New technique for in-plant high mass transfer rate electrochemical potential monitoring

Abstract

Corrosion monitoring is widely used in boiling water reactors for hydrogen water chemistry and for feedwater monitoring on the secondary side of pressurised water reactors. It is well known that water chemistry conditions can change during transport in sampling lines and also that electrochemical potential (ECP) results at low oxidant levels depend strongly on the mass transfer rate. Measurements at a low mass transfer rate may yield abnormally low corrosion potentials that are not representative of the system. In-pipe monitoring has been developed to overcome this problem, but such measurements require penetrations of the system pressure boundary. A new ECP sensor has been developed for measurements in high mass transfer rate conditions (corresponding to linear flowrates up to 3 m s^?1 at a mass flowrate of 100 kg h^?1). The sensor can be installed without penetrating the pressure boundaries of the system, using existing sampling lines. It comprises well known parts that have been used for in-plant measurements in several reactors for many years. The new technique is also able to detect small amounts of oxidants in the system by measurements in sampling lines. Local water chemistry and corrosion conditions can be accurately monitored.     

Carbon recycling for zero-emission carbon-neutral transportation

Abstract

Converting current internal combustion engines to operate in closed-cycle, lean-burn mode with pure oxygen could generate an exhaust stream of >99% carbon dioxide. The ease of capture of this carbon, relative to the dilute CO₂ in current exhaust gases, points the way to a feasible route to zero-emission transportation. Prospects for implementing such an approach, which would include re-synthesis of fuel from the captured CO₂ and water using renewable power, are considered.     

Characterisation of oxide films formed on Alloy 600 in simulated PWR primary water

Abstract

Long term exposure of austenitic nickel based alloys in the primary water of pressurised water reactors leads to the development of an oxide film that is generally considered to influence the stress corrosion cracking behaviour of the alloys. The structure, composition and thickness of the film depend on the chemical composition of the alloy and the exposure conditions. Previous laboratory tests have indicated that zinc can lower the susceptibility to initiation of primary water stress corrosion cracking of Alloy 600. In the present work, oxide films have been grown on polished samples of Alloy 600 (15 × 20 × 1 mm) under simulated beginning of cycle primary water at 303°C for periods between 485 and 725 h and their morphology, chemical composition and corrosion behaviour examined. Layers oxidised under stable chemical conditions were less porous and showed higher corrosion resistance. Samples oxidised for longer times showed higher transpassive potentials. Initial results from electrochemical data showed an influence of zinc on the corrosion susceptibility of Alloy 600 that requires further evaluation.     

Initiation of SCC in nickel base alloys in primary PWR environment: studies at Studsvik since mid 1980s

Abstract

Investigations on initiation of primary water stress corrosion cracking (PWSCC) in alloys 600 and 690 in simulated primary pressurised water reactor (PWR) environments, carried out since 1985, are reviewed. A large number of tests, mostly on reverse U bend specimens, some from steam generator tubes from operating PWRs, have been conducted for periods up to 33 000 h (4 years). Most exposures were at 365 or 330°C in either high purity water with hydrogen addition or simulated primary water (representing beginning of cycle conditions). Parameters investigated include: material (composition, heat treatment (mill annealed or thermally treated), carbide distribution, yield strength, grain size, etc.), environmental (hydrogen content, boron–lithium–pH, zinc) and experimental environment control techniques. Although the PWSCC mechanism has yet to be fully explained, these studies provide an overview of important parameters for crack initiation. The present review aims to survey the evidence for PWSCC initiation in nickel base alloys. It is concluded that, within the normal range for operating PWRs, the influence of dissolved hydrogen on initiation is small. However, a weak maximum in crack growth rate is observed at 15–25 ml H₂/kg H₂O; a corresponding minimum in crack initiation time has not been confirmed but cannot be excluded. It is concluded that hydrogen must be reduced to well below 10 ml H₂/kg H₂O to achieve significant benefits. Increasing the Li content from 2–2·5 to 3·5 ppm is shown to reduce crack initiation time by 30–50%. Lithium in the range 2·2–7·5 ppm has little effect on crack growth rate at ～1200 ppm B. Boron appears to have little influence on crack initiation or growth.     

