ABSTRACT Corrosion reactions between T. ferrooxidans and pure iron were studied by using linear polarization technique. The effect ofpH on corrosion was the main focus of this study. Experiments were performed at pH 1.0, 2.0, 3.0 and 4.0 under controlled and uncontrolled conditions. In addition to the corrosion kinetic parameters (I¢orr, Ecor~, and Rp), the redox potential and pH changes for the uncontrolled case were also monitored over time. In-situ imaging was done by an Atomic Force Microscope in tapping mode and also by a digital video camera. Biocorrosion of iron differs from abiotic corrosion in that ferric iron is generated by bacterial metabolic activity.
INTRODUCTION Corrosion is a heterogenous process involving reactions between solid metal and various chemicals present in liquid. Corrosion reactions involve the transfer of electrons. Thus, corrosion is an electrochemical process consisting of oxidation and reduction reactions. All corrosion reactions occur at the interface between the corroding (oxidizing) metal and the aqueous phase. The interface, an electrical double layer, is very complex with respect to its composition and underlying reactions. The phenomena in the electrical double layer is poorly defined and it is mainly for this reason that corrosion reactions themselves are not well understood. With the introduction of microorganisms, whose natural tendency is to adhere to a solid surface, a biofilm is formed and the properties of the electrical double layer change dramatically. The microorganisms displace previously absorbed ions and instigate a set of complex bioreactions and by-products. The presence and activities of microorganisms within the biofilm at the surface of a metal is termed microbiologically influenced corrosion (MIC).
Between abiotic and microbiologically influenced corrosion, the latter is much less understood. Substantial efforts are underway toward a better understanding of biocorrosion as can be evidenced by the literature on biocorrosion ~8.
As corrosion involves the transfer of electrons, most corrosion studies are based on electrochemical techniques (i.e., measuring corrosion current as a function of voltage.) The electrochemical studies are often supported by quantitative experimental techniques, such as chemical analysis, and qualitative experimental techniques, such as visual characterization (optical and scanning electron microscopy). Electrochemical techniques are in-situ methods of study while the experimental supporting techniques are mostly ex-situ methods. The major drawback ofex-situ studies is the change of properties at the metal/solution interface, the exact location of all corrosion reactions, once the sample is removed for characterization. Furthermore, scanning electron microscopy, as well as, all other surface chemistry characterization tools require vacuum. This requirement is not representative of a real corrosion environment. Therefore, although it may be helpful, ex-situ characterization has significant limitations. In the early 1980s, however, a major development in physics took place. Binning and Rohrer developed the Scanning Tunneling Microscope (STM) 9. Shortly after STM development, the Atomic Force Microscope (AFM) was developed 1°. The AFM and STM are commonly referred to as the scanning probe microscope (SPM). Not only do these tools provide surface characterization on an atomic level, but they also do not require vacuum. Furthermore, solid surface studies can be done even in liquids.
Comprehensive biocorrosion research utilizing the technical features of SPM microscopy is underway. For this biocorrosion study, iron was the metal of choice as it is an important representative of metals. Thiobacil
ABSTRACT The need to increase petroleum production capacity in trying to achieve self-sufficiency, has led Brazil to intensify its research and production in deep waters. The seawater injection process has proven to be an effective mechanism for improving the oil recovery factor in such cases. However, the success of the injection project depends on the quality of the injected seawater and its compatibility with the reservoir. The strategy used to minimize the risks associated with the seawater injection process and to reach production targets under environmentally safe conditions, involves: establishment of the injection water quality requirements, standardization of the techniques for on-line monitoring of the injection water quality and development of an expert system to assist the operational team in controlling the water quality and corrosion process.
This paper describes those strategies and introduces an expert system developed to seawater injection plants, that allows for the acquisition of corrosion and water quality variables, and infers diagnoses, recommendations and alarms to indicate corrective procedures as soon as a deviation occurs.
INTRODUCTION The need to increase petroleum production capacity in Brazil, in search of self-sufficiency, has led PETROBRAS to intensify its research and production in offshore deep waters. The Marlim oil field, found in 1985, located in the Campos basin, 180 Km from Rio de Janeiro, Brazil, is one of these fields, with a water depth varying from 600 to 1,100 meters. The reservoir depth is around 2,700 meters, having an oil volume in excess of one billion m 3, and a density ranging from 17 ° API to 21 ° API. Its aquifer, however, is not effective enough to maintain the reservoir pressure, whereby it becomes necessary to carry out water injection as a method for oil recovery (Figure 1).
