CN111141670A - On-line monitoring and regulating method for chloride molten salt corrosion - Google Patents
On-line monitoring and regulating method for chloride molten salt corrosion Download PDFInfo
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Abstract
The invention discloses an online monitoring and regulating method for chloride molten salt corrosion. Which comprises the following steps: obtaining a polarization curve of the working electrode in molten salt to be tested under different addition amounts of metal magnesium by a corrosion electrochemical test method; determining a corrosion control index: including corrosion potential, corrosion current density and linear polarization resistance; detecting the corrosion potential, the corrosion current density and the linear polarization resistance of the molten salt to be detected on line; and (3) carrying out online monitoring on the corrosivity of the molten salt to be detected, and when the corrosion index (corrosion potential, corrosion current density or linear polarization resistance) of online detection does not meet the molten salt corrosion control index, adding a proper amount of metal magnesium into the molten salt to be detected to enable the index of the online detection to reach the corrosion control index range, thereby realizing online monitoring and regulation of the molten salt corrosion to be detected.
Description
Technical Field
The invention relates to an online monitoring and regulating method for chloride molten salt corrosion, in particular to an online monitoring and regulating method for alkali metal molten salt corrosion containing magnesium chloride.
Background
The chloride fused salt formed by mixing alkali metal or alkaline earth metal chloride has the characteristics of good stability, high heat capacity and heat conductivity, wide working temperature range and the like, and is a solar heat collection candidate medium with high competitiveness. Researchers have conducted a great deal of research work on chloride molten salts with different compositions from the aspects of molten salt physical properties, thermal cycle stability, material compatibility, comprehensive economy and the like, and have screened out a chloride molten salt system with excellent comprehensive performance. Wherein, MgCl2NaCl-KCl fused salt (molar percentage 45.4% -33.0% -21.6%, hereinafter abbreviated to "MNKC" for the fused salt system) has a lower melting point (385 ℃), a higher heat capacity (1.180 J.g)-1·K-1) Lower cost ($ 325/ton, $ 4.95/KWh) has attracted the attention of many researchers. The low melting point has important significance in engineering, on one hand, the working temperature range of a molten salt medium can be improved, on the other hand, the possibility of freezing and blocking of a molten salt pipeline can be reduced, and the overall reliability of the heat collecting system is improved.
The problem of compatibility of molten salt materials is one of the most concerned problems of researchers, and is one of the core problems of success of solar heat collection systems based on molten salt media. Although the intrinsic corrosion of chloride to various high-temperature alloys is very low, even lower than that of fluoride to metals, the corrosion caused at high temperature is very serious in practice due to inevitable existence of water and oxygen impurities in chloride molten salt, especially under the condition of poor oxygen partial pressure control in the environment. Even under an inert atmosphere, corrosion due to impurity driven in chloride salts is not negligible. The presence of molten salt impurities is even thought to be the most important cause of aggravation of chloride or fluoride corrosion. Therefore, molten salt purification is an important means of molten salt corrosion control.
The potential of the molten salt is another important parameter in determining the corrosion rate of the molten salt to the alloy. The potential of the molten salt is determined by the composition of the molten salt, impurities and the environment. The molten salt can be kept at a lower potential by purifying the molten salt, adding redox buffer ion pairs or controlling the composition of molten salt cover gas, thereby achieving the purpose of corrosion control.
The metal magnesium has certain solubility in the chloride molten salt. Although the magnesium metal is added to carry out corrosion prevention on the magnesium chloride-containing molten salt in the prior art, the corrosion inhibition effect of magnesium on the molten salt fluctuates along with time due to the strong volatility of magnesium at high temperature, so that the online monitoring and regulation are required to be carried out, and the low corrosion of the molten salt is kept. At present, the situation of online monitoring and control of molten salt corrosion of a chloride molten salt system containing metal magnesium is not existed in the prior art. Offline sampling for analyzing the magnesium concentration is theoretically feasible, but the actual operation is difficult and the cost is high, the original condition of the molten salt sample cannot be represented after the molten salt sample is taken out and contacted with the environment, the analysis period is long, the online monitoring is not achieved, and the method is practically basically not considered.
On-line monitoring for other molten salt systems is also rare. Chinese patent document CN102235966A discloses a system and method for online monitoring molten salt corrosion, which mainly uses a plurality of electrodes to work cooperatively to obtain the molten salt corrosion condition on the monitored gas turbine, and is not specific to a specific molten salt system.
This problem is urgently needed to be solved.
Disclosure of Invention
The invention provides an on-line monitoring and regulating method for chloride molten salt corrosion, which aims to overcome the defects of high difficulty and difficult implementation of the on-line monitoring of the molten salt corrosion in the prior art.
The corrosion potential, the corrosion current density and the linear polarization resistance are conventional means for detecting metal corrosion in the field of corrosion electrochemistry, and the corrosion online monitoring method has more corrosion online monitoring application in normal temperature systems such as water systems and oil systems without molten salt. The high-temperature molten salt system has limited service life (usually only dozens of hours) of the reference electrode, so that the application of the method as an online monitoring technology is restricted. At present, the quantitative relation between the corrosion potential, the corrosion current density or the linear polarization resistance of any metal simple substance or alloy and the addition amount of the metal magnesium in the chloride molten salt containing the metal magnesium is not reported.
In addition, the problems of the service life, the cost and the like of the molten salt reference electrode are also one of the bottlenecks which restrict the application of the molten salt corrosion on-line monitoring technology. For example, NiCl for boron nitride tube used in current polarization curve testing2A Ni reference electrode, a quartz tube AgCl/Ag electrode, and the like, and such reference electrodes have problems of short life (several tens of hours), high price (several hundreds of yuan), and poor long-term stability although short-term stability (within a lifetime range of several tens of hours) is acceptable. The development of the molten salt reference electrode in the prior art is a diaphragm type reference electrode route (for example, Chinese patent documents CN104090004A or CN 110186968A). The diaphragm type reference electrode is characterized by stable potential and has the defects of relatively complex structure, high cost, limited service life and incapability of working for a long time. Some developed so-called long-life reference electrodes also have difficulty meeting the time requirements for stable operation of the reference electrode for on-line monitoring. In a specific molten salt system, a specific metal material can be directly used as a molten salt reference electrode without violating the electrochemical theory, but experiments are needed to verify whether the stability is reliable or not. The inventors have surprisingly found that the use of a magnesium rod or a magnesium dynamic reference electrode as a MgCl-containing electrode2The reference electrode of the chloride molten salt system has the advantages of low price (for example, 20 yuan/Kg of magnesium and several yuan per one magnesium rod), long service life and very stable potential (for example, the reference electrode of the invention shown in figure 7 and figure 8), can effectively solve the technical problems and realize on-line detection.
