Потенціал в області мікронерівностей поверхні металу, який працює в режимі катодного захисту

The phenomenon of endurance limit enhancement, whereby the fatigue limit of notched structural steel in sea water is increased by cathodic polarization to above the value in air, has previously been disclosed. The magnitude of enhancement has been shown to vary with potential, specimen size and frequency; and the effect itself has been rationalized in terms of calcareous deposits which form within the crack and either reduce the effective stress intensity range or interfere with surface slip. Additional experiments are reported which bear upon occurrence of endurance limit enhancement in large components and members. The results are discussed with regard to welded tubular joints of offshore structures, and a cathodic protection system design is proposed which takes advantage of endurance limit enhancement in such applications. INTRODUCTION Virtually all structural materials employed in ocean applications experience cyclic stresses, and so the problem of sea water corrosion fatigue is far-reaching in its consequences. A specific example is fixed offshore structures in deep water or rough sea exposures, where design for adequate fatigue resistance has been viewed as integrity limiting.1,2 In development of engineering systems five generalized techniques or tools have evolved historically for prevention or control of corrosion. These are 1) design for corrosion control, 2) proper materials selection, 3) cathodic (or anodic) protection, 4) coatings and 5) inhibitors. Each of these has been shown to be important with regard to corrosion fatigue also.3 Proper materials selection, although often overlooked, is fundamentally important; however, for economic reasons carbon steel is the only realistic choice in numerous sea water applications. On this basis cathodic protection may be the most effective and practical technique for corrosion fatigue mitigation in submerged exposures. A unique aspect of cathodic protection of metals in sea water is occurrence of calcareous deposits. These have been judged to be particularly significant with regard to the economics and effectiveness of this form of corrosion prevention,4,5 and it is timely that increased attention be focused upon them. CALCAREOUS DEPOSITS Either one or both of two reactions occurs at cathodic sites of a metal surface in sea water. These are: (Available in full paper) The relative importance of each of these in comprising the net cathodic current should depend upon dissolved oxygen availability, pH and potential. Since the hydroxyl ion is a product of both reactions, pH of the electrolyte immediate to the metal-sea water interface is increased in response to the cathodic current. The solubility of inorganic compounds in sea water generally decreases with increasing pH, and so solubility limit of these is exceeded immediate to the metal surface and precipitation occurs there. Humble6 showed that chemical make-up of calcareous deposits is primarily CaCO3 and Mg(OH) 2 and that the relative proportion of each is a function of cathodic current density. However, despite the generalized significance of calcareous deposits to cathodic protection, little information is available with regard to how calcareous deposits are influenced by such fundamental variables as sea water velocity, temperature and chemical and physical nature of the metal surface.

SRB-mediated corrosion of marine submerged AISI 1020 steel under impressed current cathodic protection

One way to potentially reduce the costs of concentrated solar power (CSP) central receiver systems is to operate the central receiver at high temperatures that allow for more efficient thermodynamic power conversion cycles. Molten salts are one potential heat transfer fluid at that can operate efficiently at these temperatures, but their use will introduce other technological challenges, such as corrosion and mass transfer reactions. The corrosion of alloys at high-temperature (700-1000°C) in the receivers and heat exchangers cause a reduction in heat transfer efficiency and durability. Cathodic protection may mitigate corrosion of metal surfaces by shifting the potential of the alloy below its oxidation potential. The behavior of molten salt cooled concentrated solar power (CSP) systems under cathodic protection can be obtained by developing a 3-D model. A simplified model was designed for and benchmarked against a thermosiphon reactor. This thermosiphon reactor exposed alloy coupons to non-isothermal conditions expected in concentrated solar power (CSP) plants. When the cathodic current density is equal to the protection current density and the potential is a protective, the model calculates the corrosion rate and the corrosion potential. Magnesium was used as a sacrificial anode. The rate of Mg consumption, the corrosion potential, and the corrosion current are dependent on several studied variables. Kinetic parameters were obtained from experimental coupon exposures conducted between 700-1000 °C in molten salt in the thermosiphon reactor. Results were in good agreement with experimental values at different conditions. Fig. 1 shows a comparison of the model prediction of corrosion rate distribution, i (A/cm2) at coupon surfaces for both Hot-Zone and Cold- Zone without (a) and with (b) cathodic protection. Acknowledgements The authors gratefully acknowledge the financial support for this work from the DOE EERE SunShot Initiative (DE-AC36-08GO28308) under a subcontract from SRNL to the University of South Carolina. The authors also thank the University of South Carolina Center for Fuel Cells and the CD-adapco group for their support. Figure 1

