1. INTRODUCTION
⌅Metals are being used in diverse structural or machine applications due to good strength (under fatigue and different load conditions), better manufacturing capabilities, etc (Akhtar et al., 2018Akhtar, M., Qamar, S.Z., Muhammad, M., Nadeem, A. (2018). Optimum heat treatment of aluminum alloy used in manufacturing of automotive piston components. Mater. Manuf. Process. 33 (16), 1874-1880. https://doi.org/10.1080/10426914.2018.1512128.). Steel and aluminum are being the top on the list. Steel is an alloy of iron that is the fourth most abundant material present in the earth’s crust. Kumar et al. (2019)Kumar, K.A., Natarajan, S., Duraiselvam, M., Ramachandra, S. (2019). Effect of Titanium Dioxide Particles on the Morphology, Mechanical, and Wear Properties of the Aluminum Alloy 3003-Based Composites. Arab. J. Sci. Eng. 44 (12), 10207-10217. https://doi.org/10.1007/s13369-019-04044-w. discussed the applications related to composite having titanium dioxide (TiO2) as a reinforcing particle and aluminum (Al 3003) as a matrix. A process known as friction has been applied successfully as a stir procedure for synthesizing the formation of aluminum metal composites. Efforts were done to check out the effect in different combinations of reinforcing particles on the weight percent ratios within the matrix. Typically, steel has 95% or more iron with relatively small alloying elements such as C, Mn, Si, Cr, Ni, Mo, Al, Ti, V, Nb, and Co. Steel has a wide variety of applications from simple households to complex engineering solutions like cars, bridges, mega-structures, turbines, etc. Applications made of steel have a measure issue of corrosion. Samaniego-Gámez et al. (2020)Samaniego-Gámez, P., Almeraya-Calderón, F., Martin, U., Ress, J., Gaona-Tiburcio, C., Silva-Vidaurri, L., Cabral-Miramontes, J., Bastidas, J.M., Chacón-Nava, J.G., Bastidas, D.M. (2020). Effect of sealing treatment on the corrosion behavior of anodized AA2099 aluminum-lithium alloy. Rev. Metal. 56 (4), e180. https://doi.org/10.3989/revmetalm.180. investigated the effect of corrosion of AA2099 aluminium alloys under different environments. Authors concluded that 6 wt.% Na2Cr2O7 sealing treatment resulted in improving the corrosion resistance.
Many authors investigated steels in different conditions and provide a solution for corrosion prevention (Finšgar and Jackson, 2014Finšgar, M., Jackson, J. (2014). Application of corrosion inhibitors for steels in acidic media for the oil and gas industry: a review. Corros. Sci. 86, 17-41. https://doi.org/10.1016/j.corsci.2014.04.044.; Lu et al.,2016Lu, X.F., Liao, P.Q., Wang, J.W., Wu, J.X., Chen, X.W., He, C.T., Zhang, J.P., Li, G.R., Chen, X.M. (2016). An alkaline-stable, metal hydroxide mimicking metal-organic framework for efficient electrocatalytic oxygen evolution. J. Am. Chem. Soc. 138 (27), 8336-8339. https://doi.org/10.1021/jacs.6b03125.; Deng et al., 2019Deng, L., Yan, W., Nie, L. (2019). A simple corrosion fatigue design method for bridges considering the coupled corrosion-overloading effect. Eng. Struct. 178, 309-317. https://doi.org/10.1016/j.engstruct.2018.10.028.). Finšgar and Jackson (2014)Finšgar, M., Jackson, J. (2014). Application of corrosion inhibitors for steels in acidic media for the oil and gas industry: a review. Corros. Sci. 86, 17-41. https://doi.org/10.1016/j.corsci.2014.04.044. study the number of corrosion-inhibitors for steel in the solutions of acid. Steel is kept in HCl solutions for experiments, and emphasizes is made on high temperature. A mixture of surfactants, enhancers, inhibitors of corrosion and solvents are used for the study. Mixture improves the effectiveness at high temperatures for individual compounds and