Influence of substrate topography on cathodic delamination of anticorrosive coatings

The cathodic delamination of a commercial magnesium silicate and titanium dioxide pigmented epoxy coating on abrasive cleaned cold rolled steel has been investigated. The rate of delamination was found to depend on interfacial transport from the artificial defect to the delamination front and thereby the substrate topography, whereas the coating thickness had little influence. The presence of a significant potential gradient between the anode and the cathode and the dependency of the delamination rate on the tortuosity of the steel surface suggests that cathodic delamination is controlled by migration of cations from the defect to the delamination front. This means that abrasive blasting, to some extent, can be applied to control and minimize the observed rate of cathodic delamination. The lifetime of the species causing disbondment suggested that sodium hydroxide or potassium hydroxide and not peroxide species or radicals are the causative agents at free corrosion potential (i.e. without impressed current).     

Cathodic delamination: Quantification of ionic transport rates along coating–steel interfaces

So-called cathodic delamination is one of the major modes of failure for organic coatings immersed in electrolyte solutions (e.g. seawater). Cathodic delamination occurs as a result of the electrochemical reactions, which takes place on a corroding steel surface. This means that reactants must continuously be transported from the bulk solution to the cathodic areas. The transport of sodium ions from a defect in the coating to the cathodic areas is generally considered the rate-determining step for cathodic delamination because the transport of oxygen and water through the coating is sufficient for the corrosion processes. In this work, a novel practical method, which allows direct estimation of the apparent diffusion coefficient of sodium ions in the ultrathin aqueous layer at the coating–steel interface, is described. The apparent diffusion coefficients estimated are of similar magnitude as previously reported values and show an acceptable repeatability. The method was used to obtain the apparent diffusion coefficients of sodium ions in the coating–steel interface for three commercial inert-pigmented epoxy coatings. The delamination rates predicted using the apparent interfacial diffusion coefficients and Fick's second law, under the assumption of a transport controlled mechanism, show qualitative agreement with the observed delamination rates in 0.5 M sodium chloride. This confirms that the rate-determining step of cathodic delamination is the transport of sodium ions along the coating–steel interface.     

Cathodic Delamination of an Epoxy/Polyamide Coating from Steel

X-ray photoelectron spectroscopy (XPS) has been used to determine the mechanism responsible for debonding of an epoxy/polyamide coating from steel during cathodic delamination in 3.5% aqueous NaCl solutions. Coating failure always occurred near the interface between the coating and the oxide. The nitrogen content of the free surface of the prepared coatings was about 10%. However, the nitrogen content of the free surface dropped to only 5% after exposure to 1 N NaOH for four weeks and that of the coating failure surface after cathodic delamination was only about 2%, implying that the failure involved degradation of the polyamide curing agent by hydroxide ions formed at the steel surface by reduction of oxygen. That conclusion was supported by results obtained from curve fitting of C(1s) and O(1s) spectra. The intensity of components in the C(1s) spectra due to C—N and C≡O bonds in amide functional groups decreased significantly after coatings were exposed to NaOH or subjected to cathodic delamination. Small amounts of organic materials characteristic of the coating were observed on the substrate failure surface, perhaps indicating that the failure was cohesive within the coating but very close to the interface or that some products from degradation of the curing agent precipitated on the substrate. Use of silane coupling agents to retard cathodic delamination was also investigated. Coupling agents were added directly to the coating or applied to the substrate as a primer before application of the coating. Significant reduction in the rate of cathodic delamination was seen only when the silane coupling agent was applied to the substrate and cured at elevated temperatures before the epoxy/polyamide coating was applied.     

Cathodic Delamination of an Epoxy/Polyamide Coating from Steel

X-ray photoelectron spectroscopy (XPS) has been used to determine the mechanism responsible for debonding of an epoxy/polyamide coating from steel during cathodic delamination in 3.5% aqueous NaCl solutions. Coating failure always occurred near the interface between the coating and the oxide. The nitrogen content of the free surface of the prepared coatings was about 10%. However, the nitrogen content of the free surface dropped to only 5% after exposure to 1 N NaOH for four weeks and that of the coating failure surface after cathodic delamination was only about 2%, implying that the failure involved degradation of the polyamide curing agent by hydroxide ions formed at the steel surface by reduction of oxygen. That conclusion was supported by results obtained from curve fitting of C(1s) and O(1s) spectra. The intensity of components in the C(1s) spectra due to C—N and C≡O bonds in amide functional groups decreased significantly after coatings were exposed to NaOH or subjected to cathodic delamination. Small amounts of organic materials characteristic of the coating were observed on the substrate failure surface, perhaps indicating that the failure was cohesive within the coating but very close to the interface or that some products from degradation of the curing agent precipitated on the substrate. Use of silane coupling agents to retard cathodic delamination was also investigated. Coupling agents were added directly to the coating or applied to the substrate as a primer before application of the coating. Significant reduction in the rate of cathodic delamination was seen only when the silane coupling agent was applied to the substrate and cured at elevated temperatures before the epoxy/polyamide coating was applied.     

