Passivation and pitting of 316L and HR-2 stainless steels in hydrochloric acid solution

Based on a self-designed and assembled hydrochloric acid liquid film corrosion simulation device, corrosion coupon, resistance probe, Tafel polarization and electrochemical impedance spectroscopy were used to study the corrosion behavior of 316L and HR-2 stainless steel at the concentrations of 1, 0.5 and 0.1 mol / L and temperatures of 90, 70 and 60, respectively The passivation and pitting corrosion behaviors of the samples were studied in hydrochloric acid vapor environment. The corrosion samples and products were analyzed by metallographic microscope and XRD. The results show that: the corrosion rates of the two kinds of stainless steels first accelerate, then slow down, and finally tend to be stable with time, the corrosion rate of 316L stainless steel is relatively high; the two kinds of stainless steel can form a stable passivation zone, and the difference of dimension passivation current density is not big, the passivation zone of HR-2 pitting corrosion potential is generally higher than that of 316L stainless steel, which indicates that HR-2 stainless steel is more resistant to corrosion; in addition, the surface corrosion of the two kinds of stainless steel The corrosion products on 316L stainless steel surface are more and more intensive, which is due to the adsorption of O is replaced by C1 -, the passive film on the steel surface is difficult to form or destroy, and it is more likely to cause pitting corrosion of stainless steel.

Hydrochloric acid liquid film is a kind of liquid film formed by condensation of hydrochloric acid vapor to the surface of materials, which causes faster corrosion than hydrochloric acid solution with the same concentration and temperature. Moreover, the corrosion rate is related to the concentration of hydrochloric acid and the temperature environment of the material, so the stability of the liquid film is not strong, which is difficult for electrochemical experiments. The corrosion of hydrochloric acid liquid film is equivalent to hydrochloric acid dew point corrosion. The gases causing dew point corrosion generally include SO2, SO3, HCl, NOx, H2S, etc. the sulfuric acid, hydrochloric acid, hydrogen sulfuric acid, etc. generated by condensation of these acid mixed steam are easy to cause serious pitting corrosion of industrial equipment. In practical engineering production, the acid film corrosion is mainly located at the top of the primary distillation tower, flash tower, atmospheric tower at the time of cold reflux at the top of the tower, oil and gas condensing cooler and its related pipelines with the temperature across the primary condensation zone at the top of the tower. The most serious corrosion is the overhead cooling system of atmospheric and vacuum distillation tower. Therefore, the damage caused by acid film corrosion in electric power, petrochemical and other fields has attracted much attention.

Wang Yanliang et al. [1] studied the electrochemistry of 316L stainless steel in boric acid solution with pH value of 4.7 and 11 by Tafel polarization, impedance spectroscopy and Mott Schottky technique. The results showed that stable passivation film could be formed in both solutions. The passivation range decreased with the increase of pH value, and the overpassivation potential decreased significantly. Zhou duo et al. [2] discussed the effect of chemical cleaning on corrosion perforation of 316L stainless steel tube in condenser, and Cl – in chemical cleaning agent will cause corrosion perforation of condenser material. Chen Yu et al. [3] studied the electrochemical performance of the passive film of 316L stainless steel in boric acid solution. The results showed that 316L stainless steel could form a good passive film in boric acid solution and reduce the corrosion rate. Yang Liuqing et al. [4] showed that 316L and HR-2 stainless steel showed good corrosion resistance in 6% acidic FeCl3 solution. Under the same environment and corrosion time, the pitting corrosion of 316L stainless steel was always more serious than that of HR-2 stainless steel. 316L austenitic stainless steel is commonly used in the manufacture of pipes, heat exchangers, etc; HR-2 austenitic stainless steel has the stability and corrosion resistance of general austenitic stainless steel, and has been widely used in energy and related fields [5]. The study on corrosion resistance of 316L stainless steel [6] shows that the increase of C1 – concentration, the increase of temperature or the decrease of pH value can accelerate the corrosion of 316L stainless steel [7,8,9,10]. The adsorption of C1 – on the surface of passive film is the only way for pitting nucleation. Cl – can enhance the conductivity of electrolyte, promote the activation of surface active points, accelerate the dissolution of pores, and reduce pitting potential [11,12]. However, there are few reports on the corrosion behavior of HR-2 stainless steel [13].
Tafel polarization, electrochemical impedance spectroscopy (EIS), coupon corrosion, resistance probe and X-ray diffraction were used to study the effects of hydrochloric acid concentration and temperature on the electrochemical behavior of 316L and HR-2 stainless steel in hydrochloric acid liquid film, the pitting corrosion and the properties of passive film.

