ABSTRACT A variety of austenitic alloys are available for use in flue gas desulfurization (FGD) systems. Alloys with Mo content varying from 2 to 16 percent have been used. The 6% Mo superaustenitic alloys perform significantly better in laboratory tests than 4% Mo stainless steels $31725, $31726 and N08904 and approach the performance of the nickel-base N06625 alloy, but fall short of the performance of the N10276 alloy. This presentation concentrates on one of the 6% Mo superaustenitic stainless steels, the N08367 alloy. The possibility of tailoring the PREN of the 6% Mo alloys is examined and the production of versions ofN08367 alloy having guaranteed minimum PREN values is described. L
aboratory corrosion data for N08367 alloy are presented. These data show that N08367 alloy is able to resist corrosion in high-chloride environments at temperatures of 66 to 71°C and pH levels as low as 2. The corrosion of alloys ranging from $31603 through N08367 to N10276 is shown to be aggravated by the addition of 1,000 ppm thiosulfate to 10,000 ppm chloride water at 66°C. The results of tests of N08367 alloy in a simulated SO2 absorber environment are described.
Fabrication of N08367 alloy for FGD applications is discussed and several FGD applications for N08367 alloy are described.
Overall, the 6% Mo alloys are shown to be cost-effective materials of construction to fill the gap between the 4% Mo austenitic stainless steels and higher-Mo nickel-base alloys.
INTRODUCTION Public demand for improved air quality led to the passage of the Clean Air Act and its subsequent amendments. These laws forced utilities to reduce their emissions of sulfur dioxide. Flue gas desulfurization (FGD) is one method for achieving this mandate. The use of lime or limestone slurries for "scrubbing" sulfur dioxide from flue gasses has become an established technology. Modem scrubbers are capable of removing in a cost-effective manner over 90 percent of the sulfur from the exhaust gas stream. For the FGD units to provide the desired improvement in air quality, they must operate reliably. Selection of suitable materials and proper fabrication are a vital part of the operability of the FGD process, and of power generation from fossil fuels.
A wide variety of metallic and non-metallic materials have been used for construction or repair of FGD equipment. Many different alloys have been employed. These have ranged from type 316L stainless steel (UNS $31603) through the various type 317 alloys ($31703, $31725, and $31726) and duplex stainless steels ($31803, $32550, etc.) to nickel-base alloys (N06625, N10276, N06022, etc.).
Factors involved in choosing materials for FGD include the composition of the fuel, the quality of the water used for scrubbing, the operating temperature, the build-up of impurities (such as chlorides) in the system, possible upset conditions, scaling or deposit buildup, and galvanic interactions.
Modem FGD units are divided into several zones which have different corrosion problems and may be built from different alloys. The most severe zones, such as the inlet and outlet duct regions, are now typically constructed using nickel base alloys like N10276. The absorber, sump, and mist eliminator zones are usually much less corrosive and lower alloy materials are frequently employed there.
6% Mo STAINLESS STEELS Many 6% Mo superaustenitic stainless steels have been developed in recent decades. The most common of these are $31254, N08367 and N08926. More recently developed 6% Mo alloys include N08031 and $32050. In addition to their high Mo-contents, all of the modem 6% Mo superaustenitic stainless steels are marked by deliberate additions of nitrogen
ABSTRACT This paper addresses basic issues germane to selecting materials for high-temperature aggressive environments. The overview provides first-choice options for dealing with corrosive conditions that extend beyond clean oxidation, including aggressive fluids and molten salts. Examples of material failures in various complex environments are included.
INTRODUCTION Materials are selected on the basis of the service requirements, notably strength, so corrosion resistance (stability) may not be the primary design consideration. Assemblies need to be strong and resilient to the unique loads and stresses imparted thereon which can include significant temperature changes and thermal gradients for many high-temperature applications. Of particular benefit to many users is the spin-off from materials developed for the aerospace and gas turbine sectors. Many of these high-strength alloys are also very corrosion resistant and despite high direct costs these materials can often be economically justified against overall running costs and longevity of service.
