Abstract
X80 pipeline steel was firstly aluminised at a relative low temperature then treated by a microarc oxidation process. Electrochemical behaviour of X80 pipeline steel before and after the combined treatment was evaluated by electrochemical tests in different solution. Results show that ceramics coating composed of α-Al2O3 and γ-Al2O3 can be obtained on the surface of X80 pipeline steel by the combined treatment. In neutral 3·5% NaCl solution and alkaline solution (pH = 9), X80 pipeline steel with ceramics coating shows a higher corrosion potential and a lower corrosion current density than pure steel and the X80 pipeline steel treated by one-step aluminising. However, aluminising coating presents a better corrosion protection for X80 pipeline steel than that of ceramic coating in acidic solution with pH = 5. γ-Al2O3 is considered as the main reason for the corrosion resistance degradation of ceramic coating in acidic solution.
Introduction
X80 pipeline steel is widely used for long distance transportation of oil and gas due to its excellent mechanical properties and weldability. 1 With the increasing corrosion substance in oil and gas from the ultra-deep oil well, improved corrosion resistance of X80 pipeline steel is required to ensure service security and duration. 2 Since ceramics have been proved to possess the excellent chemical stability in different environments,3–5 ceramics coating is considered as a novel choice to improve the corrosion resistance of X80 pipeline steel.
Compared with many other coating methods, like thermal spraying, 6 self-propagation high temperature synthesis, 7 chemical vapour deposition, 8 and microwave sintering, 9 in situ ceramic coating by micro-arc oxidation (MAO)10–12 is not restricted to high temperature. Therefore, the coating process by MAO will not damage the original microstructure of the treated substrate then maintain its excellent natural mechanical properties. On the other hand, in situ oxide coating by MAO is from substrate and oxygen. It is easier to obtain a superior physical and chemical compatibility between the outer ceramic coating and inner substrate. Aluminium alloy with ceramic coating composed of α-Al2O3, γ-Al2O3 and Al6Si2O13 by MAO has been reported to show a better corrosion resistance than that of stainless steel under the same work condition in the reported work. 13 Corrosion resistance of magnesium alloy with ceramic coating 14 composing of MgO, MgSiO3, MgAl2O4 and amorphous phase even is higher than those of samples after chromising. For titanium alloy, rutile and anatase hybrid TiO2 can be obtained as the working voltage of MAO over 350 V, which can effectively protect titanium alloy from corrosion as described in reference. 15 However, Fe3O4 is an electrical conductor, and it cannot accumulate charge then trigger arc. Therefore it cannot deposit in situ ceramic coating by MAO on the surface of steel directly.
Aluminising coating composed of iron aluminide is a practical prelayer to trigger arc on steel. 16 In situ ceramic coatings for steel by a combined treatment of hot-dipping aluminising and MAO have been reported in many researches.10,11,17 For X80 pipeline steel, the final annealing temperature is limited below 833 K to obtain small grain size and then to ensure excellent mechanical properties. However, hot dipping aluminising processing has to be carried out at temperature higher than the melting point of Al, which will damage original properties of X80 pipeline steel. Therefore, those above combined treatment of hot dipping aluminising and MAO cannot be applied to prepare ceramic coating on the surface of X80 pipeline steel.
In this work, we focus on a combined treatment of low temperature pack aluminising and MAO. First, an aluminising coating was prepared onto the surface of X80 pipeline steel at low temperature of 803 K. Then ceramic coating was achieved by MAO onto the surface of X80 pipeline steel with low temperature aluminide prelayer. The phase composition and microstructure of coated X80 pipeline steel were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) respectively. Furthermore, electrochemical behaviour of as treated X80 pipeline steel was investigated in different solutions.
