Numerical analysis of galvanic corrosion of Zn/Fe interface beneath a thin electrolyte

A numerical analysis of galvanic corrosion of a Zn/Fe interface beneath a thin layer electrolyte is presented. Specifically, a circular defect, where the zinc coating has been removed, is considered. It is assumed that both oxygen reduction and iron oxidation can occur on the Fe surface, while only zinc oxidation occurs on the Zn surface. The importance of electrolyte thickness and conductivity and defect radius is considered. It is assumed that the iron and zinc oxidation rates are described by a Tafel relationship. If the kinetic parameters of the oxidation reactions are known, the cathodic protection of Fe is a function of a Wagner number, the ratio of the electrolyte thickness to the defect radius, and the ratio of the radius of the defect to the outer radius of the zinc layer.     

Localized EIS characterization of corrosion of steel at coating defect under cathodic protection

Localized electrochemical impedance spectroscopy (LEIS) technique was used to investigate localized corrosion of steel at defect of coating and, furthermore, to determine the effects of cathodic protection (CP) on local electrochemical environment and the resultant corrosion reaction at the base of coating defect. The results demonstrated that corrosion of steel is dependent on CP potential and the defect geometry. For coated steel containing defect under appropriate CP potentials, cathodic reaction is dominated by reduction of oxygen. Mass-transfer of oxygen through solution layer and the defect with a narrow, deep geometry (the depth/width ratio is about 5.5) is the rate-limiting step for the corrosion process of steel. Even at a very negative potential, e.g., −1200 mV (SCE), the measured impedance spectroscopy is still associated with the diffusion-controlled charge-transfer reaction of steel at the base of defect. It is attributed to the fact that the applied CP is partially shielded by the defect with a narrow, deep geometry. With the negative increase of cathodic potential, charge-transfer resistance and the local impedance of electrode increase. It is attributed to the elevation of local alkalinity at the base of defect when the coated steel is under CP. This conclusion is subject to the condition that a significant cathodic disbonding of coating has not occurred. Furthermore, with the increase of test time, the charge-transfer resistance increases, which is attributed to the enhancement of the alkaline environment at the base of defect under CP with time.     

Synthesis and characterisation of surface gradient thin conversion films on zinc coated steel

Surface gradient layers on hot-dip galvanised steel were synthesised in order to determine the barrier properties and corrosion resistance of thin amorphous conversion coatings as a function of layer thickness and processing time. For this purpose, a dip coating procedure was established that yields well-defined gradient layers. As a model system for conversion film formation on zinc coated steel, a zirconium based bath chemistry was used. The synthesised zirconium oxyhydroxide gradient films were characterised by localised electrochemical techniques, such as Scanning Kelvin Probe (SKP) and electrochemical impedance spectroscopy using an electrochemical capillary cell. Microscopic infrared reflection absorption spectroscopy (μ-FT-IRRAS) measurements and small-spot X-ray photo electron spectroscopy (XPS) were used as complementary surface analytical techniques. The applied analysing techniques provide a spatial resolution of 100-1600 μm. Thereby, a complete variation of thin film properties, such as thickness, barrier properties, corrosion resistance and chemical composition can be measured as function of the time of film growth on a sample with a length of a few centimetres. This approach allows a precise and accurate determination of structure-to-property relationships of thin conversion films. Moreover, it could be shown that a surface gradient film analysis significantly rationalises experimental time and increases the reliability of the experimental results.     

Degradation of organic coatings in a corrosive environment: a study by scanning Kelvin probe and scanning acoustic microscope

Degradation of pigmented epoxy coatings on a steel substrate during exposure to aerated 3% NaCl solution has been investigated using the scanning Kelvin probe (SKP) and scanning acoustic microscopy (SAM). A method for processing SKP potential maps has been used to visualize the distribution of anodic and cathodic current. It was observed that rust deposited at the defects, or later beneath the coating, acted as a cathodic reactant, leading to development of new anodes beneath the coating. Exposure of rusty samples to moist air (rather than salt solution) during SKP mapping is shown to lead to more noble potentials at the defect that influence the electrochemistry of under-film processes.     

