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.     

Corrosion inhibition at galvanized steel cut edges by phosphate pigments

The corrosion mechanisms and the inhibiting effect of sodium phosphate at cut edges on galvanized steel was studied by potentiodynamic polarization, EIS and monitoring of local ionic currents made by means of the scanning vibrating electrode technique (SVET). Assessment of zinc and of iron single electrodes was also made for comparison.In the absence of phosphate, corrosion of electrogalvanized steel at the cut edge is cathodically inhibited by the precipitation of zinc corrosion products over the steel surface. Mapping of the ionic currents revealed that the cathodic area becomes laterally shifted away from the anode as zinc corrosion products precipitate along parallel lines that are dictated by the location of the maximum cathodic currents. Sodium phosphate revealed inhibiting properties on the cut edge by precipitation of zinc phosphate, in the form of gel-like zinc phosphate clusters. Estimated inhibiting efficiencies for the cut edge were slightly above 80%.     

Organic coatings for pre-treated galvanized steel technology

Fluorinated coatings are at the moment one of the best solutions to protect steel and aluminium substrate against corrosion. New VF2-based fluopolymer coatings, by the addition of OH groups in the polymeric chain and with lower curing temperature, have been formulated by ELF-ATOCHEM. Electrochemical characterization of this material has been performed by means of impedance spectroscopy. The results obtained and the comparison with the traditional PVDF system show an interesting improvement of the protective properties.     

The corrosion resistance of 316L stainless steel coated with a silane hybrid nanocomposite coating

The present work aims at evaluating the corrosion resistance of 316L stainless steel pre-treated with an organic–inorganic silane hybrid coating. The latter was prepared via a sol–gel process using 3-glycidoxypropyl-trimethoxysilane as a precursor and bisphenol A as a cross-linking agent. The corrosion resistance of the pre-treated substrates was evaluated by neutral salt spray tests, linear sweep voltammetry and electrochemical impedance spectroscopy techniques during immersion in a 3.5% NaCl solution. In addition, the effect of the drying method as an effective parameter on the microscopic features of the hybrid coatings was studied using Fourier transform infrared spectroscopy and scanning electron microscopy. Results show that the silane hybrid coatings provide a good coverage and an additional corrosion protection of the 316L substrate.     

Quality criteria of powder coated HDG steel in corrosive environment

The performance of powder coatings on hot-dip galvanized steel has been studied by testing in 5 years field exposure in marine and industrial atmospheres, as well as in accelerated tests. The aim was to assess the quality of such coating systems in corrosive environments and to establish criteria for how to obtain good quality. The test data demonstrates that polyester powder coatings are a good alternative to wet paint as top coat on hot-dip galvanized steel. The phosphate layer is identified as the most crucial step to ensure high quality. Recommendations for how to obtain good quality are provided.     

Corrosion protection of galvanized iron by polyaniline containing wash primer coating

Wash primer treatment of galvanized iron (GI) structure is widely used before painting in order to improve adhesion. Traditional wash primer contains zinc tetroxy chromate. Due to hazardous nature of chromate, alternate compounds for chromate replacements have been identified. In recent years polyaniline containing coating has been found to protect GI. In this study, a wash primer based on polyaniline has been formulated and its corrosion protection ability of GI has been compared with that of traditional chromate based wash primer by salt spray and EIS test. It has been found that the polyaniline based wash primer is able to protect GI and its corrosion protection performance is similar to chromate based wash primer coating.     

Corrosion protection behavior of AZ31 magnesium alloy with cathodic electrophoretic coating pretreated by silane

As an alternative process to phosphate and chromate conversion coatings, silane pretreatment was used to improve the performance of cathodic electrophoretic coating (E-coat) on AZ31 Mg alloy in this study. The galvanic corrosion behavior of AZ31 Mg alloy with E-coat coupled with Q235 steel was investigated. Compared to bare Mg alloy and Mg alloy with conventional painting, the corrosion properties of the AZ31 Mg alloy pretreated with silane and subsequently E-coated were studied during salt solution immersion and salt spray testing. The surface morphologies of the Mg alloy were examined in detail after immersion in NaCl solution for different times using digital photography and scanning electron microscopy (SEM). The corrosion current density of the specimens was characterized by DC polarization tests. It was found that silane pretreatment of AZ31 Mg alloy followed by subsequent E-coat led to much better corrosion protection than that without silane treatment. The silane pretreatment and E-coat delayed the galvanic corrosion of Mg alloy coupled with 235 steel bolts.     

Surface chemistry of galvanized steel sheets relevant to adhesion performance

A review is presented on the recent development of surface treatment technologies for hot-dip galvanized steels relevant to adhesion of organic coatings. Applications of surface analytical techniques have elucidated that the surface layers of the nanometer scale dramatically govern the adhesion performance of painting or adhesive bonding. Surface enrichment of aluminium in the zinc layer deteriorates paint adhesion due to the reduction in phosphatability on the galvanized steel sheets and decreases the adhesive strength of the epoxy/dicyandiamide-bonded sheets due to the loss of acid-base interaction at the adhesive-substrate interface. In addition, the co-segregation of Al and Pb into the surface layer is responsible for the intergranular corrosion of zinc and facilitates the formation of a weak boundary layer, resulting in poor bond durability in a wet atmosphere. Improved adhesion performance has been established by developing new technologies that reduce the surface enrichment of minor elements or impurities in the zinc layer on the galvanizing line or that adopt a surface conditioning process prior to pretreatment in subsequent coil coating lines.     

