Transport processes of hydrated ions at polymer/oxide/metal interfaces: Part 1. Transport at interfaces of polymer coated oxide covered iron and zinc substrates

Localised electrochemical and spectroscopic techniques were jointly applied for the evaluation of hydrated ion transport processes along polymer/oxide/metal interfaces. In situ Scanning Kelvin Probe (SKP) studies of the local interfacial potentials of organically coated oxide covered zinc and iron substrates were performed in humid nitrogen atmospheres. They were supported by ex situ small spot X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) analysis of the interfacial ion distribution. Based on the experimental results it is concluded that also in atmospheres of strongly reduced oxygen partial pressure at which no corrosive delamination takes place, a negatively charged layer of adsorbed hydroxide ions determines ion transport processes along interfaces between polymer films and oxide covered metals. No ion transport was observed for zinc substrates while hydrated cations were selectively transported along the polymer/iron interface. The reduction of oxygen molecules on the highly reactive iron oxide surface is assumed to be responsible for the generation of adsorbed interfacial hydroxide ions. On the other hand such oxygen reduction induced hydroxide formation in humid nitrogen atmospheres with strongly reduced oxygen partial pressure does not seem to take place on oxide covered zinc. The variation of free volumes at the polymer/substrate interface did not lead to a principal change of this phenomenon.     

Electrochemical studies of the inhibition of the cathodic delamination of organically coated galvanised steel by thin conversion films

Scanning Kelvin Probe (SKP) measurements of thin amorphous conversion film coated galvanised steel in combination with current density-potential curves and electrochemical impedance spectroscopy (EIS) were performed with the aim to improve the understanding of electrode potentials at the coating/metal interface and their influence on corrosive de-adhesion. The thin hybrid conversion films contained Zn-phosphates, titanates and also complexing organic compounds and led to an inhibition of the cathodic oxygen reduction and anodic zinc dissolution. In the polymer coated area the conversion film leads to a cathodic shift of the potential as measured by means of the SKP. This cathodic potential shift is explained by the substitution of the n-semiconducting Zn-oxide with an insulating inorganic layer. When the SKP detects the potential of freely corroding iron at a defect, where no protective coating layer is, the interfacial potential for the conversion film coated zinc layer is more negative than the defect potential. This leads to a diminished driving force for an oxygen reduction induced delamination process which is of relevance for the understanding of cut-edge corrosion.     

In situ studies of conversion coated zinc/polymer surfaces during exposure to corrosive conditions

This work investigates the hidden interface between a conversion-coated zinc surface and a polymer coating upon exposure to an electrolyte by simultaneous in situ ATR-FTIR and EIS. Various system properties were distinguished, such as the ingress of electrolyte constituents, and an active process of water-induced alterations of the conversion layer. The interface between a polymer film and a surface treated metal surface is of considerable fundamental and technical interest in many areas of application, and the results obtained open up the use of this method for a wide range of important applications.     

In situ ATR-FTIR studies of the aluminium/polymer interface upon exposure to water and electrolyte

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) with the Kretschmann configuration was applied for in situ studies of the transport of water and ionic species through a polymer film to an aluminium/polymer interface. The time dependent intensity changes of the infrared bands of water were used to follow the transport of water to the aluminium/polymer interfacial region and a NaSCN solution was employed as model electrolyte to follow the transport and accumulation of thiocyanate ions. Apart from water sorption and ion transport, the main processes identified were corrosion/oxidation of the aluminium surface and swelling of the polymer film. The method proved to be useful for detailed in situ studies of changes at a polymer coated metal surface, such as oxidation and surface film formation on the metal. It should also be possible to study the effects of defects and pores in the polymer film on the transport properties of water and ions to the metal/polymer interface, as well as adsorption and other chemical reactions and physical interactions in the metal/polymer interfacial region.     

Transport processes of hydrated ions at polymer/oxide/metal interfaces: Part 2. Transport on oxide covered iron and zinc surfaces

The study of ion transport processes on non-polymer coated bare oxide covered iron and zinc surfaces showed that the presence of adsorbed ions determines the ion distribution on oxide/metal surfaces in humid atmosphere. For fundamental studies of ion ingress at polymer/oxide/metal interfaces, already the transport analysis in the absence of the polymer reveals important mechanistic aspects. Sophisticated spectroscopic techniques were applied for the correlation of electrochemical data with local surface chemistry. In-situ Scanning Kelvin Probe (SKP) measurements of the local interfacial potentials of oxide covered iron and zinc substrates and ex situ time-of-flight secondary ion mass spectroscopy (ToF-SIMS) analysis showed that the surface layer charge influences the ion transport processes. A model is proposed to explain the basic mechanism of hydrated ion transport on oxide covered zinc and iron.     

