Application of polyaniline/nylon composites coating for corrosion protection of steel

In this research, we investigated the corrosion inhibition properties of polished steel plates (low carbon) coated with a polyaniline (emeraldine base form) blend with nylon 66 (termed PANi/Ny) via cast method with formic acid as the solvent. Polyaniline (PANi) was prepared chemically from aqueous solution using aniline (0.2 M) as a monomer and ammonium persulfate (0.2 M) as an oxidant. The polymer powder produced was changed into emeraldine base (EB) form after treatment with dilute ammonia solution (0.5 M) in order to do further processing. The corrosion experiments were performed in the open circuit, exposing samples to different aggressive and corrosive conditions (e.g., NaCl, HCl). To produce a good comparison, the corrosion study was performed on both polymer-coated and bare-steel samples. Corrosion monitoring was performed by simple immersion tests and determination of the concentration of iron ions and metal weight loss in test solutions. It was found that PANi/Ny coatings can provide an anodic protection against corrosive environments in which the metals are exposed. The corrosion rate for the polymer coated steel was significantly lower than the bare steel (~10–15 times).     

Aminic epoxy resin hardeners as reactive solvents for conjugated polymers: polyaniline base/epoxy composites for anticorrosion coatings

Polyaniline (PANI) has much been studied in the context of corrosion prevention, particularly on steel and aluminium. To prepare epoxy coatings consisting of PANI has turned to be nontrivial, due to its relatively rigid conformation and poor solubility. Therefore, as the aim has typically been first to dissolve PANI in the epoxy component before curing, auxiliary solvents have been required, and less attractive Lewis-type hardeners have been required if the conducting salt form has been used. In this work, we describe a particularly simple concept where emeraldine base (EB) form of PANI is first dissolved in specific aminic hardeners which are observed to be solvents for EB at low concentrations, and the mixtures are unconventionally cross-linked upon adding epoxy resin, diglycidyl ether of bisphenol-A (DGEBA). Suitable hardeners are N,N,N′,N′-tetrakis(3-aminopropyl)-1,4-butanediamine (DAB-AM-4) and trimethylhexanediamine (TMDA). Even if the subsequent cross-linking promotes phase separation, the forming cross-link sites may also control the phase separation. As a result, sufficiently homogeneous coatings are identified which contain only 1 wt% EB in the cured EB/DGEBA/TMDA composites where in aqueous 3.5% NaCl solution the corrosion front propagation is suppressed, and electrochemical impedance studies indicate the formation of a charged interface or reaction product layer between EB and steel. For reference, similar net EB/DGEBA/TMDA-compositions were prepared, where EB was first mixed in DGEBA without any solubility and which were cured by added TMDA, and they gave essentially no anticorrosion effect. We expect that the present concept opens new ways to prepare cured epoxy composites also with other conjugated or nonconjugated polymers for anticorrosion and other functional purposes.     

Effect of powder painting procedures on the filiform corrosion of aluminium profiles

In this work, the filiform corrosion behaviour of powder painted aluminium profiles was studied, and the coating barrier properties together with adhesion to the substrate were analysed. Samples coated with a traditional painting procedure (one powder layer application followed by curing), and other samples coated using a special cycle to obtain wood grain effect were compared using accelerated filiform corrosion tests. Moreover, in order to better understand the degradation mechanisms of painted metal substrates, thermal stresses were applied to accelerate the natural weathering. The effects of the thermal aging were analysed by electrochemical impedance spectroscopy.     

Anticorrosion performances of epoxy coatings modified with polyaniline: A comparison between the emeraldine base and salt forms

The protection against corrosion imparted by epoxy paints modified by the addition of polyaniline emeraldine base (0.3% w/w) and Zn₃(PO₄)₂ (10% w/w) has been investigated and compared. For this purpose, accelerated assays through corrosion cycles, which simulate the marine conditions, have been performed using a home-made robotized equipment. Results were compared with those recently obtained for a formulation constituted by the same epoxy paint modified with polyaniline emeraldine salt (0.3% w/w) [E. Armelin, R. Pla, F. Liesa, X. Ramis, J.I. Iribarren, C. Alemán, Corr. Sci. 50 (2008) 721.]. A detailed analysis of the results using the ASTM standard method D-1654-79 allows to conclude that the coating modified with polyaniline emeraldine base performs better than both the unmodified coating and the coatings modified with the inorganic corrosion inhibitors. This behaviour should be attributed to a mechanism based on the ability of polyaniline emeraldine base to store charge. On the other hand, as the epoxy paint modified with the latter polymer provides better results than the formulation containing Zn₃(PO₄)₂, we conclude that inorganic corrosion inhibitors, which may have detrimental effects on both the environment and the human health, can be replaced by a small concentration of environmentally friendly organic polymers.     

