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 Three component Polyethylene systems- epoxy, adhesive, polyethylene compound - have been used for steel pipe protection with an impressive track record in and outside Europe since the 1970's. More recently in Europe, we have seen the emergence of a new generation of High Density Polyethylene (HDPE) system and this is quickly becoming the reference for the most demanding domestic and international projects. This paper covers the technical aspects of such advanced new solutions, with comparisons with more traditional solutions, e.g. Low Density Polyethylene (LDPE) Three-component system commonly used in Europe.
INTRODUCTION In 1997, the steel pipe coating market saw the introduction of a new generation of HDPE compounds. This HDPE compound has superior properties compared to currently used polyolefin systems. Combined with a grafted adhesive and a compatible epoxy primer, this advanced three- component system shows outstanding performance concerning impact resistance (high and low temperature), indentation resistance, SCG and RCP (Slow and Rapid Crack Propagation), stress cracking resistance and UV resistance. The top layer compound is produced with an highly advanced bimodal technology. In addition to the excellent physical properties, it also allows high speed production due to its high melt strength and extrudability (+20 to 40% compared to conventional High Density & Medium Density grades). This results in more square meters coated per hour without additional costs. In Europe, this generation of HDPE three-component system is replacing LDPE solutions for onshore & offshore applications. It is also an alternative to Polypropylene (PP) systems for service temperatures up to 85°C, impact and indentation performances being in line with PP values. Because of its properties, this new system has recently been approved by domestic gas companies, Gaz de France in France and Snam in Italy, and selected for major upcoming projects. Gaz de France has been the first gas company in Europe to authorise a thickness reduction of 15 % (- 0.5 mm ) with an HDPE system compared to traditional LDPE and MDPE systems, with improved safety factors.
Figure 1: Three- Component HDPE system
Why a bimodal High Density Polyethylene system? Until 1996, there were three types of polyolefin steel pipe coating systems typically used in Europe. These were based on: Low Density Polyethylene (LDPE), Medium Density Polyethylene (MDPE) and Polypropylene (PP) for temperatures up to 110°C (230 °F). Based on decades of general experience in designing polymers for long service life (50 to 100 years) in pipe applications and the specific requirements of polyethylene for steel pipe coating application, a new high density polyethylene system evolved. In contrast to the unimodal process common for PE production, a bimodal process using dual reactor technology, as the Borstar one shown figure 2 was used in order to combine mutually opposed properties (e.g. high density with excellent impact resistance and a broad processing window). This patented technology is based on a unique combination of a slurry loop reactor and a specially designed gas phase reactor. With two truly independent reactors running in series, the process is highly flexible, allowing the production of polyethylene with controlled breadth of molecular weight and comonomer distribution. The resulting bi-modal polyethylene is tailor-made to have an optimum balance between mechanical strength, stress crack resistance and processability."
Figure 2 : Flow-chart of the Bimodal technology
The improvements gained in material performance using this new summarised in the following diagram and subsequently described in detail: bimodal tec
ABSTRACT Many conventional chemically treated closed loop hydronic systems suffer from corrosion associated with oxygen. The failures have been approached through the use of traditional chemicals such as oxygen scavengers/metal passivators and thermal mechanical deaeration pretreatment. While these approaches have worked well for high temperature hot water heating under minimal makeup conditions, the chemical eradication process eventually fails under high volume makeup conditions. Also, deaeration is not a suitable solution for smaller low temperature systems. This paper will discuss the conditions, which lead to corrosion in these systems. An outline will also be provided for corrosion inhibitor performance expectation. A number of 'tell-tale' signs or observations that can be made without extensive chemical testing or exhaustive system study will also be presented.
INTRODUCTION Closed loop hydronic systems can be found both in the commercial/institutional and industrial process sectors. By definition, closed means these systems take on minimal to no makeup, are not open to the surrounding atmosphere, and are equipped with pressure relief valves, flow control valves, air vents and a closed expansion/compression tank. The design and operation of these systems are well publicized in literature.~'2'3 The greatest majority of hydronic system failures associated with corrosion, are not related to water treatment. They are typically related to excessively high water losses, inadequate pressurization and venting. These scenarios also negatively impact both the thermal and electrical efficiency of the system regardless of the duty. The presence of entrained air and it's dissolution or operation of the system in a partially flooded state not only reduces the ability of the system to extract or give up heat, but also the transport of said heat, as the mass flow rate and hence thermal flow rates are reduced. There are many categories of closed hydronic systems, and they are listed in Table 1.
