A method to inhibit Cl-containing - Methods and applications of stainless steel corrosion in high-temperature and concentrated alkaline environments

Abstract

This invention relates to the field of metal material corrosion and protection technology, and discloses a method for inhibiting Cl-containing corrosion. ‑ A method for corroding stainless steel in a high-temperature, concentrated alkaline environment, including a temperature of 150℃ and Cl... ‑ In an alkaline solution with an ion concentration of 3.5 g / L and a NaOH mass fraction of 50%, the addition of chromate additives to the system enables the formation / maintenance of a stable passivation film with chromium enrichment characteristics on the stainless steel surface. This reduces the dissolution and damage to the film layer by the solution and improves the film's density and stability. The passivation film structure remains effectively stable even under harsh conditions of strong alkali and high temperature, improving the corrosion resistance and long-term operational reliability of stainless steel. It is suitable for corrosion control in alkaline processes, alkaline solution circulation systems, and high-temperature alkaline media equipment.

Description

Technical Field

[0001] This invention relates to the field of metal material corrosion and protection technology, specifically to a method for inhibiting the corrosion of Cl-containing materials. - Methods and applications of stainless steel corrosion in high-temperature, concentrated alkaline environments. Background Technology

[0002] Stainless steel relies on a surface passivation film for corrosion resistance. However, under high temperature and high concentration of NaOH media, the passivation film may dissolve, loosen, or be locally damaged, leading to accelerated corrosion and reduced equipment life. Existing corrosion inhibition methods include material upgrades, coatings, protective potential control, and organic / inorganic corrosion inhibitors. However, under strong alkaline and high-temperature conditions, corrosion inhibitors often suffer from insufficient thermal stability, rapid consumption, or limited improvement on the film structure.

[0003] Alkaline medium wet processes have developed rapidly due to their high reaction selectivity and high medium circulation efficiency. However, since this process is usually carried out in concentrated alkaline solutions (pH > 14), the high OH content in the high-temperature concentrated alkaline solutions presents challenges. - The concentration of dissolved oxides reacts with the surface oxide film to form soluble alkali metal salts. This ultimately leads to the destruction of the passivation film structure and dissolution corrosion, seriously affecting the service safety of the equipment.

[0004] 316L stainless steel is widely used in various industrial fields such as metallurgy, pharmaceuticals, chemicals, and shipbuilding due to its excellent corrosion resistance, mechanical properties, and price. However, the corrosion resistance of 316L remains challenging in high-temperature concentrated alkaline solutions. The continuous dissolution and separation of the passivation film in high-temperature concentrated alkaline solutions ultimately leads to the failure of stainless steel during long-term operation.

[0005] The development and utilization of corrosion inhibitors are crucial for protecting stainless steel. Among various corrosion inhibitors, chromates exhibit excellent corrosion inhibition properties for stainless steel, alloys, and other non-ferrous metals. Compared to other corrosion inhibitors, chromates can operate at low concentrations, making them economically viable for industrial applications. Furthermore, chromates exhibit excellent stability over a wide range of pH conditions and temperatures. In aerospace aluminum alloys, chromates are used as conversion coatings to improve the surface corrosion resistance of the material and enhance the adhesion of coatings and adhesives. Chromate coatings form a protective oxide film on the metal surface, acting as a barrier. Munsu Kim et al. found that chromates have the same inhibitory effect on the surface of metal substrates, improving the corrosion performance of chromate coatings by inhibiting oxygen reduction kinetics. Additionally, chromates can be used as corrosion inhibitors to enhance the material's resistance to pitting corrosion in highly corrosive environments. SAMRefaey observed that in Cl-containing... - In acidic solutions, chromate ions can prevent Cl- -Ions deposit on the passivation film, thereby improving the pitting corrosion resistance of stainless steel. It is generally believed that chromate ions in solution preferentially adsorb onto defects in the passivation film, forming insoluble Cr₂O₃ and Cr(OH)₃, thus preventing corrosion by aggressive ions. Other studies suggest that this competitive adsorption between chromate and other anions may form an adsorbent layer on the surface, reacting with dissolved metal cations to form insoluble salts, thereby preventing further invasion by aggressive ions.