Inhibition of corrosion-driven organic coating disbondment on galvanised steel by smart release group II and Zn(II)-exchanged bentonite pigments

Bentonite pigments exchanged with either zinc or group II cations are characterised as inhibitors of corrosion-driven cathodic disbondment of model polyvinylbutyral (PVB) coatings adherent to the intact zinc surface of hot dip galvanised steel. An in situ scanning Kelvin probe (SKP) technique is used to quantify rates of coating delamination as a function of pigment volume fraction (?_pt) and draw up a ranking order of inhibitor efficiency. Group II cation-exchanged bentonites show a moderate degree of inhibition, where rates of coating disbondment are reduced by up to 60-70% compared to the unpigmented case. In contrast, bentonite pigments containing exchangeable Zn²⁺ ions are markedly more effective, and no delamination is observed over periods of up to 24 h when ?_pt ≥ 0.1. The efficiency of in-coating Zn²⁺ is attributed to the ability to block underfilm cathodic oxygen reduction by reinforcing a pre-existing zinc (hydr)oxide layer.     

On the mechanism of the anodic protection of aluminium alloy AA5182 by emeraldine base coatings: Evidences of a galvanic coupling

Aluminium AA5182 coupons covered by a polyaniline film in the emeraldine base (EB) form showed increasing corrosion potential and decreasing corrosion current as a function of the thickness of the polymer layer. The cathodic reaction was proved not limited by diffusion of species inside the electrolyte solution and oxygen had no effect on the electrochemical behaviour of the coated samples. An EB coating on indium tin oxide conducting layer appeared slightly electroactive in neutral media. The IR spectra of aluminium coated samples, before and after heating in argon atmosphere, confirmed a redox reaction between the polymer film and the metal. This galvanic coupling can explain the good protective behaviour of emeraldine base against corrosion of aluminium.     

Critical aspects in the electrochemical study of unstable coated metallic substrates

Electrochemical methods are largely employed for the study of coated metallic substrates. Electrochemical techniques are very useful for investigation of coating performances also in the field of corrosion protection of aluminum alloys where replacement of chromate based coatings is an important issue. Application of sol–gel technique has recently made available thin inorganic protective coatings that can be considered as an alternative to chromate systems. Therefore, it is important to evaluate the electrochemical behaviour of defect free inorganic films which can be considered an inert material. As a consequence, investigation of this inert material might generate critical aspects related to rapid fluctuations of its open circuit potential and current density during open circuit potential and potentiodynamic polarization measurements. Moreover, the unstable nature of the sol–gel very thin films is a critical issue also in electrochemical impedance measurements carried out under potential control because variation of the open circuit potential of the system during data acquisition might lead to sample damage. Impedance measurements under current control is an alternative method for data acquisition. This technique is considered in the present paper for the investigation of AA2024 substrate coated with a thin sol–gel layer and for chromate conversion coated alloy.     

In situ electrochemical Scanning Kelvin Probe Blister-Test studies of the de-adhesion kinetics at polymer/zinc oxide/zinc interfaces

Fundamental investigations of the polymer/zinc oxide/zinc interface corrosion stability were performed in situ by means of the electrochemical Height Regulated Scanning Kelvin Probe Blister-Test (HR-SKP-BT) under controlled atmospheric conditions. A hole under an adhesive layer film served as electrolyte reservoir to initiate cathodic de-adhesion processes. Then a combinatorial approach was undertaken to simultaneously study the influence of electrolyte pressure at constant defect polarisation and of relative atmospheric humidity on the de-adhesion rate. The time resolved blister growth and the propagation of the three phase boundary polymer/oxide covered zinc/interfacial electrolyte layer could be detected. It could be proven that the oxygen reduction induced electrochemical damage of the interface precedes the subsequent mechanical de-adhesion process. By variation of the relative atmospheric humidity the water concentration within the bulk adhesive and its interphase adjacent to the metal substrate could be adjusted. These processes were further analysed by peel-tests and in situ Attenuated-Total-Reflection Infrared Spectroscopy (ATR-IR) studies of water diffusion. A decrease of the interphasial water concentration led to a deceleration of the de-adhesion kinetics for constant defect conditions and to smaller interfacial ion transport rates. This could be assigned to an inhibition of the electron transfer reactions at the front of de-adhesion and an increased adhesion force between polymer film and oxide covered metal preventing the formation of an extended interfacial electrolyte layer.     

