Electrodeposition of corrosion resistant zinc alloy coatings

With the increase use of de-icing salts on roads for safety, the need for improved corrosion resistance of the traditional galvanized automobile bodies has never been greater. In the present work, Zn alloy coatings (Zn-Ni and Zn-Co) were studied as an alternative to pure Zn coatings. The production of these deposits involved formulation of various acidic (pH of about 5.5) chloride based solutions. These showed anomalous deposition, that is, alloys were deposited much more easily than expected from the noble behaviour of Ni and Co metals. Coating compositions ranging from 0 to about 37% Ni and 20% Co were obtained. The chemical composition of the coatings depended very much on the electrolytes nature and operating conditions. The Ni content of deposits increased with increase in Ni bath concentration, temperature, pH and solution agitation but decreased considerably with increase in current density. The throwing power of the Zn-Ni solution deteriorated as Ni metal bath concentration increased. The Co content of deposits also increased with increase in Co bath concentration and temperature, and decreased with increase in current density. However, the addition of commercial organic additives to Zn-Co plating solutions suppressed considerably the amount of Co in the coatings. The Co content of deposits plated from Zincrolyte solution was found to be more sensitive to variation in current density than in the case of deposits plated from the alkaline Canning solution. The chromating procedures were carried out using laboratory formulated solution and commercially available ones. The deposit surface state was of great significance in influencing the formulation of conversion coatings. Bright and smooth deposits acquired an iridescent colour when treated with the laboratory formulated solution. However, the dull deposits acquired a brownish appearance. The correlation between the electrochemical test results and the neutral salt spray in marine environment was good. Non-chromated Zn-Ni coatings containing about 11-14% Ni increased in corrosion resistance compared to pure Zn. Non-chromated Zn-Co deposits of composition 4-8% were required to show a significant improvement in corrosion resistance Corrosion resistance was improved considerably by conversion coating. However, the type of conversion coating was very important. Samples treated in a laboratory solution performed badly compared to those treated in commercial solutions. Zn alloy coatings were superior to pure Zn, the Schloetter sample (13.8% Ni) had the lowest corrosion rate, followed by the Canning sample (1.0% Co) and then Zincrolyte (0.3% Co).Neither the chromium content of the conversion films nor the chromium state was found to have an effect on corrosion performance of the coatings.

Influence of pretreatment on corrosion behaviour of duplex zinc/polymer coatings on steel substrates

An investigation has been undertaken to determine the major factors influencing the corrosion resistance of duplex-zinc coatings on steel substrates.Premature failure of these systems has been attributed to the presence of defects such as craters and pinholes in the polymer film and debonding of the polymer film from the zinc substrate.Defects found on commercially produced samples have been carefully characterised using metallographic and scanning electron microscopy techniques. The influence of zinc substrate surface roughness, polymer film thickness and degassing of conversion coatings films on the incidence of defects has been determined.Pretreatments of the chromate, chromate-phosphate, non chromate, and alkali-oxide types were applied and the conversion coatings produced characterised with respect to their nature and composition. The effect of degassing on the properties of the films was also investigated. Electrochemical investigations were carried out to determine the effect of the presence of the eta or zeta phase as the outermost layer of the galvanized coating.Flow characteristics of polyester on zinc electroplated hot-dip continuous and batch galvanized and zinc sprayed samples were investigated using hot-stage microscopy. The effects of different pretreatments and degassing after conversion coating formation on flow characteristics were determined.Duplex coatings were subjected to the acetic acid salt spray test. The effect on adhesion was determined using an indentation debonding test and the results compared with those obtained using cross-cut/peel and pull-off tests. The locus of failure was determined using scanning electron microscopy and X-ray photoelectron spectroscopy techniques.

