Investigation on Corrosion Behaviour of Polymeric Coatings on Weathering Steel Under 3.5% NaCl Solution

Weathering steel, a low carbon alloy steel, most widely used in marine transportation and construction elds like bridges, sculptures where corrosion resistance is the main concern. Weathering steel creates an adhering protective coating on its surface in the right environmental conditions. The protective layer which is formed on the surface weathering steel known as patina. It protects the structure from further corrosion to occur. When these weathering steels are continuously exposed to moisture environment its corrosion resistance decreases. So, our investigation deals about the studies of the coatings used on weathering steel to increase its corrosion resistance in marine conditions. The coatings applied on the substrate were poly urea, polyurethane, epoxy and black enamel. Corrosion tests were performed on these coated samples to evaluate their corrosion behaviour in different environments. Corrosion tests that were performed are immersion test, salt spray test and potentio-dynamic test. The prolometry test is investigated and it conrmed the depth of the corrosion on the coatings. Based on the corrosion studies, we conclude that poly urea is the excellent coating followed by polyurethane, black enamel and epoxy.


Introduction
Weathering metallic is a high strength low alloy metallic material that became at rst evolved through United States during 1930s against corrosion and abrasion problems [1][2][3][4]. It became rst utilized in production at the John Deere World Headquarters constructing in Moline, Illinois, which opened in 1964 [5]. Since then, the usage of weathering metallic has unfold global and in Europe it's far to be had as "structural metallic". In the right environmental conditions, weathering steel quickly develops a protective rust 'patina,' which also prevents corrosion [6][7][8][9][10]. Because the rate of corrosion is so low, untreated weathering steel bridges can have a 120-year design life with very minor maintenance. Hence, a nicelyparticular weathering steel bridge in the ideal surroundings presents an attractive, very low maintenance.
Weathering steel, frequently noted through the COR-TEN steel [11]. The COR-TEN refers to the two distinguishing properties of this kind of metallic: corrosion resistance and tensile strength. The use of weathering steel is in lots of instances economically advantageous [12][13][14][15]. The fabrication, maintenance and assembly cost of weathering steel is usually 2-10% decrease while in comparison with the systems protected with conventional corrosion protection systems [16][17][18][19][20]. The economics concern of the usage of weathering steels is the removal of charges linked with upkeep or healing without corrosion safety systems. The simple precise belongings of weathering steels are their stepped forward corrosion resistance in atmospheric conditions. Polymeric coatings are coatings or paint made with polymers that offer advanced adherence and safety from corrosion [21][22][23][24][25]. The polymeric coating system applies an elastomer or different polymeric cloth onto a helping substrate. Polymer coatings may be implemented to metals, ceramics in addition to arti cial substances. They are temperature-resistant as much as approx. 535°F (280°C) and FDA-authorised for food production. Polymeric coatings made from polymeric substances may be implemented on a lot of substrates the usage of numerous special strategies inclusive of extrusion/dispersion coating and answer application. Polymeric coatings designed for corrosion safety are typically more di cult and are implemented in heavier movies than are look coatings. Polymeric coatings are required to stick nicely to the substrate and should now no longer chip effortlessly or degrade from heat/moisture/salt, or chemicals. Polymeric coatings are changing chrome and cadmium coatings partially because of growing issue approximately heavy metals and environmental factors [25][26][27][28][29][30][31][32]. The use of unique polymers in coatings permits the advent of hydrophobic surfaces and the powerful prevention of the sticking of numerous materials inclusive of adhesives/rubber/arti cial substances to those surfaces. The focused alteration of the oor shapes the usage of described roughness pro les complements the non-stick impact through lowering the touch area (e.g. Te on coating, PTFE, PFA). The chemical composition of weathering steels, IS 2062 and IRS M41 are presented in Tables 1,2,3 respectively which was con rmed by using optical emission spectroscopy (OES) technique in test point Coimbatore, Tamil Nadu, India.