The P-XVIII semi-submersible platform with a BOPD of 100,000 and an injection of 24,500 m 3 per day, was the pioneer of the Marlim oil field in the secondary oil recovery process. The injection of seawater started with the MLR-18 well in 1994. After a little over three years of injection, an injectivity loss of some wells was observed. This could impact the oil recovery targets of this field if no improvement in the quality of the injected water was implemented (Figure 2).
The presence of bacteria of different physiological groups, dissolved oxygen and suspended particles were identified as factors which determine the quality of the injection fluid, and might be responsible for the corrosion phenomena, plugging of injection wells and biogenic generation of H2S (souring).
The strategy adopted to assure a water quality suitable for injection was based on the establishment of an injection water specification for each reservoir in the Campos basin, containing the maximum limits of the acceptable determining parameters. Then the monitoring frequency for each of these factors was also established and, from the implementation of the analyses it was possible to calculate, monthly, the index of the injected water global quality (IQUAI) and to evaluate the performance of each platform. These technical recommendations, which are presented in Tables I and II, were defined on the basis of a joint study, made by reservoir, corrosion, scale, microbiology and environmental engineers.
This index is being validated every six months as a function of the measurements of the injected water effects (injection rate (m3/d), injectivity index (m3/d/kgf/cma), corrosion and pit rate, corrosion history, etc), also monitored in accordance with Table HI.
Implementation of the IQUAI brought a significant improvement in the injected water quality
ABSTRACT Increases in syngas steam to carbon ratio are generally expected to reduce the tendency of high chromium nickel-based alloys to suffer from metal dusting corrosion. This might only be true for the initiation of metal dusting. Laboratory experiments have been conducted in a pressurized autoclave to indicate that sustained metal dusting degradation of alloy 601 (UNS N06601) may also be driven by the presence of steam (oxygen partial pressure). Oxidation of matrix carbides is proposed as a possible contributing factor in metal dusting corrosion.
INTRODUCTION Metal dusting corrosion, often described as a form of catastrophic carburization ~, continues to be a significant restriction on the optimum design and construction of syngas production plants. There appears to be a significant discrepancy between claims made by fundamental metal dusting research 2.3 and the ability of operating companies to solve practical metal dusting problems 4. This is aptly reflected in the 1999 launch of a new European research program on metal dusting 5 as well as the MTI/DOE Program on Metal Dusting 6
The reasons for the above are twofold. Firstly, fundamental research requires tight control of the test environment where only one process parameter is changed at a time. While this is considered a sound scientific approach, the results often do not reflect the interaction between various changing parameters encountered in an operating plant. Secondly, the process conditions selected for fundamental metal dusting research are often selected to yield fast results and ensures results within a given time- frame. Relating data generated in this manner to plant experience and materials selection for new equipment is extremely difficult, if not impossible.
The alloys used in syngas plants are exposed to significantly different gas atmospheres during startup, compared to stable operation. The current work has aimed to conduct research under realistic process conditions (gas composition, temperature & pressure). The high pressure exposure program therefore included a simulation of both dynamic and stable process conditions.
Observations made during high temperature steam exposure (starup or trip conditions) of pre- corroded alloy 601 (UNS N06601) will be reported in this paper.
DIFFERENT EXPLANATIONS OF THE METAL DUSTING MECHANISM
Extensive research into the fundamental understanding of the various mechanisms of metal dusting has been conducted during the past forty-five years. Notable contributions to the current understanding came from Dr. R.F. Hochman and his group at the Georgia Institute of Technology, Atlanta USA 7.8, and prof. H.J. Grabke and his coworkers at the Max-Planck-Institute of Iron Research (MPI) in Dtisseldorf, Germany 9-11
Several other groups have through the years researched the mechanisms of metal dusting. Most of these reported observations that are in agreement with the mechanisms proposed by Hochman and Grabke, with some modifications. Some of these modifications include:
A. Metal dusting corrosion assisted by unreleased stresses. B. Metal dusting corrosion involving catalytic carbon deposition. C. Metal dusting corrosion as internal oxidation and carburization.