This paper will only discuss issues that pertain to chilled and low temperature hot water systems that do not contain atmospherically open expansion tanks or reservoirs. The latter requires back pressure control valves on the return main and automatic shut off valves on the recirculating pump suction, return main and the connection line to atmospheric expansion/compression tanks, if so fitted. When the recirculation pumps are stopped, then the valves automatically close in order to keep the hydronic piping network fully flooded. Closed loop and glycol systems under applied heat do not suffer from scale formation unless there is water loss resulting in high makeup quantities of hard water. Mineral scale formation is not found in chilling systems. The main concern in these systems is that of corrosion. Unfortunately, the failed specimens removed from a closed system are most often quickly diagnosed as pitting-oxygen type corrosion. The corrosion may very well be due to other causes or contributing factors, which preclude the performance realm of aqueous corrosion inhibitors. Therefore the term 'localized corrosion' better describes corrosion occurrences in closed loop systems. Corrosion in these systems may result from:
? galvanic coupling ? ennoblement of copper plating ? cavitation ? erosion of copper/copper alloys due to excessive velocity ? oxygen/air ingression ? poorly operating expansion/compression tanks, automatic (pressure set) makeup valves, pressure relief valves and automatic air vents ? microbiological activity
The typical corrosion inhibitory blends for closed loops are not designed to be substitutes for the preceding undesirable operational practices. Fo
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 Laboratory testing of chemicals is considered effective when the conditions tested resemble the operating conditions where the chemical will be used. Six inhibitors were tested under a CO2 partial pressure of 2.5 MPa, three different flow regimes, and two water/oil/methanol compositions at 40°C to show their effectiveness in limiting the corrosion rate. Turbulence under slug flow conditions produced the highest corrosion rates and one inhibitor was less effective when methanol was added to the system. Therefore, testing of inhibitors under multiphase flow conditions is recommended for pipeline inhibitor selection studies.
INTRODUCTION Choosing the appropriate inhibitor for a multiphase pipeline is an arduous task requiring information from various sources to select the one that best fits the overall conditions. Multiphase flow at high partial pressure CO2 in conjunction with a 2 cp oil produces different facets to look at with respect to the corrosion rate. First, with the increase in the CO2 partial pressure, an increase in the corrosion rate is expected (Videm and Dugstad, 1989) 1. Second, the increase in the oil/water fraction, up to 60%, has the effect of increasing tile corrosion rate (Vuppu, 1994) 2. And third, an increase in the turbulence of the flow, ie. slug flow, associated with the multiphase system will also produce a higher corrosion rate.
Two important parameters that help define the effect that slug flow has on corrosion are slug frequency and Froude number. The relationship between superficial gas velocity (Vg), superficial liquid velocity (V,1), and slug flow for this experiment is seen in Figure 1 (Cai, 1999) 3. Sun and Jepson (1992) 4 show that the Froude number gives a measure of turbulence and associated wall sheer in the mixing zone at the front of the slug. Froude number is defined as:
[Equation is available in the full version of this paper]
translational velocity average velocity of the liquid film acceleration due to gravity effective height of the liquid film Bhongale and Jepson (1996) 5 and Menezes and Jepson (1995) 6 experimented with slug flow in 10 cm diameter, horizontal flow loops at 0.27, 0.45, & 0.79 MPa partial pressure CO2 and indicated that corrosion rate increased with an increase in Froude number, carbon dioxide partial pressure, and/or temperature.
In lifts set of experiments, six inhibitors were tested under 2.5 MPa CO2 partial pressure and multiphase flow conditions for two different flow regimes, full pipe oil/water flow and three phase oil/water/gas slug flow. The liquids used in this study are 0.5% NaCI solution, laboratory grade methanol, and oil. At 40°C, the oil has a density of 800kg/m 3, a viscosity of 2 cp, and is similar to light condensate oil. Oil-water mixtures at 50% water cut were used for the liquid phase. All inhibitors were added to the system at 150 ppm concentration based upon the aqueous phase volume.
