Corrosion protection coating containing polyaniline glass flake composite for steel

Corrosion protection of steel by glass flake (GF) containing coatings is widely used in marine atmosphere. Even though, the coatings containing glass flake are highly corrosion resistant, their performance is decreased due to the presence of pinholes and coating defects. It is well established that polyaniline containing coating is able to protect the pinhole defects in the coatings due to passivating ability of polyaniline. Hence a study has been made on the corrosion protection ability of steel using polyaniline-glass flake composite containing coating with 10% loading of glass flake in epoxy binder. The polyaniline glass flake composite (PGFC) was synthesized by chemical oxidation of aniline by ammonium persulphate in presence of glass flake. The corrosion protection ability of GF and PGFC containing coating on steel was found out by salt spray test and EIS test in 3% NaCl. In both the tests, the resistance value of the PGFC containing coating has remained at 10⁸-10⁹ Ω cm² where as for the GF containing coating, the resistance values decreased to 10⁵ Ω cm². The enhanced corrosion protection ability of the PGFC containing coating is due to the passivation ability of the polyaniline present in the coating.     

A new corrosion protection coating with polyaniline-TiO2 composite for steel

Organic coating strategies for corrosion protection with inherently conducting polymers have become important because of restriction on the use of heavy metals and chromates in coatings due to their environmental problems. This work presents the synthesis of polyaniline-TiO₂ composites (PTC) and the corrosion protection behaviour of PTC containing coating on steel. PTC was prepared by chemical oxidation of aniline and TiO₂ by ammonium persulfate in phosphoric acid medium. The PTC was characterized by FTIR, XRD and SEM techniques. Suitable coating with PTC was formed on steel using acrylic resin. Using electrochemical impedance spectroscopy, the PTC containing coating's behaviour in 3% NaCl immersion test and salt spray test has been found out. Results indicate that the coating containing PTC is able to maintain the potential of steel in passive region due to its redox property. The resistance of the coating containing PTC was more than 10⁷ Ω cm² in 3% NaCl solution after 60 days and 10⁹ Ω cm² in the salt spray test of 35 days. But the resistance of the TiO₂ containing coating was found to be less than 10⁴ Ω cm² in both the cases. The high performance of PTC containing coating is attributed to the passivation of steel by polyaniline.     

Characterisation and corrosion studies of steel electrodes covered by polypyrrole/phosphotungstate using Electrochemical Impedance Spectroscopy

Polypyrrole/PW₁₂O₄₀³⁻ hybrid material was electrosynthesised on carbon steel electrodes in acetonitrile medium. The coatings obtained were characterised by Electrochemical Impedance Spectroscopy (EIS). On free-standing polypyrrole films the electrical response was mainly due to ion–ion charge transfer resistance, with a value of 175 Ω cm². A value of 2 × 10⁻⁵ S/cm was determined for the hybrid material conductivity. A charge transfer resistance about 7000 Ω cm² was obtained due to steel/oxide interface. Corrosion tests showed an important improvement in the protection against corrosion when the carbon steel electrodes were coated by these polymeric films.     

Protective properties of intact unpigmented epoxy coated mild steel under cathodic protection

Electrochemical Impedance Spectroscopy (EIS) has been used to characterise intact unpigmented epoxy coated mild steel with and without the application of cathodic protection (CP). Coated specimens were exposed to 0.6 M NaCL solution. Cathodic protection was applied at −0.78 V and −1.1 V (SCE). Coated specimens were also tested at Open Circuit Potential (OCP). The application of cathodic protection at −1.1 V was shown to affect the protective properties of the coating, causing the coating resistance to fall below the border line between fair and poor coating. The coating maintained a resistance in the order of 1 × 10⁶ Ω cm² when CP was applied at −0.78 V but a resistance of 1 × 10⁵ Ω cm² when CP was applied at −1.1 V. It was shown that water uptake by the coated specimens was considerably affected by the application of CP. The water uptake by the coated specimens was increased as a result of increasing the level of CP. The application of CP at −0.78 V, and −1.1 V was found to reduce the extent of corrosion on the coated specimens.     

