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.     

Influence of microstructure on corrosion properties of multilayer Mg–Al intermetallic compound coating

A multilayer Mg–Al intermetallic coating was fabricated on the Mg alloy in molten salts at 400 °C with treatment time range from 2 to 8 h. The coating consists of a single Al₁₂Mg₁₇ intermetallic layer or Al₁₂Mg₁₇, Al_0.58Mg_0.42 and Al₃Mg₂ intermetallic layers. The corrosion resistance of the coating which is obtained at 400 °C for 2 h is the best. When the treatment time is higher than 2 h, some cracks developed in the layers. The cracks were resulted from the thermal stress due to the different thermal expansion coefficient of the substrate and the intermetallic layer during the rapid cooling process.     

The improved performance of Mg-rich epoxy primer on AZ91D magnesium alloy by addition of ZnO

ZnO particles were added in Mg-rich epoxy primer to improve the protection for AZ91D magnesium alloy. The well dispersed ZnO particles could play a role in electrical conduction instead of Mg particles, consequently the Mg–ZnO-rich primer exhibited good conductivity while the dissolution rate of Mg particles decreased. ZnO particles also improved physical crosslink density of the epoxy matrix, which could reduce defects and enhance the barrier property and adhesion of the coating. As the results, the epoxy primer with 40 wt.% Mg and 10 wt.% ZnO showed better protection and prolonged lifetime than the primer with 50 wt.% Mg.     

Detrimental effect of particle sol-gel coating on the corrosion behavior of A380-SiC composite

In this study, the corrosion susceptibility of aluminum matrix composites reinforced with artificially oxidized SiO₂ and sol-gel Fe/TiO₂ coated silicon carbide particles (SiC_p) has been investigated. Corrosion behavior of the composites, fabricated by the liquid metal infiltration technique, was established in chloride containing alkaline environments by cyclic polarization (CP) and electrochemical impedance spectroscopy (EIS) techniques. It has been found that, sol-gel coating of SiC particles with Fe/TiO₂ has a detrimental effect on the corrosion characteristics of A380-SiC metal matrix composite.     

Comparison of surface modification with amino terminated polydimethylsiloxane and amino branched polydimethylsiloxane on the corrosion protection of epoxy coating

Epoxy coatings were designed to give a hydrophobic property on its surface by modifying it with two kinds of amino substituted polydimethylsiloxane, and then effects of the modification on the structure, surface hydrophobic tendency, water transport behavior and hence corrosion protectiveness of the modified epoxy coatings were examined using FT-IR spectroscopy, hygrothermal cyclic test, and impedance test. The phase separation tendency is more appreciable when modified by amino branched polydimethylsiloxane (ABP) then amino terminated polydimethylsiloxane (ATP). In the results of EIS, the corrosion protection of epoxy modified coating by ABP shows better than epoxy modified coating by ATP. It was well agreed with its water transport behavior and hydrophobic tendency.     

The evaluation of coating performance by the variations of phase angles in middle and high frequency domains of EIS

Electrochemical impedance spectra (EIS) of six coating systems in 3.5% NaCl solution were measured and the relations between coating resistances and phase angles at different frequencies were analyzed. The results indicated that in middle frequency range, the phase angle and the coating resistance show similar decreasing tendencies. For the coating systems studied, the variation of phase angles at 10 Hz with immersion time was very close to the variation of coating resistance, hence may qualitatively reflect the coating performance. For the studied coating systems, the phase angle at 10 Hz decreased continuously from the beginning, indicating the permeation of the coating system by the electrolytes. When the phase angle reached a relatively stable stage, for different coating systems which was below 40°–20°, meaning the coating has been permeated through and electrochemical reactions under the coatings occurred. In addition, the phase angle at 15 kHz may reflect the state of coating in later stage. These phase angle parameters may be used as quick measurements to evaluate coating performance.     

Influence on the anticorrosive properties of the use of erbium (III) trifluoromethanesulfonate as initiator in an epoxy powder clearcoat

New low curing temperature epoxy powder coatings cured cationically by the use of erbium (III) trifluoromethanesulfonate as initiator have been formulated. Their curing kinetics and anticorrosive properties have been studied and compared with a system commonly used in industry (o-tolylbiguanide/epoxy resin). Three different tests of anticorrosive properties (EIS, AC/DC/AC, and salt fog spray) have been used together with an adherence test, in order to establish the optimal system. Results show that a system employing 1 phr of erbium triflate presents good anticorrosive properties. The technique AC/DC/AC has shown its ability to evaluate properly, much faster, and in accordance to anticorrosive properties results’ of powder coatings obtained by other techniques.     

