Anticorrosive non-crystalline coating prepared by plasma electrolytic oxidation for ship low carbon steel pipes

A corrosion-resistant non-crystalline coating was fabricated by plasma electrolytic oxidation (PEO) on Q235 low carbon steel for ship pipes. The distribution and composition of chemical elements and phases of PEO coatings were analyzed by an orthogonal experiment, and the formation mechanism of PEO coatings was discussed. The corrosion current densities and corrosion potentials were measured. The results indicated that the formation of a transition layer mainly containing Fe3O4 was crucial for achieving an excellent coating quality. Furthermore, the corrosion current density of coated steel was reduced by 78% compared with the bare steel.


Scientific RepoRtS
| (2020) 10:15675 | https://doi.org/10.1038/s41598-020-72787-w www.nature.com/scientificreports/ fabrication process and formation mechanism of SiO 2 coatings are not clear. The barriers constraint the application of PEO technology in the surface treatment of low carbon steel. In this work, an orthogonal experiment was employed to thoroughly analyze the preparation process and formation mechanism of the SiO 2 PEO coating, which is significant for further promotion of the SiO 2 coating. The relationship of main PEO electric parameters, chemical composition and compactness of the PEO coatings is firstly revealed. X-Ray Photoelectron Spectroscopy (XPS) mapping and Glow Discharge Optical Emission Spectrometer (GDOES) were performed for chemical composition analysis of PEO coatings. Finally, the corrosion resistance of PEO coatings was evaluated.

experimental details
The Q235 carbon steel samples (100 mm × 10 mm × 5 mm) were used to fabricate PEO coatings by a selfdeveloping power supply with a duty cycle of 50%. The PEO electrolyte was composed of Na 2 SiO 3 ·5H 2 O (Kermel, AR), Na 2 CO 3 (15 g/L, Kermel, AR), and DI water. The cathode is a stainless steel tank (15 L). A cooling system is performed to maintain the electrolyte temperature within about 34 °C on a water chilling unit and some cooling water circulation lines.
An orthogonal experiment of three factors and four levels was designed. Because the positive voltage and frequency are the main electric parameters, and the Na 2 SiO 3 is a typical electrolyte for PEO treatment, they were chosen to optimize the PEO process. As the PEO breakdown voltage (BV) varies with the changing of frequency and concentration of sodium silicate, the levels of positive voltage were set as BV + 40 V, BV + 50 V, BV + 60 V, and BV + 70 V.The breakdown voltages were recorded by the appearance of rapid drop in current. The levels of sodium silicate concentration are based on lots of previous experiments and the literature 2 . The protocol for the orthogonal experiment is shown in Table 1. The PEO reaction time was chosen as 5 min from the positive voltage reaching the levels of the Positive Voltage, as shown in Table 1.
To achieve the deposition of SiO 2 before the PEO process, in the beginning, the positive voltage was increased from 50 to 100 V in 5 V increments every 1 min. Then, it was fast raised to the levels of Positive Voltage shown in Table 1. This operation is based on the literature 2 .
Surface analysis of PEO coatings performed by X-Ray Photoelectron Spectroscopy (XPS) on phi5000Ver-saProbe. The thickness, surface, and cross-section morphologies of PEO coatings were detected by a scanning electron microscope (SEM, VEGA 3, TESCAN). X-ray diffraction (XRD, EMPYREAN) was employed to analyze the phase composition of PEO coatings. X-ray diffraction patterns were collected using CoK α radiation: the accelerating voltage is 35 kV, scan range 2θ is 15°-130°, and step size Δ2θ is 0.039°. To analyze the patterns, the X-ray diffraction patterns were transformed to Cu-target types finally. The surface roughness and three-dimensional shape of the coatings were analyzed by a 3D measuring laser microscope (OLYMPUS, OLS4000). The element composition and distribution across coatings were detected by a glow discharge optical emission spectrometer (GDOES, SPECTRUMA, GDA 750HR) with a 2.5 mm anode. The electrochemical corrosion was evaluated by potentiodynamic polarization measurements. The polarization curves were obtained by an electrochemical system (Shanghai Chenhua, CHI604E) with 3.5 wt% NaCl and a scanning rate of 0.002 V/S. Five samples of each set of conditions were tested for each data, so as to avoid the fluctuations in the data, and the reported values are the average resulted from these measurements.

