An in situ phosphatizing coating on 2024 T3 aluminum coupons

Toxic chemicals, such as chrome, are commonly used in the application of conversion coatings to various metal surfaces. In situ phosphatizing coatings (ISPCs) are an innovative approach for eliminating the requirement of a conversion coating which ends the need of toxic materials used in a multi-step coating practice. An ISPC is formulated by predispersing an in situ phosphatizing reagent (ISPR) into a paint system. In this study, an ISPR, an arylphosphonic acid, is used in a polyester–melamine paint to react in situ with the metal surface and to provide the acidic catalyst needed, while thermally curing the paint. A second polyester–melamine paint system is used as the control that uses the standard catalyst para-toluenesulfonic acid (p-TSA) to catalyze the cross-linking reaction in the paint. These two paint systems are applied to bare 2024 T3 Al panels and to chromated 2024 T3 Al panels. The coated panels are treated in a corroding media for 2,400 h and monitored periodically using electrochemical impedance spectroscopy (EIS). Results of EIS data and the corresponding electrical equivalent circuit (EEC) show that the ISPC applied to both the chromated and untreated Al panels provide superior corrosion protection. At low frequency (0.01 Hz), the panels coated with the ISPC show 10 000 times more resistance than both the chromated and bare Al panels coated with the control polyester–melamine paint. The corresponding EEC shows that the panels with the control paint applied have double the amount of electrical components (resistors and constant phase elements). A physical interpretation is suggested for the EEC. The paint adhesion to the Al surface and the corrosion behavior are tested by salt-water immersion and salt fog. After the panels are exposed to the salt solution, a pressure tape is applied to test the adhesion. The results show that the ISPC applied on the chromated Al adheres the best. The simultaneous reaction of the ISPR catalyzing the curing of the paint and forming the metal–phosphate layer is the reason for the superior paint performance.     

In-situ phosphatizing coatings III: A water-reducible alkyd baking enamel

We report the successful formulation of stable and compatible in-situ phosphatizing coatings (ISPCs) for a waterborne alkyd-amino baking enamel applied on bare cold-rolled steel (CRS), iron phosphated Bonderite 1000 (BD), and iron phosphated plus Parcolene 60 chromated (BD+P60) coupons. The enhanced coating adhesion of water-based ISPCs is confirmed by the cathodic delamination measurements. After 100 hr of salt spray (fog) test, the corrosion resistance performance (measured by the corrosion disbondment across the “X” scribe, d in mm) of the water-based ISPC on CRS panel (d=4.0−7.0 mm) outperformed that of the control alkyd paint on B-1000 (d=26 mm) and also on BD+P60 (d=14 mm) coupons. The superior coating performance of water-based ISPCs is believed to result from the in-situ metal surface phosphatization as detected by the reflectance FTIR technique. Department of Chemistry and Biochemistry, DeKalb, IL 60115-2862.     

Optimization of phosphate coating properties on steel sheet for superior paint performance

The adhesion of electrodeposition (ED) paint on steel sheets for automobiles is highly influenced by the properties of the zinc phosphate coating which is used to improve its corrosion resistance. In the present study, a steel surface was pretreated with two types of zinc phosphate formulations followed by ED painting. The surface morphology, crystal plane, and porosity properties of phosphate coating on steel samples were studied by scanning electron microscope, X-ray diffraction, and electron probe microanalyzer, respectively. The corrosion resistance of painted samples was evaluated by an accelerated corrosion test as well as by electrochemical techniques like cathodic disbonding and AC?CDC?CAC tests. The phosphate coating enriched with a phosphophyllite structure showed small globular crystals with less porosity, whereas a hopeite structure showed coarse crystals with high porosity and comparatively thicker coating. The maximum corrosion resistance was observed in the painted sample, where the phosphate coating comprised a phosphophyllite structure. On the other hand, the painted samples phosphated with a predominantly hopeite structure showed inferior corrosion resistance performance. The unphosphated sample showed severe degradation in paint adhesion and corrosion resistance, which substantiates the importance of phosphate pretreatment.     

Influence of process parameters of oxygen cold plasma treatment on wettability ageing time of 2024 aluminium alloy

2024 aluminium alloy is widely used in aeronautical applications because it provides a high mechanical strength and low weight. Aeronautical critical environmental conditions cause corrosion on the surface in part, due to the Cu present in Al2024. To solve problems associated with corrosion, the aluminium alloy surface is commonly covered by paint; it is important to guarantee the adhesion of the paint coating to the aluminium alloy and the stability of the coating in the course of time. To achieve this aim the surface must be prepared before applying a protective coating. Cold plasma treatment represents an efficient, non-polluting and economical alternative to clean and activate aluminium surfaces. In particular, it has been demonstrated that an oxygen cold plasma treatment improves wettability and adhesion of the Al2024 surface.The aim of this work has been to value the wettability ageing time of Al2024 surfaces treated with oxygen cold plasma. The evaluation of the ageing time represents a fundamental step to plan the insertion of the plasma process inside an industrial process. In fact, once the ageing time has been determined, it is possible to establish how much time has to pass between the surface activation of a body, obtained by cold plasma treatment, and the following paint process.     