Aspects of the prevention and repair of chloride-induced corrosion of steel in concrete

Sodium formate, potassium acetate and a mixture of calcium and magnesium acetate (CMA) have all been identified as effective de-icing agents. In this project an attempt has been made to elucidate potentially deleterious effects of these substances on the durability of reinforced concrete. Aspects involving the corrosion behaviour of embedded steel along with the chemical and physical degradation of the cementitious matrix were studied. Ionic diffusion characteristics of deicer/pore solution systems in hardened cement paste were also studied since rates of ingress of deleterious agents into cement paste are commonly diffusion-controlled. It was found that all the compounds tested were generally non-corrosive to embedded steel, however, in a small number of cases potassium acetate did cause corrosion. Potassium acetate was also found to cause cracking in concrete and cement paste samples. CMA appeared to degrade hydrated cement paste although this was apparently less of a problem when commercial grade CMA was used in place of the reagent grade chemical. This was thought to be due to the insoluble material present in the commercial formulation forming a physical barrier between the concrete and the de-icing solution. With the test regimes used sodium formate was not seen to have any deleterious effect on the integrity of reinforced concrete. As a means of restoring the corrosion protective character of chloride-contaminated concrete the process of electrochemical chloride removal has been previously developed. Potential side-effects of this method and the effect of external electrolyte composition on chloride removal efficiency were investigated. It was seen that the composition of the external electrolyte has a significant effect on the amount of chloride removed. It was also found that, due to alterations to the composition of the C3A hydration reaction products, it was possible to remove bound chloride as well as that in the pore solution. The use of an external electrolyte containing lithium ions was also tried as a means of preventing cathodically-induced alkali-silica reaction in concretes containing potentially reactive aggregates. The results obtained were inconclusive and further practical development of this approach is needed.

Corrosion and passivation of steel in concrete

A study of several chemical and electrochemical factors which affect the behaviour of embedded steel in cement pastes and concrete has been made. The effects of internal and external sources of chloride ions on the pore solution chemistry of Portland cement pastes, with and without additions of anodic corrosion inhibitors, have been studied using a pore solution expression device which has enabled samples of pore solution to be expressed from hardened cement pastes and analysed for various ionic species. Samples of pure alite and tricalcium aluminate have been prepared and characterised with respect to morphology, free lime content and fineness. Kinetics of diffusion of chloride ions in hardened pastes of alite and alite blended with tricalcium aluminate have been investigated and an activation energy obtained for the diffusion process in alite. The pore structures of the hardened pastes and the chloride ion binding capacity of alite have also been determined. Concrete cylinders containing embedded steel with four different surface conditions were exposed to various environments. The electrochemical behaviour of the steel was monitored during the period of exposure by means of rest potential measurements and the steel corrosion products analysed before and after being embedded. An examination was made of the nature of the interfacial zones produced between the embedded steel and cement. Rest potential measurements were monitored for steel embedded in alite paste in the presence of chloride ions and cement paste containing various levels of inhibitors in combination with chloride ions. In the latter case the results were supported by polarisation resistance determinations.

Corrosion protection of PVD and paint coatings for selective solar absorber surfaces

The selective solar absorber surface is a fundamental part of a solar thermal collector, as it is responsible for the solar radiation absorption and for reduction of radiation heat losses. The surface’s optical properties, the solar absorption (á) and the emittance (å), have great impact on the solar thermal collector efficiency. In this work, two coatings types were studied: coatings obtained by physical vapor deposition (PVDs) and coatings obtained by projection with different paints (PCs) on aluminum substrates. The most common industrial high performing solar selective absorbers are nowadays produced by vacuum deposition methods, showing some disadvantages, such as lower durability, lower resistance to corrosion, adhesion and scratch, higher cost and complex production techniques. Currently, spectrally selective paints are a potential alternative for absorbing surfaces in low temperature applications, with attractive features such as ease of processing, durability and commercial availability with low cost. Solar absorber surfaces were submitted to accelerated ageing tests, specified in ISO 22975-3. This standard is applicable to the evaluation of the long term behavior and service life of selective solar absorbers for solar collectors working under typical domestic hot water system conditions. The studied coatings have, in the case of PVDs solar absorptions between 0.93 and 0.96 and emittance between 0.07 and 0.10, and in the case of PCs, solar absorptions between 0.91 and 0.93 and emittance between 0.40 and 0.60. In addition to evaluating long term behavior based on artificial ageing tests, it is also important to know the degradation mechanism of different coatings that are currently in the market. Electrochemical impedance spectroscopy (EIS) allows for the assessment of mechanistic information concerning the degradation processes, providing quantitative data as output, which can easily relate to the kinetic parameters of the system. EIS measures were carried out on Gamry FAS2 Femostat coupled with a PCL4 Controller. Two electrolytes were used, 0.5 M NaCl and 0.5 M Na2SO4, and the surfaces were tested at different immersion times up to 4 weeks. The following types of specimens have been tested: Aluminium with/without surface treatment, 3 selective paint coatings (one with a poly(urethane) binder and two with silicone binders) and 2 PVD coatings. Based on the behaviour of the specimens throughout the 4 weeks of immersion, it is possible to conclude that the coating showing the best protective properties corresponds to the selective paint coating with a polyurethane resin followed by the other paint coatings, whereas both the PVD coatings do not confer any protection to the substrate, having a deleterious effect as compared to the untreated aluminium reference.

