Metal-coating Interface Research Articles

Magnesium-Based Sacrificial Anode Cathodic Protection Coatings (Mg-Rich Primers) for Aluminum Alloys

Magnesium is electrochemically the most active metal employed in common structural alloys of iron and aluminum. Mg is widely used as a sacrificial anode to provide cathodic protection of underground and undersea metallic structures, ships, submarines, bridges, decks, aircraft and ground transportation systems. Following the same principle of utilizing Mg characteristics in engineering advantages in a decade-long successful R&D effort, Mg powder is now employed in organic coatings (termed as Mg-rich primers) as a sacrificial anode pigment to protect aerospace grade aluminum alloys against corrosion. Mg-rich primers have performed very well on aluminum alloys when compared against the current chromate standard, but the carcinogenic chromate-based coatings/pretreatments are being widely used by the Department of Defense (DoD) to protect its infrastructure and fleets against corrosion damage. Factors such as reactivity of Mg particles in the coating matrix during exposure to aggressive corrosion environments, interaction of atmospheric gases with Mg particles and the impact of Mg dissolution, increases in pH and hydrogen gas liberation at coating-metal interface, and primer adhesion need to be considered for further development of Mg-rich primer technology.

Water transportation through organic coatings: correlation between electrochemical impedance measurements, gravimetry, and water vapor permeability

A primary cause of coating failure is diffusion of water through organic coatings during which many corrosive species are transported to the metal-coating interface. However, water vapor permeability through the coating improves blister resistance to a certain extent. The present work describes the influence of chemical nature of the polymer on the above two properties. Attempts were also made to establish a correlation between these two properties for pigmented organic coating. Six paints were formulated and processed using six different types of binders at a constant pigment volume concentration (PVC) and specific gravity. Water ingress, water vapor permeability, and water absorption of these coatings were estimated using electrochemical impedance measurements (EIS), permeability cup method, and gravimetric method, respectively. There exists a good linear correlation between water uptake measured by EIS and water absorption measured by gravimetry. Similarly, a correlation was also noticed between water uptake by EIS and water vapor permeability. However, polyurethane type polymers did not fit into this linear correlation. Furthermore, influence of the resin chemistry on anticorrosive properties of these coatings was also studied using EIS and salt spray exposure test. Among all polymers under investigation, acrylic polyol-based polyurethane has shown the lowest water uptake, higher impedance, better salt spray resistance but higher water vapor transmission rate.

Interactions between γ-TiAl melt and Y 2O 3 ceramic material during directional solidification process

A γ-TiAl alloy with nominal composition of Ti-47%Al (molar fraction) was directionally solidified in an alumina mould with an Y 2O 3 protective coating. The effects of processing parameters (melting temperature and interaction time) on the metal-coating interface, microstructure and chemical composition of the alloy were evaluated. The result shows that the Y 2O 3 protective coating exhibits an effective barrier capability to avoid direct contact between the mould base material and the TiAl melt, although the Y 2O 3 coating is found to suffer some erosion and be slightly dissolved by the molten TiAl due to the coating-metal interactions. The directionally solidified alloys were contaminated with Y and O, and Y 2O 3 inclusions were dispersed in the metal matrix. The reason for this metal contamination is the Y 2O 3 coating dissolution by the TiAl melt. One mode of the interaction between Y 2O 3 and the TiAl melt is dissolution of yttrium and atomic oxygen in the melt by reaction Y 2O 3(s)=2Y (in TiAl melt)+3O (in TiAl melt). Both the extent of alloy contamination and the volume fractions of Y 2O 3 inclusions depend on the melting temperature and the interaction time.