Experimental
Preparation of low temperature aluminising coating and ceramics coating
Cylindrical X80 pipeline steel specimens with a dimension of Φ10×12 mm used as substrates were cut from a bulk X80 pipeline steel produced by Baosteel Corp. consisting of C: 0·058 wt-%, Si: 0·20 wt-%, Mn: 1·57 wt-%, P: 0·013 wt-%, S: 0·0033 wt-%, Cr: 0·024 wt-%, Mo: 0·23 wt-%, Ni: 0·20 wt-%, Nb: 0·050 wt-%, V: 0·035 wt-%, Ti: 0·013 wt-%, Cu: 0·09 wt-%, B: 0·0006 wt-%, Al<0·030 wt-%, balanced by Fe. Before the pack aluminising procedure, specimens polished using 600 SiC grit paper were subjected to surface mechanical attrition treatment (SMAT) on the setup as described in literature 18 under vacuum at ambient temperature. To achieve a homogenous nanostructure surface, ball milling process was accomplished by AISI 52100 steel balls with a diameter of 5 mm for 90 min with a rotating speed of 30 rev min−1. The characterisations of as SMAT X80 pipeline steel were also reported in Ref. 18. After the pre-treatment procedure, the SMAT X80 pipeline steel specimens were packed in a self-made stainless crucible with argon shield during the whole pack processing. The powder composition of pack cementation was as follows, 59·5 wt-%Al, 1·5 wt-%NH4Cl, 38·8 wt-%Zn. Zn was added into the pack powders in order to increase the wet ability of pack powder. The crucible was put into a tube electrical furnace and heated up to 803 K then kept for 2 h to form aluminising coating. X80 pipeline steel specimens with aluminising coating can be obtained by removing loose powder on the substrate after furnace cooled to room temperature.
The top ceramic coating was fabricated by MAO process on the surface of X80 pipeline steel specimens treated by low temperature aluminising. MAO equipment comprises of a 65-II computer automatic controlled microarc oxidation power source and a stainless steel electro bath. Anodic current density ranged from 0·5 to 4·6 A cm−2. Pulse frequency was kept at 1000 Hz. Deposition time was 10 min. Electrolyte solution consists of Na2AlO3 (10 g L−1), NaH2PO4 (1·5 g L−1), Na2WO4 (1·5 g L−1) in deionised water. Particularly, copper wire was used to connect the MAO equipment and X80 pipeline steel specimens with low temperature aluminising coating.
Characterisation of aluminising coating and ceramics coating
Phase evolution of X80 pipeline steel with low temperature aluminising coating and ceramic coating by MAO were assessed by XRD on Bruker AXS D8 Advance. X-ray diffraction test was operated using Cu Kα radiation at a step scanning rate with a 2θ step of 0·02° min−1. Before and after corrosion test, morphologies of low temperature aluminising coating and ceramic coating by MAO for X80 pipeline steel were characterised by a Jeol 6390 scanning electron microscope with an X-ford energy dispersive spectroscope. For samples after corrosion, they are removed from corrosion solution, use cotton yarn gently wipe the surface with acetone, and then to dry in air for SEM observation.
Electrochemical behaviour of X80 pipeline steel
Dynamic polarisation experiments were performed using PARSTAT 2273 electrochemical equipment at room temperature to test the corrosion resistance of X80 pipeline steel with low temperature aluminising coating and ceramic coating by MAO. The neutral solution prepared with deionised water and reagent grade NaCl (mass per cent 3·5%). The acidic and alkaline solution was mixed by of sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3) and deionised water with a ratio of 1∶1∶10. By adding hydrochloric acid (HCl) into the above-mentioned mixture, acidic solution (pH = 5) and alkaline solution (pH = 9) can be prepared with the testing by a starter 300 pH meter. A saturated calomel electrode was used as the reference electrode, and a platinum (Pt) plate was used as the counter electrode.
Results and discussion
Phase of coatings
Figure 1 shows the XRD pattern of X80 pipeline steel with ceramic coating by a combined treatment of low temperature aluminising and microarc oxidation. It can be seen that α-Al2O3, γ-Al2O3, Fe2Al5 and Fe are distinctly formed in the coating. Among them, α-Al2O3 and γ-Al2O3 demonstrate ceramic coating produced by MAO. Since ceramic coating was formed on the surface of aluminising ground coating, Fe2Al5 consisting of the low temperature aluminising coating can be observed. Additionally, Fe is also observed due to thin thickness of the coating.
Pattern (XRD) of X80 pipeline steel with ceramic coating by combined treatment of low temperature aluminising and microarc oxidation
Microstructure of coatings
Typical surface morphologies and EDS result of X80 pipeline steel with ceramic coating are shown in Fig. 2. From Fig. 2a, some ceramic particles with diameter of 5–7 μm can be observed. We also found round pores with diameter of 2–3 μm. They are proved to be discharge channels, which become blind after discharge. In addition, there is no crack on the surface of ceramic coating prepared by MAO.