[0006] Although the protective properties of chromates are widely recognized, most research has focused on chromates in the presence of halide ions (such as Cl-). - In terms of corrosion inhibition ability in acidic or neutral environments, in high-temperature alkaline wet processes, in addition to halide ions, high concentrations of OH- - Adsorption on the passivation film surface causes structural damage. However, the corrosion inhibition behavior of chromates on stainless steel in high-temperature, concentrated alkaline environments has not been reported. Therefore, understanding the corrosion inhibition behavior of chromates on stainless steel in high-temperature, concentrated alkaline environments is crucial. Summary of the Invention

[0007] The purpose of this invention is to provide a method for inhibiting Cl-containing... - Methods and applications for stainless steel corrosion under high temperature and concentrated alkaline conditions are proposed to address the lack of research on the corrosion inhibition behavior of chromates on stainless steel in high temperature and concentrated alkaline environments in existing technologies, which leads to unclear corrosion inhibition mechanisms and the inability to achieve film stabilization reconstruction and precise application.

[0008] Specifically, existing technologies only reveal the corrosion inhibition effect of chromates in acidic or halide-containing neutral environments, without demonstrating their ability to inhibit passivation film dissolution, promote the in-situ formation of chromium-containing stable products (such as Cr2O3 / Cr(OH)3), and resist OH in high-temperature, high-concentration NaOH solutions. - With Cl - The molecular mechanism of synergistic corrosion is unclear, and it is not clear whether it can achieve the self-healing reconstruction of the passivation film on the stainless steel surface. Furthermore, a complete technical chain from ion competitive adsorption to film structure regulation and long-term corrosion inhibition effect has not been established.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A method to inhibit Cl-containing - A method for corroding stainless steel in a high-temperature, concentrated alkaline environment, the method comprising:

[0011] (1) In Cl - Chromate additives are added to an alkaline solution with an ion concentration of 3.5 g / L and a NaOH mass fraction of 50% to form a corrosion inhibitor.

[0012] (2) Immerse the pretreated stainless steel sample completely into the corrosion inhibitor medium and keep it at 150°C for 30-60 minutes to form a stable passivation film with chromium enrichment on the stainless steel surface.

[0013] Preferably, the chromate additive is Na2CrO4.

[0014] Preferably, the concentration of the chromate additive is 0~100g / L.

[0015] More preferably, the concentration of the chromate additive is 5 to 50 g / L.

[0016] More preferably, the concentration of the chromate additive is 50 g / L.

[0017] The above methods are applied to corrosion control in alkaline processes, alkaline solution circulation systems, and high-temperature alkaline media equipment.

[0018] The present invention has the following advantages and effects compared with the prior art:

[0019] (1) The present invention can still effectively stabilize the passivation film structure under harsh conditions of strong alkali and high temperature, improve the corrosion resistance and long-term operational reliability of stainless steel, and is suitable for corrosion control of alkali process, alkali circulation system and high temperature alkali medium equipment.

[0020] (2) Chromates exhibit excellent thermochemical stability in high-temperature concentrated alkaline media and are not easily decomposed or consumed rapidly; their addition concentration can be flexibly selected in a wide range of 0~100g / L, which facilitates the optimization of corrosion inhibition effect and cost according to actual working conditions.

[0021] (3) Chromates can play an efficient corrosion inhibitory role at low concentrations and the addition method is simple. There is no need to change the main structure of the existing equipment or replace the materials. It is easy to promote and apply in alkaline wet processes in metallurgy, chemical industry, pharmaceutical industry, etc., to extend the service life of equipment and reduce maintenance costs. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0023] Figure 1 The potentiodynamic polarization curves and corrosion inhibition efficiency fitting curves of 316L stainless steel with different concentrations of Na2CrO4 added to a 50wt.% NaOH solution are provided for embodiments of the present invention; wherein, (a) is the potentiodynamic polarization curve; and (b) is the corrosion inhibition efficiency fitting curve.

