Studying inhomogeneity of organic coatings using wire beam multielectrode and physicomechanical testing

The effect of thickness, curing temperature and solvent on the structural inhomogeneity of several types of organic coatings has been investigated. The local distribution of electrochemical resistance was studied using a wire beam multielectrode array, while the inhomogeneity at a larger scale was examined using ∼3 cm2 area detached coatings. Physicomechanical tests were also employed to address the structural changes occurring because of variation in curing temperature and the type (or absence) of solvent. Results acquired by wire beam electrode and from the detached coatings showed that increasing the thickness and curing temperature improves the homogeneity of the coating as does elimination of solvent. Waterborne coatings exhibited a relatively homogeneous low resistance with a resistance lower than the threshold required for effective corrosion protection. Results of physicomechanical examination suggest that unreacted functional groups and water absorption are of the main causes of formation of structural defects in organic coatings

Atomically Thin Hexagonal Boron Nitride Nanofilm for Cu Protection: The Importance of Film Perfection

Coatings are routinely applied to protect metallic surfaces, and polymer coatings have been conventionally used where the thickness is not a dramatic issue.[1] For the next generation of nanoelectronics, nanoscale coatings are needed to accommo-date the compact design. 2D materials that can be fabricated into atomically thin film as a coating over the substrate can be a great choice. Graphene has recently been considered for this purpose, since it is robust and flexible, and the hexagonal hon-eycomb structure can effectively block any species, including helium.[2] Mixed results, however, have been reported.[3-7] Good short-term anti-corrosion performance was observed,[3-5] but over time, accelerated Cu oxidation and corrosion in air were found in the presence of graphene compared to the bare Cu substrate.[8,9] This acceleration is likely due to the high con-ductivity that assists electron transfer in the two-component galvanic cell between Cu and graphene, facilitating oxygen reduction and Cu oxidation around the defects in the long run

Electrical stimulation using conductive polymer polypyrrole promotes differentiation of human neural stem cells: a biocompatible platform for translational neural tissue engineering

Conductive polymers (CPs) are organic materials that hold great promise for biomedicine. Potential applications include in vitro or implantable electrodes for excitable cell recording and stimulation, and conductive scaffolds for cell support and tissue engineering. Here we demonstrate the utility of electroactive CP Polypyrrole (PPy) containing the anionic dopant dodecylbenzenesulfonate (DBS) to differentiate novel clinically relevant human neural stem cells (hNSCs). Electrical stimulation of PPy(DBS) induced hNSCs to predominantly β-III Tubulin (Tuj1) expressing neurons, with lower induction of glial fibrillary acidic protein (GFAP) expressing glial cells. In addition, stimulated cultures comprised nodes or clusters of neurons with longer neurites and greater branching than unstimulated cultures. Cell clusters showed a similar spatial distribution to regions of higher conductivity on the film surface. Our findings support the use of electrical stimulation to promote neuronal induction and the biocompatibility of PPy(DBS) with hNSCs, and opens up the possibility of identifying novel mechanisms of fate determination of differentiating human stem cells for advanced in vitro modelling, translational drug discovery and regenerative medicine

Developing scanning electrochemical microscopy method for studying bio-corrosion of magnesium alloys and protective surface treatments

Ways of implementing scanning electrochemical microscope (SECM) for studying corrosion and degradation of Mg in simulated biological solution has been studied in this thesis. SECM was first employed in more conventional mode of operation, namely feedback mode, to provide a baseline for comparison with other imaging modes of SECM. Four modes of SECM were in total exploited including feedback, AC, potentiometric and surface generation/tip collection modes. In feedback and AC modes electro-activity and insulating properties of surface were examined at a local scale. In the AC mode variation in conductivity of solution that was caused by corrosion reactions was also examined. A solid-state pH sensing microelectrode based of iridium/iridium oxide was fabricated and used in potentiometric mode of SECM for measuring the near surface pH during corrosion of Mg. A new method of sensing H2 at a solid surface has been introduced herein which is implemented to evaluate the H2 evolution during corrosion of Mg as a tool for acquiring mechanistic information about the corrosion process as well as measuring H2 evolution as an indicator of corrosion rate. Also a new Mg alloy named AZNd was specifically designed by Boston Scientific, funding body of this research, for use in fabrication of biodegradable coronary stents. Corrosion of AZNd in buffered media as well as effectives of surface treatments for protection against corrosion was investigated using SECM and other electrochemical and analytical methods. Chapter 4 and 5 of this thesis were case studies showcasing capabilities of SECM as a powerful characterization tool for studying corrosion of Mg and protective properties of surface treatments. In chapter 4 effect of protein (i.e. albumin) on the corrosion of Mg in buffered media was investigated and it was shown that presence of protein may reduce the corrosion rate in the early stages of contact with corrosive media via formation of a barrier layer on the surface of Mg. However in the long term presence of protein may be detrimental due to formation of inhomogeneous surface layer and/or chelation of metal ions thus promoting localized corrosion. In chapter 5 surface treatments based on praseodymium conversion coating and a biodegradable conducting polymer were applied on AZNd and corrosion protection afforded by these surface treatments were investigated

Theoretical analysis of electrochemical noise measurement with single substrate electrode configuration and examination of the effect of reference electrodes