making it very useful information for a variety of industrial areas, such as acidification procedures and the applications where steel corrosion-inhibitors are required. Dastgerdi et al. (2019)Dastgerdi, A.A., Brenna, A., Ormellese, M., Pedeferri, M., Bolzoni, F. (2019). Experimental design to study the influence of temperature, pH, and chloride concentration on the pitting and crevice corrosion of UNS S30403 stainless steel. Corros. Sci. 159, 108160. https://doi.org/10.1016/j.corsci.2019.108160. investigated the degradation of surfaces on the performance of stainless steel materials. Authors used the design of experiments to check out the effects of pH, chloride-concentration in both crevice and pitting corrosion of stainless-steel at high temperatures. The design of experiments reduces the set of experiments required to investigate the effect of different parameters on passive breakdown potential and found that temperature’s effect was more important in pitting corrosion. Hossain et al. (2019)Hossain, S.Z., Kareem, S.A., Alshater, A.F., Alzubair, H., Razzak, S.A., Hossain, M.M. (2019). Effects of Cinnamaldehyde as an Eco-Friendly Corrosion Inhibitor on Mild Steel in Aerated NaCl Solutions. Arab. J. Sci. Eng. 45, 229-239. https://doi.org/10.1007/s13369-019-04236-4. explore the cinnamaldehyde effect as an environmentally protecting inhibitor that reduces the corrosion’s damage of aerated NaCl (3% w/w) mild steel in which potentio-dynamic polarization and weight loss methods are used. Cinnamaldehyde moderate prevention efficiency reached about 70% by making an adsorption layer on the metal surface at the optimal level of 0.5 g·L-1. The corrosion rate becomes slow when the cinnamaldehyde dose reached the optimum level, but slowly increasing at moderate temperatures.
Melia et al., (2019)Melia, M.A., Nguyen, H.D.A., Rodelas, J.M., Schindelholz, E.J. (2019). Corrosion properties of 304L stainless steel made by directed energy deposition additive manufacturing. Corros. Sci. 152, 20-30. https://doi.org/10.1016/j.corsci.2019.02.029. investigated corrosion in stainless steel by the effect of processing and microstructure-defects in directed energy deposition (DED). The Materials is formed by DED having heat input of 0.45 and 0.03 kJ·mm-1 in comparison of forged 304L. Because there is no fusion pore in DED-material, the breakdown- potential (Eb) is controlled by 1st order and reduced it up to 400 mV compared with 0.6M NaCl base-material. Pradhan et al. (2019)Pradhan, S.K., Bhuyan, P., Mandal, S. (2019). Influence of the individual microstructural features on pitting corrosion in type 304 austenitic stainless steel. Corros. Sci. 158, 108091. https://doi.org/10.1016/j.corsci.2019.108091. discussed the effect of grain size, grain distribution, and residuals in corrosion by pitting in stainless steel using two types of polarization tests of potentio-static and potentio-dynamic. Authors found that the higher the dislocation density of the deformed specimen, the higher will be the pit generation rate. Haldhar et al. (2019)Haldhar, R., Prasad, D., Bhardwaj, N. (2019). Surface Adsorption and Corrosion Resistance Performance of Acacia concinna Pod Extract: An Efficient Inhibitor for Mild Steel in Acidic Environment. Arabian J. Sci. Eng. 45, 131-141. https://doi.org/10.1007/s13369-019-04270-2. researched Acacia-concinna resistive properties on mild steel when it is extracted in a medium of 0.5 M sulfuric acid. The composition and structure of the surface film were examined with SEM and an atomic-force microscope. Perform electrochemical impedance and potentiometric spectroscopy. The electrochemical analysis showed that the 250 m·L-1 inhibitor in concentration showed the best results.
