While NaNO2 addition can greatly inhibit the corrosion of carbon steel and ductile cast iron, in order to improve the similar corrosion resistance,
Since the operation period of nuclear power plants around the world increases each year, the degradations in buried pipes have become an important issue in the nuclear power industry. Many reports have been carried out on the degradation of buried pipes, such as the lining damage of buried pipe for a component cooling seawater system at Hanul #1 unit (Korea), 1998 [
Many reports have been presented on corrosion inhibition by nitrite, including Fe2O3 formation on steel by nitrite addition [
On the other hand, ductile cast iron has a very different microstructure to that of carbon steel. Spheroidized graphite forms in the matrix and galvanic corrosion occurs between the matrix and graphite. Also, cast iron does not have high corrosion resistance to various corrosion environments and it should be protected by a coating. As described above, many reports have been presented on the corrosion inhibition of carbon steel but there are few reports on cast iron. Therefore, in this work, corrosion inhibition effects of nitrite on carbon steel and ductile cast iron for nuclear power plant pipework using chemical and electrochemical methods were evaluated. This work attempts to clarify the corrosion inhibition mechanism between steel and iron by NaNO2 addition.
Commercial carbon steel (ASME SA106 Gr.B) [
Chemical compositions of experimental alloys.
Alloys | Chemical compositions, wt% | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
C | Mn | P | S | Si | Cr | Cu | Mo | Ni | V | Fe | |
CS | 0.26 | 0.86 | 0.014 | 0.005 | 0.23 | 0.04 | 0.057 | 0.033 | 0.029 | 0.008 | bal. |
DCI | 4.01 | 0.17 | 0.022 | 0.026 | 1.53 | — | 0.023 | 0.028 | 0.059 | 0.016 | bal. |
CS: carbon steel, ASME SA106 Gr.B; DCI: ductile cast iron, KS D4311.
The test solution was simulated primary cooling water used in the nuclear power plant. The standard solution was 1.6 ppm NaCl and its pH was modified with 1 N NaOH solution and the range of pH
A specimen was cut to a size of 20 × 20 × 5 mm and each surface was ground using #120 SiC paper. Immersion tests were carried out in a stagnant solution condition (500 mL glass flask) and in a circulating solution, in which test chamber has a dimension of 50 × 100 × 50 cm and the flow rate was 5 L/min. After the immersion tests, each specimen was cleaned with acetone and alcohol and was then dried; the corrosion rate was then determined.
Specimens were cut to a size of 20 × 20 mm and, after electrical connection, they were epoxy-mounted and the surface was ground using #600 SiC paper and coated with epoxy resin, except an area of 1 cm2. A polarization test was performed using a potentiostat (Gamry DC105) and the reference electrode was a saturated calomel electrode and the counter electrode was Pt wire. The test solution was deaerated using nitrogen gas at the rate of 100 mL/min for 30 minutes and the scanning rate was 0.33 mV/sec. In order to measure the AC impedance, the specimens were ground using #2,000 SiC paper and then polished using a diamond paste (3
X-ray photoelectron spectroscopy (XPS, K-alpha (Thermo VG, UK), Al-
In order to determine the difference in galvanic corrosion between the matrix and spheroidized graphite of the ductile cast iron, computer simulation was performed using COMSOL Multiphysics software. Tafel slopes of anodic and cathodic reactions were used and the rate controlling equation applied in this modeling was the secondary corrosion condition.
Figure
Effect of NaNO2 addition on corrosion rate in circulating and stagnant simulated cooling water at 25°C; (a) carbon steel and (b) ductile cast iron.
The open circuit potential with immersion time by NaNO2 addition in circulating (solid symbol) and stagnant (open symbol) simulated cooling water in the air at 25°C was shown in Figure
Open circuit potential with immersion time by nitrite addition in circulating (solid symbol) and stagnant (open symbol) simulated cooling water at 25°C; (a) carbon steel and (b) ductile cast iron.
The effect of NaNO2 addition on the polarization behavior in deaerated simulated cooling water at 25°C was revealed in Figure
Effect of NaNO2 addition on polarization behavior in deaerated simulated cooling water at 25°C (scanning rate; 0.33 mV/s); (a) carbon steel and (b) ductile cast iron.
