Electrochemical noise detection of 316L stainless steel atmospheric corrosion: theoretical model and Application
China has a vast marine area, which is one of the important areas for future resource development. The construction of islands and reefs is an important step of ocean development, in which a large number of metal materials, including stainless steel materials, are needed for infrastructure construction. In addition, stainless steel is also used as structural material for military equipment such as warships. However, the atmospheric corrosion of stainless steel is often an important factor restricting its application. Stainless steel materials are widely used as metal components in the atmospheric environment, but in the atmosphere with strong corrosiveness such as Cl -, pitting will be induced, and the further development of pitting will eventually lead to the failure of metal materials or components. Therefore, it is necessary to evaluate the corrosion degree of stainless steel materials in the atmospheric environment to provide scientific basis for the diagnosis, life prediction and health management of engineering components or equipment.
The essence of atmospheric corrosion of stainless steel is electrochemical process, so electrochemical method can effectively detect the corrosion process. In recent years, electrochemical noise (EN) technology has been widely used in atmospheric corrosion detection due to its simple and portable equipment and remote continuous monitoring. However, the data analysis of en [2,3] and the establishment of reliable electrode system in atmospheric environment  are always important factors affecting the application of en. At present, the data analysis methods of en mainly include time-domain analysis, frequency-domain analysis and time-frequency analysis . The time domain analysis mainly evaluates the corrosion degree by statistical parameters (standard deviation, noise resistance and integrated electric quantity, etc.) , the frequency domain analysis obtains the frequency spectrum characteristics of noise signals by fast Fourier transform or maximum entropy method, and then analyzes the distribution of en data in the frequency domain to obtain the characteristic parameters representing the corrosion type and corrosion degree. The time-frequency analysis method mainly uses wavelet transform  and Hilbert spectrum [10,11,12] to analyze the spectrum characteristics of noise signal at a certain time. However, pure mathematical analysis is lack of strict physical significance, so it is necessary to develop the equivalent circuit model of en technology, combined with mathematical analysis to further clarify the physical significance and influencing factors of en characteristic parameters.
The establishment of the detection electrode system of en in atmospheric environment is another key scientific problem . The traditional en detection electrode system is generally composed of two working electrodes of the same material and reference electrodes, but it can not be realized to use metal components as working electrodes directly. In the early stage of our research group, we have established an en detection system suitable for on-site monitoring and testing, which has been successfully used for on-site corrosion monitoring and testing of nuclear power key materials : the components or equipment to be tested are directly used as working electrodes, and small area platinum wires are used for electrodes, and the reference electrodes are determined according to the test environment. However, in the atmospheric environment, the corrosion detection of stainless steel materials rarely reported. The atmospheric environment is different from the aqueous environment, and the relative humidity value changes greatly, so the solution resistance value has a great influence on the test results. Therefore, it is necessary to establish an equivalent circuit model for analyzing en data of metal corrosion in atmospheric environment, and discuss the influence of solution resistance on en characteristic parameters such as potential noise power spectral density, current noise power spectral density, noise resistance and spectral noise resistance. The purpose of this paper is to study the relationship between the characteristic parameters of noise and the corrosion rate. The equivalent circuit model of en detection is established, and the factors affecting the characteristic parameters of en and the influence of solution resistance on the detection results are discussed.
The metal materials to be tested are two kinds of 316L stainless steel test pieces, the size of which is 10 cm × 20 cm × 0.4 cm. One of the samples has been exposed to the ocean atmosphere for 2 A, and the other is a blank control.
Two electrode electrochemical sensor  was used for en detection. The reference electrode was a high-purity zinc electrode with a diameter of 4 mm, and the counter electrode was a platinum niobium plated electrode with a diameter of 3 mm. During the detection, a porous plastic net with a thickness of 200 μ m is placed between the sensor and the working electrode to prevent direct contact between the sensor electrode and the working electrode . In the actual electrochemical detection, no additional water or electrolyte is added to the porous plastic mesh. If it is added, the environment of the metal material will be changed. Because the distance between the sensor and the working electrode is very close, the thin liquid film environment is mainly formed by the humid atmosphere, so the electrochemical measurement can be carried out.
Zf100 electrochemical measurement system is used for the detection of electrochemical noise, with a potential resolution of 0.25 MV and a current resolution of ≤ 50PA. The time of each test is 512s and the sampling frequency is 2Hz.
The experimental conditions are simulated atmospheric environment or outdoor atmospheric environment, in which the relative humidity is 30%, 70% and the wetting conditions are tested in the simulated atmospheric corrosion test box, and the test temperature is 22℃. During the outdoor atmospheric environment test, the real-time temperature and relative humidity are recorded by the temperature and humidity meter.
