ABSTRACT In the present study the influence of movement of the solution on the characteristics of measured electrochemical noise (EN) has been investigated. For this reason the measurements were performed in still solution, as well as in movement of electrolyte. In order to relate measured EN to the development of corrosion processes, digitized images of the electrodes were recorded continuously during these measurements. It was found that the characteristics of EN, in general, change significantly with strong movement of the solution: stirring, or laminar flow. The results of this study confirmed that main source for this change is transformation of corrosion processes: in a still solution the corrosion process tends to be localized, whereas during strong movement of the electrolyte this tendency is oriented towards uniform corrosion. It was established that the direct effect of the electrolyte movement on measured EN (modulation of signals due to spatio-temporal flow disturbances) is small.
INTRODUCTION Electrochemical noise (EN) consisted of potential and current fluctuations spontaneously generated by corrosion reactions °4). These fluctuations can be measured in freely corroding systems, and therefore the measurements and analysis of EN is one of the most promising methods for the detecting various types of corrosion: uniform corrosion, metastable pitting, pitting corrosion, crevice corrosion, stress-corrosion cracking C4t5). Several attempts were made to characterize these corrosion processes by means of measurements and analysis of EN 14"5' 16-z5), but no unified theory still exists to explain the sources of EN.
In few of numerous study of EN for monitoring corrosion in various environments it was found that stirring of solution may represent additional source of potential and current fluctuations: it was indicated that the amplitude of EN increased modestly as the flow rate was increased C26"27). It was established during previous investigations (2s) that the nature of EN measured with stirring of the electrolyte is rather different than without it. In the case of stirring the high-frequency part was ordinarily quite high, whereas EN measured in the still solution usually consisted of low-frequency fluctuations of electrochemical potential and current. The influence of different types of electrolyte movement on measured EN, however, has not yet been exactly determined.
Since the EN is generated by corrosion reactions, the influence of the electrolyte flow on corrosion processes has to be considered as a first. It was established (27-32) that corrosion processes may involve a number of mass transport stages which can be influenced by solution flow: a) delivery of reactants to the anodic site, b) removal of products from the anodic site, c) analogous mass transfer at the cathodic site, property changes due to anodic film repair or breakdown. The superimposed effect of aeration and consequent oxygen reduction was also confirmed in certain systems. These observations could explain the changes in measured EN due to electrolyte flow mentioned above (2s'26), but some direct effect of the electrolyte hydrodynamics (modulation of EN signals due to spatio-temporal flow disturbances in electrolyte) could also be possible.
The main aim of this study was to investigate the influence of various types of solution movement on the characteristics of measured EN. In order to determine the main source of certain types of fluctuations in EN, digitized images of electrode surfaces were recorded at the same time as the measurements of EN were made. The emphasis was made also to assess the eventual direct influence of the solution movement on measured EN: it was assumed that the turb
ABSTRACT An ultrasonically induced cavitation facility was used to study the cavitation corrosion behaviour of L-80 carbon steel in sea water. The work included measurements of free corrosion potentials, and mass loss in the presence and absence of cavitation. The cavitation tests were made at a frequency of 20 kHz and at a temperature of 50°C. Cavitation conditions caused an active electronegative shift in the free corrosion potential of this alloy. Cavitation also increased the rate of mass loss of this alloy as a function of exposure time. Cavitation made the surface of this alloy very rough, exhibiting large cavity pits in the middle region of the attacked area as revealed by the scanning electron microscope (SEM). Mechanical factors were determined to be the leading cause of metal loss,
INTRODUCTION It has been planned to use seawater from the northern Arabian (Persian) Gulf for the purpose of secondary oil recovery for some Kuwaiti oil fields to enhance production and maintain reservoir pressure 1. Initially, 1 million barrels per day of seawater from a seawater injection plant will be injected into the reservoirs of high flow rate 2. Carbon steel L-80 alloy has been considered for this waterflooding scheme because of its corrosion resistance under sweet conditions. Therefore, the aim of this study is to characterize the effect of cavitation conditions on the corrosion behaviour of L-80 carbon steel utilizing Arabian Gulf seawater.
EXPERIMENTAL METHOD Apparatus The vibratory apparatus used for this test method works at a frequency of 20 kHz and an amplitude of 25 ~m. This test method produces axial oscillations of a test specimen inserted to a specified depth in the test liquid. The vibrations are generated by a magneto strictive or piezoelectric transducer, driven by a suitable electronic oscillator and power amplifier. Figure 1 shows a schematic view of such apparatus.