Evaluation of thick organic coatings degradation in seawater using cathodic protection and thermally accelerated tests

Organic coatings are often associated with cathodic protection to fight against the corrosion of metallic structures when immersed in seawater. However, cathodic protection leads to the generation of a strong alkalinity at the metal/coating interface, which causes the degradation of the coating. It is then necessary to develop a reliable method to evaluate the compatibility between organic coatings and the application of cathodic protection.

On one hand, cathodic disbonding tests (ASTM G-8 and G-80) can be driven with an artificial defect but this defect is mainly responsible for the electrochemical response. In addition, calcareous deposit rapidly forms onto the defect zone when cathodic protection is applied which can make difficult the evaluation of coating delamination. On the other hand, immersion of defect-free specimens requires very long testing periods (several months or even years) in order to detect the coating degradation.

In this work, an attempt to accelerate the coatings degradation by imposing a high temperature and thermal cycles were made in order to decrease the test-time duration. The influence of the applied cathodic potential was also investigated. The coating degradation was evaluated by EIS, considering the defect-free zone of coatings. It was shown that the coating degradation is faster in the presence of a defect and for high temperature (45 °C). Moreover, thermal cycles allow to greatly accelerate the degradation of defect-free coatings and then to compare the compatibility of both coatings with cathodic protection.     

Cathodic delamination of seawater-immersed anticorrosive coatings: Mapping of parameters affecting the rate

Cathodic delamination is one of the major modes of failure for organic coatings immersed in seawater and refers to the weakening or loss of adhesion between the coating and the substrate. The diminished adhesion is the result of electrochemical reactions occurring at the coating–steel interface, where solid iron is oxidized to ferrous ions and oxygen is reduced to hydroxyl ions. In this work, the effects of various parameters on cathodic delamination have been investigated. The parameters are: permeability of the coating, concentration of dissolved oxygen and cations, polarization potential, type of binder, degree of curing, and pigment loading, shape and type. The results show that cathodic delamination increases with increasing concentration of cations up to the point where the concentration of dissolved oxygen becomes insufficient to maintain the corrosion rate. The rate of cathodic delamination is inversely proportional to the magnitude of polarization potential when ions can penetrate the coating, while cathodic polarization does not affect cathodic delamination when the ionic transport is restricted to the coating–steel interface. Increasing the pigment loading or partial replacement of spherical pigments with flake-shaped micaceous iron oxide or aluminium pigments reduces the rate of cathodic delamination. Finally, binders with an increasing amount of secondary hydroxyl groups in the polymer backbone reduce the rate of cathodic delamination while increasing the initial molar ratio of amide to epoxide increases cathodic delamination.     

Evaluation of the effects of surface treatments on the cathodic delamination and anticorrosion performance of an epoxy-nanocomposite on steel substrate

The St-37 type steel substrates were pretreated with Cr(VI) and Cr(III) conversion coatings where the latter was then post-treated with Co(II) and Ni(II) chemical treatments. The epoxy coatings containing 3.5 wt% nano-sized ZnO particles were applied over the chemically treated steel samples. The corrosion resistance of the samples was studied by a DC polarization technique. A scanning electron microscope (SEM) was utilized to investigate the morphology of the pretreated and post-treated samples. Electrochemical impedance spectroscopy (EIS) was utilized to investigate the corrosion resistance of the epoxy nanocomposites for different immersion times in 3.5 wt% NaCl solution. The adhesion strengths of the coatings were measured before and after 120 days of immersion in the corrosive electrolyte using a pull-off test. The cathodic delamination (CD) of the painted samples was also investigated. Results showed that conversion coatings can significantly increase the corrosion resistance and adhesion strength of the epoxy coating on the steel, and also reduce the rate of CD in comparison with an untreated sample. The adhesion strength and corrosion resistance of the epoxy coating on the Cr(III) pretreated samples were significantly greater than on the Cr(VI) sample. The increase in adhesion strength and corrosion resistance was more pronounced on the samples that were post-treated with Co(II) and Ni(II) chemical treatments. The cathodic disbonded areas of the Cr(III)–Co(II) and Cr(III)–Ni(II) post-treated samples were significantly lower than the Cr(III) and Cr(VI) pretreated samples. Results showed that Cr(III)-based conversion coatings can improve the anticorrosion performance and reduce CD compared with those with Cr(VI).     