Impedance evaluation of permeability and corrosion of Al-2024 aluminum alloy coated with a chromate free primer

The corrosion of AA-2024 aluminum alloy protected with a chromate free primer is investigated after immersion in a 0.5 M NaCl aqueous solution. The water uptake by the coating increases continuously when the film, applied on an aluminum AA-2024 substrate, is placed in the 0.5 M NaCl solution. This increase is attributed to corrosion reactions taking place at the alloy/coating interface when water molecules reach the interface. The maximum water volume fraction absorbed by a similar coating applied on platinum substrate is 3.5 vol% and the permeability is 7.6 × 10⁻¹² m² s⁻¹. After 72 h immersion in the 0.5 M NaCl solution, the Nyquist representation of impedance data shows transmission line behavior that can be assigned to percolation pathway along the filler particles after water uptake. Charge transfer and diffusion of corrosion reactants and products occur, but no delamination was observed for immersion longer than 172 h. Furthermore, the coating resistance is still close to 10⁸ Ω cm⁻² after this immersion time. This accounts for the good protective performance of the coating.     

Studies on novel silicone/phosphorus/sulphur containing nano-hybrid epoxy anticorrosive and antifouling coatings

The objective of the present work is the development and characterization of silicone, phosphorous and sulphur containing nano-coatings using diglycidyl ether of bisphenol-A type epoxy resin (DGEBA) as base material, tris (p-isocyanatophenyl) thio phosphate (DESMODUR) as modifier and POSS-NH₂ (polyhedraloligomeric silsesquioxane) as nano-reinforcement. The nano-coatings were cured by Aradur 140 (polyamidoimidazoline) and XY 54 (polyamidoamine) curatives. The corrosion and fouling resistant properties of these coatings were evaluated by potentiodynamic polarization, electrochemical impedance, salt-spray and antifouling tests. It was interesting to observe that the molecular structure of curing agents as well as the nano-reinforcing effect of POSS-NH₂ significantly influenced their corrosion and fouling protection behaviour. For example, coating system ‘1’ alone retained an impedance value of 10⁹ ohm cm² even after 30 days of immersion indicating no deterioration on the coating. However the impedance values of other specimens decreased from 10⁹ to 10⁷ and 10⁶ ohm cm². This observation clearly indicates the synergistic effect of curing agent and nano-reinforcing effect of POSS-NH₂ towards corrosion resistance. Similar observation was made from the antifouling study, with a marked inhibition of bacterial adhesion on such coated panels.     

Electrochemical and surface analytical studies of the self-assembled monolayer of 5-methoxy-2-(octadecylthio)benzimidazole in corrosion protection of copper

5-Methoxy-2-(octadecylthio)benzimidazole (MOTBI) monolayer was self-assembled on fresh copper surface obtained after etching with nitric acid at ambient temperature. The optimum conditions for formation of self-assembled monolayer (SAM) were established using impedance studies. The optimum conditions are methanol as solvent, 10 mM concentration of the organic molecule and an immersion period of 24 h. The MOTBI SAM on copper surface was characterized by contact angle measurements, X-ray photoelectron spectroscopy and reflection absorption FTIR spectroscopy and it is inferred that chemisorption of MOTBI on copper surface is through nitrogen. Corrosion protection ability of MOTBI SAM was evaluated in aqueous NaCl solution using impedance, electrochemical quartz crystal nanobalance, potentiodynamic polarization and weight-loss studies. While bare copper showed a charge-transfer resistance (R_ct) value of 1.89 kΩ cm² in 0.20 M NaCl aqueous environment, the R_ct value for SAM covered copper surface is 123.4 kΩ cm². The MOTBI SAM on copper afforded corrosion inhibition efficiency of 98-99% in NaCl solution in the concentration range and in the temperature range studied. The SAM functions as a cathodic inhibitor. Quantum chemical calculations showed that MOTBI has relatively small ΔE between HOMO and LUMO and large negative charge in its benzimidazole ring, which facilitate formation of a polymeric [Cu⁺-MOTBI] complex on copper surface.     

Synthesis and evaluating corrosion protection effects of emeraldine base PAni/clay nanocomposite as a barrier pigment in zinc-rich ethyl silicate primer

In the present work, polyaniline/clay nanocomposite (PAniCN) was synthesized by chemical oxidative polymerization of aniline monomers in the presence of Closite30B powders. XRD and SEM examinations were used to examine the intercalation and morphologies of PAniCN, respectively. Electrical conductivity test showed that the conductivity of final PAniCN was higher than pristine PAni by one order of magnitude. Synthesized nanocomposite was added to the zinc rich ethyl silicate primer to modify its barrier properties. The corrosion protection performances of modified and unmodified primers were evaluated using open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) in 3.5% sodium chloride solution for a period of 120 days. It was found that the modified primer had higher barrier properties than original primer. After 120 days of immersion, resistance of modified and unmodified primers reaches 5.565 × 10³ Ω cm² and 6.056 × 10² Ω cm² respectively. The OCP of both primers were lower than −800 mV/SCE during the immersion. Besides, the OCP of modified primer was higher than the original primer due to the passivation and barrier effects of PAniCN. Results revealed that the performance of modified primer improved strongly.     