Material Specimens were made from a tubular section of L-80 carbon steel pipe. Each specimen had a diameter of 1.59 cm and a thickness of about 0.27 cm. Before experimental testing, specimens were mechanically polished with silicon carbide papers up to 1200 grit. For morphological examination, some specimens were etched before testing to reveal their microstructure. The test specimens were fixed on a special holder which was placed at a distance of 0.125 cm from the apparatus horn. At the end of each cavitation test, detailed morphological examinations were carried out on the specimens. Optical and scanning electron microsocopy (SEM) were used to identify the initiation and mode of damage in addition to the role played by the constituent phases of the alloy.
Test Solution The chemical composition of the seawater used in this study is shown in Table 1. The seawater was contained in an open 600 ml glass beaker surrounded by a copper coil in a water bath. Inside the beaker, the seawater was maintained at 50_+1°C
RESULTS Mass-Loss of L-80 Carbon Steel Figure 2 shows the rate of mass loss of alloy C-80 carbon steel specimen exposed to the seawater at 50°C. It is evident that the rate of mass loss under cavitation conditions increases initially until it reaches a maximum value after about 1 hr of testing. Then it gradually decreases up to 4 h of cavitation after which it jumps to another value of 1.6 mg/hr.cm 2 after 14 h of testing. The rate of mass loss again begins to decrease after 15 h of testing, reaching a steady state value of about 1.0 mg/h cm 2 after 40 h of cavitation testing.
To study the effect of cavitation on the free corrosion potential of L-80 carbon steel exposed to seawater, potential measurements were
ABSTRACT Approximately 10% of all preliminary failures of copper tubes used in the HVAC industry are the direct result of ant-nest corrosion on a worldwide scale. This unusual form of localized corrosion has been detected in new tubes from some manufacturers, as well as early in service (i.e., less than one year). Laboratory studies have replicated service failures. The sources of the corrosive species that promote this form of attack are considered.
INTRODUCTION This is the continuing saga into a very insidious form of corrosion apparently limited to refrigeration grade copper (i.e., DHP copper, UNS C12200) although other forms of attack, similar in appearance, have been observed in other alloys. (1 - 5) This very localized attack is known as "ant-nest corrosion" (a.k.a. formicary corrosion). A previous review of this phenomenon (6) describes the mechanism where the copper base-metal oxidizes to form copper oxides and copper carboxylates (primarily copper formate). The literature provides several attempts to classify the morphology of ant-nest corrosion (6, 7) although no individual pattern has been identified.
Laboratory studies (7, 9 - 14) have been successful in replicating ant-nest corrosion found in the field using carboxylic acids, hydrochloro- solvents, and petroleum- and water-based drawing and finning lubricants. Case studies reported in the past two years suggest that this form of attack continues to be a problem in the heating, ventilation and air conditioning (HVAC) industry. (6,15) It is reiterated that approximately 10% of all preliminary failures of copper tubes used in the HVAC industry are the direct result of ant-nest corrosion on a worldwide scale. (6, 9, 16,17) This paper reviews not only the current understanding of the mechanism of attack but cites new case studies including a thorough investigation into the cause of over 160 crates of new copper condenser and evaporator tubes that were affected by this form of attack.
MECHANISM REVIEW Residual organic compounds that remain on copper tubes during production and fabrication into chiller units, are able to advance to ant-nest corrosion only with the simultaneous presence of moisture, air and the decomposition of the organics to acids. It is concluded that the mechanism of the ant-nest corrosion is a modified pitting process involving a very small pit (termed a "micro-anode") where the copper base-metal oxidizes and dissolves according to:
Cu ° "-) Cu ++ e- (1)
In the presence of carboxylic acid (e.g., formic acid) the copper ions react to form an unstable copper (I) complex:
Cu + + HCOO" ~ Cu(CHOO) (2)
This species is further oxidized to form a copper (II) carboxylate (e.g., copper (II) formate) and copper (I) oxide (cuprite):
4 Cu(CHOO) + ½ 02 @ 2 Cu(CHOO)2 + Cu20 (3)
Copper (II) formate has a monoclinic crystalline form and is blue in color. Micro-cracks develop and radiate outward within the pit due to the wedging effect of the deposited copper (I) and copper (II) complexes. The micro-cracks expose more surfaces of copper and the process proceeds within the micro-crack to give the copper (I) complex according to:
Cu(CHOO)2 + Cu ° ") 2 Cu(CHOO) (5)
Thereon, reactions 3, 4 and 5 repeat over and over until tunnels are formed leading to ultimate through- wall penetration.