The invention adopts a corrosion electrochemical method to obtain the MgCl content of the metal electrode to be measured under different magnesium adding amounts2Corrosion potential of the chloride molten salt system of (E)corr) Corrosion current density (i)corr) And linear polarization resistance (R)p) Data and the rationality of a magnesium reference electrode or a magnesium dynamic reference electrode as a molten salt reference electrode of an MNKC system is proved through experiments, and on the basis, a MgCl-containing electrode is provided2The method for monitoring and controlling the corrosion of the chloride molten salt system on line.
The invention solves the technical problems through the following technical scheme.
The invention provides an online monitoring and regulating method for chloride molten salt corrosion, which comprises the following steps:
(1) obtaining polarization curves of the working electrode in molten salt to be tested under different addition amounts of metal magnesium by a corrosion electrochemical test method, and determining a corrosion control pre-index; the working electrode is made of a metal simple substance or an alloy; the molten salt to be detected contains MgCl2The alkali metal chloride molten salt of (1), the MgCl2The content of the compound is more than 30 percent, and the percentage is relative to the mole percentage of the total amount of chloride in the molten salt to be detected; the corrosion control pre-determined indicator comprises a corrosion potential E relative to the polarization curve reference electrodecorrCorrosion current density icorrAnd a linear polarization resistance Rp；
(2) Using a magnesium reference electrode or a magnesium dynamic reference electrode as a working electrode, adopting an auxiliary electrode and a reference electrode in the polarization curve testing process, and obtaining the potential E of magnesium relative to the reference electrode in the polarization curve testing process in the molten salt to be testedcorr；
Determining a corrosion control index: the corrosion control index includes a corrosion potential Ecorrvs Mg, corrosion current density icorrAnd the linear polarization resistance Rp(ii) a The corrosion potential Ecorrvs Mg corrosion potential E of step (1) versus the polarization curve reference electrodecorr-the potential E of the magnesium during the step (2) with respect to the reference electrode during the polarization curve testcorr；
(3) Detecting the corrosion potential, the corrosion current density and the linear polarization resistance of the molten salt to be detected on line; in the online detection process, the adopted corrosion probe comprises a reference electrode, an auxiliary electrode and a metal electrode to be detected; the reference electrode is a magnesium reference electrode or a magnesium dynamic reference electrode, and the type of the metal electrode to be detected is the same as that of the working electrode in the step (1);
when the corrosion potential, the corrosion current density or the linear polarization resistance of the online detection do not meet the corrosion control index determined in the step (2), adding magnesium metal into the molten salt to be detected to enable the online detection index to be within the corrosion control index range determined in the step (2).
In the step (1), the addition amount of the magnesium metal can be (0.1 x 10)-3～1.4*10-3) g/1g of the molten salt to be tested, e.g. 0.2 x 10-3g/1g of the molten salt to be measured, 0.4 x 10-3g/1g of the molten salt to be measured, 0.6 x 10-3g/1g of the molten salt to be measured, 0.8 x 10-3g/1g of the molten salt to be measured, 1 x 10-3g/1g of the molten salt to be tested or 1.2 x 10-3g/1g of the molten salt to be detected.
In a preferred embodiment, for 300g of molten salt to be measured, the addition amount of magnesium metal can be 300 × 10 (100-1400) ×-6g (i.e., 0.03g to 0.42g), such as 0.06g, 0.12g, 0.18g, 0.24g, 0.3g, or 0.36 g.
In the step (1), the metal simple substance or alloy may be any kind of solid metal simple substance or alloy which is in a solid state all the time in the steady-state polarization curve test process, for example, 316H stainless steel.
In the step (1), the diameter of the metal simple substance or the metal alloy can be conventional in the art, and is preferably 0.5-1.5 mm, such as 1 mm.
In step (1), the molten salt to be tested may be conventional MgCl in the art2Conventional alkali chloride molten salts having a molar content of greater than 30%, except for MgCl2Besides, the molten salt to be tested can also comprise one or more of LiCl, NaCl and KCl, and MgCl is preferred2-NaCl-KCl mixed molten salt, more preferably the MgCl2In a mixed molten salt of-NaCl-KCl, MgCl2The content of (A) is 45.4%, the content of NaCl is 33.0%, and the content of KCl is 21.6%, wherein the percentages are respectively the mole percentages of the components relative to the total amount of chloride in the molten salt.
In the step (1), the molten salt to be measured generally needs to be dried before use. The drying operation and conditions may be conventional and are generally carried out in a shaft furnace. The drying temperature is preferably 200-300 ℃, for example 250 ℃. The drying time is preferably 36h or more, for example 48 h.
In step (1), those skilled in the art know that polarization curves of the working electrode in molten salt to be tested can be obtained by conventional electrochemical testing methods in the art under different magnesium addition amounts. The means for detecting the polarization curve may be conventional in the art, such as a three-electrode electrochemical test system, preferably employing a Switzerland AutoLab 302N type electrochemical workstation, and its own Nova2.1 software.
In the test process of the polarization curve, the adopted auxiliary electrode can be a conventional auxiliary electrode which is not influenced by a system to be tested in the field, and preferably a glassy carbon rod or a platinum rod. The auxiliary electrode is typically 6mm in size.
