CN109338447B - Stainless steel super-corrosion-resistant treatment method based on electrochemical removal of surface inclusions - Google Patents

Abstract

The invention belongs to the field of material surface treatment, and provides a stainless steel super-corrosion-resistant treatment method based on electrochemical removal of surface inclusions: (1) cleaning to remove oil, and pickling and passivating to obtain a pretreated surface; (2) placing the pretreated surface in NaNO with the concentration of 4-10 mol/L and the temperature of 40-60 DEG C₃In the solution, a multi-period potential step control method is adopted to remove impurities on the surface of the pretreatment to thicken the passive film, so that the protection of the passive film is improved; (3) washing with water, drying, and storing. The invention adopts a method of combining chemical passivation of nitric acid and multicycle potential step control of sodium nitrate, and realizes the ultrahigh corrosion resistance of the stainless steel at high temperature and high salinity by a comprehensive means of removing impurities by electrochemistry and thickening a passivation film. The passivation solution and the electrochemical passivation method used by the invention are simple and convenient and easy to operate, and can meet the harsh requirements of stainless steel on corrosion resistance in a high-temperature and high-salt environment.

Description

An ultra-corrosion-resistant treatment method for stainless steel based on electrochemical removal of surface inclusions

technical field

The invention belongs to the field of material surface treatment, and relates to an electrochemical treatment method for removing inclusions on the surface of stainless steel and improving the corrosion resistance of a passivation film.

Background technique

430, 443, 304 and other stainless steels have become widely used materials in marine engineering, nuclear engineering, aerospace, medicine, high-speed rail and other industries due to their low price, good corrosion resistance and machinability. The quality of the passive film on the surface and the physical and chemical properties of the inclusions are the keys to affecting the corrosion resistance of stainless steel. In engineering, passivation methods are often used to strengthen the quality of the surface passivation film to improve the corrosion resistance of stainless steel.

Traditional passivation methods include nitric acid-based chemical passivation methods and other electrochemical passivation methods in solution. Nitric acid, mixed reagents of nitric acid and hydrofluoric acid, mixed reagents of nitric acid and sodium dichromate, etc. are the basic reagents commonly used in chemical passivation at present, which can increase the content of chromium-rich oxides in the dense passivation film on the surface of stainless steel, thereby improving corrosion resistance. . The electrochemical method forms a dense passivation film on the surface by methods such as potentiostatic polarization, carrier passivation, cyclic voltammetry or galvanostatic passivation in a relatively low concentration of sodium sulfate solution or nitric acid solution. The dense passivation film obtained by the above passivation method can greatly improve the corrosion resistance of the stainless steel and meet the corrosion resistance requirements of the stainless steel in the conventional environment.

However, for stainless steel serviced in high temperature, high salt, harsh atmospheric service environment or high temperature and high concentration halogen environment in the South China Sea, a single passivation method cannot meet the corrosion resistance requirements; when the ambient temperature is higher than 40 degrees, the passivated iron Elemental and austenitic stainless steels still experience significant pitting corrosion. From the point of view of the formation principle of pitting corrosion, surface inclusions are the preferred location for pitting corrosion initiation. How to combine the inclusion removal process with the existing passivation process to provide an environmentally friendly and ultra-corrosion-resistant solution is one of the ways to expand the application range of 430, 443, 304 and other stainless steels and manufacture high-performance stainless steel parts in harsh service environments. one.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to overcome the fact that the corrosion resistance of stainless steel after conventional passivation technology cannot meet the corrosion resistance requirements of high temperature and high salinity harsh environment, and realize the stainless steel through the comprehensive means of electrochemical removal of inclusions and passivation film thickening. Super corrosion resistance at high temperature and high salinity.

In order to achieve the above object, the technical scheme adopted in the present invention is as follows:

A stainless steel super-corrosion-resistant treatment method based on electrochemical removal of surface inclusions, comprising the following steps:

(1) After degreasing and cleaning the stainless steel surface, it is placed in a conventional pickling passivation solution for passivation treatment to obtain a pretreated surface.

(2) Remove the pretreatment surface inclusions to thicken the passivation film and improve the protection of the passivation film.