Carbon steel pipelines are provided with corrosion prevention systems, namely an insulating coating and a cathodic protection (CP) system that reduces corrosion rate below 10 μm/a. CP is applied by a stationary cathodic current, which effectiveness on the metal surface is twofold: oxygen consumption and alkalinization (pH > 10) at the metal-to-electrolyte interface. The increase of pH is beneficial, promoting passive condition. In this paper, a preliminary study of the effect of intermittent CP has been carried out in order to investigate the effect of a temporary current interruption on the potential monitoring and on the residual corrosion of the metal. During the on period, oxygen is consumed and alkaline pH is established, during off period, the alkalinity and the slow oxygen replacement assure corrosion rates lower than in free corrosion condition. Test has been performed both in normal and overprotection condition, varying the off period duration, monitoring weekly the protection potential.

Self-powered metal surface anti-corrosion protection using energy harvested from rain drops and wind

Repaired reinforced concrete structures often suffer from a reinforcement corrosion problem known as the incipient anode (or halo) effect. The diagnosis of this problem is widely reported to be macrocell activity caused by small changes in reinforcement potential in the parent and repair concrete. Cathodic protection is a technique that is used to prevent or arrest such corrosion by inducing small potential changes on the reinforcement. This study looks at the effects potential in both causing and arresting corrosion in concrete. Potential measurements on repaired structures suggest that potential changes on passive steel cannot cause corrosion. Further, the protective effects of a negative steel potential shift induced by the relatively small practical current densities applied in cathodic protection are negligible when oxygen has easy access to the reinforcement. Other factors are suggested to affect both of these processes.

Prestressed Concrete steel Cylinder Pipe (PCCP) is extensively used as seawater pipes for cooling in nuclear power plants. The internal surface of PCCP is exposed to seawater, while the external surface is in direct contact with underground soil. Therefore, materials and strategies that would reduce the corrosion of its cylindrical steel body and external steel wiring need to be employed. To prevent against the failure of PCCP, operators provided a cathodic protection to the pre-stressing wires. The efficiency of cathodic protection is governed by the anodic performance of the system. A mixed metal oxide (MMO) electrode was developed to meet criteria of low over potential and high corrosion resistance. Increasing coating cycles improved the performance of the anode, but cycling should be minimized due to high materials cost. In this work, the effects of RuCl3 concentration on the electrochemical properties and lifespan of MMO anode were evaluated. With increasing concentration of RuCl3, the oxygen evolution potential lowered and polarization resistance were also reduced but demonstrated an increase in passive current density and oxygen evolution current density. To improve the electrochemical properties of the MMO anode, RuCl3 concentration was increased. As a result, the number of required coating cycles were reduced substantially and the MMO anode achieved an excellent lifespan of over 80 years. Thus, we concluded that the relationship between RuCl3 concentration and coating cycles can be summarized as follows: No. of coating cycle = 0.48*[RuCl3 concentration, M] -0.97 .

Primers are customarily used to prevent further corrosion of rebar during the repair process for corrosion-damaged concrete structures on the coast of Yucatán. However, there are not enough data about their effectiveness in this environment. This work presents laboratory data of mortar specimens where primers to the reinforcement were used as they are currently in localized repairs in tropical marine coasts. Four different, locally available primers were used according to their hypothetical protection mechanism (barrier, cathodic protection, inhibition, and repassivation). Primers were applied to the rebars and then embedded in mortar and subjected to two exposure conditions: partial immersion to different solutions of sodium chloride (NaCl; 0.4, 2.0, and 5.0%) and wet and dry cycles (12 h wet and 12 h dry) to a NaCl solution (3.5%) at 50°C. The electrochemical behavior of the specimens was followed through polarization resistance, corrosion potential, and electrical resistance. In practical terms, the use of primers proved to be a good option for economical and fast repairs or in cases where concrete was of bad quality (for example, concrete with a water-to-cement [w/c] ratio higher than 0.5). A degree of primer effectiveness was obtained according to the electrochemical behavior of the repair. Barrier-type primers were more degradation-resistant than the others. Although primers proved to be a reasonable option for localized repair, future behavior must be followed carefully.

