Electrochemical behaviour of Ni-base alloys exposed under oil/gas field environments

The electrochemical behaviour of Ni-base alloys (Inconel 625, Inconel 718, G3 and Incoloy 825) is carried out at 80 °C in CO₂/H₂S corrosion environments using cyclic potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. The passivity mechanisms are analysed and discussed. In addition, some significant characterisation parameters such as E_corr, I_pass, E_pit, E_pp, ΔE and I_pass in cyclic polarisation curves are analysed and compared to reveal the corrosion resistance of various Ni-base alloys. The equivalent circuit model and ZsimpWin software are utilised to discuss the Nyquist plots of various Ni-base alloys. The diffusion mechanism in EIS measurement is discussed. The result shows that the corrosion resistance of the Ni-base alloys to CO₂ corrosion or CO₂/H₂S corrosion follows the sequence: Inconel 625 > G3 > Inconel 718 > Incoloy 825. H₂S works as a cathodic depolariser with accelerating initiation of the corrosion process.     

Corrosion Protection Properties of 4-[(E)-[(2,4-Dihydroxy phenyl)methylidene] amino]-6-methyl-3-sulfanylidene-2,3,4,5-tetrahydro-1,2,4-triazin-5-one [DMSTT] Toward Mild Steel in Sulfuric Acid

The inhibition of mild steel corrosion in aerated 0.5 N H₂SO₄ solution was investigated using potentiodynamic polarization studies (Tafel), linear polarization studies, electrochemical impedance spectroscopy studies, adsorption studies, and surface morphological studies. The effect of inhibitor concentration on corrosion rate, the effect of temperature, degree of surface coverage, adsorption kinetics, and surface morphology are investigated. The inhibition efficiency increased markedly with increase in the additive concentration and decreased slightly with increasing temperature. The presence of DMSTT decrease the double-layer capacitance and increase the charge transfer resistance. The value of activation energy (E _a) of metal corrosion, adsorption equilibrium constant (K _ads), and free energy of adsorption (ΔG _ads) were calculated from the temperature dependence of corrosion current. The adsorption of inhibitor molecule on mild steel surface follow Langmuir isotherm. DMSTT offers excellent inhibition properties and acts as a mixed-type inhibitor.     

The Inhibition Effect of Synthesized 4-Hydroxybenzaldehyde-1,3propandiamine on the Corrosion of Mild Steel in 1 M HCl

The corrosion inhibition of mild steel in 1 M HCl by 4-hydroxybenzaldehyde-1,3propandiamine (4-HBP) has been investigated using potentiodynamic polarization, electrochemical impedance spectroscopy and chronoamperometry measurements. The experimental results suggest that this compound is an excellent corrosion inhibitor for mild steel and the inhibition efficiency increases with the increase in inhibitor concentration. Polarization curves reveal that this organic compound is a mixed-type inhibitor. The effect of temperature on the corrosion behavior of mild steel in 1 M HCl with the addition of the Schiff base was studied in the temperature range from 25 to 65 °C. The experimentally obtained adsorption isotherms follow the Langmuir equation. Activation parameters and thermodynamic adsorption parameters of the corrosion process such as E _a, ΔH, ΔS, K _ads, and ΔG _ads were calculated by the obtained corrosion currents at different temperatures and using the adsorption isotherm. The morphology of mild steel surface after its exposure to 1 M HCl solution in the absence and in the presence of 4-HBP was examined by AFM images.     

A newly synthesized glycine derivative to control uniform and pitting corrosion processes of Al induced by SCN anions - Chemical, electrochemical and morphological studies

A newly synthesized glycine derivative (termed GlyD), 2-(4-(dimethylamino)benzylamino)acetic acid hydrochloride, was used to inhibit uniform and pitting corrosion processes of Al in 0.50 M KSCN solutions (pH 6.8) at 25 °C. For uniform corrosion inhibition study, Tafel extrapolation, linear polarization resistance and impedance methods were used, complemented with SEM examinations. An independent method of chemical analysis, namely ICP-AES (inductively coupled plasma atomic emission spectrometry) was also used to test validity of corrosion rate measured by Tafel extrapolation method. GlyD inhibited uniform corrosion, even at low concentrations, reaching a value of inhibition efficiency up to 97% at a concentration of 5 × 10⁻³ M. Results obtained from the different corrosion evaluation techniques were in good agreement. This new synthesized glycine derivative was also used to control pit nucleation and growth on the pitted Al surface based on cyclic polarization, potentiostatic and galvanostatic measurements. The pitting potential (E_pit) and the repassivation potential (E_rp) increased by the addition of GlyD. Thus GlyD suppressed pit nucleation and propagation. Nucleation of pit was found to take place after an incubation time (t_i). The rate of pit nucleation and growth decreased with increase in inhibitor concentration. Morphology of pitting was also studied as a function of the applied anodic potential and solution temperature. Cross-sectional view of pitted surface revealed the formation of large distorted hemispherical and narrow deep pits. GlyD was much better than Gly in controlling uniform and pitting corrosion processes of Al in these solutions.     