The passivation pretreatment sample in step (1) is used in a 4-10 mol/L NaNO ₃ solution to remove inclusions and thicken the passivation film by using a multi-period potential step control method to improve the protection of the passivation film, specifically: in a In the cycle, linear anodic polarization is first used to dissolve surface inclusions and thicken the passivation film, and its potential control range is +0.5V _SSE ~ 0.8V _SSE ; then control the potential cathodic polarization to dissolve iron oxides in the passivation film , which is conducive to the dissolution of inclusions from the passivation film, and its cathodic polarization potential is selected between -0.1V _SSE ~ -0.8V _SSE ; finally, cycle the above cycle to 10min ~ 3h; the single cycle time is 15s ~ 30s. The temperature of the NaNO ₃ solution is 40-60°C.

(3) The stainless steel after the inclusions are removed in step (2) is rinsed with clean water, dried and stored to avoid being seriously scratched.

In the step (2), the passivation pretreatment sample in the step (1) can also be used in a 4-10 mol/L NaNO ₃ solution to remove inclusions and thicken the passivation film by a multi-cycle electrochemical pulse method, so as to improve the passivation film. Protection, specifically: in one cycle, the anode potential range of the pulse method is between +0.5V _SSE ~ 0.8V _SSE , the cathode potential is between -0.1V _SSE ~ -0.8V _SSE , and the above cycle is cycled to 10min ~ 3h; the single cycle time is 15s～30s. The temperature of the NaNO ₃ solution is 40-60°C.

In the step (2), the ratio of the time of the linear anodic polarization potential to the time of the cathodic polarization is 3-10.

In the step (1), the objects to be treated are ferritic stainless steel and austenitic stainless steel.

The counter electrode used in the multi-period potential step control method or the multi-period electrochemical pulse method is one or more combinations of duplex stainless steel, titanium plate or graphite.

Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention first adopts a traditional passivation method to achieve a relatively complete passivation film; and then according to the electrochemical treatment in the high _- concentration NaNO solution, not only the passive film is increased The content of chromium oxide is medium, and the inclusions that are prone to pitting corrosion on the surface can be removed. The passivation solution and the electrochemical passivation method used in the present invention are simple and easy to operate, and can meet the strict requirements on the corrosion resistance of stainless steel in a high temperature and high salt environment.

Description of drawings

FIG. 1 is a graph of potentiodynamic polarization curves of 304 stainless steel in Example 1 after different passivation treatments in 3.5 wt.% NaCl solution.

Figure 2 is a graph of the critical pitting temperature of 304 stainless steel after passivation by Example 1 and standard nitric acid at different NaCl concentrations.

Detailed ways

The present invention will be further described below with reference to specific embodiments.

In the present invention, taking 304 stainless steel as an example, different passivation treatment methods and specific implementation steps are given in detail by the following examples. 304 stainless steel was selected as the experimental material, and the stainless steel wire was cut into 50mm x 50mmx5mm test pieces, which were polished to 1500# by sandpaper.

Example 1 Multi-period potential step control method to remove inclusions

The solution used in the inventive method is: 20% nitric acid by mass, 5mol/ _L NaNO solution, the water used for configuring the solution is deionized water, the mass percentage concentration of the chemical reagent nitric _acid used is 68%, and NaNO is an analytical solution. pure reagent.

Use different passivation treatment steps in above-mentioned embodiment 1 as follows:

(1) Put the sample into a 20% nitric acid solution at 50°C for 0.5h passivation and then wash;

(2) The cleaned sample was subjected to multi-cycle potential step treatment in 5 mol/L NaNO ₃ solution at 50 °C, and the counter electrode was made of duplex stainless steel. The single-cycle electrochemical treatment included the following three stages: first, the anode The step potential was linearly polarized from 0.5V _SSE to 0.8V _SSE , and the treatment time was 17s; then the potential was reduced to -0.5V _SSE , and the cathodic polarization was treated for 5s; finally, the step was stepped to the initial potential of 0.5V _SSE , and the treatment time was 2s. The cycle time is 24s.

(3) After the above-mentioned samples were treated by the multi-cycle potential step control method for 300 cycles, they were cleaned and stored to prevent serious scratches on the surface.