Preparation of Ni-P electroless and its corrosion resistance against sea water

Electroless coating is much appropriate process at engineering surface. Since the discovery of electroless nickel-phosphorus coating in 3996, due to technical and economic advantages as well as desirable characteristics such as resistance to corrosion, abrasion and a high hardness has found wide application at engineering industries. Properties of electroless nickel-phosphorus coating depend on the characteristics of the used bath and heat treatment. In this study, optimal conditions and concentration of sodium citrate, sodium acetate and lactic acid in the bath of electroless nickel-phosphorus coating to the steel ck67 surface was determined. Structure, chemical composition and phases occurring in the coating were investigated using Scanning Electron Microscope (SEM) and X-ray diffraction analysis (XRD). The corrosion behavior of coatings in solution 3.5٪ NaCl was studied using electrochemical. The results showed that corrosion resistance of the coating with composition sodium citrate, sodium acetate and lactic acid decreased respectively.

Galvanic corrosion of two non noble dental alloys

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Intergranular Corrosion and Stress Corrosion Cracking of Extruded AA6005A

A research program focused on understanding the intergranular corrosion (IGC) and stress corrosion cracking (SCC) behavior of AA6005A aluminum extrusions is presented in this dissertation. The relationship between IGC and SCC susceptibility and the mechanisms of SCC in AA6005A extrusions were studied by examining two primary hypotheses. IGC susceptibility of the elongated grain structure in AA6005A exposed to low pH saltwater was found to depend primarily on the morphology of Cu-containing precipitates adjacent to the grain boundaries in the elongated grain structure. IGC susceptibility was observed when a continuous (or semi-continuous) film of Cu-containing phase was present along the grain boundaries. When this film coarsened to form discrete Cu-rich precipitates, no IGC was observed. The morphology of the Cu-rich phase depended on post-extrusion heat treatment. The rate of IGC penetration in the elongated grain structure of AA6005A-T4 and AA6005A-T6 extrusions was found to be anisotropic with IGC propagating most rapidly along the extrusion direction, and least rapidly along the through thickness direction. A simple 3-dimensional geometric model of the elongated grain structure was accurately described the observed IGC anisotropy, therefore it was concluded that the anisotropic IGC susceptibility in the elongated grain structure was primarily due to geometric elongation of the grains. The velocity of IGC penetration along all directions in AA6005A-T6 decreased with exposure time. Characterization of the local environment within simulated corrosion paths revealed that a pH gradient existed between the tip of the IGC path and the external environment. Knowledge of the local environment within an IGC path allowed development of a simple model based on Fick's first law that considered diffusion of Al3+ away from the tip of the IGC path. The predicted IGC velocity agreed well with the observed IGC velocity, therefore it was determined that diffusion of Al3+ was the primary factor in determining the velocity of IGC penetration. The velocity of crack growth in compact tensile (CT) specimens of AA6005A-T6 extrusion exposed to 3.5% NaCl at pH = 1.5 was nearly constant over a range of applied stress intensities, exposure times, and crack lengths. The crack growth behavior of CT specimens of AA6005A-T6 extrusion exposed to a solution of 3.5% NaCl at pH = 2.0 exhibited similar behavior, but the crack velocity was ~10.5X smaller than that those exposed to a solution at pH =1.5. Analysis of the local stress state and polarization behavior at the crack tip predicted that increasing the pH of the bulk solution from 1.5 to 2.0 would decrease the corrosion current density at the crack tip by approximately 11.8X. This predicted decrease in corrosion current density was in reasonable agreement with the observed decrease in SCC velocity associated with increasing the solution pH from 1.5 to 2.0. The agreement between the predicted and observed SCC velocities suggested that the electrochemical reactions controlling SCC in AA6005A-T6 extrusions are ultimately controlled by the pH gradient that exists between the crack tip and external environment.