Experimental methods

The experimental device is shown in Figure 1. The electrodes are Ag / AgCl reference electrode, graphite electrode, working electrode and probe thermometer from left to right. The temperature of oil bath is set at 130 ℃, and hydrochloric acid solution of certain concentration is prepared in evaporation bottle. Under the heating of oil bath, HCl gas and water vapor are produced. After the two mixed gases are cooled on the surface of the three electrode system of the condenser tube, a liquid film or droplet is formed. The electrode sample is connected to the external electrochemical workstation through the upper copper wire. The cotton ball soaked in NaOH solution is plugged at the top outlet of the condenser tube for tail gas absorption. The solution used in the test is 1, 0.5 and 0.1mol/l hydrochloric acid solution, and the solution volume is 0.4L.
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Fig.1 Electrochemical test experimental device

316L and HR-2 austenitic stainless steels were used as experimental materials, and their chemical composition (mass fraction,%) is shown in Table 1.

Table.1 Compositions of 316L and HR-2 stainless steel (mass fraction / %)

Steel C Si Mn P S Cr Ni Mo N Fe
316L 0.021 0.512 1.391 0.028 0.003 16.88 12.65 2.1 0.012 Bal.
HR-2 0.033 0.23 9.235 0.005 0.004 19.3 7.4 0.32 Bal.

Metal corrosion weight loss coupon was used in the experiment. The coupon was placed at the temperature of 90 ℃, the diameter of coupon was 4mm, and the height of coupon was 50mm. The metallographic sandpaper of 240 #, 360 #, 400 #, 600 #, 800 #, 1000 #, which was polished from coarse to fine, then cleaned with deionized water, acetone and alcohol, and then dried in the drying oven. The weight was accurate to 0.1mg. The size of electrochemical test electrode is Φ 4mm × 2mm. After welding with Cu wire, it is sealed. After grinding the working face step by step with metallographic sandpaper, it is cleaned with deionized water, anhydrous ethanol and acetone and blown dry for standby.
Two kinds of stainless steel (316L and HR-2 stainless steel) are used as the resistance probe samples, and the Φ 0.4mm × 12mm linear probes are made by wire cutting. Before the corrosion coupon test, the two probes made of the same material are washed with acetone and ethanol in turn, dried to room temperature, and the resistance stability value at the position of 10mm from one end to the other end is measured and recorded. Tie a string at the measuring position of 10 mm, and hang it together with the coupon sample into the 90 ℃ position of the heated boiling condenser tube, which is not in contact with each other, and the bottom end is flush. At the end of the experiment, take out the probe, wash it gently with distilled water, blow it dry, and measure the resistance value after it is reduced to room temperature, then bring in the formula to calculate the corrosion rate, and draw the corrosion kinetics curve of the two materials.
In this experiment, three concentrations (1, 0.5 and 0.1mol/l) of hydrochloric acid solution were designed. Under the set conditions, the resistance probe samples were hung with thin wire and placed at the same height (t = 90 ℃), and the hanging time was 24, 48, 96 and 144H; two pieces were hung for each material at each time, and the average value of the experimental data was calculated. After the hanging test, the corrosion sample was cleaned with deionized water, dried in the drying oven for 24h, and weighed to 0.1mg. Bm-200c metallographic microscope and D / max-2500 / PC X-ray diffraction (XRD) were used to analyze the surface corrosion products and determine the chemical composition of the corrosion area.
Tafel polarization and electrochemical impedance spectroscopy (EIS) tests were carried out on cs350 electrochemical workstation. The classical three electrode system, 316L and HR-2 stainless steel samples were used to make the working electrode. The graphite electrode was used as the auxiliary electrode. The probe thermometer was used to measure the end temperature of the three electrode system in the corrosion system. The three electrodes were closely connected and always at the same level with the bottom of the probe thermometer It’s on. All potentials in this paper are relative to the reference electrode used unless otherwise specified. In Tafel polarization experiment, the scanning range was – 0.1 ~ 0.2V. EIS test disturbance potential is 10mV, frequency test range is 105 ~ 10-2hz, voltage disturbance amplitude is 50mV, scanning rate range is 105 ~ 10-2hz.