The formation and corrosion behavior of a zirconium-based conversion coating on the aluminum alloy AA6061

To understand the mechanism of the coating formation, the formation process of a zirconium-based conversion coating on aluminum alloy 6061 has been studied by means of AFM in PeakForce Kelvin Probe Force Microscope (PF-KPFM) mode which could provide direct evidence for the existence of driving force for the film formation. In addition, various techniques including SEM, XPS, EIS, salt spray test, and scanning electrochemical microscope were used to investigate the surface state and corrosion behavior of the conversion film. The direct driving force for the coating formation is the Volta potential difference between the intermetallic particles and matrix. That difference produces an ocean of micro electrochemical cells in which the intermetallic particles act as cathodic sites for the film deposition. However, the precipitation of the layer is a self-limited process in which the driving force gradually decreases as the conversion layer covers the surface of the aluminum alloys. The anti-corrosion performance of the film is unfavorable compared to the conventional chromate conversion coatings due to the pitting corrosion that occurs when exposed to harsh environment containing chloride. Furthermore, the zirconium-based conversion coating possesses no self-healing ability leading to the continuous degradation of the film until it completely lose efficacy.     

Scanning Kelvin Probe Blister test measurements of adhesive delamination – Bridging the gap between experiment and theory

The delamination of an epoxy-adhesive film from a zinc coated steel substrate was studied by means of the electrochemical Height Regulated Scanning Kelvin Probe Blister Test (HR-SKP-BT¹) under controlled atmospheric conditions, applied pressure and interfacial electrode potential. The experimental studies focused on the analysis of the critical environmental water activity that leads to a corrosive delamination process under applied mechanical load and the analysis of the corrosion and delamination mechanisms at the front of delamination. The influence of applied pressure and relative humidity on the increase in the maximum blister height and the delamination rate was measured under constant polarization of the defect. 90° peel-tests were performed in order to correlate the water activity with the resulting peel force. The corrosion products that formed across the delamination front were analyzed by Raman microscopy. Through these HR-SKP-BT studies, a critical value was found for relative humidity for the delamination process. A transition zone was detected in which electrochemical degradation precedes mechanical delamination. In addition to the experimental studies, the critical energy release rates of the blister were calculated in finite element (FE²) simulations so as to enable a better understanding of the delamination of adhesives on metal surfaces. The combined experimental and theoretical studies show that the delamination process is controlled by the interfacial electrochemical reactions at the delamination front and that a transition area of few hundred micrometers exists in which the adhesion strength is lowered by the cathodic oxygen reduction process to a value which can be overcome by the mechanical stress in this area.     

Tribomechanical and microstructural properties of cathodic arc-deposited ternary nitride coatings

Sintered carbides are promising materials for surfaces that are exposed to extreme wear. Owing to their high service load, ceramic-based thin films are coated on carbides using different techniques. In this study, non-toxic and cobalt-free powder metallurgy-sintered carbide samples were coated with TiN, TiAlN, CrAlN, and TiSiN ceramic-based thin film coatings by cathodic arc physical vapor deposition. The microstructure (phase formation, coating thickness, surface roughness, and topography), mechanical properties (hardness, modulus of elasticity, and plasticity indices), and tribological properties (nanoscratch and wear behavior) of the thin film coatings were investigated. No cracks or defects were detected in these layers. The ceramic-based ternary nitride thin film coatings exhibited better mechanical performance than the TiN coating. The TiN thin film coating had the highest average surface roughness, which deteriorated its tribological performance. The ternary nitride thin film coatings exhibited high toughness, while the TiN thin film coating exhibited brittle behavior under applied loads when subjected to nanoscratch tests. The wear resistance of the ternary nitride coatings increased by nearly 9–17 times as compared to that of the TiN coating and substrate. Among all the samples investigated, the substrate showed the highest coefficient of friction (COF), while the TiSiN coating exhibited the lowest COF. The TiSiN thin film coating showed improved mechanical and tribological properties as compared to other binary and ternary nitride thin film coatings.     