Modes and mechanisms for the degradation of fusion-bonded epoxy-coated steel in a marine concrete environment

Fusion-bonded epoxy is used extensively to protect steel reinforcing bars from corrosion. This coating has proven to be a cost-effective material for increasing the service life of reinforced concrete structures. However, field observations have reported premature corrosion of fusion-bonded epoxy-coated reinforcing bars used in marine concrete environments, leading to severe cracking and spalling of the reinforced concretes. This study was initiated to provide a better understanding of the modes and mechanisms of the corrosion of fusion-bonded epoxy-coated steel exposed in a marine concrete environment. Grit-blasted steel panels, were coated with two commercial fusion-bonded powder epoxy coatings at two thicknesses. Half of the coated panels were scribed, the other half were non-scribed. The panels were immersed in a saturated calcium hydroxide aqueous solution containing 0.6 mole/liter sodium chloride maintained at either 35°C or 50°C. Degradation was characterized and followed by infrared thermography, wet adhesion, and microscopic and analytical chemical techniques. Unscribed panels exhibited only water-induced adhesion loss, most of which was recovered after drying. However, in addition to the water-induced adhesion loss, scribed panels exhibited two modes of failure: anodic blistering near the scribe mark and cathodic delamination around the anodic blisters. Anodic blistering was attributed to localized crevice corrosion under the coating followed by blistering via an osmotic pressure mechanism. Cathodic delamination was probably induced by the alkaline cathodic reaction products, and water-induced adhesion loss was due to the presence of a water layer at the coating/steel interface. Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 12–14, in Philadelphia, PA.     

Determination of the oxygen reduction products on ASTM A516 steel during cathodic protection

The electrochemistry of steel in aerobic and anaerobic aqueous alkaline solutions was studied with or without forced convection to investigate the cathodic processes occurring on steel exposed by defects in polymer coated steel pipe. The results are relevant to the mechanistic understanding of the effect of cathodic protection on the disbonding of fusion bonded epoxy (FBE) coatings on steel. Moderate (pH9.8) and strongly (pH14) alkaline aqueous solutions were used to simulate the water layers at the cathodically polarized steel surface on the soil-side of buried pipe. A rotating gold ring and steel disc electrode (RRDE) in alkaline aqueous electrolyte equilibrated with 1atm oxygen over solution was used to measure the rotation rate dependent current for the electroreduction of oxygen, O2, on an ASTM A516 steel disc and the resulting peroxide generation, which was determined by monitoring the oxidation current on the gold ring. An appreciable fraction of the oxygen reduction current on the steel disk gave rise to peroxide generation over a wide range of potentials, from –0.2 to –0.9V vs SCE in 1M KOH. The observation of peroxide generation is noteworthy, because oxidizing agents, such as peroxide and its decomposition products, superoxide and hydroxy radical, can degrade the polymers used for coating pipelines. As result, oxidative degradation of polymer or interfacial compounds may be a cause of the accelerated disbonding observed for protective coatings on steel pipelines under cathodic protection.     

Investigating the role of key ingredients on cathodic delamination resistance of high-build pigmented epoxy coatings

Organic coatings applied on cathodically protected metallic structures must have good resistance to cathodic delamination or disbonding (CD). Both environmental conditions and coating composition influence the CD resistance. In the present study, the effect of types of epoxy resin, curing agents and their mixing ratio on cathodic delamination rate was studied in a high-build pigmented coating. Furthermore, the influence of platey fillers on CD resistance was also studied. In order to bring out correlations, if any, between adhesion and CD resistance, pull-off adhesion strengths (both dry and wet) of these coatings were also measured. Fairly good correlation was found between residual (wet) pull-off adhesion strength and CD resistance. When tested at 60 and 90°C, all the coatings under investigation showed chalking. Among the coatings under investigation, the one based on Bisphenol F epoxy and modified cycloaliphatic amine adduct exhibited excellent CD resistance.     

Electrodeposition and characterization of Zn–Ni alloys as sublayers for epoxy coating deposition

The chemical composition and phase structure of Zn–Ni alloys obtained by electrodeposition under various conditions were investigated. The influence of the deposition solution and deposition current density on the composition, phase structure, current efficiency and corrosion properties of Zn–Ni alloys were examined. It was shown that the chemical composition and phase structure affect the anticorrosive properties of Zn–Ni alloys. A Zn–Ni alloy electrodeposited from a chloride solution at 20 mA cm^–2 exhibited the best corrosion properties, so this alloy was chosen for further examination. Epoxy coatings were formed by cathodic electrodeposition of an epoxy resin on steel and steel modified with a Zn–Ni alloy. From the time dependence of the pore resistance, coating capacitance and relative permittivity of the epoxy coating, the diffusion coefficient of water through the epoxy coating, D(H₂O), and its thermal stability, it was shown that the Zn–Ni sublayer significantly affects the electrochemical and transport properties, as well as the thermal stability of epoxy coatings. On the basis of the experimental results it can be concluded that modification of a steel surface by a Zn–Ni alloy improves the corrosion protection of epoxy coatings.     

