Protection of Galvanized steel from corrosion in salt media using sulfur nanoparticles

: The characteristics of sulfur nanoparticles were studied by using atomic force microscope (AFM) analysis. The atomic force microscope (AFM) measurements showed that the average size of sulfur nanoparticles synthesized using thiosulfate sodium solution through the extract of cucurbita pepo extra was 93.62 nm. Protecting galvanized steel from corrosion in salt media was achieved by using sulfur nanoparticles in different temperatures. The obtained data of thermodynamic in the presence of sulfur nanoparticles referred to high value as compares to counterpart in the absence of sulfur nanoparticles, the high inhibition efficiency (%IE) and corrosion resistance were at high temperature, the corrosion rate or weight loss decreased with increasing temperature in the presence of sulfur nanoparticles. The positive value of enthalpy ∆H* for galvanized steel with and without sulfur nanoparticles indicates that the reaction was endothermic. Therefore, the sulfur nanoparticles can be suggested as good inhibitor for galvanization in salt media.


Introduction:
The small size and high ratio of surface to volume of nanoparticles have novel characteristics compared with bulk material 1 . The shape and size of nanoparticles depend on many properties, such as applications ranging from catalysts, transistors of computer, activity of antimicrobial, sensors of chemical and electrometers. The nanoparticles were used in many fields for example nanocoposites, drug, medical and hyperthermia 2 . The sulfur nanoparticles were used in diseases of plant for example scab of apple, grapes culture, strawberry and vegetables. In agriculture, sulfur nanoparticles were used due to good efficiency pesticide also black spot and powdery mildew diseases. The sulfur nanoparticles have high quantum size properties and surface area 3 . They are also used in animals and humans diseases such as dandruff causing malassezia furfur. There are many techniques for the synthesis of sulfur nanoparticles such as hydrolysis methods by using sodium polysulphide, precipitation technique for unique membrane-assisted also liquid phase and microemulsion method for water -oil, these techniques involve physical and chemical approaches 4 . Using galvanized steel compared with other materials due to that galvanized steel has good strength, formability and production economics. The reason why steel is the preferred material is because of its resilience, strong, recyclable and economical form. The steel forms stable bonds of iron and oxide salts because it interacts with substances in the atmosphere, and these reactions depend on the concentration of the substances and the nature of the corrosive substances present in the environment. The steel is protected from corrosion by using the galvanizing method, and this method is the most used, as the use of the galvanizing method reduces the replacement of the metal and thus saves energy 5 . Because galvanized steel is resistant to corrosion, it is therefore used in many magazines, including under sea water, car parts, and others. Due to the zinc coating, the metal is rust-resistant, but when the zinc surface is wet with rain, an electrochemical process begins, causing atmospheric corrosion of the galvanized steel structure 6 . Cucurbitaceous is family of cucurbita pepoL, cucurbita pepo L which consist of fatty acids, vitamins E, phytosterol, flavonoids and mineral rich of zinc. The best time to sowing cucurbita pepoL in spring is when temperature is 15 o C and the sowing is at depth 10 cm 7 . Summer squash (Cucurbita pepo L.) is an annual vegetable crop which belongs to Cucurbitaceous family. It is the main source of carbohydrates, dietary fibers, minerals (Ca, Mg, P and Zn) and many important vitamins 8 . The sea water corrosivity depends on multiple factors 5 , such as: salinity, dissolved oxygen, temperature, different types of biological factors 6 (bacteria, fungus, algae), relative displacement of corrosion medium, exposed type of metal. The type of corrosion depends on the mineral exposed to the influence of sea water, so marine corrosion is known as electrochemical corrosion, i.e. an oxidation process (anodizing) and a reduction process (cathodic). This corrosion is wet erosion and has many effects, as the metal is classified in a specific order, known as the galvanic chain, because the corrosion is bimetallic 9 .

2.1-Material
The galvanized steel which has been used in the work had the following composition shown in Table  1 10 . When the carbon steel coated with zinc by using hot-dip process with thickness equal 0.080 nm, then the chemical composition of zinc layer becomes as shown in Table 2.

2.2-Preparation of metal
The galvanized steel was treated by emery paper of different grades from 300 to 1200 up grit ,this process gave smooth surface of metal then washed with Ion-free water, then an ultrasound device is used to clean the metal with acetone, and then it is dried with warm air.

2.3-Preparation of Test Solution
The test solution (3.5% NaCl) is prepared by dissolving 35 g NaCl in 1 L distilled water.

2.4-Preparation of Cucurbita pepo leaf extra
The first step in this work was wishing the leaves with tap water then deionized water and dried at room temperature. The leaves were cut into small pieces, after that 20g of Cucurbita pepo leaf was added to 100ml deionized water and boiled for 1 h at100 o C. The mixture was departed by using centrifuge for 30 min., the solute was kept to use.

2.5-Preperation of sulfur nanoparticles
The mixture of 20ml of Cucurbita pepo leaf extra with 50ml sodium thiosulfate pentahydrate, was stirred for 10min. Then 25 ml of 20% citric acid was added with keeping stir for 30 min. The formation precipitation was collated and kept until use 11 .