Zhang et al. (2018)Zhang, F., Ju, P., Pan, M., Zhang, D., Huang, Y., Li, G., Li, X. (2018). Self-healing mechanisms in smart protective coatings: A review. Corros. Sci. 144, 74-88. https://doi.org/10.1016/j.corsci.2018.08.005. review some benefits and limitations of the self-healing procedure for preventive organic-coatings that can be used for corrosion resistivity purposes. Autonomous healing methods are mostly possible by implanting polymeric healing materials or deterioration preventers used in the coating matrix. In the case of a non-autonomous procedure, the effects of healing are activated by an outside source that becomes the cause of compound reactions that are useful for bonding. For the current study A-36 low carbon steel is used that has a density of 7800 kg·m-3, Young’s modulus value of 200 GPa, Poisson’s ratio of 0.26, and shear modulus of 75 GPa (Dai et al., 2017Dai, D., Prussin, A.J., Marr, L.C., Vikesland, P.J., Edwards, M.A., Pruden, A. (2017). Factors shaping the human exposome in the built environment: Opportunities for engineering control. Environ. Sci. Technol. 51 (14), 7759-7774. https://doi.org/10.1021/acs.est.7b01097.; Li et al., 2017Li, W., Landon, J., Irvin, B., Zheng, L., Ruh, K., Kong, L., Pelgen, J., Link, D., Figueroa, J.D., Thompson, J., Nikolic, H. (2017). Use of carbon steel for construction of post-combustion CO2 capture facilities: a pilot-scale corrosion study. Ind. Eng. Chem. Res. 56 (16), 4792-4803. https://doi.org/10.1021/acs.iecr.7b00697.). Many authors investigated the corrosion rate of material using experimental methods like linear polarization resistance (LPR), weight-loss method, etc. Erfani et al. (2015)Erfani, A., Boroojerdi, S., Dehghani, A., Yarandi, M. (2015). Investigation of carbon steel and stainless steel corrosion in a MEA based CO2 removal plant. Petroleum & Coal 57 (1), 48-55. discussed the use of the LPR method to investigate corrosion in the operation of CO2 removal units. The study shows the effect of temperature, inhibitor, and CO2 content on mild-steel and stainless steel in both laboratory and long duration tests in an operating plant. The result shows 1 to 11 mpy corrosion rate for temperature ranges from 60 °C to 80 °C. The use of salicylic acid decreases the corrosion rate by 43%. Zaferani and Shishesaz (2014)Zaferani, S.H., Shishesaz, M.R. (2014). Corrosion Inhibition of Carbon Steel in Acidic Solution by Alizarin Yellow GG (AYGG). J. Pet. Environ. Biotechnol. 5 (4), 1000188. https://doi.org/10.4172/2157-7463.1000188. used a dye named AYGG (Alizarin yellow GG) as an inhibitor. This method is used to study the corrosion prevention in carbon steel in an acidic environment. The results show better resistance to corrosion by newly made inhibitors even in the highly corrosive environment. Barranco et al. (2004)Barranco, V., Feliu Jr, S., Feliu, S. (2004). EIS study of the corrosion behaviour of zinc-based coatings on steel in quiescent 3% NaCl solution. Part 1: directly exposed coatings. Corros. Sci. 46 (9), 2203-2220. https://doi.org/10.1016/j.corsci.2003.09.032. investigated the behavior of different paints along with zinc-based coatings on steel in3% solution of sodium chloride. The results of this study give better corrosion prevention for painted specimens as compared to bare samples. Zinc-based coatings also give very good prevention against corrosion. Parhizkar et al. (2018)Parhizkar, N., Ramezanzadeh, B., Shahrabi, T. (2018). The epoxy coating interfacial adhesion and corrosion protection properties enhancement through deposition of cerium oxide nanofilm modified by graphene oxide. J. Ind. Eng. Chem. 64, 402-419. https://doi.org/10.1016/j.jiec.2018.04.003. studied the effect of epoxy-based cerium nano-coatings on steel samples and produce coatings like Ce, Ce-AGO, and Ce-GO, and changes in morphology were investigated using field emission SEM. The outcome of the experimental study showed that cerium nano-coatings can enhance adhesion, prevention against corrosion, and delaminating of the films. Osarolube et al. (2008)Osarolube, E., Owate, I.O., Oforka, N.C. (2008). Corrosion behaviour of mild and high carbon steels in various acidic media. Sci. Res. Essays 3 (6), 224-228. studied the effect of corrosion on high-to-low carbon steels in different acidic environments. Samples are exposed for a week and investigated the corrosion rate using a weight loss technique. It was concluded that the fastest corrosion rate occurs in nitric acid, medium in per chloride, and slowest in hydrochloric acid. Techniques used by different authors provide the solution to many problems against corrosion but they usually need complex, and expensive methods or sometimes required more time and effort. Sambathkumar et al. (2021)Sambathkumar, M., Gukendran, R., Dineshkumar, K., Ponappa, K., Harichandran, S. (2021). Investigation of mechanical and corrosion properties of Al 7075/Redmud metal matrix composite. Rev. Metal. 57 (1), e185. https://doi.org/10.3989/revmetalm.185. studied corrosion resistance of Al 7075 matrix alone and Al 7075-redmud based composite. Corrosion rate is found out to be decreased in case of redmud-matrix composite as compared to matrix alone.