Studies of the impedance models and water transport behaviors of cathodically polarized coating

Cathodic protection (CP) is usually combined with organic coatings to protect metallic structures exposed to seawater. However, the application of CP would enhance coating failure, such as cathodic delamination. To date, there has been few works characterizing the impedance models and water transport behaviors of cathodically polarized coating. In the present article, the analyses of impedance models and water uptake processes of chlorinated rubber coating subjected to various levels of cathodic protection were studied during coatings aging process by electrochemical impedance spectroscopy (EIS).Four distinguished electrical equivalent circuits (EEC) were used to fit the EIS plots of coatings without CP, while only two were employed for samples with CP. Since no corrosion was expected to take place at the metal/coating interface for sample which was polarized cathodically. Coating capacitance was used to investigate the sorption characteristic of water in coating since the increase of C_c was associated with water penetration into the coating. Compared with the sample without CP, those coating systems under CP have a smaller water diffusion coefficient and a further water uptake process after the saturation period.     

Surface treatment of carbon steel substrates to prevent cathodic delamination

Steel structures immersed in seawater are protected from corrosion using cathodic protection systems (sacrificial anodes or impressed current), by paint coatings or by duplex systems in which cathodic protection hinders corrosion at coating's failure sites. The main problem related to those duplex systems is how to avoid cathodic delamination due to the alkalinity generated at cathodic sites.The present paper analyses the effectiveness of red mud suspensions as pre-treatment to avoid cathodic delamination. The study was conducted using mainly electrochemical impedance spectroscopy (EIS). The evolution of the electrode potential with immersion time was also recorded because it concerns the practical aspect of current density necessary to reach the protection potential.The study shows that red mud pre-treatments are effective in avoiding coating's delamination.     

石墨烯改性涂层防护性能研究

为减少团聚,提高石墨烯在涂层中的分散性,研究采用纳米分散技术预先制备了石墨烯分散液,再将其分散至环氧树脂中获得石墨烯改性复合涂层。通过对石墨烯含量为 0、0.3%、0.6%的复合涂层进行盐水浸泡、盐雾、阴极剥离实验及电化学性能测试,证明石墨烯的加入显著增强了涂层的防护性能。石墨烯复合涂层在 3.5%盐水中浸泡 1 008 h后,涂层低频阻抗仍大于 106 Ω·cm²比未添加石墨烯的涂层提高了 3个数量级,且盐雾实验 6 000 h后涂层表面仍保持完好;含 0.6%石墨烯,的涂层耐蚀行为劣于石墨烯含量为 0.3%的涂层。     

The influence of ageing of epoxy coatings on adhesion of polyurethane topcoats and protective properties of coating systems

The dependence of adhesion and protective properties of coating systems on surface properties of epoxy intermediate coatings, aged and non-aged before an application of polyurethane topcoats, were examined. The intermediate coatings were aged 500 h in UV chamber. The surface free energy and polar groups were estimated after ageing. After applying polyurethane topcoats on aged and non-aged epoxy coatings, resistance to salt spray and thermal shocks were tested as well as internal stresses were measured before and after corrosion tests.The results showed that adhesion in coating systems with polyurethane topcoats applied on aged epoxy coatings depends strongly on the degradation degree of epoxy intermediate coatings and the value of generated internal stresses. Coatings with good adhesion retention in corrosion environments have good protective properties even when temporary blistering has occurred.     

海水压力对深海用环氧涂层防护性能的影响

采用电化学阻抗谱(EIS)技术与局部交流阻抗技术(LEIS)研究了深海环境用重防腐环氧涂层H44-61在深海模拟环境(青岛海水,常压以及6 MPa交变压力)下的腐蚀电化学行为,探讨了交变压力对深海用涂层防护性能的影响。结果表明,涂层在6 MPa交变压力下的涂层电容较常压下高且涂层电阻较低,涂层的防护性能下降,但低频阻抗膜值均在10⁷ Ω·cm²以上,说明涂层仍有较好的防护性能;LEIS的研究表明交变压力下人造缺陷区域的阻抗值较小,缺陷周围涂层的剥离面积较大,说明压力交变能加快电解质溶液向涂层金属界面扩散,加速涂层下金属的腐蚀过程,降低涂层的防护性能。     