The objectives of this work are to measure corrosion rates under multiphase conditions with a high partial pressure of CO2, to compare the effectiveness of 6 different inhibitors under similar conditions of high partial pressure of CO2 and in different multiphase flow regimes, and to compare the emulsion tendencies of the 6 inhibitors in the study.
ABSTRACT The hydrogen embrittlement controlled stage II crack growth rate of AA 7050 (6.1% wt. Zn, 2.1% wt. Mg, 2.2% wt. Cu) was investigated as a function of temper and alloyed copper level in a humid air environment at various temperatures. Three tempers representing the underaged, peak aged, and overaged conditions were tested in 90% relative humidity (RH) air at temperatures between 25 and 90 °C. At all test temperatures, an increased degree of aging (from underaged to overaged) produced slower stage II crack growth rates. The stage II crack growth rate of each alloy and temper displayed Arrhenius-type temperature dependence with activation energies between 53 and 97 kJ/mol. For both the normal copper and low copper alloys, the fracture path was predominately intergranular at all test temperatures (25-90 °C) in each temper investigated.
Comparison of the stage II crack growth rates for normal and low copper alloys in the peak aged and overaged tempers showed the beneficial effect of copper additions on stage II crack growth rate in humid air. In the 2.2 wt.% copper alloy, the significant decrease (-10 times at 25 °C) in stage II crack growth rate upon overaging is attributed to an increase in the apparent activation energy for crack growth. In the 0.06 wt.% copper alloy, overaging did not increase the activation energy for crack growth but did lower the pre-exponential factor, vo, resulting in a modest (-2 times at 25 °C) decrease in crack growth rate. These results indicate that alloyed copper and thermal aging affect the kinetic factors which govern stage II crack growth rate. Overaged, copper bearing alloys are not immune to hydrogen environment assisted cracking but are intrinsically more resistant due to an increased apparent activation energy for stage II crack growth.
BACKGROUND Hydrogen Controlled Crack Growth in AI-Zn-Mg-(Cu)Alloys Precipitation hardened AI-Zn-Mg-(Cu) alloys are susceptible to intergranular environmentally assisted cracking (EAC) when exposed to wet gaseous environments [1-7]. Crack growth kinetics in moist gases are controlled by the relative humidity level, independent of the gas composition as shown by Hyatt and Speidel [1, 2]. Dry molecular gases such as hydrogen (H2), oxygen (O2), nitrogen (N2), and air (-78% N2, 21% 02) do not initiate or support EAC crack growth. However, EAC readily initiates and propagates when precracked specimens loaded to near Kic are exposed to wet gases. [1, 2]. In contrast to molecular hydrogen gas, intergranular cracking in 7XXX series alloys has also been observed in ionized hydrogen gas . Since molecular hydrogen does not readily dissociate on 7XXX series alloy surfaces , ionizing likely promotes hydrogen uptake . These results indicate that internally dissolved hydrogen is embrittling to 7XXX series aluminum alloys.
The water vapor content of gases has a limited effect on stage I of the crack velocity vs. stress intensity (or "v-K"), curve but stage II crack growth depends linearly on the water vapor pressure in A1- Zn-Mg alloys [1, 2]. Crack growth is observed at low relative humidities, where water condensation is unlikely at the crack tip and hydrogen embrittlement is implicated as the embrittlement mechanism. The linear dependence of crack growth rate on relative humidity from -1-99% ~ strongly suggests that the crack tip is not filled with condensed water and metal dissolution does not control crack growth at relative humidities _< 99%. Instead, the reaction of aluminum with water vapor to produce high fugacity hydrogen gas is thought to control EAC of A1-Zn-Mg-(Cu) alloys via hydrogen embrittlement (see Equation 1 where X is the degree of hydration) [1, 2] and cracking in humid air is more accurat
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 development of new oil production areas and deep water activities requires more efficient and reliable cathodic protection (CP) designs. In this regard, laboratory tests that simulate service conditions can be very valuable. Preliminary results have shown that it is possible to obtain, on a short term basis, meaningful data that can be applied to the design of offshore CP systems. Current practice and challenging aspects of the methodological aspects of using laboratory electrochemical techniques to develop CP guidelines for offshore structures are addressed. The discussion focuses on those aspects that are more relevant to obtain on a short-term basis useful information that could provide for the successful long-term operation of new CP systems. A protocol recently introduced by Hartt et al. was utilized to evaluate the potential of relatively short-term laboratory data for predicting mean current densities. New ways of predicting long-term current densities based on accelerated laboratory tests are discussed.