In the test process of the polarization curve, the adopted reference electrode is a diaphragm type reference electrode commonly used in the field, such as a NiCl hot-pressed boron nitride tube2a/Ni reference electrode or a quartz glass tube AgCl/Ag reference electrode, preferably a hot-pressed boron nitride tube NiCl2a/Ni reference electrode. The hot-pressed boron nitride tube NiCl2the/Ni reference electrode comprises: the device comprises a nickel wire electrode, a reference electrode tube and a sealing plug for sealing the reference electrode tube; the bottom of the reference electrode tube is provided with reference salt, the nickel wire is fixed on the sealing plug, and one end of the nickel wire passes through the sealing plug to extend into the reference electrode tube and is immersed in the reference salt; wherein the reference electrode tube is made of a hot-pressing boron nitride tube, and the reference salt is NiCl2Molten salt and mixed molten salt of the molten salt to be detected, wherein NiCl is contained in the mixed molten salt2The molar ratio of the molten salt is 8-12%, and the percentage is the molar ratio relative to the total amount of the chloride in the mixed molten salt. The thickness of the hot-pressed boron nitride tube can be conventional in the field, and is preferably 0.5-1.5 mm, such as 1 mm. The nickel wire is typically a pure nickel wire as is conventional in the art. The hot-pressed boron nitride tube NiCl2When the/Ni reference electrode is used, one end of a nickel wire passes through the sealing plug and extends into the reference electrode tube and is soaked in the reference salt, the other end of the nickel wire is connected with a test instrument, and the nickel wire passes through the reference electrode tube(i.e., hot-pressed boron nitride tube) is contacted with the molten salt to be tested.
Wherein, the reference electrode adopted in the polarization curve test process is a hot-pressed boron nitride tube NiCl2When the/Ni reference electrode is used, in the corrosion control prefabrication index in the step (1), the corrosion potential corresponding to the polarization curve reference electrode is recorded as Ecorrvs NiCl2and/Ni. Marking the potential of the magnesium obtained in the step (2) relative to a reference electrode in the polarization curve testing process as EMgvs NiCl2and/Ni. In the corrosion control index of step (2), corrosion potential Ecorrvs Mg＝Ecorrvs NiCl2/Ni-EMgvs NiCl2/Ni。
The scanning speed of the polarization curve is generally 0.001-0.010V/s, and preferably 0.005V/s. In the polarization curve testing process, the temperature of the molten salt to be tested can be selected according to common knowledge in the field, and is generally higher than the melting temperature and lower than the decomposition temperature, and is smaller than the melting point 648.8 ℃, preferably 450-648 ℃, for example 600 ℃ of the magnesium metal.
And reading the corrosion potential of the metal simple substance or the alloy in the molten salt to be detected under different magnesium addition amounts through the polarization curve, and drawing a curve of the corrosion potential of the metal simple substance or the alloy in the molten salt to be detected along with the magnesium addition amount. And respectively obtaining the corrosion current density of the metal simple substance or the alloy in the molten salt to be detected or the change curve of the linear polarization resistance along with the addition amount of magnesium by combining a Tafel epitaxy method carried by Nova2.1 software and a linear polarization resistance calculation function. Based on the change curve of the corrosion potential with the magnesium addition amount, the change curve of the corrosion current density with the magnesium addition amount, and the change curve of the linear polarization resistance with the magnesium addition amount, a person skilled in the art can determine the corrosion potential, the corrosion current density and the linear polarization resistance under a certain metal magnesium addition amount as corrosion pre-control indexes of the system according to common knowledge.
In a preferred embodiment of the present invention, when the working electrode used in the corrosion test is 316H stainless steel and the reference electrode is a hot pressed boron nitride tube NiCl2When used as a Ni reference electrode, the magnesium metal is selected to be added in an amount of 0.5 to 10-3g/1g corrosion potential of molten salt to be measuredCorrosion current density and linear polarization resistance as corrosion control pre-indicators at 600 deg.C, specifically corrosion potential Ecorrvs NiCl2/Ni<-0.80V vs NiCl2Ni, corrosion Current Density icorr<25μA/cm2Linear polarization resistance Rp>800Ω·cm2。
In step (2), the magnesium reference electrode may be a reference electrode of pure magnesium material conventionally used in the art, such as a magnesium rod. The specification of the magnesium reference electrode can be conventional in the art, and the diameter is preferably 5-10mm, and more preferably 8 mm.
In the step (2), the magnesium Dynamic reference electrode (Dynamic reference electrode) generally refers to a material obtained by in-situ deposition of a magnesium plating layer on an electrode material other than magnesium.
The electrode material may be of the kind conventional in the art, and nickel or molybdenum is preferred. The diameter of the electrode material is preferably 5-10mm, and more preferably 8 mm.
Wherein the thickness of the magnesium plating layer is preferably 10 to 150 μm.
In the step (2), the magnesium dynamic reference electrode is preferably prepared by the following steps: and (2) taking a nickel rod or a molybdenum rod as a cathode and a glassy carbon or platinum rod as an anode, carrying out constant current deposition in the molten salt to be detected, and depositing a magnesium coating on the cathode in situ, wherein the molten salt to be detected contains metal magnesium. If the reference electrode is a magnesium dynamic reference electrode, a metal magnesium coating is obtained in an electrochemical deposition mode every time a test is carried out.
And (3) in the molten salt to be tested, the content of the metal magnesium is preferably the same as the addition amount of the metal magnesium in the polarization curve testing process in the step (1). The operation and conditions of the deposition may be conventional in the art. In the deposition process, the current density is preferably 0.05-0.2A/cm2E.g. 0.1A/cm2. The deposition time is preferably 5-15 min, for example 10 min. In the deposition process, the temperature of the molten salt is preferably 450-648 ℃, for example 600 ℃.
In step (2), the potential E of the magnesium relative to a reference electrode in the polarization curve test processcorrIn the test process, the test operation and conditions are the same as those of the polarization curve test process in the step (1). Such as the temperature of the molten salt to be measured.
In a specific embodiment, the boron nitride tube NiCl is relatively hot pressed by magnesium in MNKC molten salt at 600 DEG C2Potential E of/Ni reference electrodeMgvs NiCl2The value of/Ni is-1.74V.