In situ ATR-FTIR studies of the aluminium/polymer interface upon exposure to water and electrolyte

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) with the Kretschmann configuration was applied for in situ studies of the transport of water and ionic species through a polymer film to an aluminium/polymer interface. The time dependent intensity changes of the infrared bands of water were used to follow the transport of water to the aluminium/polymer interfacial region and a NaSCN solution was employed as model electrolyte to follow the transport and accumulation of thiocyanate ions. Apart from water sorption and ion transport, the main processes identified were corrosion/oxidation of the aluminium surface and swelling of the polymer film. The method proved to be useful for detailed in situ studies of changes at a polymer coated metal surface, such as oxidation and surface film formation on the metal. It should also be possible to study the effects of defects and pores in the polymer film on the transport properties of water and ions to the metal/polymer interface, as well as adsorption and other chemical reactions and physical interactions in the metal/polymer interfacial region.     

Adhesive strength of powder coated aluminium substrates

Adhesive strengths of differently pretreated powder coated aluminium substrates were evaluated using the pull-off and the tape tests in the dry and wet conditions. Various accelerated test chambers were employed to evaluate the corrosion behaviour of the samples. Scanning electron microscopy (SEM), with energy dispersive X-ray analysis (EDX) were also employed to the better understanding of the nature and morphology of the differently pretreated aluminium substrates prior to and after the accelerated tests.

Accelerated tests and wet adhesion measurements showed that the degreased samples demonstrated a better performance than Polyacrylic acid (PAA) treated samples. The chromate/phosphate conversion coating (CPCC) treated samples gave the best performance and the Polyacrylic acid/ hexafluorozirconic acid treated samples (PZr) gave marginally second best performance in anti-corrosive behaviour.

Based on the results obtained from the different experiments, it is argued that these studies may provide sufficient evidence to show that hydrolysis may destroy interface bonds between PAA and aluminium substrate. Moreover, SEM/EDX analysis, suggest that, the thin PAA polymeric layer was replaced by a crystalline, thick hydrated oxide layer, when samples were exposed to humid conditions.     

Coatings based on electronic conducting polymers for corrosion protection of metals

In this work, corrosion protection of mild steel by a novel epoxy resin (EP)-based coating system containing polyaniline (PAni) as an anticorrosive agent was studied. The corrosion behavior of mild steel samples coated with an EP/PAni-EB (emeraldine base), EP/PAni-ES (emeraldine salt), EP/SPAN (PAni sulfonated), EP/PAni-fibers, EP/PhoZn (zinc phosphate), EP/ChroZn (zinc chromate) or EP/Charge was investigated in 3.5% NaCl solution. For this purpose, electrochemical impedance spectroscopy measurements were utilized. It was found that the addition of three forms of PAni—undoped, sulfonated and fibers—to the EP resin increased its corrosion protection efficiency.     

Undoped polyaniline–surfactant complex for corrosion prevention

Due to the strict regulations on the usage of heavy metals as the additives in the coating industries, the search for effective organic corrosion inhibitors in replacement of metal additives has become essential. Electrically conducting polymers have been shown to be effective for corrosion prevention, but the poor solubility of these intractable polymers has been a problem. We have explored a polyaniline–4-dodecylphenol complex (PANi–DDPh) to improve the dissolution, and it has been shown to be an effective organic corrosion inhibitor. With the surfactant, DDPh, PANi could be diluted into the coatings, and the properties of the coatings were affected. An emeraldine base (EB) form of PANi was also found to be oxidized by the hardener. The oxidized form of polyaniline provides improved corrosion protection of metals than that of emeraldine base since the value of the standard electrode potential for the oxidized form of PANi is higher than that of EB. Additionally, the surfactant improves the wet adhesion property between the coating and the metal surface. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2849–2856, 1999     