Results and discussion

Electrochemical analysis of two materials at different temperatures and concentrations

Analysis of Tafel polarization test

It can be seen from table 2, 3 and 4 that the self corrosion potential Ecorr of the two materials gradually decreases with the increase of temperature. The decrease of corrosion potential is due to the local accumulation of C1 – on the steel surface and the hydrolysis of chloride, which leads to the local acidic environment [14], resulting in the decrease of corrosion potential. In addition, with the increase of Cl – concentration, the corrosion current density IP of 316L and HR-2 stainless steel increases. According to Farady’s second law, the corrosion current density is proportional to the corrosion rate, which indicates that the corrosion rate of 316L and HR-2 stainless steel increases with the increase of temperature. At the same time, it was found that the IP of 316L stainless steel was always higher than that of HR-2 stainless steel at the same concentration, indicating that the corrosion rate of 316L stainless steel was always higher than that of HR-2 stainless steel.

Table.2 Tafel fitting data of 316L and HR-2 stainless steel at 1 mol/L HCl solution of different temperatures

Sample T /  Ecorr / V Em / V Eb / V Passive potential range / V Ip / A·cm-2 d / mm·a-1
316L 90 0.162 0.238 2.163 0.383~2.163 0.226 0.049
70 0.409 0.695 1.282 0.695~1.282 0.207 0.038
60 0.468 0.738 1.494 0.738~1.494 -0.354 0.008
HR-2 90 0.448 0.568 1.483 0.568~1.483 -5.916 0.485
70 0.586 0.766 1.556 0.766~1.556 -0.822 0.024
60 0.72 0.87 1.708 0.870~1.708 -0.162 0.008

Table.3 Tafel fitting data of 316L and HR-2 stainless steel at 0.5 mol/L HCl solution of different temperatures

Sample T /  Ecorr / V Em / V Eb / V Passive potential range / V Ip / A·cm-2 d / mm·a-1
316L 90 0.383 0.571 1.43 0.571~1.430 0.22 0.066
70 0.514 0.722 1.441 0.722~1.441 0.241 0.03
60 0.584 0.606 1.47 0.606~1.470 0.282 0.001
HR-2 90 0.448 0.568 1.483 0.568~1.483 -5.916 0.485
70 0.266 0.506 1.16 0.506~1.160 -0.056 0.141
60 0.318 0.677 1.221 0.677~1.221 0.001 0.035

Table.4 Tafel fitting data of 316L and HR-2 stainless steel at 0.1 mol/L HCl solution of different temperatures

Sample T /  Ecorr / V Em / V Eb / V Passive potential range / V Ip / A·cm-2 d / mm·a-1
316L 90 0.007 0.092 1.018 0.092~1.018 0.22 0.22
70 0.48 0.631 1.56 0.631~1.560 0.214 0.112
60 0.617 0.777 1.55 0.777~1.550 0.182 0.03
HR-2 90 0.563 0.973 1.585 0.973~1.585 0.22 0.152
70 0.719 0.989 1.743 0.989~1.743 0.196 0.102
60 0.742 0.992 1.317 0.992~1.317 0.067 0.001