Figure
Comparison of (a) corrosion rate (circulation condition) from Figure
In order to determine the resistance of the passive film formed on the surface of carbon steel and ductile cast iron by NaNO2 addition, the AC impedance was measured. Figure
Effect of NaNO2 addition on Nyquist plot obtained from AC impedance measurement in deaerated simulated cooling water at 25°C; (a) carbon steel at +400 mV (SCE) and (b) ductile cast iron at 0 V (SCE).
It was revealed that the difference in the effect of corrosion inhibition due to NaNO2 addition between carbon steel and ductile cast iron was about 100 times through the immersion test and electrochemical tests as described above. As shown in Table
Optical microstructures (a, b) and SEM images (c, d); (a, c) carbon steel and (b, d) ductile cast iron.
In order to determine the effect of NaNO2 addition on the corrosion morphologies of ductile cast iron after immersion, the corroded surface was observed. The effect of NaNO2 addition on the surface appearance of ductile cast iron after the immersion test in simulated cooling water for 3 hours at 25°C was presented in Figure
Effect of NaNO2 addition on surface appearance of ductile cast iron after the immersion test in simulated cooling water for 3 hours at 25°C; (a) 0 ppm NaNO2 and (b) 10,000 ppm NaNO2.
Corrosion morphologies of ductile cast iron corroded in stagnant simulated cooling water (10,000 ppm NaNO2) for 3 hours at 25°C; (a) 3D microscope and (b) SEM image.
The galvanic corrosion between graphite and matrix iron was simulated using a COMSOL Multiphysics program. Anodic and cathodic Tafel slopes (+108 mV and −206 mV, resp.) were applied to calculate the corrosion behavior of the corroding and noncorroding areas. Figure
Surface electrolyte potential (V(SCE), the right vertical color bar) obtained by computer 3D simulation (the unit of
Figure
Elemental distribution analyzed by EPMA on the surface of ductile cast iron passivated in simulated cooling water (100,000 ppm NaNO2) at 25°C for 72 hours; (a) SEM image, (b) Fe, (c) C, (d) O, and (e) N.
Elemental distribution analyzed by EPMA on the surface of ductile cast iron corroded in simulated cooling water (10,000 ppm NaNO2) at 25°C for 72 hours; (a) SEM image, (b) Fe, (c) C, (d) O, and (e) N.
The depth profile on the passivated surface was obtained using XPS to determine the role of nitrite ion on the passivation of steel and iron. Figure
Depth profile by XPS on passive film of carbon steel passivated for 24 hours in simulated cooling water (1,000 ppm NaNO2) at 25°C; (a) depth profile, (b)
Depth profile by XPS on passive film of ductile cast iron passivated for 24 hours in simulated cooling water (100,000 ppm NaNO2) at 25°C; (a) depth profile, (b)
Deconvolution of the chemical species determined by XPS on the surface of (a, a′, c) carbon steel passivated in 1,000 ppm NaNO2 and (b, b′, c′) ductile cast iron passivated in 100,000 ppm NaNO2; (a) and (a′) Fe 2p, (b) and (b′) Fe 2p, and (c) and (c′) N 1s.
Therefore, as discussed above, corrosion and its inhibition model can be proposed as follows. Figure
Corrosion and inhibition steps with nitrite addition of ductile cast iron; (a) without corrosion inhibitor, (b) with insufficient corrosion inhibitor (10,000 ppm NaNO2), and with sufficient corrosion inhibitor (100,000 ppm NaNO2) (G; graphite, red line; metallic oxide, dot line; nitrogen compound).
On the other hand, passivated film of various alloys exhibits semiconductive properties [
Effect of NaNO2 addition on Mott-Schottky plot for the passive film formed in deaerated simulated cooling water at 25°C; (a) carbon steel at +400 mV (SCE) and (b) ductile cast iron at 0 mV (SCE).
While NaNO2 addition can greatly inhibit the corrosion of carbon steel and ductile cast iron, in order to improve the similar corrosion resistance, The passive film of carbon steel and ductile cast iron, formed by NaNO2 addition, showed N-type semiconductive properties and its resistance is increased; the passive current density is thus decreased and the corrosion rate is then lowered. In addition, the film is mainly composed of iron oxide due to the oxidation by
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the Nuclear Power Core Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (no. 20131520000100).