Investigation on anticorrosion performance of nano and micro polyaniline in new water-based epoxy coating

In this research, a micro polyaniline (MicroPAni) and a dispersion of nano polyaniline particles (NanoPAni) were used as anticorrosion additive in a new water-based epoxy coating. Both materials were added directly to a new water-based hardener (RIPI-W.B.H.). The resulted mixtures were homogenized and sonicated for size reduction and better particle dispersion. The average particle size of polyaniline in the hardener was determined using dynamic light scattering (DLS). Transmission electron microscopy (TEM) was used to observe the particle size of polyaniline in the final dried film. After applying the coatings on metal surface, anticorrosion performance and adhesion properties of the coatings were compared together with a reference coating using salt spray and cross-cut adhesion tests. The results showed that anticorrosion performance of coating containing NanoPAni was better than the one for coating containing MicroPAni. In addition, investigation on the morphology of metal-coating interface by scanning electron microscopy (SEM) technique in samples containing polyaniline after salt spray test showed stable oxide layer formation on metal surface, which will enhance the corrosion resistance.

Cathodic disbonding of an unpigmented epoxy coating on mild steel under semi- and full-immersion conditions

Cathodic disbonding of an unpigmented epoxy coating around a defect was studied with samples fully- and semi-immersed in 3.5 wt.% NaCl solution. Disbonding and the growth of blisters were monitored by scanning acoustic microscopy, which showed growth of blisters within the disbond for both type of samples. Scanning Kelvin probe potential maps suggest that blisters first form at local anodes. For semi-immersed samples, disbonding of the immersed part and the non-immersed part propagate with parabolic kinetics at identical rates, similar to fully-immersed samples. This implies that ion transport is along the coating–metal interface in both cases.

Degradation of rubber to metals bonds during its cathodic delamination, validation of an artificial ageing test

Rubber-metal assemblies used for submarine applications are often associated to cathodic protection which can lead to cathodic disbonding of the polymer/metal interface. The degradation mechanisms of polychloroprene/epoxy adhesive/copper–aluminium substrate systems associated to an accelerated ageing test are compared to the degradations resulting from natural ageing conditions in the Mediterranean sea. From destructives tests, a kinetic in four steps is identified for both ageing conditions. This kinetic is confirmed by Electrochemical Impedance Spectroscopy data during ageing. In order to further identify the failure mode, the disbonded surfaces on polymer and steel sides are analyzed using SEM and FTIR. Both ageing conditions present the same type of degradation which validates the fact that the accelerated ageing protocol reproduces the natural conditions. The chemical transformations in delaminated epoxy are very similar to the chemical transformation in epoxy coatings under high energy electron-beam irradiation. However, bidendate metal carboxylates rather than carboxylic acid chain ends are the main oxidation products due to the alkaline environment at the coating-metal interface. It was shown that the metal/polymer interface is not destroyed by an alkaline attack but by a strongly oxidative attack caused by the intermediates of the oxygen reduction.

Salt Exclusion in Silane-Laced Epoxy Coatings

The corrosion protection mechanism of a one-step silane-laced epoxy coating system was investigated using neutron reflectivity. Pure epoxy and silane-laced epoxy films were examined at equilibrium with saturated NaCl water solution. The results demonstrate that the addition of silane introduces a salt-exclusion effect to epoxy coating. Specifically, the addition of silane densifies the epoxy network, which leads to exclusion of hydrated salt ions by a size effect. The effect is particularly significant at the metal-coating interface. Exclusion of ions improves the corrosion resistance, particularly for metals susceptible to pitting.

Heat-resistant glass-ceramic coatings protecting low-alloyed steels against high-temperature gas corrosion

Glass-ceramic coating aimed at protecting low-alloyed and low-carbon steels under technological heating is developed; it decreases the metal losses to scale by a factor of 12, as well as substantially decreases the decarbonizing and dealloying of the metal surface. The coating is characterized by an optimum ratio of the refractory to glasslike phases, which provides a reliable adhesion of the coating to the metal. As a result of sintering solid and liquid phases, a barrier layer forms that prevents the oxygen penetration to the metal and, hence, decelerates the corrosion processes at the coating-metal interface under heating conditions.