Epoxysilane as adhesion promoter in duplex system

Coatings are one of the most used protection methods for metals. Metallic coatings, such as zinc and its alloys, are used to protect steel in mild corrosive environments. In aggressive environments, on the other hand, organic coatings must be employed in the so-called duplex systems. However, the galvanized steel/organic coating adhesion is a problem and many attempts had been done to solve it with the incorporation of a chromate-based or phosphate-based interlayer. Nowadays, the use of these compounds is questioned due to their environmental impact and new adhesion promoters, like silanes, are being investigated. The aim of this paper was to study the adhesion and the anticorrosive behavior of a duplex system with a layer of glycidoxypropyltrimethoxysilane (γ-GPS) between the zinc and the coating. Polarization tests and corrosion potential measurements were done on the γ-GPS/galvanized steel to select the better anticorrosive pretreatment conditions for the application of an organic traditional paint. Dried and wet adhesion of the coating to the pretreated substrate was studied by the standard tape test. Salt spray test and electrochemical noise technique were employed to study the corrosion behavior of the duplex systems. Results showed that the films of γ-GPS formed on galvanized steel diminished the corrosion current of the metal, but they do not protect the substrate by a barrier effect. The incorporation of the pretreatment in the duplex system increased the adhesion of the paint, especially when the pretreated substrate was cured 1?h at 200?°C.     

Silane and epoxy coatings: A bilayer system to protect AA2024 alloy

This work has proved that a good combination of a simple and fast metal pre-treatment, followed by the deposition of a thin layer of an organic–inorganic silane coating and further layer of epoxy coatings, are able to protect the aluminium alloy AA2024-T3 against corrosion in high concentrations of NaCl solution. The alloy AA2024 is one of the most employed aluminium alloy in structural applications due to its good mechanical properties. However, AA2024 alloy series commonly presents galvanic corrosion due to the rich content of copper element. The influence of different surface pre-treatments, the presence of a silane layer as pre-coating treatment and the influence of phosphonic acids combined with the silane layer on the corrosion protection and adhesion to the aluminium alloy have been examined using accelerated corrosion tests. High roughness and the presence of a pre-coating film between the metal surface and the organic coating were essential for a good protection and resistance to blistering appearance in the surface of AA2024-T3.     

Effect of composite coating with poly-dopamine/PCL on the corrosion resistance of magnesium

We report the use of poly(ε-caprolactone) (PCL) and poly-dopamine (PD) as a protective coating that inhibits corrosion of the underlying magnesium metal. The PD coating layer also improved the adhesion of the PCL layer, which has been found to have a significant effect on corrosion behavior. In this study, electrochemical methods were employed to investigate the corrosion behavior of Mg after applying PCL composite coatings. Potentiodynamic polarization measurements determined that the PCL coating pretreated with PD effectively inhibited metal corrosion. In addition, the coating layer with improved adhesion has shown the possibility of inhibiting metal corrosion.     

Ultra thin layers as new concepts for corrosion inhibition and adhesion promotion

The adsorption and self-organization process as well as the surface reactions of several bifunctional adhesion promoters on different oxide surfaces have been investigated. The aim was to improve the adhesion between metal oxides and different organic coatings. We developed a large number of bifunctional compounds, which are able to adsorb spontaneously on different pre-treated metal (oxide) surfaces. The second group can be designed for grafting different otherwise incompatible layers.

Therefore, a special two-step procedure has been developed: (1) adsorption of the designed bifunctional molecules on the substrate and (2) surface reaction of the terminal reactive group with a polymeric top coating or with further monomers resulting in a strongly bond composite.

For this purpose substances were chosen having a surface reactive group, an aliphatic spacer and a reactive group for a suitable top layer. Phosphonic acids forming strong bonds with several metal surfaces were chosen as surface-active groups on metal oxide substrates.

The termination of these compounds with further reactive groups opened a wide range of possible applications. Functionalities like amino or carboxylic groups allowed reactions with commercial lacquers (e.g., polyurethane) for improving adhesion promotion and corrosion inhibition of the metal substrates. By using polymerizable groups like thiophene and pyrrole an in situ surface polymerization with further monomers is possible directly on the substrate.