The effects of zinc ions on the performance of epoxy coated mild steel under cathodic protection (CP)

This work aimed to address the issues that arise when cathodic protection is applied to a coated structure. The use of zinc chloride (ZnCl₂) as a corrosion inhibitor was suggested; the idea was to bridge the weakly cross linked areas on the coating by depositing a film of zinc compounds in order to seal them from the corrosive environment.     

Application of EIS and SKP methods for the study of the zinc/polymer interface

Electrochemical impedance spectroscopy (EIS) and Scanning Kelvin probe (SKP) were applied to study the zinc/polymer interface. The coating capacitance and the drop of potential across the zinc/epoxy interface are investigated as a function of water penetration and hydrolysis of adhesion bonds. Water penetrates to the interface, decreasing thus the potential drop and increasing the capacitance. Further removal of water leads to the restoration of bonds accompanied by a decrease in capacitance and the return to the initial potential distribution across the interface. Commercial high-performance coil coatings applied to galvanized steel were studied in order to correlate the interface stability and the tendency to blistering. EIS and SKP measurements allowed the evaluation of the electrochemical conditions at the interface. Local adhesion failures caused non-uniformity in the potential profile measured by SKP. Monitoring of changes in impedance at low frequency related to the interface during temperature cycling may be useful for the evaluation of the tendency to blistering.     

SECM study of steel corrosion under scratched microencapsulated epoxy resin

Scratch tests were performed on epoxy coated steel samples with and without microcapsulated linseed oil, known to develop a polymerized protective layer after having been released from the capsules at the damaged sites. The scanning electrochemical microscope has been applied to monitor the protection efficiency of this self-healing coating in aqueous acidic solution. In the proximity of the local damage, both the anodic metal dissolution and the cathodic oxygen reduction showed that the coating with microcapsules substantially and spontaneously decreased the rate of corrosion proving the self-repair concept of the coating. Electrochemical impedance measurements underline the protective effect of the coating as well.     

Preparation of electrolytic ceramic films on stainless steel conversion coatings

An electrolytic method for the synthesis of an alumina barrier on stainless steel with strong interfacial bonding is described for a Fe-17%Cr alloy. The deposit was laid down electrochemically after a specific conversion treatment by chemical oxidation of the substrate in acid solution. The conversion coating was very porous and had excellent adhesion at the substrate interface. Alumina was obtained by thermal dehydration of aluminium hydroxide deposited from an aqueous solution of an aluminium salt, according to a two-step mechanism: generation of hydroxyl ions at the cathodic substrate by reduction of H₂O or dissolved oxygen and a precipitation reaction forming aluminium hydroxide. Thermal treatment induced interfacial reactions between aluminium oxide and conversion coating compounds which led to spinel formation beneath the superficial alumina layer. The coating presented chemical composition gradients suitable for strong adhesion. Thermal oxidation resistance was studied in air at 1000° C.     

Investigation of Processes Occurring at the Metal/Polymer Coating/Electrolyte Interface

The methodical approach and the cell to study electrochemical processes occurring during cathodic disbondment of a polymer coating are worked out. They permit one to investigate the role of each process separately when supervising the metal substrate potential, electrolyte and polymer coating composition at a metal/polymer/electrolyte interface. The cathodic disbondment of ethylene-vinyl acetate copolymer, polyisoprene and poly(vinyl chloride) coatings are studied. It is found that the cathodic disbondment rate for ethylene-vinyl acetate copolymer coatings depends on double layer parameters at the interface. These parameters are determined by specific volume charge of hydrated cations of the electrolyte, potential of the substrate, the presence of oxygen, surface active substances, etc. Based on the data of IR spectroscopy in internal reflection applied to disbonded films, it is established that during the cathodic disbondment an electron transfer to polymer functional groups, as well as an attacking of the adhesion bonds by active intermediates of oxygen reduction, occurs resulting in an electrochemical degradation of the polymer and an adhesion loss. It is shown that the electrochemical transformations at the steel/poly(vinyl chloride) interface can lead to the appearance of new adhesion bonds, increasing adhesion strength and decelerating the cathodic disbondment.     

