(1) CO2+H2O corrosion system [1]: due to CO2+H2O
The formation of carbonic acid directly reacts with the iron-based tube bundle to produce corrosion.
Its reaction formula is as follows:
CO2+H2O→H2CO3
Fe+H2CO3→FeCO3 åH2↑
At this time, most of the H2CO3 in the solution exists as H+ and HCO2-3, and therefore, most of the reaction product is Fe(HCO3)2 and is further decomposed at a high temperature to:
Fe(HCO3)2=FeCO3+H2O+CO2
In fact, the corrosion product carbonate (FeCO3CaCO3) or the scale product film has different coverage in different areas of the steel surface, and a strong autocatalytic corrosion couple is formed between the different coverage areas. The local corrosion of CO2 is this. The result of a corrosive galvanic action. This mechanism also explains well the water chemistry and the phenomenon that local corrosion suddenly becomes very serious once the above process occurs in the field [2].
The most favorable evidence to verify this is the discovery of CaCO3 and FeCO3 in both the energy spectrum analysis and the X-ray diffraction analysis. In addition, through macroscopic inspection, it is also found that the corrosion morphology characteristic of CO2 corrosion is worm-like corrosion morphology.
(2) The presence of dissolved oxygen exacerbates corrosion: Oxygen corrosion is the most common type of corrosion. The electrochemical process of oxygen corrosion is as follows:
This is oxygen depolarization corrosion. Iron, oxygen and water combine to form rust during the corrosion process. The rate of oxygen corrosion is affected by the dissolved oxygen content in the water, and the corrosion rate increases as the dissolved oxygen content in the water increases.
The damage caused by the dissolved oxygen in the shell-side process medium in the environment of 80~130 °C is even greater than the damage caused by CO2+H2.
Secondly, CO2 generates carbonic acid (H2CO3) in water, which can make the pH value reach 3.3. When O2 exists in the system, O2 can adsorb on the metal surface to act as a corrosion inhibitor, preventing the overall corrosion of steel due to carbonic acid. . At this time, if there is stress, a step is formed on the surface due to slip between the crystal lattices to expose the new surface, and the metal begins to dissolve the new surface, which is an anode, and the CO adsorption layer (<10) around it is a cathode [3]. Therefore, the presence of oxygen (O2) acts to exacerbate corrosion of CO2+H2O. The TS-212 partial step shape is shown in Figure 1.
(3) External influence factors of two corrosion systems: The CO2 corrosion rate is affected by many factors such as CO2 partial pressure, flow rate, temperature, protective film and solution composition.
a CO2 partial pressure: Among the various factors affecting the corrosion rate of CO2, the partial pressure of CO2 plays a decisive role, which directly affects the solubility of CO2 in corrosive media and the acidity of the solution, ie the acidity and corrosion rate of the solution. It increases with the increase of CO2 partial pressure; when CO2 partial pressure is greater than 0·2MPa, corrosion will occur, and when the partial pressure is less than 0·021MPa, the corrosion is negligible.
b Effect of flow rate: High flow rates promote corrosion because of turbulence at high flow rates and pitting under uneven conditions.
In addition, the fluid causes the damage of the CO2 corrosion product FeCO3 film at high speed, causing the metal interface to be exposed to the corrosive medium and subject to strong erosion and corrosion of the fluid.
c Effect of temperature: The effect of temperature on the corrosion rate is not only reflected in the influence of temperature on the solubility of various chemical components of the gas and composition solution, the pH value of the solution, but also the effect of temperature on the protective material. This is because the FeCO3 film formed in the CO2 corrosion reaction is protective at high temperatures, and the temperature corresponding to the protective effect of the film is called the film temperature [4].
In general, below 60 ° C, a small amount of soft and non-densified FeCO3 film is formed on the surface of carbon steel. At this time, the corrosion is uniform corrosion; at 100 ° C, the corrosion product is thick but still loose, and the corrosion rate increases. At this time, a deep pit or ring corrosion is formed; at a temperature higher than 150 ° C, corrosion can be substantially prevented by the formation of a dense and highly adherent FeCO 3 film.
Metal surfaces are subject to high flow rates and turbulent fluids, and are subject to wear and corrosion, known as wear and corrosion. Impact corrosion is the main form of wear and corrosion. Under the impact of high-speed fluid, the protective film breaks and the exposed metal at the break accelerates corrosion. If the fluid contains solid particles, the wear and corrosion is more serious. Its appearance is: local grooves, corrugations, round holes and valleys, usually showing directionality [5].