INTRODUCTION This paper is intended to discuss some of the methodological aspects of applying laboratory electrochemical testing to develop offshore cathodic protection guidelines. A new protocol recently introduced by Hartt et all will be utilized to evaluate the potential of relatively short-term laboratory data for predicting mean current densities. The advantages of using electrochemical laboratory techniques should not be underestimated if we take into account the substantial economical impact that may result by anticipating long-term cathodic protection performance from relatively short-term data.
The problem of selecting the most appropriate methods to provide data of good quality for offshore CP designs in new development areas is not an easy task. The current state of the art in the instrumentation technology allows the monitoring of the performance of offshore CP systems. The experience and data accumulated from such monitoring systems can be of great value in order to improve the performance of upcoming CP designs. However, the expansion of oil production operations to deeper waters and new development areas needs quality data that could provide for the success~l long-term operation of the new CP systems.
The exposure of instrumented panels in the targeted areas is a good alternative, the data can be obtained in the real conditions where the structure will operate. It is an approach that several major oil companies and research institutes have chosen to obtain their current density requirements. Another alternative is to conduct laboratory testing. The advantage of obtaining these requirements with short- term laboratory techniques are obvious. The use of such techniques would imply cost savings and also would provide quality data with the degree of confidence that the CP designers need. Experimentation with natural environments in the laboratory is a technical challenge. The experimental design should be capable of reproducing similar kinetics of nucleation and growth of calcareous deposits in the specific geographical areas. It is well recognized that it is the quality of the calcareous deposits that determines the current requirements for CP designs.
Additionally, when sacrificial anode materials are involved in such experiments, care should be taken to provide the anodic material the minimum conditions for a proper activation and steady dissolution. Deepwater operations started in the early nineties accompanied by a dramatic move to deeper waters between the years 1994 and 1997. Such a short period of time is clearly not enough to accumulate all the necessary data to accomplish new designs. The most experience in offshore has bee
ABSTRACT Performance enhancing chemicals were applied to metallized zinc anodes on three reinforced concrete field structures: a parking garage, a condominium, and a motorway substructure crosshead. In the parking garage, a structure that contained pre- stressed, post-tensioned reinforcing steel, 4-hr potential decays averaging 146 mV were measured on tendon anchorages. On the motorway crosshead, polarization was satisfactory 6 months after energizing, but decreased after 9 months operation in galvanic mode. Use of the enhancing agent at the condominium resulted in significant improvement in both protective current and steel polarization.
INTRODUCTION Corrosion of steel reinforcement is recognized as one of the major contributors to the deterioration of reinforced concrete structures. The technique of cathodic protection (CP) has been shown to be a highly effective technique for the prevention of corrosion of steel in concrete due to chloride contamination from deicing salt, set accelerators or seawater.l"2 Several CP systems using various types of anodes have been developed and applied over the past 24 years for the delivery of protective current to reinforcing steel embedded in concrete structures.l
California Department of Transportation researchers investigated several types of coatings for use as anodes on concrete in 1983. 3.4 The results of their study of fifteen different painted and metallized coatings indicated that flame sprayed zinc provided the best combination of cost, effectiveness, acceptability and coating consumption rate for cathodic protection. Following this study, improvements to arc-spray metallizing equipment greatly improved the application rate of metallized zinc which, therefore, decreased the costs of this coating.
Arc-sprayed zinc has recently found favor as an anode on bridge substructures, principally because the zinc coating can be easily applied to the complex shapes of substructure surfaces. A survey conducted in 1994 identified 22 systems in 7 states and 2 countries using metallized zinc anodes on over 46,000 square meters (500,000 fiE) of concrete surface. 5 The gray coloring and thin aspect of the zinc coating has made it particularly useful for the architecturally complex surface found on historic bridges. 6 The low resistivity of zinc (about 10 6 times more conductive than carbon-based coatings) allows uniform distribution of CP current, and simplifies current distribution requirements.