[0024] Figure 2 The surface morphology of 316L stainless steel provided in the embodiments of the present invention; wherein, (a4) is the surface morphology of 316L stainless steel containing 3.5 g / L Cl - Surface morphology of 316L stainless steel soaked in 50wt.% NaOH solution with 0g / L Na2CrO4 added for 168h (b4) is 0g / L. - Surface morphology after soaking in 50wt.% NaOH solution with 50g / L Na2CrO4 for 168h;

[0025] Figure 3 The images provided in this embodiment of the invention are cross-sectional TEM images of the passivation film formed after soaking in a 50 wt.% NaOH solution without Na2CrO4 for 168 h; wherein, (a) is the HADDF image of the cross section; (b) is the elemental distribution on the cross section; (c), (d) and (e) are the elemental distributions of Fe, Cr and Ni, respectively; (f) is a magnified image and FFT image showing region A in (a); (g) is a magnified image and FFT image showing region B in (a); (h) and (i) represent EDS analysis of regions A and B in (a), respectively.

[0026] Figure 4 The images provided in this embodiment of the invention are cross-sectional TEM images of the passivation film formed after soaking in a 50 wt.% NaOH solution containing Na2CrO4 for 168 h; wherein, (a) is the HADDF image of the cross section; (b) is the elemental distribution in the cross section; (c), (d) and (e) are the elemental distributions of Fe, Cr and Ni, respectively; (f) is a magnified image of the passivation film; 1 and 2 are FFT images of (f); (H) and (I) represent the energy dispersive spectroscopy analysis of regions 1 and 2 in (f);

[0027] Figure 5 The embodiments of the present invention provide the 3D morphology and voltammetric potential difference after soaking in 50 wt.% NaOH solution without Na2CrO4 and containing Na2CrO4 for 168 h; wherein, (a) is the 3D morphology and voltammetric potential difference after soaking in 50 wt.% NaOH solution without Na2CrO4 for 168 h; (b) is the 3D morphology and voltammetric potential difference after soaking in 50 wt.% NaOH solution containing Na2CrO4 for 168 h. Detailed Implementation

[0028] The present invention will be further described below with reference to embodiments, but this is not intended to limit the present invention in any way. Any modifications or substitutions made based on the teachings of the present invention shall fall within the protection scope of the present invention.

[0029] The concentrated alkaline solution used in the following examples has the following composition by mass percentage: NaOH 50%, Cl - The concentration is 3.5 g / L, with the remainder being water.

[0030] Furthermore, the following concentrations of corrosion inhibitor Na2CrO4 were added to the above concentrated alkaline solution: 0 g / L, 5 g / L, 10 g / L, 20 g / L, 50 g / L, and 100 g / L, respectively.

[0031] Example 1

[0032] Electrochemical polarization curve test

[0033] The high-temperature concentrated alkaline solution in this embodiment has the following composition by mass percentage: 50% NaOH and the remainder is water.

[0034] In this embodiment, the concentrations of the corrosion inhibitor Na2CrO4 are 0 g / L, 5 g / L, 10 g / L, 20 g / L, 50 g / L, and 100 g / L.

[0035] The experimental solutions provided in this embodiment are as follows:

[0036] 1. The mass of NaOH is 50%, and the remainder is water;

[0037] 2. The mass of NaOH is 50%, with 5 g / L Na2CrO4, and the remainder is water;

[0038] 3. The mass of NaOH is 50%, 10 g / L Na2CrO4, and the remainder is water;

[0039] 4. The mass of NaOH is 50%, 20 g / L Na2CrO4, and the remainder is water;

[0040] 5. The mass of NaOH is 50%, which is 50 g / L Na2CrO4, and the remainder is water;

[0041] 6. The mass of NaOH is 50%, 100g / L Na2CrO4, and the remainder is water.