The adsorbed films of bifunctional phosphonic acids on metal (oxide) substrates were characterized by contact angle measurements, X-ray photoelectron spectroscopy (XPS), the surface polymerized films were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), cyclovoltammetry (CV), and electrochemical impedance spectroscopy (EIS). The results showed that monolayers were formed, which were correctly oriented on the surfaces: the phosphonic acid group was attached to the substrate whereas the terminal group was free standing for further reactions. Surface polymerization with additional monomer was possible either chemically or electrochemically resulting in smooth polymer layers of adjustable thickness. The conducting polymers were found to be p-conductive with a doping level of about 30%. Conductivity measurements revealed a conductivity of about 0.13 S/cm for the best films.

Based on this principle two possible applications are given: firstly, a corrosion protecting system for steel, and secondly, a model release system for protecting steel after damage of the coating.     

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.     

Comparative EIS study of pretreatment performance in coated metals

Various coated metal samples with different pretreatments were investigated by electrochemical impedance spectroscopy (EIS). Variables were the substrate (cold-rolled steel and hot-dipped galvanized steel), phosphate system (iron and zinc phosphate), post rinse (chromate and silane/zirconium rinse) and paint systems. The corrosion performance was determined on the basis of coating degradation, water uptake and interface delamination of the tested samples. The zinc phosphate performed better than iron phosphate on CRS. The silane/Zr rinse did not perform well in the CRS/iron-phosphate system. However, it showed a better performance than the chromate when used as a post rinse of zinc phosphate. Salt spray test (SST) and adhesion test results of the same samples are also reported in this paper and compared to the EIS data. The correlation among three test methods was poor.     

The influence of aluminium surface pretreatment on the corrosion stability and adhesion of powder polyester coating

One of the most important factors in corrosion prevention by protective coatings is the coating adhesion loss under environmental influence. Thus, adhesion strength is often used when characterizing protective properties of organic coatings on a metal substrate. In order to improve the adhesion of organic coating the metal substrate is often pretreated in some way. In this work, the adhesion of polyester coatings on differently pretreated aluminium surface (by anodizing, with and without sealing, by phosphating and by silane film deposition) was examined. The dry and wet adhesion of polyester coatings were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test. It was shown that under dry test conditions all polyester coatings showed very good adhesion, but that aluminium surface pretreated by silane film showed superior adhesion. The overall increase of wet adhesion for polyester coating on aluminium pretreated by silane film was maintained throughout the whole investigated time period. The different trends in the change of adhesion of polyester coatings were observed for different aluminium pretreatments during exposure to the corrosive agent (3% NaCl solution). The highest adhesion reduction was obtained for polyester coating on aluminium pretreated with phosphate coating. The corrosion stability of polyester coated aluminium was investigated by electrochemical impedance spectroscopy in 3% NaCl solution. The results confirmed good protective properties of polyester coating on aluminium pretreated with silane film, i.e. greater values of pore resistance and smaller values of coating capacitance were obtained in respect to other protective systems, whereas charge-transfer resistance and double-layer capacitance were not measurable during 2 months of exposure to a corrosive agent.     

Deposition of zinc–zinc phosphate composite coatings on steel by cathodic electrochemical treatment

The present work aims at the development of an energy-efficient and eco-friendly approach for the deposition of zinc phosphate coatings on steel. The study describes the possibility of preparing zinc–zinc phosphate composite coatings by cathodic electrochemical treatment using dilute phosphoric acid as an electrolyte and zinc as an anode. The methodology enables the preparation of coatings with different proportions of zinc and zinc phosphate by suitably varying the applied current density, pH, and treatment time. Adhesion of the coating on mild steel and adhesion of paint film on the phosphate coating were found to be good. The surface morphology of the coatings exhibited platelet-type features and small white crystals (agglomerated at some places) which represented zinc and zinc phosphate, respectively. An increase in current density (from 20 to 50 mA/cm²) increased the size of the zinc crystals, and coatings prepared at 40 and 50 mA/cm² resembled that of electrodeposited zinc. Since the proportions of zinc and zinc phosphate could be varied with applied current density, pH, and treatment time, it would be possible to use this methodology to prepare coatings that would offer different degrees of corrosion protection.     