Monitoring of stress corrosion cracking of sensitised 304H stainless steel in nuclear applications by electrochemical methods and acoustic emission

Abstract

A hybrid monitoring technique for stress corrosion cracking (SCC) has been developed that employs simultaneously localised corrosion monitoring, electrochemical noise and acoustic emission (AE) techniques. The application of the hybrid technique for detection of SCC initiation and propagation in sensitised 304H stainless steel in dilute tetrathionate solutions at ambient temperature is reported. Initial result shows that SCC initiation and its early stage propagation can be detected by the localised corrosion monitoring and electrochemical noise methods. The dimensions of the crack can be estimated from the charge values derived from the detected transients. The locations of AE events determined using two sensors are in good agreement with the locations of cracks observed in the specimen. The AE technique is sensitive to rapid crack propagation, but does not appear to be sensitive to SCC initiation and early stage propagation for the present material environment load combination. It is postulated that AE is sensitive to SCC propagation involving a relatively large volume of plastic deformation. On the basis of test results and on information from the literature, it is suggested that in this material environment system SCC cracks initiate via slow anodic dissolution at the chromium depleted grain boundaries. Subsequently, elemental sulphur adsorbed on the surface around the crack tip catalyses the entry of hydrogen atoms produced by the hydrogen reduction reaction into the steel matrix ahead of the crack tip; this hydrogen accumulates gradually over a relatively long period of time and preferentially at carbide/matrix interfaces, eventually causing hydrogen induced brittle fracture along grain boundaries.     

Contribution to understanding of stress corrosion cracking of Alloy 600 in PWR primary water

Abstract

The stress corrosion cracking mechanism of Alloy 600 in pressurised water reactor (PWR) primary water has been investigated. U-bend specimens (extracted from tubular products) were tested in simulated PWR primary water containing H₂¹⁸O. After testing, crack tips was characterised by nanosecondary ion mass spectrometry (nanoSIMS) and unstressed areas were analysed by standard SIMS. The results allowed oxygen (¹⁶O, ¹⁸O) penetration into the material to be evaluated and the oxide present at crack tips to be characterised. Experimental data were compared with results obtained by calculations performed according to Fickian diffusion laws and Whipple Le Claire theory.     

Optimisation of HVOF thermal spray coatings for their implementation as MSWI superheater protectors

Abstract

The aim of this paper is to the study high velocity oxygen fuel thermal spray coatings as superheater tubes protectors against degradation problems in municipal solid waste incinerators. Ni based and Fe based high velocity oxygen fuel coatings have been sprayed through optimised spray parameters with the help of online monitoring technology. All these coatings have been tested under laboratory conditions simulating the boiler erosion and corrosion mechanisms. The influence of the spray parameters on the porosity and oxidation has been studied in order to optimise the coatings structure. Interconnected porosity and spray oxidation must be avoided to attain the best coating resistance under corrosion and erosion mechanisms identified on the municipal solid waste incinerators. Tribological and electrochemical coating properties have been determined under standardised tests. Different erosion tests have been carried out in order to determine coatings resistance. X-ray diffraction studies show the main formed phases under corrosion mechanisms for the different studied alloys. The Ni based Inconel 625 coatings have been reported as a good superheater tubes protector for its industrial application.     

Factors affecting energy efficiency of waste to energy plants

Abstract

Improvement of the energy efficiency of waste to energy (WtE) plants demands closer attention to detail in order plant integrity is not compromised. Availability for operation of WtE plants of >90% is a basic requirement to ensure the continuous supply of waste can be managed without the need for excessive storage or unnecessary transport. The drive to increase energy efficiency comes from the need of plant operators to maximise economic income, but also from European targets to meet the challenges of climate change and security of energy supply. Energy efficiency in WtE plants is reviewed based on experiences in Europe. While in the 1990s the emphasis for WtE operators was reducing emissions, more recently major efforts have been made to increase the export of energy. In the main, this has been approached by increasing steam temperature. However, the subsequent rate of corrosion can be catastrophically high and there is thus a need to improve corrosion resistance of heat exchange components to achieve acceptable service lifetimes. Trials have been conducted using more corrosion resistant boiler steels, but the preference is now for use of corrosion resistant coatings and claddings (e.g. alloy 625) to ensure plant reliability. Spray coating and weld cladding techniques are described. Other energy efficiency improvement methods described include online boiler cleaning, inhibiting fouling, achieving complete combustion and optimising flue gas cleaning (to reduce plant internal energy consumption). From what was until recently considered a good net efficiency of electricity production of 18%, measures being taken aim to achieve 30% in the most optimistic case.     