Galvanic reactions during localized corrosion on stainless steel

During localized (crevice and pitting) corrosion, a local cell is established between an anode within a crevice or pit and a cathode on the surrounding passive surface. Data are presented to show that concentrated acidic chloride solutions, simulating corrosion product hydrolysis within a crevice or pit, produce potentials which are active (negative) to the normal surface passive potential. This behaviour explains the previously observed active drift of corrosion potential after initiation of crevice or pitting attack in dilute chloride solutions. The active state in concentrated chloride solutions was quite noble (positive) compared to the active state in more dilute solutions. Thus, there is no need to invoke ohmic resistance effects to account for the active state within a crevice or pit.Experiments were devised in which the local anode within a crevice was physically separated from the nearby passive-surface cathode. When the two were coupled together electrically, the cathode surfaces were polarized nearly to the unpolarized local anode potential, with only a few millivolts anodic polarization at the anode within the crevice. The rate of localized corrosion appears from the data to be limited by the rate of dissolved-oxygen reduction on the cathode surfaces. Thus, localized corrosion in dilute chloride solutions will be increased by (a) raising the temperature, (b) adding an oxidizer such as Fe³⁺ ions, or (c) substituting external anodic polarization for dissolved oxidizers.The overall potential, E_corr acquired by a specimen undergoing pitting or crevice corrosion is demonstrated to be near the protection potential, E_p below which pitting corrosion cannot propagate. Any potential active to E_corr and E_p results in cathodic polarization and suppression of the anode reaction in a crevice or pit. Since both E_corr and E_p vary with the extent of previous localized attack, E_p is not a unique property of the alloy as has been sometimes suggested and is of limited value in classifying alloy resistance to localized corrosion.     

Electrochemical and theoretical investigation on the corrosion inhibition of mild steel by thiosalicylaldehyde derivatives in hydrochloric acid solution

Inhibitory effect of three Schiff bases 2-{[(2-sulfanylphenyl)imino]methyl}]phenol (A), 2-{[(2)-1-(4-methylphenyl)methylidene]amino}-1-benznethiol (B), and 2-[(2-sulfanylphen-yl)ethanimidoyl)]phenol (C) on corrosion of mild steel in 15% HCl solution has been studied using weight loss measurements, polarization and electrochemical impedance spectroscopy (EIS) methods. The results of the investigation show that the compounds A and B with mean efficiency of 99% at 200 mg/L additive concentration have fairly good inhibiting properties for mild steel corrosion in hydrochloric acid, and they are as mixed inhibitor. All measurements show that inhibition efficiencies increase with increase in inhibitor concentration. This reveals that inhibitive actions of inhibitors were mainly due to adsorption on mild steel surface. Adsorption of these inhibitors follows the Langmuir adsorption isotherm. Thermodynamic adsorption parameters (K_ads, ΔG_ads) of studied Schiff bases were calculated using the Langmuir adsorption isotherm. Activation parameters of the corrosion process such as activation energies, E_a, activation enthalpies, ΔH^∗, and activation entropies, ΔS^∗, were calculated by the obtained corrosion currents at different temperatures. Obvious correlation was found between the corrosion inhibition efficiency and the calculated parameters. The obtained theoretical results have been adapted with the experimental data.     