In a preferred embodiment of the present invention, when the working electrode used in the corrosion test is 316H stainless steel, the corrosion potential, the corrosion current density and the linear polarization resistance at the magnesium metal addition of 500ppm are selected as the corrosion control indexes at 600 ℃, specifically the corrosion potential Ecorrvs Mg<0.94V, corrosion current density icorr<25μA/cm2Linear polarization resistance Rp>800Ω·cm2. Wherein E iscorrvs Mg＝Ecorrvs NiCl2/Ni-EMgvs NiCl2/Ni＝-0.80-(-1.74)＝0.94V
Once the corrosion control index is determined, its accuracy can also be verified by immersion corrosion testing methods that are conventional in the art. Preferably according to the following steps: and placing the working electrode in the molten salt to be tested, wherein the molten salt to be tested contains the addition amount of metal magnesium corresponding to the corrosion control index, preserving the heat for at least more than 100h at the temperature in the polarization curve testing process, and if the weight loss rate of the working electrode before and after soaking is less than 0.2% and no metal is corroded under an electron microscope, the corrosion control index is proved to be reasonable.
In step (3), the online detection device may be a portable electrochemical analyzer or a corrosion tester with polarization curve testing function, which is conventional in the art. If an electrochemical analyzer is adopted for online detection, after a polarization curve is tested, the corrosion potential, the corrosion current density and the polarization resistance can be obtained through calculation by a Tafel epitaxy method and a linear polarization resistance function carried by Nova2.1 software. If the corrosion monitor is adopted for online detection, the principle is consistent with that of an electrochemical analyzer, but the automation level is higher, and the corrosion potential, the corrosion current density and the polarization resistance can be directly read without manual calculation through software.
In the step (3), when the magnesium dynamic reference electrode is used as the reference electrode, in order to avoid the influence of electrochemical reaction products on adjacent electrodes on the stability of the reference electrode and the polarization curve test of the working electrode, the layout of three electrodes in the corrosion probe is preferably as follows: the reference electrode and the metal electrode to be detected are parallel and located at the upstream of the flowing direction of the molten salt to be detected, and the auxiliary electrode is located at the downstream of the flowing direction of the molten salt to be detected.
In the step (3), the auxiliary electrode may be a conventional auxiliary electrode which is not affected by the system to be measured in the field, and is preferably a glassy carbon rod or a platinum rod.
In the step (3), the size of the metal electrode to be measured can be conventional in the art, and the diameter is preferably 2-6mm, and more preferably 4 mm. In the invention, in the on-line detection process, the material of the pipeline or the equipment through which the molten salt to be detected flows is generally the same as that of the metal electrode to be detected. For example, when the metal electrode to be measured is 316H stainless steel, the material of the pipe through which the molten salt to be measured flows is also 316H stainless steel.
In the step (3), different metal electrodes to be measured can be replaced according to actual conditions, and the corrosion condition of the metal electrodes to be measured in the molten salt to be measured is measured.
In the step (3), the magnesium reference electrode is the same as the magnesium reference electrode in the step (2). In the step (3), the magnesium dynamic reference electrode is the same as the magnesium dynamic reference electrode in the step (2).
In the step (3), when the corrosion potential, the corrosion current density or the linear polarization resistance which are detected on line do not meet the corrosion control index determined in the step (2), the corrosion potential of the on-line test is larger than the corrosion potential E of the corrosion control index determined in the step (2)corrvs Mg, and the corrosion current density of the online test is larger than the corrosion control index determined in the step (2) and the corrosion current density icorrAnd, the linear polarization resistance of the online test is larger than the linear polarization resistance R of the corrosion control index determined in the step (2)p。
In step (3), the operation and conditions for adding the metallic magnesium may be conventional in the art. The addition amount or the addition frequency of the metal magnesium can be judged according to actual conditions, for example, in the actual experiment process, the addition amount of the metal magnesium is not determined, the metal magnesium can be slowly added until the online detection index is qualified, the metal magnesium can also be added once, and after the online detection index is adjusted to be close to the corrosion control index, the metal magnesium is added in a small amount until the online detection index is qualified.
On the basis of the common knowledge in the field, the above preferred conditions can be combined randomly to obtain the preferred embodiments of the invention.
The reagents and starting materials used in the present invention are commercially available.
The positive progress effects of the invention are as follows:
the invention discloses an online monitoring and regulating method for molten salt corrosion, which comprises the steps of firstly obtaining polarization curves of metal simple substances or alloys in molten salt to be tested under different magnesium addition amounts by an electrochemical test method, and determining corrosion control indexes comprising corrosion potential, corrosion current density and linear polarization resistance; then, a magnesium rod or a magnesium dynamic reference electrode is taken as a reference electrode, and the reference electrode, an auxiliary electrode and a metal electrode to be detected jointly form a corrosion probe which is arranged in a molten salt system to be detected; and (3) carrying out online monitoring on the corrosivity of the molten salt to be detected, and when the corrosion index (corrosion potential, corrosion current density or linear polarization resistance) of online detection does not meet the molten salt corrosion control index, adding a proper amount of metal magnesium into the molten salt to be detected to enable the index of the online detection to reach the corrosion control index range, thereby realizing online monitoring and regulation of the molten salt corrosion to be detected.
Drawings
FIG. 1 is a steady state polarization curve of 316H stainless steel in MNKC-Mg molten salt system at 600 ℃ with different magnesium addition amounts.
FIG. 2 shows the corrosion potential E of 316H stainless steel in MNKC-Mg molten salt system at 600 DEG Ccorrvs NiCl2/Ni、Ecorrvs. Mg change curves with the addition of Mg.
FIG. 3 is a graph showing the corrosion current density of 316H stainless steel in MNKC-Mg molten salt system as a function of the addition of magnesium at 600 ℃.
FIG. 4 is a curve of linear polarization resistance of 316H stainless steel in MNKC-Mg molten salt system with the addition of magnesium at 600 ℃.
FIG. 5 is a cross-sectional electron micrograph of a 316H stainless steel sample soaked in MNKC molten salt system for 100H at 600 ℃.
FIG. 6 is a cross-sectional electron micrograph of a 316H stainless steel sample soaked in MNKC-500ppm Mg molten salt system for 100H at 600 ℃.
FIG. 7 shows the relative NiCl of magnesium rod electrode in MNKC molten salt at 600 deg.C2Potential versus time curve of the/Ni reference electrode.
FIG. 8 is a graph showing the effect of the polarization current of the MNKC molten salt system on the potential of a magnesium rod electrode at 600 ℃.
FIG. 9 is a graph of the change of the potential of a magnesium dynamic reference electrode relative to a pure metal magnesium rod electrode over time.
