السبت، 1 مارس، 2014

Corrosion inhibitors for carbon steel



Corrosion inhibitors for carbon steel

Introduction:

The electrochemical behavior of steel in stagnant alkaline solutions, especially saturated Ca (OH) 2, has been previously investigated (1, 2). It was found that the oxidation processes that take place previously on steel are determined by the degree of surface oxidation of the sample and by dissolved oxygen, but not by the type of caption present. In aerated solution, ferrosoferric oxide (Fe3O4) is the intermediate oxidation product on the steel surface while ferrous hydroxide Fe (OH) 2 is the intermediate product in de-aerated solution (1), therefore immersed steel in aerated or de-aerated solution of ­Ca (OH) 2­ always becomes passivated. Such passivity is destroyed, however, when chlorides Cl- and other corroding materials are present as admixtures in the alkaline solution (3).
Corrosion inhibitor by NaNO2 is a problem of both practical and theoretical significance. Corrosion of steel can be prevented in nutral and alkaline solutions by NaNO2. It is of advantage over chromates in that it has no known effect on the skin. NaNO2 is of theoretical interest in that it is an inhibiting material which does not form an insoluble with iron (4).
It is believed, that the present study of the corrosion behavior of steel in aerated & de-aerated saturated Ca (OH) 2 is similar in many respects to the aqueous phase of cement in reinforcement, so it will contribute to better understanding of the behavior of embedded reinforcement. Keeping in mind that the induced applied current which causes polarization of the immersed mild steel in the anodic direction compared to its free corrosion potential in this study was about 10μA/cm2 using Galvan static pulse technique (5) .


Experimental Work:
The evaluation of an anodic corrosion inhibitor for mild steel immersed in stagnent alkaline solution was investigated using Half-Cell and Galvanostatic polarization techniques.
Steel coupons of (3*1*0..1)cm3 dimensions and chemical composition ( C:0.04, Mn:0.309, Si:0.004, P:0.005, S:0.007, Cr:0.021, Ni:0.01, Mo:0.009, Cu:0.012, Al:0.004, Fe:The reminder ) %wt were used .
The coupons were first polished using emery paper of grades no : 220,320,400 and 600 , degreased by benzene and acetone , rinsed with distilled water & then pickled in 50% HCl acid .The clean steel was coated with epoxy in such away that only about 3cm2 area was always exposed to corrosive media .
De-aerated and aerated Ca (OH) 2 at 30 C0 were used as a corrosive media (i.e., saturated Ca (OH) 2 was prepared by dissolving 1.53g Ca (OH) 2 crystals in 1 liter of pure distilled water. All potential values mentioned are with respect to saturated calomel electrode. For de-aerated test solution, high purity (99.9%) N2 was bubbled into the test solution for ≈ 1.5 h.

Polarization Measurements:

Galvan static technique was used to made these measurements. The circuite diagram and procedure is shown in details (6). The potential of the steel electrode was recorded relative to that of the saturated calomel electrode, at current density of 10μA/cm2 as function of time .The experiments were carried out at 30 C0.



Result and Discussion :
Typical corrosion potential measurements as referenced to saturated calomel electrode (S.C.E), for low carbon steel totally immersed in aqueous solutions of saturated Ca(OH)2 , in absence and presence of both sodium nitrite (NaNo­2) and sodium chloride (NaCl) at different concentrations are tabulated in table (1) , with pH value for each experiment between brackets . The variation of electrode potential with time is shown in figures (1,2,3,4,and5). These figures associated with corrosion potentials in table (1) showed that:
(1) in absence of NaCl. Increasing the inhibitor concentration (NaNO2) from 0% to 0.3% wt., shifts the Ector to more positive direction indicating the effectiveness of NaNo2 to polarize the metal anemically and shifting the Ector to open circuit potential of the cathode. While increasing the concentration of NaNo2 to 3%wt through 1% leads to shift the Ector to negative direction.
(2) Generally, the variation of steel potential with time in alkaline solution containing 0.1,0.3,1&3%wt NaCl, shifts to more positive direction at all inhibitor concentration levels , compared with 0% NaNO2 .