Corrosion inhibition of mild steel in hydrochloric acid solution by some double Schiff bases

The inhibition effect of four double Schiff bases on the corrosion of mild steel in 2 M HCl has been studied by polarization, electrochemical impedance spectroscopy (EIS) and weight loss measurements. The inhibitors were adsorbed on the steel surface according to the Langmuir adsorption isotherm model. From the adsorption isotherm, some thermodynamic data for the adsorption process were calculated and discussed. Kinetic parameters activation such as E_a, ΔH∗, ΔS∗ were evaluated from the effect of temperature on corrosion and inhibition processes. Quantum chemical calculations have been performed and several quantum chemical indices were calculated and correlated with the corresponding inhibition efficiencies.     

Taguchi Optimization of Pulsed Current GTA Welding Parameters for Improved Corrosion Resistance of 5083 Aluminum Welds

In this study, the Taguchi method was used as a design of experiment (DOE) technique to optimize the pulsed current gas tungsten arc welding (GTAW) parameters for improved pitting corrosion resistance of AA5083-H18 aluminum alloy welds. A L9 (3⁴) orthogonal array of the Taguchi design was used, which involves nine experiments for four parameters: peak current (P), base current (B), percent pulse-on time (T), and pulse frequency (F) with three levels was used. Pitting corrosion resistance in 3.5 wt.% NaCl solution was evaluated by anodic polarization tests at room temperature and calculating the width of the passive region (?E _pit). Analysis of variance (ANOVA) was performed on the measured data and S/N (signal to noise) ratios. The “bigger is better” was selected as the quality characteristic (QC). The optimum conditions were found as 170 A, 85 A, 40%, and 6 Hz for P, B, T, and F factors, respectively. The study showed that the percent pulse-on time has the highest influence on the pitting corrosion resistance (50.48%) followed by pulse frequency (28.62%), peak current (11.05%) and base current (9.86%). The range of optimum ?E _pit at optimum conditions with a confidence level of 90% was predicted to be between 174.81 and 177.74 mV_SCE. Under optimum conditions, the confirmation test was carried out, and the experimental value of ?E _pit of 176 mV_SCE was in agreement with the predicted value from the Taguchi model. In this regard, the model can be effectively used to predict the ?E _pit of pulsed current gas tungsten arc welded joints.     

Influence of flow on the corrosion inhibition of St52-3 type steel by potassium hydrogen-phosphate

Influence of hydrodynamic conditions on the corrosion of St52-3 type steel rotating disc electrode, RDE, in 3.5% NaCl and its corrosion inhibition using K₂HPO₄ have been studied. Results showed that by rotating the electrode in blank and inhibited solutions, corrosion current density, i_corr, increased, corrosion potential, E_corr, shifted toward more positive values and charge transfer resistance, R_ct, decreased. The inhibition efficiencies increased with electrode rotation rate. This increase was attributed to the enhanced mass transport of inhibitor molecules toward the metal surface and formation of more protective films. Little decrease of efficiencies at higher rotation speeds was probably because of the separation of protective films due to high shear stresses.     

The inhibition of mild steel corrosion in acidic medium by 1-methyl-3-pyridin-2-yl-thiourea

The effect of 1-methyl-3-pyridin-2-yl-thiourea on the corrosion resistance of mild steel in H₂SO₄ solution was investigated by different techniques. The results show that the inhibition efficiency increases with the increase of inhibitor concentration. This compound affects both the anodic dissolution of steel and the hydrogen evolution reaction in 0.5 M H₂SO₄. The adsorption of this inhibitor is also found to obey the Langmuir adsorption isotherm. From the adsorption isotherm, value of the ΔG_ads for the adsorption process was calculated. From the corrosion rate obtained at 25-45 ± 1 °C E_a, ΔH_a and possible mechanism have been proposed.     

Water soluble conducting polymer composite of polyvinyl alcohol and leucine: An effective acid corrosion inhibitor for mild steel

A new corrosion inhibitor namely poly(vinyl alcohol‐leucine) composite (PVAL) has been synthesized and its influence on corrosion inhibition of mild steel in 1 M hydrochloric acid solution has been studied by weight loss and potentiodynamic polarization techniques. The composite (PVAL) showed more than 95% inhibition efficiency (IE) at an optimum concentration of 0.6% by weight. The inhibition efficiency of inhibitor has been found to vary with inhibitor concentration, solution temperature, and immersion time. Various kinetic and thermodynamic parameters (E_a, ΔH_o, ΔS_o for corrosion and ΔG_ads, ΔH_ads, ΔS_ads for adsorption) reveal a strong interaction between inhibitor and mild steel surface. The negative values of ΔG_ads indicate the spontaneous adsorption of the inhibitor on mild steel surface. Potentiodynamic polarization studies showed PVAL as mixed type inhibitor. It inhibits mild steel corrosion by blocking the active sites of the metal. Electrochemical impedance spectroscopic (EIS) techniques were also used to investigate the mechanism of corrosion inhibition.     