Mechanism by R.F. Hochman - Georgia Institute of Technology
Most of the research conducted at the Georgia Institute of Technology was on the interaction of pure metals and alloys with carbon monoxide at high temperatures. Only a limited amount of research has been conducted using hydrocarbon or complex gas environments. A comprehensive summary of this research was published in 1973 7. This was followed up another publication in 1976, summarizing the
ABSTRACT Two model surfactant corrosion inhibitors, cetyltrimethylammonium chloride (CTAC) and cetylpyridinium chloride (CPC) were chemically modified to drag reducing agents by exchange of the chloride with salicylate counterion. The modified surfactants (cation exchanged) and the corresponding halides were compared in a corrosion test under conditions were drag reduction occurred. Drag reduction promoted corrosion inhibition in standard corrosion test.
INTRODUCTION Drag reduction is a term used to characterize the reduction in friction in turbulent flow through pipes resulting in an increase in fluid flow and/or decrease in pressure loss. Drag reduction in turbulent flow is an important phenomenon both for practical applications in fluid transport H and for basic studies in fluid mechanics 51°. The main oilfield applications of chemicals capable of affecting drag reduction are in crude oil transportation and water reinjection lines. Drag reducing agents (DRA's) used in crude oils are usually characterized as high molecular weight oil soluble polymers, while hydrolyzed polyacrylamide and polyacrylate have been used as DRA's in aqueous systems. An important and fast growing area of oilfield production, namely multiphase flow systems, is normally not treated with DRA' s. The development and use of DRAs for multiphase flow application may offer significant reductions in turbulence and/or modification in flow regime, in which case corrosion inhibition may be enhanced. The goal of the current work was to develop a molecule, which shows simultaneously drag reduction and corrosion inhibition properties. In this paper we have studied the effect of model drag reducers based on surfactant type molecules on corrosion inhibition. While this type of drag reducers have been recently studied as prospective additives for reducing energy requirements in recirculating heating systems 1H3, such as apartment buildings, little work was done on their use as drag reducers in oil field application 14. Drag reduction by surfactant type molecules is affected by the chemical structure of the counterion 11-13. The general accepted theory is that by a judicious choice of the counterion the shape of the micelles can be altered. Our initial study has focused on two surfactants, cetyltrimethylammonium and cetylpyridinium ammonium salts. We have studied the effect of the counterion on drag reduction, critical micelle concentration (CMC), and corrosion inhibition on those two surfactants.
EXPERIMENTAL Chemicals Cetyltrimethylammonium salicylate (CTAS) was prepared from a 25% aqueous solution of cetyltrimethylammonium chloride and sodium salicylate in equimolar ratio. In addition CTAS with a 0.42 molar excess of sodium salicylate was prepared. Cetyltrimethylammonium hydroxycoumarate (CTAHCo) was prepared by ring opening of dihydrocoumarin with a 50% aqueous solution of sodium hydroxide in a 1:1 molar ratio followed by reaction with cetyltrimethylammonium chloride in equimolar ratio. Cetyltrimethylammonium 2-coumarononate (CTAC) was prepared by ring opening of 2- coumaranone with a 25% aqueous solution of sodium hydroxide in a 1:1 molar ratio followed by reaction with cetyltrimethylammonium chloride in equimolar ratio. Cetyltrimethylammonium o-hydroxycinnamate (CTAHCi) was prepared by first ring opening of coumarin with a 50% aqueous solution of sodium hydroxide in a 1:1 molar ratio followed by reaction with cetyltrimethylammonium chloride in equimolar ratio. Cetylpyridinium salicylate (CPS) was prepared from cetylpyridinium chloride monohydrate and sodium salicylate as a 25 % active solution in water/isopropanol (1/1 weight) in equimolar ratio.
Drag Reduction Measurements Two screening methods for dra
ABSTRACT Several commercial products are marketed either as water additives to aid salt removal and provide a residual protective film or as a corrosion resistant barrier coating to retard the rate of metal deterioration in marine environments. Faced with the need to reduce high costs of corrosion related repairs and extend the lifetimes of ground vehicles operating in harsh marine environments, maintenance facilities at military bases are currently investigating the use of such products on new equipment or during routine maintenance cycles. In this study the relative effects of five water wash additives and two coatings on the corrosion rates of representative steel and aluminum specimens in seawater were evaluated. Results show significant differences in the corrosion behavior of steel and aluminum specimens. For three of the water additives the corrosion loss of steel was reduced by more than 50%, however, only one of these products showed beneficial effects for aluminum when used according to manufacturers directions. It is important to note that increased aluminum corrosion (pitting) was observed for each of the other additives tested. Significant corrosion inhibition of steel and aluminum was observed for one of the barrier coatings. These results provide a measure of guidance in the selection of corrosion maintenance protection for military vehicles containing steel and aluminum constructions.
