Polyaniline, polypyrrole and poly(3,4-ethylenedioxythiophene) as additives of organic coatings to prevent corrosion

The aim of this work is to compare the protection against corrosion imparted by different conducting polymers when these materials are used as anticorrosive additives in the formulation of conventional epoxy paints. Specifically, the polymers employed as anticorrosive additives are polyaniline emeraldine salt, polyaniline emeraldine base, polyaniline emeraldine salt composite with carbon black, polypyrrole composite with carbon black and poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulphonate). Initially, the structural, thermal and mechanical properties of the unmodified epoxy paint as well as the modified coatings, obtained by the addition of 0.3 wt.% of conducting polymers, have been characterized. After this, controlled accelerated corrosion assays in an aggressive solution medium were developed using coated steel panels. Results indicate that the protection against corrosion imparted by the formulations modified by the addition of poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulphonate), polyaniline emeraldine salt and, especially, polyaniline emeraldine base is significantly higher than that of the unmodified paint. In contrast, the use of conducting polymer composite with carbon black reduces the efficacy of the coating. Results indicate that some conducting polymer compositions should be considered as a suitable alternative to replace inorganic anticorrosive pigments currently used in paint formulations.     

The effect of epoxy coating containing emeraldine base and hydrofluoric acid doped polyaniline on the corrosion protection of AZ91D magnesium alloy

Corrosion protection of epoxy coatings containing emeraldine base polyaniline (PANI) or hydrofluoric acid doped PANI on AZ91D magnesium alloy were studied by EIS and Pull-Off Adhesion Test. The results indicated that the addition of emeraldine base PANI or hydrofluoric acid doped PANI could improve the corrosion resistance of epoxy coating. The epoxy coating containing hydrofluoric acid doped PANI had the best performance of the corrosion protection among three systems under investigation. The corrosion product film was analyzed by XPS indicating that PANI changed the chemical structure of the corrosion film. The protective mechanism imparted by PANI was discussed.     

Comparison of selected inhibitor doped sol–gel coating systems for protection of mild steel

Current legislation around the use of antifouling and anticorrosion coatings has increased the need to find an environment friendly alternative to existing biocide containing pretreatments for steel structures exposed to sea water. Sol–gel technology offers a solution to fouling and corrosion inhibition since sol–gel chemistry lends itself ideally to functionalisation with a variety of components, for example, corrosion inhibitors. In this study, we have utilised the above mentioned approach in order to produce an inhibitor doped sol–gel protective coating on mild steel. The corrosion protection performance of three inhibitors, i.e. SD (Shieldex303), MOLY (Moly-White101ED-PLUS) and ZAPP (Heucophos ZAPP), added individually to a sol–gel, and applied to mild steel Q panels, were evaluated using electrochemical impedance spectroscopy. The inhibitor containing coatings were shown to protect against corrosion of mild steel; the coatings containing the inhibitors Moly and ZAPP were the most effective of the systems tested.

Highlights

? Sol–gel coatings are potential candidate systems for the protection of mild steel substrates.

? Sol–gel chemistry allows a variety of corrosion inhibitors to be incorporated into the sol–gel coating matrix.

? Evaluation of corrosion inhibition efficiency of sol–gel coatings can be assessed using electrochemical impedance spectroscopy (EIS).     

Corrosion inhibition of aluminum alloy in chloride mediums by undoped and doped forms of polyaniline

Corrosion inhibition of Al 3003 alloy by pure undoped PANI (emeraldine base) and PANI doped with p-toluene-sulfonic, camphorsulfonic and dodecylbenzenesulfonic acids was investigated. Corrosion resisting properties of PANI coatings with an artificially created hole defect were evaluated by electrochemical impedance spectroscopy (EIS) in aqueous 3.5% NaCl and 0.1 N HCl solutions. The highest corrosion inhibition factor was obtained for undoped PANI being equal to 12 and 4.4 in neutral and acidic media, respectively. The results indicated that corrosion protection of the bare aluminum alloy surface resulting from a defect of the PANI coating is in line with an increase of the thickness of the oxide layer protecting the aluminum alloy surface. The efficiency of corrosion protection of mild steel and aluminum alloy by polyaniline coatings was compared.     