Condition monitoring and prognosis of utility scale wind turbines

Abstract

The state of the art in condition monitoring in wind turbines, and related technologies currently applied in practice and under development for aerospace applications, are reviewed. Condition monitoring systems estimate the current condition of a machine from sensor measurements, whereas prognosis systems give a probabilistic forecast of the future condition of the machine under the projected usage conditions. Current condition monitoring practice in wind turbine rotors involves tracking rotor imbalance, aerodynamic asymmetry, surface roughness and overall performance and offline and online measurements of stress and strain. Related technologies for monitoring of load history and fatigue crack growth in aircraft structures are evaluated for their applicability to wind turbine blades. Similarly, condition monitoring practice in wind turbines is compared with monitoring and prognosis in helicopter gearboxes. The state of the art in condition monitoring of electronic controls, power electronics and towers is also evaluated and compared with the state of the art in aerospace. Based on these comparisons, technology needs and future challenges for the development of condition monitoring and prognosis for large wind machines, both onshore and offshore, are summarised.     

Materials problems and solutions in biomass fired plants

Abstract

Owing to Denmark's pledge to reduce carbon dioxide emissions, biomass is being increasingly utilised as a fuel for generating energy. Extensive research and development projects, especially in the area of material performance for biomass fired boilers, have been undertaken to make biomass a viable fuel resource. When straw is combusted, potassium chloride and potassium sulphate are present in ash products, which condense on superheater components. This gives rise to specific chlorine corrosion problems not previously encountered in coal fired power plants. The type of corrosion attack can be directly ascribed to the composition of the deposit and the metal surface temperature. In woodchip boilers, a similar corrosion rate and corrosion mechanism has on some occasions been observed. Cofiring of straw (10 and 20% energy basis) with coal has shown corrosion rates lower than those in straw fired plants. With both 10 and 20% straw, no chlorine corrosion was seen. The present paper will describe the results from in situ investigations undertaken in Denmark on high temperature corrosion in biomass fired plants. Results from 100% straw firing, woodchip and cofiring of straw with fossil fuels are summarised and compared.     

New experience in field of application of corrosion resistance protection in flue gas desulphurisation plant absorbers

Abstract

Absorbers form the main part of the flue gas desulphurisation plants for 360 MW units at the Belchatow Power Station in Poland. The plant working cycle and service life are dependent on the grades of the corrosion resistance, protection materials used as an absorber lining in individual zones. The objective of manufacturers activities in this field has been to extend the service life of the flue gas desulphurisation plant. This has been implemented by testing 'wallpapering' technology in the absorber cylindrical part, using plate sheets of Hastelloy nickel alloys, 0·5 to 1·6 mm in diameter. Experiments involving Hastelloy nickel alloys, grades C-22 and C-2000, among others, were used to evaluation the resistance of the base material, 'wallpaper' plate sheet, welded joints and heat affected zones (HAZ) towards corrosion. The corrosion tests were executed for three different conditions of corrosive environment: (a) actual boiler outlet environment: chemical composition of boiler outlet flue gases, with addition of 10 wt-%K₂SO₄; a testing temperature of 70°C; and a testing time of up to 1000 h; (b) actual boiler outlet environment: chemical composition of boiler outlet flue gases, with addition of 6 wt-%K₂SO₄ and 4 wt-%NaCl; a testing temperature of 70°C; and a testing time of up to 1000 h; (c) simulated waste incineration environment, produced during thermal utilisation of wastes with the following chemical composition: N₂–9%O₂–0·08%SO₂–0·2%HCl; testing temperatures of 450 and 600°C; and a testing time was up to 1150 h. For the purpose of comparison, corrosion resistance tests were carried out on boiler steel types T22, E911, HCM12 and TP347H. This paper includes results of the evaluation of welded joints structure and HAZ, joint corrosion resistance characteristics, corrosion product test results, as well as requirements and recommendations with regard to fabrication of welded joints, the preparation degree of lining plate join surfaces, and requirements in field of weld joints.     

