05 Testing of Polymeric Materialsfor Corrosion ControlB. THOMSON and R. R CAMPIONMERL Ltd.Hertford, UKA. INTRODUCTIONThis chapter reviews the material properties that influence the selection of a polymer for corrosioncontrol or limitation duty. Mechanical integrity, thermal resistance, fluid permeation, chemicalageing, and fluid resistance are some of the factors which should be considered and, if possible,measured under service conditions using realistic samples. Tests developed to assess these and otherproperties are discussed below, accompanied by background theory where relevant. The extent towhich a polymer resists service fluids, hostile or otherwise, largely determines its suitability as aprotective barrier in static applications (e.g., a chemical storage tank liner). Considerable importanceis attached in this chapter to permeation, the phenomenon which underpins the ultimate success (orfailure) of a polymeric material employed to separate a metal surface from contained fluids. Thepolymer must have sufficient strength to withstand the mechanical stresses of the application, and forbarrier materials operating in a dynamic service environment, additional testing to evaluate stressand cyclic loading effects on material performance must also be undertaken.The selection of a polymer for anticorrosion purposes depends primarily on service conditions,but cost and processability can also be factors. No polymer is entirely immune to interaction(physical and/or chemical) with contacting fluids (liquids, high pressure gases) but, with correctselection and design, the permeation rate can be extremely slow—hence, the widespread use ofpolymers to limit the access of hostile fluids to the surfaces of underlying metallic structures. Hereinlies the dilemma for the evaluation of anticorrosion materials—in many applications they areexpected to have multiyear service lives, but testing in the laboratory over such periods is clearlyunrealistic.To resolve the dilemma as best as possible, accelerated procedures are necessary. However, theirapplication requires knowledge of the basic mechanisms involved, whether chemical, physico-chemical or physical. As an example, and in the interests of covering as much ground as possible inthis chapter, the upcoming discussion will outline one specific, and commercially important,polymeric anticorrosion application to illustrate the scope of testing requirements. The exampleselected is the internal pressure sheath (or liner) of unbonded flexible pipe, used to transport crude oilin offshore oil production systems, which is introduced below, and then is later referred to at intervalsthrough the chapter.Uhlig's Corrosion Handbook, Second Edition, Edited By R. Winston Revie.ISBN 0-471-15777-5 © 2000 John Wiley & Sons, Inc.FIGURE 1. Schematic representations showing the composite layer structure of a typical unbonded flexiblepipe: (1) carcass; (2) internal pressure sheath; (3) pressure armour; (4) back-up pressure armour; (5) inner tensilearmour; (6) outer tensile armour; (7) outer sheath.1. Example of Polymer Barrier ApplicationUnbonded flexible pipes are employed offshore to convey pressurized hot crude oil and gas from thewellhead on the seabed to the surface for transport to processing facilities. In addition, at intervalsthey carry a variety of injection and service fluids, which are necessary to maintain optimumproduction rates, and are subjected to wide variations in fluid pressure and temperature. Some of thefluids can be very hostile to certain polymers, particularly at elevated temperatures. \"Flexible\" isa relative term: these pipes are enormously stiff compared with a garden hose, but are flexiblecompared with steel pipes of similar dimensions.A typical unbonded flexible pipe comprises discrete metallic and polymeric layers that areallowed a degree of slip relative to one another (Fig. 1). In addition to the illustrated layers there mayalso be included between the armour windings, thin (usually polymeric) antifriction layers. Theinternal pressure sheath is a critical component of the pipe [1] and acts to contain production fluids,transmit internal fluid pressure to the surrounding layers of metallic armour reinforcement, andprevent corrosion of the armour wires by the transported fluids. The armour layers providemechanical resistance to pressure and tension. The internal diameter of a typical current pressuresheath might be ~ 25 cm, with a wall thickness of 6 to 10 mm; a requirement of the industry is thedevelopment of ever larger diameter pipes. Bonded flexible pipe technology is also in use. In bondedflexile pipes, the inner polymer layer, usually an elastomer (a rubbery polymer), is bonded to theunderlying metal; the whole pipe is a composite of bonded metal cord and polymer layers. Bondedpipes have their own requirements for testing; in particular, blistering/splitting of the inner elastomeras a result of rapid gas decompression events assumes great importance. This phenomenon is alsopossible in unbonded flexible pipes based on thermoplastics.For risers (those flexible pipes that leave the seabed to rise to the rig, as opposed to flowlinesrunning from wellheads along the sea floor), the transport of multiphase production fluids in themanner described above is clearly a dynamic service environment, and the flexible pipe experiencesloads from vessel