Novel isocyanate-free self-curable cathodically depositable epoxy coatings: Influence of epoxy groups on coating properties

Novel self-curable cathodically depositable coatings were developed from glycidyl functional epoxy ester-acrylic graft co-polymer (EEAG) without using any external crosslinking agents. The EEAG-amine adducts (EEAGAs) were prepared by reacting EEAG with varying amount of diethanolamine (DEoA) which are neutralized with acid and dispersed in deionised water to give stable dispersion for cathodic electrodeposition (CED) coatings. The dispersions were cathodically electrodeposited on phosphated steel panels and thermally cured to give uniform coating. The coatings were evaluated for different mechanical, chemical and corrosion resistance properties. The coatings were evaluated for their thermal properties using thermo gravimetric analysis (TGA). The final properties of the coatings were found to be affected by the amount of amine reacted with epoxy. The coating films showed good overall performance properties for their use in coating industry.     

海洋环境下抗阴极剥离重防腐涂层的研究

海洋环境中浸泡在海水中的涂层由于阴极保护而产生的剥离是涂层失效的主要原因之一。针对这种失效现象,讨论了影响涂层抗阴极剥离的主要因素并研制了适应于阴极保护的海洋浸泡涂层。涂料包含不同环氧值的环氧树脂基料、烷基酚醛胺固化剂以及相适应的颜填料。通过调整涂料的颜基比,得到抗阴极剥离性能优异的涂层,涂层能够满足海洋环境中涂层与阴极保护相结合使用的需要。     

Study of degradation of organic coatings in seawater by using EIS and AFM methods

In this article, electrochemical behaviors and their topography observation for four organic coatings used in seawater, by using both electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM) methods to study environment behaviors of different coatings as well as the effects of their film formation, pigments, and fillers on anticorrosion behaviors, were measured. The results show that polyurethane, epoxy, and chlorinated rubber coatings all present one capacitive loop in their tested EIS which contains phenomenally only one time constant, whereas alkyd coating presents two capacitive semicircle arcs. With two capacitive loops, the capacitive semicircle in the high frequency range represents barrier layer property, but the semicircle in the low frequency range represents corrosion reaction of metals under the film. Polyurethane coating used in seawater has well anticorrosion property in seawater immersion test. The appearance features of different layers are visible different between different layers of tested coatings at their surface topography. The property of polyurethane paint film coated on metal is better than other layers, and film of alkyd coating has many pits at its surface by observing the layer's images. AFM photos imaged have also been used to further detail surface topography for four organic coatings, and to approve effects of topography of these coatings on its electrochemical behaviors, from two views of both height and phase modes. It is beneficial to explain deeply the environment behaviors and degradation mechanism of organic coatings. To further study failure of these organic coatings and dynamic processes of corrosion of metal under the film, two equivalent circuit models, according to these tested EIS, have been suggested to explain the corrosive kinetics of these four coatings. To polyurethane, epoxy, and chlorinated rubber coatings used in seawater which have good protection effects for substrate metal, the diffusion process for water, from their layer's surface to interface of film/metal, is mainly controlled factor for degradation. However, the electrochemical reaction process has may become a control procedure for corrosion of alkyd coated metal. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Corrosion and mechanical behavior of fusion bonded epoxy (FBE) in aqueous media

In recent years there have been many reported cases of corrosion failure in cement concrete pipelines. In the majority of cases, the failures have been attributed to rebar corrosion which is caused by the permeability of chloride from low resistivity soil and subsequent attack on a passive layer on an iron bar in the structure. As a possible alternative to cementitious materials, some organic coatings based on olefin, vinyl or epoxy-based polymers have been considered. However, due to a paucity of data on the behavior of these coatings in aqueous media— particularly product water—the possibility of their application in water transmission systems in the Kingdom has not been fully exploited. This paper deals with the studies carried out on the corrosion and mechanical behavior of fusion bonded epoxy (FBE) coating on steel in aqueous media which include product water, distilled water and saline water. The mechanical testings on coating include adhesion, bending and cathodic disbondment testings. The corrosion studies include immersion testing under static and dynamic conditions, autoclave tests and accelerated (salt-fog) tests. The analysis of results indicates chemical inertness of FBE coating in either of the aforementioned water used during testing, good adhesion and no damage to the coating during bending. Cathodic disbondment tests indicate that FBE coating sustains under cathodic protection (CP) conditions. In general, the results of mechanical and corrosion tests indicate that FBE is a promising material for internal coating on steel in water transmission systems.