2.6-Potentiodynamic polarization measurements
The measurements of electrochemical parameter were used potentiostatic Set -Up, which consisted of the thermostat, host computer, magnetic stirrer and M lab potentiostat-galvanostat. The medium was 3.5%NaCl solution at different temperatures. Corrosion cell have three electrodes (name of electrodes), in the working electrode paced galvanized steel holder with caver opening size 1cm, counter electrode was Pt wire and the reference electrode, that consisted of silver-silver chloride placed near surface working electrode, the test solution was filled in the reference electrode. The open circuit potential (Ecor) was calculated from steady state potential then recorded the cathodic direction of potential and the anodic direction. Fig. 1 shows potentiostatic Set -Up.

Figure 1. Potentiostatic Set -Up
Results and Discussion:

3.1-Atomic Force Microscope (AFM ) measurements
The technique AFM was scanning microscope probe, the major task of Atomic Force Microscope is to account the force between the sample and tip.
Force of repulsive between sample and tip was the basic idea. Atomic Force Microscope explained the 349 scan of the surface sample for image rebuilding from lines acquiring successive sulfur nanoparticles 12 . Fig.2 shows all of AFM images of the sulfur nanoparticles, when the size was 93.62 nm, also the topography displayed adhesion force and peak current. The recognition of surface materials by image area of different properties, is shown in Fig3.The particle size distribution and the average particles size around 93.62 nm. 13 .

3.2-Potentiodynamic polarization
The explained polarization curves of galvanized steel with sulfur nanoparticles and without sulfur nanoparticles in salt media at different temperatures are shown in Figs. (4 and 5). Table 3 shows corrosion current density (Icorr), corrosion potential (Ecorr), anodic Tafel line (ba), cathodic Tafel line (bc), inhibition efficiency (IE) and surface coverage (θ) for galvanized steel with and without sulfur nanoparticles in salt media at different temperatures (298, 308, 318 and 328). It has been found that increasing the temperature in the presence and absence of sulfur nanoparticles caused shifting of the tafel lines to negative potential for anodic processes and positive potential for cathodic processes, due to that sulfur nanoparticles effectiveness on processes, Table 3 and Figs. (4 and 5) show the results. When applying external current, it has been obtained that when anodic was more polarized, then sulfur nanoparticles were anodic inhibitors. The values Ecorr was more positive and (Icorr) decreased with increasing the temperature. This is attributed to the presence of sulfur nanoparticles, which induce an adsorption on surface galvanized steel 14 .  The protection efficiency (IE%) and surface coverage(θ) calculated from equation (1and 2 ) 15 .
Where, IE% = inhibition efficiency, Io and I are the corrosion current density without and with the inhibitor respectively. Table 3 shows the results of increasing temperature, increasing protection efficiency and surface coverage due the sulfur nanoparticles adsorption on surface galvanic steel in salt media at different temperature. It can be noticed that the tafel lines shifted to more positive potential and more negative potential during cathdic processes and anodic processes 16 .
The resisters polarization (Rp) was increasing in the presence of sulfur nanoparticles and increased with increa nanoparticles therefor the sulfur nanoparticles can be a good inhibitor for galvanized in salt media 17 .

3.3-Thermodynamic study
The data listed in Table 4 will be used as reference to evaluate the thermodynamic parameter for corrosion galvanized steel in absence and presence of sulfur nanoparticles in salt media at different temperature.

3.3.1-The activation energy
The data listed in Table 4 will be used as a reference to evaluate the thermodynamic parameter for corrosion galvanized steel in the absence and presence of sulfur nanoparticles in salt media at different temperature.

3.3.1-The activation energy
The activation energy was estimated from equation 3 18 . log CR = − Ea 2.303RT + log A…………..3 Where (C R ) The corrosion rate, T is absolute temperature (K), R is constant of universal gas (8.314 J/mol K) and A is the Arrhenius preexponential constant. According to the equation 3, plots of log C R versus 1/T, the solpe of the straight line is equal to the activation energy and intercept with y axes is log A. Figs (6 and 7) show the result of increasing the activation energy in the presence of sulfur nanoparticles compering with the results in the absence of sulfur nanoparticles due to forming thin film on the surface of metal, the metal increased energy barrier. Therefore the adsorption of sulfur nanoparticles on surface of galvanic steel was physical adsorption.

3.3.2-Enthalpy and Entropy
Enthalpy and entropy functions can be calculated from equation 4 19 .
log(icorr/T) = log(R/Nh) + ∆S 2.303R Where h is the Plank constant, N is Avogadro number, R is the constant of universal gas (8.314 J/mol K), ∆H is the enthalpy activation and ∆S is the entropy activation. Figs.(8 and 9 ) show a plot of ln (Icorr/T) against 1/T for the galvanized steel only and galvanized steel with sulfur nanoparticles. Straight lines are obtained with a slope of (-∆H /R) and an intercept of (ln R/Nh + ∆S /R) from which the values of ∆H and ∆S are calculated and listed in Table 4 19 . Analysis of the values listed in Table 4 show that the positive values of the activation enthalpy (+ ΔH) of the dissolution reaction of galvanized steel in 3.5% NaCl in the presence of the sulfur nanoparticles are greater compared to the case of the free. The positive sign of these enthalpies reflect the endothermic nature of the galvanized steel dissolution process, because decreasing the corrosion rate of galvanized steel was controlled by the activation of kinetic parameters also the value of activation energy was larger than the value enthalpy 20 . The value of entropy was positive value, this is indicates that the complexity in the rate determination steps was activated rather than dissociation steps, the activated complexity became more associated in the rate determination when the inhibitor was in a higher concentration 21 .