The development of the single substrate technique presents a realistic possibility of utilising electrochemical noise measurement for on-site evaluations of corrosion behaviour. A theoretical model was developed to further understand the effects of reference electrodes on the acquired data to increase confidence in the technique. The model demonstrates that the reference electrodes can have a significant impact on the measurements, resulting in erroneous values if the reference electrodes are not selected carefully. Furthermore, the derived equations elucidate how to limit/remove such influence, facilitating accurate application of the single substrate technique on bare metal and coated substrates both in laboratories and in the field

An assessment of intrinsic noise of pseudo-reference electrodes and instrumental noise to enable reliable electrochemical noise measurements in situ on organically coated metal

Electrochemical noise measurement (ENM) is well-established as a standard test procedure for laboratory use to assist in assessing organic coatings. However, its non-destructive nature makes it particularly appealing for field use i.e. the monitoring of actual structures. A fundamental requirement for collecting valid electrochemical noise data in any situation is the use of a stable reference electrode that produces as little noise as possible. This is particularly important for organically coated substrates, where the currents being measured are very small due to attenuation of the signal by the high-impedance coating. Standard electrodes, e.g. the saturated calomel electrode, are not suitable for field measurements. In the present study, a method of assessing candidate electrodes and their noise characteristics is proposed leading to a suggestion for a suitable pseudo-reference electrode (PRE). An important conclusion is that when used as a PRE, the area of the electrode influences the noise characteristics. In general, the smaller the electrode the lower the current noise. But this has to be balanced against small electrodes having higher impedance, the latter being undesirable, so an optimization is needed

Steel surface preparation prior to painting and its impact on protective performance of organic coating

The influence of surface preparation on performance of alkyd coatings on steel was investigated. Degreasing, abrasion with emery, acid pickling, hydroblasting and wet abrasive blasting methods were used to modify the surface of steel before painting. Steel surfaces were analytically and morphologically characterized before painting. DC resistance and pull-off adhesion methods were used to evaluate the performance of alkyd coatings on differently prepared surfaces. Results implied the key role of oxide layer in maintaining the high level of corrosion protection. Also the detrimental influence of the inherent contamination caused by wet abrasive blasting method was shown as a cause of early failure of protective properties. Comparison between electrochemical and adhesion results showed that they may not indicate towards similar directions in terms of protective properties which is due to the different factors affecting them

Inhomogeneity of organic coatings and its effect of protection

It has been revealed that when metal is protected by an organic coating, corrosion proceeds through the ionic conduction pathways within the coating layer. An important aspect of polymeric coatings from the protection point of view is their inhomogeneous nature. Phenomena such as lack of cross-linking, inappropriate pigmentation, quick solvent evaporation etc may introduce local micro defects. The rate of diffusion of ions at these micro defects is proportional to the ionic concentration of the solution at which the coating is immersed and therefore they have been known as D or direct type areas. On the other hand much of the coating is composed of the highly resistive I type area where the ionic resistance increases with increasing the ionic concentration of environment. Traditionally it is believed that a certain level of adhesion is a fundamental requirement to avoid anodic and cathodic areas connecting underneath the coating and this adhesion will be reduced at D area Therefore the ionic resistance and specifically D type areas play an important role to determine the anti-corrosive properties of paint. In the present study a set of alkyd coatings in the form of detached coatings has been examined using DC technique to determine the ratio of D to I types. The significance of thickness, curing condition, solvent type, nano particle incorporation and multi-layer application on D area formation has been investigated. Subsequently a small group of these coatings were applied on steel panels and the long term corrosion behaviour was looked at. Also the adhesion strength of this group of coatings was examined by pull-off method. Results indicate that various factors affect the D to I ratio and these factors therefore will also influence the protection efficiency of the layer but is relatively insensitive to the adhesion strength. Copyright (2012) by the Australasian Corrosion Association

Inhomogeneity of organic coatings and its effect of protection

It has been revealed that when metal is protected by an organic coating, corrosion proceeds through the ionic conduction pathways within the coating layer. An important aspect of polymeric coatings from the protection point of view is their inhomogeneous nature. Phenomena such as lack of cross-linking, inappropriate pigmentation, quick solvent evaporation etc may introduce local micro defects. The rate of diffusion of ions at these micro defects is proportional to the ionic concentration of the solution at which the coating is immersed and therefore they have been known as D or direct type areas. On the other hand much of the coating is composed of the highly resistive I type area where the ionic resistance increases with increasing the ionic concentration of environment. Traditionally it is believed that a certain level of adhesion is a fundamental requirement to avoid anodic and cathodic areas connecting underneath the coating and this adhesion will be reduced at D area Therefore the ionic resistance and specifically D type areas play an important role to determine the anti-corrosive properties of paint. In the present study a set of alkyd coatings in the form of detached coatings has been examined using DC technique to determine the ratio of D to I types. The significance of thickness, curing condition, solvent type, nano particle incorporation and multi-layer application on D area formation has been investigated. Subsequently a small group of these coatings were applied on steel panels and the long term corrosion behaviour was looked at. Also the adhesion strength of this group of coatings was examined by pull-off method. Results indicate that various factors affect the D to I ratio and these factors therefore will also influence the protection efficiency of the layer but is relatively insensitive to the adhesion strength. Copyright (2012) by the Australasian Corrosion Association