Energy drivers for materials research and development

Abstract

An overview is given of the significant materials aging issues for nuclear power reactors. Despite the fact that many of these issues were not anticipated at the design stage, the development of active management techniques, backed by improved understanding and modelling of the underlying processes, has permitted many nuclear plants to run safely and efficiently well beyond their design lifetime. The mechanisms of aging are reviewed and management strategies for aging plant assessed. In conclusion, the requirements and options for future nuclear plant are considered.     

Corrosion issues related to disposal of 316 SS–zirconium metal waste form under simulated repository conditions

Abstract

Metal waste form (MWF) alloy of D9 stainless steel with 8·5 wt-% zirconium was cast and evaluated for the corrosion behaviour. The microstructure, phase analysis, corrosion resistance and passive film properties of the alloy were evaluated using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and electrochemical methods. The corrosion performance of MWF alloy was carried out in demineralised (DM) water at pH 1, 5 and 8 and also in simulated Kalpakkam and Rajasthan ground water systems. In X-ray diffraction analysis, the identified phases are mostly iron based solid solutions of γ-austenite, Zr–Fe type and Ni–Zr type intermetallics. The typical microstructure of MWF alloy showed the presence of solid solution and the intermetallics. The anodic polarisation results of MWF alloy in Rajasthan ground water and in DM water at pH 5 and 8 showed higher breakdown potential compared with DM water at pH 1 and in Kalpakkam ground water. The electrochemical impedance spectroscopic results of MWF alloy in different media showed the influence of ground water media. The surface morphology after passivation and polarisation revealed the possibility of different types of deposition and dissolution. The corrosion behaviour of MWF alloy in different simulated media and its relation to microstructure and surface morphology are discussed based on the results obtained in relevance with geological repository conditions.     

Corrosion monitoring techniques for detection of crack initiation under simulated light water reactor conditions

Abstract

Monitoring the early stages of environmentally assisted cracking under light water reactor conditions is a challenging task but is getting more important in the context of a proactive aging management approach. From a rather large number of available corrosion monitoring techniques, only very few appear suitable for the detection of crack initiation and application in high temperature water environments (～300°C). The most promising of these techniques (electrochemical noise, acoustic emission, electrochemical impedance spectroscopy, alternating/direct current potential drop and high temperature imaging autoclave) are summarised and assessed in the current paper.     

Impact of climate change on corrosion risks

Abstract

According to most pundits on the topic, climate change will have a serious impact on many aspects of our life on the planet. At the onset it is not obvious how most of the predicted changes will take their toll on the integrity of the equipment and systems presently in use. This paper will examine three aspects of climate change that may alter the corrosive behaviour of the environment and increase the risk of corrosion failures: changes in precipitation patterns, corrosivity of coastal regions, and increased stresses on marine systems.     

Experience of surveillance schemes adopted for magnox steel reactor pressure vessels

Abstract

Surveillance or monitoring schemes are recognised to be an important part of any strategy to demonstrate that reactor pressure vessels used in civil nuclear power stations are operated within a safe regime. In the paper the authors describe the experience obtained from the surveillance schemes adopted for the UK's magnox nuclear power stations that were constructed with C–Mn steel reactor pressure vessels. These power stations were constructed in the late 1950s and 1960s and the last ceased generating in 2006. During the lifetime of the fleet with steel pressure vessels, there were developments in testing, observed changes in properties and understanding of radiation damage process that challenged the safety cases to support the operation of the stations. At the time the reactors were designed the concept of fracture toughness was only beginning to be investigated yet, during the lifetime of the stations, fracture toughness testing was successfully adopted as an input to fracture mechanics based assessment of the steel vessels. Over the operating life, a series of challenges emerged that were successfully addressed, including both hardening and non-hardening embrittlement, the latter due to impurity phosphorous segregation in weld metal and contributions from thermal nuclear embrittlement. These challenges led to the adoption of sophisticated statistical techniques to assess changes in embrittlement properties of the most critical construction material – submerged arc weld metal. A large scale sampling and testing programme of submerged arc weld metal removed from a decommissioned reactor pressure vessel validated the assessment process. As a result of successfully addressing these, and other challenges when the last two steel pressure vessel stations closed in December 2006, they had achieved lifetimes of nearly 40 years.     

