Effect of current density on the corrosion protection performance of polyaniline coated AISI 4140 steel

The present work reports on electropolymerisation of aniline onto AISI 4140 steel substrate at room temperature in oxalic acid media by the potentiodynamic electrodeposition method. The effect of coating current density on the morphology of conductive polymer films and their corrosion behaviour has been investigated. The electrolyte solution comprised 0.1 M aniline?+?0.3 M oxalic acid. The effects of deposition current density changes (0.1, 0.2, 0.3, 0.4 and 0.5?mA cm^?2) on the films were investigated. The PANI film was characterised using cyclic voltammetry and optical microscopy; it was covered with a dark green-yellow homopolymer film of strongly adherent homogeneous characteristic. The corrosion behaviour of steel electrodes with and without polyaniline (PANI) film in 3.5 wt-% NaCl solution was investigated through anodic polarisation curves. The results indicated that the PANI coating obtained under 0.2?mA cm^?2 was the most corrosion protective.     

Effect of electrolyte and monomer concentration on anticorrosive properties of poly(N-methylaniline) and poly(N-ethylaniline) coated mild steel

The electrosynthesis of poly(N-methylaniline) (PNMA) and poly(N-ethylaniline) (PNEA) coatings on mild steel in aqueous oxalic acid solutions was carried out by potentiodynamic synthesis technique. The effects of monomer and electrolyte concentrations on electrochemical growth of PNMA and PNEA coatings on mild steel substrates were investigated. Repassivation peak did not appear during electrosynthesis of PNMA and PNEA coatings from solutions containing 0.1 M monomer and 0.1 M electrolyte. The tests for corrosion protection of the polymer coated and uncoated mild steel substrates were done in 3% NaCl solutions by dc polarization and electrochemical impedance spectroscopy (EIS) techniques. Corrosion tests revealed that PNMA and PNEA coatings exhibited effective anti-corrosive properties. The acidity of the polymerization solution was found to influence the anticorrosive behavior of the polymer coating.     

Electrochemical synthesis and corrosion performance of poly o-anisidine on 304 stainless steel

The corrosion inhibition of the synthesis poly o-anisidine on 304 SS has been studied by using a different electrochemical technique such as potential-time measurement, EIS and polarization methods in 0.5 M HCl, and 3.5% NaCl solutions. The poly o-anisidine layer has been obtained on SS electrochemically from 0.5M H₂SO₄ acid solution by using cyclic voltammetric (CV), potentiostatic and galvanostatic techniques. The polymer layer provides around 93% protection in acid solution by potentiostatic coating. Nevertheless, the poly o-anisidine coating is not protected in 3.5% NaCl media by cyclic voltametry and galvanostatic coating. The examination of polymer surface by SEM confirms the results obtained. For characterization of polymer layer the UV-vis spectra and Fourier transform infrared (FTIR) spectra of the polymer films were measured. This work was aimed to study the effect of electrodeposition method of poly o-anisidine on corrosion protection of 304 SS.     

Zinc modified polypyrrole coating on mild steel

Polypyrrole (PPy) films (∼ 1.7 μm thick) have been electrodeposited on mild steel (MS) substrates from 0.1 M pyrrole containing aqueous oxalic acid solution, by using cyclic voltammetry technique. Then, the polymer coatings were modified with deposition of zinc particles (∼ 1 mg/cm²), at a constant potential value of − 1.20 V in 0.2 M ZnSO₄ solution. The corrosion performance of zinc modified PPy coating has been investigated in 3.5% NaCl solution, by using electrochemical impedance spectroscopy and anodic polarisation curves. Also, the corrosion behaviours of zinc modified PPy coated platinum and single PPy coated MS samples have been investigated, for comparison. It was shown that zinc modified coating exhibited very low permeability and provided important cathodic protection to MS for considerably long immersion period. The voluminous zinc corrosion products are formed during exposure time in aggressive solution, giving rise to a blocking effect on the porous structure and led to effective barrier behaviour of zinc modified PPy coating, even after 96 h of exposure time to corrosive solution.     