The original en data is first fitted with quintic polynomials to eliminate DC components, then statistical analysis and frequency domain analysis are carried out. Statistical analysis mainly calculates the standard deviation of potential noise and current noise and noise resistance. The power spectral density and resistance of potential noise and current noise are calculated in frequency domain. The calculation formula can be found in reference .
The macromorphology of the specimen was observed by hand scanner, and the corrosion morphology was collected by vhx-2000 ultra depth of field three-dimensional microscope (KEYENCE company).
Electrochemical noise test results of 316L stainless steel in typical atmospheric environment
Drying conditions (temperature 22 ℃, relative humidity 30%)
Figure 1 shows the electrochemical noise detection results of 316L stainless steel measured by the electrochemical sensor in the atmospheric environment of 22 ℃ and 30% relative humidity (after DC component is removed). When the humidity is low, the amplitude of the current noise is very small (< 0.1na, close to the minimum resolution of the instrument current), on the one hand, because the solution resistance is large, on the other hand, because the polarization resistance is large. The amplitude of potential noise is large, which is mainly due to the fact that when the humidity is low, the metal surface is in a passive state, unable to form a stable double electric layer and corrosion potential. In addition, when the humidity is low, the potential noise of the reference electrode may also be large and affect the test results. By comparing Fig. 1a and B, it can be found that when the humidity is low, the current noise amplitude of the two test pieces is basically the same, which is mainly because the stainless steel is basically non corrosive at this time; for the stainless steel that has been exposed for 2 A, the corrosion hole is in a blunt state when the humidity is low. Therefore, when the humidity is low, the corrosion degree of stainless steel cannot be detected.
Fig.1 ECN (a, b) and EPN (c, d) detection results of 316LSS blank specimen (a, c) and corroded 316LSS (b, d) when the relative humidity is 30%
Humid environment (relative humidity 70%)
Figure 2a and b show the electrochemical noise detection results of 316L stainless steel measured by the electrochemical sensor in the atmospheric environment of 22 ℃ and 70% relative humidity (after DC component is removed). When the relative humidity increased to 70%, there was a difference between the two samples. The current noise amplitude of blank test piece is very small (< 0.1na), but the current noise value of exposed test piece is 0.2na. The fluctuation amplitude of potential noise of blank test piece is much larger than that of exposed test piece. The results show that the corrosion resistance of the exposed specimens is less than that of the blank specimens when the humidity value increases.
Fig.2 ECN (a, b) and EPN (c, d) detection results of 316LSS blank specimen (a, c) and corroded 316LSS (b, d) when the relative humidity is 70%
Total wet conditions
Figure 3a and figure b show the test results after the test piece is completely wetted, simulating the situation that the surface of the test piece is completely wetted during rainfall. When it is fully wetted, the fluctuation amplitude of potential noise standard is only 3 MV, which is due to the stable corrosion potential formed by the metal when it is completely wetted. The fluctuation amplitude of current noise is quite different. The fluctuation amplitude of corrosion test piece is 10 Na, while that of blank test piece is only 1 Na. The difference of current noise reflects the different corrosion state of the two specimens. The passivation film on the blank sample surface is complete, so the protection performance is good, and the fluctuation amplitude of current noise and potential noise is small; the passivation film in the etched hole in the exposed sample for 2 years is broken and repaired, resulting in the fluctuation amplitude of potential and current noise.
Fig.3 ECN (a, b) and EPN (c, d) detection results of 316LSS blank specimen (a, c) and corroded 316LSS (b, d) when the specimen surface is totally wet
Electrochemical noise detection under different concentration of CL liquid film
In order to further study the influence of corrosive ions on atmospheric corrosion, the electrochemical noise detection results of corrosion samples under 0.01 mol / L and 0.1 mol / L CL liquid film were studied, as shown in FIG. 4A and B. Compared with FIG. 3b and FIG. 4A, when there is 0.01 mol / L Cl -, the fluctuation amplitude of current noise increases greatly, from 10 Na to 15 Na. When the concentration of Cl – increased to 0.1 mol / L, the fluctuation amplitude of current noise increased from 15 Na to 30 Na. The results show that the existence of Cl – decreases the stability of passive film, and the increase of Cl – concentration will accelerate the rupture of passive film, which may lead to further development of pitting.
Fig.4 ECN (a, b) and EPN (c, d) detection results when the specimen surface is totally wet of 0.01 mol/L Cl- (a) and 0.1 mol/L Cl- (b)
Electrochemical noise detection in outdoor haze environment
Figure 5 shows the original electrochemical data of 316L test piece continuously monitored for 4 hours in the haze atmosphere of Tianjin. The relative humidity value rises from 70% to 80%, and the temperature drops from 3 ℃ to 1 ℃. With the increase of humidity, the fluctuation amplitude of current noise increased from 20 Na to 40 Na, and the corrosion potential increased from – 0.2 vZN to 0.8 vZN. The results show that the corrosion sensitivity increases with the increase of humidity.