Electrochemical Principles And Potentio-dynamic Polarization Measurements
Weathering steel specimen is immersed in a corrosive environment, corrosion reactions such as reduction and oxidation approaches arise on its surface. Moreover, the specimen corrodes and the solvent is reduced. It is observed that hydrogen ions are reduced in acidic condition. Each anode and cathode, as well as each anodic and cathodic current, must be visible on the specimen surface. Anodic currents are usually the cause of any corrosion processes that occur. When a specimen comes into contact with a corrosive liquid and is not linked to any instrumentation-as it would be "in service"-the specimen known as the corrosion potential, or E CORR . E CORR of the specimen has each anodic and cathodic currents present on its surface. Nevertheless, those currents are precisely identical in importance so there is no net current present to be measured. The ability at which the rate of oxidation is exactly equal to the rate of reduction is known as E CORR . Figure 1 show the three-electrode setup for studying electrochemical measurements.
The corrosion current can be directly associated to the corrosion rate using the equation ( Figure 2 show the optical pro lometry. It is a method used to extract topographical information on the surface of the specimen. A single unconnected point, a line experiment, or even a whole 3D experiment is also possible. The surface morphology and its step height and surface roughness will be observed clearly in pro lometry experimentation that is the main purpose of doing pro lometry study. Surface and structural materials analysis includes microstructural characterization of materials, including polymers, lms, coatings, geological and mineral samples, pharmaceuticals, raw materials, metals, plastics, ceramics, glass, food, dust, contaminants and other sample types.

Immersion test
In order to understand the corrosion behaviour of the unique polymeric coatings used on weathering steels by performing an immersion test. Base metal IRS M41, base metal IS 2062, polyurea IS2062, polyurea IRS M41, epoxy, black enamel. The immersion test has been conducted in accordance with ASTM G31 standard. The samples density is measured through Archimedes principle.

Procedure for Immersion test
The deionized water was mixed with the appropriate amounts of analytical-grade reagents (3.5wt. percent) NaCl solution with a pH of around 7.1, and they were utilised to replicate neutral seawater. The temperature of the solution has been maintained at 25°C for corrosion process, room temperature kept into average of 17°C. The samples had been subjected to cyclic conditions, with each cycle of every 24 h consisting of a moist period of 24 hours at 25 o C. The specimen's surfaces had been ground in order to obtained at and smooth surface. Later, the specimens were polished with silicon carbide emery papers of various grades like 80grit to 1200 grit, and then ultrasonically wiped clean in acetone, and dried and saved in a desiccator. All the samples weight and surface area had been calculated and then image also recorded before immersion test. Table 4,5,6 representing the density values of the samples, initial dimensions of the samples and initial area of the samples respectively. The samples had been immersed in accordance with ASTM G31 standard which is shown in gure 3. Twelve specimens had been examined for every period. The dissolved oxygen and pH of the solution was detected and recorded. Chemical cleaning processes in accordance with ASTMG1-03-E for the elimination of corrosion product, followed by ultrasonically cleaning with distilled water for 10 minutes and drying with hot air. Table 7 and 8 representing the weight measurement of samples of immersion test with and without scratch samples respectively.  Table 9 and 10 representing the weight measurement of samples of salt spray test with and without scratch condition. Figure 4 show the samples are placed inside the salt spray fog chamber after exposed 24 h under 3.5wt.% NaCl solution.

Potentio-dynamic polarization
In order to get the corrosion current of the given specimens with the aid of using necessary potential difference. Experimental apparatus for the potentio-dynamic polarization (PDP) ACM Gill AC, (NIT Trichy) ACM instrument has been used. The specimens are base IS 2062, polyUrea IRSM41. For PDP test, ACM instrument has been potentiated, samples were polished using emery papers to remove the scratches in order to get at surface prior to the test, then polished in disc polishing. For coated sample the test was conducted directly. The sample is xed to the electrochemical cell using tightening screw. The sample to be evaluated is a working electrode with a reference electrode of saturated calomel and an auxiliary electrode of platinum foil. The potential of the solution sample interface is measured by the reference electrode, while the current ow is measured by the auxiliary electrode. The samples were immersed in the test solution until the open circuit potential was reached (OCP). For this, 3.5% NaCl solution is used as an electrolytic solution. Power supply is turned on after the proper circuit connections, using the software the cyclic sweep program parameters. On starting the cyclic sweep, the voltage across the circuit and current passing through the sample is measured by the equipment for every few seconds. After the end of the cyclic sweep the plots of potential Vs current or Tafel polarization plot for the sample in given electrolyte is obtained from the graph using the software. Table 11 and 12 representing the parameters for electrochemical polarization test and density values of coated samples respectively.  The density of the coated samples obtained after coatings was determined by measuring the mass of the samples using a weighing balance and the dimensions of the coated samples using Vernier calipers. The sample density is reported in the table above. The ASTM G31 standard provides a formula for calculating corrosion rate. It can be calculated using the following formula: 3.5 Observations from immersion test: Figure 6 and 7 show the macrostructure images of samples before and after immersion test and corrosion rate (mpy) vs Time(h) comparative graph between different coatings with and without scratched respectively. From the values of corrosion rate, we can deduct that poly urea has the least corrosion rate and at the same time epoxy, an old coating used on railway tracks found to exhibit more corrosion rate when compared to other coatings. Base metals IS 2062 found to exhibit more corrosion rate than other samples because of less percentage of Cu and absence of coating on it. On the other hand, polyurethane exhibits medium corrosion rate in between polyurea and epoxy coating. Even in the scratched samples the same scenario repeats i.e. poly urea has better corrosion resistance than any other coatings, but poly urethane exhibits more corrosion rate in scratched conditions, it can clearly be seen in the images of the sample, that polyurethane surface peels off in scratch conditions.