INTRODUCTION Operational maneuvers for military ground transport vehicles routinely include submersion in seawater and prolonged exposures to saltspray. Residual salt deposits combined with humidity during downtimes or storage promote the accelerated deterioration of metal components. As a result it has been shown that the operational lifetimes of military vehicles can be reduced by more than 30% and the costs for replacing damaged parts can be as much as 10% of the initial cost of the equipment. For example, replacement of steel framerails on Marine Corps HMMWV's costs between $5000 to $7000 per vehicle.~ As equipment ages maintenance and life cycle costs become significant.
In order to remedy the problems associated with corrosion, maintenance facilities on military bases are now exploring the use of commercial corrosion inhibitors to augment the recommended procedure of fresh water rinses following exposures. It should be noted that fresh water rinses are not always practical for the military either because of the time constraints of an active military operation or the availability of fresh water. Selection of appropriate treatments is complicated by the facts that the quantitative benefits from maintenance freshwater rinses have not been documented, it is very difficult to construct a "typical" set of conditions for usage of military equipment and manufacturer's claims for their products are inconclusive at best. Product claims are often backed by testimonials from users of recreational or sports equipment under poorly characterized or infrequent circumstances. For proprietary reasons, the inhibitor ingredients and their function are not indicated
Two types of corrosion preventatives offer potential and the convenience necessary for military use. The first type is a water additive that could enhance and prolong the effects of fresh water rinses. The second is a surface application which could provide a long-term barrier coating to moisture and oxygen. In this work we have selected four water additives and two surface coatings for testing. Tests were designed to reduce the complexity of the problem by directly studying only the endurance and ability of each type of inhibitor to retard corrosion of bare aluminum and steel specimens under conditions of total seawater immersion. Und
ABSTRACT The effective use of both oxidizing and non-oxidizing biocides for control of MIC in cooling water systems was evaluated. Traditional off-line methods of monitoring and control were compared to a new on-line method. The on-line method utilizes a patented localized corrosion monitor (LCM) to monitor changes in localized corrosion rates within laboratory scale and pilot cooling tower systems under various biocide treatment regimes. Under many of these conditions, the changes in localized corrosion rates can be directly correlated to specific biocide feed. Standard culture techniques for indicator microorganisms of MIC such as Acid Producing Bacteria (APB) and Sulfate Reducing Bacteria (SRBs) were also used to evaluate biocide performance. These tests were useful for selecting the best biocides against anaerobes associated with MIC, but the on-line LCM monitor was more effective in adequately providing the correct biocide dosage.
INTRODUCTION Microbiologically Influenced Corrosion (MIC) is a well-known problem in many industries 1-6. It is characterized by rapid or accelerated localized corrosion rates. Documented cases of MIC have shown failures in new systems in as little as 6 months. Despite its well-known occurrences, little progress has been made in the prevention of failures from MIC. This is mostly the result of inefficient monitoring, rather than lack of effective treatments. Many techniques that claim to measure MIC 7-9 have been developed but to date, there is still no industry standard for measuring MIC like there is for measuring general corrosion rates. Many techniques that have proven useful for teasing apart some of the fundamental electrochemical properties of MIC under laboratory conditions are neither practical nor successful under field conditions 10-16. Measurements of specific and/or generic microorganisms and their unique metabolic products represent indirect evidence for MIC but their presence does not necessarily correlate to a localized corrosion rate. Biocide efficacy tests aimed at these specific indicator microorganisms are useful for identifying potential antimicrobials for MIC control. Unfortunately the mere presence of these indicator microorganisms does not always equate with enhanced localized corrosion rates. Electrochemical techniques seem to have the most promise for measuring some component of localized corrosion that can be attributed to microbial processes. The three most popular electrochemical techniques for measuring MIC are ElectroChemical Noise (ECN), 15-17 galvanic generated current measured with a Zero Resistance Ammeter (ZRA) 8, and Electrochemical Impedance Spectroscopy (EIS). 13, 18 EIS has shown excellent promise as a field tool for measuring microbial degradation of coatings but not corrosion rates. This is mainly due to the fact that EIS requires knowing the actual corrosion attack area in order to calculate the corrosion rate. When MIC is present, corrosion is usually highly non-uniform. Thus, knowing the electrode sample surface area used in the measurement is often not sufficient. ECN techniques have shown great promise under laboratory conditions but have failed to routinely predict MIC events accurately under field conditions. Furthermore, the ECN based methods described in the literature have not been able to determine the localized corrosion rates associated with MIC accurately and reliably, even under laboratory conditions. ZRA measurements have been used for many years to monitor MIC processes in laboratory experiments 19. Recently this technique has been used to measure localized corrosion events under field conditions with various electrode configurations. These electrode configurations take on two basic designs: 1) concentric ele
ABSTRACT The design, fabrication, transportation and testing of large dual laminate tanks for high purity and corrosion resistant applications requires coordination across many disciplines. In addition, it is critical that all the appropriate design scenarios are investigated and considered.