The effect of hot-dipped aluminum coatings on Fe-8Al-30Mn-0.8C alloy

The morphology and microstructure of an intermetallic layer formed on the surface of Fe-8Al-30Mn-0.8C alloy by hot-dip aluminization treatment have been examined in detail. The phases present in the coating are unambiguously identified by means of transmission electron microscopy. After aluminization, a two layer coating was formed consisting of an external Al layer and a (Fe, Mn)₂Al₅ intermetallic on top of the substrate. The (Fe,Mn)₂Al₅ compound has an orthorhombic structure with lattice parameters a = 0.752 nm, b = 0.667 nm and c = 0.417 nm. The activation energy (E_FeMnAl) for the growth of such an intermetallic layer is calculated to be 52.7 kJ/mol. These results are different from those observed in aluminized low-carbon steel (E_Fe). The difference between E_FeMnAl and E_Fe is attributed to the alloying elements (Mn) in the present alloy. Environmental salt fog corrosion and high temperature oxidation tests were carried out to examine the corrosion and oxidation resistance. The results indicated that both the corrosion and oxidation resistance of the Fe-8Al-30Mn-0.8C alloy treated by hot-dip aluminization can be significantly increased.     

Effect of 4-(N,N-diethylamino)benzaldehyde thiosemicarbazone on the corrosion of aged 18 Ni 250 grade maraging steel in phosphoric acid solution

4-(N,N-diethylamino)benzaldehyde thiosemicarbazone (DEABT) was studied for its corrosion inhibition property on the corrosion of aged 18 Ni 250 grade maraging steel in 0.67 M phosphoric acid at 30–50 °C by potentiodynamic polarization, EIS and weight loss techniques. Inhibition efficiency of DEABT was found to increase with the increase in DEABT concentration and decrease with the increase in temperature. The activation energy E_a and other thermodynamic parameters (Δ, Δ, Δ) have been evaluated and discussed. The adsorption of DEABT on aged maraging steel surface obeys the Langmuir adsorption isotherm model and the inhibitor showed mixed type inhibition behavior.     

Evaluation of corrosion rates by the faradaic rectification method

Accurate evaluation of corrosion currents from polarization data has been the aim of many workers in the recent past. This, however, requires both polarization resistance and Tafel slope measurements. Although many accurate methods have been developed for polarization resistance measurements a certain amount of ambiguity still exists regarding the measurement and the validity of Tafel slopes at corrosion potential. A method for evaluation of corrosion currents is presented in this paper based on the faradaic rectification technique. According to this method the corrosion current (ICorr is given by

where R_p is the polarization resistance and (δE/V²)_ω→O and (δE/V²)_ω→∞ are the rectification ratios at low and high frequency a.c. signals. The method does not require Tafel slope measurements, hence the controversy generated as to the validity of Tafel slopes at E_Corr is completely eliminated.     

Effect of palladium on the corrosion behavior of titanium

The effects of palladium (Pd) additions on the localized and uniform corrosion of titanium (Ti) were examined by comparing the corrosion behavior of Ti Grade 2 (UNS R50400) to that of Pd-bearing Ti Grade 7 (UNS R52400). Pd additions were found to increase the pitting (E_pit) and repassivation (E_rp) potentials such that E_pit for Ti Grade 2 was significantly lower than E_rp for Grade 7 in chloride (Cl⁻) solutions. The effect of Pd on Ti can be explained through the effects Pd has on the hydrogen evolution reaction. Though Pd additions did significantly affect the localized corrosion resistance of Ti, Pd did not appear to influence the passive corrosion rate nor did it mitigate the deleterious effects of fluoride (F⁻). Fluoride was found to dramatically increase the measured corrosion current above a critical concentration of about 0.5 mM.     