[0042] The composition of the 316L stainless steel sample used in this embodiment, expressed as a percentage by mass, is shown in Table 1 below:

[0043] Table 1 Composition of 316L stainless steel samples

[0044]

[0045] The experimental method in this embodiment is carried out according to the following steps:

[0046] Step (1): Cut the material into samples with dimensions of 10mm × 10mm × 3mm. Then, weld the samples to copper wires and seal them with epoxy resin. Select a 10×10mm area as the working electrode surface. Grind the electrode surface with 150~2000 grit silicon carbide sandpaper and polish it to a mirror finish with 1-micron polishing paste. After polishing, clean the samples with distilled water and ethanol and dry them in cold air.

[0047] Step (2): Set up the electrochemical system: a three-electrode system is adopted: the working electrode is the prepared 316L stainless steel sample, the reference electrode is the MnO2 / MnOOH electrode, and the auxiliary electrode is a platinum sheet; nitrogen gas is passed through the solution to remove oxygen for 30 minutes before the test.

[0048] Step (3): Potentiodynamic scanning: Before each electrochemical test, all working electrodes were pre-polarized at −1.4V (MnO2 / MnOOH) for 1 min at the cathode to remove the air passivation film, and the open circuit potential was measured (stabilized for 30-60 min). The potentiodynamic scanning rate was 1 mV / s, scanning from an initial potential approximately 0.5V below the open circuit potential towards the anode until the current showed a significant increase. To ensure accuracy, each experiment was performed at least three times.

[0049] Step (4): Obtaining the curve: The instrument automatically records the potential and current density, and plots a curve with potential as the abscissa and the logarithm of current density as the ordinate, thus obtaining the potentiodynamic polarization curves under different Na2CrO4 concentrations. The corrosion inhibition efficiency is calculated, and a corrosion inhibition efficiency fitting curve is plotted. The potentiodynamic polarization curve and the corrosion inhibition efficiency fitting curve are shown below. Figure 1 As shown. The formula for calculating corrosion inhibition efficiency is:

[0050]

[0051] In the formula, To improve corrosion inhibition efficiency, The corrosion current density is the value without Na2CrO4. The values ​​represent the corrosion current density at different concentrations of Na2CrO4.

[0052] The calculation results are shown in Table 2:

[0053] Table 2 Corrosion inhibition efficiency at different Na2CrO4 concentrations

[0054]

[0055] Results Analysis: Table 2 shows that as the Na2CrO4 concentration increases from 0 g / L to 50 g / L, the self-corrosion potential (E) of 316L stainless steel increases. corr The corrosion current density (i) gradually shifts positively from -1.27V to -0.73V. corr From 2.55×10-4 A·cm -2 Significantly decreased to 0.28×10 -4 A·cm -2 The corrosion inhibition efficiency (η) subsequently increased to 96.47%, indicating that chromate can effectively inhibit anodic reactions and promote the stability of the passivation film. When the concentration was further increased to 100 g / L, the corrosion current density slightly rebounded to 0.34 × 10⁻⁶. -4 A·cm -2 The corrosion inhibition efficiency decreased slightly to 94.51%, indicating that under the experimental conditions, 50 g / L Na2CrO4 had the best corrosion inhibition effect.

[0056] Example 2

[0057] 1. Plate hanging test

[0058] The composition and mass percentage of the 316L stainless steel sample used in this comparative example are shown in Table 1. The experimental method in this embodiment was carried out according to the following steps:

[0059] Step (1): Sample Preparation: According to the testing standards, prepare metal samples of a certain size, a total of 6 samples, and number them sequentially as 1#, 2#, 3#, 4#, 5#, and 6#. Polish the surface of all samples progressively using 150-grit, 320-grit, 600-grit, 1000-grit, and 1500-grit sandpaper, respectively. Then, polish the sample surfaces using polishing cloth and polishing agent to remove surface oxides, contaminants, etc. Measure the length (l), width (w), and height (h) of each sample using vernier calipers and calculate the surface area S. Then, accurately weigh the initial mass m1 of each sample using a micrometer balance and record the data.