In making a choice it is necessary to know what materials are available (and when) and to what extent they are suited to the specific application. The decision tree is quite involved (Figure 1) and choice is significantly affected by the circumstances of the environment and the intended use, (reactor vessel, tubes, supports, shields, springs, etc.). Further material factors include the fabricability and costs. The user (or designer) needs to properly define (or recognize) that the environment dictates the materials selection process at all stages in the process or application. For example, an alloy that performs well at the service temperature may corrode because of aqueous (dewpoint) corrosion at lower temperatures during off-load periods, or through some lack of design detail and/ or poor maintenance procedure that introduces local air draughts (or similar) that cool the system, (e.g., at access doors, inspection ports, etc.). The merits of using alloys that are considered both corrosion resistant and high-temperature resistant are obvious. Demand for strong, high-temperature materials has continued over the past twenty years, as has research focussed on improving performance and reliability. Improvements in casting (vacuum melting) and fabrication techniques, (powder metallurgy, mechanical alloying and the use of oxide dispersions to not only strengthen an alloy but also to improve scale adhesion (e.g., rare earth oxides such as yttria and ceria) can be noted. Not to be excluded from these developments are the continued use of composite materials (including co- extruded tubes for reactors or boiler tubes) and of overlay weld procedures. Also, advances in the use of surface coatings or modifications. There are many publications that herald the arrival of "new" alloys, some of which are capable of withstanding complex environments even at temperatures above 1100 C (2012 F), but probably not for too long in some cases.
A frustration in dealing with many end users is their expectation that a failing system can be made to last indefinitely by simply changing to another material. There are times when miracles occur, but it is important to recognize that there is no single panacea for all applications and typically each case has to be viewed on its own merits. In order to provide as optimum a performance as possible it is necessary for a supplier to be aware of the application, and for the user to be aware of the general range of available materials. In general, the oil and chemical industries are better aware of these parameters than in other sectors, including, general engineering, food-meat processing (broilers, ovens) and others. I
ABSTRACT Internal corrosion has long been of major concern to pipeline owners, having spent millions of dollars and countless man-hours combating its destructive effects. Pipeline failures, due to internal corrosion, have produced environmental and legal issues that have resulted in huge fines and judgements, creating even greater concerns about its ambiguous nature. Over the years, a number of technologies have been introduced to deal with the variety of corrosion problems that exist in oilfield pipelines. One such option is a 50 year old technology known as in-situ coating. The in-situ coating process is a relatively simple procedure, whereas, the internal pipe wall is thoroughly cleaned, either by means of abrasive blasting or by a system that combines progressive pigging and chemical cleaning. This is then followed by the application of several coats of a suitable protective coating. The most common coating used is typically a high performance epoxy. The process is unique, whereas, pipeline pigs, instead of conventional spray equipment apply the coating.
The objective of this paper is to address the overall process of in-situ coating, which includes; cleaning options and effectiveness, coating selection and application, its limitations and residual benefits and the pipeline owners or operators involvement.
INTRODUCTION Pipeline companies continue to look for maintenance-free technologies to control internal corrosion. In-situ coating is viewed by some as a maintenance-free technology, whereas, a protective barrier is placed between the pipe wall and the corrosive environment. However, some minimal maintenance is recommended, such as periodic pigging, along with a minimal inhibition program. Pigging may be used to maintain the efficiency, should deposits and liquids effect flow, when left to settle in low lying areas. The use of inhibitors will provide additional assurance, should the coating become damaged due to improper pig use.
In-situ coating has proved equally effective in new and existing pipelines, both onshore and offshore. When applied to new lines, the system is an excellent first line of defense, preventing corrosion before it has a chance to start. However, in-situ coating is most commonly used when few options remain and total line replacement appears imminent. Rehabilitation of a pipeline with the in- situ coating process, will normally cost 30 % or less, than total pipeline replacement, depending on condition of the pipeline, number of lines to be coated, whether onshore or offshore or other location adversities. There has been articles written, indicating that properly applied coating could extend pipeline life as much as 40 to 50 years, depending on environmental conditions and minimal supplemental maintenance. The major advantage of in-situ coating, over other rehabilitation processes, is the ability to rehabilitate many miles of pipe, without having to cut or segment the pipeline. This is essential when working offshore. One major disadvantage, is that the unit price can be very expensive for pipelines less than three (3) miles in length. This is due to the base amount of equipment, materials, and personnel required for the process. The pipeline diameters are not extremely restrictive, although, most lines that have been in-situ coated range between the nominal size of 4" and 30".
