Recent developments in characterising electrodeposition of anti-corrosion coatings using the wire beam electrode method

New progresses have been made during recent years in the application of the wire beam electrode (WBE, a coupled multielectrode array) for studying electroplating of metallic coatings, for monitoring the electrodeposition of polymer coatings, and for evaluating the performance of anti-corrosion coatings. The WBE allows localized electrode processes to occur over different locations of its surface under external anodic or cathodic polarization and permits monitoring of nonuniform electrodeposition processes. Several typical experiments are presented in this paper. One sample experiment is the characterization of nonuniform electroplating of nickel coating, which was achieved by mapping the distributions of currents over a WBE surface that was under cathodic polarization. Various characteristic current distribution patterns, which indicate different electrodeposition mechanisms or low covering-power, have been observed. These patterns were found to correlate with the effects of several affecting factors such as electrolyte concentration, temperature and agitation flow. Another sample experiment is the investigation of nonuniform anodic electrodeposition of polyaniline (PANI) coatings and the understanding of their anti-corrosion performance and mechanisms. Anodic polarization currents were measured from various locations over the WBE surface in order to produce anodic polarization current maps under PANI deposition.

Characterising localised corrosion inhibition by means of parameters measured by an electrochemically integrated multielectrode array

Electrochemical parameters including maximum anodic current density, total anodic current density, the number of anodic sites and the localised corrosion intensity index have been extracted from galvanic current distribution maps that were acquired using an electrochemically integrated multielectrode array, namely, the wire beam electrode. Experiments have been carried out to demonstrate the application of these new electrochemical parameters for characterising localised corrosion inhibition of metals. A typical corrosion inhibitor, potassium dichromate, was found to affect localised corrosion processes in various ways, for instance in sodium chloride solutions, it was found to inhibit localised corrosion of aluminium alloy AA 2024-T3 by suppressing galvanic corrosion activities occurring over the alloy surface, whereas it was found to control localised corrosion of AA 1100 by creating a large number of minor anodes distributing randomly over the metal surface.

The corrosion inhibition of aluminium alloy 7075-T651 with cerium and Mischmetal di phenyl phosphate

Previous studies have shown that cerium diphenyl phosphate (Cedpp) 3 is a very effective inhibitor of corrosion of aluminium alloys in chloride solutions. This paper describes the results of further studies using electrochemical and constant immersion corrosion tests to compare the effectiveness of Ce(dpp) 3 and Mischmetal diphenyl phosphate Mm(dpp) 3 as inhibitors of corrosion pitting on AA7075-T651 aluminium alloy. The results shows that both Ce(dpp) 3 and Mm(dpp) 3 are excellent inhibitors of pitting corrosion of this alloy in very aggressive environments of continuously aerated 0.1M and 1.0M sodium chloride (NaCl) solutions. Polarisation tests indicate that these compounds act as a cathodic inhibitors by reducing the rate of the oxygen reduction reaction, which results in a decreased corrosion current density and a separation of the corrosion potential from the pitting potential. This inhibition is thought to be due to the formation of a surface film consisting of rare earth metal oxide, aluminium oxide and a cerium-aluminium organo-phosphate complex. Surface analysis data from scanning electron microscopy and X-ray Energy Dispersive Spectroscopy show the complex nature of this protective film. This work further develops our understanding about the mechanisms through which these complex films form, and how inhibition occurs in the presence of these compounds.

