Effects of CaO and MgO on Anticorrosive Performance of Aluminum

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Effects of CaO and MgO on anticorrosive performance is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced


of aluminum dihydrogen tripolyphosphate on mild steel is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced


Weiqiang Song, Lantao Yang, Xi Ma, and Gaoqiang Liang



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Page 1 of 17 Industrial & Engineering Chemistry Research 3

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Effects of CaO and MgO on Anticorrosive Performance of Aluminum Dihydrogen Tripolyphosphate on mild steel Weiqiang Song*, Lantao Yang, Xi Ma, Gaoqiang Liang School of Materials Science and Engineering, Henan University of Technology, Zhengzhou, China 450001; * Corresponding author: +86 18623719057; +86 371 67758729 Email: [email protected] ABSTRACT CaO and MgO were used to neutralize aluminum dihydric tri-polyphosphate (ADTP) by co-grinding, and the effects of CaO and MgO on anticorrosive performance of ADTP on mild steel were studied. The efficiency of the resulted multi-component pigments, as corrosion inhibitors for mild steel boards in 3.5% w/w NaCl solution, was determined by electrochemical measurements. The results showed that the anticorrosive performance depended on the component of the pigments and the feeding order of ADTP, CaO and MgO in grinding. ADTP/MgO/CaO displayed more high performance than others, which was attributed to the relatively integrated passivating film constructed on the surface of the test board. And the anticorrosion of waterborne epoxy coatings was effective and persistent. INTRODUCTION Lead- and chromate-containing pigments have excellent anticorrosive characteristic, but the use thereof has been regulated while taking into consideration, for example, environmental pollution and hygiene.1-3 Most particularly, it is proved that they may hurt the human health.4 For this reason, borates (i.e., zinc borate, barium metaborate),5-7 molybdates (basic zinc molybdate, basic calcium zinc molybdate),8-11 calcium-exchange silica,12,13 or phosphates14-20 as well as with various types of phosphate powders21-27 have been exploited as environmentally pollution-free alternative pigments. Although these alternatives are much less problem concerning the safety, their anticorrosive properties are in general inferior to those of lead- and chromate-containing pigments.28, 29 Furthermore, they require a high production cost, and their physical properties, dispersibility and storage stability still need further improvement to meet the requirements. In general, these alternatives comprise solid acids as the principal components easily dissolved out, and solid base components as secondary components to maintain the pH value of the resulting coated film at the neutral level. Among the alternatives, the best rust inhibiting properties were observed for the paints with zinc-based phosphates.30, 31 Thus, zinc oxide is currently used as such a solid base component in commercial environmentally pollution-free pigments.32-34 Nevertheless, a few papers reported a detrimental influence of zinc-containing pigments on aquatic life.35,36 Thus, there have been developed environmentally pollution-free, anticorrosive pigments, which do not contain zinc oxide at all.37-40 When making alternatives for zinc-containing pigments, the emphasis is on calcium and magnesium phosphate compounds.41-47 However, these zinc-free pigments are still insufficient in both anticorrosive properties and wide-spread applicability. In order to meet the demands of high technology applications, phosphates are usually replaced by tripolyphosphates. Among tripolyphosphates, aluminum dihydric tri-polyphosphate (ADTP) is preferred for preparation of special anticorrosive coatings, such as the interior coatings of metal cans.48-51 ADTP has a multi-layered configuration with the terminal hydroxyl groups connecting to PO4 units. The hydroxyl groups protrude into the inter-lamellar region and form hydrogen bonds with the inter-lamellar water molecules which in turn hold the layers and structure together.52, 53 1