Fundamentals of Adhesion Failure for a Model Adhesive (PMMA/Glass) Joint in Humid Environments

The origins for the abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces were explored using a poly(methyl methacrylate) (PMMA) film on silicon oxide as a model system. The interfacial and bulk water concentrations within the polymer film were quantified as a function of D₂O partial pressure using neutron reflectivity. The adhesive fracture energies of these PMMA/SiO₂ interfaces at the same conditions were determined using a shaft-loaded blister test. Discontinuities in the adhesive fracture energy, bulk moisture solubility, and the width of the interfacial moisture excess near the interface were observed at the critical RH. A mechanism based on the coupling of bulk swelling-induced stresses with the decreased cohesive strength due to moisture accumulation at the interface is proposed and is consistent with all experimental observations.     

Fundamentals of Adhesion Failure for a Model Adhesive (PMMA/Glass) Joint in Humid Environments

The origins for the abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces were explored using a poly(methyl methacrylate) (PMMA) film on silicon oxide as a model system. The interfacial and bulk water concentrations within the polymer film were quantified as a function of D₂O partial pressure using neutron reflectivity. The adhesive fracture energies of these PMMA/SiO₂ interfaces at the same conditions were determined using a shaft-loaded blister test. Discontinuities in the adhesive fracture energy, bulk moisture solubility, and the width of the interfacial moisture excess near the interface were observed at the critical RH. A mechanism based on the coupling of bulk swelling-induced stresses with the decreased cohesive strength due to moisture accumulation at the interface is proposed and is consistent with all experimental observations.     

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.     

An electromechanical perspective on the metal/solution interfacial region during the metallic zinc electrodeposition

The difficulty of studying the metal/solution interfacial region makes the use of non-conventional measurement techniques indispensable. In this way, a careful in situ study by means of acoustic impedance techniques coupled with nano-electrogravimetric techniques allowed this interface to be monitored during the metallic zinc electrodeposition process. This paper proves the formation of a viscoelastic layer consisting of ultra-hydrated Zn(II)/Zn(I) salts as a key step in the metallic zinc electrodeposition mechanism in sulfate aqueous solutions. Surprisingly, this layer is located in the metal/solution interfacial region and not on the reaction substrate. The chloride ions effect on the metallic zinc electrodeposition mechanism at these experimental conditions lies mainly in the stabilization of the zinc ions inside this layer.     

Molecular Studies of Adhesion and De-Adhesion on ZnO Nanorod Film-Covered Metals

This paper aims at the investigation of the effect of ZnO nanorod film deposition on the adhesion between a zinc surface and a model epoxy-based adhesive, with and without the adhesion promoter 3-aminopropylphosphonic acid (APPA) treatment. The stability of octadecylphosphonic acid (ODPA) monolayers on ZnO nanorod surfaces was investigated by means of contact angle measurements and FT-IRRAS to simulate wet de-adhesion conditions at the phosphonate-zinc oxide interface. Peel tests were performed under controlled humidity to study the wet de-adhesion of the model epoxy-amine coating from the ZnO nanorod surface. The deposition of ZnO nanorod films resulted in a significant increase of peel forces in comparison with bare zinc. In the case of APPA-treated ZnO nanorod films the increase of the macroscopic adhesion forces was more pronounced. The high surface area ZnO nanorod films provide for the adsorption of polymers as well as adhesion promoters and makes them promising candidates for improving the adhesion properties of engineering metals.     

The anodic dissolution of zinc and zinc alloys in alkaline solution. I. Oxide formation on electrogalvanized steel

The polarization behaviour of zinc in alkaline solution has been investigated using atomic emission spectroelectrochemistry. By independently measuring the oxidation rate of zinc (electrical current) and the rate of Zn²⁺ dissolution (partial elemental current) it is possible to calculate the amount of insoluble zinc cations produced at any instant. Assuming the insoluble cations are present as a zinc oxide film, the growth of this film as a function of potential and time was determined. On the basis of kinetic evidence, it was found that at least three forms of zinc based oxide/hydroxide films form during polarization experiments. Type I oxide formation occurs when the metal/electrolyte interface becomes locally saturated with Zn²⁺ ions. Type II oxide forms on the metal surface underneath the film of Type I oxide but has little inhibiting effect on zinc dissolution. Type III oxide is produced in much smaller quantity and results in a transition to the passive state. This may be due to a potential induced transition of Type II → Type III oxide.     

In situ infrared spectroscopic and scanning Kelvin probe measurements of water and ion transport at polymer/metal interfaces

Scanning ATR-spectroscopy and scanning Kelvin probe studies are introduced as new analysis techniques for the study of water and ion transport at polymer/metal interfaces. The new approach of scanning ATR spectroscopic analysis of water transport at the adhesive/metal interface is combined with scanning measurements in transmission mode of water transport along adhesive joints and non-scanning ATR-measurements of the water transport in a vertical direction through the adhesive. D₂O was chosen in some cases instead of water due to its excellent traceability in the infrared spectra. A scanning Kelvin probe was chosen for the detection of the transport kinetics of hydrated alkali ions along the adhesive/metal interface based on the local measurement of interfacial electrode potentials.The complimentary FT-IR techniques showed that the interfacial diffusion of water, in the case of epoxy adhesives on iron, is about two orders of magnitude faster that the transport through the adhesive itself. Similar transport kinetics at the interface is also shown by hydrated ions. Moreover, the here presented FT-IR-ATR and SKP results reveal more information on how adhesion promoting organosilane layers and organosilanes as additives act at polymer/metal interfaces in the presence of water incorporated in the interphase zone.     