Electrochemical Oxidation Of Landfill Leachate In A Flow Reactor: Optimization Using Response Surface Methodology.

Response surface methodology based on Box-Behnken (BBD) design was successfully applied to the optimization in the operating conditions of the electrochemical oxidation of sanitary landfill leachate aimed for making this method feasible for scale up. Landfill leachate was treated in continuous batch-recirculation system, where a dimensional stable anode (DSA(©)) coated with Ti/TiO2 and RuO2 film oxide were used. The effects of three variables, current density (milliampere per square centimeter), time of treatment (minutes), and supporting electrolyte dosage (moles per liter) upon the total organic carbon removal were evaluated. Optimized conditions were obtained for the highest desirability at 244.11 mA/cm(2), 41.78 min, and 0.07 mol/L of NaCl and 242.84 mA/cm(2), 37.07 min, and 0.07 mol/L of Na2SO4. Under the optimal conditions, 54.99 % of chemical oxygen demand (COD) and 71.07 ammonia nitrogen (NH3-N) removal was achieved with NaCl and 45.50 of COD and 62.13 NH3-N with Na2SO4. A new kinetic model predicted obtained from the relation between BBD and the kinetic model was suggested.

Determinação da densidade de corrente de corrosão em meios de baixa condutividade: uso de microeletrodos para minimizar a queda ôhmica

The gravimetric and electrochemical tests are the most common techniques used in determining the corrosion rate. However, the use of electrochemical polarization is limited to electrolytes with sufficient conductivity for which Tafel curves are linear. In this study, we investigated a technique in which working microelectrodes of AISI 1020 steel were used to obtain the Tafel curves in diesel oil. The strategy was to reduce the electrode area and hence the ohmic drop. The diameter of the microelectrode was reduced to a value where the compensation of the Tafel curves became unnecessary. The results showed that for electrodes with diameters below 50 μm, the ohmic drop tends to a minimum and independent of the microelectrode diameter.

Estudo do comportamento eletroquímico de substâncias complexantes utilizando eletrodo de cobre

Copper electrode can be used for determination of complexing compounds through complexation reactions between Cu(II) and the analites. In this work some studies with three compounds were performed: glycine (precursor of glyphosate synthesis), herbicide glyphosate and aminomethylphosphonic acid (main metabolite of glyphosate). These compounds are complexing agents for Cu electrodes. Through simple experiments (cyclic voltammetry and corrosion studies) the applicability of the copper electrode as electrochemical sensor for complexing compounds in flow systems was presented.

Electrochemical behavior of parent and photodegradation products of some selected pesticides

Electrochemical behavior of pesticides is extensively studied, but little attention has been given to the study of their degradation products (by-products) by electrochemical methods. However, the degradation products of pesticides can be even more toxic then the parent products and such studies should be encouraged. Therefore, the objective of this work was to evaluate the electroactivity of by-products of imazaquin, methylparathion, bentazon and atrazine, generated by UV irradiation and measured using cyclic and differential pulse voltammetry and UV-visible absorption spectrophotometry. Results have shown that several by-products exhibit electroactivity, allowing, in some cases, the simultaneous determination of both parent and degradation products.