If, however, there is any disruption in the presence of the basic three components (i.e., moisture, air and organic acid), then the micro-cell mechanism shuts down and ant-nest corrosion ceases to propagate. Therefore, a tube may be infected with ant-nest corrosion but failure has not resulted in a leak.
ABSTRACT A real time electrochemical noise corrosion monitoring field trial was conducted at the Cahn 3 Water Treatment Plant in Lost Hills, California in late 1995. Five corrosion-monitoring probes were installed at different locations in the plant that had a history of weight loss corrosion between 0.5 to 450 mils per year (mpy). The five-element probes simultaneously performed Linear Polarization Resistance (LPR) and Electrochemical Noise (EN) measurements. Instantaneous and time averaged corrosion rates were calculated and recorded along with various other statistically relevant parameters for each probe. While absolute measurements of corrosion rates differed from weight loss measurements made on probe elements by factors of between 101 to 103 , the on-line corrosion monitoring system detected and measured changes in plant operation and process chemistry which impacted corrosion in the plant system. Problems encountered with this field trial of corrosion monitoring probes and systems included, probe fouling due to iron sulfide and sludge, data collection volume and awkward data manipulation routines for post collection processing of data. Modifying the probe electrode design reduced probe fouling, but regular cleaning was still required. The project showed that real-time corrosion monitoring in a production plant environment was feasible and provided valuable plant operating information.
INTRODUCTION AND BACKGROUND The Lost Hills field is unique in that the oil is deposited in a diatomite formation that has a very high porosity, but little or no natural permeability [1 ]. Because of the low permeability, primary oil recovery was limited and the field was not significantly developed until a program of hydraulically fracturing the reservoir was begun in the late 1980's. Hydraulic fracturing is a process where an aqueous/complex sugar (guar polymer) gel with coarse sand is pumped into the well, fractures the formation, and forms a permeable path for oil to follow to the well. Due to the fracturing operation, the produced fluids contain fracture sands that accumulated 1 to 3 inches deep in the six o'clock position in the main lines coming into the Cahn 3 plant. The sand is conducive to microbiologic growth, especially acid producing and sulfate reducing bacteria that cause microbial influenced corrosion (MIC). Nitrates and some sulfates from the Tulare wellwater and the guar polymer from the fracturing operation provide a food supply on which the bacteria can flourish. In addition, the produced water in the field contains approximately 15 part per million of soluble iron which reacts readily with hydrogen sulfide and oxygen to form particulate iron sulfide (FeS). The main 8", 12" and 16" lines failed in 1993 due to MIC . Replacement costs for the failed piping were substantial. Chemical inhibitor costs in 1993 were substantial and problems persisted despite an aggressive chemical treatment program. In 1994, a pigging facility was added to the front of the plant to remove solids in the system. The net effect was to reduce the cost of the biocide treatment program by approximately two thirds. However, corrosion rates, as measured by weight loss coupons in the system, are highly variable, between 0.5 to 450 mils per year (mpy). Chemical treatment operating costs for Cahn 3 continued to be significant.
OBJECTIVES OF THE PROJECT 1. Implement EN and LPR corrosion monitoring techniques to investigate the process corrosion problems. 2. Determine the effectiveness of the corrosion inhibitors and biocides used. 3. Correlate changes in measured corrosion rate with plant operations or changes in plant conditions.
OVERVIEW OF PLANT OPERATION AND PROCESS FLOWS The Cahn 3 wate
ABSTRACT This paper reviews different types of computer applications utilized for solving problems in corrosion science and engineering. Brief descriptions of different types of computer applications, including expert systems, neural networks and object-oriented software systems are provided. A description of some of the currently available computer tools for modeling corrosion and cracking problems, selection of materials/equipment as well as for corrosion management, monitoring and control is also given.
INTRODUCTION Computers and computer-based information systems have revolutionized our approach to problem-solving, information access and knowledge processing in every domain of human endeavor. Corrosion science and engineering has benefited from the application of numerous computer-based systems and tools, promoting automated data/information access and efficient problem-solving.