The most obvious use of in-situ coating is to combat the ravages of internal corrosion; however, it has also been used solely for its residual benefits, such as, reducing friction to flow and maintaining product purity. Some pipelines have experienced an increase in efficiency after coating of 6% or more. In-situ coating is also a viable option when a pipeline requires a
ABSTRACT The effects of steel microstructure and composition on the inhibition of CO2 corrosion were studied at 25 °C, pH 5, 1 bar CO2 in 3 % NaC1 solutions. Generic inhibitor compounds cocoalkyl- dimethyl-benzyl ammonium chloride (QUAT) and sodium thiosulfate (THIO) were used. Sixteen different carbon steels were tested with a blend of 40 ppm QUAT and 0.5 ppm THIO. The test specimens were precorroded for six days in the corrosive medium prior to inhibitor addition. The effect of inhibitors was assessed by electrochemical polarization tests. The inhibitor efficiency ranged from 84 to 98 %. The variations in the inhibitor efficiency were attributed to differences in the amount of carbon (as pearlite) and copper in the steel. The inhibition of the cathodic reaction on cementite particles exposed by corrosion was identified as an important factor in the inhibition of CO 2 corrosion of carbon steel.
INTRODUCTION Carbon steel used in combination with corrosion inhibitors constitutes in many cases a favorable economic alternative for oil and gas pipelines compared to the use of corrosion resistant materials. However, inhibitors do not always perform as intended in the field. TM The reasons for such problems are complex and not yet well understood. Research into the factors that limit the inhibitor performance is thus of great importance. Furthermore, the development of reliable and representative testing methods are important for the selection and qualification of inhibitors, in order to give higher confidence in the use of these chemicals.
In laboratory tests the inhibition is often studied on freshly ground surfaces. Laboratory experience shows that inhibitors perform differently on corroded surfaces and freshly ground surfaces, s'~2 In the field, inhibitors encounter steel surfaces that are covered with different kinds of corrosion products, such as mill scale from pipe production and rust from storage and pipeline testing.
Furthermore, a pipeline may have been in operation for several years before increasing water cut necessitates inhibition. In this case the metal surface might be covered with iron carbonate precipitates, uncorroded iron carbide, and different types of scale. These products may significantly affect the performance of the inhibitor.
Previous inhibitor tests carried out in our laboratory 12 showed that long term precorrosion strongly affected the performance of the tested inhibitors. Four commercial, water-soluble corrosion inhibitors (imidazoline based and amine based) were tested. In general, the inhibitor efficiency decreased with increasing precorrosion time. However, large differences in performance were found between different steels and different inhibitor products. Severe localized corrosion attack was observed after long time exposure. The results indicated that the adverse effect of precorrosion was connected to the presence of a corrosion product film at the steel surface.
The objective of the present work was to shed more light on the interaction between the inhibitor compounds and the steel composition and microstructure. In order to achieve this goal, generic inhibitor compounds were used. Cocoalkyl-dimethyl-benzyl quaternary ammonium chloride (QUAT) was used in combination with sodium thiosulfate (THIO). QUAT is a cationic film forming inhibitor. 13 The term cocoalkyl denotes a mixture of carbon chains in the range C12 - C15. THIO is a common additive in many water soluble inhibitor products, due to its synergistic inhibition effect when it is used together with cationic nitrogenous inhibitor compounds. THIO probably works through the formation of a mixed oxide/carbonate film at the steel surface.
ABSTRACT A brief description of the process utilized in flue gas scrubbing systems is presented and the corrosion risks are analyzed in relation with the local environments which are supposed to exist in the main parts of the installation.
Then, Uniform and Localized corrosion performance of welded and non welded super-austenitic and super-duplex stainless steels are investigated and compared to nickel base alloys in laboratory conditions simulating actual flue gas desulfurization environments.
The main corrosion determining parameters investigated are pH, chloride and fluoride concentration as well as temperature.
Results gained from these tests were combined to field experience to design materials selection charts taking into account corrosion risks in various zones of scrubbers. Finally, field experience and references are presented.