Evaluating the corrosion behaviour of magnesium alloy in simulated biological fluid by using SECM to detect hydrogen evolution

Scanning electrochemical microscopy (SECM) in surface generation/tip collection mode is investigated as an assessment tool for studying the corrosion behaviour of magnesium in simulated biological fluid. The technique provides a local map of hydrogen (H2) evolution which alone can be used as a direct measure of corrosion. The H2 generated during corrosion of magnesium is oxidized at the probe(i.e. a Pt ultra micro-electrode);with the magnitude of the current generated due to oxidation being indicative of the intensity of H2 evolution at a local scale on the magnesium surface. This method was calibrated using a cathodically polarized Pt disk to simulate H2 evolution in a controlled condition on a homogeneous surface. Potential interference from dissolving Mg or high local pH was also investigated. The technique was implemented for studying H2 evolution at the surface of AZ31 as a model Mg alloy.SECM results combined with SEM-EDX and profilometry data revealed that local domains of higher H2 evolution on the surface of AZ31 are in close proximityof the observed pitting sites.

A review of techniques for the monitoring of cathodic shielding and corrosion under disbonded coatings

The mitigation of external corrosion of energy pipelines by a combination of barrier coatings and Cathodic Protection (CP) is not always effective. Even when design specifications are properly met, the shielding of cathodic protection current from reaching steel surface by disbonded barrier coatings, often referred to as cathodic shielding, may lead to severe corrosion problems such as deep pitting, high and near neutral pH Stress Corrosion Cracking (SCC) and Microbiologically Induced Corrosion (MIC). Unfortunately, current indirect assessment methods used in the pipeline industry have serious difficulties in detecting such corrosion problems. This paper provides a brief review of current techniques and their limitations when being applied under complex buried pipeline environmental conditions. The main purpose is to identify potential methods that could be utilised in the design of new monitoring probes specific for the monitoring of cathodic shielding and corrosion of disbonded coatings in the pipeline industry.

An electrochemical method for measuring localized corrosion under cathodic protection

 A new method has been developed to measure metal corrosion rates and their distribution under cathodic protection (CP). This method uses an electrochemically integrated multi-electrode array to take local measurements of cathodic current density while simulating a continuous metallic surface. The distribution of cathodic current densities obtained under CP was analyzed to estimate the anodic current component at each electrode of the array. Corrosion patterns determined by this new method have shown good correlation with visual inspection and surface profilometry of the multi-electrode array surface.

An overview of new progresses in understanding pipeline corrosion

An approach to achieving the ambitious goal of cost effectively extending the safe operation life of energy pipeline to 100 years is the application of health monitoring and life prediction tools that are able to provide both long-term remnant pipeline life prediction and in-situ pipeline condition monitoring. A critical step is the enhancement of technological capabilities that are required for understanding and quantifying the effects of key factors influencing buried steel pipeline corrosion and environmentally assisted materials degradation, and the development of condition monitoring technologies that are able to provide in-situ monitoring and site-specific warning of pipeline damage. This paper provides an overview of our current research aimed at developing new sensors and electrochemical cells for monitoring, categorising and quantifying the level and nature of external pipeline and coating damages under the combined effects of various inter-related variables and processes such as localised corrosion, coating cracking and disbondment, cathodic shielding, transit loss of cathodic protection.

Monitoring cathodic shielding and corrosion under disbonded coatings

This work presents a novel corrosion monitoring probe designed for simulating the conditions developed under disbonded coatings and for measuring current densities and their distribution over a simulated pipeline surface. The probe’s concept was experimentally evaluated via immersion tests under Cathodic Protection (CP) in high resistivity aqueous solution. Under the disbonded area, anodic currents as well as cathodic currents were both measured. Anodic current densities were used to calculate metal losses by means of Faraday’s law. Calculated corrosion patterns were compared with corrosion damage observed at the probe’s surface after a period of test. The probe’s working principles are explained in terms of simple electrochemistry.

Microstructure and corrosion of AA2024

AA2024-Tx is one of the most common high-strength aluminium alloys used in the aerospace industry. This article reviews current understanding of the microstructure of sheet AA2024-T3 and chronicles the emergence of new compositions for constituent particles as well as reviews older literature to understand the source of the original compositions. The review goes on to summarise older and more recent studies on corrosion of AA2024-T3, drawing attention to areas of corrosion initiation and propagation. It pays particular attention to modern approaches to corrosion characterisation as obtained through microelectrochemical techniques and physicochemical characterisation, which provide statistical assessment of factors that contribute to corrosion of AA2024. These approaches are also relevant to other alloys.