This paper provides an overview of computer applications utilized for solving corrosion-related problems, data storage and data analysis. An introduction to computer-based corrosion problem- solving is followed by a description of types of computer programs employed in the domain of corrosion. This includes brief descriptions of different types of computer applications, including expert systems, neural networks and object-oriented software systems. A description of currently available computer tools for modeling corrosion and cracking problems, selection of materials/equipment as well as for corrosion management, monitoring and control is also provided.
BACKGROUND Using computer tools to model and represent corrosion processes is a challenging task since characterizing corrosion processes requires a fundamental understanding of principles underlying multiple disciplines, from electrochemistry and fluid mechanics to material science and engineering. The complexity of characterizing corrosion-related tasks has necessitated use of computer tools in corrosion science and engineering, from modeling to data acquisition and analysis. Computers, in the current day environment, are an intrinsic part of both data representation and automated problem solving. In this context, computer-based corrosion problem solving systems may be classified as,
? Systems for modeling corrosion/cracking processes ? Material selection and equipment specification programs ? Systems for design, analysis and inspection ? Computer-based corrosion monitoring systems ? Computer-based systems for control of corrosion testing equipment ? Databases and hyper-text systems ? Internet-based databases and software programs
A large number of early programs in corrosion were billed as expert systems, primarily because the programs typically attempted to capture human expertise in corrosion , and these programs represented research-based development efforts normally lacking rigorous software engineering foundations necessary for commercial distribution. Most of these programs were developed using software platforms called shells  that supported easy implementation of heuristic rules (rules of thumb) and representation of common concepts of reasoning. The list below provides a few well known computer programs in corrosion developed in the late eighties and early nineties [3-8]. It is interesting to note that none of these early systems were implemented in commonly used programming languages (such as C, C++, Fortran etc.) and many were implemented by corrosion/materials specialists with little or no formal training in software development. 
List of Early software (expert) systems in corrosion
The single most popular application of computers in corrosion stems fro
ABSTRACT The excellent corrosion resistance of nickel-alloys has been put to good use in marine engineering for many years. Some applications, such as bolting, require high levels of strength as well as corrosion resistance. New high strength nickel- alloys and their weldments exhibit excellent resistance to hydrogen embrittlement and seawater corrosion. Solid solution nickel-based alloys such as alloy 686 (UNS N06686) obtain their strength through cold work. Other highly corrosion resistant nickel-alloys such as alloy 925 (UNS N09925) and alloy 725 (UNS N07725) are precipitation hardened. Both the cold worked and the precipitation hardened alloys exhibit exceptional strength, ductility and toughness.
INTRODUCTION The U.S. Navy often uses corrosion resistant fasteners with corrosion sensitive materials such as steel, which requires cathodic protection. For example, MONEL alloy K-500 (UNS N05500) fasteners are used with alloy steel in a seawater environment. The steel receives cathodic protection flom sacrificial anodes. The protection is extended to the alloy K-500 fasteners. Failures of the alloy K-500 have occurred due to hydrogen embrittlement problems and also due to corrosion resulting from galvanic interaction with more noble materials. The U.S. Navy currently has a need to replace alloy K-500 fasteners, which can suffer hydrogen embrittlement, with a high strength corrosion resistant alloy. INCOLOY alloy 925, and INCONEL alloys 686 and 725 are highly corrosion resistant nickel-based alloys, which exhibit high strength, toughness and superior corrosion resistance and therefore are excellent candidate materials to replace alloy K-500 as a fastener material for the Navy in various applications.
Alloy 686 is a solid solution nickel-base alloy capable of being cold worked to high yield strengths, such as 90 to 100 ksi (690 MPa). Alloy 686 was originally developed for Flue Gas Desulfurization (FGD) and chemical process applications. Alloys 925 and 725 are age-hardenable nickel-base alloy capable of being aged to the minimum yield strengths of 110 ksi and 120 ksi (758 MPa and 827 MPa), respectively. Alloy 925 and alloy 725 are strengthened by precipitation of gamma prime [Ni3 (Ti, Al)] and gamma double-prime [Ni3 (Nb, Ti, A1)], respectively. Alloys 925 and 725 were developed for Oilfield applications such as tubing hangers, subsurface safety valves, Christmas trees, valve trim, packers, and other down hole equipment for severe sour service.