The results show that the two kinds of stainless steel can form a stable passivation zone in three different hydrochloric acid concentrations, and the potential EM has little difference. With the increase of HCl concentration, the passivation interval usually increases. If the polarization potential exceeds the overpotential, the corrosion current will increase rapidly, indicating that the passive film is unstable and dissolved. In addition, with the increase of Cl – concentration, the pitting potential EB of the electrode decreases, the metal passive film is unstable and easy to damage, and the material is more prone to pitting corrosion, which accelerates the corrosion rate. The pitting potential of HR-2 is higher than that of 316L stainless steel, and the passivation range of HR-2 stainless steel is higher than that of 316L stainless steel, indicating that HR-2 stainless steel is more resistant to corrosion.

EIS test results

Research [14] shows that the radius of capacitive reactance arc not only reflects the resistance of charge mass transfer, the larger the capacitive reactance arc is, the greater the resistance of charge mass transfer is, and the more corrosion resistant it is; on the contrary, the smaller the resistance of charge mass transfer is; it also reflects the impedance of passive film and the strength of passivation ability. The larger the radius of capacitive reactance arc is, the greater the impedance of passive film is, and the stronger the passivation ability is. Figure 2 shows the equivalent circuit of EIS, and figures 3, 4 and 5 show the EIS fitting diagrams of 316L and HR-2 stainless steel under the conditions of HCl concentration of 1, 0.5 and 0.1 mol / L and temperature of 60, 70 and 90 ℃, respectively.
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Fig.2 Equivalent circuit diagram of EIS

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Fig.3 Electrochemical impedance diagram of 316L (a) and HR-2 (b) stainless steel 1 mol/L HCl solutions at 90, 70 and 60 ℃

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Fig.4 Electrochemical impedance diagram of 316L (a) and HR-2 (b) stainless steel at 0.5 mol/L HCl solutions at 90, 70 and 60 °C

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Fig.5 Electrochemical impedance diagram of 316L (a) and HR-2 (b) stainless steel at 0.1 mol/L HCl solutions at 90, 70 and 60 °C

For the same stainless steel, at the same concentration, with the increase of temperature, the smaller the capacitive arc, the smaller the solution resistance Rs, the smaller the solution polarization resistance RP in the etching point, and the cpe-t representing the capacitance increases gradually. Therefore, HR-2 stainless steel is more resistant to corrosion than 316L stainless steel, which is consistent with the results of Tafel polarization curve. At the same temperature and material, with the increase of HCl concentration, C1 – concentration continued to increase. With the progress of the reaction, the corrosion product film on the metal surface is continuously thickened, which hinders the mass transfer process of oxygen and Cl -, and then causes the corrosion process to slow down; in the later stage of corrosion, with the increase of corrosion products, the metal surface can not be evenly covered, and Cl – will be enriched in the gap, promoting the generation of pitting corrosion, so that the corrosion rate gradually becomes faster. Therefore, the corrosion rate of two kinds of stainless steel in HCl liquid film environment first decreases and then gradually increases.

