ANTI-CORROSION PERFORMANCE OF 1 , 3-BENZOTHIAZOLE ON 410 MARTENSITIC STAINLESS STEEL IN H 2 SO

The corrosion inhibition e®ect of synthesized 1,3-benzothiazole at very low concentrations on 410 martensitic stainless steel in 3MH2SO4 solution was studied through potentiodynamic polarization and weight loss measurements. The observation showed that the organic compound performed e®ectively with average inhibition e±ciencies of 94% and 98% at the concentrations studied from both electrochemical methods due to the inhibition action of protonated inhibitor molecules in the acid solution. The amine and aromatics functional groups of the molecules active in the corrosion inhibition reaction were exposed from Attenuated total re°ectance-Fourier transform infrared spectroscopy (ATR-FTIR) spectroscopic analysis. Thermodynamic calculations showed cationic adsorption to be chemisorption adsorption, obeying the Langmuir adsorption isotherm. Images from optical microscopy showed an improved morphology in comparison to images from corroded stainless steel. Severe surface deterioration and macro-pits were observed in the uninhibited samples.


Introduction
H 2 SO 4 is an important industrial chemical extensively applied over a wide range of applications involving the use of stainless steels, such as in fertilizer production, paper bleaching for chlorine dioxide generation, water treatment facilities, steel manufacturing, petroleum re¯nery and so on. 1 Due to the presence of sulfate anions (SO 2À 4 Þ and other impurities in these environments, stainless steels su®er from severe surface deterioration, pitting and general corrosion and in some cases cracks due to hydrogen embrittlement leading to catastrophic failure and huge cost overheads. 2,3There are numerous corrosion control measures for stainless steels in acidic media encountered in industry. 4Chemical compounds known as inhibitors, especially those of organic origin, are most often used, being the most practical and cost-e®ective method for corrosion control.Selection and application of appropriate inhibitors are very important due to the complex nature of corrosive environments. 5,6][9][10] Benzothiazole is an organic compound used as accelerators for the vulcanization of rubbers, as an insecticide, in the production of dyes and pharmaceutical drugs and as a food-°avoring agent. 113][14] 410 martensitic stainless steel is applied in petroleum fractionating structures, mine ladder rungs, gas turbines, shafts, pumps, valves and as bolts, screws, bushings and nuts. 15This research aims to study the corrosion inhibition e®ect of 1,3benzothiazole on 410 martensitic stainless steel (41000SS) in 3 MH 2 SO 4 solution through the use of potentiodynamic polarization and weight loss technique.The inhibitor functional groups responsible for adsorption will be determined through ATR-FTIR spectroscopy, and surface morphology of inhibited and corroded steel specimens will be analyzed through optical microscopy.Corrosion kinetic and thermodynamic properties will be evaluated.

Potentiodynamic polarization technique
Polarization measurements were carried out at 30 C using a three-electrode system and glass cell containing 200 mL of the corrosive test solution at predetermined concentrations of BEZT with Digi-Ivy 2311 electrochemical workstation.Cylindrical S41000SS electrodes mounted in acrylic resin with an exposed surface area of 0.50 cm 2 were prepared according to ASTM G59-97(2014). 17Polarization plots were obtained at a scan rate of 0.0015 V versus Ag/AgCl/s between potentials of À1:5 V versus Ag/ AgCl and þ1:5 V versus Ag/AgCl, according to ASTM G102-89(2015). 18Platinum rod was used as the counter electrode and silver chloride electrode (Ag/AgCl) as the reference electrode.Corrosion current density (J cr , A/cm 2 Þ and corrosion potential (E cr , V versus Ag/AgCl) values were obtained using the Tafel extrapolation method, whereby the estimated corrosion current, I cr , was obtained from the intercept of the two linear segments of the Tafel slope from the cathodic and anodic polarization plots. 19,20he corrosion rate (C R Þ was calculated from the mathematical relationship: where E qv is the sample equivalent weight in grams, 0.00327 is a constant for corrosion rate calculation in mm/y 21 and d is the density in g.The inhibition e±ciency ( 2 , %) was determined from the corrosion rate values according to Eq. (2): where C R1 and C R2 are the corrosion rates without and with BEZT compound, respectively.

R. T. Loto
Polarization resistance (R p , Þ was calculated from Eq. (3) below: where B a is the anodic Tafel slope and B c is the cathodic Tafel slope, both are measured as (V versus Ag/AgCl/dec).