INTRODUCTION Energy production using coal or fuel oil boilers generates exhaust gas polluted by sulphur compounds (SO2, SO3) and halide species (CI-, F-) which have to be treated to avoid the deleterious effects on human health and environment due to sulphuric (H2SO4) and hydrochloric (HCI) acid production in air. To achieve that, Flue Gas Desuiphurization (FGD) systems have been installed for about 20 years now mainly in United States of America, later in European countries and more recently in Korea, Taiwan and Thailand.
The most common technology applied in more than 80% of existing installations consists of scrubbing the polluted gas with a slurry of lime Ca(OH)2 or limestone CaCO3 in water (Wet FGD Process). The aggressive constitutants of the raw gas are SO2, SO3, HCI and HF but SO2 is the most concentrated. SO2 and SO3 react respectively with Ca++ and H20 lea- ding to sulfate compounds which precipitate as CaSO4. HCI and HF give CaCI2 and NaF which remain in the slurry. The chemical composition of the slurry is continuously monitored in order to insure an efficient neutralizing effect. Then the cleaned gas is blown into the atmosphere.
The process generates aggressive conditions in some parts of the FGD Unit which can cause severe corrosion problems ; as a consequence, high corrosion resistant stainless steels have to be selected in order to insure the efficiency of the system.
PROCESS CONDITIONS and CORROSION PROBLEMS
The raw gas produced by burning coal or fuel oil goes through an electrostatic filter in which fly ash is removed. Then, the hot gas ( generally 150-200°C/300-390°F) enters the scrubber unit and goes up while it is sprayed by the neutralizing slurry (generally 3 stages).
If the gas is HC1 rich, a pre-scrubber unit can be installed to remove this acid before the main scrubbing stage; a mist eliminator is generally placed between the pre-scrubber and the scrubber to reduce gas pollution by wet HC1. When sea water is used to wash the polluted gas, the mist eliminator is compulsory. The reaction tank where the neutralizing slurry is prepared is placed either at the bottom or near the scrubber ; a pump feeds the spraying system and insures the recirculation. A demister is placed inside the scubber above the upper spray level in order to remove liquid particles from the clean gas which reduces salt deposits. Then the clean gas is blown into the chimney.
As the clean gas is not completely depolluted, some acid condensates can be produced in the chimney causing some corrosion problems. So, in some cases, the clean gas is reheated in order to minimize such a risk. Moreover, during start-up periods, the polluted gas is generally partly by-passed ; this means that by-pass ducts and the chimney are submitted to very aggressive conditions due to high temperature and ac
ABSTRACT The increase in use of high quality and expensive pipeline coatings has heightened the need for field joint coating systems to match the quality of the factory coating. Recent developments in field joint coating technology have gone a long way to address this need. This paper describes one major Middle Eastern oil and gas company's experience with a number of field joint coating systems for three layer polyethylene and polypropylene coated pipes. This experience includes coating well over a 100,000 field joints in some of the toughest conditions (extreme heat and humidity, coupled with sand storms) existing in any oil & gas field. A comparison is made between the different field joint coating systems in terms of technical characteristics, cost and ease of application in the field. The relative scarcity of international standards, and hence the importance of pre-qualification trials & production testing in the field is also highlighted.
INTRODUCTION Underground steel pipelines in the Middle East, carrying oil and gas, have traditionally been coated and often supplemented by cathodic protection. One of the most common types of coating for these pipelines has been coal tar epoxy. Environmental concerns with coal tar have led to limiting, and in most European and North American countries, total banning of the use of coal tar based coatings. An attractive alternative appeared to be cold wrapped tapes. These offer the advantage of easy field application and negate the requirement for speciality field joint coatings. The experience with these coatings has, at best, been mixed in the Gulf desert environment 1. A major reason for this is the existence of Sabkha areas in the Arabian deserts. These are highly saline marshy soils which are both highly corrosive and subject to much movement. The coatings in these areas are therefore subjected to saline water, stress and high temperatures. Cold wrapped tapes 1, and even in some instances fusion bonded epoxy (FBE) 2, have been found to undergo rapid deterioration in these Sabkha areas. This can cause accelerated corrosion of the pipeline leading to leaks. Normally it should be possible to avoid leaks, at least in the short term, by adjusting the level of cathodic protection. However, if the coating deterioration is on a large scale, or if the CP levels are not continuously monitored, leaks can still occur. These factors, and the need for coatings with higher temperature resistance, has led to the increasing use of three layer polyethylene (PE) and three layer polypropylene (PP) coating systems in the Gulf countries. In Abu Dhabi Company for Onshore Oil Operations (ADCO) the majority of new steel underground pipelines are coated with these three layer coatings. Coatings can degrade due to a number of reasons. These include:
? Pipe movement and soil stress ? High temperatures ? Ultra Violet radiation ? Bacterial attack ? Saline moisture in the soil ? Chemicals
The damage can also be done before pipeline installation during transportation from factory to site. Perhaps the greatest contributor to coating failures is poor coating application. The main factors that determine the selection of a suitable pipeline coating system are:
? Soil resistivity ? Design life ? Pipeline design temperature ? Cost ? Climatic conditions ? Ease of application & repair ? Pipeline laying method ? Availability of skilled applicators ? Equipment requirements ? Coating of field Joints ? Track record ? Health, safety & environmental regulations
In soils with very high resistivity it may be possible to bury a pipeline without any coating, particularly if it is being cathodically
ABSTRACT The atmospheric corrosion of the Zn-2 lwt%AI-2wt%Cu (Zn-21AI-2Cu) alloy exposed to an urban atmosphere was investigated after different outdoor exposition periods. The corrosion rate, determined by weight loss, was slightly higher for the ternary alloy than that for zinc and very superior to the one measured with aluminum. The results of the electrochemical methods used, such as corrosion potential, polarization resistance (Rp), potentiostatic and potentiodynamic polarizations, demonstrated a greater corrosion resistance of the surveyed materials as a consequence of the presence of corrosion product layers on the metallic surface, this greater corrosion resistance increased with the outdoor exposure time. However, in a direct comparison between both types of methods, the Rp values showed results that are contrary to those obtained by weight loss. Through scanning electron exfoliation or layer corrosion was detected in the ternary alloy, which had not been reported up to now in these alloys with relatively low aluminum contents. By means of x-rays diffraction the compounds present in the corrosion products for the ternary alloy (essentially water-soluble sulfates) could be identified whether those present on zinc and aluminum could not.
INTRODUCTION Wrought alloys based on the eutectoid composition Zn-22AI are of commercial interest because of their superplastic properties. In tile superplastic state this alloy can, as well as thermoplastics, be easily molded at moderately elevated temperatures. The superplastic materials are generally fine-grained materials and the deformation is associated with the gram boundary processes. Because of this, the superplastic alloys are exposed to a potential danger of intergranular stress corrosion cracking under susceptible service conditions ~. Another important disadvantage presented by this Zn-AI alloy is its ambient temperature creep resistance that is worse than that of pure lead 2. Ternary additions have been tested to overcome these disadvantages. Dollar et al. 3 determined the effect of ternary element additions on the susceptibility of this alloy to intergranular corrosion; twenty different alloys involving six ternary elements were evaluated in their study. Although it is well-known that copper reduces the corrosion resistance of aluminum more than any other alloying element 4 the results pointed out that, for this Zn-22AI alloy, copper additions appear to be the most adequate solution for the intergranular corrosion problem since at 1.0 at. %0 virtually no intergranular corrosion was noted in a stream/water environment. An example of zinc-aluminum-copper alloys is the family of zinc-aluminum eutectoid alloys modified with 2 to 5 wt % Cu, developed by the National University of Mexico (UNAM). Copper not only improves the mechanical properties of the binary eutectoid alloy but also its resistance to intergranular corrosion, alone 3, 5-~ or in combination with magnesium 6-8 In addition, Goodwin 9 claims that copper additions up to 2% can improve atmospheric corrosion of zinc by up 20%. The most common composition used, of the alloy family, is Zn-21AI-2Cu which has a steel-like strength due to the controlled presence of the intermetallic compound CuZn4 and a T phase enriched in copper and aluminum lo. Another interesting property of this alloy, in addition to its superplasticity, could be found which is its biocompatibility.
When discussing the atmospheric corrosion of Zn-AI alloys, which combine the superior corrosion resistance of A1 with the galvanic protection offered by zinc 12, the obligatory references are the alloys developed as steel coatings. The work of the Bethlehem Steel Corp. research team was particularly important in the dev
ABSTRACT Due to increasing demand of free maintenance, longer life cycle with free trouble for air pollution control equipment such as FGD and Chimney Liners, the metallic solution is being more popular and more important than before. 111, I~l, 131 In metallic solutions, clad plates can offer the most economical and reliable solution, which has the equivalent corrosion resistance and reliability as solid material, and better than lining.