Investigation of anti-corrosion behavior of waterborne organosilane–polyester coatings for AA6011 aluminum alloy

Thermal-cured, sol–gel derived, waterborne organosilane–polyester coatings (SiE) have been developed using methyltrimethoxysilane, 3-glycidoxytrimethoxysilane and polyester resin for corrosion protection of aluminum AA6011. The structural and morphological features of the coatings were analyzed by Fourier transform infrared spectroscopy (FT-IR) and atomic force microscopy (AFM). Results show that the coatings on aluminum were smooth, continuous and defect-free. Performance of the SiE coatings were investigated and compared with pure organosilane coating and polyester coating using potentiodynamic polarization studies, contact angle measurement and pencil hardness test. Results from polarization studies have shown that the SiE coated substrate (4.6–13.1 × 10⁻⁷ A/cm²) provided a better corrosion protection than the polyester coated substrate (7.8 × 10⁻⁶ A/cm²) due formation of aluminum–oxygen–silicon covalent bond at aluminum-coating interface. Furthermore, SiE coatings provided better hydrophobicity and hardness than the polyester coating.     

Electrochemical synthesis of polypyrrole (PPy) and PPyǀmetal composites on copper electrode and investigation of their anticorrosive properties

This study reports corrosion protection behaviour of various metal cations electrodeposited onto polypyrrole (PPy) coated copper (Cu) electrode. Before electropolymerization of pyrrole, the Cu electrode was passivated in 0.1 M oxalic acid via cyclic voltammetry method. After the coating process, metal cation electrodeposition onto PPy coating was carried out in 10⁻² M CuCl₂, ZnCl₂, FeCl₂ and NiCl₂ solutions. Corrosion behaviour of uncoated, PPy and PPy|metal coated Cu electrodes was studied in 0.1 M H₂SO₄ solution by using potentiodynamic polarization, chronoamperometric and impedance spectroscopic measurements. Surface morphologies were examined by scanning electron microscope (SEM). All the electrochemical measurements were in good agreement showing that metal electrodeposited PPy coated Cu electrodes have a higher corrosion resistance. Furthermore, SEM results show that while all the samples have a homogeneous distribution of metal cations, zinc and nickel have a much better homogeneous distribution compared to copper and iron. It was found that the best corrosion protection is provided by PPy|Zn and PPy|Ni coatings and there is a significant increase in their polarization resistance with increasing amounts of electrodeposited cations.     

Metal separators coated with carbon/resin composite layers for PEFCs

A new type of metal separator coated with corrosion-resistant and electronically conductive carbon/resin composite layers has been developed. A flat, stainless steel plate was coated with a thin composite layer, and then ribs were formed of a similar composite over the thin layer as gas flow channels. The composite consisted of graphite, epoxy resin and a phenol hardener. By optimizing the combination and composition of materials, target values for the bulk electric conductivity and the chemical stability in hot water were cleared. The separator pieces exhibited a good corrosion resistance during soaking tests in 0.1 M H₂SO₄ at 90 °C over 2000 h or even at 120 °C over 1200 h. The area-specific resistance of the separator coated with the thin protecting layer and the rib layer was less than 13.8 mΩ cm².     

Modification of copper with self-assembled organic coatings

Self-assembled monolayers of n-alkanethiols (CH₃(CH₂)_n−1SH; n = 12 and 18) were prepared on a copper surface, and the quality and protection efficiency of resulting coatings against aqueous corrosion of copper were investigated. A combination of physical (scanning electron microscopy, energy dispersive X-ray spectroscopy and contact angle measurements) and electrochemical (cyclic voltammetry and impedance spectrometry) methods was used to correlate the structure of the coatings with their barrier properties during exposures in an aqueous solution. Impedance results reveal that the coatings behave almost like an ideal dielectric, with a resistance of several MΩ cm² and a coating capacitance that agree well with the value calculated according to the theory of dielectrics. Protection efficiency of chemisorbed alkanethiol coatings determined from dc and ac measurements were above 99%.     

