Corrosion resistance evaluation of carbon steel plates protected by zirconium and titanium nanoceramic coatings

Metal surface pre-treatment is a known process and is used to increase corrosion performance as well as improve adhesion between the substrate and the paint layer. The present paper evaluated the corrosion resistance of carbon steel before and after treatment with nanoceramic coatings. The comparison was between a pure zirconia nanoceramic compound (Bonderite NT-1), with the addition of a dispersant (polyacrylic acid) and another nanoceramic coating developed from titanium oxide. Additionally, salt spray, open circuit potential (OCP), polarization and impedance tests were performed to obtain a methodology to quantitatively assess the quality of protection. The zirconia coating presented better corrosion protection than the titanium coating and carbon steel without coating. The corrosion potential of this coating was about twice as low as carbon steel without coating, while for the titanium coating it was about 1.5 times less. The addition of dispersant produced no significant improvements Research, Society and Development, v. 9, n. 3, e183932715, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i3.2715 3 in corrosion resistance and was similar to uncoated carbon steel, possibly due to the high concentration used.


Introduction
Metals are fundamentals compounds for infrastructure development and daily life products. Therefore, it is of great relevance to the technological progress aiming to obtain greater resistance to aggressive conditions, expanding the range of these compounds applications (Gentil, 2011).
The carbon steel excels at industrials applications by its mechanical properties.
However, it is unavoidable the occurrence of corrosion on harsh surroundings and means (Guo, Kaya, Obot, Zheng, & Qiang, 2017). This iron alloy has between 0.05 and 2.0 % per carbon mass besides small quantities of others compounds and it receives considerable attention for the solutions of problems involving corrosion (Bossardi, 2007;Guo et al., 2017).
The phosphating and chromatization are common processes used as this purpose, however, it may present various environmental concerns and problems with the low surface cover (Popić et al., 2011;Ramanauskas et al., 2015). While the phosphating needs heating, resulting in an energetic cost, the chromate is known for being a toxic and carcinogenic compound (Milošev & Frankel, 2018). Insight of the need to use sustainable process, the technology of nanoceramics surfaces coatings is proposed and applied to reduce the environmental impacts caused by conventional treatments.
The main advantages of this new process encompass the high reactivity of the nanostructured compounds, the better utilization as well as the minor residues generation. The high reactivity makes possible the greater processing reduction time on the cold or room temperature processes. While the small residues generation solves one of the serious problems of phosphating. These two advantages together lead to an economy of water and energy Roman and contributors (2011) performed a comparative study about the temperature variation used during the formation stage of a thin nanofilm and the obtained corrosion resistance. Regarding the temperature, it was verified that the major the variable is, the major will be the obtained corrosion resistance. However, the room temperature bath presented satisfactory quality, a fact of great importance for industrial applications due to is charge generated by having an energy supply.
Ramanathan and Balasubramanian (2016) studied the mechanism of nanoceramics coatings of low carbon steels. The researchers found a deposition of hydrated nano zirconia under the carbon steel surface. The formation of this thin and uniform layer of oxide gives the same special properties after the coating of the paint layer, comparing to the process involving phosphate and zinc.
According to Droniou and Fristad (2005), the high spontaneity of the process results Research, Society and Development, v. 9, n. 3, e183932715, 2020(CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i3.2715 of the deposition quality with the increase of the dispersant or with the use of the titanium coating. Thus, it becomes extremely relevant to the impedance and polarization curve tests analyses. The test was not followed up because the results of the impedance and polarization curve tests analyses show the difference in corrosion resistance between the coatings.

Open Circuit Potential (OCP) Test
The Open Circuit Potential (OCP) test has the purpose to express if the system analyzed is in equilibrium after one hour of metal excitation, which it searches for the dynamic equilibrium of the system (metal-solution) before the variation tests of current and potential start. Figure 1 presents the graphics of OCP for the unprotected carbon steel with the zirconia coating with or without dispersant addition and the titanium base. Based on Figure 1, it is noticed that the potential reached the expected dynamic equilibrium. Since after the end of the 1h immersion, the potential did not present considerable variations. According to Wolynec (2003), for the unbalance or the passive layer break occur, it must have a variation superior to 0.5 V on the final 5 minutes of the test, which had not to happened on the performed tests.
It must be pointed out that the lower the Open Circuit Potential, the most active the material on the site and the lower the corrosion resistance (Hadinata et al., 2013). By Figure 1, it is noted corrosion potentials are similar among the coated steels and with superiors values than the pure carbon steel. Consequently, the coated steels have presented more resistant to corrosion.
In spite of the steel with zirconia coating and titanium coating presenting a similar behavior, the Open Circuit Potential the zirconia coating was greater at the end of the test. Therefore, this coating has a better resistance to corrosion in relation to the titanium coating.
The Open Circuit Potential of the zirconia coating with dispersant addition had decreased with time and it reached the value of the uncovered carbon steel corrosion potential. This behavior is due to the addition of the dispersant that had produced a very thin coating that did not resist for a long time to corrosion and eventually it left the steel uncovered.

Linear Polarization Tests
For the polarization analyses, it was performed the arithmetic mean of the data relative to the triplicate of each coating. Figure 2 resumes the results of the polarization obtained for the carbon steel without coating, with zirconia coating, with zirconia coating and dispersant and with titanium coating. The current density is represented on the graphic abscissa. Source: Authors. Research, Society and Development, v. 9, n. 3, e183932715, 2020(CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i3.2715 10 The carbon steel without coating, with nanoceramic zirconia coating and dispersant and with titanium coating have corrosion potential raised in the module, around 0.89 V, -0.88 V and -0.75 V, respectively. The carbon steel with nanoceramic zirconia coating was the one that presented major corrosion resistance with a corrosion potential of -0.47V. The minor titanium performance is because the literature presents that the TiCl 4 baths produce more uniforms layers than the ones produced by the H 2 TiF 6 baths (Milošev & Frankel, 2018), consequently, these layers are more resistant to corrosion.
The carbon steel without coating presented high current density and therefore, greater corrosion rates (Behzadnasab, Mirabedini, & Esfandeh, 2013;Ramanathan & Balasubramanian, 2016). It is noticed that the corrosion potential indicated by the polarization curves shows that the carbon steel with titanium coating presented lower current density, while the carbon steel with nano-ceramic coating and dispersant presented a greater density, thus, it is verified that the dispersion acted negatively on the substrate protection. It was not observed the presence of "current shots", which indicated the absence of local corrosion on the samples analyzed.
Similarly, to Ramanathan and Balasubramanian (2016), the carbon steel with nanoceramic zirconia coating presented a small corrosion potential and inferiors current densities when compared to the steel without coating, being this one the coating with the greater performance on the tests accomplished at this article.

Impedance Tests
Figures 3 and 4 exhibits the tests results for the impedance test through the Nyquist and Bode graphics, respectively.