[0060] Step (2): Divide the samples into groups: Samples 1#, 2#, and 3# are divided into control groups, and samples 4#, 5#, and 6# are divided into experimental groups. The control groups are immersed in a solution without corrosion inhibitor, and the experimental groups are immersed in a solution with corrosion inhibitor.

[0061] Step (3): Prepare two test solutions: The control group solution is a 50% NaOH solution with a chloride ion concentration of 3.5 g / L, without the addition of chromate corrosion inhibitor; the experimental group solution is also a 50% NaOH solution with a chloride ion concentration of 3.5 g / L, but with an additional 50 g / L Na2CrO4 as a corrosion inhibitor. Measure 1000 mL of the control group solution and 1000 mL of the experimental group solution and pour them into two high-temperature reaction vessels.

[0062] Step (4): Install the samples on the specimen fixing device of the high-temperature reactor: Samples 1#, 2#, and 3# are installed in the control group reactor, and samples 4#, 5#, and 6# are installed in the experimental group reactor. Three parallel samples are set in each group. After sealing the reactor, the temperature is maintained at 150℃, and the soaking period is 168h.

[0063] Step (5): After soaking for 168 hours, remove all samples and clean the sample surface with a cleaning solution (such as water or acetone) to remove corrosion products. After cleaning and drying, accurately weigh each sample again using the same thousandth-place balance (m2) and record the data. After the experiment, organize the data and calculate the relevant data. The calculation formula is as follows:

[0064]

[0065]

[0066]

[0067]

[0068] Where ∆m is the mass difference of the sample before and after corrosion, in g; S is the sample surface area, in m². 2 T represents corrosion time in hours (h); ν represents corrosion rate in g / m³. 2 ·h -1 Dt represents the corrosion depth in µm; D represents the annual corrosion depth in mm / year. Based on the above calculations, the corrosion resistance of the samples is evaluated. The following grading standards are typically used:

[0069] (1) D<0.1mm / a, corrosion resistance is excellent; (2) 0.1≤D<1mm / a, corrosion resistance is good; (3) 1≤D<10mm / a, corrosion resistance is average; (4) D≥10mm / a, corrosion resistance is poor.

[0070] The experimental results are shown in Table 3:

[0071] Table 3 Corrosion Rate of Samples in Comparative Examples and Examples

[0072]

[0073] Note: D' is the average annual corrosion depth; Dt' is the average corrosion depth; W' is the average corrosion rate.

[0074] Results Analysis: Based on the corrosion data calculated from the experimental data, it can be seen that after adding Na2CrO4, 316L stainless steel performs better, with a corrosion rate of about 1 mm / a and corrosion resistance rising from general to good. This indicates that adding Na2CrO4 provides good protection for 316L stainless steel in a concentrated alkaline environment.

[0075] Example 3

[0076] To further understand the effects of Na₂CrO₄ on the surface morphology, cross-sectional structure, and micropotential distribution of the passivation film on stainless steel under high-temperature concentrated alkaline conditions, this example provides 50wt.% NaOH solutions (containing 3.5g / L Cl₂) with and without 50g / L Na₂CrO₄. - The results of surface morphology, cross-section transmission electron microscopy (TEM), and Kelvin probe force microscopy (KPFM) observations of 316L stainless steel samples after immersion at 150℃ for 168h are shown in the figure.

[0077] 1. Surface morphology observation: After the experiment in Example 2 was completed, the macroscopic / microscopic morphology of the sample surface was observed using a scanning electron microscope (SEM), such as... Figure 2 As shown.

[0078] 2. Cross-sectional TEM analysis: After the experiment in Example 2, a focused ion beam (FIB) was used to cut and extract a cross-sectional thin section from the sample surface. The cross-sectional morphology of the passivation film was observed using a transmission electron microscope (TEM). Elemental distribution and crystal structure were analyzed using high-angle annular dark-field imaging (HADDF), energy dispersive spectroscopy (EDS), and fast Fourier transform (FFT), such as... Figure 3 and Figure 4 As shown.