Fig. 10 is a structural diagram of online corrosion monitoring of MNKC molten salt system 316H stainless steel, wherein the arrow direction is the molten salt flow direction.
Fig. 11 is a schematic diagram of a three-electrode layout of the corrosion probe, wherein 1 is an auxiliary electrode, 2 is a working electrode, 3 is a reference electrode, and the arrow direction is the molten salt flowing direction.
Detailed Description
The invention is further illustrated by the following examples, which are not intended to limit the scope of the invention. The experimental methods without specifying specific conditions in the following examples were selected according to the conventional methods and conditions, or according to the commercial instructions.
The combination of steady polarization curve and Tafel epitaxy method is a common test means for corrosion electrochemistry, and can quickly obtain the corrosion potential (E) of metal or alloy in corrosion mediumcorr) Corrosion current density (i)corr) And linear polarization resistance (R)p). Taking 316H stainless steel structural material preferentially considered by an MNKC molten salt system as an example, firstly, the steady-state polarization curve of the 316H stainless steel at 600 ℃ in the MNKC molten salt system is obtained by the method under different magnesium addition amounts. Obtaining the corrosion potential E of the 316H stainless steel in a corrosion medium according to a steady-state polarization curvecorrCorrosion current density icorrAnd a linear polarization resistance RpData are related to the amount of magnesium metal added.
Example 1
A device for testing a molten salt corrosion electrochemical steady-state polarization curve is disclosed in patent publications CN102553664A and CN108956744A, and mainly comprises a glove box and a shaft furnace, wherein the atmosphere in the furnace is communicated with the atmosphere in the glove box, the glove box is an inert atmosphere glove box with a self-purification function, and the sum of water and oxygen in the atmosphere in the box is controlled to be below 10 ppm. 300g of analytically pure MgCl were used before testing2And (3) mixing NaCl-KCl molten salt (the mol percentage is 45.4% -33.0% -21.6%), placing the mixture into a corundum crucible, drying the mixture in a well type furnace for 48 hours at the drying temperature of 250 ℃, and then heating and melting the mixed salt and keeping the temperature at the testing temperature. The three electrodes in the steady-state polarization curve test are a 316H stainless steel working electrode (phi 1mm), a glass carbon rod (phi 6mm) auxiliary electrode and NiCl2a/Ni reference electrode. The reference electrode is: a pure nickel wire electrode, a reference electrode tube and a sealing plug for sealing the reference electrode tube; the bottom of the reference electrode tube is provided with reference salt, the nickel wire is fixed on the sealing plug, and one end of the nickel wire passes through the sealing plug to extend into the reference electrode tube and is immersed in the reference salt; wherein the reference electrode tube is a hot-pressed boron nitride tube with a thickness of 1mm, and the reference salt is NiCl2Mixed molten salt of molten salt and molten salt to be detected, wherein NiCl is contained in the mixed molten salt2The molar ratio of the molten salt was 10%, and the above percentages are molar ratios relative to the total amount of chlorides in the mixed molten salt. In the test for inspecting the electrode potential of the metal magnesium rod, the working electrode is the magnesium rod (phi 8mm), and the auxiliary electrode and the reference electrode are the same as those in the steady-state polarization curve test. The electrochemical measuring device adopted by the invention is a Switzerland Wantong AutoLab 302N type electrochemical workstation and Nova2.1 software carried by the workstation.
(1) Obtaining corrosion potential, corrosion current density and linear polarization resistance:
method for testing polarization curve of 316H stainless steel in MNKC molten salt: the MNKC molten salt temperature is kept at 600 ℃, the diameter of a 316H stainless steel wire to be detected is 1mm, the wire is immersed into the molten salt for 3cm, and the 316H stainless steel working electricity is measuredPolar and NiCl2The open circuit potential OCP between the/Ni reference electrodes is then scanned from a potential 0.15V lower than the OCP potential to a potential 0.15V higher than the OCP potential at a scanning speed of 5mV/s, a steady polarization curve of the 316H stainless steel in MNKC molten salt is obtained, and the measurement is repeated for 3-5 times by replacing the working electrode of the 316H stainless steel until a polarization curve with good reproducibility is obtained. Then taking out the three measuring electrodes, adding 200ppm, 400ppm, 600ppm, 800ppm, 1000ppm and 1200ppm of metal magnesium powder (ppm represents mass percent, and for 300g of MNKC molten salt, 200ppm is that 300g multiplied by 200 multiplied by 10-60.060g Mg), and the above measurement procedure was repeated after stirring and holding for 1 hour to obtain polarization curves of 316H stainless steel in MNKC molten salt at different magnesium addition amounts (as shown in fig. 1, 0, 200, 400, 600, 800, 1000, 1200 in fig. 1 represent the addition amounts of magnesium as 0ppm, 200ppm, 400ppm, 600ppm, 800ppm, 1000ppm, 1200ppm, respectively). From the polarization curve of FIG. 1, the corrosion potential of 316H stainless steel in MNKC molten salt with different magnesium addition amounts is recorded as E in FIG. 2corrvs NiCl2Ni; corrosion potential (E) of the reference electrode of FIG. 2 relative to magnesiumcorrvs Mg) data is obtained by conversion calculation, and the potential ratio of magnesium to NiCl2the/Ni is 1.74V lower (FIG. 7), so Ecorrvs Mg＝EcorrvsNiCl2and/Ni + 1.74. Meanwhile, by utilizing a Tafel epitaxy method and a linear polarization resistance calculation function carried by Nova2.1 software, the corrosion current density and the linear polarization resistance of the 316H stainless steel in MNKC molten salt with different magnesium addition amounts can be obtained from the polarization curve of figure 1 (figures 3 and 4).
As can be seen from FIGS. 1 to 4, the corrosion potential of 316H stainless steel in MNKC molten salt gradually decreases with the increase of the addition amount of magnesium; the corrosion current density is first greatly reduced and then slightly increased at a lower level; the polarization resistance of the 316H stainless steel is firstly greatly increased and then slightly reduced along with the increase of the addition amount of magnesium. From the analysis of corrosion electrochemistry, when the corrosion current density and the polarization resistance under a certain corrosion potential are measured to be lower, the metal is proved to have low electrochemical reaction activity in the molten salt and the corrosion reaction is in a suppression state. Thus, the corrosion characteristics indicators described above may be applied to monitor the corrosivity of the molten salt. For example, when the amount of magnesium metal added exceedsAt 500ppm, the corrosion potential of 316H stainless steel in MNKC molten salt system is lower than-0.80V (vs NiCl)2Ni), corrosion current density lower than 25 muA/cm2Linear polarization resistance higher than 800 omega cm2It is shown that MNKC molten salt exhibits good material compatibility with 316H stainless steel.