Images (SEM) and EDS results of X80 pipeline steel with ceramic coating: a surface; b cross-sectional surface; c EDS of point 014; d EDS of point 015
According to EDS patterns of two points marked in Fig. 2c and d, the outer layer next to the substrate is composed of Al and Fe with a ratio of 5∶2. For the outmost layer in Fig. 2b, it consists of Al and O with a ratio of 3∶2. Combined with XRD and EDS analysis results, a three-layer structure including X80 pipeline steel substrate, aluminising coating and ceramic coating were indicated. The thickness of outmost ceramic coating is only 10 μm, therefore Fe2Al5 generated from inner aluminising coating and Fe from X80 pipeline steel substrate can be detected by XRD as analysed in the section on ‘Phase of the coatings’. Due to natural brittleness of ceramics, the thickness of outmost ceramic coating is not continuous and uniform. Especially on the edge of the sample, there are some ceramic pieces flaked off from coating, but they are still on the surface of X80 pipeline steel substrate. By observing the interfaces of different layers in Fig. 2b, there are no distinct cracks or discrete combinations among them. It illustrates a good connectivity between coatings and substrate.
Electrochemical behaviour of coated X80 pipeline steel
In neutral 3·5% NaCl solution
Figure 3 shows three electrochemical polarisation curves of X80 pipeline steel without and with different coating in neutral 3·5% NaCl solution. By Tafel method, self-corrosion potential (Ecorr) and self-corrosion current density (icorr) can be calculated from those plots. ΔEcorr of original X80 pipeline steel and ceramic coated one is 199·896 mv. The corresponding Δicorr between them is 3·213 μA cm−2. Results show that ceramic coating improves the Ecorr by 31% and decreases the icorr to 41%. It is well known that the higher self-corrosion potential means the more difficult corrosion tendency, while the lower self-corrosion current density represents the slower corrosion rate. As a result, X80 pipeline steel with ceramic coating has a better corrosion resistance than that of original X80 pipeline steel.
Electrochemical polarisation curves of X80 pipeline steel in neutral 3·5% NaCl solution: a original X80 pipeline steel; b X80 pipeline steel with aluminising coating; c X80 pipeline steel with ceramic coating
For the corrosion system consisting of original X80 pipeline steel and Pt counter electrode, electrochemical reaction can occur in neutral 3·5% NaCl solution. In this case, oxygen reaction dominates in the cathode, and it will accelerate the corrosion of original X80 pipeline steel served as anode. For X80 pipeline steel with aluminising coating, insulating Al2O3 film can be formed by Fe2Al5 during corrosion processing, which will weaken and block the electron transportation then protect X80 pipeline steel from corrosion. Compared with ceramic coated X80 pipeline steel, the integrity and adhesion of Al2O3 film formed by Fe2Al5 is not satisfying. Consequently, the corrosion resistance of X80 pipeline steel with aluminising coating is higher than that of original X80 pipeline steel but lower than that of with Al2O3 ceramic coating.
In alkaline solution with pH = 9
The electrochemical polarisation curves of X80 pipeline steel without and with different coating in alkaline solution with pH = 9 are shown in Fig. 4. By means of Tafel methods same as Fig. 3, it can be calculated that ΔEcorr of original X80 pipeline steel and ceramic coated one is 295·557 mV, and their corresponding Δicorr is 22·336 μA cm−2. Results show that the self-corrosion potential of X80 pipeline steel with ceramic coating is 40% higher than that of original X80 pipeline steel, and its self-corrosion current density decreases to 4% of original X80 pipeline steel. For X80 pipeline steel with aluminising coating, its self-corrosion potential is 16% higher than that of original X80 pipeline steel, and its self-corrosion current density decrease to 17% of original X80 pipeline steel. In this way, ceramic coating is proved to provide a better corrosion protection for X80 pipeline steel substrate than that of aluminising coating. The reason is considered the same with the above-mentioned analysis of corrosion behaviour tested in neutral 3·5% NaCl solution.