Performance of polyaniline pigmented vinyl acrylic coating on steel in aqueous solutions

Using electrochemical impedance spectroscopy method, the performance of vinyl acrylic coating containing polyaniline on steel in 0.1 N HCl, 3% NaCl and cement extract (pH 13.0) has been evaluated. The resistance of the paint coating in all the media has been found to decrease initially from 10⁸ to 10⁶ Ω cm². On subsequent exposure to the aqueous media, the resistance of the coating has been found to increase steadily. Further, the open circuit potential values have been found to shift in active direction initially and then move in noble direction with longer immersion period due to the passivating effect of polyaniline pigment.     

Preparation and characterization of thermally stable epoxy-titanate coatings

Epoxy-polyamide coatings are used to protect metallic substrates in corrosive atmosphere. Thermal stability of the coating can be improved by the addition of inorganic cross-linking agent. Epoxy resin is incorporated with small percentage of silicone resin and cured with two types of hardeners such as polyamide and butyl titanate. The physical properties, heat resistance properties and electrochemical impedance behaviour of these coatings on steel in 0.5 M NaCl solution have been studied. The result implies that the heat resistant character of the titanate-cured coating is increased from 260 to 370 °C. The impedance study has shown that the coating resistance exerted by both the systems is in the range of 10⁵ Ω cm² after 6 days of immersion in 0.5 M NaCl. FTIR and Raman spectroscopy analysis confirm the presence of titanate linkage in the cured polymer coating. Thermal stability data indicate that the epoxy silicone resin cured with titanate hardener possesses higher thermal stability than that cured by polyamide hardener.     

Improvement of the electrochemical behavior of steel surfaces using a TiN[BCN/BN]n/c-BN multilayer system

The aim of this work is to improve the electrochemical behavior of AISI 4140 steel substrates by using a TiN[BCN/BN]_n/c-BN multilayer system as a protective coating. We grew TiN[BCN/BN]_n/c-BN multilayers via reactive r.f. magnetron sputtering technique, systematically varying the length period (Λ) and the bilayer number (n), maintaining constant the total thickness of the coating and all other growth parameters. The coatings were characterized by FTIR spectroscopy that showed bands associated to h-BN bonds, and c-BN stretching vibrations centered at 1385 cm^− 1 and 1005 cm^− 1, respectively. Film composition was studied via X-ray photoelectron spectroscopy where typical signals for C1s, N1s and B1s are shown. The electrochemical properties were studied by electrochemical impedance spectroscopy and Tafel curves. In this work, the maximum corrosion resistance for the coating with (Λ) equal to 80 nm was obtained, corresponding to n = 25 bilayers. The polarization resistance and corrosion rate were around 10.1 kOhm cm² and 0.22 mm/year; these values were 83 and 15 times higher, respectively, than uncoated AISI 4140 steel substrate (0.66 kOhm cm² and 18.51 mm/year). Optical microscopy was used for surface analysis after corrosive attack. The improvement of the electrochemical behavior of the AISI 4140 coated with this TiN[BCN/BN]_n/c-BN multilayer system can be attributed to the presence of several interfaces that offer resistance to diffusion of Cl⁻ of the electrolyte toward the steel surface.     

High performance polyaniline containing coating system for wet surfaces

Application of paint coatings on wet surfaces is rather difficult due to poor adhesion of coatings. For painting of wet surfaces, moisture curable coating systems based on epoxy resin and ketimine are found to be useful. Hence a study has been made on the corrosion protection ability of coating on wet surfaces using epoxy resin, ketimine and polyaniline. Paints with 20–30% PVC were prepared and applied over the wet steel surface and the corrosion protection performance of the coating was found out by salt spray and electrochemical impedance spectroscopic techniques. Coating with 20% PVC is found to offer very high protection since the impedance values are remained at greater than 10⁹ Ω cm² after immersion and salt spray tests.     