Designing in-house cathodic protection system to assess the long-term integrity of natural gas pipelines

AC interference on metallic structures has been a major challenge to the operational services of utilities around the world. As a result of frequent installation of metallic pipelines in the energy utility corridor of transmission lines, AC interference problem is prominent. A major research area is the development of an optimum cathodic protection (CP) potential for protecting the pipeline in the presence of AC. In this study, an investigation on the influence of AC potential variation on the stability nature of some CP potentials was conducted using a small experimental test beds derived from the real situation of pipeline-power line right of way. A promising result was observed when a CP of −1150 mV is applied to the steel pipe specimen. At this CP potential, a better stability is observed, among other potentials, under the presence of the same varying AC voltages. Detailed graph is presented herein which could be useful for utility engineers.

The questions of organization of cathodic protection against corrosion of pipelines laid by the «pipe in pipe» method are considered. The variants of the organization of the cathodic corrosion system for pipelines laid by this method are determined. A laboratory stand simulating a pipe-in-pipe gasket with the installation of coppersulphate electrodes of comparison of small size between the pipes has been prepared. Laboratory studies to verify the protection of the main pipeline under various schemes of organization of cathodic corrosion protection were carried out. The conditions that must be observed when laying the pipeline to ensure the operation of the cathodic protection system, the main ones are the presence of electrolyte in the medium between the pipelines, as well as the absence of direct electrical contact between the pipelines. Negative features of joint protection of both pipelines are revealed. The optimal schemes of organization of cathodic corrosion protection during pipeline laying by this method, which are schemes with the use of extended anode grounding between pipelines or schemes without cathodic protection of external pipe, are determined. Solutions are proposed to eliminate the «failure» of the protective potential in contact with the metal between the pipelines, which consists in laying an extended anode grounding and local displacement of the potential of the internal pipeline relative to the external one.

In vitro corrosion behaviour and metallic ion release of different prosthodontic alloys

In general, the hydrocarbon transporting pipelines are protected against external corrosion using coatings and with cathodic protection systems. However, even with this protection, there have been some failures due to external corrosion. The soil and environmental factors can be very important, because they may increase the conditions for some microorganisms to grow, including sulfate reducing bacteria (SRB), which play a very important role during the microbiologically influenced corrosion processes. The SRB can grow under the pipeline coating, causing a depolarization effect and increasing the corrosion rate, even if the pipeline is cathodically protected. This is due mainly to the bacteria action, that forms a dielectric film and the cathodic protection can not reach the metal surface. The pipelines located in the southeast of Mexico are buried and/or immersed, and microbial analysis of the surrounding environment (soil, mud and water) have shown that microorganisms, including the SRB, can grow easily next to the pipelines, and the metal surface may be exposed to microbiologically influenced corrosion processes, which can occur easily. According to these observations, it is very important to identify the possible effect of these microorganisms on the pipelines integrity. In this work, typical d.c. laboratory techniques were carried out, along with field and laboratory weight loss experiments, in order to study the influence of the Sulfate Reducing Bacteria (SRB) on the cathodic protection system of the pipelines that transport hydrocarbon from Atasta to Cd. PEMEX, in México. The results were complemented with some surface examination.

The localized corrosion susceptibility of stainless steels, such as type 316L, immersed in natural seawater is higher than those in synthetic seawater. It is widely observed that the corrosion potential of stainless steels in natural seawater becomes noble over +0.3V vs SCE. This phenomenon is caused by the influence of the microorganisms in the natural seawater, and therefore it is considered as Microbially Influenced Corrosion (MIC). In this paper, the laboratory reproduction of the corrosion potential ennoblement was examined on the basis of the metabolize of aerobic bacteria (pH, dissolved oxygen and metabolite). The electrochemical measurements in natural seawater and synthetic seawater with bacterial mediums were carried out. The corrosion potential of stainless steels became noble in only natural seawater, and cathodic current density increased around corrosion potential, these results suggest that the presence of oxidizer on the metal surface which has higher redox potential than that of oxygen. As the corrosion potential became noble during the logarithmic phase of bacterial growth in the result of the examination with isolated bacteria, it is suggested that the aerobic metabolism of bacteria effects the ennoblement of corrosion potential. The ennoblement was also reproduced in laboratory by the addition of oxidase simulated the metabolism of aerobic bacteria. The oxidase produced the peroxide by the reduction of oxygen which has higher redox potential than that of oxygen, therefore it is considered that the corrosion potential ennoblement is caused by the peroxide generated by the metabolism of aerobic bacteria included in the biofilm formed on the surface of the stainless steels in natural seawater.

Multi-walled carbon nanotubes decorated polyaniline- α-Fe2O3 nanocomposite as an efficient photoanode for photoelectrochemical water splitting and photoelectrochemical cathodic protection of mild steel