motions, waves, currents, self-weight and internal fluid pressure. With requiredservice lives of up to 20 years, almost continuous contact with a range of aggressive fluids at hightemperatures and pressures, numerous decompression cycles, sporadic combinations of bending,tensile and compressive strains, and no opportunities for repair should failure occur, the require-ments for a sheath polymer are stringent. Needless to say, comprehensive evaluation of candidatematerials for such a role is absolutely essential prior to deployment offshore [2].For various reasons, thermoplastic polymers (Section A.I) are most suited to this application,with the choice of thermoplastic governed to a large extent by service temperature. At moderatelyelevated temperatures (up to 9O0C 1940F) PA 11 (nylon 11) and crosslinked polyethylene (PE) aresuitable pressure sheath materials. At higher temperatures, up to 12O0C, plasticized and modifiedunplasticized types of poly(vinylidene fluoride) (PVDF) are current preferences. For the future, thereis a need to evaluate polymers that will operate safely at temperatures as high as 20O0C, andpressures up to 10000 psi, to enable the exploitation of deeper wells. The investment necessary toselect and test suitable materials is currently being made by sections of the offshore oil and gasindustry. The testing requirements for a dynamic flexible pipe pressure sheath are clearly moreextensive and involved than those for a \"simple\" polymeric lining for a chemical storage tank.The means of accelerating testing to allow residual product life to be estimated is of considerablecurrent interest. Arrhenius plots can be used to assess ageing effects and fluid permeation rates, butonly if a single activation energy is involved. Chemical kinetic effects should also be examined. Inaddition, sudden changes can occur at critical temperatures for physicochemical reasons: forexample, a severe deterioration of PVDF in methanol at vapor pressure and 14O0C has been observedafter relatively short periods [2].B. POLYMERS FOR CORROSION CONTROLBefore discussing the testing of polymers, a summary of their status as materials is essential. Withthe exception of natural rubber and a few other materials, all polymers are synthetic (i.e., manmade)and are usually derived from oil cracking products. Polymers [3-6] can be divided into three maincategories: elastomers (rubbers), thermoplastics, and thermosets. In general terms, thermosets resistdeformation whereas elastomers elongate readily under a small applied stress, with most ofthe energy being recoverable-they are elastic. Thermoplastics exhibit intermediate stress/straincharacteristics, are nonelastic, and can be further subdivided into amorphous and semicrystallinetypes. The majority of the latter will soften when heated (crystalline melting) and can thus be rapidlyprocessed into useful shapes. However, service temperatures are limited by the melting point.Elastomers and thermosets on the other hand are characterized, after processing and curing at, say,130-18O0C, by the presence of permanent cross-links within the polymer matrix; these tie individualmacromolecules into a three-dimensional network that will not soften when heated (especiallythermosets), and in the case of elastomers, is capable of tolerating large reversible extensions. Amore recent innovation is the thermoplastic elastomer (TPE), the melt processability of which arisesfrom the existence of thermally labile cross-links, but it is more prone to high temperature servicecreep/extrusion than normal elastomers. The means of deterioration differs between the abovepolymer classes. For example, thermosets resist swelling better than elastomers, but have poorerimpact strength.The choice of polymer to function in a particular anticorrosion application depends on the serviceconditions. In static applications, it may not matter which polymer type is selected, as long as thefluid is safely contained. However, other factors are relevant, including ease of manufacture and cost.For elastomeric tank linings, it is common to coat the metal surface requiring protection withbonding agent (adhesive), after suitable preparation to remove grease and debris, dry and then applyan uncured rubber layer. When the tank is sealed, superheated water can be used to apply someconsolidating pressure and heat; for this arrangement, active recipes which bring about curing attemperatures as low as 10O0C must be used. Work has been performed to show that strong, well-bonded, rubbers can be formed at temperatures down to 4O0C with the correct procedures [7].1. Types of Polymeric Anticorrosion BarrierA polymer coating is similar in one respect to the continuous oxide layer that develops on somemetal surfaces when exposed to air; it imparts protection against further corrosion, or at least acts toslow the corrosion rate. Rubber (e.g., halobutyl) tank linings are common, fabricated as outlinedabove. However, for the most aggressive fluids, the high chemical resistance shown by fluorinatedthermoplastics makes them preferable.There are two general categories of polymer protection [8, 9]. Barrier coatings/linings, which areclassified as thin (<0.64 mm) and thick (> 0.64 mm), can be applied in a number of ways to a metal(or other) substrate; examples of application techniques include thermal/chemical curing, spray-and-bake