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The presence of the terminal hydroxyl groups makes ADTP have strong acidity, and accordingly, in order to keep the long-term storage stability of the coating system, alkaline metal oxides were usually used to neutralize ADTP before ADTP was introduced into the system. However, not all alkaline metal oxides are suitable. From the aspects of reactivity, alkalinity, toxicity, environmental protection and so on, calcium oxide and magnesium oxide are preferred.54-60 Since a few papers reported excellent corrosive resistance of calcium and magnesium phosphates/tripolyphosphates on metal substrates, 43, 61-69 calcium oxide and metal oxide were used to modify ADTP by co-grinding in the present study. The aim is to prepare an environmentally pollution-free and zinc-free, white anti-corrosive pigment composites. EXPERIMENTAL Preparation of ADTP Alumina oxide (Al2O3), calcium oxide (CaO), magnesium oxide (MgO), phosphoric acid (H3PO4) and sodium chloride (NaCl) were purchased from Shanghai Aladdin Industrial Corporation. In a typical procedure, aluminum hydroxide slurry and phosphoric acid solution (mole ratio of P2O5 / Al2O3 is 3:1) were mixed at 90 o C in a container. The container was stirred at 100 o C for 1.5 hours and the mixture turned to translucent viscous slurry. The slurry was immediately placed into a reacting furnace set at 310oC. After reacting for 10 hours, the obtained solid was taken out of the furnace and splashed with distilled water. The resulted power was treated by dehydration drying and further crushing after the hydration reaction. The product was used as ADTP in the following study, which was not further separated and purified. Multi-component pigment slurries Composites of ADTP, CaO and MgO were prepared by co-grinding in the aqueous medium. The grinding was accomplished in a closed grinding mill with a rotation speed of 700 rpm at the room temperature. ADTP slurry was prepared in the mill by grinding for 60 min. ADTP/CaO slurry was prepared by adding CaO into ADTP slurry in the mill and grinding for a further 30 min. ADTP/CaO/MgO slurry was prepared by adding MgO into ADTP/CaO slurry in the mill and grinding for another 30 min. ADTP/MgO and ADTP/MgO/CaO slurries were prepared by the similar method. The slurries comprised at least 50% solids. A weight ratio of 30, 60 and 30 for ADTP, CaO and MgO, respectively, was used in grinding. Electrochemical measurements for the supernatants of the multi-component pigment slurries Steel substrate preparation: Mild steel panels with 120  50  0.28mm size were used for measurements. The surfaces of each steel panel were polished down to a grid size of 1000 by silicon carbide (SiC) abrasive papers in distilled water and rinsed in acetone. Each edge of the steel panel was sealed with epoxy resin adhesive. EIS measurements: The prepared panels were immersed in the supernatants of the multi-component pigment slurries for 2 hours. Afterwards, the immersed panels were taken out, and exposed to 3.5% NaCl solution. RST5200F electrochemical working instrument (Zhengzhou shiruisi Instrument Technology Co., Ltd.) was used in the frequency range of 100000 Hz to 0.001 Hz with perturbation amplitude of 5mV around the open circuit potential. The 3.5% NaCl solution was employed as the working solution. The electrochemical cell with a three-electrode configuration including Ag/AgCl reference electrode (RE), the mile steel panel as the working electrode (WE) and platinum counter electrode (CE) was used to run the tests. Surface analysis: The surfaces of the test panels were examined by scanning electron microscopy (SEM). Before the examination, the panels were washed with double distilled water and dried in dry condition. An Inspect F50 Scanning Electron Microscope was operated 2



at an accelerating voltage of 20kV and an 18mm working distance. Electrochemical measurements for epoxy coatings Coating preparation: A waterborne two component epoxy/amine coating system was purchased from The Epoxy Resin Division of Baling Petrochemical Co., Ltd. The prepared pigment slurry and the coating were mixed with a pigment-to-binder ratio of 15:100, and the mixture was stirred mechanically at a speed of 1000rpm for 10 min. In this way, white waterborne epoxy coatings were obtained. Coating application: The mild steel panels prepared by the aforementioned method were used as steel substrates. The panels were dipped into the coatings for 5min and then placed in an electrothermal blowing dry box at 120°C for 60min to make the coating films be cured. By adjusting the viscosity of the coating systems with deionized water, the film thickness of cured coating systems was limited to 80±3microns. EIS measurements: The measurements were carried out by the similar method as aforementioned. However, 3.5wt% NaCl aqueous solution was used as the corrosive media, and the coated panels with the cured film containing the aforementioned pigments were used as the working electrode (WE). RESULTS AND DISCUSSIONS Electrochemical performance of the pigment supernatants The X-ray diffraction (XRD) spectrum of the product of alumina oxide and phosphoric acid in the section of experimental was recorded using an Analytical Diffractometer (Bruker D8 Advance) with Cu Kα radiation (=0.154 nm) at 40kV and 35mA. The spectrum is shown in Figure 1, which revealed a coexistence of two main structural phases, ADTP and AlPO4 in the product. Still, the product was mainly composed of ADTP. 4000 Diffraction intensity / PSD