It can be cocluded that: Under the particular conditions of interest here, the low carbon steel at high pH, a film of oxides probably of few nino meters (nm) thickness covers the surface and is responsible for the passive nature of the metal at the low levels of %NaCl concentrations  with increasing NaNO2 from 0.1 to 0.3%wt . This situation becomes more pronounced at high levels of NaCl %wt.
Rosenberg and Gaidis (7) in their study on the mechanism of nitrite inhibition of Cl- attack on reinforcing steel in alkaline aqueous environments concluded that : Nitrite ion rapidly oxidizes Fe+2 ion to Fe+3 ion , blocking further passage of Fe+2 ion from the metal into the electrolyte .

Carbon steel corrosion inhibitors

The prevention of corrosion on the surfaces of metallic pipes, heat exchangers, and the like which are in contact with industrial cooling waters, and particularly industrial cooling waters containing low levels of hardness is controlled utilizing a corrosion inhibiting amount of a composition including an alkali metal silicate, a hydroxycarboxylic acid or its water soluble salts, an organophosphonate, and a water soluble polymer which acts as a dispersant. Superior corrosion inhibition is achieved using the compositions of this invention as corrosion inhibitors, particularly in cooling water systems employing mild steel metallurgy.

The secondary cooling system dissipates the heat produced in the primary circuit of the second Egyptian research reactor (ETRR-2) to the environment through a cooling tower. The secondary system utilizes tap water of ETRR-2 site. It consists of carbon steel piping and fittings, cast iron pumps, and the cooling tower. The corrosion behavior of carbon steel (AISI 1010) in the secondary untreated water and its inhibition by different inorganic anodic (phosphate, molybdate, and unstated) as well as only one cathodes (polyphosphate) inhibitors was studied. Three different methods were used in this study: potentiodynamic polarization tests, simulated tests, and immersion tests. In the simulated test a special test rig was constructed to simulate the conditions existing in the secondary system. The tested concentrations ranged between 20 and 40 ppm. Two types of waters were investigated: untreated secondary water of ETRR-2 site as well as secondary water to which extra amounts of aggressive ions (chloride and sulphate) were added to investigate the effect of their presence on the performance of the tested inhibitors. The effect of using two inhibitors simultaneously (synergism) was also studied: the effect of adding three organic additives to unstated/ molybdate was investigated.
Potentiodynamic polarization and ac impedance studies were carried out on the inhibition of carbon steel in 0.1 M hydrochloric acid solution by various Schiff bases containing heteroaromatic substituents. The examined Schiff bases were 2-((1E)-2-aza-2-pyrimidine-2-ylvinyl)hyphened, 2-((1Z)-1-aza-2-(2-pyridyl)vinyl)pyramiding, 2-((1E)-2-aza-2-(1,3-thiazol-2-yl)vinyl)thiophene, 2-((1Z)-1-aza-2-(2-thienyl)vinyl)benzothiazole. Polarisation curves indicate that studied Schiff bases act essentially as anodic inhibitor. The variation in inhibitive efficiency mainly depends on the type and nature of the substituents present in the inhibitor molecule. Potentiodynamic polarization and ac impedance measurements carried out at different concentration of studied Schiff bases reveal that these compounds are adsorbed on steel surface and the adsorption obeys Temkin’s adsorption isotherm.
In this work the influence of alkylimidazoles on the electrochemical behavior of carbon steel in stirred and aerated NaCl solution is studied. For all the organic compounds tested, the corrosion inhibition efficiency increases with concentration. The inhibition effect has been noticed to increase with the number of carbon atoms in the hydrophobic chain. The maximum corrosion inhibition efficiency was obtained with 11 carbon atoms.