Polyaniline inhibition of filiform corrosion on organic coated AA2024-T3

Dispersions of the polyaniline emeraldine salt (ES) of paratoluene sulphonic acid (PAni-pTS) effectively inhibit filiform corrosion (FFC) affecting polyvinyl butyral (PVB) coated AA2024-T3 aluminium alloy. An in-situ scanning Kelvin probe (SKP) technique is used to study the effect of systematically varying PAni-pTS volume fraction ( ϕ pa) on FFC initiation and propagation. For ϕ pa < 0.15, there is no evidence of FFC inhibition and E corr values recorded for the intact coated aluminium ( E intact) remain similar to those measured for unpigmented PVB. At ϕ pa ≥ 0.15, a marked rise in E intact is observed, FFC propagation rates decrease and significant oxide growth is observed at the coating-metal interface. For emeraldine base (PAni-EB)-containing coatings, there is no evidence of interfacial oxide film formation, no ennoblement of E intact and minimal inhibition of FFC. Conversely, when a PAni-pTS induced oxide covered surface is re-coated using unpigmented PVB and FFC is initiated as per normal, a substantial reduction in the rate of FFC propagation is observed. It is therefore proposed that inhibition of FFC by PAni-pTS arises principally as a result of the protective nature of the oxide film formed at the metal-coating interface.

Electrochemical Quartz Crystal Microbalance Technique to Monitor External Polymeric Pipeline Coatings

Abstract Electrochemical Quartz Crystal Microbalance (EQCM) can be used to monitor changes in several material properties, including mass, electrochemical, viscosity, adsorption, and deposition. In this paper, EQCM was evaluated to determine if it can monitor changes to the metal-coating interface well before any visible/physical changes to the coating. Modified cathodic disbondment (CD) experiments were carried out over a period of 14 months on 13 coatings used to protect the external surfaces of oil and gas pipelines. During the experiments, EQCM measurements were recorded by placing the quartz crystal at the coating-steel interface. After the experiments, the CD area was measured and compared with the EQCM measurements. Of the 117 panels of 13 different pipeline coatings tested, for 67 % of coatings EQCM correctly predicted the coating performance. Twenty-five percent of coatings performed better than that predicted from EQCM data. Only for 8 % of coatings EQCM measurements predicted better performance but the coatings disbonded during CD area measurement. Based on these observations it was concluded that microscopic changes to the metal-coating interface could be assessed from oscillation frequency of the EQCM crystal placed at the metal-coating interface. However, in order to obtain useful information, extreme care should be exercised in placing the fragile EQCM crystal at the metal-coating interface.

Asymptotic approach to the constant velocity of hydrogen delamination growth

We consider delamination crack growth controlled by gas diffusion into the crack. If the gas is accumulated inside the delamination, after some incubation period, the crack starts growing under the pressure of the accumulated gas. An important example is given by hydrogen-induced delamination. Hydrogen absorbed by a metal is typically dissolved in the proton form within the lattice. Some of the protons reach the surface of pre-existing or freshly created cracks or delaminations where they recombine with electrons and form molecular hydrogen in the crack cavity. Since the molecular form of hydrogen is thermodynamically more stable, this process leads to accumulation of hydrogen gas inside the delamination crack. Under the excessive hydrogen pressure fracture often takes place even in the absence of any additional external loading. It is especially dangerous if the metal is covered with coating, where because of the dissimilarity of materials, microscopic voids appear more frequently. This leads first to hydrogen precipitation within these coating–metal interface voids and then to their development. This results in the delaminating of the coating. As it is well known, in the absence of aggressive media the cracking resistance of a material is characterized by a unique constant: so that if the stress intensity factor is smaller than this constant, the crack is fixed, and if it is larger, the crack moves in a dynamic regime. But in the presence of aggressive media, particularly hydrogen, the crack development is characterized by a smooth kinetic function v( K I), which is dependence of the crack velocity on the stress intensity factor, with the lower threshold value of resistance K scc smaller than the static critical stress intensity factor K I c . As numerically shown in the author's earlier work for the internal crack growth, the crack velocity first increases in accordance with the kinetic function, until reaching some value of v s , and remains at the same level afterwards. Since, as has been obtained by the author in the previous paper, kinetic equations for internal and delamination cracks are essentially identical, the same conclusion can be derived for the latter. In this paper the stability and asymptotic approach to the constant velocity of delamination growth is proved analytically. If v s is known, the corresponding value of the stress intensity factor, K s , is obtained by substituting v s into the kinetic function v( K I).