This paper introduces the advantages of clad plate, especially hot roll bonded clad plates based on its manufacturing method, and some data for welding of High-Mo and Ni-Cr-Mo alloy clad plates. Some economical comparison of several materials is also introduced.
INTRODUCTION In recent years, in the field of air pollution control equipment, it has been accepted to apply the metallic solutions such as high-Mo or Ni-based highly corrosion/abrasion resistance alloys considering the life cycle cost, even though it spends higher initial cost. As the environment for FGD systems and Chimney Liners is quite severe and different, many kinds of metallic alloys from 300 series of austenitic stainless steel to high-Ni based alloys are applied in each parts to meet in each condition. Many alloys have been manufactured and supplied in the form of clad steel plates due to its economic and advantages in many industrial fields and also in this air pollution control equipment. In this air pollution control equipment, considering above demand such as free maintenance and free trouble during the operation, clad plates are being more popular and important than before. , . I~]
MANUFACTURING OF HOT-ROLL BONDED CLAD PLATE Clad steel is a material in which part or the entire surface of base metal is covered with a different metal (cladding alloy) and both metals are metallurgically and completely bonded together. Clad steel is thus different from lining. Cladding the base metal or making more than two different metals into composite steel offers many strong advantages.
A few method of bonding the base metal to the cladding alloy has been industrialized. They are (1) hot roll bonding, (2) explosion bonding, and (3) weld overlay welding. Although each method has its own advantages, hot roll bonding is the most suitable for mass- producing of lighter gauge and wide-width and long length steel plates, and therefore the most advantageous for air pollution control equipment such as FGD. Figure 1 shows a typical manufacturing processes for hot-roll bonding clad steel. Ingot or slabs for base metal and cladding alloy are hot-rolled into plate or sheet to the thickness required for assembling. In this process one side of the cladding alloy sheet which is bonded is plated with Nickel. The two cladding alloy plates whose surface has been nickel-plated are placed between two base metal plates. The base metal plates are then welded together on all four edges to ensure maximum airtightness. According to the grade or plate thickness, the space between the two surfaces should be vacuum-treated. The assembled, welded clad pack is reheated in the furnace, then hot-rolled. During this hot- rolling process, metallurgical bonding takes place. At the same time, the material is rolled into the specified order size. The hot-rolled clad pack is then heat-treated under the optimum conditions taking into consideration the corrosion resistance of the cladding alloy and the mechanical properties of the base metal.
Figure 1 Typical manufacturing process for hot-roll bonding clad plate
The merit and advantage of this process for the material for FGD or Chimney Liners are; -Using this so called sandwich assembling and rolling method, thin and wi
ABSTRACT The opportunities of improving the performance of plastics in corrosive applications by knowing better their corrosion properties, and the factors that determine the service lifetime or need for repairs, are discussed. The importance of having relevant design corrosion data is stressed and is exemplified with cases from practice. In particular, the corrosion behavior of FRP in chlorine dioxide, chlorine and sodium chlorate environments is presented. The performance of FRP in flue gas scrubber plants environments is briefly summarized. Comparisons of the corrosion behavior of metals and plastics are made both from a fundamental point of view considering corrosion mechanisms, types of corrosion, coixosion testing, etc. and from points of view of practical experiences regarding service performance in different applications. Typical reasons for choosing plastics within the process industry are given, as well as typical environments for plastic applications.
INTRODUCTION Plastics can be utilized in solid construction, as well as for linings or coatings. They have for a long time and to a great extent been employed in the trials of solving different industrial corrosion problems. From the fact that plastics are often used to solve problems of corrosion on various metallic materials one may be led to believe that this material cannot be attacked by corrosion. But that is not true. It is often well known in a pulp mill or a chemical plant, particularly by the maintenance staff, that in time corrosion damage may appear on plastic structures and components, as well as on other materials. In many instances, the lifetime or the need for repair of the structures is determined by corrosion attack. Depending on the type of plastic material, fabricating factors, type of medium and its composition, temperature and other factors, the corrosion rate or changes in mechanical properties may be negligible over a period of twenty years or more, or may lead to a failure in a couple of weeks. From that point of view, metals and plastics or other materials are equal. For both metal and plastic structures, we can find numerous corrosion related failures as well as numerous successful applications. A shortcoming for plastics compared to metals is that the corrosion science of metals is much more developed than that of plastics. One of the reasons for that is certainly that plastic materials are younger than, e.g., steels. The present situation concern- ing polymeric materials and corrosion is somewhat similar to that of metallic rnaterials 50-60 years ago. When the so-called stainless steels were developed and introduced in the 1920's and 30% most corrosion problems were supposed to be solved.
