Cause analysis on Corrosion of 2507 duplex stainless steel potash evaporation tank
Taking the actual use of evaporation tank in two projects designed by a certain institute as the starting point, the causes of corrosion cracking of 2507 duplex stainless steel equipment were analyzed from the use environment and manufacturing process of 2507 duplex stainless steel equipment, and then how to avoid corrosion of duplex stainless steel equipment in the process of design, manufacture and use was discussed. The results show that the service environment and manufacturing process of duplex stainless steel evaporation tank have great influence on its corrosion resistance, especially when there are welding defects and stress concentration caused by assembly residual stress, or in the environment of strong acid, high temperature and High Cl – content medium solution, the equipment is prone to stress corrosion cracking.
Evaporation tank plays an important role in potash industry, and its use directly affects energy consumption, product yield and product quality. The corrosion leakage of evaporation tank will not only affect the equipment life and production safety, but also lead to overall shutdown. In the early stage, pure carbon steel, 316L stainless steel or 316L stainless steel composite plate were mainly used in the early stage of potassium salt evaporation tank. However, due to the poor corrosion resistance of these materials, they usually need to be replaced in a few months. In the 1980s, titanium and its composite plate began to be used to manufacture potassium salt evaporation tank, and achieved good results. However, the high cost of manufacturing potassium salt evaporation tank with titanium and its composite plate restricted the wide application of the material.
In this context, duplex stainless steel and its composite plate are considered to be one of the most suitable materials in vacuum salt industry as they have good corrosion resistance and relatively low cost. Although duplex stainless steel has excellent resistance to pitting corrosion, crevice corrosion, stress corrosion and fatigue corrosion, its corrosion resistance will be reduced by processing and manufacturing. Moreover, the evaporation tank usually works under the joint action of stress and environmental medium, and the potassium salt evaporation tank of duplex stainless steel often suffers from serious corrosion damage.
In this paper, combined with the engineering project example, the corrosion causes of 2507 duplex stainless steel potassium salt evaporation tank are analyzed, and some suggestions on how to prevent corrosion are given.
Our institute has successively designed two KCl projects abroad, which are 500000 T / a (hereinafter referred to as project a) and 120000 T / a (hereinafter referred to as project B). Although different mining and production processes are used in the two projects, they both involve the evaporation process of decomposition mother liquor, and the evaporation tank adopts the forced external circulation evaporation tank (mainly including the evaporation tank body and circulation pipe, and the heating chamber is not considered here). Specifically, three effect evaporation process is adopted in project a. the tank body material of each effect evaporation tank is 2507 duplex stainless steel composite plate, and the circulating pipe material is pure 2507 duplex stainless steel. Project B adopts two-stage preheating plus one effect evaporation process, and the preheater equipment is not discussed here. The materials of evaporation tank body and circulation pipe are consistent with those of project a. The evaporation process of the two projects is shown in Figure 1.
|Brine into the tank comes from the previous process||Triple effect evaporation||
|Two effect evaporation||
|One effect evaporation||Go to the next process|
|Brine into the tank comes from the previous process||Primary preheating||
|One effect evaporation||Go to the next process|
Fig. 1 Evaporation flow charts of project A and project B
Equipment use environment
See Table 1 for the operating temperature, operating pressure, feed liquid composition and pH of the first, second and third effect evaporators in project a.
Tab.1 The application envtronment of proJect A
|Equipment||Temperature / ℃||Gauge pressure / Mpa||Feed liquid composition (mass fraction) /%||PH||Remarks|
|Discharge of first effect evaporation tank||110-145||0.2||3||8||26||＜3.5||Containing flotation agent, hydrochloric acid, acetic acid, Fe3 +, Fe2 +|
|Discharge of two effect evaporation tank||80-105||-0.08||–||–||–||–|
|Discharge of three effect evaporation tank||45-55||-0.1||5.5||6||23||4.5|
See Table 2 for operating temperature, operating pressure, feed liquid composition and PH of evaporation tank in project B.
Tab.2 The application environment Of project B
|Equipment||Temperature / ℃||Gauge pressure / Mpa||Feed liquid composition (mass fraction) /%||PH|
|Discharge of evaporation tank||110-145||0.2||4||7||23||6-7|
- 1) Due to the construction status of the project in the underdeveloped Southeast Asia, the traffic is backward, and the evaporation tank equipment diameter of the two projects is large, and the transportation is difficult. Therefore, the evaporation tank body and circulation pipe of the two projects are made on site, and only the heating chamber is manufactured in China and transported to the project site.
- 2) In the manufacturing process of circulation pipe and evaporation tank, the plate is welded after cold roll forming.
- 3) Due to the limited site construction conditions, no acid pickling and passivation treatment was carried out on the welds of dual phase steel.
After less than one month of operation, obvious cracks were found in the circulating pipe of the first effect evaporation tank of project a, especially at the weld joint. After cleaning the outer wall of the circulating pipe, serious stress corrosion cracking was even found, as shown in Fig. 2. Corrosion cracking occurred at the welding seam and near the welding seam of the contact part between the first effect evaporation tank and the liquid phase, as shown in Fig. 3; no obvious corrosion was found at the contact part between the tank body and the gas phase.