1‐Methyl‐3‐pyridine‐2‐yl‐thiourea as inhibitor for acid corrosion of stainless steel

The inhibiting effect of 1‐methyl‐3‐pyridine‐2‐yl‐thiourea (MPT) on the corrosion of stainless steel in 0.5 M H₂SO₄ solution was investigated using weight loss and potentiostatic polarization techniques. The percentage inhibition efficiencies and surface coverage degrees increased with increasing additive concentration. Potentiostatic polarization studies revealed that MPT is of the mixed‐type inhibitor in 0.5 M H₂SO₄ solution. The adsorption of this inhibitor is also found to obey the Langmuir adsorption isotherm. From the adsorption isotherm, value of the ΔG_ads for the adsorption process was calculated. From the corrosion rate obtained at 25–55 °C E_a, ΔH_a and possible mechanism have been proposed. Scanning electron microscopy (SEM) was done from the surface of the exposed sample indicating uniform film on the surface.     

Inhibitive effect of diethylcarbamazine on the corrosion of mild steel in hydrochloric acid

Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), weight loss measurements and atomic force microscopy techniques were used to investigate the inhibitory effect of diethylcarbamazine (DECM) on corrosion of mild steel in HCl solution. The inhibitor showed >90% inhibition efficiency at 5.01 × 10⁻⁴ M. Results obtained revealed that inhibition occurs through adsorption of inhibitor molecules on metal surface without modifying the mechanism of corrosion process. Potentiodynamic polarization studies suggested that it is a mixed type inhibitor, predominantly controls cathodic reaction. Activation parameters (E_a, ΔH and ΔS) and thermodynamic parameters (, and ) were calculated to investigate mechanism of inhibition.     

Effect of plasma nitriding of electroplated chromium coatings on the corrosion protection C45 mild steel

Plasma nitriding is a promising posttreatment technique to create a nitride layer on electroplated chromium coatings for improving their corrosion resistance. In the present study, the effects of plasma nitriding on the corrosion properties of electroplated chromium/C45 mild steel were investigated using electrochemical characterization. The chromium plated samples were nitrided using a pulsed direct current glow discharge in an NH₃ atmosphere. The polarization curve measurement results showed that the plasma nitrided samples exhibited more positive corrosion potentials (E_corr), smaller corrosion currents (I_corr), and evident passivation when compared with unnitrided chromium plating/substrate system. The high value of E_corr and low value of I_corr imply an improvement of the corrosion resistance of the coating/substrate system after plasma nitriding.     

Effect of annealing on microstructure and properties of Cu–30Ni alloy tube

The microstructure, mechanical properties and seawater corrosion resistance of annealed Cu–30Ni alloy tube were investigated using mechanical test, optical microscope, scanning electronic microscope and electrochemical measurement system, respectively. The recrystallizations gradually increased with the increase of annealing temperature and holding time. The hardness and tensile strength, which maintained invariability with annealing temperature at 680–720 °C, dramatically decreased with annealing temperature at 720–770 °C. As annealing temperature and holding time increase, corrosion potential (E_C) increased while corrosion rate (i_C) decreased at the beginning of seawater immersion. But after 15 days’ seawater immersion, as annealing temperature and holding time increase, E_C firstly increased and then decreased, on the contrary, i_C firstly decreased and then increased. The Ni-rich surface film and the Ni-rich sub-grains were responsible for the initial and extended immersion, respectively. It was found that the Cu–30Ni alloy tube annealed at 720 °C for 30 min exhibited favorable comprehensive mechanical properties and seawater corrosion resistance.     

Inhibitory effect of some amino acids on corrosion of Pb-Ca-Sn alloy in sulfuric acid solution

The present article describes the inhibition effect of amino acids cysteine (Cys), methionine (Met) and alanine (Ala), towards the corrosion of lead-alloy (Pb-Ca-Sn) in H₂SO₄ solution by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), weight loss measurement and scanning electron microscopy (SEM) methods. The influence of inhibitor concentration, temperature and time on inhibitory behavior of the amino acids was investigated. The corrosion data including corrosion current density (I_corr), corrosion potential (E_corr) and charge transfer resistance (R_ct) were determined from Tafel plots and EIS. Recording impedance spectra showed that the charge transfer resistance is increased by increasing adsorption time. The SEM micrographs revealed that the corroded surface area is decreased in the presence of amino acids. Meanwhile, the inhibition efficiency (IE) was found to be depending on the type of amino acid and its concentration. The IE for 0.1 M Cys in 0.5 M H₂SO₄ is greater than 96%. Adsorption isotherms were fitted by Langmuir isotherm.