Results and discussion
The coating-forming process. As shown in Table 2, the changes of thickness and surface roughness of PEO coatings are not significant in the orthogonal experiment. According to the range analysis method, the sequence of the three factors for the thickness of coatings is the positive voltage (range 9.37), the concentration of sodium silicate (range 5.15), and the frequency (range 2.76). The sequence for the surface roughness is also the positive voltage (range 0.44), the concentration of sodium silicate (range 0.14), and the frequency (range 0.13). Therefore, the positive voltage is the most significant factor for the preparation of PEO coatings. Simultaneously, the thickness and surface roughness become higher with increasing the positive voltage due to the high energy of plasma arcs. Hence, the positive currents in the last 5 min of PEO processes are higher while the difference between the positive voltage and breakdown voltage are more significant, as shown in Table 3.
The changes of positive current with increasing the positive voltage for Test 8 are shown in Fig. 1.
The PEO process can be divided into three phases: Phase A, Phase B, and Phase C. In Phase A, the positive voltage is increased from 50 to 100 V with 5 V increments per 1 min; during the Phase A, the anode produces a mass of oxygen accompanied with emitting a large number of bubbles; meanwhile, the cathode produces a large amount of hydrogen.
Meanwhile, a SiO 2 film is also deposited on the surface of the anode, according to the literature 2 . The SiO 2 and oxygen film are crucial for the PEO reaction, as they provide discharge channels for the PEO process. Because the SiO 2 passive film is produced on the anode, the current decreases from 30 A to around 22 A in Phase A. After  Table. 3. The positive voltage and range of positive current in the last 5 min of PEO processes.  Table 3 decreases from 43.3 A to 8.5 A. By the end of the PEO process, the discharge becomes more and more intense due to the coating growth, the microarcs become more large and bright, but the density of microarcs decreases correspondingly as shown in the Fig. 1. The other samples of the orthogonal experiment also exhibit a similar reaction phenomenon with Test 8 during PEO processes, but the values of positive current are different from each other (see in Table 3). Because the thickness and compactness of the SiO 2 passive film affect the difficulty level of breakdown, they are also determined by the frequency and concentration of sodium silicate. Therefore, the PEO coatings exhibit kinds of characteristics in the orthogonal experiment, such as elements distribution, morphologies, and phase constituents. The composition profiles of elements for the PEO coatings of Test 4 and 16 are shown in Fig. 2. Because the stand sample was not fabricated and detected, the values of Fig. 2 are only for analyzing the changing trends of each element. According to the results of GDOES (Glow Discharge Spectrometer), the main  The morphology and composition of PEO coatings. The surface morphologies of PEO coatings are shown in Fig. 3. The cross-section morphologies of PEO coatings are given in Fig. 4. As shown in Fig. 3, the PEO coatings exhibit similar surface morphologies with the traditional PEO coatings (Crater-shaped). However, most of PEO coatings have some defects, such as cracking and remained large discharge channels, which are not suitable for anti-corrosion. Meanwhile, as shown in Fig. 4, the majority of PEO coatings do not have excellent adhesion conditions. However, the PEO coating prepared in Test 8 has a smooth surface morphology and a good adhesion condition with the steel substrate. To analyze deeply, the XRD (X-ray Diffraction) is carried out for detecting the phase compositions of the PEO coatings. Figure 5 exhibits the phase compositions of the PEO coatings of Test 3, 7, 8, 13, and 16. The main peaks in Fig. 5 are for Fe. Because the PEO coatings are relatively thin, the X-ray already reaches the substrate of samples. Apart from the peaks for Fe, there are several weak peaks for Fe 2 O 3 , Fe 3 O 4 , FeO, Fe 2 SiO 4 , and SiO 2 . As the peaks have much lower intensity, these phases do not have high crystallinity, and the grains dimension are relatively small. Therefore, the PEO coatings are a kind of non-crystalline coatings. It is observed in Fig. 5 that there is a small bread peak at around 2θ = 25° in the XRD curve of sample 3 and 7. The bread peaks are ascribed to amorphous SiO 2 . However, bread peaks are not detected in the curves of sample 8, 13, and 16. Furthermore, there are several lower peaks of SiO 2 in the XRD patterns of sample 8, 13, and 16. Therefore, the PEO coatings prepared in Test 8, 13, and 16 have higher crystallinity. Moreover, the XRD curve of sample 8 contains higher intensity of peaks than the other curves, for instance, at around 2θ = 65.2° there is a peak for Fe 3 O 4 , which has the highest intensity of Fe 3 O 4 peaks. Thus, the PEO coating produced in Test 8 has the highest crystallinity among these coatings. This is one reason for the PEO coating produced by Test 8 exhibiting relatively excellent surface morphology and adhesion condition. www.nature.com/scientificreports/ Figure 6 shows XPS mapping results of the PEO coating fabricated in Test 8. XPS mapping is a more effective method than normal XPS spot scanning, as the scanning time is longer than that of normal XPS spot scanning, and each of the colored dots is a normal XPS spot scanning in the mapping. The XPS mapping result is the superposition of all the spot scannings. The atomic concentration is the average value of the scanning area. The XPS mapping results correspond with the results of GDOES. On the surface of the PEO coating, the content of Si is much higher than that of Fe. Furthermore, according to the fine spectrum of Fe2p, the PEO coating mainly contains Fe 3 O 4 with a small amount of Fe 2 O 3, which also corresponds with the results of XRD 2 .

