海洋环境下抗阴极剥离重防腐涂层的研究

海洋环境中浸泡在海水中的涂层由于阴极保护而产生的剥离是涂层失效的主要原因之一。针对这种失效现象,讨论了影响涂层抗阴极剥离的主要因素并研制了适应于阴极保护的海洋浸泡涂层。涂料包含不同环氧值的环氧树脂基料、烷基酚醛胺固化剂以及相适应的颜填料。通过调整涂料的颜基比,得到抗阴极剥离性能优异的涂层,涂层能够满足海洋环境中涂层与阴极保护相结合使用的需要。     

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.     

Disbondment Mechanisms for a Heat Shrink Polyethylene Coating on a Cathodically Protected Hot Pipeline

Interfacial adhesive failure of a pipeline coating was found to be related to the operating temperature of the pipeline, the presence of moisture at the coating/steel interface and cathodic polarisation of the steel. The application of cathodic protection was found to be more detrimental to a pipeline coating than was the immersion of the coating specimens in alkaline environments without polarisation. It is suggested that in the system examined cathodic disbondment is initiated at a coating holiday by the electrochemical reduction of Fe₃O₄ in the interfacial oxide film and that propagation of the disbondment is associated with electrocapillary action which reduces the surface tension between the steel and the crevice solution. This process increases the thermodynamic disbonding force between the adhesive and the steel in the aqueous environment.     

Disbondment Mechanisms for a Heat Shrink Polyethylene Coating on a Cathodically Protected Hot Pipeline

Interfacial adhesive failure of a pipeline coating was found to be related to the operating temperature of the pipeline, the presence of moisture at the coating/steel interface and cathodic polarisation of the steel. The application of cathodic protection was found to be more detrimental to a pipeline coating than was the immersion of the coating specimens in alkaline environments without polarisation. It is suggested that in the system examined cathodic disbondment is initiated at a coating holiday by the electrochemical reduction of Fe₃O₄ in the interfacial oxide film and that propagation of the disbondment is associated with electrocapillary action which reduces the surface tension between the steel and the crevice solution. This process increases the thermodynamic disbonding force between the adhesive and the steel in the aqueous environment.     

Electrocatalysis of oxygen evolution at stainless steel anodes by electrosynthesized cobalt hydroxide coatings

Controlled growth of cobalt hydroxide coatings (1–2 m thick) on stainless steel electrodes was achieved by electrogeneration of base by cathodic reduction of aqueous cobalt nitrate solutions. These coatings catalyse the oxygen evolution reaction in alkaline medium (1 M KOH) by as much as 100 mV, making cobalt hydroxide a candidate catalyst for these applications.     

An electrochemical study of the dissolution of gold in thiosulfate solutions Part I: Alkaline solutions

The anodic dissolution of gold in alkaline thiosulfate solutions has been studied by using a rotating gold disc. Experimental results have shown that the gold dissolves at measurable rates in thiosulfate solutions at potentials above 0.2 V. It has been confirmed that dissolution occurs in parallel with oxidation of thiosulfate with a dissolution current efficiency that varies with time and with the experimental conditions and which is generally in the range of 0.3 to 0.6. Although oxygen could be used as an oxidant for gold in the thiosulfate system, the rate of the cathodic reduction of oxygen in the relevant potential region is too slow for practical purposes. It was found that in the potential region investigated, thiosulfate ions undergo oxidative decomposition leaving a sulfur-like film on the surface of gold, which inhibits the rate of dissolution of gold and results in a low anodic current efficiency for the dissolution of gold. The factors (temperature, pH, thiosulfate concentration and ammonia) have been found to have positive effects on the kinetics of gold dissolution. The rates of gold dissolution in oxygenated alkaline thiosulfate solutions have been estimated to be much lower than in the copper–ammonia–thiosulfate and cyanide systems.     

Evaluation of thick organic coatings degradation in seawater using cathodic protection and thermally accelerated tests

Organic coatings are often associated with cathodic protection to fight against the corrosion of metallic structures when immersed in seawater. However, cathodic protection leads to the generation of a strong alkalinity at the metal/coating interface, which causes the degradation of the coating. It is then necessary to develop a reliable method to evaluate the compatibility between organic coatings and the application of cathodic protection.

On one hand, cathodic disbonding tests (ASTM G-8 and G-80) can be driven with an artificial defect but this defect is mainly responsible for the electrochemical response. In addition, calcareous deposit rapidly forms onto the defect zone when cathodic protection is applied which can make difficult the evaluation of coating delamination. On the other hand, immersion of defect-free specimens requires very long testing periods (several months or even years) in order to detect the coating degradation.