FTIR spectroscopy and optical microscopy characterization
BEZT/3 MH 2 SO 4 solution (before and after the corrosion test) was exposed to speci¯c range of infrared ray beams from Bruker Alpha FTIR spectrometer at a studied wavelength range of 375-7500 cm À1 and resolution of 0.9 cm À1 .The transmittance and re-°ectance of the infrared beams at various frequencies were decoded and transformed into an FTIR absorption plot consisting of spectral peaks.The spectral pattern was evaluated and equated according to the FTIR absorption table to identify the functional groups involved in the corrosion inhibition reaction.Images of corroded and inhibited S41000SS surface morphology from optical microscopy were analyzed after weight-loss measurement with Omax trinocular through the aid of ToupCam analytical software.

Weight loss measurement
Measured S41000SS steel coupons separately immersed in 200 mL of the dilute acid test solution for 240 h at 30 C were weighed every 24 h according to ASTM G31-72( 2004).The corrosion rate (C R Þ is determined as follows 22 : where ! is the mass loss in mg, D is the density in g/ cm 3 , A is the total surface area of the coupon in cm 2 and 87.6 is a constant for corrosion rate determination in mm/y.t is the time in h.Inhibition e±ciency (Þ was determined from the mathematical relationship: where ! 1 and ! 2 are the mass losses at speci¯c BEZT concentrations.Surface coverage was determined from the following relationship 23,24 : where is the degree of the BEZT compound adsorbed per gram of the steel samples.! 1 and ! 2 are the mass losses of each steel coupon at speci¯c concentrations of BEZT in the acid solution.

Potentiodynamic polarization
Polarization results for the e®ect of BEZT on S41000SS corrosion are shown in Table 1 and Fig. 2 depicts the polarization plots.The values in show the signi¯cant variation in corrosion rates and other potentiostatic parameters between the uninhibited and BEZT-inhibited samples.BEZT performed e®ectively at all concentrations (very low concentrations) studied with a maximum inhibition e±ciency of 86.07% at 0.5% BEZT concentration.This is due to the adsorption of BEZT molecules on the steel surface forming a protective barrier ¯lm and blocking the active sites on the metal surface. 25,26hanges in BEZT concentration had no signi¯cant in°uence on the corrosion rate and inhibition e±ciency values.This observation aligns with the values obtained for polarization resistance (Table 1) and corrosion current densities, which generally remained similar at all BEZT concentrations.The corrosion potential of S41000SS at 0% BEZT at À0:330 V versus Ag/AgCl di®ers from values obtained at 0.13-0.75%BEZT due to the presence of an e®ective BEZT ¯lm as mentioned earlier, which inhibits the electrolytic transport of corrosive sulfate anions (SO 2À 4 Þ and metallic cations which may result from oxidation. 27espite the minimal variation observed for cathodic tafel slopes, the inhibitor did alter the hydrogen evolution and oxygen reduction mechanism of the cathodic reaction process; however, values obtained for anodic Tafel slope show that BEZT had a stronger in°uence on the anodic dissolution mechanisms. 28he potentiodynamic polarization plots in Fig. 2 show active and anodic-passivation between the uninhibited and inhibited S41000SS samples.The polarization graph for 0% BEZT with a corrosion potential of À0:330 V versus Ag/AgCl corresponds to the active redox electrochemical mechanism, resulting in accelerated deterioration of the alloys.At 0.13-0.75%BEZT, the corrosion potentials shift to the anodic potentials signifying anodic inhibition through electrostatic interaction between the negatively charged Fe surface and the protonated inhibitor.This process sti°es the oxidation reactions responsible for S41000SS dissolution.[31]

ATR-FTIR spectroscopy analysis
Identi¯cation of the functional groups responsible for BEZT inhibition on S41000SS in H 2 SO 4 solution was done through ATR-FTIR spectroscopy and matched with the ATR-FTIR table 32,33 for identi¯cation.The ATR-FTIR spectra of 3 MH 2 SO 4 /BEZT solution before and after the corrosion tests are shown in Fig. 3.The spectral diagrams show the same peak con¯guration for both test solutions but vary in intensity, suggesting that the dominant anodic inhibition properties of BEZT are through surface coverage and suppression of the redox electrochemical process.The spectral peaks for the test solution after corrosion decreased signi¯cantly at some wavenumbers between 3348.98 and 579.33 cm À1 due to adsorption as a result of the electrochemical reaction of speci¯c molecules and functional groups of BEZT with S41000S surface in inhibiting the corrosion of the steel.The spectra peaks of 3348.96,1633.25

amides) and C-H \oop" (aromatics)
. Past research has shown amines, alcohols and hydroxides functional groups to be e®ective corrosion inhibitors. 34,35EZT being an aromatic heterocyclic compound with the chemical formula C 7 H 5 NS consists of a 5-membered 1,3-thiazole ring attached to a benzene ring.Considering its structure, several points of inhibitor/metal interaction can be identi¯ed.The free electron pairs on N and S and the -electrons from the aromatic rings are capable of forming covalent bonds with Fe substrate metal.The double bonds in the molecule enable reverse donation of metal d-electrons to the pi-orbitals.S-atoms in the BEZT molecular structure due to their lower solubility, excess free lone pair of electrons and greater polarizability enable the formation of the d-d bond, resulting from the overlapping of 3d-electrons which enhances the adsorption of the inhibitor onto the metal.The N atom of the amino group attached to the thiazole ring enhances the BEZT to adsorb onto the steel. 36,37