Fig. 2 Corrosion morphology of circulation pipe wall (a) and weld (b) of the 1st stage evaporation tank In project A
Fig. 3 Corrosion morphology Of the 1st stage evaporation tank Contacting With liquid Phase in Project: （a) weld；（b) near weld A
No obvious corrosion was found on the body of the secondary effect evaporation tank of project a, but obvious cracks were visible on the pipe wall and weld joint after spraying colorant and developer on the circulating pipe surface of the second effect evaporation tank, as shown in Fig. 4. The tank body and circulation pipe of three effect evaporation tank are in good condition.
After running for one year, the circulating pipe wall and weld of evaporation tank in project B are smooth and in good condition. After spraying colorant and developer on the surface, there is no visible crack and leakage phenomenon. See Fig. 5.
Fig.4 Corrosion morphology of circulation pipe wall (a) and weld（b) of the 2nd stage evaporation tank in project A (developed)
Fig.5 Internal situations of circulation pipe wall (a) and weld（b) in evaporation tank of project B
Analysis and discussion
In project a, the cracks of circulating pipes of primary and secondary effect evaporators are vertical tree cracks, and most of the cracks are concentrated in the weld seam, and the corrosion of the tank body of the first effect evaporation tank in project a is also found in the weld. These two kinds of corrosion show typical characteristics of stress corrosion cracking. Theoretically speaking, there are three main factors for stress corrosion cracking of vessels: the existence of certain tensile stress; the sensitivity of materials to stress corrosion; and the existence of media that can cause stress corrosion of metals.
The 2507 duplex stainless steel used in the evaporation tank of the two projects is the same batch product of a German company, so the influence of material difference can be excluded. The stress corrosion cracking phenomenon mentioned above can only be caused by the existence of medium which can cause stress corrosion of metal (related to the service environment of evaporation tank) and the existence of tensile stress (related to the manufacturing process of evaporation tank).
Influence of evaporation tank on Environment
The operating temperature of evaporation tanks of project a and project B is close, the maximum operating temperature is not more than 150 ℃, the low temperature is 110 ℃ and not lower than 80 ℃. There is no overtemperature phenomenon during the operation. In actual use, it is found that the service temperature of three effect evaporation tank of project a is 45 ~ 55 ℃, and the operating temperature is relatively low.
The operation pressure of evaporation tank in two projects is not high. The maximum operating pressure of the first effect evaporation tank of project a is 0.2MPa, and the minimum operating pressure of the second effect evaporation tank is -0.08mpa under vacuum condition; the maximum operating pressure of evaporation tank of project B is 0.2MPa. One effect of two projects
The main steam pipe of evaporation tank is equipped with safety valve to avoid overpressure.
The main compositions of the two projects are basically the same, including K +, Na +, Mg2 +, Cl – and a small amount of Ca2 +. The total ion content in the feed liquid is close, and the Cl – content in the feed liquid is relatively high, more than 23% (mass fraction)
More than. However, the feed liquid of project a contains a small amount of flotation agent, hydrochloric acid and acetic acid brought from the previous process, and the pH adjustment device is not installed according to the design requirements, which directly leads to the low pH of feed liquid of project a. The pH value of feed liquid of project a is about 4.5, and the acidity is strong, which is more than 100 times of that of project B. Due to the higher content of Cl -, Mg2 + in feed solution of project a, MgCl2 will react with water to generate HCl at high temperature, as shown in formula (1) and formula (2). Therefore, the HCl content in the mother liquor of the first effect evaporation tank in project a is more than that in project B.
In addition, acetic acid and Fe3 + and Fe2 + are also contained in the feed solution of project a. Acetic acid is a kind of strong organic acid. The presence of Cl – in acetic acid will lead to pitting corrosion. When the concentration of Cl – reaches 1mol / L, the pitting corrosion resistance of 2507 duplex stainless steel decreases obviously. At the same time, the sampling inspection of feeding liquid of project a shows that the mass fraction of Fe3 + and Fe2 + reaches 0.1810% and 0.1355% respectively. This is mainly due to the carbon steel equipment in the previous process, and the mother liquor is stored in the underground pool with high iron content for a long time.
It can be seen that the maximum operating temperature and pressure of project a and project B are almost the same, the main components of the feed liquid are basically similar, and the total ion concentration of the feed liquid is also very close, but the acidity of the feed liquid in project a is much higher than that in project B, and there are acetic acid, Fe3 + and Fe2 + in project a.
In conclusion, high temperature, high content of Cl – and strong acid environment are the main reasons for corrosion. In the high content CL environment, with the decrease of pH, the surface film of the duplex stainless steel is destroyed, and its corrosion resistance gradually decreases, and the stress corrosion cracking is easy to occur. The existence of high content of Cl – in acetic acid will lead to pitting corrosion. The existence of Fe3 + and Fe2 + makes 2507 duplex stainless steel easy to electrochemical corrosion, which aggravates the corrosion.