In this work, an attempt to accelerate the coatings degradation by imposing a high temperature and thermal cycles were made in order to decrease the test-time duration. The influence of the applied cathodic potential was also investigated. The coating degradation was evaluated by EIS, considering the defect-free zone of coatings. It was shown that the coating degradation is faster in the presence of a defect and for high temperature (45 °C). Moreover, thermal cycles allow to greatly accelerate the degradation of defect-free coatings and then to compare the compatibility of both coatings with cathodic protection.     

Electrochemical behaviour of iron and iron oxide thin films in alkaline (1 m KOH) aqueous solution: a voltammetry study for cathodic instability of coating/metal interface

The electrochemical behaviour of thin film metallic iron and the iron oxides in 1 m KOH anaerobic aqueous solution was studied by cyclic voltammetry technique. The material studied was of thin film (50 nm) iron supported on a glass substrate, and the oxide thermally formed on the iron film. The voltammetry of thin film iron on glass was similar to that of low carbon steel and gave a convenient means for preparing specimens for obtaining information relevant to the degradation of structural steel. In this report, the formation of various types of oxides on the iron surface are identified, and the electrochemical stability of the oxides and their relevance to the cathodic disbonding of organic coating from cathodically protected steel pipelines are discussed.     

Cathodic disbonding of a thick polyurethane coating from steel in sodium chloride solution

The cathodic disbonding of a thick, pigmented polyurethane coating from steel in 3.5 wt.% NaCl solution was studied by using an electrochemical AC impedance technique. Double-cylinder electrolyte cells were designed to separate the measurements of cathodic disbonding process from the influence of the impedance of an artificial defect. It was found that for a thick, pigmented polyurethane coating, the more important transport pathway of the reactive species is along the coating/steel interface rather than through the coating. There existed a delay time for the cathodic disbonding process, and cathodic polarization was not a predominant factor in determining the cathodic disbonding behavior in the early stages. The thick polyurethane coating, which was applied on a well sand-blasted steel surface, had excellent resistance to cathodic disbonding.     

Cathodic disbondment of epoxy coating with zinc aluminum polyphosphate as a modified zinc phosphate anticorrosion pigment

As an approach to improve the resistance of protective coatings to the disbondment, modification of the formulation through incorporation of zinc aluminum polyphosphate anticorrosion pigment representing third generation phosphates was examined in this paper. The data obtained from cathodic disbonding test, electrochemical impedance spectroscopy and pull-off indicated that introduction of zinc aluminum polyphosphate within epoxy coating could provide improved resistance to cathodic disbonding as well as superior adhesion strength. The superiority in the presence of the modified pigment was connected to deposition of a layer at the disbonding front and locally controlled pH as well. The precipitation restricting active zone available for electrochemical reaction was confirmed by SEM.     

Localized EIS characterization of corrosion of steel at coating defect under cathodic protection

Localized electrochemical impedance spectroscopy (LEIS) technique was used to investigate localized corrosion of steel at defect of coating and, furthermore, to determine the effects of cathodic protection (CP) on local electrochemical environment and the resultant corrosion reaction at the base of coating defect. The results demonstrated that corrosion of steel is dependent on CP potential and the defect geometry. For coated steel containing defect under appropriate CP potentials, cathodic reaction is dominated by reduction of oxygen. Mass-transfer of oxygen through solution layer and the defect with a narrow, deep geometry (the depth/width ratio is about 5.5) is the rate-limiting step for the corrosion process of steel. Even at a very negative potential, e.g., −1200 mV (SCE), the measured impedance spectroscopy is still associated with the diffusion-controlled charge-transfer reaction of steel at the base of defect. It is attributed to the fact that the applied CP is partially shielded by the defect with a narrow, deep geometry. With the negative increase of cathodic potential, charge-transfer resistance and the local impedance of electrode increase. It is attributed to the elevation of local alkalinity at the base of defect when the coated steel is under CP. This conclusion is subject to the condition that a significant cathodic disbonding of coating has not occurred. Furthermore, with the increase of test time, the charge-transfer resistance increases, which is attributed to the enhancement of the alkaline environment at the base of defect under CP with time.     

Reduction of cathodic delamination rates of anticorrosive coatings using free radical scavengers

Cathodic delamination is one of the major modes of failure for anticorrosive coatings subjected to a physical damage and immersed in seawater. The cause of cathodic delamination has been reported to be the result of a chemical attack at the coating–steel interface by free radicals and peroxides formed as intermediates in the cathodic reaction during the corrosion process. In this study, antioxidants (i.e., free radical scavengers and peroxide decomposers) have been incorporated into various generic types of coatings to investigate the effect of antioxidants on the rate of cathodic delamination of epoxy coatings on cold rolled steel. The addition of <5 wt% free radical scavengers to epoxy coatings improved the resistance toward cathodic delamination by up to 50% during seawater immersion, while peroxide decomposers had a limited effect. Testing using substrates prepared from stainless steel, copper, aluminum, galvanized steel, and brass also showed a reduction in the rate of cathodic delamination when the coating was modified with a free radical scavenger. The protective mechanism of free radical scavengers investigated for the primers are similar to that of antioxidants used for protection against photochemical degradation by UV-radiation of top coatings. Both substrate corrosion and degradation of a coating exposed to UV-radiation lead to the formation of free radicals as reactive intermediates.     