Also, the inherently negative potential of zinc allows it to function, at least in theory, as a galvanic anode without the need for impressed current. Operation in galvanic mode has the potential to greatly simplify CP systems, since no rectifiers, AC power, or DC wiring and conduit are necessary. This simplicity reduces initial cost, as well as long-term costs associated with monitoring and maintenance. Furthermore, CP of prestressing steel based on the use of a galvanic zinc anode is generally considered safe from the standpoint of hydrogen embrittlement. But some studies have shown that a galvanic zinc anode is unlikely to deliver adequate CP current unless the anode is subjected to direct periodic wetting. 7'8
Recent studies have confirmed that current delivered by metallized zinc anodes decreased quickly at low humidity to values unlikely to meet accepted CP criteria, but could be easily restored by direct wetting of the anode. 9 This same study reported that certain chemicals, when applied to the exterior surface of the anode, were capable of migrating by capillary action to the anode-concrete interface where they served to maintain the interface conductive and the zinc electrochemically active.
ABSTRACT Even though several stainless steels and a few nickel based alloys have shown promise and are used in marine environments, under very severe crevice corrosion conditions, most of these have suffered from localized crevice attack. The search for alloys that are essentially immune to crevice corrosion attack in marine environment led the industry to increase the alloy content of nickel based alloys primarily in chromium and molybdenum. One such alloy, alloy 59 (UNS N06059) having a typical chemical composition of 59% nickel, 23% chromium, 16% molybdenum and iron levels of less than 1%, appears to have fulfilled this need. Extensive laboratory and field tests by various companies and corrosion laboratories in USA, U.K., Norway, France and the U.S. Navy have shown this alloy to be essentially immune to crevice corrosion attack. Based on the excellent crevice corrosion resistance of alloy 59, the U.S. Navy has selected this alloy for testing a prototype component in a butterfly valve and is conducting further tests for overlay welding application as a superior alternative to alloy 625 and C-276. This paper presents a brief description of this alloy's development, its physical metallurgical characteristics and localized corrosion data from various test programs. Other companies are also evaluating this alloy for use in a weld overlay application on off-shore platforms.
INTRODUCTION Materials used in the marine industry, such as the U.S. Navy and offshore platforms, encounter numerous corrosion problems. The corrosion problems of primary concern are uniform corrosion, localized corrosion (pitting and crevice), stress corrosion cracking, galvanic corrosion, corrosion fatigue, and erosion corrosion. A large amount of corrosion data has been generated over the last few decades and is well publicized in the technical literature. (~9) Even though the precise determination of all corrosion variables as related to site specific marine corrosion is not fully categorized, there is ample laboratory, field, and case history experience available to make cost effective and functionally reliable maintenance-free selection. Table 1 lists the various classes of materials, usually specified and used in seawater service, whereas Table 2 lists the nominal chemistry of some of these alloys. Coated carbon steel, along with most of the materials listed in Table 1 and Table 2, have been successfully used in marine applications although in certain very specific severe crevice corrosion conditions, the performance has not been totally satisfactory. The following sections describe the general metallurgical characteristics, corrosion resistance, mechanical properties and results of marine testing programs conducted at or by various institutions on alloy 59 (UNS N06059) along with a few applications in media with very high chloride contents.
METALLURGICAL AND CORROSION CHARACTERISTICS OF ALLOY 59 Alloys of the Ni-Cr-Mo family, starting with alloy C, date back to the 1930's. Since then improvements in the melting technology and a better fundamental understanding of the role of various alloying elements have led to newer Ni- Cr-Mo alloys. Their typical chemical composition is given in Table 3. The physical metallurgy and corrosion resistance (uniform corrosion, localized corrosion, thermal stability) of Ni-Cr-Mo alloys are very well documented in the open literature, including many applications of alloy 59 in chloride containing environments. °°-~2) Alloy 59 is one of the highest nickel containing alloy of the Ni-Cr-Mo family without any addition of other alloying elements such as tungsten, copper, or titanium and hence can be classified as the purest ternary form of a "Ni-Cr- Mo" alloy. It also has the highest