A study on mechanism of corrosion protection of polyaniline coating and its failure

Polyaniline (PANI) coatings were electrochemically deposited on substrates of stainless steel and platinum in solutions of 0.2 M H₂SO₄ and 0.1 M aniline by cyclic voltammetry. The corrosion protection of the PANI coatings and their failure were investigated in 0.2 M H₂SO₄ solution. It was observed that the corrosion protection ability of the coating to steel substrate was increased with the increase of the coating thickness. The corrosion protection ability was mainly attributed to the passivating effect of PANI due to its oxidizing ability in its emeraldine state. During its operation, the PANI coating in emeraldine state tended to gradually lose its corrosion protection ability. This gradual failure of the PANI coating, but faster than expected, was confirmed to be related to a gradual reduction of the emeraldine PANI and a gradually increased resistance between the PANI coating and the stainless steel substrate. These findings lead to a new mechanism for the corrosion protection of PANI coating and its failure.     

The effects of polyethylene glycol (PEG) as an electrolyte additive on the corrosion behavior and electrochemical performances of pure aluminum in an alkaline zincate solution

The effects of zinc oxide and/or polyethylene glycol (PEG) as electrolyte additives on the corrosion and electrochemical performances of pure aluminum in 4.0 M KOH solutions were investigated by means of hydrogen collection, polarization curve, galvanostatic discharge, scanning electron microscopy (SEM), and energy dispersive analysis of X‐ray (EDAX). The addition of ZnO markedly inhibited the corrosion of aluminum in 4.0 M KOH solutions, resulting from the deposition of zinc with high hydrogen evolution overpotential in aluminum surfaces. The introduction of PEG in the alkaline zincate solution obviously improved the deposition of zinc by increase in the overpotential of zinc deposition, thus the corrosion rate of aluminum in the alkaline zincate solutions with PEG was further decreased. The enhancement effect of PEG on the inhibition of zinc oxide first increased and then decreased with increasing the content of PEG in the electrolyte. The electrolyte system with 0.2 M ZnO and 2.0 mM PEG presented the highest inhibition efficiency (98.8%) for the corrosion of aluminum. The results of galvanostatic discharge indicated that the aluminum anode shows excellent discharge performances in the 4.0 M KOH solution with 0.2 M ZnO and 2.0 mM PEG.     

Corrosion behavior of lanthanum-based conversion coating modified with citric acid on hot dip galvanized steel in aerated 1 M NaCl solution

Rare earth conversion coating is one of the most promising substitutes to the toxic chromate coating. The corrosion resistance of lanthanum conversion coating modified with citric acid on hot dip galvanized (HDG) steel was investigated in aerated 1 M NaCl solution by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. Equivalent circuits were subsequently developed from the measurements to elucidate the corrosion behavior of the coating. The surface morphology of the lanthanum conversion coating was observed by scanning electron microscopy (SEM), and the chemical composition of the coating was characterized by energy dispersive spectroscopy (EDS). The results showed that the corrosion process of the modified lanthanum conversion coating consisted of three stages. The overall corrosion resistance of the coating was excellent.     

Wear and corrosion behavior of CrCN/CrN coatings deposited by cathodic arc evaporation on nitrided 42CrMo4 steel substrates

In the paper, the results of wear and corrosion tests of the CrCN/CrN multilayer coatings, formed by cathodic arc evaporation on 42CrMo4 (AISI 4140) steel substrates are presented. The substrates were subjected to thermo-chemical treatment–nitriding with various nitriding potential. The results of nitriding were determined by XRD and the hardness profile in the samples cross-section. The morphology of thin coatings was examined with SEM. A Vickers FV-700 and Fisherscope HM2000 hardness testers enabled to investigate hardness of steel substrates and CrCN/CrN coatings respectively. A pin-on-disc wear tests were used to determine the hardness and tribological parameters of the coatings: the coefficient of the friction and wear rate. The scratch test and Rockwell test were applied to assess the adhesion of the coatings to the substrates. The corrosion properties of coating–steel substrate systems were investigated using potentiodynamic polarisation tests. Corrosion potential, corrosion current density and polarization resistance were determined. It was found that that the nitriding of steel substrates improves properties of the coating–substrate system. The nitriding 42CrMo4 steel substrate with low nitriding potential enable to obtain substrates without surface “white layer” what favours good adhesion of the coating to the substrate. The CrCN/CrN multilayer coating–steel substrate systems show good mechanical and tribological properties and corrosion resistance.     

Electrochemical noise resistance evaluation of 304L SS in nitric acid and simulated nuclear high level waste

The paper focuses on corrosion monitoring of 304L stainless steel (SS) (solution annealed, sensitised) in nitric acid (4, 8 and 12M) at 298 and 323 K electrolyte temperatures, and simulated high level waste (HLW, 323 K), using electrochemical noise (EN) measurements. Double loop electrochemical potentiokinetic reactivation test was conducted to assess the degree of sensitisation. Electrochemical noise records revealed passivation process during the monitoring period, under all conditions studied, except for the sensitised specimen in 4M nitric acid (323 K), which showed localised attack. The results showed an inverse relation between EN resistance and corrosion activity. An increase in nitric acid concentration resulted in an increase in corrosion activity, while increasing the electrolyte temperature increased the corrosion activity marginally. A profound increase in corrosion activity occurred for the sensitised specimen in nitric acid when compared to the solution annealed specimen, but the increase was marginal in simulated HLW. The results are detailed in the paper.     