Evaluation of the corrosion resistance of an epoxy-polyamide coating containing different ratios of micaceous iron oxide/Al pigments

The corrosion resistance of an epoxy coating reinforced with different ratios of MIO/Al pigments was studied. The coatings properties were investigated by an electrochemical impedance spectroscopy (EIS), salt spray test, cathodic disbonding and a scanning electron microscope (SEM). The corrosion resistance of the epoxy coating was improved using MIO (micaceous iron oxide) and Al pigments. The corrosion resistance of the purely Al pigmented coating was considerably greater than the purely MIO pigmented coating. The cathodic disbonded area of coating was decreased using MIO and Al pigments. The decrease in disbonded area was more pronounced in the presence of Al particles.     

The effect of temperature and concentration on the corrosion inhibition mechanism of an amphiphilic amido-amine in CO2 saturated solution

The corrosion inhibition mechanism of the N-[2-[(2-aminoethyl)amino]ethyl]-9-octadecenamide on mild steel surface in CO₂-saturated 5% NaCl solution has been studied. The inhibition efficiency decreases with increasing temperature. Adsorption of the inhibitor studied is found to follow the Frumkin adsorption isotherm. EIS results show that the mechanism of its corrosion inhibition at concentrations higher than critical micelle concentration is by forming a protective porous bi-layer. The activation energy, thermodynamic parameters and electrochemical results reveal a change in the adsorption mode of the inhibitor studied: the inhibitor could primarily be physically adsorbed at low concentrations, while chemisorption is favoured as concentration increases.     

Characterization of corrosion of X70 pipeline steel in thin electrolyte layer under disbonded coating by scanning Kelvin probe

Scanning Kelvin probe technique was used to characterize the electrochemical corrosion behavior of X70 steel in a thin layer of near-neutral pH and high pH solutions, respectively. Results demonstrate that passivity can be developed on steel in the near-neutral pH solution layer as thin as 60 μm, which is attributed to the fact that Fe²⁺ concentration in aqueous phase could reach saturation in the thin solution layer. The solubility of FeCO₃ is reached to drop out of solution as a precipitate. With the increase of solution layer thickness, it becomes more difficult for Fe²⁺ concentration to reach saturation. Consequently, the passivity cannot be maintained, and the steel shows an active dissolution state. Anodic dissolution rate of steel increases with the immersion time. The electrochemical polarization behavior of X70 steel in high pH solution is approximately independent of the solution layer thickness and immersion time. In thin solution layer, diffusion and reduction of oxygen dominate the cathodic process, as demonstrated by the presence of cathodic limiting diffusive current. In the bulk solution, the absence of limiting diffusive current density in cathodic polarization curve indicates that the main cathodic reaction is reduction of H₂CO₃ and , and the formed film is thus mainly FeCO₃.     

Heat treatment effect on the corrosion behaviour of black passivated ZnNi alloys

Electrochemical corrosion behaviour of ZnNi and black passivated ZnNi alloys in aerated and partially deaerated 5% NaCl solution was studied using dc and ac techniques. The heat treatment (24 h, 120 °C) effect on black CCC properties was analyzed. No changes in the R_p values were observed, but differences in the potentiodynamic curves and in EIS diagrams suggested a change in the corrosion mechanism. Comparative studies by AFM and electrochemical techniques showed that these changes in the corrosion behaviour could be explained by taking into account the different effect of heat treatment on the inner and outer layers of the CCC.     

The effect of molybdate concentration and hydrodynamic effect on mild steel corrosion inhibition in simulated cooling water

The effects of two important factors namely concentration and hydrodynamics on the inhibition efficiency of molybdate were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy tests. Results showed that molybdate was capable of inhibiting the corrosion of mild steel in simulated cooling water. The inhibition efficiency of molybdate was increased with increasing both its concentration and water circulation velocity. These two factors seem to promote the adsorption of molybdate and oxygen ions on the metal surface, leading to the formation of a protective layer with a greater charge transfer resistance and lower permeability. A model is also proposed which facilitates the probable mechanism of inhibition.     

Disbonding of underwater-cured epoxy coating caused by cathodic protection current

Cathodic disbonding of the underwater-applied, ultra-thick, solvent free epoxy coating subjected to various levels of cathodic protection was investigated during the period of the coating cure. The results indicate that the partially cured coating was of low resistivity, between 10³ and 10⁵ Ω cm² for the cathodic polarization of on-potentials between −0.98 and −1.4 V_Ag/AgCl/sw. The coating was shown to be capable of withstanding normal levels of cathodic protection between off-potentials of −0.8 and −1.1 V_Ag/AgCl/sw while the IR drop, introduced by the coating in the same potential range, increased from 0.06 to 0.1 V and has to be taken into account at the design stage of the cathodic protection system. Beneficial influence of calcareous deposit formation on the cathodic protection current was confirmed, particularly for the failed coating. The initial period (1 week) of coating cure was shown as the most critical for disbonding processes caused by the excessive cathodic polarization.     