Electrochemical method for studying localized corrosion beneath disbonded coatings under cathodic protection

This paper presents a new method for measuring localized corrosion under disbonded coatings by means of an electrochemical sensor, denoted differential aeration sensor (DAS). It measures the distribution of electrochemical currents over an electrode array surface partially covered by a crevice that simulates a disbonded coating. The DAS has been evaluated using immersion tests at open circuit and under cathodic protection (CP) conditions. Under both conditions, anodic as well as cathodic current densities were detected within the crevice. A fundamental understanding for the detection of anodic currents under CP has been explained in terms of basic electrochemistry. Based on the current distribution data provided by the sensor, two different analysis methods have been used to estimate corrosion and its distribution. These methods consisted of a direct application of Faraday's Law to the anodic currents detected by the array, and on a sensor-specific method denoted corrected currents' method. It has been demonstrated that under diffusion controlled conditions this latter method produces a better corrosion estimation than the direct application of Faraday's Law. The corrected currents' method allowed the estimation of corrosion patterns outside the crevice under CP. Good correlation between electrochemical calculations and surface profilometry results has been obtained.

Monitoring corrosion of reinforced concrete beams in a chloride containing environment under different loading levels

Corrosion has significant adverse effects on the durability of reinforced concrete (RC) structures, especially those exposed to a marine environment and subjected to mechanical stress, such as bridges, jetties, piers and wharfs. Previous studies have been carried out to investigate the corrosion behaviour of steel rebar in various concrete structures, however, few studies have focused on the corrosion monitoring of RC structures that are subjected to both mechanical stress and environmental effects. This paper presents an exploratory study on the development of corrosion monitoring and detection techniques for RC structures under the combined effects of external loadings and corrosive media. Four RC beams were tested in 3% NaCl solutions under different levels of point loads. Corrosion processes occurring on steel bars under different loads and under alternative wetting - drying cycle conditions were monitored. Electrochemical and microscopic methods were utilised to measure corrosion potentials of steel bars; to monitor galvanic currents flowing between different steel bars in each beam; and to observe corrosion patterns, respectively. The results indicated that steel corrosion in RC beams was affected by local stress. The point load caused the increase of galvanic currents, corrosion rates and corrosion areas. Pitting corrosion was found to be the main form of corrosion on the surface of the steel bars for most of the beams, probably due to the local concentration of chloride ions. In addition, visual observation of the samples confirmed that the localities of corrosion were related to the locations of steel bars in beams. It was also demonstrated that electrochemical devices are useful for the detection of RC beam corrosion.

Electrochemically monitoring localized corrosion patterns and CP effectiveness under disbonded coatings

© 2015 by Nace International. This paper presents new experimental evidences on the capability of a novel electrochemical corrosion monitoring sensor, which was recently conceived, for measuring localized corrosion under disbonded pipeline coatings. The sensor's design includes an artificial crevice for simulating the conditions developed under disbonded coatings and an electrode array for measuring current density distribution over its surface. The sensor capabilities were further evaluated by studying the dependency of corrosion patterns and current density distribution on the Cathodic Protection (CP) potential applied upon immersion in an aqueous environment. At the less negative CP potential, a good correlation was found between the inhomogeneous corrosion distribution under the disbonded coating as measured by the sensor and actual metal loss and corrosion attack observed on its surface at the end of the test. At more negative CP potentials no corrosion was detected or observed on the sensor's surface. In addition, characteristic changes in the cathodic current distribution at different CP potentials illustrated the possibility of employing the sensor to obtain valuable feedback on the performance of a given CP setup, without requiring its interruption or compensation of IR-drops. Furthermore, the sensor's capability to detect some of the effects of overprotection were shown at the most negative CP potential applied.