Alloys 686, 925 and 725 are resistant to hydrogen embrittlement in the NACE International TM0177 t sulfide stress cracking test and are listed in the NACE MR01752 document "Standard Material Requirements- Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment" and to chloride stress corrosion cracking in severe sour brine environments. Sulfide stress cracking is considered in the Oilfield to be the most severe for hydrogen embrittlement. Depending on the alloy, applications include use in chemical and food processing, marine and offshore platform equipment, oilfield wellhead and subsurface equipment and tubular goods, salt plant evaporators, air pollution control systems, condenser tubing, service water piping and feedwater heaters in the power industry.
DISCUSSION Testing Unless otherwise specified, duplicate corrosion specimens were tested, and alloys 925 and 725 were tested in the standard solution annealed plus age-hardened conditions listed in Table 3.
Composition and Mechanical Properties The complete limiting chemical composition is given in Table 1.
Table 2 exhibits the Room Temperature Tensile properties for cold worked alloy 686. The material displays excellent stren
ABSTRACT An understanding of the characteristics and the handling and protection requirements for the materials used in building water systems is essential to reliability of the building systems. This requires selection of the proper metals and alloys for proper and uninterrupted service through the life of the system. System reliability is also based on effective water treatment and service programs plus ongoing monitoring to minimize corrosion, deposition and microbiological problems that can impact system performance.
INTRODUCTION An active building has many service water systems in use for HVAC purposes and many different materials in those systems. Piping, tankage and equipment systems in buildings are a major part of this support system in every building. Without the HVAC and utilities equipment and associated piping systems, buildings could not cool, heat, ventilate, humidify, supply water, supply energy, remove waste products, or be protected from fire. In short, the buildings as we know them, could not be. It is critically important to select proper materials and coatings for these applications, effectively treat or protect these systems and evaluate the condition of water-carrying and fuel/steam/process pipes in buildings. The object of these procedures and evaluations is to determine the performance of the systems and to determine the extent of any deterioration that may have occurred, establish the cause(s) of this deterioration, and provide information needed to make informed decisions concerning future operations to protect the building. In this paper, a basic educational summary, we will review properties and corrosion behavior of metals and materials commonly used in buildings, types of corrosion, methods of chemical water treatment monitoring and service programs, and inspection requirements necessary to maintain cost-effective reliability.
BUILDING SYSTEMS MATERIALS The major water utilities systems in any building consist of cooling systems (i.e., piping, condensers, cooling towers, storage tanks, chillers) and heating systems (i.e., steam system, hot water piping, condensate piping and boilers). The predominant piping material in HVAC systems is mild steel. Copper is commonly used for heat transfer tubes because of its heat transfer properties and its inherent corrosion resistance. Other materials commonly used are: brasses and other copper alloys, cast irons (gray, ductile and malleable) for valves and fittings, stainless steels, and plastics. Table 1 shows general application of materials in building water systems. The following discussion covers general types of corrosion that can impact these materials, and a detailed review of specific material properties and corrosion problems for each metal.
TYPES OF CORROSION There are a number of different types of corrosion:
Uniform or general corrosion takes place at a generally equal rate over the entire surface and usually is corrosion resulting from acids in a water environment on metals having minimal to no protective properties.
Pitting corrosion is nonuniform, occurs at a localized anodic area, may be sharp and deep, and is an example of an environment offering some protective properties but not complete corrosion inhibition. It is associated with concentration-cell corrosion, galvanic corrosion, and crevice corrosion.
Galvanic corrosion occurs as a result of the exposure of two dissimilar metals in the same environment and is most noticeable when they are directly connected. On the basis of the relative potential of the two metals, the one less noble will corrode (the anode), leaving the other more noble metal (the cathode) intact, thus offering protection for the catho
ABSTRACT The Colville River cathodic protection (CP) system is unique in many ways. The cased pipelines are isolated from the mainline CP system and are directionally drilled 85 feet (26 m) below the river surface. Both thawed soils under the active channel and frozen soils under the floodplain and river banks characterize the crossing. These conditions make for differential thermal and soil resistivity states over time. The CP system is designed to protect the casings from external corrosion, and must function in a very dynamic and aggressive environment. High temperature and high chloride content characterize the operating environment for the pipelines. The design of this system utilized laboratory testing of soils, a Finite Element Analysis (FEA) modeling effort, and a unique anode and reference electrode design. The anode is constructed out of an 8 inch (220 mm) seamless steel pipe that has a series of internal redundant positive connections as well as five (5) reference electrodes used for monitoring.