Mathematical model for cathodic delamination using a porosity–pH relationship

A mathematical model was developed to simulate the transient propagation of the cathodic delamination on a coated metal. The model employed porosity and the polarization kinetics at the metal-coating interface that were assumed to be dependent on the local pH. The simulations provided qualitative agreement with published experimental results for a zinc coated system characterized by extremely rapid cathodic delamination. The mathematical model described in this work provides a foundation on which more sophisticated models can be constructed.

Influence of galvanised surface state on the duplex systems behaviour

The influence of galvanised surface conditions on the duplex systems behaviour is analysed. A paint system was applied over new and weathered galvanised steel samples. A comparative study was performed by adhesion tests and electrochemical impedance spectroscopy (EIS) technique. Previously, the roughness profiles of galvanised surfaces were evaluated. The obtained results point out that a certain weathering degree is necessary for optimum adherence between paint and metallic surface and for good corrosion protection. Nevertheless, when the weathering degree is too high, the protection efficiency decreases. This behaviour can be explained in terms of galvanised surface roughness. The smooth surface of fresh galvanised steel makes difficult the establishment of adhesive bonds at coating–metal interface, leading to poor behaviour, at initial stages. If the weathering degree is too high, the surface roughness can hinder optimum coverage by the organic coating and the passive layer remains partially bare; as a consequence, the protection features are lower than those obtained for an average weathering degree where good adhesion and complete coverage are reached.

Corrosion Processes at the Organic Coating-metal Interface Studied by Scanning Kelvin Probe, Scanning Acoustic Microscopy and Electrochemical Impedance

not Available.

Structural and electrochemical characterization of sputter-deposited nitrided NiCr alloys

Nickel-based coatings are potential candidates for the protection of electrochemical dissolution of steel surfaces. Such coatings, elaborated by magnetron sputtering in a nitrogen atmosphere, offer good corrosion protection, good adherence as well as stability for metallic structures. NiCr alloys with almost constant composition have been deposited with different nitrogen contents on stainless steel and carbon steel surfaces. The coating uniformity, homogeneity, composition and crystallinity have been studied by scanning electron microscopy, energy-dispersive X-ray spectrometry, atomic force microscopy and X-ray diffraction techniques. The corrosion degradation behavior of all the samples was tested in NaCl and NaCl and CO2 mixture exposures using electrochemical impedance spectroscopy measurements. Nitrided NiCr alloys on a stainless steel substrate resulted with better adhesion than carbon steel, by delaying the corrosion mechanism when exposed to NaCl and CO2 solution. A comparison of the corrosion resistive behavior of the substrates (stainless steel, carbon steel) and the coatings is made by using the electrical capacitance concept from a double-layer model for the coating–metal interface.

Inhibition of corrosion-driven organic coating delamination on zinc by polyaniline

The polyaniline emeraldine salt of para-toluenesulphonic acid (PAni-PTS) is dispersed in polyvinylbutyral (PVB) coatings adherent to a zinc surface. Such dispersions are shown to effectively inhibit corrosion-driven (cathodic) coating delamination when aqueous chloride electrolyte contacts a penetrative coating defect. A scanning Kelvin probe (SKP) is used to determine the influence of PAni-PTS volume fraction ( φ pa) on both delamination kinetics and the potential of the intact (undelaminated) coated surface ( E intact). At relative humidities (rh) of ca. 95% no delamination is observed over a 48 h period when φ pa⩾0.2. However, ennoblement of E intact does not persist for longer than 6 h at the same rh. Over a similar period (6 h) a zinc oxide layer develops at the coating-metal interface and attains a thickness proportional to φ pa. An inhibition mechanism is proposed in which the PAni-PTS induced zinc oxide layer stifles cathodic O 2 reduction and is itself protected from alkaline dissolution through PAni-PTS mediated pH buffering.