Weight loss and optical microscopy
Data from weight loss analysis of S41000SS in 3 MH 2 SO 4 at 0-0.75% BEZT are shown in Table 2. Figures 4 and 5 show the plot of S41000SS corrosion rate and BEZT inhibition e±ciency versus exposure time in the acid solution.The optical micrographs for the steel specimen before and after corrosion are shown in Figs.6(a)-8(b).Observation of Fig. 4 shows the signi¯cant contrast between the uninhibited and inhibited stainless steel during the exposure hours.The corrosion rate of the uninhibited steel remained generally high.At 24 h, the corrosion rate  which reacts with the steel surface.Observing the corrosion rate values for the inhibited steel specimens at speci¯c BEZT concentrations (Fig. 4), similar electrochemical behavior can be deduced from the plot from 24 to 240 h.Their corrosion rates remained generally the same throughout the exposure period at signi¯cantly low values.The electrochemical behavior of the inhibited steel specimens in interaction with the BEZT compound is clearly depicted in Fig. 5.At all BEZT concentrations, the inhibition e±ciency increased progressively with time till about 72 h, showing that the inhibiting action of BEZT is time-dependent, although at high e±ciency values.Between 72 and 192 h, their inhibition e±ciencies were generally constant before increasing progressively again to the end of the exposure period.The micrographs of the inhibited steel specimens con¯rm their corrosion behavior on the weight loss plot, as the surface morphology shows a slightly corroded or etched surface due to the competitive action of corrosive ions and the protonated inhibitor molecule before the inhibitor completely adsorbed onto the steel and inhibited further action of the corrosive species.

Adsorption isotherm
Electrostatic adsorption of the BEZT compound onto the alloy surface inhibits the corrosion process.
Adsorption isotherm gives useful information on the quantitative adsorption of BEZT molecules for a given set of state variables.The amount of the adsorbed material is known as surface coverage .The Langmuir   The Langmuir isotherm is conventionally expressed as 38,39 : where is the degree of surface coverage of the inhibitor on the alloy surface, C is the BEZT concentration in H 2 SO 4 acid media and K ads is the equilibrium constant of the adsorption process.The plots of C versus the BEZT concentration were linear (Fig. 9), con¯rming the Langmuir adsorption.
Based on the assumptions of Langmuir isotherm, all adsorption sites are equivalent and have the same binding energy to the surface.BEZT-protonated molecules adsorb over the entire surface at the metal/ solution boundary without causing any change in the slope from unity (Fig. 9). 40Increase in BEZT concentration caused changes in its energy of adsorption in comparison with water molecules as BEZT molecules became more concentrated on the steel.

Thermodynamics of the corrosion inhibition mechanism
The equilibrium constant of adsorption K ads is related to the standard free energy of adsorption, Á G ads by the expression in Eq. ( 8).Data obtained for ÁG o ads as a result of BEZT adsorption on S41000SS (Table 3) were calculated from the following relationship: where 55.5 is the molar concentration of water in the solution, R is the universal gas constant, T is the absolute temperature and K ads is the equilibrium constant of adsorption for BEZT.The negative values of ÁG ads show that the adsorbed layer on the S410SS surface is stable and the adsorption process is spontaneous. 41The calculated values ranged between À45:39 < ÁG ads < À48:88 kJ/mol, which is higher than the threshold value (À40 kJ/mol) necessary for chemisorption adsorption resulting from charge sharing or a transfer from the BEZT cations to the charged metal surface. 42The varied properties of the S41000SS surface are responsible for the di®erences in ÁG o ads for BEZT inhibitor in direct proportion to changes in surface coverage values as shown in Table 3. 43,44

Conclusion
BEZT e®ectively inhibited the corrosion of the martensitic stainless steel in the acid media through electrostatic attraction and covalent bonding, resulting from chemisorption mechanism onto the steel surface.The inhibition e±ciency values remained su±ciently high from the onset of the exposure hours till the end due to the inhibition reaction of the molecular functional groups and heteroatoms of the compounds, which strongly altered the mechanism of the electrochemical process, protecting the steel from corrosion.The inhibition property of the compound was determined to be anodic-type inhibitor.

Table 3 .
Results for standard free energy, surface coverage and equilibrium constant of adsorption for 0.13-0.75%BEZT/3 MH 2 SO 4 .