The above-treated samples were subjected to potentiodynamic polarization tests in 3.5wt.% NaCl solution at 25°C and 60°C, and compared with the samples passivated by 25wt.% nitric acid recommended by ASTM standard. The results are shown in Figure 1. (Fig. 1 shows the potentiodynamic polarization curves of 304 stainless steel in 3.5 wt.% NaCl solution at 25°C and 60°C after passivation by Example 1 and standard nitric acid). The results show that in the NaCl solution, the samples treated by the two methods have good corrosion resistance at 25 ℃, and their breaking potentials are in the overpassivation potential range; when the temperature rises to 60 ℃, the conventional nitric acid passivation The pitting corrosion potential of the treated sample is lower than 0.3V _SSE ; however, through the sample of the present invention, under this high temperature condition, its breaking potential is close to 1.2V _SSE , and the rupture of its passivation film belongs to overpassivation dissolution, indicating that it is processed by this embodiment. The samples have good pitting resistance in high temperature solutions.

Example 2 Multi-period potential step control method to remove inclusions

The solutions used in the inventive method are: a nitric acid solution with a mass percentage of 20%, a NaNO solution with a concentration of ₅ mol/L, the water used for configuring the solution is deionized water, the mass percentage concentration of the chemical reagent nitric acid used is 68%, and the NaNO ₃ is analytical reagent.

Use different passivation treatment steps in above-mentioned embodiment 2 as follows:

(1) A part of the sample was passivated in 20% nitric acid solution at 50°C for 0.5h, and ultrasonically cleaned with deionized water;

(2) The sample was subjected to 300 cycles of potential step treatment in a 5mol/L NaNO ₃ solution at 40°C: the counter electrode was a titanium plate, the anode step potential was from 0.5V _SSE to 0.8V _SSE , and the treatment time was 17s , the cathode potential is -0.5V _SSE , the treatment time is 5s, and the final step is to the initial potential of 0.5V _SSE , and the holding time is 2s, and the time of a single cycle is 24s.

After 304 stainless steel was passivated by nitric acid and then treated by sodium nitrate multi-cycle potential step control, the critical pitting temperature measurement was carried out in 0.6mol/L and 5mol/L NaCl solutions, respectively. The heating rate of the critical pitting temperature measurement was 1 ℃/min, the applied constant potential is 0.75V relative to the open circuit potential; the detection results show that: in 0.6mol/L NaCl solution, the critical pitting temperature of nitric acid passivation is 56℃; after the passivation method of the present invention is processed, Even after the temperature of the NaCl solution was raised to 95 °C, the current density did not reach the critical pitting current density value, and no obvious pitting was found. In 5mol/L NaCl solution, the critical pitting temperature of nitric acid passivation is lower than 36°C; for the samples treated by the passivation method of the present invention, the critical pitting temperature in 5mol/L NaCl solution is increased to above 70°C, Still has good corrosion resistance.

Example 3 Multi-period potential step control method to remove inclusions

A stainless steel super-corrosion-resistant treatment method based on electrochemical removal of surface inclusions, comprising the following steps:

(1) After degreasing and cleaning the stainless steel surface of the treatment object, it is placed in a 50° C., 5wt.% sulfuric acid solution for passivation treatment to obtain a pretreated surface.

(2) Remove the pretreatment surface inclusions to thicken the passivation film and improve the protection of the passivation film

The sample was subjected to 20 cycles of potential step treatment in 10mol/L NaNO ₃ solution at 60°C: graphite was used as the counter electrode, the anode step potential was from 0.5V _SSE to 0.8V _SSE , the treatment time was 20.5s, and the cathode potential was It is -0.1V _SSE , the processing time is 7.5s, the final step is to the initial potential of 0.5V _SSE , and the hold is 2s, and the single cycle time is 30s.

(3) The stainless steel after the inclusions are removed in step (2) is rinsed with clean water, dried and stored to avoid being seriously scratched.

Example 4 Multi-cycle potential step control method to remove inclusions

A stainless steel super-corrosion-resistant treatment method based on electrochemical removal of surface inclusions, comprising the following steps:

(1) After degreasing and cleaning the stainless steel surface of the treatment object, it is placed in a 50° C., 5wt.% sulfuric acid solution for passivation treatment to obtain a pretreated surface.