Manufacturing process impact
The corrosion of the first effect evaporation tank and the second effect evaporation tank of project a mainly occurs at the weld joint, so the corrosion should also be related to the processing and manufacturing process of the evaporation tank. Specifically:
- 1) The welding quality of duplex stainless steel equipment is closely related to the welder’s technical level and whether the welding process of dual phase steel is correctly mastered. At the same time, the selection of welding materials, the control of welding line energy and interlayer temperature are very important to ensure whether the welding joint has the same performance as the base metal. In addition, due to the long production cycle of evaporation tank, the production site is in the rainy season, and the rainstorm often falls suddenly, and the humidity and temperature change at any time. These will cause adverse effects on welding quality, such as weld roughness, burning loss, phase transformation, slag inclusion and so on.
- 2) Although the operation is convenient, the forming force is large and the strength of duplex stainless steel is high, so it is easy to produce cold work hardening and surface defects induced cracking. When the circulating pipe and the evaporation tank are assembled, a large external force will be exerted, resulting in a large assembly residual stress. Therefore, the duplex stainless steel equipment needs to be properly heat treated to eliminate the stress after the fabrication. The temperature and time of heat treatment should be strictly controlled, otherwise the toughness and corrosion resistance of the material will be affected by the precipitation of brittle phase, especially the local corrosion resistance.
- 3) After the evaporation tank is manufactured, the welds of 2507 duplex stainless steel and its composite plate on the inner wall of the evaporation tank shall be cleaned, and then acid pickling passivation or coating with professional pickling paste shall be carried out. After the requirements are met, the passivation residues shall be removed by washing with clean water. However, due to the lack of on-site construction conditions and no treatment, the surface of 2507 duplex stainless steel and its welded joint lost the protection of passive film, resulting in a significant reduction of its corrosion resistance.
How to avoid corrosion
In the initial stage of design, the most severe working condition of the equipment should be confirmed, and materials should be selected reasonably or adjusted to the allowable range of materials according to this working condition. For vessels under internal pressure, the equipment shell is mainly subjected to tensile stress. Therefore, in order to reduce and control the stress corrosion, it is necessary to control the working stress and residual tensile stress of the vessel; for vessels under external pressure, the stress state after the combination of stress caused by external pressure and residual tensile stress should be considered. In structural design, stress concentration should be avoided as far as possible. For equipment with large temperature difference during installation and use, the stress caused by temperature difference should be reduced as much as possible.
Because duplex stainless steel is not widely used in China, and the relevant manufacturing experience is relatively lack, so in the manufacturing process, such as rolling, welding, assembly, there should be perfect construction guidance, including perfect cold working process, welding process, reasonable post weld heat treatment process, etc. Since the production and installation of evaporation tank are all outdoor construction, in order to avoid the influence of humidity on welding quality, it is recommended to avoid construction in rainy season, which is also conducive to the operation and installation of construction personnel.
Pickling and passivation is the last line of defense to ensure the safe operation of evaporation tank. It is necessary to overcome the limitation of production site conditions and conduct acid pickling and passivation treatment timely and comprehensively according to the requirements, so as to form a uniform protective film on the surface of duplex stainless steel and strengthen the anti-corrosion ability of the surface and welded joint of the duplex stainless steel.
During the operation of duplex stainless steel equipment, the equipment shall be operated in strict accordance with the design parameters, so as to avoid over temperature and overpressure. In the environment of strong acid and Cl -, the stress corrosion resistance of duplex stainless steel is greatly reduced. Therefore, it is necessary to take real-time sampling to measure the pH of the feed liquid with low pH, and take reasonable measures to neutralize the acidity of the feed liquid, and then enter the evaporation tank after passing the test.
Strictly control the content of Fe3 + and Fe2 + in the feeding liquid to avoid Fe pollution and electrochemical corrosion on the surface of duplex stainless steel.
Since the stress corrosion cracking occurs in two sensitive potential regions of activation passivation and passivation over passivation, the occurrence of stress corrosion can be prevented by controlling the potential.
If necessary, methods such as adding corrosion inhibitor, promoting film-forming, preventing hydrogen or harmful substances from adsorption can be adopted according to the project situation to prevent the occurrence of stress corrosion.
The service environment and manufacturing process of duplex stainless steel evaporation tank have a great influence on its corrosion resistance, especially when there are welding defects and assembly residual stress which cause stress concentration, or when it operates in the environment of strong acid, high temperature and high content of Cl – medium solution, the equipment is prone to stress corrosion cracking.
Master the material characteristics of duplex stainless steel, and avoid the corrosion of duplex steel from the aspects of design, manufacture and use. In China, the corrosion resistance of two-phase steel will be widely used in large-scale evaporation environment.
Author: Yang Xiaofeng, Chen Weilai
Source: China Evaporation Tank Manufacturer – www.secmachinery.com
www.secmachinery.com is one of the leading china filtration equipment & sanitary stainless steel pressure vessels manufacturer, with professional factory. We focus on the sanitary pressure vessels, filtration equipment research and development, manufacturing, sales and service since the company been established. Our products is widely used on bio-pharmaceutical, food and beverage, fine chemical industry.
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