INTRODUCTION Location ARCO Alaska Inc. is constructing the Alpine Development to produce oil that is located beneath the Colville River delta on the North Slope of Alaska. The development is approximately 34 miles (55 km) west of the established North Slope Kuparuk oilfield. Kuparuk oil is transported to the 48 inch (1,200 mm) Trans-Alaska Pipeline that connects the North Slope to the Valdez, Alaska marine terminal. Above ground insulated pipelines will connect the Alpine Development to the Kuparuk pipeline system. To exit the Colville River delta the pipelines must cross the 3,600 ft (1,100 m) wide east channel of the Colville River. Several studies of alternative crossing techniques pointed to horizontal directional drilling (HDD) as a cost effective and environmentally sound means of constructing this pipeline crossing. The crossing is located 8 miles (13 km) southeast from the Alpine Development that is approximately centered within the Colville River delta (70°14'41 '' N, 150051'28" W). The Colville River is the largest river on Alaska's North Slope and drains approximately 20,700 square miles (53,600 km2). At the crossing the river is 3,600 feet (1,100 m) wide between high banks. The length of HDD bore is approximately 4,300 ft (1,310 m). Two-2,000 ft (600 m) sections characterize the river; the deeper "active" channel to the east and a floodplain to the west. Figure 1 illustrates a profile section of the river crossing, facing north or down river.
FIGURE 1 - Colville River Crossing, Profile Section System Description
The four crossings, consisting of a 14 inch (350 mm) crude oil pipeline, 12 inch (325 mm) seawater pipeline, 8 inch (220 mm) utility casing, and an 8 inch (220 mm) impressed current anode, have been installed over two winter seasons. The crude oil and seawater pipelines are housed within 20 inch (500 ram) and 18 inch (450 mm) casings, respectively, providing secondary containment and leak detection under the environmentally sensitive Colville River. The three pipeline casings (Oil, Water, Utility) are coated with a dual layer fusion bonded epoxy and cathodically protected with a continuous parallel anode driven by impressed-current rectifiers. The anode is 8.625 inch (220 mm) diameter, 0.5 inch (13 mm) wall, steel pipe running parallel to, and positioned in order to optimize current distribution to the three casings. The anode is made of API Grade 5LX-65 seamless pipe. Current from the anode will be impressed onto the three pipeline casings from full-wave rectifiers located on the west bank of the Colville River.
Monitoring of the CP system will utilize reference electrodes placed along, and isolated from, the anode as well as a re
ABSTRACT The control of bacteria attached to surfaces - commonly known as biofilm - is becoming recognized as perhaps the most important function of an industrial microbiological program. We describe here a versatile laboratory method for the generation of reproducible biofilms and demonstrate its utility in assessing the biofilm eradication ability of several common industrial oxidizing and non-oxidizing biocides.
INTRODUCTION A major function of a good industrial biocide program is to control bacteria and other microorganisms in the bulk water and on system surfaces. Control of bacteria on surfaces - commonly referred to as biofilm - is becoming recognized as perhaps the most important function of an industrial biocide. This is because biofihn development can significantly impact the economics, service life, and safety of recirculating and once-through water systems.
Biofilms pose numerous problems in industrial water systems. Biofilm growth on heat exchanger surfaces can dramatically decrease heat transfer efficiency and lead to increased energy consumption and operating costs. 1 Biofilm deposits collect on cooling tower film fill surfaces - high efficiency film fill types are particularly prone to this - and can compromise cooling tower efficiency to the point of clogging or even collapsing the entire fill structure. 2'3 Biofilms lead to under-deposit corrosion through the formation of differential aeration cells and through the process of biomineralization which concentrates corrosive inorganic chemical species on the metal surface. 4 In addition, biofilms create a low-oxygen environment for undesired anaerobic organisms such as sulfate reducing bacteria which cause accelerated microbiologically induced corrosion (MIC) and pitting. 5 Finally, biofilms can provide a food source for protozoa and other single-cell organisms which essentially function as bioamplifiers for Legionella pneumophila, the bacteria responsible for Legionnaire's disease. 6'7
It is apparent that good control of biofilm is a critical requirement for an effective industrial water treatment program. Conventional techniques for biofilm growth and evaluation include continuous bacterial culture methods using the Robbins device and similar devices, 8j° annular reactors, ~ flow-through tubes, 12 as well as batch-type methods employing coupons, slides, or disks mounted in jars, flasks, or aquaria/TM Recently, a non-destructive method for biofilm growth and evaluation was disclosed based on real time monitoring of changes in heat transfer resistance, dissolved oxygen, and pH.~7 Deposition monitors and electrochemical probes provide additional dynamic methods to assess biofilm formation and the action of biocides. ~8 In general, it has proved difficult to reproducibly grow a number of biofilms in the laboratory and determine the effects of various biocide programs without investing in elaborate equipment and resorting to rather tedious and time-consuming microbiological evaluation methods such as most probable number and standard plate count methods.