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Figure 2. Nyquist (a) and Bode (b and c) plots in 3.5% NaCl solution for mild steel panels which were immersed beforehand in supernatants of the pigment slurries for 30 min. As shown in Figure 2 (a), each of the Nyquist plots consists of a capacitive arc in the high frequency region and an inductive arc in the low frequency region. The capacitive arc indicates the formation of the passivating film on the surface of the steel panel, while the inductive arc indicates the exfoliating process of the passivating film during testing, and the increase in the inductive arc radius for pigment-free through to ADTP/MgO/CaO implies a decrease in the exfoliating rate of the corresponding film. The corresponding impedance increased in the order of pigment-free, ADTP, ADTP/MgO, ADTP/CaO, ADTP/CaO/MgO and ADTP/MgO/CaO as shown in Figure 2, which implies gradually increasing inhibition of the passivating films on the surface of the steel panels. For the pigment-free solution, no passivating film was formed on the surface of the steel panel during immersion in distilled water without pigments. Instead, the surface layer was composed of iron oxide derived from the reaction of iron to oxygen and water. This reaction is usually considered to be the rusting process of iron. Fe  H 2O  Fe(OH)2 Fe(OH)2  H 2O  O2  Fe(OH)3 Fe  H 2O  O2  Fe(OH)3 The resulting rust layer is not corrosion-resistant, so a low impedance value is displayed in 4



Figure 2. In the supernatant of ADTP slurry, the passivating film was composed of chelates of iron ions with a variety of phosphate ions. In literatures, 69 the protective mechanism of ADTP for steel was proposed as follows: When it is dissolved in aqueous solution, the tri-phosphoric ion 5-

( P3O10 ) is formed, which exhibits strong chelation ability and can chelate with various metal ions, such as ferrous iron and ferric iron. Correspondingly, the passivating film is formed by chelates on the surface and provides protection to the steel substrate. 5AlH 2 P3O10  Al3  2H   P3O10 5Fe2  Fe3  P3O10  Fe2 P3O10 5-

Even if P3O10 depolymerized, the formed orthophosphate ion ( PO34- ) can also construct protective film. 5P3O10  2H 2O  3PO34-  4H 

xFe2，Fe3   yPO34-  Fe x PO4 y ADTP displayed a corrosive inhibition performance as shown in Figure 2, but the performance is inferior in practice mainly due to its acidity. In order to increase the performance, metallic oxides were recommended to offset the acidity. CaO and MgO were used as the metallic oxides in the present study, and the performance was raised as shown in Figure 2 for ADTP/CaO and ADTP/MgO. But the comparison reveals a more effective modification of CaO on ADTP than that of MgO. In aqueous solution, CaO reacted with water to form Ca(OH)2. Ca(OH)2 was soluble in water, and further reacted with acid ADTP. CaO  H 2O  Ca(OH)2 5AlH 2 P3O10  Ca(OH)2  Al3  Ca 2  P3O10  2H 2 O The acid-base neutralization raised the concentration of the tri-phosphoric ion in solution, which facilitated the formation of the passivating film on the surface of the steel panel. Although MgO exhibited alkalinity, the inferior performance of ADTP/MgO was attributed to the low reactivity of MgO with water and the low solubility of the resulting Mg(OH)2, which resulted a low concentration of the tri-phosphoric ion and other phosphate ions in aqueous solution. MgO  H 2O  Mg(OH)2 5AlH 2 P3O10  Mg(OH)2  Al3  Mg2  P3O10  2H 2 O The representative Nyquist and Bode plots in supernatants of ADTP/CaO/MgO and ADTP/MgO/CaO slurries are also presented in Figure 2. The plots show more effective performance for three-component pigments than that for two-component pigments, which can be defined as a synergetic modification of CaO and MgO on ADTP. In addition, ADTP/MgO/CaO was more effective than ADTP/CaO/MgO. Figure 3 shows anodic and cathode polarization curves measured for the multi-component pigment slurries from which corrosion potential Ecorr and corrosion current icorr are determined. All the polarization curves were essentially comparable to each other. This suggested similar polarization mechanism for the multi-component pigment slurries. However, the variations of the curve positions were evidently observed. The values of Ecorr increased in the order of pigment-free, ADTP, ADTP/MgO, ADTP/CaO, ADTP/CaO/MgO and ADTP/MgO/CaO, and the derived icorr for ADTP/MgO/CaO was lowest. Generally, a material with a lower icorr and a higher Ecorr is held to possess a lower tendency to be oxidized and corroded and therefore 5


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higher anticorrosion performance. for ADTP/MgO/CaO.