Anodic behaviour of arsenopyrite and cathodic reduction of ferrate(VI) and oxygen in alkaline solutions

This paper presents the results of an electrochemical study of the anodic characteristics of arsenopyrite in strongly alkaline solutions and of the cathodic reduction of ferrate(VI) and of dissolved oxygen at an arsenopyrite surface at potentials which are relevant to the oxidation reactions. Cyclic voltammetry at both arsenopyrite disc and arsenopyrite disc/platinum ring electrodes has shown that arsenic(III) is the main product of the anodic process at potentials in the region of the rest potential during oxidation by either ferrate(VI) or oxygen. Evidence for partial passivation of both the anodic and cathodic reactions has been obtained from potentiostatic current–time transients. The initial stage of oxidation by ferrate(VI) has been shown to be mass-transport controlled and this is also true of the oxidation by oxygen in dilute solutions of sodium hydroxide.     

Disbonding of Epoxy Resin Coatings

Measurements are reported of the rate of cathodic disbonding of 2 ethyl, 4 methyl imidazole cured epoxy resin coatings with variation of the electrolyte system. Parallel studies of the elemental distribution in the disbonding region, using EDAX§ indicate that significant permeation of electrolyte occurs ahead of the detached zone. The changes of rate of cathodic disbonding are discussed with respect to the changes in the type of electrolyte being used.     

Disbonding of Epoxy Resin Coatings

Measurements are reported of the rate of cathodic disbonding of 2 ethyl, 4 methyl imidazole cured epoxy resin coatings with variation of the electrolyte system. Parallel studies of the elemental distribution in the disbonding region, using EDAX§ indicate that significant permeation of electrolyte occurs ahead of the detached zone. The changes of rate of cathodic disbonding are discussed with respect to the changes in the type of electrolyte being used.     

Electrooxidation of 1024 mild steel in slightlyalkaline phosphate and bicarbonate solutions

The electrochemical behaviour of 1024 mild steel electrodes was investigated in the presence of 0.05–0.5m disodium phosphate aqueous solution, in the pH range 8.4 to 9.7 and from 5 to 50°C. Bicarbonate solutions were also considered for comparison. Voltammograms were obtained with a rotating gold ring steel disc electrode, and the effects of the phosphate concentration, pH, electrode rotation speed, anodic potential limit and temperature were considered. For potential sweeps in the anodic direction in phosphate solutions, the voltammograms display an oxidation peak current at low potentials and a passivity region at higher potentials. The rate-determining step of the oxidation peak current was also investigated. The oxidation current in the passivity region is virtually independent of the applied potential, temperature, pH and phosphate concentration.     

浅谈埋地管道的防腐设计

埋地钢管外壁腐蚀是引起管道穿孔发生事故的主要原因之一,研究管道的腐蚀机理与防护技术,对延长管道的使用寿命以及保证工业生产的顺利进行具有重要意义。本文通过埋地管道腐蚀问题分析,介绍管道表面处理方法、管道内外防腐蚀涂层设计和管道阴极保护设计。     

地下钢制管道阴极保护技术现状与应用

针对地下钢质管道腐蚀新情况、新问题,从延长油气输送管道的使用寿命角度出发,详细分析钢质管道腐蚀形态、腐蚀机理、防腐方案;以某地区埋地钢质管道为例,主要采用阴极保护和防腐涂层联动保护体系,是一种进行钢制管道防腐的有效方法,并得到了牺牲阳极、强制电流和阳极井等阴极保护技术相应各项指标和要求,以及对阴极保护技术系统管理经验,为类似的地下钢质管道防腐提供了技术支持。同时通过介绍国内外阴极保护技术研究现状,指出阴极保护与强电干扰软件的开发将是我国现阶段发展方向。     

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.     

Inhibition of corrosion of mild steel by homopolymer and bilayer coatings of polyaniline and polypyrrole

Homopolymers and bilayers of polyaniline (PAni) and polypyrrole (PPy) coatings have been electropolymerized on mild steel by potentiodynamic synthesis technique in aqueous oxalic acid solutions. Characterization of coatings was carried out by cyclic voltammetry. Corrosion behavior of the polymer coated mild steel electrodes was investigated using linear anodic potentiodynamic polarization, Tafel test and electrochemical impedance spectroscopy (EIS) techniques in various aqueous corrosive solutions. Corrosion test results clearly showed that PPy and PAni/PPy bilayer coatings served as a stable host matrix on mild steel against corrosion. However, bilayers of PAni and PPy did not exhibit good combined properties of each polymer, unlike expected. PPy exhibited the best corrosion resistance among all coatings.