Electrochemical polarisation curves of X80 pipeline steel in alkaline solution with pH = 9: a original X80 pipeline steel; b X80 pipeline steel with aluminising coating; c X80 pipeline steel with ceramic coating
Additionally, compared with the self-corrosion potential and the self-corrosion current density of ceramic and aluminising coated X80 pipeline steel in neutral 3·5% NaCl solution, it can be found that ceramic and aluminising coating can protect X80 pipeline substrate from corrosion more effectively in alkaline solution (pH = 9). This is due to the absence of Cl− ion preventing the depassivation of coating when the solution is composed of Na2CO3 and NaHCO3 instead of NaCl.
In acidic solution with pH = 5
Figure 5 shows three electrochemical polarisation curves of X80 pipeline steel without and with different coating in acidic solution with pH = 5. From the figure, we found that ceramic coating and aluminising coating can both improve self-corrosion potential and decrease self-corrosion current density of original X80 pipeline steel, similar as Figs. 3 and 4. By further calculation, ΔEcorr of original X80 pipeline steel and ceramic coated one is 98·058 mV, and their corresponding Δicorr is 11·443 μA cm−2. On the other hand, ΔEcorr of original X80 pipeline steel and aluminised one is 246·324 mV, and their corresponding Δicorr is 0·566 μA cm−2. Results show that X80 pipeline steel with aluminising coating is more difficult to corrode than X80 pipeline steel with ceramic coating in acidic solution with pH = 5. In particular, it is different from its corrosion characteristics in neutral 3·5% NaCl solution and in alkaline solution with pH = 9.
Electrochemical polarisation curves of X80 pipeline steel without and with different coatings in acidic solution with pH = 5: a original X80 pipeline steel; b X80 pipeline steel with aluminising coating; c X80 pipeline steel with ceramic coating
In acidic solution with pH = 5, electrochemical will occur in the corrosion system consisted of ceramic coated X80 pipeline steel and Pt counter electrode due to high concentration of H+. Since H+ can easily penetrate the γ-Al2O3 part of the coating with natural porous microstructure, it will degrade the ceramic coating with γ-Al2O3. Moreover, hydrogen evolution is dominant by the cathodic reaction, which will accelerate the corrosion of original X80 pipeline steel acted as anode. Although above-mentioned corrosion also exists in the corrosion system consisted of aluminised X80 pipeline steel and Pt counter electrode, Al2O3 film can generate with the corrosion processing and suffer from slighter damage of H+ invasion. Consequently, aluminising coating shows a more effective corrosion protection for X80 pipeline steel substrate than the ceramic coating in acidic solution with pH = 5.
Corrosion morphology of X80 pipeline steel
Original X80 pipeline steel
Figure 6 shows corrosion surface morphologies of original X80 pipeline steel in different solution including neutral 3·5% NaCl solution, alkaline solution with pH = 9 and acidic solution with pH = 5. It can be seen that there are some microcracks and stacking corrosion products with bright colour in Fig. 6a–c. Based on the analysis in the section on ‘Electrochemical behaviour of coated X80 pipeline steel’, original X80 pipeline steel can be severely corroded in above mentioned solutions. Once corrosion occurs on the surface of X80 pipeline steel substrate, stress between corrosion products and X80 pipeline steel substrate will produce microcracks. On the other hand, Fe2+ can be formed by Fe losing their electrons when X80 pipeline steel substrate acts as anode. In neutral 3·5% NaCl solution and acidic solution with pH = 5, Fe2+ ions will be further oxidised. In alkaline solution with pH = 9, ferric hydroxide dominates in the corrosion product. Due to the poor electrical conductivity of both iron oxide and ferric hydroxide, they will show a bright colour when imaging by the secondary electron.