Stellite 21 coatings on AISI 410 martensitic stainless steel by gas tungsten arc welding

Abstract

Degradation of AISI 410 martensitic stainless steel, a typical alloy for many applications such as steam turbine blade, could impair its efficiency and lifetime. To overcome this problem, critical surfaces could be modified by weld cladding via gas tungsten arc welding technique. In the present research, a comparative study of Stellite 21 weld overlays deposited in three different thicknesses, i.e. dilutions, at various preheat and post-weld heat treatment temperatures on the surface of AISI 410 martensitic stainless steel, has been made. The surface of coatings has been examined to reveal their microstructures, phase characterisation and mechanical properties using XRD, microhardness tester and metallographic techniques. The results showed that the deposition of Stellite 21 coating on AISI 410 martensitic stainless steel improved its corrosion resistance. Moreover, the volumetric dilution had a considerable effect on the hardness, microstructure and electrochemical corrosion behaviour of Stellite 21 weld overlays.     

Exploring the potential of wind energy for a coastal state

Adequate recognition of the wind energy potential of coastal states may have far-reaching effects on the development of the energy systems of these countries. This study evaluates wind energy resources in Taiwan with the aid of a geographic information system (GIS), which allows local potentials and restrictions such as climate conditions, land uses, and ecological environments to be considered. The findings unveiled in this study suggest a significant role for offshore wind energy resources, which may constitute between 94% and 98% of overall wind resources in Taiwan. Total power yield from wind energy could reach between 150 and 165 TWh, which would have, respectively, accounted for between 62% and 68% of Taiwan's total power generation of 243 TWh in 2007. Based on the Taiwan's current emission factor of electricity, wind energy has the potential to reduce CO₂ emissions by between 94 and 102 million ton per year in Taiwan, which is, respectively, equivalent to 28% and 31% of the national net equivalent CO₂ emissions released in 2002. However, the challenge of managing the variability of wind power has to be addressed before the considerable contribution of wind energy to domestic energy supply and CO₂ reduction can be realized.     

Fireside issues in advanced power generation systems

Abstract

The traditional trend towards the development and use of power plants with ever increasing efficiencies is now being coupled to the use of a wider range of fuels and technologies designed to minimise CO₂ emissions. Alternative solid fuels such as biomass and waste products, which can be classified as CO₂ neutral, are being used alone or cofired with fossil fuels. The cofiring of biomass and coal is currently the most efficient and effective method for using biomass to generate power. CO₂ capture technologies include systems for either precombustion or postcombustion CO₂ removal. Gasification of fuels (using either oxygen or steam as the oxidant) produces a gas that can be conditioned to enable precombustion CO₂ removal. Post-combustion CO₂ capture can be carried out using either solid or aqueous sorbent processes. Oxy firing of fuels is a technology that would enable more efficient post-combustion CO₂ capture. The various combinations of new fuels, novel technologies and higher temperature component operating conditions are producing challenging operating environments for components. Deposition, erosion and corrosion issues for hot gas path components in these advanced power generating systems, which are potentially life limiting, are reviewed. Reduction in heat transfer owing to high rates of deposition can significantly reduce heat transfer and increase the need for component cleaning. Depending on the system, component parts can include various heat exchangers, gas cleaning systems and gas turbines.     

Current corrosion monitoring trends in the petrochemical industry

Corrosion monitoring has traditionally been defined as the recording of historical events relating to materials degradation. This provided plant engineers and maintenance personnel with information only after damage has occurred and often integrated short term damage over the total exposure time. Modern petrochemical plants have, by nature of their size and subsequent operating costs, a definite need to predict and measure materials degradation due to corrosion during early stages and preferably on-line. An overview of traditional corrosion monitoring techniques such as corrosion coupon exposure and wall thickness measurements is provided. These techniques are now supplemented by modern electrochemical corrosion monitoring and measurement techniques.

Modern corrosion monitoring techniques have found applications at Mossgas ranging from general corrosion in cooling water systems to severely localised stress corrosion cracking in gas systems. Three different case studies to illustrate this are presented.