The effect of electrolyte anions incorporated into anodic oxide films on the experimental transport numbers of ions

The growth of barrier anodic film is considered theoretically with regard to the migration of three ionic carriers: oxygen and metal ions and electrolyte anions. It is shown that the consideration of anion transport leads to the conclusion that the film grows at three interfaces: the metal/oxide and oxide/electrolyte interfaces and the interface between an oxide layer containing electrolyte anions (contaminated layer) and the oxide layer free of them (“pure” layer). The error in the measured transport numbers of metal and oxygen, which is caused by ignoring a contribution of electrolyte anions to the total charge transport, is maximum in the absence of anion motion.     

Study of the interfacial adhesive strength of a heat-shrinkable multilayer film

A heat-shrinkable multilayer film is widely employed as labels of plastic bottles. A new heat-shrinkable multilayer film without an adhesive layer was designed in this study. The interfacial adhesive strength between the layers was controlled to avoid layer separation. We assumed a polyethylene terephthalate glycol-modified (PETG)/styrene-co-butadiene block copolymer/PETG shrinkable film substitute as the general poly(ethylene terephthalate) (PET)/polystyrene/PET shrinkable film. The interlayer adhesive strength between the layers was retained for industrial utilization even after drawing. Additional polybutadiene (PB) infiltrated the butadiene layer in the microphase-separated structure. Further addition of PB could not infiltrate the butadiene layer. The excessive PB contents coexisted with the interface between the layers, as observed by transmission electron microscopy (TEM). The segregated PB enhanced the interfacial adhesive strength. We concluded that the selective distribution of adhesive functional materials along the interface could appropriately retain its adhesive strength.     

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.     

Comparative study on surface behaviors of copper current collector in electrolyte for lithium-ion batteries

The chemical and electrochemical stability of Cu current collectors in electrolyte for lithium-ion batteries is investigated. During long-term storage, the surface section of Cu foil is oxidized to copper compounds along with the reduction reaction of electrolyte. A continuous surface film can be formed on the Cu current collector after the foil is immersed in electrolyte for lithium ion batteries at room temperature for 30 days. This surface film is composed of inorganic compounds located in the inner layer and organic/inorganic mixed components stayed outside. It comes from the spontaneous reaction at the interface between Cu foil and electrolyte for the existence of trace water in electrolyte. Different from SEI film spontaneous formation during storage, surface film generated on Cu foil during electrochemical process shows different characteristic and mechanism. By using metal lithium as counter electrode, SEI film on Cu foil in Cu foil/metal Li battery is formed from surface chemical species floating from lithium counter electrode and electrochemical oxidation/reduction process. In contrast, thinner SEI film can be generated merely from electrochemical electrolyte decomposition and precipitation. All the evidences reveal that the structure of SEI film from different conditions is similar, which shows inorganic fluorides located in the inner layer and organic/inorganic mixed lied in the outer layer.     

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.     

A model for adhesion-producing interactions of zinc oxide surfaces with alcohols,amines, and alkenes

The interactions between paint/adhesive polymers and metal surfaces that are critical for adhesion have been studied theoretically. This study used zinc oxide as a model of a galvanized steel surface, and ammonia, water, and ethylene as models for amino, hydroxy, and unsaturated functionalities in paint/adhesive polymers. Ab initio molecular orbital calculations were carried out on zinc oxide and zinc oxide dimer. Geometries were optimized at the HF/3-21G level and relative energies were calculated by CASSCF/3-21G and by MP2 with the DZP basis set of Wachters and Hay. Ethylene forms a stable complex with zinc oxide dimer that has a stabilization energy of 24.9 kcal/mol. Insertion of ethylene into zinc oxide dimer to form a stable six-membered ring adduct occurs with a surprisingly low activation energy of 8.8 kcal/mol. The binding energy of ammonia with zinc oxide dimer is 38.5 kcal/mol and the activation energy for insertion of ammonia forming covalent Zn-NH₂ and O-H bonds is calculated to be 9.6 kcal/mol. Aminolysis of zinc oxide dimer with two ammonia molecules has a predicted barrier height of 6.7 kcal/mol. The transition structure for Zn-O bond rupture with one NH₃ and one H₂O molecule is only 1.5 kcal/mol higher in energy than the reactant cluster. The calculations suggest that alkenes, amines, and alcohols could readily form covalent bonds with the ZnO surface, thereby facilitating adhesion of the polymer containing these functional groups to a galvanized surface.