Corrosion surface morphologies of X80 pipeline steel without coating in different solutions: a neutral 3·5% NaCl solution; b alkaline solution with pH = 9; c acidic solution with pH = 5
X80 pipeline steel with aluminising coating
Figure 7 shows corrosion surface morphologies of X80 pipeline steel with aluminising coating in different solutions. It can be seen that smooth glassy Al2O3 barrier films are formed in Fig. 7a–c by the corrosion of iron aluminides instead of X80 pipeline steel substrate. In Fig. 7a, there are still some microcracks and accumulated corrosion products. It is due to Cl− in neutral 3·5% NaCl solution breaking the Al2O3 barrier film and causing corrosion of X80 pipeline steel substrate. For surface morphologies of X80 pipeline steel with aluminising coating after corrosion in alkaline solution with pH = 9 (in Fig. 7b) and acidic solution with pH = 5 (in Fig. 7c), few corrosion products were observed. It indicates a better corrosion resistance of aluminising coating without Cl−. Although some microcracks still exist from Fig. 7b and c, they will be self-healed by continuous propagation of Al2O3 barrier film along with corrosion processing. In addition, the self-healing ability of X80 pipeline steel with aluminising coating in acidic solution with pH = 5 is better than that in alkaline solution with pH = 9. We explain that the more effective Al2O3 film can be generated by H+ stimulation, consistent with the reason of better corrosion resistance of aluminised X80 pipeline steel than that of ceramic coated X80 pipeline steel in acidic solution with pH = 5.
Corrosion surface morphologies of X80 pipeline steel with aluminising coating in different solutions: a neutral 3·5% NaCl solution; b alkaline solution with pH = 9; c acidic solution with pH = 5
X80 pipeline steel with ceramic coating
Figure 8 shows the corrosion surface morphologies of X80 pipeline steel with ceramic coating in different solutions. In Fig. 8a, some microcracks can be observed which illustrate ceramic coating will provide a short-time corrosion protection for X80 pipeline steel substrate in neutral 3·5% NaCl solution. In Fig. 8b, the corrosion surface of X80 pipeline steel with ceramic coating in alkaline solution with pH = 9 is smooth. It can well explain why X80 pipeline steel with ceramic coating in alkaline solution with pH = 9 exhibits the highest self-corrosion potential and the lowest self-corrosion current density. For X80 pipeline steel in acidic solution with pH = 5, substantial amount of corrosion pits appear in Fig. 8c. Those pits are supposed to be rapid corrosion channels providing by γ-Al2O3 in ceramic coating due to the H+ existence. Based on the above analysis, as prepared ceramic coating in our work shows the best corrosion protection for X80 pipeline steel substrate in alkaline solution with pH = 9. In conclusion, ceramic coating can be obtained by the combined low temperature aluminising and MAO. Ceramic coating is a promising choice to improve the corrosion resistance of X80 pipeline steel in alkaline solution with pH = 9. Meanwhile, further investigation is required for more effective corrosion protection of ceramic coating for X80 pipeline steel substrate in neutral 3·5% NaCl solution and acidic solution with pH = 5.
Corrosion surface morphologies of X80 pipeline steel with ceramic coating in different solutions: a neutral 3·5% NaCl solution; b alkaline solution with pH = 9; c acidic solution with pH = 5
Conclusions
Ceramic coating for X80 pipeline steel mainly consisting of α-Al2O3 and γ-Al2O3 with the thickness of 10 μm can be prepared by a combined treatment of low temperature aluminising and microarc oxidation. Compared original X80 pipeline steel with X80 pipeline steel with aluminising coating, the electrochemical behaviour of X80 pipeline steel with ceramics coating can be greatly improved in neutral 3·5% NaCl solution and in alkaline solution with pH = 9. In acidic solution with pH = 5, the corrosion resistance of X80 pipeline steel with ceramics coating is higher than that of original X80 pipeline steel but lower than that of X80 pipeline steel with aluminising coating. The existence of γ-Al2O3 results in the degradation of corrosion resistance of ceramic coating in acidic solution. Without Cl− and H+, X80 pipeline steel with ceramic coating in alkaline solution with pH = 9 exhibits the highest self-corrosion potential and the lowest self-corrosion current density. There are no distinct microcracks and corrosion pits on the corrosion surface of X80 pipeline steel with ceramic coating in alkaline solution with pH = 9.
Footnotes
Acknowledgements
This work was supported in part by the PetroChina Innovation Foundation (Grant No. 2012D-5006-0607), Scientific Research Plan Projects of Shaanxi Education Department (Grant No.12JK0451) and the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201210). The authors acknowledge the assistance from Dr Xianghong Lv and Dr Yani Zhang in Xi'an Shiyou University.