We report here a new device for growing number of reproducible biofilms together with a simple method for determining biofilm viability after biocide challenge. Results are obtained quickly - within twenty four hours - which enables rapid changes in formulations. This new technique was used to study the effects of biocide concentration, biocide contact time, and biofilm age on biofilm eradication for a series of oxidizing biocides. It was also used to compare the biofilm eradication properties of several commercial biocide systems used in industrial and recreational water treatment. The results obtained with the new technique are compared to some of tho
ABSTRACT Most laboratory corrosion tests simulating waterwall corrosion in coal-fired boilers indicate corrosion rates of carbon and low alloy steels well below those recently found in boilers retrofitted with staged low NOx burner systems, especially those found in supercritical boilers. In this paper we studied the effect of boiler deposits on waterwall corrosion. It was found that FeS rich deposits can increase corrosion rates up to tenfold, under conditions in which iron sulfide can decompose to corrosive gas species. This generally occurs under oxidizing, mildly reducing or alternately oxidizing and reducing conditions. The actual presence of reduced sulfur species was confirmed in separate tests.
INTRODUCTION Waterwall corrosion has been an occasional problem in coal-fired boilers for as long as such boilers have been used in the industry. The basic failure mechanisms and perceived root causes have been described in great detail in EPRI report TR-105261, Volume 2, Chapter 18 by Dooley and McNaughton (~). Since a significant increase in the extent and severity of waterwall corrosion has occurred since the introduction of low NOx burner systems, especially those featuring overfire airports (OFA's), it is useful to briefly review the generally accepted understanding of the root causes of the wastage.
Under normal, oxidizing operating conditions low alloy or carbon steel waterwalls are protected from rapid wastage by the formation of an iron oxide, usually Fe304 scale. 3 Fe + 2 02 ---~ Fe304 (eq. 1)
The scale thus formed is dense, impermeable to gases and strongly adheres to the tube. Such a scale grows slowly and the growth rate decreases with time. Thus, the corrosion loss by oxidation is so low that metal loss becomes generally negligibly small and tube life is not determined by fireside corrosion. Since the scale strongly adheres to the tube, it is also not easily removed by normal sootblowing operations or mechanical and thermal cycling. The presence of SO2 in the fluegas may increase the porosity and decrease the strength of the scale slightly, but has no major effect on service life.
When the fluegas contacting the waterwall does not contain excess oxygen, and contains significant amounts of CO, sulfur in the coal may be partially converted into H2S instead of SO2. Under such conditions there is also some H2 present in the fluegas and the reactions FeS2 + CO + H20 --+ FeS + H2S + CO2 (eq. 2) S(org) + H2 ~ H2S (eq. 3)
become possible. When H2S is present in the fluegas it will preferentially react with iron in the waterwall tubes to form FeS.
Fe + H2S ---) FeS + H2 (eq. 4)
Already formed Fe304 may also be transformed to FeS.
Fe304 + 3 H2S + CO ~ 3 FeS+ 3 H20 + CO2 (eq. 5)
Thus depending on the relative amounts of H2S, SO2, CO and CO 2 present in the fluegas, the scale formed on the steel may consist of iron oxide (Fe304), mixtures of Fe304 and FeS or nearly pure FeS. The strength and adherence of the scale decrease with increasing FeS content, while its growth rate and permeability increase significantly. The result is increased metal wastage, which becomes the dominant factor in tube life. The weak, sulfur rich scale is also more easily removed by sootblowing or thermal cycling, which may further increase metal wastage.
The major concerns are how high the metal wastage can be for a given boiler condition or low NOx burner system, and what is practically possible to reduce high wastage rates.
From experience in coal gasifiers, it is known that corrosion rates of low alloy steels under extremely reducing conditions, i.e., CO = 30-60%, H2S 2000-10,000 ppm, are a function of the H2S content of the