Long-term anticorrosion behaviour of polyaniline on mild Steel

Anticorrosion performances of polyaniline emeraldine base/epoxy resin (EB/ER) coating on mild steel in 3.5% NaCl solutions of various pH values were investigated by electrochemical impedance spectroscopy (EIS) for 150 days. In neutral solution (pH 6.1), EB/ER coating offered very efficient corrosion protection with respect to pure ER coating, especially when EB content was 5-10%. The impedance at 0.1 Hz of the coating increased in the first 1-40 immersion days and then remained constant above 10⁹ Ω·cm² until 150 days, which in combination with the observation of a Fe₂O₃/Fe₃O₄ passive film formed on steel confirmed that the protection of EB was mainly anodic. In acidic or basic solution (pH 1 or 13), EB/ER coating also performed much better than pure ER coating. However, these media weakened the corrosion resistance due to breakdown of the passive film or deterioration of the ER binder.     

Role of dissolved oxygen diffusion in coating defect protection by emeraldine base

The corrosion protection of emeraldine base (EB) on mild steel in the artificial defect exposed to 1 wt% NaCl solution was studied using in situ optical microscope and scanning electron microscopy (SEM). The EB coating displayed good corrosion protection on the exposed bare steel, which was difficult to understand by traditional anodic protection mechanism or inhibitory protection mechanism, or cathodic protection mechanism, since steel usually cannot be passivated in brine, and EB coating cannot release inhibitory anions to protect exposed bare steel, and further, the galvanic coupling measurement demonstrates that EB cannot provide cathodic protection to exposed bare steel. The importance of dissolved oxygen diffusion caused by EB in corrosion protection was disclosed in this paper, a related protection model was therefore proposed, which was qualitatively verified by corresponding electrochemical measurements.     

Corrosion protection with polyaniline and polypyrrole as anticorrosive additives for epoxy paint

Accelerated immersion tests have been carried out in the laboratory to investigate the performance of polyaniline emeraldine salt and polypyrrole composite with carbon black as additives of an epoxy paint coating. The steel panels coated with paint modified with polyaniline emeraldine salt presented the best protection after 720 h of exposure in 3.5% NaCl solution. The results indicated that this conducting polymer might works as both corrosion inhibitor and adhesion promoter. The protective mechanism imparted by conducting polymers is also discussed.     

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.     

Chlorinated rubber paints for corrosion prevention of mild steel: A comparison between zinc phosphate and polyaniline pigments

The anticorrosion performance of plasticized chlorinated rubber coated mild steel sheets incorporating polyaniline emeraldine salt or zinc phosphate as active pigments were compared using salt spray and immersion in 3.5% NaCl solution. The results obtained by different electrochemical methods indicate the superiority of polyaniline in comparison with zinc phosphate in terms of corrosion protection. The time of the emergence of the first rust spot in 3.5% NaCl solution for the film containing 1.5 wt% polyaniline reach 960 h, which is six times higher than that of the film without polyaniline. The protective mechanism by zinc phosphate and polyaniline pigments were discussed.     

Corrosion Protection of Mild Steel with Polyaniline–Thiokol Rubber Composite Coatings

The anti-corrosion performance of polyaniline-thiokol rubber (PANI/TR) composite coating applied to mild steel samples in artificial brine and hydrochloric acid environments is for the first time evaluated in this paper. The PANI/TR composite -coatings on mild steel were electropolymerized in a nonaqueous solution. EDS, XPS and SEM were concurrently used to characterize the structure, as well as composition of the coating and show that the structure is favorable to the corrosion protection. Both potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) demonstrated that the PANI/TR coating outperforms PANI coating both in the adhesion to the substrate and its corrosion protection, making the free corrosion potential by about 100mV more positive. As a result, the mild steel corrosion was greatly impeded by applying PANI/TR composite conductive polymer coating.     