Polyaniline for corrosion prevention of mild steel coupled with copper

Polyaniline emeraldine base/epoxy resin (EB/ER) coating was investigated for corrosion protection of mild steel coupled with copper in 3.5% NaCl solution. EB/ER coating with 5-10 wt% EB had long-term corrosion resistance on both uncoupled steel and copper due to the passivation effect of EB on the metal surfaces. During the 150 immersion days, the impedance at 0.1 Hz for the coating increased in the first 1-40 days and subsequently remained constant above 10⁹ Ω cm², whereas that for pure ER coating fell below 10⁶ Ω cm² after only 30 or 40 days. Immersion tests on coated steel-copper galvanic couple showed that EB/ER coating offered 100 times more protection than ER coating against steel dissolution and coating delamination on copper, which was mainly attributed to the passive metal oxide films formed by EB blocking both the anodic and cathodic reactions. Salt spray tests showed that 100 μm EB/ER coating protected steel-copper couple for at least 2000 h.     

Polyaniline inhibition of filiform corrosion on organic coated AA2024-T3

Dispersions of the polyaniline emeraldine salt (ES) of paratoluene sulphonic acid (PAni-pTS) effectively inhibit filiform corrosion (FFC) affecting polyvinyl butyral (PVB) coated AA2024-T3 aluminium alloy. An in-situ scanning Kelvin probe (SKP) technique is used to study the effect of systematically varying PAni-pTS volume fraction (?_pa) on FFC initiation and propagation. For ?_pa < 0.15, there is no evidence of FFC inhibition and E_corr values recorded for the intact coated aluminium (E_intact) remain similar to those measured for unpigmented PVB. At ?_pa ≥ 0.15, a marked rise in E_intact is observed, FFC propagation rates decrease and significant oxide growth is observed at the coating-metal interface. For emeraldine base (PAni-EB)-containing coatings, there is no evidence of interfacial oxide film formation, no ennoblement of E_intact and minimal inhibition of FFC. Conversely, when a PAni-pTS induced oxide covered surface is re-coated using unpigmented PVB and FFC is initiated as per normal, a substantial reduction in the rate of FFC propagation is observed. It is therefore proposed that inhibition of FFC by PAni-pTS arises principally as a result of the protective nature of the oxide film formed at the metal-coating interface.     

Raman spectroscopic study on the structural changes of polyaniline during heating and cooling processes

Polyaniline (PANI) in three ground states—emeraldine base (EB), leucoemeraldine base (LEB), and pernigraniline base (PNB)—were studied by Raman spectroscopy in the temperature scale of ?195 to about 375°C. The Raman spectral results demonstrated that a crosslinking reaction occurred on EB chains at high temperatures, whereas the phenyl torsion angles of EB chains decreased during the cooling process. LEB was unstable and was gradually converted to EB during the heating process, although its chain conformation remained stable by cooling. The spectral change tendency of PNB was similar to that of EB during the heating process. Although the torsion angle of PNB also decreased with decreasing temperature, like that of EB, this behavior was hard to observe experimentally because of the relatively “free ” rotation of its phenyl rings. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96:732–739, 2005     

Thermal aging of undoped polyaniline: Effect of chemical degradation on electrical properties

Aging experiments, with a special emphasis on the atmosphere effect, have been carried out on undoped polyaniline, in its half‐oxidized state, namely emeraldine base (EB). The polymer has been aged under vacuum and in air atmosphere. The chemical degradation has been analyzed by several complementary techniques such as viscosity measurements, FTIR, XPS, and UV‐Vis‐nIR spectroscopy. We show that emeraldine base exhibits two different degradation mechanisms. The first one is intrinsic to the polymer and occurs independently on aging conditions (vacuum or air). It consists of crosslinking via tertiary amine groups created from imine nitrogen via double‐bond breaking. The second mechanism is extrinsic and occurs concomitantly with the first one upon aging in air. It consists of oxygen incorporation in a form of carbonyl groups and chain scission. Both degradation pathways result in a decrease of the electrical conductivity of the polymer due to the lowering of the average conjugation length. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 395–404, 2002     