Structural health monitoring: a practical approach to achieving in-situ and site-specific warning of pipeline corrosion and coating failure

An approach to achieving the ambitious goal of cost effectively extending the safe operation life of energy pipelines to, for instance, 100 years is the application of structural health monitoring and life prediction tools that are able to provide long-term remnant pipeline life prediction and in-situ pipeline condition monitoring. A critical step in pipeline structural health monitoring is the enhancement of technological capabilities that are required for quantifying the effects of key factors influencing buried pipeline corrosion and environmentally assisted materials degradation, and the development of condition monitoring technologies that are able to provide in-situ monitoring and site-specific warning of pipeline damage. This paper provides an overview of our current research aimed at developing new sensors for monitoring, categorising and quantifying the level and nature of external pipeline and coating damages under the combined effects of various inter-related variables and processes such as localised corrosion, coating damage and disbondment, cathodic shielding. The concept of in-situ monitoring and site-specific warning of pipeline corrosion is illustrated by a case of monitoring localised corrosion under disbonded coatings using a new corrosion monitoring probe. A basic principle that underpins the use of sensors to monitor localised corrosion has been presented: Localised corrosion and coating failure are not an accidental occurrence, it occurs as the result of fundamental thermodynamic instability of a metal exposed to a specific environment. Therefore corrosion and coating disbondment occurring on a pipeline will also occur on a sensor made of the same material and exposed to the same pipeline condition. Although the exact location of localised corrosion or coating disbondment could be difficult to pinpoint along the length of a buried pipeline, the ‘worst-case scenario’ and high risk pipeline sections and sites are predictable. Sensors can be embedded at these strategic sites to collect data that contain ‘predictor features’ signifying the occurrence of localised corrosion, CP failure, coating disbondment and degradation. Information from these sensors will enable pipeline owners to prioritise site survey and inspection operations, and to develop maintenance strategy to manage aged pipelines, rather than replace them.

Corrosion behavior of Ti-13Nb-13Zr alloy used as a biomaterial

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Corrosion behavior of Ti-6Al-4V alloy treated by plasma immersion ion implantation process

Ti-6Al-4V alloy is one of the most frequently used Ti alloys with diverse applications in aerospace and biomedical areas due to its favorable mechanical properties, corrosion resistance and biocompatibility. Meanwhile, its surface can stiffer intense corrosion caused by wear processes due to its poor tribological properties. Thus in the present study, PIII processing of Ti-6Al-4V alloy was carried out to evaluate its corrosion resistance in 3.5% NaCl solution. Two different sets of Ti-6Al-4V samples were PIII treated, varying the plasma gases and the treatment time. The corrosion behavior is correlated with the surface morphology, and the nitrogen content. SEM micrographs of the untreated sample reveal a typical two-phase structure. PIII processing promotes surface sputtering and the surface morphology is completely different for samples treated with N-2/H-2 mixture and N-2 only. The highest penetration of nitrogen (similar to 88 nm), corresponding to 33% of N-2 was obtained for the sample treated with N-2/H-2 mixture for 1:30 h. The corrosion behavior of the samples was investigated by a potentiodynamic polarization method. A large passive region of the polarization curves (similar to 1.5 V), associated with the formation of a protective film, was observed for all samples. The passive current density (similar to 3 x 10(-6) A cm(-2)) of the PIII-treated Ti-6Al-4V samples is about 10 times higher than for the untreated sample. This current value is still rather low and maintains good corrosion resistance. The anodic branches of the polarization curves for all treated Ti-6Al-4V samples demonstrate also that the oxide films break down at approximately 1.6 V, forming an active region. Although the sample treated by N-2/H-2 mixture for 1.30 It has thicker nitrogen enriched layer, better corrosion resistance is obtained for the PIII process performed with N, gas only. (c) 2007 Elsevier B.V. All rights reserved.