However, it soon became evident that the stainless steels could also suffer severely from corrosion aRack, and even new types of corrosion were discovered. Since then, metal corrosion research has increased significantly and a well-developed corrosion science for metals has been formed. This progress has been beneficial for the development of new and more corrosion resistant grades of stainless steel, for selecting the proper grades of steel for specific applications, for corrosion design, etc. By learning from the history of metal corrosion, we can foresee the necessary future developments in the field of polymer corrosion science. The confidence and the general status of plastic materials would probably be significantly increased if their so-called "chemical resistance" could be presented in technical corrosion terms analogous to metals. An important step towards an extended and correct use of plastics in various corrosive environments is to have relevant corrosion design data. An increa
ABSTRACT A comparison was made between hydrogen-induced corrosion fatigue crack growth (FCG) rates and critical distances ahead of the growing crack. This comparison was made by using results obtained from previous experiments on the corrosion FCG behavior of high-strength steels, titanium alloys, and a magnesium alloy. The critical distances included the distance to the location of maximum triaxial stress ahead of the crack, the sizes of cyclic and monotonic plastic zones, the depths of hydrogen penetration due to lattice diffilsion and dislocation sweeping. Hydrogen-induced corrosion FCG rates were presented as increments of crack growth per cycle. Critical distances ahead of the crack were calculated from the equations of continuum mechanics of solids. Some of possible cases that were discovered from the comparison did not agree with well-known and conventional theoretical views on hydrogen-induced cracking and hydrogen-induced FCG mechanisms.
INTRODUCTION During the past three decades, the subject of corrosion fatigue crack growth (FCG) of metallic materials has attracted considerable attention from researchers throughout the worldJ "2 This stems from the fact that corrosion fatigue, along with stress corrosion cracking (SCC), (° is one of the major causes, if not the major cause, for failure of engineering structures and components in a wide variety of industries. 5"6 By now, the influence of mechanical, environmental, and metallurgical variables on corrosion FCG rates has been studied in some detail. Nevertheless, despite efforts which have been exerted, there is still no effective method for preventing corrosion FCG failures and it is not possible to predict which combinations of material and environment will result in the corrosion FCG under realistic operating conditions. The theory of corrosion FCG is far from comprehensive.
The search for prevention methods, including the selection of materials resistant to the failure mode, is complicated by the fact that quite a number of fundamental questions regarding the possibility, mode and rate of corrosion FCG remain unanswered. Foremost among these questions is the problem of the corrosion FCG mechanism. This problem contains at least two sub-problems: identification of the corrosion FCG mechanism and studying the processes (mechanisms) responsible for the crack growth. Up to now, no firm consensus exists on the corrosion FCG mechanism in any one material-environment combination and the roles of the two processes - hydrogen-induced cracking (HIC, or "hydrogen embrittlement") and stress-assisted dissolution (SAD) of metal at the crack tip - are discussed as possible mechanisms of corrosion FCG. Discussion of this problem is mainly of a speculative character. Most authors argue for the dominant role of one of the mechanisms but either do not consider, or reject, the possibility of crack growth by another mechanism. It seems that this discussion, rather than studying a specific mechanism (HIC or SAD) for understanding the corrosion FCG phenomenon, has continued because of the absence of a conventional approach to the identification of the crack growth mechanism that could be applied to any material regardless of its composition and microstructure. It is possible to choose correctly from among the various possibilities - alloying elements, heat treatment, parameters of cathodic/anodic protection, and inhibitors - for the prevention of corrosion FCG failures only if reliable identification and quantitative estimation of the role of HIC and SAD in corrosion fatigue crack propagation can be made.
In previous studies, 7-s undertaken as a step towards the elaboration of this approach, a new electrochemical method was devised to quan