(2) Remove the pretreatment surface inclusions to thicken the passivation film and improve the protection of the passivation film

The samples were subjected to 720 cycles of potential step treatment in 5 mol/L NaNO ₃ solution at 50 °C: the anode step potential was from 0.5V _SSE to 0.8V _SSE , the treatment time was 11.6s, and the cathode potential was -0.8V _SSE , the processing time is 1.4s, the final step is to the initial potential of 0.5V _SSE , the hold is 2s, and the single cycle time is 15s.

(3) The stainless steel after the inclusions are removed in step (2) is rinsed with clean water, dried and stored to avoid being seriously scratched.

Example 5 Multi-period potential step control method to remove inclusions

A stainless steel super-corrosion-resistant treatment method based on electrochemical removal of surface inclusions, comprising the following steps:

(1) After degreasing and cleaning the stainless steel surface of the treatment object, it is placed in a 50° C., 5wt.% sulfuric acid solution for passivation treatment to obtain a pretreated surface.

(2) Remove the pretreatment surface inclusions to thicken the passivation film and improve the protection of the passivation film

The samples were subjected to 360 cycles of potential step treatment in 5mol/L NaNO ₃ solution at 40°C: the anode step potential was from 0.5V _SSE to 0.8V _SSE , the treatment time was 25.3s, and the cathode potential was -0.5V _SSE , the processing time is 2.7s, the final step is to the initial potential of 0.5V _SSE , the hold is 2s, and the single cycle time is 30s.

(3) The stainless steel after the inclusions are removed in step (2) is rinsed with clean water, dried and stored to avoid being seriously scratched.

Example 6 Multi-cycle potential step control method to remove inclusions

A stainless steel super-corrosion-resistant treatment method based on electrochemical removal of surface inclusions, comprising the following steps:

(1) After degreasing and cleaning the stainless steel surface of the treatment object, it is placed in a 50° C., 5wt.% sulfuric acid solution for passivation treatment to obtain a pretreated surface.

(2) Remove the pretreatment surface inclusions to thicken the passivation film and improve the protection of the passivation film

The sample was subjected to 40 cycles of potential step treatment in 5mol/L NaNO ₃ solution at 60°C: the anode step potential was from 0.5V _SSE to 0.8V _SSE , the treatment time was 9.25s, and the cathode potential was -0.5V _SSE , the processing time is 3.75s, the final step is to the initial potential 0.5V _SSE , the hold is 2s, and the single cycle time is 15s.

(3) The stainless steel after the inclusions are removed in step (2) is rinsed with clean water, dried and stored to avoid being seriously scratched.

Example 7 Multi-cycle electrochemical pulse method to remove inclusions

A stainless steel super-corrosion-resistant treatment method based on electrochemical removal of surface inclusions, comprising the following steps:

(1) After degreasing and cleaning the stainless steel surface of the treatment object, it is placed in a 50° C., 20% nitric acid solution for passivation treatment to obtain a pretreated surface.

(2) Remove the pretreatment surface inclusions to thicken the passivation film and improve the protection of the passivation film

The samples were subjected to 300 cycles of electrochemical pulse treatment in 5 mol/L NaNO ₃ solution at 50 °C: the anode pulse potential was controlled from 0.5V _SSE to 0.8V _SSE , the treatment time was 17s, the cathode potential was -0.5V _SSE , and the treatment The time is 5s, the final control pulse potential is 0.5V _SSE , the hold is 2s, and the single cycle time is 24s.

(3) The stainless steel after the inclusions are removed in step (2) is rinsed with clean water, dried and stored to avoid being seriously scratched.

Example 8 Multi-cycle electrochemical pulse method to remove inclusions

A stainless steel super-corrosion-resistant treatment method based on electrochemical removal of surface inclusions, comprising the following steps:

(1) After degreasing and cleaning the stainless steel surface of the treatment object, it is placed in a 50° C., 5wt.% sulfuric acid solution for passivation treatment to obtain a pretreated surface.

(2) Remove the pretreatment surface inclusions to thicken the passivation film and improve the protection of the passivation film

The samples _were treated with 20 cycles of electrochemical pulses in a 5mol/L NaNO3 solution at 60 °C: the anode pulse potential was controlled from 0.5V _SSE to 0.8V _SSE , the treatment time was 20.5s, the cathode potential was -0.1V _SSE , The processing time is 7.5s, the final control pulse potential is 0.5V _SSE , the hold is 2s, and the single cycle time is 30s.