(2) The immersion corrosion test method comprises the following steps:
in the same device of the electrochemical test, 300g of MNKC molten salt is newly prepared, dried and melted for standby application according to the same method. Polishing 3 pieces of 316H stainless steel sheets (20mm multiplied by 7mm multiplied by 2mm) by 400-mesh, 800-mesh and 1200-mesh silicon carbide abrasive paper in sequence for brightening, washing and drying by distilled water, immersing in 600 ℃ and 300g of MNKC molten salt, preserving heat for 100H, taking out and weighing, and observing the microscopic appearance of the cross section surface of the sample (figure 5). To 300g of MNKC molten salt, 500ppm of metallic magnesium was added, the above operation was repeated, and the surface morphology of the cross section of the 316H stainless steel after the addition of the metallic magnesium was observed (FIG. 6).
As can be seen from fig. 5-6, the MNKC molten salt added with 500ppm of magnesium metal has no substantial corrosion effect on the 316H stainless steel, and the surface of the sample is smooth and undamaged (fig. 6), while the 316H stainless steel without magnesium metal under the same conditions loses 3.1mg (accounting for 0.18% of the weight of the sample) after being soaked for 100H, and the surface of the 316H stainless steel has obvious corrosion pits.
Therefore, the corrosion potential E of the 316H stainless steel sample in MNKC molten salt can be determinedcorr<-0.80V vs NiCl2[ Ni ] corrosion Current Density icorr<25μA/cm2Linear polarization resistance Rp>800Ω·cm2As the basis for corrosion regulation. Namely, when the corrosion characteristic index of the 316H stainless steel sample in the MNKC molten salt is monitored to be not capable of meeting the requirement on line, a proper amount of metal magnesium can be added to enable the metal magnesium to reach the standard, so that the corrosion of the molten salt to a metal material is inhibited. Other conditions or other material corrosion control indicators may be obtained in the same manner.
(3) Reference electrode of the chloride molten salt system AgCl/Cl or NiCl used in the test of the invention2the/Ni reference electrode has a complex structure and short service life, and does not meet the requirement of long-time continuous work required by on-line monitoring. In the invention, a metal magnesium rod with a simple structure is directly adopted as molten salt reference electricity for an MNKC molten salt systemThe extreme possibilities were experimentally verified. Based on electrochemical theory, the metal magnesium rod and the large amount of Mg in the MNKC molten salt2+Can form a good reversible electrode and has a theoretical basis as a reference electrode of the system. In practical application, only consideration is needed to be given to whether molten salt impurities or certain polarization current can affect the stability of the reference electrode potential.
The implementation method of the stability and sensitivity investigation test of the magnesium rod reference electrode comprises the following steps: in the same device of the electrochemical test, 300g MNKC molten salt is newly prepared according to the same method, dried and melted for standby. The stability of the electrode potential of the magnesium rod adopts an open circuit potential-time (OCP-t) curve measuring method to record the electrode potential of the magnesium rod relative to NiCl within 3 hours2Potential variation curve of/Ni reference electrode (FIG. 7). Sensitivity test adopts a chronopotentiometry method to apply 10 seconds of 40mA/cm to the working electrode of the magnesium rod respectively2The anodic and cathodic polarization currents, the open circuit potential before and after the polarization process and the electrode potential during the polarization process were recorded (fig. 8).
FIG. 7 shows the relative NiCl in MNKC molten salt at 600 ℃ of a magnesium rod electrode2The potential of the/Ni reference electrode is plotted against time, and it can be seen from the graph that the potential of the magnesium rod is very stable (-1.74V vs NiCl)2and/Ni), the potential is only increased by 9mV in 3h observation, the potential average drift rate is 3mV/h, and the requirement of corrosion electrochemical test is completely met.
FIG. 8 is a graph showing the change of the influence of the polarization current of the MNKC molten salt system on the potential of the magnesium rod electrode at 600 ℃. As shown in FIG. 8, the potential of the magnesium rod can be rapidly restored to a stable value of-1.74V even after a certain current polarization. Therefore, magnesium rods are directly adopted in an MNKC molten salt system to replace AgCl/Cl or NiCl which have complex structure, short service life and high cost2the/Ni reference electrode is feasible.
(4) Preparing a magnesium dynamic reference electrode:
another way to obtain a magnesium reference electrode is to obtain a magnesium metal coating by electrochemical deposition on other metallic materials and to maintain the same potential as a pure magnesium rod for several minutes, during which time it is sufficient to perform a corrosion measurement as a reference electrode, which is commonly referred to as a magnesium dynamic reference electrode.
Taking a pure nickel rod (phi 8mm) as a cathode and a glassy carbon or platinum rod as an anode, and carrying out constant current deposition in MNKC molten salt at the temperature of 600 ℃, wherein the current density is controlled to be 0.1A/cm2And controlling the deposition time to be 10 minutes to obtain the magnesium dynamic reference electrode. FIG. 9 is a graph of the change of the potential of a magnesium dynamic reference electrode relative to a pure metal magnesium rod electrode over time. As can be seen from FIG. 9, the MNKC molten salt system at 600 ℃ passes through 0.1A/cm on a pure nickel rod (phi 8)2After 10 minutes of galvanostatic deposition, the potential of the nickel rod electrode was equal to that of the magnesium metal rod (i.e., 0V vs Mg) within 5 minutes, after which the potential of the nickel rod electrode was gradually increased. The stabilization time was approximately 5 minutes. The primary polarization curve test can be completed at a scan rate set of 0.005V/s for 5 minutes.
The test of the magnesium rod reference electrode and the magnesium dynamic reference electrode shows that the two electrodes can replace NiCl with complicated structure, short service life and high cost2a/Ni or AgCl/Ag reference electrode.