Salt spray test studies:
3.7 Corrosion rates of unscratched samples: 3.8 Corrosion rates of scratched samples: to epoxy coatings these showed better corrosion resistance due to the formation of the patina layer on the base metals. These samples are exposed to aggressive environment their corrosion rate will equalize to epoxy coatings and patina layer will be peeled off from the surface.
Epoxy coatings show poor corrosion resistance both in immersion and salt spray test due to the less resistance of the epoxy towards the chemical attack and high temperatures. Even in the scratched states it shows high corrosion rate due to the openings of the bonds at OH areas. Even we can observe in macro images that the surface of the epoxy is peeling off due to reaction of the OH groups with the moisture.
Because of its superior adhesion and mechanical qualities, a poly urea coating has a stronger corrosion resistance than any other coating. Furthermore, the polyurea molecule's amide bond is responsible for these features. These bond vander waal forces act between the surface and the coatings, holding them together. Polyurea may be more resistant to chemical attack, allowing for increased corrosion resistance even in coastal environments. Figure 12 and 13 show the pro lometry analysis for base IS 2062 and poly urea coated sample respectively. Polyurethane coatings show better corrosion resistance in unscratched state but in the unscratched state, polyurethane coatings have a stronger corrosion resistance, but in the scratched state, bond breaks on the functional groups imply poor overall performance shown in gure 12 and 13. For metallic substrate applications, these offer numerous bene ts. In compared to other coatings, polyurethanes have a wet appearance, high mechanical performance, chemical resistance, and UV resistance. Without sacri cing performance, polyurethanes can be supported as higher solids or powder coatings. Polyurethanes are now the industry leader for coating metallic and plastic substrates in a wide range of high-end and highly visible applications. In the future, the performance of urethane coatings will allow them to be used in a broader spectrum of application. Black Enamel coatings shows same scenario as poly urethane. Coatings are peeled off in scratched conditions it might be due to the poor adhesion. Poor adhesion is due to the increase in the temperature while drying. Drying time and temperature play a vital role in determining corrosion rate.

Conclusion
On weathering steels: In increasingly corrosive atmospheres, the period for corrosion layer stability (consolidation) reduces as the corrosivity of the exposed atmosphere grows. The corrosivity of the environment raises the steady-state corrosion rate of weathering steel. Unpainted weathering steel can only be used in non-marine environments with low corrosivity. Due to the lack of essential wet/dry cycles, continuous moist exposure does not allow the creation of protective corrosion layers.
On coated samples: Because of the existence of amide bonds, poly urea coatings have better corrosion resistance than other coatings such as polyurethane, black enamel, and epoxy, even in maritime circumstances. Poly urethane shows good corrosion resistance in unscratched state regardless of the environment due to its better mechanical properties and good adhesion, but in scratched condition its performance is poor due to the bond breakage at CO-O areas. Epoxy coatings shows poor corrosion resistance due to its poor adhesion nature and its properties changes according to the temperature. In scratched conditions, poly urea coatings offer good corrosion resistance, while epoxy and poly urethane show poor phenomena. So, it is recommended to restrict its usage in service conditions at high temperatures. Figure 2 Optical Pro lometry  Macro Structure images of samples before and after Immersion test  Stereo macro structure analysis of samples before and after salt spray test   Pro lometry results for Base IS 2062