The adhesion properties and corrosion performance of differently pretreated epoxy coatings on an aluminium alloy

The influence of various blends of hexafluorozirconic-acid (Zr), polyacrylic-acid (PAA) and polyacrylamide (PAM) pretreatment on the performance of an epoxy coated aluminium substrate was investigated and compared to that of a so-called chromate/phosphate conversion coating (CPCC).Adhesive-strength of epoxy coated substrates was evaluated using pull-off and tape tests. Salt spray, humidity chambers and EIS were employed to characterize corrosion performance of coated substrates with different initial surface pretreatments. Among the Zr-based formulations, PAA/Zr and PAA/PAM/Zr showed the best adhesion strength, while the later revealed a good corrosion performance as well. However, CPCC pretreated sample was still superior in these aspects.     

Microstructure and corrosion performance of a cold sprayed aluminium coating on AZ91D magnesium alloy

A pure Al coating was deposited on AZ91D magnesium alloy through cold spray (CS) technique. The microstructure of the coating was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the grain interfaces and subgrains formed close to the particle/particle boundaries. Electrochemical tests revealed that the cold sprayed pure Al coating had better pitting corrosion resistance than bulk pure Al with similar purity in neutral 3.5 wt.% NaCl solution. In addition, a mass-transfer step was found to be involved in the corrosion during 10 days immersion.     

Study of the corrosion inhibition effect of sodium silicate on AZ91D magnesium alloy

The poor corrosion resistance of magnesium alloys is a major impediment to their applications in many fields. In this paper, sodium silicate as a corrosion inhibitor is studied on the inhibition effect of AZ91D magnesium alloy. From the results of the corrosion tests, sodium silicate could effectively improve the corrosion resistance of alloy at the optimum concentration 10 mmol/L, while the pH value range from 10.5 to 12.5 is preferable. The corrosion inhibition mechanism of the protective layers is also discussed. These results can provide a guide for the protection of magnesium alloy in the cooling water systems, etc.     

Multifunctional epoxy coatings combining a mixture of traps and inhibitor loaded nanocontainers for corrosion protection of AA2024-T3

This work aims at investigating the corrosion protection effectiveness of multifunctional epoxy coatings modified with pigments such as ceramic nanocontainers loaded with corrosion inhibitor, chloride and water traps, applied on AA2024-T3. Characterizations on the morphology, composition and structure of the coatings were conducted. The corrosion resistance was studied by electrochemical impedance spectroscopy, localized electrochemical impedance spectroscopy and scanning vibrating electrode technique. The mechanical behaviour of the coatings was examined through nanoindentation and nanoscratching tests. Electrochemical and nanomechanical testing results, evidenced the improvement of the corrosion protective properties and mechanical behaviour of the coatings in the presence of the various pigments.     

Studies on enhancement of surface durability for steel surface by camphor oil modified epoxy polyamide coating

Epoxy polyamide coatings are generally used to protect mild steel structures from corrosive atmosphere due to their better adhesion over under prepared surface and effective barrier protection. But the coating has the ability to disintegrate due to UV radiation and high humidity condition. To improve the weatherability and chemical resistance performance of epoxy polyamide, there is a need to modify it with suitable cross linking agent. In this work, it has been found that camphor oil at 5 wt.% as the optimum level to protect the mild steel structures from corrosive electrolyte. Further the impedance study has shown that the resistance exerted by the Camphor oil incorporated coating in 0.5 M NaCl solution after 60 days is 3 × 10⁷ Ω cm² where as the resistance of the coating without this modifier is 3 × 10⁶ Ω cm². The FTIR spectral study indicates that the formation of ether linkages in the dried film and also the other functional groups present in the epoxy polyamide polymer is completely disappeared in the modified coating. Similarly the TG and DTA analysis showed that considerable shift in the degradation temperature has been noticed for the polymer coating with modifier.     

The role of zinc aluminum phosphate anticorrosive pigment in Protective Performance and cathodic disbondment of epoxy coating

Protective performance and cathodic disbondment of epoxy coating pigmented with zinc aluminum phosphate (ZPA) were studied in this work. In solution, superior corrosion inhibition of ZPA extracted from EIS and electrochemical noise data was connected to deposition of a protective layer. EIS evaluation of the pigmented coatings indicated significant effect of modification of zinc phosphate on the protective performance as well as resistance to cathodic disbonding. Compared to ZPA, introduction of zinc phosphate resulted in inferior performance in cathodic disbonding test. In presence of ZPA, precipitation at disbonding front inferred from EIS data was confirmed by SEM.     

A study of the corrosion behavior of gamma titanium aluminide in 3.5 wt% NaCl solution and seawater

Gamma titanium aluminide (γ-TiAl) was evaluated for corrosion resistance in 3.5% NaCl and seawater using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Polarization plots indicate very low corrosion rates although in both electrolytes the control samples lost passivity at low polarization potentials. Surface modification treatments were employed by oxidizing the samples in air at 500 °C and 800 °C with the purpose of improving corrosion resistance. While the surface treatments rendered γ-TiAl passive in the polarization experiments in both electrolytes, EIS tests showed that the oxide formed at 800 °C was not protective. EEC models are proposed to explain the EIS results for the three surface conditions tested.