Composite zinc-fly ash coating on mild steel

Zinc-fly ash composite coatings were deposited on mild steel substrates with the help of electrodeposition technique. Metallographic and chemical characterization of the produced composite coatings was performed with the aid of Scanning Electron Microscopy (SEM) and Electron Dispersive X-ray Analysis (EDX). The corrosion behaviour of the composite coated mild steel substrate was studied in a 0.3 M NaCl solution (pH = 5.5) by means of anodic polarization curves. The wear of the zinc-fly ash coating was also investigated by using a pin-on-disk apparatus. The composite coating exhibited increased wear resistance, compared to the pure zinc coating and the mild steel substrate. The adhesion strength between the zinc-fly ash coating and the mild steel substrate was examined with a scratch testing apparatus. The adhesion strength between zinc-fly ash composite coating and the mild steel substrate was found to be higher than that of the pure zinc coating to mild steel.     

Characterisation of phosphate coatings obtained using nitric acid free phosphate solution on three steel substrates: An option to simulate TMT rebars surfaces

Phosphate coatings have been obtained on three steel substrates, (a) ferritic-pearlitic (F-P), (b) tempered martensitic (T-M) and (c) tempered martensitic containing oxide scale (T-M-O) at the top to simulate TMT (thermo mechanical treatment) rebar surfaces which are extensively used for composite concrete structure. Nitric acid free phosphate solution was used for the coating purpose. Scanning electron microscopy (SEM), glow discharge optical emission spectrometry (GDOES) and X-ray diffraction (XRD) techniques were used to characterise phosphate coatings. Acicular phosphate microstructure was obtained on T-M-O and T-M steel substrates, whereas coarser phosphate microstructure was obtained on F-P steel substrate. Thinner to thicker coatings were obtained on F-P, T-M and T-M-O steel substrates respectively. Oxide scale, on the T-M-O steel substrate promoted for deposition of phosphate compounds and thereby obtained thickest coating on T-M-O steel substrate. Zinc phosphate (hopeite) on T-M-O steel substrate and zinc phosphate (spencerite) on F-P steel substrate were detected as main phosphate respectively, whereas both zinc phosphates (hopeite and speccerite) on T-M steel substrate were detected in the same proportion. In addition to zinc phosphate, zinc iron phosphate (phosphophyllite) was detected on F-P and T-M steel substrates, whereas iron phosphate (beraunite) was detected on T-M-O steel substrate. A comparative performance against corrosion of all the phosphate coated steel substrates was evaluated by salt spray, Tafel and electrochemical impedance spectroscopy (EIS) studies. Test for extended exposure in open atmosphere as well as simulated highly humid condition were also conducted to identify coating performance. Phosphate coated T-M-O steel substrate showed 4-5 times improved resistance against corrosion than the phosphate coated F-P steel substrate. The bond strength of coated steel substrate with concrete was increased 2-26% for phosphate coating on T-M-O and T-M steel substrates while the same was decreased 4-12% for phosphate coating on F-P steel substrate.     

Inhibition effect of 4-phenylthiazole derivatives on corrosion of 304L stainless steel in HCl solution

Some 4-phenylthiazole derivatives were tested as corrosion inhibitors for 304L stainless steel in 3.0 M HCl using weight loss and galvanostatic techniques. The results showed that the inhibition efficiency increases with increasing the concentration of the 4-phenylthiazole derivatives. Polarization measurements showed that these derivatives act as mixed-type inhibitors. The adsorption of these inhibitors on the surface of stainless steel follows Temkin’s adsorption isotherm and kinetic model. The effect of temperature on the corrosion was investigated by the weight loss method and some thermodynamic parameters were calculated. The synergistic influence caused by thiocyanate ion on the inhibition of corrosion of 304L stainless steel in 3.0 M HCl in the presence of 4-phenylthiazole derivatives has been studied using weight loss method. The inhibition action of these 4-phenylthiazole derivatives was discussed in terms of its adsorption on the metal surface.     