Fig.5 Electrochemical noise monitoring results of the corroded specimen in outdoor haze environment test time from 2019/1/12/23:00 to 2019/1/13/3:00: (a) ECN, (b) EPN
Figure 6 shows the surface morphology of the two specimens. Fig. 6a is a 316L stainless steel specimen exposed for 2 A in the atmosphere. The corrosion pattern is mainly local corrosion, and there are many corrosion holes on its surface. Fig. 6b is a blank sample with bright surface and no corrosion. Fig. 6C is the enlarged 3D picture of local corrosion. It can be clearly seen that the corrosion morphology of 316L stainless steel is mainly pitting, and the depth of pitting is about 10 μ M.
Fig.6 Surface image of corroded sample (a, c) and blank sample (b)
Analysis and discussion
Qualitative characterization of different corrosion degree
Figure 7a shows the statistical results of potential noise at different relative humidity. When the relative humidity is 30% and 70%, the potential noise is much more accurate than that when the humidity is wet. The reason is: on the one hand, when the humidity is less than 70%, the thin liquid film on the electrode surface is not continuous, at this time, the metal can not form a stable corrosion potential, so the potential fluctuation amplitude is large. The lower the relative humidity is, the greater the solution resistance is, and the smaller the standard deviation of potential noise is. The actual test results are the comprehensive results of this aspect. The standard deviation of potential noise of 30% relative humidity is less than 70% relative humidity, which may be due to the increase of solution resistance. When the electrode surface is all wetted, the standard deviation of potential noise decreases sharply. The standard deviation of the potential noise of the blank sample is less than that of the exposed sample, which is mainly due to the potential fluctuation caused by the rupture and repair of the passive film on the surface of the corroded sample.
Fig.7 Statistical analysis of the electrochemical noise data: (a) standard deviation of potential noise, (b) standard deviation of the current noise, (c) noise resistance
Figure 7b shows the statistical results of current noise at different relative humidity. With the increase of relative humidity, the standard deviation of current noise increases, which is mainly due to the increase of electrochemical reaction intensity. When the relative humidity value is 30%, the standard deviation of the current of the two samples is very small and basically the same, which is mainly because the corrosion reaction activity is low when the humidity value is low, even if there are corrosion holes on the surface, the corrosion holes are also in a passive state. When the relative humidity increased to 70%, the standard deviation of the current of the corroded sample was slightly larger than that of the blank sample, indicating that the electrochemical activity of the corroded sample increased. When the electrode surface is completely wetted, the standard deviation of current between them is one order of magnitude, indicating that the corrosion resistance of the exposed specimen is poor.
Figure 8 shows the frequency domain analysis results of electrochemical noise data. Figure 8A shows the power spectral density of potential noise, which decreases with the increase of humidity value and atmospheric environment corrosiveness. Fig. 8b shows the power spectral density of current noise, which is basically opposite to Fig. 8A. With the increase of atmospheric humidity, the power spectral density of current noise increases. When the relative humidity is 30%, the high frequency white noise of PSD is 10-23 ~ 10-22, which increases to 10-18 when the PSD is completely wet, and the high frequency white noise of corrosion sample is higher than that of blank sample. Figure 8C shows the spectral noise resistance RSN under different conditions. With the increase of relative humidity, the RSN decreased, indicating that the corrosion resistance decreased, and the RSN with Cl – was the lowest.
Fig.8 Frequency domain analysis of EN data: (a) power spectral density of potential noise, (b) power spect-ral density of current noise, (c) spectral noise resista-nce (1: 30%RH, blank sample; 2: 30%RH, corroded sample; 3: 70%RH, blank sample; 4: 70%RH, corro-ded sample; 5: totally wet, blank sample; 6: totally wet, corroded sample; 7: 0.01 mol/L Cl-; 8: 0.1 mol/L Cl-)
Figure 9 shows the outdoor continuous monitoring results of the corrosion test piece and the temperature and humidity values in the corresponding period. It can be found that the atmospheric corrosion is mainly affected by the humidity value, and the temperature is less affected. With the increase of humidity, the standard deviation of potential noise decreases, the standard deviation of current noise increases, and the noise resistance decreases, indicating that the corrosion activity of the material increases.