and electrostatic powder coating, rotolining, and adhesive bonding. The choice of a thin or thicklining depends on the expected rate of corrosion rate of the steel to be protected. The second form inwhich polymers are deployed in an anticorrosion role is as self-supporting structures. Since a largenumber of thermoplastics are suitable for extrusion and/or moulding, components such as pipes,valves and fittings can be fabricated to carry hostile fluids or to protect metallic structures.Thermosets on their own are not employed as thick (> 0.65 mm) coatings for pipes and tanks, butepoxy and phenolic resins are utilised as thin (< 0.3 mm) protective linings. Fiber reinforced plastics(FRP) are widely employed to fabricate piping and tanks for protection against corrosion andaggressive chemicals. These are composite materials, a combination of thermosetting resin andfibre reinforcement (usually glass), and are outside the scope of this chapter. Of the thermoplastics,it is the perfluoropolymers, in which all C-H bonds that can be \"replaced\" are fluorinated, whichpossess the highest combined levels of chemical and thermal resistance; examples include PTFE(poly(tetrafluoroethylene)), PFA (perfluoroalkoxy), MFA (copolymer of TFE and perfluoromethyl-vinylether), and FEP (fluorinated ethylene-propylene). Partially fluorinated polymers such as PVDF(poly(vinylidene fluoride)), ETFE ((ethylene-tetrafluoroethylene), and ECTFE (ethylene-chlorotri-fluoroethylene) are also widely used by the chemical process industry [1O].Polymer coats that are chemically bonded to the metal via an adhesive system [11] or thatintimately contact the metal as a result of the application method (e.g., dipping, spraying, electro-static spraying, vacuum coating [12]) generally provide better corrosion protection (because air andwater tend to be excluded from the polymer-metal interface during fabrication) than polymeric linersthat operate by being in close physical contact with the metal surface. An example of the latter is thelining of steel pipe with PE pipe [13, 14]. In this case, fluid molecules could slowly gather in thisinterfacial region, following permeation through the polymer, possibly allowing metal corrosion tocommence. For elastomers bonded to metal, the bond joining the two material types is often strongerand more durable than the rubber itself [15, 16].C. RELEVANT POLYMER PROPERTIESThere are numerous factors to be considered when selecting a polymer to control corrosion. Of theutmost importance is the nature of the service fluid and the conditions likely to be experienced by thepolymer during service (temperature, pressure, flow, stress, etc.). The criteria which influence thechoice of polymer for a particular fluid environment have been outlined in Chapter 54.In an amorphous polymer at sufficiently low temperature, all large scale chain movement is\"frozen\" and no long range order exists; the polymer is said to be in a glassy state. With heating, atemperature is reached at which segmental chain motion becomes possible, and the polymer standson the threshold of a rubbery condition. The apex of this region, which varies with polymer type, isknown as the glass transition temperature (Tg); the vast majority of polymers possess a Tg. Ingeneral, the more flexible a polymer chain (a function of its chemical structure), the lower its Tg.During dynamic motions, there is a high energy absorption range in the vicinity of the Tg, envisagedas the \"internal friction\" between the glassy and rubbery regions.1. Polymer/Fluid CompatibilityA polymer selected to protect a metal surface should be incompatible, in the chemical sense, with thecontacting fluid; that is, the fluid should not interact with the polymer to a significant degree. Thechemical compatibility of a liquid and a polymer can be estimated empirically from the proximity oftheir solubility parameters (symbol 5; units usually ca!1/2/cm3/2; multiplying by 2.05 changes them toMPa) [17-19]. This is a measure of the energy of attraction between the different molecular speciesinvolved. If the 6 values differ significantly there is no chance of swelling but if they are similar,within ~ 2 units, swelling may occur. For example, a nitrile rubber with a low acrylonitrile content(8 ~ 9) would not be considered an appropriate lining for a tank containing toluene (6 also ~ 9) asconsiderable swelling (and therefore weakening) of the rubber could result. Structural (entropic)considerations are important here and, particularly with elastomers, there is scope to counteract theswelling expected from solubility parameter similarities at the compounding stage of processing.Increased filler loadings and/or increased cross-link density will reduce the propensity of a givenelastomer to swell in a compatible liquid; in the first instance, the volume of rubber present in thecompound is lowered, in the second, tighter network formation reduces free volume (as wouldstarting with a higher Tg base rubber) and restricts the volume of solvent that the material canaccommodate; passage of solvent through the material is also impeded. Increasing the crystallinityof a semicrystalline thermoplastic, perhaps by altering the molecular weight distribution and/orprocessing conditions, has the same effect. The viscosity of the contained fluid should also beconsidered, with more viscous liquids ultimately