Study on corrosion characteristics of two kinds of materials at different time

Analysis of weight loss corrosion rate

Figure 6 shows the corrosion kinetics curves of the two materials in 1 and 0.5mol/l HCl solution after different simulation time, and the hanging position is in the region of 85 ~ 90 ℃. It can be seen from Fig. 6 that the corrosion rates of the two materials are similar in the two concentration environments. The results show that the corrosion rate reaches the maximum at 48h after 24-48h; the corrosion rate gradually slows down at 48h-96h; because the corrosion products are unlikely to form a film at 48h before corrosion, the corrosive medium can completely contact the steel body, and the corrosion rate is accelerated. After 48 ~ 96h corrosion, a stable and dense passive film was formed on the metal surface, which enhanced the protection of the steel. It hinders the further penetration of corrosive medium and slows down the corrosion rate. After 96h, the corrosion rate increased, and the passive film formed on the surface of steel was invaded by a large amount of Cl – in the liquid film, which destroyed the passive film and accelerated the corrosion rate. From the overall corrosion situation, it can be seen that the overall corrosion weight loss of HR-2 stainless steel is small and the corrosion rate is slow; moreover, the corrosion rate of the two steels at each stage of corrosion in 1mol / lhcl solution is higher than that in 0.5mol/lhcl solution; due to the high content of Cr in HR-2 steel, Cr has oxidation resistance and can promote the formation of stable phase α – FeOOH in the inner rust layer [15], which makes the corrosion rust layer more tightly combined with metal Therefore, HR-2 is more resistant to corrosion than 316L steel.
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Fig.6 Corrosion kinetics curves of two materials after immersion in 1 mol/L (a) and 0.5 mol/L (b) hydrochloric acid solution for different simulation time

Corrosion rate analysis of resistance probe

Figure 7 shows the corrosion kinetics curves of the two kinds of resistance probes after different simulation time in 1 and 0.5 mol / L HCl solution, and the hanging position is 85 ~ 90 ℃. The corrosion rate of the two materials increased continuously in 24-48 h, reached the maximum in 48 h, and decreased gradually in 48-96 H. Due to less corrosion products 48 h before corrosion and failure to form a film, the corrosion medium can fully contact with the steel body, and the corrosion rate is accelerating; in the later stage of corrosion, that is, 48 ~ 96 h, a stable and dense rust layer is formed on the metal surface, which strengthens the protection of the steel body and further hinders the penetration of the corrosion medium, and the corrosion rate is slowing down. After 96 h, the corrosion rate increased, and the passive film formed on the surface of steel body was invaded by a large amount of Cl – in the liquid film, which destroyed the passive film and accelerated the corrosion rate. This is consistent with the corrosion weight loss data.

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Fig.7 Corrosion kinetics curves of two material resistance probes after immersion in 1 mol/L (a) and 0.5 mol/L (b) hydrochloric acid solution for different simulation time

Observation of microstructure

The surface corrosion morphology of the two kinds of stainless steel after 24, 96 and 144 h corrosion is shown in Fig. 8. The observation shows that there are few corrosion pits on the surface of 316L and HR-2 stainless steel after 24 h corrosion in 0.5 mol / L HCl solution, and the corrosion pits are relatively scattered; after 96 h corrosion, there are many obvious corrosion pits on the surface of the sample, which are connected in a regional shape, and the color is similar to dark red rust layer; corrosion 144 The results show that there are more corrosion layers and more corrosion pits on the surface of the two kinds of stainless steel. At the same time, the corrosion phenomenon of 316L stainless steel is more obvious than that of HR-2 stainless steel, so HR-2 stainless steel is more resistant to corrosion than 316L stainless steel, which is consistent with the previous electrochemical, corrosion coupon and resistance probe results.

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Fig.8 Metallographic morphologies of 316L (a, c, e) and HR-2 (b, d, f) stainless steels at a concentration of 0.5 mol/L hydrochloric acid solution for 24 h (a, b), 96 h (c, d) and 144 h (e, f)

XRD analysis of corrosion products

Figure 9 shows the XRD patterns of 316L and HR-2 stainless steels corroded in 0.5 mol / L HCl solution for 144 h. It can be seen from the figure that the corrosion products of the two kinds of pipeline steels include Fe2O3, Fe3O4, FeOOH, Cr and fec12 · 4H2O in hydrochloric acid liquid film environment. For the two kinds of stainless steels with different corrosion resistance, the corrosion product layers are basically the same, indicating that there is no significant effect on the composition of corrosion products.