Corrosion behaviour of TaN thin PVD films on steels

Abstract

The corrosion behaviour of tantalum nitride (TaN) thin films deposited on AISI 1018 carbon steel, AISI M2 tool steel and AISI 304 stainless steel has been investigated in an aqueous sodium chloride solution. The films were produced by means of magnetron sputtering plasma deposition. Morphological and microstructuctural characteristics of the films were studied by atomic force microscopy and X-ray diffraction analysis. Potentiodynamic polarisation, electrochemical impedance spectroscopy and cyclic voltammetry techniques were used for electrochemical characterisation. The TaN films displayed lower corrosion current densities than those of the substrates. A relaxation time in the impedance analysis was observed in the 1018/TaN system, two relaxation times were observed in the M2/TaN system, and a diffusion process was found in the 304/TaN system. This indicates that the substrates were not completely isolated from the solution and the defects in the films appear to be detrimental owing to electrolyte penetration through the defects which caused localised corrosion. The corrosion behaviour of the films depends mainly on the electrochemical properties of the substrate.     

Corrosion properties of HVOF-coated steel in simulated concrete pore electrolyte and concentrated chloride environments

The corrosion susceptibility of steel and HVOF-coated steel in solutions simulating the alkaline concrete pore environment and with the addition of chloride was investigated using potentiodynamic polarization and potential step techniques. The surface characterization was performed using SEM and the surface elemental analysis was determined by EDS. The concentration of chloride was 2.8 M to simulate the concentration of chloride spread in many local regions of Saudi Arabia and called Sabkha. It was found that, in the case of the simulated concrete pore electrolytes, the HVOF coating resulted in an anodic shift of the corrosion potential with marginal effect on the corrosion current. However, upon addition of 2.8 M chloride solution, the corrosion rate of the HVOF-coated steel was found to increase by a factor of two. SEM showed a network of pores within the coating which provides a path for the electrolyte. This would result in preferential corrosion around splat boundaries and confirmed by EDS which showed that the corroded splats have higher oxide contents. Potential step experiments at 400 mV above open circuit potential showed a suppressed current of the HVOF-coated steel compared to the steel substrate alone. The corrosion potential versus time experiments resulted in a more anodic Ecorr which decreased with time and became equal to the Ecorr of the bare steel after 34 h. After that, the corrosion potential of the HVOF-coated steel decreased due to the increase in galvanic coupling between the steel and the HVOF coating.     

The protective effect of polypyrrole coating on the corrosion of steel electrode in acidic media

In this study, the corrosion parameters of stainless steel containing 12% Cr, have been determined by Tafel extrapolation method in 1 M HCl, H₂SO₄ and H₃PO₄ media. Later, steel was coated with polypyrrole in 0.1 M Pyrrole + 0.3 M Oxalic acid solution by cyclic voltametric method. The corrosion parameters and percentage inhibition efficiencies of coated electrodes were investigated according to immersion times in the same media. In all acidic media studied, increases in immersion time, produced increased corrosion densities and a decrease in percentage inhibition efficiencies were determined.     

Microstructure and Electrochemical Properties of CoCrCuFeNiNb High-Entropy Alloys Coatings

The CoCrCuFeNiNb high-entropy alloys coatings were prepared by using plasma-transferred arc cladding process. The microstructure and electrochemical behaviors of the coating were investigated in detail. The experimental results indicated that the coating consists of a simple fcc solid solution phase and an order（Co Cr）Nb-type Laves phase. The polarization curves, obtained in 1 and 6 mol/L hydrochloric acid solutions, clearly indicated that the general corrosion resistance of the coating at ambient temperature was better than that of 304 stainless steel. The coating displayed a lower corrosion current and lower corrosion rate. Electrochemical impedance spectroscopy demonstrated that the impedance of the coating was significantly higher than that of the 304 stainless steel.     

Influence of pH on the deterioration of plasma electrolytic oxidation coated AM50 magnesium alloy in NaCl solutions

The corrosion deterioration process of plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy prepared from two different based electrolytes, i.e., an alkaline phosphate electrolyte and an acidic fluozirconate electrolyte, were investigated using electrochemical impedance spectroscopy (EIS) in a 0.1 M NaCl solution with pH of 3, 7 and 11, respectively. It was found that the PEO coating formed in alkaline phosphate electrolyte, which was composed mainly of MgO, suffered from rapid chemical dissolution and lost its protection capability very quickly in acidic NaCl solution (pH 3). The chemical dissolution of this PEO coating was retarded in neutral NaCl solution (pH 7) and the corrosion damage was localized in this environment. On the other hand, in the alkaline NaCl solution (pH 11), the MgO coating underwent only slight degradation. The PEO coating produced in acidic fluozirconate electrolyte, the failure was marked by the flaking-off of the large areas of coating in acidic NaCl solution (pH 3). However, in the neutral and alkaline NaCl solutions, the coating underwent only a slight degradation without any observable corrosion damage in the 50 h test. The results showed that the deterioration process of PEO coated magnesium alloy was governed mostly by the pH of NaCl solution and it was also strongly related to the microstructure and composition of the PEO coatings.     