The current study is based on providing a simpler, quick, and relatively cheaper method for corrosion prevention. Low carbon steel (A-36) is being used for experimental investigation of corrosion rate under different coatings. Five different coatings of red oxide primer, oil paint, primer plus oil paint, paint mixed with TiO2, and ZnO based nanoparticles are applied. To explore the effect of environmental conditions all tests were performed in acidic, basic, and neutral environments. The following sections provide details of experiments performed along with the results obtained.
2. EXPERIMENTAL METHOD AND SETUP
⌅2.1. Linear Polarization Resistance (LPR)
⌅LPR method is an electrochemical process that is widely used to determine the polarization resistance of a material that helps in determining the corrosion level (ElBatanouny et al., 2013ElBatanouny, M.K., Mangual, J., Ziehl, P.H., Matta, F. (2013). Early corrosion detection in prestressed concrete girders using acoustic emission. J. Mater. Civ. Eng. 26 (3), 504-511. https://doi.org/10.1061/(ASCE)MT.1943-5533.0000845.). Figure 1 gives the experimental setup of LPR used for the current study.
The electrical conductivity of the fluid is related to the ability to produce corrosion. This technique establishes the relationship between the current of charged electrons and electrochemical potential. It generally includes two or more electrodes which on erosion generates polarization resistance that ultimately gives the degree of corrosion. Material is first polarized in an open circuit to ±10 to ±20 mV. Current usually flows between electrode made of material under consideration and counter electrode (Amin et al., 2010Amin, M.A. (2010). Chemical and Electrochemical Techniques for Evaluation of Performance of Some Amino Acids as Corrosion Inhibitors for Low Alloy ASTM A213 Grade T22 Boiler Steel in HCl Solutions. Jordan Journal of Chemistry 5 (4), 371-388. https://jjc.yu.edu.jo/index.php/jjc/article/view/278.; Boikanyo et al., 2018Boikanyo, D., Masheane, M.L., Nthunya, L., Mishra, S.B., Mhlanga, S.D. (2018). Carbon-supported photocatalysts for organic dye photodegradation. In New Polymer Nanocomposites for Environmental Remediation. Elsevier, pp. 99-138. https://doi.org/10.1016/b978-0-12-811033-1.00005-6.; Lu et al., 2017Lu, Y., Geng, J., Wang, K., Zhang, W., Ding, W., Zhang, Z., Xie, S., Dai, H., Chen, F.R. Sui, M. (2017). Modifying surface chemistry of metal oxides for boosting dissolution kinetics in water by liquid cell electron microscopy. ACS Nano 11 (8), 8018-8025. https://doi.org/10.1021/acsnano.7b02656.). Polarization resistance (PR) is calculated by determining the slope of electrochemical potential and current. Using the Stern Geary equation (Angst and Büchler, 2015Angst, U., Büchler, M. (2015). On the applicability of the Stern-Geary relationship to determine instantaneous corrosion rates in macro-cell corrosion. Mater. Corros. 66 (10), 1017-1028. https://doi.org/10.1002/maco.201407997.) this PR can be converted into a corrosion rate.
where, Β is the proportionality constant for the particular corrosion system and ιcorr is its corrosion current density (in units of µΑ·cm−2). Tafel plot is used to determine proportionality constant (Β) by measuring the slopes of anode (βα) and the cathode (βb) empirically.
Tafel constants are dependent on electrode used and the environment. Many authors determined these constants for many electrodes under different acidic or basic solutions (Bates and Pinching, 1949Bates, R.G., Pinching, G.D. (1949). Acidic dissociation constant of ammonium ion at 0º to 50 ºC, and the base strength of ammonia. J. Res. Natl. Bur. Stand. 42 (5), 419-430.).