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Figure 3. Polarization curves in 3.5% NaCl solution for mild steel panels which were immersed beforehand in supernatants of the pigment slurries for 30 min. Micro-morphologies of the multi-component pigments The micro-morphologies of the pigments are shown in Figure 4. The poor performance of MgO can also be indirectly validated by the SEM micrographs. ADTP and ADTP/MgO (50/5 w/w) had a similar flaky structure with clear edges and corners as displayed in Figure 4 (a) and (b), which indicates a poor modified effect of MgO on ADTP. ADTP/CaO (50/5 w/w) had a flaky structure with relatively blurry edges and corners as displayed in Figure 4 (c), and a lot of small crystal particles of calcium phosphates can clearly be seen on the surface of the flaky structure, which shows more effective modification of CaO on ADTP. In other words, calcium oxide firstly reacted with water to produce calcium hydroxide and then the product reacted with ADTP on its slice surface. As a result, the surface was coated by calcium phosphate and/or tripolyphosphate. The different alkalinity and reactivity activity between MgO and CaO probably resulted the similar impedance of ADTP and ADTP/MgO but the higher impedance of ADTP/CaO as shown in Figure 2. The SEM micrographs of ADTP/CaO/MgO (50/5/5 w/w/w) and ADTP/MgO/CaO (50/5/5 w/w/w) pigments are also shown in Figure 4 (d) and (e). ADTP/MgO/CaO (50/5/5 w/w/w) pigment has more numerous crystal particles of calcium phosphates than ADTP/CaO (50/5 w /w).

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Figure 4. Micro-morphologies of ADTP (a), ADTP/MgO (b), ADTP/CaO/MgO (d) and ADTP/MgO/CaO (e)

ADTP/CaO (c),

Surface characterization of the test panels In order to characterize the surfaces of the test panels, SEM measurements were introduced. The micrographs for the panels beforehand immersed in the supernatants of ADTP, ADTP/CaO and ADTP/MgO/CaO slurries are displayed in Figure 5 (A), (B) and (C), respectively. Despite a small number of defects, the passivating films on the surfaces were relatively intact in general for all panels. After immersion of 2 hours in the slurries, the panels were exposed to 3.5% NaCl solution for 9 days. The micrographs for the resulted panels are displayed in Figure 6 (a), (b) and (c) corresponding to Figure 5 (A), (B) and (C), respectively. The micrographs in Figure 6 show that the surfaces of the test panels were covered by broken films with apparent cracks, which can be attributed to corrosion of the corrosive medium. Otherwise, the cracks in Figure 6 (a) were wider and denser than those in Figure 6 (b), and the cracks in Figure 6 (b) were wider and denser than those in Figure 6 (c). Because the presence of cracks reduced the barrier ability of the passivating film to the penetration of the corrosive medium to the steel panel, the wider the cracks were, the worse the barrier ability was. This can explain the original reason why the impedances in Figure 2 are in the order of ADTP, ADTP/CaO and 8



ADTP/MgO/CaO.

Figure 5. Surfaces of mild steel panels after immersion of 2 hours in the slurries of ADTP (A), ADTP/CaO (B) and ADTP/MgO/CaO (C)

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Figure 6. Surfaces of the resulted mild steel panels after exposure to 3.5% NaCl solution for 9 days for ADTP (a), ADTP/CaO (b) and ADTP/MgO/CaO (c) Anticorrosive performance of epoxy coatings The anticorrosive performance of waterborne epoxy coatings containing ADTP/MgO/CaO pigments was also evaluated by ESI measurements. The obtained Nyquist and Bode plots are shown in Figure 7. The plots show that the coatings had good anticorrosive performance in the long term of immersion. The frequencies of the maximum phase angles covered a wide range for all time of immersion mainly attributed to a composite protective mechanism of the coating composition. 1.0x107

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Figure 7. Nyquist (a) and Bode (b) plots for mild steel panels coated by epoxy coatings containing ADTP/MgO/CaO exposed to 3.5% NaCl solution For better understanding of the comparation of the anticorrosive performances of the multi-component pigments, the Nyquist plots obtained for epoxy coatings containing different component pigments are presented in Figure 8. The tested mild steel panels were exposed to 3.5% NaCl solution for 24 hours. The general shapes of the plots were the same, which revealed a similar electrochemical process, although the impedance values were different to each other. 1.2x107