Evaluation of an environmentally friendly anticorrosive pigment for alkyd primer

An alkyd formulation containing zinc phosphate (10 wt.%) was prepared and subsequently modified replacing the latter anticorrosive additive by a very low concentration of conducting polymer. Specifically, three modified paints, which contain polyaniline emeraldine base (undoped form), polyaniline emeraldine salt (doped form) and an eco-friendly polythiophene derivative (partially oxidized), were formulated. The properties and corrosion resistance of the four alkyd coatings have been characterized. Among the three modified paints, the one containing polythiophene shows the best adherence and the highest corrosion resistance. This has been attributed to the fact that the miscibility of the polythiophene derivative with the alkyd formulation is better than that of polyaniline. Furthermore, accelerated corrosion assays and electrochemical impedance spectroscopy measurements revealed that the corrosion resistance of the paint with polythiophene is several orders of magnitude higher than that with zinc phosphate. The polythiophene derivative has been found to induce the formation of a passivating and well-adhered layer between the coating and the surface, preventing the access of chloride anions and oxygen to the substrate.     

Preparation of core–shell structured alumina–polyaniline particles and their application for corrosion protection

Polyaniline (PANI) was synthesized by chemical oxidative polymerization of aniline (ANI) in the presence of alumina (Al₂O₃) particles. The polymerization of ANI occurred preferentially on the surfaces of the particles, resulting core–shell structured alumina–polyaniline (Al₂O₃‐PANI) particles. Morphology examination showed that with decreasing of the weight ratio of Al₂O₃/ANI in the reactants, the thickness of the PANI layer increased and changed from an even surface morphology to a particulate morphology. UV–vis and Fourier transformed infrared (FTIR) spectra indicated that there is no chemical interaction between the PANI layer and the Al₂O₃ surfaces. The PANI layer adhered well to the particles and can be used as anticorrosive fillers for polymer coatings. Enhanced corrosion protection performance was achieved for the emeraldine base (EB) form of PANI deposited Al₂O₃ particles (Al₂O₃‐EB) filled epoxy coating on carbon steel in 3.0 wt % aqueous NaCl solution. The particles demonstrate both excellent corrosion protection performance and lower cost, which will be of great importance in practical applications. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4372–4377, 2006     

A comparative study of the available electrochemical methods for the investigation of epoxy and fluoropolymer coatings deterioration

The corrosion behaviour of phosphatized galvanized steel coated with both epoxy films of different thickness and fluoropolymer films has been studied by means of a.c. impedance spectroscopy (EIS), break-point frequency, potentiodynamic measurements and faradaic distortion methods as well as by the salt spray test. It was observed that the degradation of protective films appears after a long-lasting initial period but once the process starts, the area of defects increases with exposure time. The rate of degradation depends both on type of polymer and of film thickness for the same type of polymer. The same behaviour can be observed from the decrease in pore resistance and charge-transfer resistance (EIS) and increase in double-layer capacitance (a.c. impedance measurements) and corrosion current (potentiodynamic measurements and harmonic analysis).     

微弧氧化表面的溶胶-凝胶涂料封孔及其耐腐蚀性研究

使用紫外光辅助溶胶 -凝胶的光化学工艺,在微弧氧化的铝合金纤维表面涂敷一层封孔层,封孔层能够有效地将多孔的微弧氧化膜封闭,显著提高铝合金纤维的耐腐蚀性能。对微弧氧化后和封孔后的铝合金纤维进行电化学分析,结果显示,封孔后铝合金纤维的腐蚀电位从 -0.651 V正移至 -0.368V。腐蚀电流密度 Icorr从 6.02×10^-6 A/cm²降低到 9.58×10^-10 A/cm²,极化电阻 R_p从 5.4×10³ Ω·cm²增加到 4.5× 10⁴Ω·cm²,紫外光处理过的封孔膜层的腐蚀速率显著降低,铝合金纤维的耐腐蚀性能显著提高。     

Processable polyaniline-HCSA/poly(vinyl acetate-co-butyl acrylate) corrosion protection coatings for aluminium alloy 2024-T3: A SVET and Raman study

Scanning vibrating electrode technique (SVET) analysis of processable polyaniline-HCSA/poly(vinyl acetate-co-butyl acrylate) coated on to aluminium alloy 2024-T3 exhibited strong interaction between the polymer coating and the underlying metal. A scribed defect in the coating surface resulted in rapid oxidation of the exposed metal within a coating defect while the overlying polymer coating underwent both reduction and dedoping. Under the conditions investigated no protective oxide was observed to form. Raman spectroscopy of the polymer surface confirmed that dedoping was the dominant process which was accompanied by a clear reduction of the emeraldine salt form of the coating to the leucoemeraldine base form within the defect. This coating/metal interaction was observed to be dependant of the proximity to an artificial defect, initially yielding a more reduced material in close proximity to the coating defect, providing evidence of an electrochemical interaction between the polyaniline co-polymer system rather than a barrier effect.     