2.2 Experimental setup
⌅A-36 structural steel is used in the present study for the determination of the corrosion rate (CR). CR is typically stated as a degree of penetration deep inside the material. This phenomenon takes time hence it is normally given in terms of the degree of penetration occurred over a period like inches per year or melts per year (MPY) where melts equals to 0.0254 mm (1/1000 inch) (Mendonça, et al., 2017Mendonça, G.L., Costa, S.N., Freire, V.N., Casciano, P.N., Correia, A.N., de Lima-Neto, P. (2017). Understanding the corrosion inhibition of carbon steel and copper in sulphuric acid medium by amino acids using electrochemical techniques allied to molecular modelling methods. Corros. Sci. 115, 41-55. https://doi.org/10.1016/j.corsci.2016.11.012.). Test specimens of dimensions 1 cm x 1 cm are cut from the plate of A-36 steel material. The specimens are grinded and polished by using different grades of sandpapers from coarse to finer grit sizes. The final finishing step is done using the cloth-wheel machine with Alumina powder. Specimen than soldered with copper wire (10 inches long) to make connections and later molded in epoxy material as shown in Fig. 2. Corrosion testing has been done using LPR equipment.
For the experimental investigation, different coatings were made and resistance to corrosion in each condition is evaluated. Coatings are applied using Primer (red-oxide), Paint, Primer and Paint, Paint with zinc oxide nanoparticles, and Paint with titanium oxide nanoparticles. Figure 3 gives the Scanning electron microscope (SEM) images of zinc oxide (ZnO) and titanium oxide (TiO2) nanoparticles that were used in coatings. SEM image gives the average size of ZnO nanoparticles of 97.5 nm and for TiO2 average size comes out to be 106.8 nm. To investigate the correct response for zinc-oxide and titanium oxide-based coatings, nano-particles for about 5-10% by weight are added in the paint as described by Schaefer and Miszczyk (2013)Schaefer, K., Miszczyk, A. (2013). Improvement of electrochemical action of zinc-rich paints by addition of nanoparticulate zinc. Corros. Sci. 66, 380-391. https://doi.org/10.1016/j.corsci.2012.10.004.. Three environmental conditions are provided to the specimen to investigate the effect in different situations like acidic, basic, and neutral. Standard 1M solution of NaOH is used to produce a base environment, 1M solution of HNO3 is used to provide the acidic environment and the neutral condition is maintained by distilled water. For each environment, six conditions were tested (five with coatings and one bare sample).
Coatings are applied manually to each specimen. The thickness of coatings is measured using an electrometer and found to be 50 to 60 microns in each case. The next section discusses different results obtained by experimental investigation and analysis of those results.
3. RESULTS AND DISCUSSION
⌅In this section results obtained by the LPR, the experiment is described. One sample without coating (bare) and five with different coatings are tested in acidic, basic, and neutral environments.
3.1. Acidic environment
⌅To provide an acidic environment, specimens are kept under 1 molar HNO3 solution in LPR equipment. Sample without coating is tested first. Equipment through controller sends the data to computer software that gives a graph between the voltage in millivolts vs reference current in milli-ampere. For illustration purposes, the linear polarization graph for the bare specimen is shown in Fig. 4 as a sample. The graph shows a linear increasing trend.
The above curve is used to determine polarization resistance using the Stern-Geary equation as described above. For the bare sample, the value of the corrosion rate is found to be 191.5 mpy. This value shows a relatively higher amount of corrosion. For the remaining samples, a similar process is used to determine the melts per year. Following Fig. 5 shows the corrosion rate in melts per year for all the specimens in the acidic environment.
It can be seen that the bare sample showed the fastest corrosion rate as expected. While the sample in the paint gives the corrosion rate of 44.31 mpy, primer alone gives 56.96 mpy, paint plus primer gives 61.3 mpy in acidic environment respectively. The addition of nano-particles in paints gives a significant improvement in the corrosion rate. For titanium-based nano-particles paint, coatings resulted in a huge decrease in corrosion rate i.e. 0.6084 mpy that further dropped to 0.302 mpy in case of zinc oxide-based nano-particles paint coatings. The addition of zinc oxide nanoparticles in paints improves the cathodic protections by creating additional conductive paths between the nano-particles. These nano-particles start to be oxidized confirming better corrosion prevention to the steel specimen. Titanium also gives good prevention against corrosion due to its chemical affinity to form to oxides that help in cathodic protection.