Hydrophobic interfacing layers for improvement of corrosion protection by polymeric coatings

Water sorption of coating materials is the main cause of coating deterioration, adhesion loss and substrate corrosion. By introducing alkanethiol self-assembled monolayers (SAMs), a hydrophobic interfacing layer between coating and substrate metal can be constructed. The effect of the hydrophobic SAMs interfacing layer on the corrosion protection of epoxy coatings was evaluated using electrochemical techniques including Tafel polarization, electrochemical impedance spectroscopy and impedance–time transition measurement. It was found that the SAMs interfacing layer improved the corrosion protection of the coating significantly. The improvement was attributed to the strong interaction between SAMs and the metal substrate, the compact structure and low water affinity of the SAMs interfacing layer, which prevent water absorbed by the coating from reaching the coating–metal interface and spreading along the interface.

Polyurethane/Polysiloxane Ceramer Coatings: Evaluation of Corrosion Protection

A series of polyurea and polyurethane ceramer coatings were formulated using hexamethylene diisocyanate (HDI) isocyanurate, alkoxysilane-functionalized HDI isocyanurate, tetraethyl orthosilicate (TEOS) oligomers and cycloaliphatic polyesters. The coatings were prepared as a function of alkoxysilane-functionalized HDI isocyanurate and TEOS oligomers concentration. Also, the effect of acid catalyst was investigated. The corrosion resistance of polyurea or polyurethane ceramer coating systems were evaluated using a prohesion chamber on aluminium alloy 2024-T3 substrate. The polyurethane ceramer coatings were compared with the chromate pretreatment and the epoxy-polyamide primer containing the chromate pigment. In addition to prohesion, the interface between the coating and substrate was characterized using X-ray photoelectron spectroscopy (XPS). The prohesion data showed that the corrosion was inhibited by the TEOS oligomers. However, high concentrations of TEOS oligomers and acid catalyst produced blistering in the polyurea/polysiloxane ceramer coatings. The prohesion data also showed that the corrosion protection of ceramer coatings performed as well as the chromate pretreatment and competitively with the epoxy primer. From the XPS and prohesion data, a self assembling silicon oxide layer at the metal-coating interface was proposed.

Electrochemical impedance spectroscopy as a tool for investigating the quality and performance of coated food cans

In the present work, electrochemical impedance spectroscopy (EIS) is explored as a tool for evaluating the quality of coatings on food cans. The properties of the coating-metal interface are evaluated using an equivalent electrical circuit representing intact and defect coating areas. The element corresponding to the charge transfer capacitance under pores, proportional to the wetted metal surface, is found to be the most useful element for quality evaluation. It is shown that with EIS, differences can be detected between new and filled cans from different suppliers, as well as between different product formulations. Three measurement techniques, i.e., initial state characterization, rapid dc aging, and long-term electrolyte aging, are applied to a series of petfood and beverage cans, and the results compared with the performance of the same cans after filling and storage. It is shown that all three measurement techniques give basic guidance as to the long-term performance of the filled cans. However, the electrolyte-aging test was found to give the best precision in the performance prediction.

Discrimination by EIS of degradation mechanisms in lap joints of coated metal sheet

The information derived from inspedance diagrams obtained by using electrochemical sensors specifically designed for lap joints testing of coated metallic sheets is analyzed. The electrochemical sensors were made by means of steel or zinc sheets, uncontaminated or contaminated with variable amounts of sodium chloride, and overcoated with organic coatings. Such sensors show the effect of the nature of the metallic substrate, the underfilm saline contamination, and the relative humidity of the environment outside the lap joint. The results show that the information obtained from electrochemical impedance spectroscopy (EIS) on the behavior within the lap joints can be assessed by the criteria traditionally used for plain painted metal surfaces. By modeling the data, it is possible to discriminate between effects due to the organic coating and to the corrosion reaction at the metal-coating interface.