INTRODUCTION An experienced fabricator is ideally suited to take the leadership role in the subject processes. The following will reveal a typical project and depict the essential elements for success. It is intended to set the "expectation level" for end users and engineers on participation by a fabricator in the entire process from specification development to a completed installation.
SPECIFICATION and PROPOSAL During the initial stages the end user must gather the essential application data to develop a specification. The User's Basic Requirement Specification, (UBRS), contained in the ASME RTP-1 standard is well suited for this purpose, as it is a checklist of the information that needs to be addressed. In fact, this completed document is the only information an ASME accredited shop needs to provide a proposal. Refer to Table 1-1 User's Basic Requirement Specification contained in the latest edition ofASME RTP -1. Lacking a checklist such as the UBRS the following information would be considered essential:
1. Size: 20' Diameter x 26' Tall 2. Configuration: Vertical 3. Service: 32% NaOH 4. Pressure: +/- 10" WC 5. Operating Temperature: 194 Degrees F 6. Design Temperature: 244 Degrees F/ 10 Degrees F 7. Specific Gravity: I. 35 8. Materials of Construction: Thermoplastic Lined FRP
Although the end user is ultimately responsible for material selection it is prudent to engage the assistance of fabricators and raw material suppliers for the selection process. Both can provide published data and case histories pertinent to the subject application. Immersion coupons can also be provided to the end user if exposure to the process is feasible. The best basis for material selection is always actual experience by the end user for the application being considered.
In the case of a dual laminate structure the liner material involves the liner resin manufacturer, sheet supplier, and the fabricator. The RTP structural thermoset resin manufacturer and reinforcement suppliers may also be involved if the application dictates. The fabricator can coordinate this process through his routine involvement with the end user and the raw material suppliers.
Each step from Specification thru Installation should become more and more refined. For example, ifa proposal from the fabricator is less specific and less detailed than the RFQ something is wrong. The level of detail is to be such that a complete commercial and technical evaluation can be made. Special design features, exceptions, and alternates should also be part of a complete proposal. It should be evident from the fabricator's proposal that the preliminary design work is complete via confirmation of major component thickness, laminate sequence, and equipment weight. A vessel or equipment data sheet provided with the proposal is an efficient way to summarize and organize the pertinent details of the equipment offered.
ENGINEERING Once an order is placed the engineering package including complete calculations and drawings is submitted to the end user by the fabricator. Depending on the timing of project funding, it may be advisable to order the engineering phase in advance of an equipment need. In these cases the engineering is completed during waiting periods and project approval dictates the fabrication release.
The design considerations vary with the application; the fol
ABSTRACT Dual laminate piping and ducting have been primarily used world wide in electrochemical processes such as chlor- alkali, sodium chlorate and hydrometallurgy. There are many recognized international standards which have been in use since the 1950's which include the German DIN 16965 part 2 for Type B pipes and the British standards BS 6464 and BS 7159. The physical and mechanical properties and testing requirements involve many different materials, fusion and bonding methods and resin test methods. Design criteria will be discussed for temperature limitations, pressure and vacuum ratings (in relation to bond strength) pipe span distances and proof testing to ASME B31.3, Chapter 7, Part 9.