Electrochemical studies of the inhibition of the cathodic delamination of organically coated galvanised steel by thin conversion films

Scanning Kelvin Probe (SKP) measurements of thin amorphous conversion film coated galvanised steel in combination with current density-potential curves and electrochemical impedance spectroscopy (EIS) were performed with the aim to improve the understanding of electrode potentials at the coating/metal interface and their influence on corrosive de-adhesion. The thin hybrid conversion films contained Zn-phosphates, titanates and also complexing organic compounds and led to an inhibition of the cathodic oxygen reduction and anodic zinc dissolution. In the polymer coated area the conversion film leads to a cathodic shift of the potential as measured by means of the SKP. This cathodic potential shift is explained by the substitution of the n-semiconducting Zn-oxide with an insulating inorganic layer. When the SKP detects the potential of freely corroding iron at a defect, where no protective coating layer is, the interfacial potential for the conversion film coated zinc layer is more negative than the defect potential. This leads to a diminished driving force for an oxygen reduction induced delamination process which is of relevance for the understanding of cut-edge corrosion.     

Effect of impurities on the electrochemical behaviour of aluminium electrodes anodized in phosphoric acid

The electrochemical behaviour of anodic films formed on pure aluminium (99.999% and 1100) and 6061-Al alloy substrates in phosphoric acid has been investigated in an aluminium saturated phosphate solution of pH 6.23. The polarization resistance data obtained after one day immersion in the electrolyte showed a strong correlation with the pit number, assessed from an outdoor atmospheric exposure test after 4 months. The cathodic polarization measurements of anodized aluminium in the electrolyte were also carried out. Electrochemical behaviour, eg corrosion, hydrogen evolution reaction, was interpreted in terms of “active sites” within the anodic film. It is suggested that the active sites were mainly formed in the presence of impurities or alloy constituents in aluminium substrates.     

Compatibility between cataphoretic electro-coating and silane surface layer for the corrosion protection of galvanized steel

The compatibility between a cataphoretic electro-coating and a silane layer applied on galvanized steel was evaluated by performing electrochemical impedance measurements on coated and uncoated samples. During electro-deposition, the water electrolysis induces hydrogen production. This process can induce degradation or destruction of the silane layer. This process was simulated by reproducing the application conditions of electro-coating in an aqueous solution of same pH (6) and conductivity (1600 μS) than the electro-coating bath, but without any pigments and binder. A current of 2 mA/cm² was applied between the sample and the counter-electrode during 10 and 20 s. These conditions are representative of the mean real application conditions just before the coating formation. The loss of the barrier effect offered by the silane layer was evaluated by EIS before and after simulation. This simulation shows whether it is possible to conveniently design the properties of the silane layer to maintain its protection and adhesion promotion properties after polarization. The barrier properties and the water uptake of the electro-coated samples were evaluated by EIS as a function of immersion time in a sodium chloride solution (0.1 M). The coated silane pre-treated samples show a good behaviour compared to the samples coated without pre-treatment. By properly managing the deposition conditions of sol–gel films it is possible to obtain cataphoretic coating with improved corrosion resistance. Silane sol–gel films of different thicknesses and curing temperature were produced. It was demonstrated that a 120 nm thick silane sol gel film cured at 180 °C ensures a very good compatibility with the electro-coat. In fact, this system shows a very high corrosion resistance even after 50 days of immersion in a sodium chloride solution. Also the resistance in the salt spray chamber of the electro-coated thin silane layer cured at 180 °C is remarkable. The results confirm that, if conveniently designed, silane sol–gel film properties, the silane layer is a good adhesion promoter of the cataphoretic coating on galvanized steel and this property is maintained for long exposure times.