The low corrosion inhibition efficiency with long hydrophobic chains can probably be interpreted by deformation of the hydrophobic head destabilizes the adsorbed layer. Results obtained with N-undecylimidazole showed that it acts catholically at low concentrations and anemically at high concentrations. The maximum corrosion inhibition efficiency was observed about the critical micellar concentration (c.m.c). The establishment of a bimolecular layer film leads to a modification of the cathodic and anodic processes. The electrochemical behavior of 1-(2-ethylamino)-2-methylimidazoline (imidazoline), its precursor N-[3-(2-amino-ethylamino-ethyl)]-acetamide (amide) and its derivative 1-(2-ethylamino)-2-methylimidazolidine (imidazolidine), is evaluated by using potentiodynamic polarization curves and electrochemical impedance spectroscopy, EIS, techniques in deaerated acid media to compare their corrosion inhibition efficiency. The experimental results suggest that imidazoline is a good corrosion inhibitor at different concentrations whereas amide shows low efficiency values; however, the properties of a corrosion inhibitor were not found in imidazolidine. The reactivity of these compounds was analyzed through theoretical calculations based on density functional theory (DFT) to explain the different efficiencies of these compounds as corrosion inhibitors both in the neutral and proton Ted form. The theoretical results indicate that imidazoline is the more efficient corrosion inhibitor because of its two very active sites (two nitrogen atoms) and the plane geometry of the heterocyclic ring, thus promoting coordination with the metal surface.
The corrosion rates in the presence of mimosa tannin as a low carbon steel corrosion inhibitor in sulfuric acid media were measured by the weight loss method, in the range of temperatures from 20 to 60 °C. The Temkin, Frumkin and Freundlich isotherms were tested for their fit to the experimental data. The free energies and enthalpies for the adsorption process and the apparent activation energies, enthalpies and entropies of the dissolution process were determined. The fundamental thermodynamic functions were used to glean important information about the mimosa tannin inhibitory behavior. The results were explained in terms of chemical thermodynamics
inhibition effect of property trephine phosphonium bromide (PgTPhPBr), with a structure (CH6-point triple bond; length half of m-dashC–CH2–)(C6H5–)3 P+Br, on the electrochemical and corrosion behavior of three 0.1, 0.5 and 1.0% C-steels and pure iron in deaerated 1 M H2SO4 solution at different temperatures was studied using linear polarization resistance, polarization curves and SEM investigations. It was found that 0.5% C-steel (CS2) has high density of active sites on the surface leading to high corrosion rate. PgTPhPBr acted as a mixed type inhibitor with anodic predominance in case of pure iron and a mixed type inhibitor in case of C-steels. The inhibition efficiency of PgTPhPBr increased with increasing temperature up to 60 °C. Adsorption of the inhibitor was found to follow the Longmuir's isotherm. This inhibitor was more efficient for CS2 than for other samples at different temperatures. It was proposed that PgTPhPBr decreases the corrosion rate through the reduction mechanism of metal reactivity.
Inhibition effects have been carried out on carbon steel in solutions containing different amines. These compounds were dissolved in some petroleum–water corrosive mixtures containing of acetic acid and NaCl. Corrosion inhibition afforded by ethylenediamine (EDA), hexylamine (HA), butyl amine (BA), tert-butylamine (t-BA), polyamine (PA) and iso-propylamine (i-PA) has been studied by cyclic polarization (CP), electrochemical impedance spectroscopy (EIS) and optical micrography with a carbon steel rotating disk electrode at 25 °C. The best corrosion inhibition was obtained for the 4% of EDA, which is able to act as polarizing agent on the carbon steel surface. Thus, analysis of the change of inhibitor structure and its concentration in the solution allowed us to find the optimum parameters for obtaining the best inhibition. The observed difference in behavior of the additives can be attributed to the differing of solubility in various solvents ratio and to the differing of strict hindrance of the compounds to metal surface.

The influence of cetyl trim ethyl ammonium bromide (CTAB) on the corrosion of carbon steel in HCl and H2SO4 solutions has been studied using several techniques such as weight loss, Tafel polarization, linear polarization and open circuit potential. Inhibition efficiencies have been obtaining from weight loss measurement; the effect of temperature on corrosion inhibition and the effectiveness of the inhibitor at higher acid strength have been examined. Polarization studies reveal that the inhibitor behaves as an effective inhibitor in H2SO4 as well as in HCl solutions. Measurements of values of polarization resistance (Rp) have also been carried out. The open circuit potential curve was shifted to less negative potential contrary to the blank.

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