(3) The stainless steel after the inclusions are removed in step (2) is rinsed with clean water, dried and stored to avoid being seriously scratched.

Example 9 Multi-cycle electrochemical pulse method to remove inclusions

A stainless steel super-corrosion-resistant treatment method based on electrochemical removal of surface inclusions, comprising the following steps:

(1) After degreasing and cleaning the stainless steel surface of the treatment object, it is placed in a 50° C., 5wt.% sulfuric acid solution for passivation treatment to obtain a pretreated surface.

(2) Remove the pretreatment surface inclusions to thicken the passivation film and improve the protection of the passivation film

The samples _were treated with 360 cycles of electrochemical pulses in 5mol/L NaNO3 solution at 40°C: the anode pulse potential was controlled from 0.5V _SSE to 0.8V _SSE , the treatment time was 25.3s, the cathode potential was -0.8V _SSE , The processing time is 2.7s, the final control pulse potential is 0.5V _SSE , the hold is 2s, and the single cycle time is 30s.

(3) The stainless steel after the inclusions are removed in step (2) is rinsed with clean water, dried and stored to avoid being seriously scratched.

The technical scheme of the present invention can overcome that the corrosion resistance of 430, 443 and 304 stainless steels after conventional passivation technology cannot meet the corrosion resistance requirements of high temperature and high salinity harsh environments, and the inclusions can be removed through electrochemical removal and the passivation film is thickened. The comprehensive means can realize the super corrosion resistance of stainless steel under high temperature and high salinity.

The above-mentioned embodiments only represent the embodiments of the present invention, but should not be construed as limiting the scope of the present invention. It should be pointed out that for those skilled in the art, without departing from the concept of the present invention , several modifications and improvements can also be made, which all belong to the protection scope of the present invention.

Claims (5)

1. A stainless steel super corrosion resistant treatment method based on electrochemical removal of surface inclusions is characterized by comprising the following steps:

(1) after degreasing and cleaning the surface of the stainless steel, putting the stainless steel into a conventional pickling passivation solution for passivation to obtain a pretreated surface;

(2) removing impurities on the surface of the pretreatment to thicken the passive film and improving the protection of the passive film;

placing the passivation pretreatment sample obtained in the step (1) in NaNO₃The method for removing the impurities in the solution by adopting a multi-period potential step control method specifically comprises the following steps: in one period, linear anode polarization is adopted firstly to dissolve surface inclusions and thicken a passive film, and the potential control range of the passive film is +0.5V_SSE～0.8V_SSE(ii) a Then controlling the cathode polarization of the potential to dissolve the iron oxide in the passive film, which is beneficial to the dissolution of the inclusion from the passive film, wherein the cathode polarization potential is selected to be-0.1V_SSE～-0.8V_SSETo (c) to (d); finally, the period is circulated to 10 min-3 h; the time of the single period is 15-30 s;

the NaNO₃The concentration of the solution is 4-10 mol/L, and the temperature is 40-60 ℃;

(3) and (4) washing the stainless steel with the impurities removed in the step (3) with clear water, drying and storing.

2. The method for super corrosion resistance treatment of stainless steel based on electrochemical removal of surface inclusions as claimed in claim 1, wherein the step (2) is to place the passivation pretreatment sample of the step (1) in NaNO₃The method for removing the impurities in the solution by adopting a multi-period electrochemical pulse method instead of a multi-period potential step control method comprises the following steps: in one period, the anode potential of the pulse method is in the range of +0.5V_SSE～0.8V_SSEBetween the cathode potential and-0.1V_SSE～-0.8V_SSEThe period is circulated to 10 min-3 h; the time of the single period is 15-30 s.

3. The method for super corrosion resistance treatment of stainless steel based on electrochemical removal of surface inclusions according to claim 1, wherein the ratio of time of linear anodic polarization potential to time of cathodic polarization in step (2) is 3-10.

4. The method of claim 1, wherein the counter electrode used in the potential step control method is one or more of duplex stainless steel, titanium plate or graphite.

5. The method of claim 2, wherein the counter electrode used in the electrochemical pulse method is one or more of duplex stainless steel, titanium plate or graphite.

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