Fig.9 Statistical analysis of the electrochemical noise data for the corroded specimen when exposed to have environment in Tianjin (test began from 23:00, Jan 12, 2019)
Equivalent circuit model analysis of electrochemical noise
The electrode system of electrochemical noise detection mainly includes working electrode (we), counter electrode (CE) and reference electrode (RE). In the electrochemical noise detection of zero resistance current mode, the working electrode is coupled with the counter electrode, the working electrode is anode a and the counter electrode is cathode C. According to the Thevenin electrochemical equivalent circuit model of en proposed by bertocc et al. , the current noise sources of anode a and cathode B are represented by IA and IC, and the potential noise sources are represented by EA and EC, respectively, as shown in Fig. 10a and B. The current △ I from anode to cathode is recorded as a positive value. When instrument noise can be ignored, potential noise △ V and current noise △ I can be expressed as:
Fig.10 Equivalent circuits of the electrochemical system using Pt as counter electrode (CE), 316L SS as working electrode (WE), and high-purity Zn as reference electrode: (a) equivalent circuit for potential noise, (b) equivalent circuit for current noise
When the solution resistance can be ignored,
The noise resistance RN can be expressed as:
Where, σV is the standard deviation of potential noise △V, and σI is the standard deviation of current noise △ I. It is not difficult to see that all factors affecting △V and △I can affect Rn. When the atmospheric humidity is low, the calculation of Rn is greatly affected by Rs.
The physical quantity in the formula is complex and related to frequency.
The power density spectrum can be expressed as:
When the solution resistance can be ignored,
The spectral noise resistance can be expressed as:
Formula (5) shows that the Rsn(f) value of spectral noise resistance is affected by Za、Zc、Ψia(f)、Ψic(f) and Rs. Therefore, when the solution resistance value is larger, the value of the spectral noise resistance is larger, and the detection results are greatly affected.
Only when the solution resistance can be ignored is discussed below, mainly in three cases:
(1) If the current noise value on cathode C is far greater than that on anode a (for example, H2 reduction reaction occurs on cathode and uniform corrosion occurs on anode a), then:
At this time, the current noise mainly reflects the current fluctuation on cathode C, and the spectral noise resistance is approximately equal to that of anode a.
(2) If the current noise value on anode a is much higher than that on cathode C (for example, the anode is pitted and the cathode is oxygen reduced), there are:
At this time, the current noise mainly reflects the current fluctuation on anode a, and the spectral noise resistance is approximately equal to the spectral noise resistance of cathode C.
(3) If the electrochemical noise values of anode and cathode are equal, Zn values are between Za and Zc. If it is equal to the polarization resistance Rpa and Rpc of anode and cathode reaction respectively, then:
Therefore, when Pt electrode is used as the electrode for electrochemical noise detection, the spectral noise resistance RSN mainly reflects the situation of Pt electrode, so it can not be used as an index to evaluate atmospheric corrosion.
In the system studied in this paper, the main reaction on cathode C is oxygen reduction. Compared with anode, the noise level of cathode is very small and can be ignored. Therefore, the main reaction of current noise is corrosion on anode a.
Influence of relative humidity value of atmospheric environment on test results
The influence of atmospheric relative humidity on electrochemical detection is mainly reflected in two aspects: one is the influence of solution resistance, the other is the influence of polarization resistance. It can be seen from formulas (1) and (2) that when other factors remain unchanged, the values of △I(f) and △V(f) decrease with the increase of RS value of solution resistance, resulting in the decrease of signal-to-noise ratio. Therefore, the reference electrode and the working electrode should be close to the working electrode as much as possible, that is, the thickness of the plastic mesh between the sensor and the working electrode should be as small as possible. With the increase of atmospheric relative humidity, Rs value decreases. When Rs <Za(f) or Zc(f), the detection results are not affected by rs.
In addition to the potential and current fluctuation caused by the breakdown and repair of passivation film is the main source of en, according to curioni et al. , the electrochemical noise sources detected in the local corrosion of passivation metal include:
- (1) local passivation film damage / passivation film thinning;
- (2) anode reaction at the defect before passivation film damage;
- (3) local impedance suddenly reduced due to passivation film damage;
- (4)The decrease of Faraday resistance in anode process leads to the potential drop, while the cathode Faraday resistance remains unchanged;
- (5) the re passivation process leads to the potential drop;
- (6) the increase of Faraday resistance in anode process.
- (1) The electrochemical behavior and corrosion rate of 316L stainless steel specimens with two different corrosion states in different relative humidity were studied. The experimental results show that the relative humidity has an important influence on the test results. When the relative humidity is low, the solution resistance is large, resulting in the measured current noise fluctuation amplitude is small; when the relative humidity is high, the effect of solution resistance on the test results can be ignored.
- (2) The influence factors of potential noise and current noise are analyzed by Thevenin electrochemical equivalent circuit model. The results show that the potential noise and current noise are mainly affected by the solution resistance Rs, the working electrode and the impedance mode values (Za and Zc) of the electrode.
- (3) The fluctuation of current noise mainly reflects the corrosion of the working electrode, so it can be used as a characteristic parameter to evaluate the corrosion degree. Other parameters such as spectral noise resistance are easily affected by the cathode impedance, so it is necessary to be cautious as a parameter to evaluate the corrosion degree.
Source: China 316L Pipe Fittings Manufacturer – Yaang Pipe Industry (www.metallicsteel.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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