Effect of Al on the galvanic ability of Zn-Al coating under thin layer of electrolyte

The effect of Al on the galvanic ability of Zn-Al coating has been studied under thin electrolyte layers by measuring surface potential and surface pH. The changes of surface potential and surface pH over Zn-Al/steel galvanic couple corroding in artificial sea water (ASW) were measured at 60% and 90% RH at 298 K. In the initial stage of corrosion, Zn-55Al coating has shown better galvanic protection ability than Zn-5Al coating in both 60% and 90% RH. However, Zn-5Al coating was better in long term corrosion. The better galvanic ability of Zn-55Al coating in the initial stage of corrosion was related to the observation of pH as low as low as 2 on its surface. The low pH value was due to hydrolysis of Zn²⁺ and Al³⁺ ions. The low pH value was further confirmed by observing evolution of gas due to H⁺ reduction on the Zn-55Al coating. With the progress of corrosion, the low pH region of coating layer extended towards the base steel. This helped expand the deposition of zinc corrosion products on the steel surface. The enhanced dissolution of zinc in Zn-55Al coating led to the formation of a barrier layer which limited the galvanic protection of remaining steel. This was not the case in Zn and Zn-5Al coating. The X-ray analyses of the corroded samples have shown the deposition of zinc corrosion products on the steel surface, which greatly depended on the RH value. The part of the steel surface covered with zinc corrosion products has shown relatively less noble potential than other part indicating that zinc corrosion products took a role to protect the base steel against corrosion. The results from surface potential and surface pH measurements were substantiated by the surface observation of the corroded sample during and after the corrosion test.     

Corrosion behavior of AISI 316 stainless steel coated with modified fluoropolymer in marine condition

In the maritime industry, stainless steel corrosion resistance requires further surface improvement and further enhanced protection using surface coatings. In this study, an engineered copolymer of polyvinylidene fluoride (PVDF) with polymethylmethacrylate (PMMA)-blended coating was found to provide outstanding corrosion resistance for metal surfaces affording protection against severely corrosive marine environments. Electrochemical measurements indicate that corrosion protection of 316 stainless steel was drastically increased when utilizing the KP blend (a combination of PVDF + PMMA), producing a new set of corrosion properties and morphological characteristics. The corrosion protection of the KP coating proved to be very effective in reducing the passive region current density from 2.19 × 10^?5 A/cm² (for bare stainless steel) to 2.63 × 10^?10 A/cm² and the breakdown passive region potential at 0.25 V. This was followed by a significant decrease in corrosion rate, when compared to pure PMMA and PVDF films, during exposure to artificial marine seawater. With the KP film, impedance measurements surpassed those of other films, with a noticeable nonpeak straight line in the phase angle diagram. Optical observation showed that corrosion pits and delamination areas existed under the coatings.     

Active coating

Preventing the corrosion of metal substrates by applying an active coating was studied. The active coating was prepared by addition of metal powder and polyaniline in xylene solution. Trials were conducted by coating a blast-cleaned, mild steel round bar with a brush. The coating was dried in the air and the steel samples were immersed in water collected from the open sea. Open circuit potential of steel was measured with silver/silver chloride reference electrode. The potential in seawater was in the range of — 740mV to — 956mV, indicating complete protection of steel against corrosion. This potential meets the criteria for protection as per clause 5.4.1 of Det Norsk Veritas Recommended Practice RPB 401 (1993), page 8.¹ The active coating system can be used for protecting the steel against corrosion in marine and any other highly corrosive environment. Accelerated tests in accordance with ASTM D5894 — 96² with freeze cycle developed by U.S. Federal Highway Administration are planned.     

Capped inhibitor-loaded halloysite nanoclay-based self-healing silica coatings for corrosion protection of mild steel

The effect of polymeric nanocapsule capping in benzotriazole encapsulated into halloysite nanoclay (HNTs) dispersed into hybrid silica coatings was investigated for corrosion protection of mild steel. Optimization of the amount of inhibitor-loaded halloysite nanotubes with and without capping in the coating sol was carried out. The prepared formulations were dip-coated on mild steel substrates using dip-coater and then cured at 130 °C for 1 h. Surface morphology and elemental analysis of the nanoclay were studied using scanning electron microscopy and energy dispersive X-ray spectroscopy. X-ray diffraction and Fourier Transform Infrared spectroscopy analyses were carried out to confirm the encapsulation and capping of the halloysite nanoclay. The anti-corrosion and autonomic-healing properties of bare and coated substrates in 3.5 wt% NaCl solution were studied using electrochemical impedance spectroscopy, potentiodynamic polarization measurements and scanning vibrating electrode technique for varying exposure times. The coatings generated from the capped inhibitor-loaded HNTs dispersed sol-gel matrix was seen to provide higher corrosion resistance when compared to uncapped HNT based silica coatings. Electrochemical studies carried out for capped inhibitor-loaded HNT based coatings have shown an increase in charge transfer resistance to 10⁸ Ω cm² from 10⁶ Ω cm² of uncapped inhibitor-loaded HNT based coatings.