NiCl used for the polarization curve test of FIG. 1 in view of the potential of the magnesium reference electrode2the/Ni reference electrode has a potential difference of 1.74V, namely the corrosion potential corresponding to 316H stainless steel is 1.74V higher than the original test value when the magnesium or magnesium dynamic reference electrode is used as the reference electrode (as shown in figure 2). Therefore, when corrosion monitoring is carried out by taking magnesium or a magnesium dynamic reference electrode as a reference electrode, the corrosion potential of 316H in 600 ℃ MNKC molten salt is controlled to be below 0.94V vs Mg (corresponding to-0.80V vs NiCl)2/Ni). The other control indicators need not be changed.
(5) The online monitoring device and the online monitoring and regulating method of the molten salt corrosion comprise the following steps:
fig. 10 shows a schematic diagram of an online corrosion monitoring device, in which a 316H stainless steel electrode, a magnesium rod reference electrode or a magnesium dynamic reference electrode (obtained after plating magnesium on a nickel rod), a glass carbon rod or a platinum rod auxiliary electrode for corrosion monitoring are packaged in a corrosion probe through high-temperature-resistant insulating ceramic, and are respectively connected with a portable electrochemical analyzer or a corrosion tester with a polarization curve testing function through leads for corrosion monitoring. In the manner shown in fig. 10 and 11, the corrosion probe is mounted on a mounting base on an actual running MNKC molten salt system (such as a molten salt pipeline) through a sealing flange and self-locking screws.
In the test, if a magnesium rod reference electrode is used, the measurement can be directly carried out. If the magnesium dynamic reference electrode is used, the dynamic reference electrode needs to be subjected to pre-polarization treatment, namely, a metal magnesium coating is obtained in an electrochemical deposition mode every time the test is carried out, namely, the concentration of the metal magnesium coating is 0.1A/cm2And depositing for 10 minutes under constant current density to prepare a magnesium dynamic reference electrode, and then carrying out corrosion test on the magnesium dynamic reference electrode. In order to avoid the influence of the anode product on the measuring electrode during the pre-polarization treatment, when the corrosion probe is installed, the auxiliary electrode should be located at the downstream of the molten salt flowing direction, and the working electrode and the reference electrode are arranged at the upstream of the molten salt flowing direction in parallel (fig. 11, wherein 1 is the auxiliary electrode, 2 is the working electrode, and 3 is the reference electrode).
Taking the corrosion control of 316H stainless steel in 600 ℃ MNKC molten salt as an example, when the corrosion potential of the 316H stainless steel is higher than 0.94V vs Mg and the corrosion current density is higher than 25 muA/cm2Linear polarization resistance RpLess than 800 omega cm2) When the method is used, metal magnesium is required to be gradually added into a molten salt system to enable the related monitoring value to reach the expected index.
While specific embodiments of the invention have been described above, it will be appreciated by those skilled in the art that this is by way of example only, and that the scope of the invention is defined by the appended claims. Various changes and modifications to these embodiments may be made by those skilled in the art without departing from the spirit and scope of the invention, and these changes and modifications are within the scope of the invention.
Claims (10)
1. The chloride molten salt corrosion on-line monitoring and regulating method is characterized by comprising the following steps:
(1) obtaining polarization curves of the working electrode in molten salt to be tested under different addition amounts of metal magnesium by a corrosion electrochemical test method, and determining a corrosion control pre-index; the working electrode is made of a metal simple substance or an alloy; the above-mentionedThe molten salt to be tested contains MgCl2The alkali metal chloride molten salt of (1), the MgCl2The content of the compound is more than 30 percent, and the percentage is relative to the mole percentage of the total amount of chloride in the molten salt to be detected; the corrosion control pre-determined indicator comprises a corrosion potential E relative to the polarization curve reference electrodecorrCorrosion current density icorrAnd a linear polarization resistance Rp；
(2) Using a magnesium reference electrode or a magnesium dynamic reference electrode as a working electrode, adopting an auxiliary electrode and a reference electrode in the polarization curve testing process, and obtaining the potential E of magnesium relative to the reference electrode in the polarization curve testing process in the molten salt to be testedcorr；
Determining a corrosion control index: the corrosion control index includes a corrosion potential Ecorrvs Mg, corrosion current density icorrAnd the linear polarization resistance Rp(ii) a The corrosion potential Ecorrvs Mg corrosion potential E of step (1) versus the polarization curve reference electrodecorr-the potential E of the magnesium during the step (2) with respect to the reference electrode during the polarization curve testcorr；
(3) Detecting the corrosion potential, the corrosion current density and the linear polarization resistance of the molten salt to be detected on line; in the online detection process, the adopted corrosion probe comprises a reference electrode, an auxiliary electrode and a metal electrode to be detected; the reference electrode is a magnesium reference electrode or a magnesium dynamic reference electrode, and the type of the metal electrode to be detected is the same as that of the working electrode in the step (1);
when the corrosion potential, the corrosion current density or the linear polarization resistance of the online detection do not meet the corrosion control index determined in the step (2), adding magnesium metal into the molten salt to be detected to enable the online detection index to be within the corrosion control index range determined in the step (2).
2. The chloride molten salt corrosion on-line monitoring and controlling method of claim 1, wherein in the step (1), the addition amount of the metal magnesium is (0.1 x 10)-3～1.4*10-3) g/1g of the molten salt to be detected;
and/or in the step (1), the simple metal or alloy is 316H stainless steel;
and/or in the step (1), the diameter of the working electrode is 0.5-1.5 mm;
and/or, in step (1), except MgCl, in the molten salt to be tested2Besides, one or more of LiCl, NaCl and KCl can be included;
and/or, in the step (1), drying the molten salt to be detected before use;
and/or, in the step (1), the auxiliary electrode adopted in the test process of the polarization curve is a glassy carbon rod or a platinum rod.