[0079] 3. KPFM Observation: Another sample from the same batch after immersion was used to measure the three-dimensional morphology and volt-ampere potential difference of the passivation film surface using a Kelvin probe force microscope (KPFM). Figure 5 As shown.

[0080] Results analysis:

[0081] 1. From Figure 2 It can be seen that the surface corrosion was severe (pitting corrosion, peeling, etc.) when no corrosion inhibitor was added, while the surface was smoother and there was no obvious corrosion after adding 50g / LNa2CrO4, indicating that chromate effectively inhibited local corrosion.

[0082] 2. From Figure 3As can be seen from the cross-sectional TEM image of the passivation film without the addition of Na2CrO4, the passivation film has uneven thickness, loose structure, and local cracks or voids; the elemental distribution shows that the chromium content is low and the film layer lacks a stable chromium-enriched phase; FFT and EDS analysis further confirm that the film layer is mainly composed of Fe oxides and has poor compactness.

[0083] 3. From Figure 4 As can be seen from the cross-sectional TEM image of the passivation film after adding 50 g / L Na2CrO4, the film layer is uniform and dense, and has good bonding with the substrate; the elemental distribution shows that chromium is significantly enriched in the film layer, forming a continuous Cr2O3 / Cr(OH)3 protective layer; FFT and EDS analyses show that the film layer structure is stable and effectively blocks OH. - and Cl - The erosion.

[0084] 4. According to Figure 5 Analysis of the 3D morphology reveals a significant height difference on the passivation film surface in the absence of Na₂CrO₄. The highest point reaches 64.7 nm, while the highest point of the passivation film formed with added Na₂CrO₄ is only 13.6 nm. The arithmetic mean roughness Ra and root mean square roughness Rq can more intuitively represent this difference. Figure 5 As shown in (a), the average surface roughness Ra of the passivation film without Na2CrO4 is 13.1 nm, and Rq is 16.3 nm. However, when Na2CrO4 is added ( Figure 5 (b) The average surface roughness Ra of the passivation film formed decreased to 3.01 nm, and Rq decreased to 3.77 nm. The surface roughness showed significant differences in both environments, indicating that the passivation film formed with Na₂CrO₄ exhibits excellent densification and smoothness. This difference is also reflected in the surface potential difference. Comparing the voltammetric potential diagrams of the passivation films formed in the two environments, it can be seen that the passivation film formed without the addition of Na₂CrO₄ shows obvious differences in bright and dark areas. The passivation film formed after adding Na₂CrO₄ during the passivation process exhibits a uniform distribution of bright and dark areas. This phenomenon can be attributed to the fact that without the addition of Na₂CrO₄, corrosion products accumulate and aggregate, resulting in a higher potential response. Conversely, the passivation film formed after adding Na₂CrO₄ exhibits a lower potential response.

[0085] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for inhibiting Cl-containing compounds - A method for corroding stainless steel in a high-temperature, concentrated alkaline environment, characterized in that... The method includes: (1) In Cl - Chromate additives are added to an alkaline solution with an ion concentration of 3.5 g / L and a NaOH mass fraction of 50% to form a corrosion inhibitor. (2) Immerse the pretreated stainless steel sample completely into the corrosion inhibitor medium and keep it at 150°C for 30-60 minutes to form a stable passivation film with chromium enrichment on the stainless steel surface.

2. The method according to claim 1, characterized in that, The chromate additive is Na2CrO4.

3. The method according to claim 1, characterized in that, The concentration of the chromate additive is 0~100g / L.

4. The method according to claim 3, characterized in that, The concentration of the chromate additive is 5–50 g / L.

5. The method according to claim 4, characterized in that, The concentration of the chromate additive is 50 g / L.

6. The application of the method described in any one of claims 1 to 5 in corrosion control of alkaline processes, alkaline solution circulation systems, and high-temperature alkaline media equipment.

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