Test code Positive voltage (V) Range of positive current (A)
The general rule for the oxidation of Fe is as follows: the first product is FeO; then, the FeO can be oxidized to Fe 3 O 4 very easily; at a higher temperature, the Fe 3 O 4 will be oxidized to Fe 2 O 3 ; at around 1535 °C, the Fe 2 O 3 can be reduced to Fe 3 O 4 again. Moreover, molten Fe 2 O 3 and SiO 2 in the plasma discharge channels can produce   . The thermal expansion coefficient of SiO 2 is around 0.5 × 10 -6 /°C. Q235 steel has a thermal expansion coefficient of about 12 × 10 -6 /°C, which is also higher than thermal expansion coefficients of Fe 3 O 4 , Fe 2 O 3 , and Fe 2 SiO 4 at ambient temperature. Therefore, to achieve an excellent coating quality, it is necessary to form a compact and compatible transition layer between SiO 2 and the steel substrate.
The compactness of the SiO 2 coatings affects the breakdown voltage of PEO. Because Test 8 exhibits the highest breakdown voltage (170 V), the SiO 2 coating produced in Test 8 is more compact than other coatings in phase A. Furthermore, the positive currents affect the PEO reaction temperatures. The temperature of PEO reactions determines the cooling speed of SiO 2 , which affects the crystallinity of SiO 2 coatings. Under the higher frequency (2000 Hz) and a lower concentration of sodium silicate (19 g/L), Test 8 has the nearly lowest final positive current (8.5 A, see Table 3), and its reaction temperature is relatively low. Thus, the surface morphology of Test 8 is relatively smooth and has no obvious defects [see Fig. 3(8)].Besides, the PEO reaction temperatures   (13)]. The cracking is generated by the stress induced from different thermal expansion coefficients, and loose structure of FeO and Fe 2 O 3 . In conclusion, the specific phase composition is the key factor to affect the coating quality of PEO coatings. Figure 7 displays the potentiodynamic polarization curves of the Q235 steel substrate and the PEO coating of Test 8.

Potentiodynamic polarization measurements.
The corrosion current density of the PEO coating of Test 8 is 52.6 μA/cm 2 , and its corrosion potential is − 0.693 V. The Q235 steel substrate has a corrosion current density of 244 μA/cm 2 , and a corrosion potential of − 0.983 V. After the PEO treatment, the corrosion potential of Q235 steel moves to a noble direction; meanwhile, the corrosion current density of PEO coating is 22% of bare Q235 sample. Therefore, the anti-corrosion property of Q235 steel is greatly improved by PEO surface treatments.

conclusion
In this work, a non-crystalline anticorrosive coating was prepared on Q235 low carbon steel by PEO. To promote the application of this technology in engineering practice and improve the coating quality, the coating-forming mechanism, elements and phase composition of the PEO coatings were analyzed deeply by an orthogonal experiment. The results indicated that: 1. The positive voltage is the main factor for preparing the anticorrosive PEO coatings. 2. The PEO process has similar characteristics with the traditional PEO process for Al, in terms of number and color changes of arcs, and surface crater-shaped morphology. 3. There are two layers between the coatings surface and the steel substrate, including SiO 2 and a transition layer. The transition layer contains FeO, Fe 2 O 3 , Fe 3 O 4 , and Fe 2 SiO 4 ; they can transform into each other during PEO processes. Furthermore, for the optimized PEO coating, the transition layer mainly contains Fe 3 O 4 . 4. The protocol of Test 8 of the orthogonal experiment produces an excellent PEO coating, which has the best coating quality, a corrosion current density of 52.6 μA/cm 2 , and higher corrosion potential of − 0.693 V.