Study of the interfacial chemistry of poly(vinyl chloride) paint on steel exposed to the ultraviolet- water condensation test

The effect of exposure to the UVCON (humidity and UV radiation) accelerated corrosion test of a poly(vinyl chloride) paint applied over cleaned, abraded, and pre-rusted mild steel substrates has been investigated from the interfacial chemistry and adhesion standpoints. The X-ray photoelectron spectroscopy (XPS) data reveal significant differences in the locus and mode of adhesion loss, which depend on the time of exposure and the initial presence of a rust layer on the mild steel. The adhesion values obtained with the pre-rusted mild steel were greater than those obtained with the cleaned and abraded mild steel before exposure. After 15 days of exposure, it was observed that changes in the interfacial surface composition of the PVC/cleaned and abraded mild steel systems might have important positive effects on the adhesion strength values. The XPS data suggest the oxidation of both interfacial surfaces, incorporating polar C-O and C=O species, which would be expected to enhance the adhesion of paint to metal. In contrast, the PVC paint/pre-rusted mild steel system showed no change in adhesion values or interfacial surface composition over the entire period of exposure.     

Über einige wesentliche Unterschiede zwischen der Aufbringung von konventionellen Anstrichen und der Elektrotauchlackierung,insbesondere hinsichtlich der Vorbehandlung der Werkstücke

A Few Major Differences Between Coating with Conventional Paints and Electrical Dip Coating Especially with Respect to Pretreatment of the Article to be Coated The basic difference between conventional coatings and paints for electrical dip coating is that in the latter case a high voltage is applied during coating of the metallic surface. Consequently, a low and uniform resistance of the metal is required, which can be achieved by thorough cleaning for removal of the surface impurities and by coating of the surface with a thin layer of fine-sized phosphate or chromate. Paints for electrical dip coating are more susceptible to defects of the surface and various qualities of steel than the conventional paints. The problems in the single layer white painting by the electrical dip process and in the pigmentation process are discussed. The advantages of electrical dip coating over conventional coatings, which have led to a greater use of the former technique, are discussed. Best use of this process can be made by optimizing the steps, such as surface cleaning, washings before and after phosphatizing, passivation, drying, and by a suitable choice of the paint.     

Study on an elaborated method to improve corrosion resistance of zinc phosphate coating

Coating with dense and fine particles containing fewer cracks and lower porosity shows more improved protective properties due to limiting pathways between the environment and base metal. The main aim of present research is to introduce an innovative method that is called rephosphating to achieve this morphology. The outstanding point of the present investigation is to highlight the significant effect of surface pretreatment by secondary grinding of phosphated surface and then rephosphating of this surface to obtain a coating with appropriate properties. The SEM observations showed that this method has an obvious influence on the formation of a very uniform zinc phosphate coating on the plain carbon steel compared with the traditional method of phosphating. Furthermore, the protecting properties of phosphated and rephosphated samples were described and compared using the neutral salt spray and the electrochemical polarization tests. The results showed that rephosphating method had a beneficial effect on improving the corrosion resistance. As well, improved paint adhesion of rephosphated sample was observed compared with that of the phosphated sample. Finally, it was concluded that when rephosphating method can be used to repair damaged phosphated areas, the coating with more compact morphology and improved properties can be achieved.     

Adhesion promotion and corrosion prevention using thin anisotropic coatings

Using sol-gel technology, thin organic/ceramic (ceramer) coatings have been applied to metal surfaces to enhance such surface properties as adhesion promotion and corrosion prevention. Isotropic coatings have been found to be effective in certain applications such as corrosion prevention, but the formation of anisotropic (functionally gradient) coatings permits greater flexibility over the resulting properties. Isotropic coatings derived from tetraethoxysilane, for example, have been found to effectively inhibit corrosion while being only 100-1000 A thick. These coatings do not, however, promote adhesion. Thin coatings made from traditional silane adhesion promoters alone are unable to prevent corrosion of metallic substrates. Using monomers with appropriate reactivities permits the single-step synthesis of anisotropic coatings that can both promote adhesion and prevent corrosion. These types of anisotropic coatings allow the physical and chemical properties of a coating to be varied as a function of the distance from the substrate and confer properties to the substrate that would not be possible from a single isotropic coating. The principle behind the construction of these anisotropic coatings is general enough that it can be used in many applications where microengineering of surface structures is important.     

Fabrication of hydrophobic surface with hierarchical structure on Mg alloy and its corrosion resistance

A hydrotalcite/hydromagnesite conversion coating with hierarchical structure has been fabricated on a Mg alloy substrate by in situ hydrothermal crystallization method. A MgO layer existing between the hydrotalcite/hydromagnesite film and the substrate was formed prior to the hydrotalcite/hydromagnesite film during the crystallization process. After surface treatment with silane coupling agent, the surface of conversion coating changes from hydrophilic to hydrophobic. Scanning electron microscopy (SEM) revealed that the silylated conversion coating with hierarchical structure maintains the original rough surface of which was composed of numerous micro-scale flakes and beautiful flower-like protrusions. Polarization measurements have shown that the hydrophobic conversion coating exhibited a low corrosion current density value of 0.432 μA/cm², which means that the hydrophobic conversion coating can effectively protect Mg alloy from corrosion. Electrochemical impedance spectroscopy (EIS) showed that the impedance of the hydrophobic conversion coating was 9000 Ω. It means that the coating served as a passive layer with high charge transfer resistance.     