3. The chloride molten salt corrosion on-line monitoring and controlling method of claim 2, characterized in that in the step (1), the addition amount of the metal magnesium is 0.2 x 10-3g/1g of the molten salt to be measured, 0.4 x 10-3g/1g of the molten salt to be measured, 0.6 x 10- 3g/1g of the molten salt to be measured, 0.8 x 10-3g/1g of the molten salt to be measured, 1 x 10-3g/1g of the molten salt to be tested or 1.2 x 10-3g/1g of the molten salt to be detected;
and/or, in the step (1), the diameter of the working electrode is 1 mm;
and/or in the step (1), the molten salt to be detected is MgCl2-NaCl-KCl mixed molten salt, preferably said MgCl2In a mixed molten salt of-NaCl-KCl, MgCl2The content of (A) is 45.4%, the content of NaCl is 33.0%, and the content of KCl is 21.6%, wherein the percentages are mole percentages of the components relative to the total amount of chloride in the molten salt;
and/or the drying temperature is 200-300 ℃;
and/or the drying time is more than 36 h.
4. The chloride molten salt corrosion on-line monitoring and controlling method of claim 1, characterized in that in the step (1), the reference electrode adopted in the polarization curve testing process is hot-pressed boron nitrideTube NiCl2a/Ni reference electrode or a quartz glass tube AgCl/Ag reference electrode, preferably a hot-pressed boron nitride tube NiCl2a/Ni reference electrode;
and/or in the step (1), in the test process of the polarization curve, the scanning speed is 0.001-0.010V/s;
and/or in the step (1), in the polarization curve testing process, the temperature of the molten salt to be tested is higher than the melting temperature and lower than the decomposition temperature, and is lower than the melting point 648.8 ℃ of the metal magnesium.
5. The chloride molten salt corrosion on-line monitoring and controlling method of claim 4, characterized in that in the step (1), the hot-pressed boron nitride tube NiCl is adopted2the/Ni reference electrode comprises: the device comprises a nickel wire electrode, a reference electrode tube and a sealing plug for sealing the reference electrode tube; the bottom of the reference electrode tube is provided with reference salt, the nickel wire is fixed on the sealing plug, and one end of the nickel wire passes through the sealing plug to extend into the reference electrode tube and is immersed in the reference salt; wherein the reference electrode tube is made of a hot-pressing boron nitride tube, and the reference salt is NiCl2Molten salt and mixed molten salt of the molten salt to be detected, wherein NiCl is contained in the mixed molten salt2The molar ratio of the molten salt is 8-12%, and the percentage is the molar ratio relative to the total amount of chlorides in the mixed molten salt; wherein the thickness of the hot-pressing boron nitride tube is preferably 0.5-1.5 mm;
and/or, in the step (1), during the test of the polarization curve, the scanning speed is 0.005V/s;
and/or in the step (1), in the polarization curve testing process, the temperature of the molten salt to be tested is 450-648 ℃.
6. The chloride molten salt corrosion on-line monitoring and controlling method of claim 1, wherein when a working electrode adopted by a corrosion test is 316H stainless steel, and a reference electrode is a hot-pressing boron nitride tube NiCl2When used as a Ni reference electrode, the magnesium metal is selected to be added in an amount of 0.5 to 10-3g/1g corrosion potential, corrosion current density and linear polarization electricity of molten salt to be detectedThe corrosion resistance is used as a pre-index for corrosion control at 600 ℃, specifically corrosion potential Ecorrvs NiCl2/Ni<-0.80V vs NiCl2Ni, corrosion Current Density icorr<25μA/cm2Linear polarization resistance Rp>800Ω·cm2；
When the working electrode used in the corrosion test is 316H stainless steel, the addition amount of the metal magnesium is selected to be 0.5 to 10-3The corrosion potential, corrosion current density and linear polarization resistance of g/1g molten salt to be measured are used as corrosion control indexes at 600 ℃, specifically the corrosion potential Ecorrvs Mg<0.94V, corrosion current density icorr<25μA/cm2Linear polarization resistance Rp>800Ω·cm2。
7. The chloride molten salt corrosion on-line monitoring and regulating method of claim 1, characterized in that in the step (2), the magnesium reference electrode is a magnesium rod;
and/or, in the step (2), the diameter of the magnesium reference electrode is 5-10 mm;
and/or, in the step (2), the magnesium dynamic reference electrode is a material obtained by in-situ deposition of a magnesium coating on an electrode material except magnesium; the electrode material is nickel or molybdenum.
8. The chloride molten salt corrosion on-line monitoring and controlling method according to claim 7, characterized in that in the step (2), the diameter of the magnesium reference electrode is 8 mm;
and/or in the step (2), the diameter of the electrode material is 5-10mm, preferably 8 mm;
and/or in the step (2), in the magnesium dynamic reference electrode, the thickness of the magnesium plating layer is 10-150 μm;
and/or, in the step (2), the magnesium dynamic reference electrode is prepared by the following steps: and (2) taking a nickel rod or a molybdenum rod as a cathode and a glassy carbon or platinum rod as an anode, carrying out constant current deposition in the molten salt to be detected, and depositing a magnesium coating on the cathode in situ, wherein the molten salt to be detected contains metal magnesium.
9. The chloride molten salt corrosion on-line monitoring and regulating method of claim 8, wherein in the preparation process of the magnesium dynamic reference electrode, the content of metal magnesium in the molten salt to be tested is the same as the addition amount of metal magnesium in the polarization curve testing process in the step (1);
and/or in the deposition process, the current density is 0.05-0.2A/cm2E.g. 0.1A/cm2；
And/or the deposition time is 5-15 min, such as 10 min;
and/or the temperature of the molten salt in the deposition process is 450-648 ℃, such as 600 ℃.
10. The chloride molten salt corrosion on-line monitoring and controlling method as claimed in claim 1, characterized in that in step (3), the on-line detection equipment is a portable electrochemical analyzer or a corrosion tester with polarization curve testing function;
and/or, in the step (3), when a magnesium dynamic reference electrode is used as the reference electrode, the layout of three electrodes in the corrosion probe is as follows: the reference electrode and the metal electrode to be detected are parallel and are positioned at the upstream of the flowing direction of the molten salt to be detected, and the auxiliary electrode is positioned at the downstream of the flowing direction of the molten salt to be detected;
and/or, in the step (3), the auxiliary electrode is a glassy carbon rod or a platinum rod;
and/or, in the step (3), the diameter of the metal electrode to be measured is 2-6mm, preferably 4 mm;
and/or, in step (3), the magnesium reference electrode is the same as the magnesium reference electrode in step (2);
and/or, in step (3), the magnesium dynamic reference electrode is the same as the magnesium dynamic reference electrode in step (2).
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