The effect of benzimidazole and zinc acetylacetonate mixture on cathodic disbonding of epoxy coated mild steel

Electrochemical behavior of mild steel in the presence of zinc acetylacetonate (Zn(acac)₂) and benzimidazole (BIMIDA) was evaluated by electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution and compared to zinc phosphate (ZP) pigment and zinc potassium chromate (ZPC) pigment extracts. Results showed superior performance of Zn(acac)₂ and BIMIDA mixture (ZBM) compared to ZP pigment, while it introduced inferior inhibitive action compared to ZPC pigment. The epoxy coatings were formulated with ZBM as anticorrosive additive, ZP and ZPC pigments. The disbonding rate of coated steel follows the order: Blank > ZP > ZBM > ZPC. EIS results showed a complex film could be formed at the hole area for the coatings formulated with ZP, ZBM and ZPC. It is concluded that the stronger the complex film on the surface, the lesser the cathodic disbonding rate would be.     

Effect of SO4 on the corrosion behavior of cerium-based conversion coatings on galvanized steel

The cerium-based conversion coatings on galvanized steel were investigated and the influence of SO₄²⁻ on the corrosion behavior was evaluated. The coatings were prepared by a simple immersion of galvanized steels in an aqueous solution composed of cerium nitrate and sodium sulfate. The corrosion behavior was studied by means of potentiodynamic polarization measurements and neutral salt spray tests. The addition of SO₄²⁻ to the coating solution considerably improved the corrosion resistance of the conversion coatings. Atomic force microscope observation shows that deposits prepared from the solution with SO₄²⁻ have smaller crystalline size than those prepared without SO₄²⁻. The results of X-ray photoelectron spectroscopy and titration curves demonstrate that SO₄²⁻ ions are incorporated in the coating during the conversion process. This indicates that SO₄²⁻ acts as a grain refiner and/or growth inhibitor, thus enhancing the corrosion resistance.     

Electrodeposition of silane films on aluminum alloys for corrosion protection

In this paper, three types of protective silane films, methyltrimethoxysilane (MTMS), vinyltrimethoxysilane (VTMS) and dodecyltrimethoxysilane (DTMS) were prepared on aluminum alloys AA 2024-T3 by electrodeposition technique. The Reflection-Absorption Fourier Transform IR (FTRA-IR) measurements showed that, the silane films were successfully deposited through chemical bonding between silane agents and Al alloys. Electrochemical impedance spectroscopy (EIS) tests indicated that in comparison with those by conventional “dip-coating” method, silane films electrochemically prepared at cathodic potentials exhibited obviously higher corrosion resistances. “Critical potential” was all observed for each silane system. Silane films prepared at this potential performed the highest corrosion resistance. The scanning electron microscopy (SEM) images indicated a potential dependence of surface morphology of silane films. The highest compactness was obtained at the “critical potential”. Due to the presence of long hydrophobic dodecyl chain in bone structure, DTMS films displayed the highest barrier properties.     

Integrated electrochemical approach for the investigation of silane pre-treatments for painting copper

Copper surfaces are sometimes protected with an organic coating to improve the durability or the aesthetic properties. Examples of industrial applications are household appliances and heat exchanger components.     

Adhesion enhancement of powder coatings on galvanised steel by addition of organo-modified silica particles

The addition of organo-modified silica particles (OSP) to organic monolayer coatings has been investigated as an alternative to the use of primers or surface pretreatments in galvanised steel substrates. A commercial additive consisting of trifunctional organosilane (alkyl-triethoxysilane) grafted on silica particles was directly incorporated at different concentrations (1, 2.5, 3.5 and 4.5 wt%) as an integral additive in a polyester powder coating. The OSP were characterised physicochemically by means of FTIR and TGA, and the coating formulated by DSC. The anticorrosive properties of the systems were evaluated by means of electrochemical impedance spectroscopy (EIS), showing improvements with all the formulations containing the OSP, especially in the coating with 2.5% OSP. In order to explain this behaviour, morphological (using SEM) and adhesion studies were done. The formation of agglomerates in the powder coatings was detected when the concentration was over 2.5%. There was an improvement in the adhesion of the coating to the substrate for all the samples containing the OSP but especially for that containing 2.5%. The impact resistance was increased too, especially in the formulations with 2.5% and 3.5%.     