INTRODUCTION The design and construction of dual laminate vessels is a growing industry in North America and has been addressed in the new ASME RTP-1 Dual Laminate Appendix M-14 which is now fully operational. However, there is currently a void in North America for a Dual Laminate Standard for piping, the solution for which is still years away) The best hope for a voluntary standard is probably with the ASME RTP Committee which has a new task force working with the ASME B31.3 Task Group F/RTP to produce a new FRP piping standard and later a dual laminate piping standard. (See Table 1 for existing international standards)
DESIGN CRITERIA FOR DUAL LAMINATE PIPING TO INTERNATIONAL STANDARDS
Dual Laminate piping in the chemical industry is often exposed to severe stresses such as temperatures from approximately -40°C to 100°C, or even up to 130°C a, complete with thermal cycling, pressures between full vacuum to 220 psi (15 Bar), and huge surge pressures. Critical service rated corrosive media covers an extreme range of chemical compositions, inorganic and organic, as well as abrasion and erosion forces through high solid contents, salts and slurries.
Bond strength requirements is the most misunderstood issue in dual laminate piping as the method is generally chemical bonding for the PVC and CPVC vinylchloride group 3 and mechanical bonding for the Polyolefin and Fluoropolymer groups. The difference between bonding requirements of vessels and pipe is the method of bonding in pipe is sometimes by melt bond impregnation, and is best described in the DIN 16964 Part 1 General Quality Requirements of Glass Fiber Reinforced Polyester Resin (UP-GF) Pipes and tested in accordance with DIN 53769 Part 1, determination of the adhesive shear strength of Type B pipeline components and/or ASTM D-1781-98 for peel strength.
Besides the problem of chemical resistance for a dual laminate pipe (which is affected by temperature and the resistance to oxidizing media), is the affect of long term creep. 4 The term "long term creep strength" refers in the case of plastic pipes to the failure pressure or comparative failure stress as a function of time under internal pressure. The test for long term creep strength can be carried out at room temperature or at elevated temperatures, the temperature of the internal and external medium being generally the same. 5 This method of assessing dual laminate pipes is particularly suitable because this stress is very similar to that occurring in practice. The long term creep strength of a pipe is the last decisive analysis to its suitability for a specific application. 6 The final test for the pipe is the ring shear test that is generally carried out by a third party (TUV in Germany).
Another important property for suitability of a material for a particular application is the permeability in addition to its chemical resistance. As is well known, organic polymers are not completely impermeable to low molecular substances e.g. O2, H2, C12, HC1 or or
ABSTRACT Microbiologically Influenced Corrosion (MIC) is a result of many factors, including the formation of biofilms which can cause severe, under-deposit corrosion. This corrosion phenomenon is exacerbated in the cases where anaerobic conditions exist beneath the biofilm that can lead to the proliferation of sulfate reducing bacteria (SRB's). These SRB's metabolize sulfate to sulfide, which is known to be a strong aggressor against a variety of metals including higher alloy stainless steels.
Glutaraldehyde has a long history of established in-field effectiveness against SRB's and the penetration and removal of biofilms. Described herein are a number of laboratory and field test results that address control of MIC through the use of glutaraldehyde. In addition, a few significant case history examples will be highlighted.
INTRODUCTION Microbiologically Influenced Corrosion (MIC), sometimes referred to as biocorrosion, is a particularly aggressive form of under-deposit corrosion that has its genesis in microbiological presence and activity. MIC is a subset of a larger class of corrosion phenomena which includes electrochemical and galvanic corrosion; both of which are based on differential oxygen concentration cells. Recent research on MIC has identified many species of bacteria that can contribute to metal corrosion. Almost every common metal and alloy used today, with the possible exception of titanium, has been shown to be susceptible to MIC I.
The steps leading up to an MIC event are well defined and sequential (refer to Figure 1). Planktonic, or free-floating bacteria are prone to form sessile, or attached biofilms when their numbers are sufficiently high. The biofilm environment is one that is relatively impenetrable to dissolved oxygen in the bulk water and hence, creates an anaerobic condition. The anaerobic environment becomes ideal for the proliferation of sulfate reducing bacteria (SRB's) which metabolize sulfate ion into sulfide ion. The generated sulfide ion can then become protonated to form the corrosive acid, hydrogen sulfide. In addition to the SRB's, acid-producing and iron-oxidizing bacteria can also contribute to MIC related events.