Controlled surface modification of Ti6Al4V using biomimetic mineralization via thermo-chemical route improves bioactivity

Ti alloy (Ti6Al4V) sheets were bio-activated by a two-step thermo-chemical treatment followed by biomimetic mineralization. The samples were then characterized by standard techniques and evaluated of their mechanical properties, electrochemical corrosion potential and biological performance. The intermediate layer corresponding to thermo-chemical treatment displayed anatase TiO₂ peaks and the final bio-mineralization resulted in a globular hydroxyapatite (HAP) layer. Thermo-chemical treatment yielded a two-fold increase (98.79% increment) in microhardness value, whereas, the biomimetically activated samples showed a very small decrease in the same owing to their ceramic behavior. The surface hydrophobicity of the bio-activated surface was found reduced significantly, might assist to facilitate improved cell adhesion. Electrochemical corrosion measurements exhibited an increase in corrosion potential and decrease in current density of the samples, suggested increased corrosion resistant. The surface coating on the Ti6Al4V sheet also demonstrated enhanced cytocompatibility as no toxic effect of the samples could be perceived to human keratinocyte cell line (HaCaT). Similarly, the samples showed higher hemocompatibility and enhanced bactericidal activity. Our study concluded that the surface coating of Ti6Al4V sheets significantly improved corrosion resistance and bioactivity of the substrates, which can be applied for various biomedical applications.     

Corrosion stability of polyester coatings on steel pretreated with different iron–phosphate coatings

The influence of steel surface pretreatment with different types of iron–phosphate coatings on the corrosion stability and adhesion characteristics of polyester coatings on steel was investigated. The phosphate coating was chemically deposited either from the simple novel plating bath, or with the addition of NaNO₂, as an accelerator in the plating bath. The morphology of phosphate coatings was investigated using atomic force microscopy (AFM). The corrosion stability of polyester coatings on steel pretreated by iron–phosphate coatings was investigated by electrochemical impedance spectroscopy (EIS) in 3% NaCl solution, while “dry” and “wet” adhesion were measured by a direct pull-off standardized procedure. It was shown that greater values of pore resistance, R_p, and smaller values of coating capacitance of polyester coating, C_c, on steel pretreated with iron–phosphate coating were obtained, as compared to polyester coating on steel phosphated with accelerator, and on the bare steel. The surface roughness of phosphate coating deposited on steel from the bath without accelerator is favorable in forming stronger bonds with polyester coating. Namely, the dry and wet adhesion measurements are in accordance with EIS measurements in 3% NaCl solution, i.e. lower adhesion values were obtained for polyester coating on steel phosphated with accelerator and on the bare steel, while the iron–phosphate pretreatment from the novel bath enhanced the adhesion of polyester coating on steel.     

Investigation of fluoroacid based pretreatments on aluminium

Conclusions  Effective alternatives to chromium phosphate are available for the pre-treatment of extruded aluminium. Blends of fluoroacids and polymers may be selected to provide equivalent corrosion protection and facilitate the adhesion of paint. Polymer choice is critical in assuring good performance. Electrochemical evaluation of unpainted, fluoroacid/polymer treated aluminium provided some clues concerning the effectiveness as corrosion inhibitors. Polarisation curves showed a pseudo-passive region in the anodic curve that was not observed for untreated aluminium. Polarisation resistance was sensitive to choice of fluoroacid and polymer, with the combination of fluorozirconic acid and 7:3 AA/AM polymer providing the lowest corrosion rate. EIS demonstrated that the addition of polymer improved both the stability and the corrosion resistance provided by fluoroacid treatments. This behaviour was retained for long immersion times (up to 6 days). Microscopic examination of the treated aluminium surface, showed a heterogeneous coating when treated without polymer. Fluorotitanic acid was especially non-uniform, with titanium-enriched 0.5 μm particules on the metal surface. Incorporation of polymer into the pre-treatment enhanced surface uniformity. FTIR spectroscopy (and to a limited extent, XPS) gave evidence of poly(acrylic acid) ionisation on fluoroacid/polymer treated aluminium. Acid ionisation was not observed on evaporation of either treatment or polymer solution. Other workers have proposed that PAA complexation with zirconium enhances pre-treatment performance.