The influence of zinc surface pretreatment on the adhesion of epoxy coating electrodeposited on hot-dip galvanized steel

The adhesion and electrochemical properties of epoxy coatings electrodeposited on hot-dip galvanized steel with and without passive films were investigated during exposure to 3% NaCl. The passive films were formed in hot air, in boiling water and by chromating. Adhesion was measured both by a standardized pull-off method and by swelling in N-methyl pyrrolidone. Pretreatment of hot-dip galvanized steel with passive film formed in hot air increases both dry and wet adhesion strength of the epoxy coating compared to pretreatment with passive film formed in boiling water and chromate coating. The overall increase of wet adhesion for this sample was maintained throughout the whole investigated time period. It was shown that the change in adhesion of epoxy coating on a chromate coating is smallest of all investigated samples, although the initial value of adhesion on this surface had the lowest value. The corrosion stability of coated Zn samples pretreated by different methods, was investigated by electrochemical impedance spectroscopy and in the initial time of exposure to NaCl the highest values of pore resistance were also obtained for the epoxy coating on Zn pretreated in hot air, whereas the epoxy coating on a HDG steel with a chromate coating showed the smallest change in electrochemical properties (pore resistance, coating capacitance, charge-transfer resistance) during prolonged exposure time.     

Corrosion protection of epoxy‐coated steel using different silane coupling agents

To suppress steel corrosion at elevated temperature and in humid condition, silane coupling agents N‐β‐aminoethyl aminopropyltrimethoxysilane (AAPS), γ‐glycidoxypropyltrimethoxysilane (GPS), and bis[3‐(trimethoxysilyl)‐1‐phenylpropyl]tetrasulfide (RC‐2) were introduced as primers into an epoxy/steel system. Silane coupling agents and epoxy were coated onto the steel surface using the solution casting method. The polymer degradation and steel corrosion formation after heat and humid treatment were investigated by Fourier transform infrared reflection and absorption spectroscopy (FTIR–RAS) and scanning electron microscopy (SEM). Compared to various silane treated epoxy/steel systems, the AAPS‐treated epoxy/steel (AAPS/epoxy = 6 : 4) system suppressed steel corrosion at 400°C for 10 min in air and for 5 days at 60°C in 100% relative humidity. This is due to the formation of Si O Si linkage and Fe O Si bond on steel surface, which are resistant to water diffusion and thermally stable at elevated temperature. The relationship between chemical bonding at the steel–epoxy interface and corrosion protection on the steel surface was also investigated. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 585–593, 1999     

SKP and FT-IR microscopy study of the paint corrosion de-adhesion from the surface of galvanized steel

Scanning Kelvin Probe (SKP) and FTIR microscopy were applied to study the atmospheric corrosion of galvanized steel coated by electrophoretic epoxy resin (ED) at a defect.     

Corrosion behaviour of epoxy coatings electrodeposited on galvanized steel and steel modified by Zn–Ni alloys

The electrochemical and transport properties and thermal stability of epoxy coatings electrodeposited on hot-dip galvanized steel and steel modified by Zn–Ni alloys were investigated during exposure to 3% NaCl solution. Zn–Ni alloys were electrodeposited on steel by direct and pulse current. From the time dependence of pore resistance, coating capacitance and relative permittivity of epoxy coating, diffusion coefficient of water through epoxy coating, D(H₂O) and thermal stability, it was shown that Zn–Ni sublayers significantly improve the corrosion stability of the protective system based on epoxy coating. Almost unchanged values of pore resistance were obtained over the long period of investigated time for epoxy coatings on steel modified by Zn–Ni alloys, indicating the great stability of these protective systems, due to the existence of the inner oxide phase layer and the outer layer consisting of basic salts.     

Inhibition of nickel coated mild steel corrosion by electrosynthesized polyaniline coatings

Polyaniline (PANI) coatings were electrochemically synthesized on nickel (Ni) coated mild steel (MS) and their corrosion protection properties were investigated. In this work, the Ni layer (∼1 μm thick) was electrodeposited on MS under galvanostatic condition. Thereafter, the PANI coating was deposited over the Ni layer from aqueous salicylate medium by using cyclic voltammetry. These bi-layered composite coatings were characterized by cyclic voltammetry, UV–vis absorption spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The corrosion protection properties of Ni coated MS (Ni/MS) and PANI coated Ni/MS (PANI/Ni/MS) were investigated in aqueous 3% NaCl by using open circuit potential (OCP) measurements, potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS). It was shown that the top layer of PANI exhibits a lower porosity behavior with respect to Ni coating and reduces the corrosion rate of Ni/MS almost by a factor of 3500 and increases the lifetime of Ni coating.     

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.     

Corrosion protection assessment of sacrificial coating systems as a function of exposure time in a marine environment

The present investigation assessed the corrosion protection performance of 17 different Zn and Al sacrificial coating system configurations during marine atmospheric exposure at Kure Beach, NC. The coating systems incorporated several conversion coating layers, primers and organic topcoats. Visual observations and electrochemical measurements (including electrochemical impedance spectroscopy, EIS) were made on six different occasions throughout the 20-month exposure time. Milled scribes on each of the coating specimens allowed for defect protection as well as barrier protection to be investigated. A novel corrosion analysis technique utilizing a specialized conducting agar (SCAR) cell enabled impedance measurements to be made on both intact and defect areas. Visual observations, E_oc's, and EIS as a function of atmospheric exposure time provided complementary results. Impedance results were found to be useful in determining a coating's barrier protection and scribe damage analysis accurately represented defect protection.