20210423010250 27238 - Passivation and pitting of 316L and HR-2 stainless steels in hydrochloric acid solution

Fig.9 XRD patterns of 316H and HR-2 steels after imm-ersion in 0.5 mol/L hydrochloric acid solution for 144 h

Under the condition of high concentration Cl, the stainless steel has the anode dissolution [16], and the electrochemical reaction is as follows:

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Fe (OH) 2 is easy to further oxidize to FeOOH.

20210423011143 57061 - Passivation and pitting of 316L and HR-2 stainless steels in hydrochloric acid solution
Fe2O3 and Fe3O4 are generated after FeOOH dehydration [17].

Because of C1 equivalent acid anion, the process makes the stainless steel surface weak acid, and then promotes the active sites of stainless steel surface to be corroded first.

20210423011153 12429 - Passivation and pitting of 316L and HR-2 stainless steels in hydrochloric acid solution

The other is competitive adsorption theory. Because the adsorption of O is replaced by C1 competition, the surface of stainless steel is partially deprived of oxygen, and then the oxidation of Cr is greatly reduced. Therefore, passivation film on the surface of steel body is difficult to form or destroy, and it is more likely to cause pitting corrosion of stainless steel. In addition, c1- will penetrate the passivation film on the steel surface to consume oxygen, and then Pitt directly.

Conclusion

  • (1) The trend of weight loss corrosion rate of the two materials was similar under two HCl concentrations. After 24-48 h of corrosion, the corrosion rate increased; The corrosion rate reached the maximum at 48 h; At 48-96 h, the corrosion rate gradually slows down and a stable and dense passive film forms on the metal surface; After 96 h, the corrosion rate increased, and the passive film formed on the surface of steel body was invaded by a large amount of Cl – in the liquid film, which destroyed the passive film and formed pitting corrosion, which accelerated the corrosion rate. From the overall corrosion situation, it can be seen that the corrosion weight loss of HR-2 stainless steel is small and the corrosion rate is slow.
  • (2) The results show that both 316L and HR-2 stainless steel can form a stable passivation zone in two different concentrations of solution and three different temperatures, and there is no significant difference in the dimensional passivation current density. At two different concentrations, 316L stainless steel has a higher self corrosion current density than HR-2 stainless steel. With the increase of concentration, the passivation interval was shortened and the self corrosion potential shifted negatively. With the progress of the reaction, Cl – is enriched in the gap of the metal surface, which promotes the formation of pitting corrosion, and the corrosion rate gradually increases; In the same concentration environment, the higher the corrosion temperature is, the smaller the capacitive reactance arc is, the smaller the charge transfer resistance is, the smaller the passive film impedance is, the weaker the passivation ability is, the lower the pitting potential is, and the more serious the corrosion is. The pitting potential of HR-2 stainless steel is generally higher than that of 316L stainless steel, and the passivation range of HR-2 stainless steel is also higher than that of 316L stainless steel, and the passivation film impedance is larger, which indicates that HR-2 stainless steel is more resistant to corrosion.
  • (3) XRD analysis shows that the corrosion products of the two kinds of stainless steels include Fe2O3, Fe3O4, FeOOH, Cr and fec12 · 4H2O. For the two kinds of stainless steels with different corrosion resistance, the corrosion product layer is basically the same. C1 – competitive adsorption leads to local anoxia on the surface of stainless steel, which hinders the oxidation of Cr on the surface of stainless steel, thus hinders the formation of Cr rich passive film, As a result, the dynamic balance of the passivation film of stainless steel becomes invalid, and the passivation film becomes thinner, which promotes the pitting corrosion in stainless steel.

Author: HE Zhuang, WANG Xingping, LIU Zihan, SHENG Yaoquan, MI Mengxin, CHEN Lin, ZHANG Yan, LI Yuchun. Passivation and Pitting of 316L and HR-2 Stainless Steel in Hydrochloric Acid Liquid Membrane Environment. Journal of Chinese Society for Corrosion and Protection[J], 2020, 40(1): 17-24 DOI:10.11902/1005.4537.2019.221

Source: China 316L Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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