Polyaniline by new miniemulsion polymerization and the effect of reducing agent on conductivity

A new stable miniemulsion of aniline was prepared using sodium dodecyl sulphate (SDS) as the surfactant and cetyl alcohol as the cosurfactant. The oxidative polymerization reaction was initiated by ammonium peroxy disulphate (APS). At the end of the polymerization reaction polyaniline (PAni) formed was treated with stannous chloride (SnCl₂) and finally doped with p-toluene sulphonic acid (PTSA). The monomer to oxidant and polymer to dopant mole ratio were optimized. Maximum conductivity is obtained with monomer to oxidant mole ratio1:1 and polymer to dopant mole ratio 1:0.5. There is a decrease in particle size and increase in the solubility in miniemulsion process. Dramatic increase in the conductivity is due to the treatment with SnCl_2. Part of the pernigraniline (PNB) form of PAni formed was reduced by SnCl₂ to emeraldine base (EB) form having higher conductivity. Plausible reduction mechanism with SnCl₂ is proposed. The conduction mechanism is explained using EPR measurement. The reason for difference in conductivity of different forms of PAni is explained with the help of polaron formation and hole theory.     

Polyaniline-coated iron: studies on the dissolution and electrochemical activity as a function of pH

Polyaniline coatings were electrodeposited from an oxalic acid solution onto iron and their electrochemical activity and corrosion protection properties studied as a function of pH. It was found that the coating (emeraldine salt) had a limited effect on the corrosion protection of iron in acidic solutions. However, in an alkaline borate solution, where the conducting polyaniline was converted to the emeraldine base, the coating had a clear beneficial effect on the local breakdown of iron by chloride anions; much higher pitting potentials were recorded following a 2 h immersion period for the polyaniline-coated substrate relative to the uncoated electrode. Relatively small anions, such as acetates, nitrates and borates, were transported readily across the polymer interface. However, the emeraldine base inhibited the transport of the much larger ethylenediamine tetraacetate (EDTA) species to the iron interface, preventing complexation of the iron by EDTA.     

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.     

On the use of conducting polymers to improve the resistance against corrosion of paints based on polyurethane resins

This work analyzes the physical properties of several paints and the resistance against corrosion that they impart after being modified by adding a conducting polymer. Five different formulations were selected: four polyurethane resins (two varnish and two aqueous based) and one multicomponent system containing polyester, melamine and cellulose acetobutyrate. The physical properties of the coatings were examined by FTIR, thermal analyses and viscosity measurements. Corrosion resistance of carbon steel coated with these paints was studied by means of accelerated laboratory tests. The results provided by the original formulations and those modified by adding 1 % w/w of polypyrrole or poly(3‐decylthiophene‐2,5‐diyl) regioregular were compared. Although the general behavior was irregular, showing dependence with the paint formulation, excellent results were achieved for an aqueous based polyurethane resin modified with polypyrrole. Accelerated immersion experiments in salt and acid solutions showed fast degradation of the film without conducting polymer, while the polymeric film with polypyrrole offered a higher resistance.     

Corrosion protection properties of copper‐ and carbon‐containing polyethylene films in humid air climates

The corrosion protection properties of commercially available polyethylene (PE) films containing copper and carbon were investigated and compared with pure PE films of comparable thickness. The composition of these films was analyzed by thermoanalysis and atom emission spectroscopy (ICP‐AES). Electrochemical impedance spectroscopy (EIS) and packaging tests were used to determine the ability of copper‐ and carbon‐containing PE films to protect mild steel against corrosion in humid air climates. For the here‐investigated samples, no electrochemical activity of copper could be found under these conditions. In contrast to the added copper only the inserted carbon decreased the surface resistance. Additional abrasive tests imply that copper is encapsulated inside the films. For all samples, the permeability to vapor and the insertion of water were similar to the copper and carbon free reference film. The tests in the climate cabinet revealed that mild steel samples packed in copper‐ and carbon‐containing films reached the same degree of corrosion than those in the reference films.     