The pressure of the TS212 tube bundle shell is 0·75~0·49MPa, and the temperature is 215~130°. This operating condition is the temperature at which the water phase of the crude benzene in the process medium undergoes a violent phase transition. Due to the action of the vacuole, the wear is caused. Corrosion is very serious. The formation of cavitation is caused by the partial pressure drop caused by liquid turbulence or temperature change. The cavitation contains only a small amount of water vapor, and the existence time is very short. The shock wave pressure generated when the bubble is broken can be as high as 400 MPa, which causes the metal protective film to be destroyed and can cause Plastic deformation, even tearing metal particles. The bare metal at the break of the film is corroded, and then the film is re-formed. At the same point, a new bubble is formed and rapidly ruptured. This process is repeated, resulting in a dense and deep hole in the surface of the metal, and the appearance is rough. The bubble bursts and collapses to form a "lip" around it, or the size ripples are connected into a sheet to form a "C" shaped corrugation, see Figure 2.
As for the TS102 shell-side baffle plate, the corrosion reduction is less than 1 mm, which should be related to the phase transition and turbulence of the process medium fluid in the baffle plate. The baffles in the bubble-prone explosion and phase change parts are severely corroded. .
The scale-induced corrosion induced by fouling deposits is a corrosion failure of the TS-212 tube bundle, which is a secondary factor in the corrosion of TS212.
The corrosion mechanism of the TS-212 tube bundle tube (water side) is still depolarized corrosion of oxygen (not to be described). The flow rate of the tube is low, which increases the deposition of the above corrosion products, forming an objective condition for promoting the corrosion under scale. This is illustrated by the fact that corrosion products and scales contain iron oxides with a mass fraction of more than 70%. In the circulating water system, since Cl- has strong adsorption and permeability, it can damage the passivation film on the surface of carbon steel equipment or adsorb it on defects, causing local damage and forming pores. After the formation of the etched holes, the outside of the holes is blocked by corrosion products, internal and external convection forms a block, and an occluded battery is formed in the holes. A large number of positively charged metal ions are accumulated in the holes, in order to maintain electrical neutrality, relying on electrophoresis, negatively charged Cl -migration into the pores, the increasing concentration of Cl- increases, the H+ produced by the hydrolysis of metal ions, and the formation of hydrochloric acid with Cl-, forming strong corrosion, the internal gradual acidification of the scale core, the pH value can even reach 1, further exacerbating the corrosion process .
4 Discussion
Through the analysis of X-ray diffraction inside and outside the tube, a more abnormal problem was found, and the Cu6·26Sn5 component was detected. It is inferred that the contamination caused by cutting the sample may be small, and the detected components are very typical and regular. It is impossible to obtain such a typical and regular composition due to random sample contamination or instrument operation, so it is recommended to investigate carefully. The presence or absence of rehydrator leakage and the corrosion of the copper alloy equipment associated with the system occur. If necessary, the chemical anti-corrosion film should be treated as soon as possible to prevent the corrosion of the copper alloy equipment due to the destruction of the protective film or the corrosion of the new copper alloy equipment. ".
5 Anti-corrosion measures and recommendations
(1) reducing the flow rate of the shell-side process medium;
(2) Minimize the amount of oxygen delivered on the basis of not affecting the process operation;
(3) adding a process inhibitor or neutralizer to the system to reduce corrosion of the carbon steel matrix by H2CO3;
(4) It is conceivable to use the 08Cr2AMl o material to make the tube bundle. Studies have shown that a carbon steel matrix containing a certain amount of Cr can reduce the corrosion rate of CO2. After the low carbon steel is added with a mass fraction of 0.5% Cr, the corrosion rate of carbon steel can be reduced by more than 50% [6].
(6) It is not advisable to use aluminum-magnesium alloy tube bundles. It is possible to consider 20th carbon steel bundles without affecting heat transfer. It is not necessarily suitable to make bundles of SUS321 or SUS316L stainless steel.
(7) The Al-shell sacrificial anode block is welded to the shell-side baffle plate, and the cathodic protection can reduce the damage of the equipment caused by the cavitation corrosion;
(8) Increasing the cathodic protection of the Mg-based sacrificial anode on the water tank side of the pipe box and the floating head can also reduce the corrosion leakage of the equipment and prolong the service life of the equipment [7].
(9) Check whether the system is adjacent to the equipment, whether it is a rehydrator made of navy copper or brass. Pay attention to the comparison of the concentration of Zn2+ and Cu2+ in the condensate inlet and outlet to judge whether there is corrosion leakage in the copper alloy equipment. Measures have been taken as soon as possible, and the chemical film coating method adopted in recent years in China has achieved good results in suppressing corrosion of copper alloy equipment.
references
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2 Zhang Zhongyu, Guo Jinbao·CO2 Corrosion Law of Oil and Gas Pipeline and Research Progress at Home and Abroad[J]. Baosteel Technology, 2000, (4): 54-58
3 China Petrochemical Equipment Management Association, Equipment Anticorrosion Professional Group·Petrochemical Plant Equipment Corrosion and Protection Manual [M]·Beijing: China Petrochemical Press, 1994·3
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