Electrochemical and structural properties of a polyurethane coating on steel substrates for corrosion protection

Electrochemical impedance spectroscopy (EIS) has been employed in the investigation of the corrosion characteristics of polyurethane polymeric film applied on metallic substrates during immersion in 0.5 M NaCl aqueous solution. Galvanized steel and mild steel coupons were studied. EIS has made it possible to monitor the penetration of the surrounding electrolyte into the organic coating and determine the extent of the coating breakdown. The EIS spectra were modeled in terms of equivalent electric circuits to explain the impedance results. The time course of the impedance parameters was used to establish the effectiveness of the anticorrosive properties of the various paint-substrate systems. Pull-off adhesion and microhardness tests were conducted on the samples both before and after exposure to the corrosive environment, in order to investigate the existence of a relationship between the changes in mechanical properties of the coating/metal system and the degradation process as monitored by EIS measurements. Polyurethane coating shows higher adhesion and enhanced anticorrosion protection characteristics when applied on carbon steel than on galvanized steel.     

Improvement of the Corrosion Resistance of High Alloyed Austenitic Cr-Ni-Mo Stainless Steels by Solution Nitriding

Characteristic features of austenitic steel grades combine a good corrosion resistance with a low hardness, wear resistance and scratch resistance. An interesting possibility for improving the wear behaviour of these steels without loss of their corrosion resistance lies in enriching the near surface region with nitrogen. The process of a solution nitriding allows the rise of the solution of nitrogen in the solid phase. On this state nitrogen increases the corrosion resistance and the tribilogical load-bearing capacity. The aim of the study was, to investigate the improvement of the pitting corrosion behaviour by solution nitriding. A special topic was to observe the effect of nitrogen by different molybdenum content. So austenitic stainless steels (18% Cr, 12% Ni, Mo gradation between 0.06 to 3.6%) had been solution nitrided. The samples could be prepared with various surface content of nitrogen from 0.04 to 0.45% with a step-by-step grinding. The susceptibility against pitting corrosion of these samples had been tested by determination of the stable pitting potential in 0.5M and 1M NaCl at 25℃. For the investigated steel composition and the used corrosion system there is no influence of molybdenum on the effectiveness of nitrogen. The influence of nitrogen to all of the determined parameters can be described well by PRE = Cr 3.3 * Mo 25 *N. XPS analysis of the sample surfaces support the results of the pitting corrosion tests.Additionally surface investigations with an acid elektolyte (0.1M HCl 0.4M NaCl) were performed. In this case the passivation effective nitrogen content increases markedly with rising molybdenum concentration of the steel.Obviously an interaction of Mo and N is connected with a strongly acid electrolyte.     

The effect of processing gas on corrosion performance of electroless Ni-W-P coatings treated by laser

An investigation of the microstructural and corrosion characteristics of electroless Ni-5.5 W-6.5P coatings on steel substrates after laser treatment in argon and air is presented. The microstructural characteristics of the coatings, in terms of crystallisation, grain size, microstrain, porosity as well as surface chemistry, were examined using quantitative X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Electrochemical tests, using potentiodynamic polarisation in 0.5 M H₂SO₄ solution and electrochemical impedances spectroscopy (EIS) in 3.5% NaCl solution, were undertaken to evaluate the corrosion behaviour of the coatings. The results indicated that the laser-treated coatings consisted of nanocrystalline Ni and Ni₃P phases, along with retained amorphous phase; further, the dimensions of the Ni crystallites were larger than those of Ni₃P. The laser-treated coating in argon revealed the presence of submicron scale porosity, while no porosity was evident in the coating surface treated by laser in air. The uniform corrosion revealed in 0.5 M H₂SO₄ solution is mainly determined by the microstructural characteristics of the coating. Pitting corrosion in 3.5% NaCl solution depended on the amount of porosity on the surface. The laser-treated coating in air exhibited better corrosion resistance in both acidic and chloride environments than that laser-treated in argon.