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Figure 8. Nyquist plots for mild steel panels coated by epoxy coatings containing different component pigments exposed to 3.5% NaCl solution CONCLUSION As common alkaline earth metal oxides, CaO can improve the anticorrosive performance of acidic ADTP on carbon steel by co-grinding, but MgO hardly. Still, ADTP/MgO/CaO and MgO/ADTP/CaO display better anticorrosive performance than ADTP/CaO mainly attributed to a synergetic modification of CaO and MgO on ADTP. Furthermore, ADTP/MgO/CaO was more effective than ADTP/CaO/MgO, which indicates that the performance of 12



multi-component pigments depends on the order of feeding in co-grinding. According to SEM measurements, the inferior performance of ADTP was attributed to the incomplete passivating film constructed by ADTP on the surface of carbon steel, and the superior performance of ADTP/CaO and ADTP/MgO/CaO was attributed to their integrated passivating film constructed on the surface. In addition, the anticorrosion of waterborne epoxy coatings containing ADTP/MgO/CaO on mild steel is effective and persistent. ACKNOWLEDGMENTS This study was supported by the Provincial Scientific Project of Henan (Grant no. 152102210069) and the Science and Technology Innovation Team Project of Zhengzhou City (Grant no. 131PCXTD615) REFERENCES (1) Brokbartold, M.; Temminghoff, E. J. M.; Weng, L.; Marschner, B. Unique characteristics of Pb in soil contaminated by red lead anti-corrosion paint. J. Soil Contam. 2013, 22, 839. (2) Monico, L.; Janssens, K.; Cotte, M.; Sorace, L.; Vanmeert, F.; Brunetti, B.; Miliani, C. Chromium speciation methods and infrared spectroscopy for studying the chemical reactivity of lead chromate-based pigments in oil medium. Microchem. J. 2016, 124, 272. (3) Sørensen, P. A.; Kiil, S.; Dam-Johansen, K.; Weinell, C. E. Anticorrosive coatings: a review. J. Coat. Technol. Res. 2009, 6, 135. (4) Freund, A. Listing occupational carcinogens. Environ. Health Perspect. 2005, 112, 1447. (5) Hu, J.; Zhao, X. H.; Tang, S. W.; Ren, W. C.; Zhang, Z. Y. Corrosion resistance of cerium-based conversion coatings on alumina borate whisker reinforced aa6061 composite. Appl. Surf. Sci. 2007, 253, 8879. (6) Bae, B.; Tamura, S.; Imanaka, N. Novel environment-friendly yellow pigments based on praseodymium(iii) tungstate. Ceram. Int. 2017, 43, 7366. (7) Şen, F.; Şentürk, İ. İ.; Kahraman, M. V. Bisphenol S and bisphenol A cyanate ester/barium metaborate composites. Polym. Compos. 2016, 37, 1312. (8) Shkirskiy, V.; Keil, P.; Hintzebruening, H.; Leroux, F.; Vialat, P.; Lefèvre, G.; Ogle, K.; Volovitch, P. Factors affecting MoO42- inhibitor Release from Zn2Al Based Layered Double Hydroxide and Their Implication in Protecting Hot Dip Galvanized Steel by Means of Organic Coatings. ACS Appl. Mater. Inter. 2015, 7, 25180. (9) Karekar, S. E.; Bhanvase, B. A.; Sonawane, S. H.; Deosarkar, M. P.; Pinjari, D. V.; Pandit, A. B. Synthesis of zinc molybdate and zinc phosphomolybdate nanopigments by an ultrasound assisted route: advantage over conventional method. Chem. Eng. Process. Process Intensification 2015, 87, 51. (10) Alipour, K.; Nasirpouri, F. Smart anti-corrosion self-healing zinc metal-based molybdate functionalized-mesoporous-silica (MCM-41) nanocomposite coatings. RSC Adv. 2017, 7, 51879 (11) Singh, H. K.; Yeole, K. V.; Mhaske, S. T. Synthesis and characterization of layer-by-layer assembled magnesium zinc molybdate nanocontainer for anticorrosive application. Chem. Eng. J. 2016, 295, 414. (12) Aghzzaf, A. A.; Rhouta, B.; Rocca, E.; Khalil, A.; Steinmetz, J. Corrosion inhibition of zinc by calcium exchanged beidellite clay mineral: a new smart corrosion inhibitor. Corros. Sci. 2014, 80, 46. (13) Hussain, A.; Lyon, S. Evaluating novel inhibitor pigment blends using electrochemical scanning techniques (EIS, SVET and SKP). J. Polym. Compos. 2013, 2013, 1. 13


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Effects of CaO and MgO on Anticorrosive Performance of Aluminum

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