Since the steps leading up to an MIC event are sequential, treatment of the problem early on in the sequence will reduce the chances for MIC to occur. Glutaraldehyde has been shown to be an effective treatment strategy for eliminating many of the steps leading up to an MIC event. Specifically, this paper will discuss how glutaraldehyde is particularly effective against biofilm formation and removal, elimination of both planktonic and sessile SRB's, and the removal of acid-producing and iron-oxidizing bacteria.
BIOFILM CONTROL It's been well established that microorganisms that live within a biofilm community are much less susceptible to microbicides than are the same microorganisms living in the free-floating, or planktonic state. A prevailing theory on why this occurs is that the microbicide can't penetrate the biofilm due to poor diffusion within the biofilm matrix. However, biofilms are estimated to contain 98% water z with diffusion rates approaching 20-80% of those in pure water for solutes that are similar in size to common microbicides. 3 Biofilms are also known to possess channels within the matrix that allow solutes to move freely within them.
In many cases, barriers to biofilm penetration by microbicides occur when the microbicide r e a c t s within the biofilm before it can penetrate. This appears to be the prevailing mechanism for oxidants such as hydrogen peroxide and chlorine. 4 Another theory postulates that biofilm bacteria are in a slowo growth or non-growth phase, which makes them
ABSTRACT The susceptibility of alloy 600 to intergranular stress corrosion cracking (IGSCC) has been tested in pressurized water reactor (PWR) simulated primary water chemistry at 3400 C by slow strain rate testing (SSRT). The electrochemical noise (EN) in current and potential was monitored during these tests. SSRT was also done in solutions in which the specific conductivity was increased by an order of magnitude by increasing the content of boric acid and lithium hydroxide, in order to increase the efficiency of monitoring of EN signals. The presence of 100 ppb each of chloride and sulphate ions in the high conductivity solution caused extensive pitting damage on the sample. Distinct EN signals were monitored before, during and after SSRT and were due to the pitting damage. The characteristics of these signals are reported in this paper. No EN signals were monitored due to the IG crack initiation events. The results are analyzed to deliberate on the mechanism of IGSCC in this environment.
INTRODUCTION The nickel base alloy, alloy 600 has been commonly used as a construction material for the tubing in steam generators (SG) of pressurized water reactors (PWR). Experience over the last four decades has shown that the mill annealed alloy 600 cracks during service from the primary and the secondary side ~' 2. While the cracking from the secondary side has been controlled by changing the water chemistry and the design of the SG, the primary water stress corrosion cracking (PWSCC) necessitates a change of the material for reliable operation to those more resistant to cracking, e.g. thermally treated alloy 600 and alloy 690. Laboratory studies have established the material, environment and stress parameters that make alloy 600 prone to intergranular stress corrosion cracking (IGSCC) in high temperature water 37. However, there exists no single model or mechanism that explains all the aspects of PWSCC and allows the prediction of the crack propagation and the remaining life of the SG tubing 6-8.
There is even less information available on the initiation of PWSCC. Development of methodologies to detect initiation of cracks and early stages of their growth would help in the life management of SG and reliable operation of power plants. The slip dissolution model 9 has been used to explain the mechanism of cracking in stainless steel (SS) and other materials in boiling water reactor (BWR) environment and to predict the crack growth rates based on the crack tip strain rates and the repassivation rates.
Electrochemical noise (EN) measurement has been used to detect and study the general and different forms of localized corrosion of materials in a variety of environments ~°' 1~. However, the use of EN to study the SCC in pressurized high temperature water has been limited. The studies reported so far are for the cracking of austenitic stainless steels in BWR environments ~2-L' and for sub and super critical water applications 16. These studies suggest that it is possible to identify the initiation and early stages of the propagation of cracks by EN monitoring. However, different EN parameters (based on electrochemical current noise (ECN) or electrochemical potential noise (EPN)) were used in these studies to correlate to the cracking of materials. It was shown in an earlier study that the initiation of IGSCC of sensitized SS in BWR environment gave rise to distinct EN signals. The shape of the ECN during a cracking event was correlated to the slip dissolution model ~ 5. The value of the current decay rate measured from the ECN had been shown )5 to match well with that of the repassivation constant 'n', commonly determined by the potential pulse technique. The only reported study 17 on the cr