Correlation of content and corrosion protection of volatile corrosion inhibitors in packaging material – regaining trust in VCI

Two representative types of commercial volatile corrosion inhibitor (VCI) packaging materials without a potential for health hazards according to TRGS 615 were investigated: a paper containing ethanol amine and a polyethylene foil containing sodium nitrite as main VCI ingredients. For a variation of VCI concentration the packaging material was thermally treated at 60–100 °C and the amount of VCI remaining was determined after this accelerated desorption. Additionally a nitrite‐free polyethylene foil was impregnated with various amounts of sodium nitrite. Correlations of VCI content – of both the amine and nitrite – with corrosion protection of mild steel were observed. For ethanol amine a color change reaction is proposed to indicate the remaining potential of corrosion protection. This work is aimed at regaining trust in VCI technology.     

Structure and properties of selenious acid doped polyaniline with varied dopant content

In this study, the effects of dopant content on the structure, especially crystalline structure, and properties of selenious acid doped polyaniline (PAni) were investigated. The structure and properties of PAni were characterized by X-ray diffraction, FTIR spectroscopy, UV–vis spectroscopy, DSC, and TGA. By varying the dopant/monomer molar ratio (D/M), the morphology as well as the crystalline structure of PAni was significantly changed. The presence of water in the crystalline structure of PAni prepared with a low D/M ratio caused a change in the d-spacing of the PAni crystalline structure. The oxidation level and doping degree of PAni were also changed by varying the D/M ratio. The electrical conductivity of PAni increased with increasing D/M ratio, and the thermal stability of PAni in the doped state was about 200 °C for different D/M ratios.     

Corrosion performance of conducting polymer coatings applied on mild steel

Conducting polypyrrole electrodes were obtained by galvanostatic electropolymerisation on mild steel electrodes from an aqueous solution. The electrochemical response of the coated electrodes in doped and in undoped state was compared with bare mild steel electrodes. The undoped polypyrrole coated electrode offered a noticeable enhancement of protection against corrosion processes.     

The inhibition effect of the new Schiff base,namely 2,2′‐[bis‐N(4‐choloro benzaldimin)]‐1,1′‐dithio against mild steel corrosion

2,2′‐[bis‐N(4‐choloro benzaldimin)]‐1,1′‐dithio has been synthesized and its inhibiting action on the corrosion of mild steel in 0.5 M sulfuric acid was investigated by various corrosion monitoring techniques, such as weight loss and potentiodynamic polarization techniques. The experimental results suggest that this compound is a good corrosion inhibitor for mild steel and the inhibition efficiency increases with the increase in inhibitor concentration. This organic compound is a mixed type inhibitor in the acid solution, and its adsorption on the mild steel surface is also found to obey the Langmuir adsorption isotherm.     

The effect of calcination on the corrosion performance of TiO2 sol–gel coatings doped with benzotriazole on steel CK45

Sol–gel method under dip coating process and tetra‐n‐butyle orthotitanate as precursor were used to produce titanium oxide coating on steel CK45. The effect of calcination at 400 °C, after drying at 120 °C, on the morphology and corrosion performance of the coatings was investigated. Benzotriazole (BTA) as an inhibitor was doped on titanium oxide coatings to improve corrosion performance of the coatings as well as its self‐healing properties. The morphology and structure of the coatings were studied by X‐ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive X‐ray spectroscopy (EDS) techniques. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques were used to study the corrosion behavior of the coatings. The results revealed that after calcination the dominant crystalline phase was anatase. The results obtained from potentiodynamic polarization curves and electrochemical impedance spectra showed that the calcinated and doped with BTA coatings possessed higher corrosion resistance than non‐calcinated although doped with BTA coatings.