3.2. Basic environment
⌅Figure 6 gives the results obtained by LPR testing for different coatings to determine the corrosion rate in the basic environment. Samples were tested under 1 molar sodium hydro-oxide solution that serves as the base environment. Corrosion rates obtained are 0.1044 mpy, 0.0281 mpy, 0.0129 mpy, and 0.0307 mpy in the bare sample, painted, primer based and paint plus primer samples respectively. Paint coating with titanium oxide-based nano-particles gives 0.07083 mpy that is slightly higher than the paint/primer samples. Zinc oxide nano-particles based paint gives the minimum corrosion rate of 0.000261. As expected, in base condition overall better resistance to corrosion is achieved. That’s the reason that in highly corrosive environment inhibitors were used to provide a basic environment.
3.3. Neutral environment
⌅Figure 7 gives the LPR results obtained for different coatings in a neutral environment. Neutral condition is provided by distilled water. It can be seen that the bare sample gives the corrosion rate of 12.98 mpy, paint gives 0.022 mpy, primer gives 0.0255 mpy, and primer plus paint gives 0.0204 mpy. Zinc-oxide nano-particles and titanium oxide nano-particles based paint coatings give the corrosion rate of 0.00265 mpy and 0.435 mpy, respectively. Again it can be observed that nano-particles based coatings give the best resistance against corrosion.
The bare sample gives the highest corrosion rate of 191.5 mpy in the acidic, the medium of 12.98 mpy in neutral and lowest of 0.1044 mpy in the base environment. This similar trend follows nearly in all samples coated with paint, primer, and primer plus paint. Titanium oxide nanoparticles based paint coating gives highest to lowest corrosion rate in acid, neutral, and base environment respectively. Zinc oxide nano-particles based paint coating follows the similar trend giving maximum resistance to corrosion in the base environment and minimum resistance in an acidic environment. In order to have better understanding and study the effect at a glance a three-dimensional plot is added (Fig. 8), that relates corrosión rate, coatings and environments in a single graph. On the comparison between the acidic, base, and neutral environments, it can be observed that the highest corrosion rate occurred in an acidic environment. The neutral environment also shows more resistance to corrosion as compared to the acidic environment. Best cathodic protection is found in the base environment. Amongst different coatings, bare samples gave the worst corrosion resistance while zinc oxide-based nano-particle paint coatings give minimum corrosion rate (or maximum prevention against corrosion) because of results already explained above. Zinc oxide nano-particles based paint provide coatings that tremendously decreases the corrosion rate from 191.5 mpy to 0.302 mpy in the worst environmental condition (acidic). The current study provides a simple, easy, and less expensive method of synthesis of protective coatings that provide excellent prevention against corrosion.
4. CONCLUSIONS
⌅The current study investigated the corrosion prevention capabilities of different coatings. Low carbon steel is selected for evaluating the corrosion rate using paint, primer, primer plus paint, special nano-particles based (zinc-oxide and titanium oxide) paint coatings. Three environmental conditions are provided (acidic, base, and neutral) to all coated specimens. All experiments were conducted using LPR technique for determining the corrosion rate. The bare sample was also tested to observe the worst scenario of corrosion prevention. In the acidic environment, the bare sample gives 191.5 mpy, neutral condition gives 12.98 while the base environment gives 0.1044 mpy corrosion rate.
The corrosion rate showed a decreasing trend in paint, primer, and primer plus paint in an acidic environment. Titanium oxide-based coating gives a 0.6084 mpy corrosion rate. The lowest rate is seen in zinc oxide nanoparticles based coatings (0.302 mpy). As compared to the acidic environment, samples showed a lesser corrosion rate in the basic medium for all bare and coated samples. In distilled water, again bare sample showed the largest corrosion rate as estimated.
Experimental observations anticipated the analogous drop in corrosion rate in primer, paint, primer plus paint, titanium oxide, and zinc oxide-based coatings respectively. Hence it can be resolved that the base environment gives the best prevention against corrosion. On comparison amongst different environmental conditions provided, acidic medium gives the largest corrosion rate in bare samples as well as in all coatings. Zinc oxide based coating gives the maximum cathodic protection amongst all the coatings while the bare sample gives the worst.
It can be concluded from current research that zinc oxide nanoparticles coating made by mixing with paint shows the maximum resistance against corrosion in all acidic, base, and neutral environments. Hence in different applications having the worst environmental conditions, this coating provides easier, reliable, and cheaper solutions against corrosion.