In situ electrochemical studies of forming-induced defects of organic coatings on galvanised steel

A new electrochemical setup is presented for in situ measurements during uniaxial forming of thin film coated metal substrates. This approach allows the formability analysis of a zinc pigmented organic coating on a galvanised steel substrate. The aim is to monitor the formation of defects during the forming process. The setup comprises an electrochemical microcapillary cell in a three-electrode arrangement and a miniaturised linear stretching device. The development of forming-induced defects is monitored in situ by applying electrochemical impedance spectroscopy (EIS) and also microscopically analysed by means of field emission scanning electron microscopy (FE-SEM). The studies were supported by GOM^® grid measurements and finite element simulations of model sample forming degrees. The established technique enables the evaluation of the correlation between forming degree and degradation of the barrier properties of organic coatings.Finally a phosphating process on the unformed and formed specimen is electrochemically and microscopically analysed to correlate the respective defect size with its local reactivity. The results show that stretching-induced defects occur at the interface between spherical Zn particles and the epoxy binder matrix. The defect size increases with increasing strain values. The phosphating process leads to the nucleation of phosphate crystals especially in the forming-induced defects and thereby reduces the free zinc in the defect area. The kinetic of the phosphating is accelerated with increasing size of the defect.     

Electrochemical state conversion model for occurrence of pitting corrosion on a cathodically polarized carbon steel in a near-neutral pH solution

Pitting corrosion occurs frequently on oil/gas pipelines which are protected by both coating and cathodic protection (CP). Since the steel is in an active state, the classic theories attributing pitting to the passive film breakdown do not apply. This work is anticipated to advance the understanding of the fundamentals of pitting corrosion of carbon steel pipelines occurring in an active system. In this work, a square wave polarization method was used to simulate the CP fluctuation and its effect on pipeline steel pitting in a near-neutral pH solution. Moreover, an electrochemical state conversion model was developed to illustrate the pit initiation and growth on the cathodically polarized steel. According to ESCM, when a potential fluctuation is generated on the steel electrode, the local double-charge layer structure is disturbed. The defect area undergoes a temporary anodic potential field, resulting in the local anodic dissolution to nucleate pits. Thus, local anodic dissolution (pitting) of steel would occur under an unstable cathodic polarization. Furthermore, it is found that the potential shifting range plays an important role in pitting initiation and growth. Pits nucleate in high-density under square wave polarization if the polarization potential range is relative narrow and the upper potential is not so negative, i.e., close to OCP. When under a wide potential range and a relatively negative upper potential, pits tend to grow into big size with a low density. Therefore, although the pipeline is protected by CP, the CP fluctuation could occur during service, which would introduce the polarization fluctuation on the steel, resulting in pitting corrosion.     

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.     

Corrosion protection properties of silane pre-treated powder coated galvanized steel

Silane based products are becoming an interesting material for pre-treatment deposition, because, for the environmental compatibility, they can be used as substitutes of traditional pre-treatments like chromates. Silanes have been studied as new pre-treatments before organic coating deposition for many different metals, including aluminium, copper and zinc.In this work, some results concerning the properties of water-based silane pre-treatments on galvanized steel will be presented.Galvanized sheets obtained by continuous hot dip process were considered. A silane based bath containing a mixture of three different silanes were used for the pre-treatment deposition (Glycidoxypropiltrimethoxysilane, Tetraethoxysilane and Methyltriethoxysilane).The obtained pre-treatments were characterized by SEM observations, FT-IR and ToF-Sims analysis. The corrosion protection properties of the pre-treated galvanized samples were studied using industrial accelerated tests (like salt spray exposure) and electrochemical measurements (polarization curves and electrochemical impedance spectroscopy (EIS) measurements), as a function of the different curing conditions. The pre-treated galvanized sheets were further coated with an epoxy-polyester powder coating, in order to verify the adhesion promotion properties and the corrosion protection performances of the complete protective system.The coated samples were characterized by EIS measurements with artificial defect in order to study the interfacial stability (adhesion) in wet conditions and monitor the coating delamination.The electrochemical data were compared with adhesion measurements obtained by cathodic delamination tests. The electrochemical tests showed that the silane layer acts not only as a coupling agent between the inorganic substrate and the organic coating, but it also ensures a good barrier effect against water and oxygen.