absorbed in smaller amounts and at a slower ratethan low viscosity solvents.2. Mechanical Strength and Fatigue ResistanceThe mechanical strength of semicrystalline thermoplastics originates from their crystalline domains;crystallites are the strength-giving elements, literally holding the material together above Tg andfurnishing the polymer with reasonable mechanical properties below the melting point. Crystallites,dense regions of highly ordered polymer chains, are dispersed in a less dense amorphous (randomlyoriented or disordered) polymer phase: some polymer molecules, which are very long, run in and outof the two phases. Crystallites will not admit solvent molecules and diffusing species mustcircumvent such obstacles, increasing their pathlength (tortuosity) and residence time in thepolymer. In articles fabricated by extrusion of a polymer melt (e.g., flexible pipe internal pressuresheath), polymer chains tend to orient in the flow direction. Subsequent cooling of the extruded meltmeans that developed crystallites also tend to lie in the flow direction. In other words, properties ofthe extruded article may be anisotropic. The degree of anisotropy depends on many factors includingpolymer type and processing conditions and can range from being extreme (as in liquid crystallinepolymers) to being insignificant.In the example of Section A.I, the flexible pipe concept requires the correct balance between thecompact self-organization of crystalline regions and chain mobility within amorphous domains, toprovide strength together with some flexibility. Mechanical property requirements depend on serviceconditions, particularly temperature, but there is scope at the pipe design stage to accommodate arange of material attributes. For example, if a rigid (high modulus) polymer is being assessed as apotential pressure sheath, a reduction in pipe wall thickness may be one way in which to compensatefor the increased material stiffness.3. Thermal StabilityAlthough thermoplastics are generally less permeable than elastomers as a direct result of structuraldifferences, destruction of the crystallites by melting weakens the material further and removes thesebarriers to solvent passage. Hence, the requirement that service temperature not come close to themelting point, or other transition point: for instance, the continuous use temperature (CUT) for thePVDF example following is brought about by a transition of crystalline form at 150-1550C. In fact,the optimum CUT of a semicrystalline thermoplastic is typically well below its melting point, andvaries with exposure conditions. For example, the CUT of plasticized PVDF in air is 145-15O0C butthis falls to 120-13O0C in contact with oilfield production fluids. Another factor that must not beoverlooked is increase in size experienced by most materials with temperature. Since polymersgenerally have higher coefficients of thermal expansion than do metals, glass or ceramics, thermalstresses resulting from mismatched joined materials should be taken into consideration at the designstage.4. Chemical Resistance and DegradationThe next question concerns interaction, that is, does the fluid or any of its components chemicallyreact with the polymer? Polymers do not corrode in the sense that corrosion is seen as thedeterioration of metals [12], but nevertheless they can degrade chemically in contact with hostilechemicals. The main point of issue is that, to retain a barrier function, a polymer lining must beaffected less rapidly than the metal being protected from a particular hostile fluid. In addition, it isnecessary to know the residual life of the polymer to avoid fluid eventually reaching the metal.Degradation (by definition) chemically alters polymer structure at the point of contact. The chemicalchanges are usually irreversible and surface alterations may or may not enhance the entry of fluidsinto the polymer bulk. Properties (physical, thermal, and mechanical) are thus changed. For example,the discoloration and embrittlement of PVDF by n-butylamine is well known. Although muchinformation about the effect of specific chemicals on polymers has been published, it is by no meanscomplete, especially for complex formulations such as the corrosion inhibitors, scale inhibitors,biocides, and drilling fluids used in offshore oil and gas production. These fluids contact the flexiblepipe inner sheath, some for short periods neat, others continuously in trace amounts. Appropriateassessments of their impact on polymeric components (pipes, seals) must be made beforedeployment.5. PermeationBecause of its relevance to all anticorrosion linings and barriers, this section is devoted to adiscussion of permeation and its measurement. Other properties and test methods germane to theflexible pipe example are outlined briefly in Section D.Permeation is a molecular phenomenon, involving the passage of a fluid through a material [2O].Due to their nature, a degree of permeation is an unavoidable consequence of polymer contact with aliquid. The driving force is the chemical potential (|i), which is normally quantified as a concentra-tion gradient. When the fluid is a gas or vapour, the concentration becomes pressure dependent (i.e.,the driving force is now the applied partial pressure). In all polymeric barrier/lining applications,including the unbonded flexible pipe inner sheath, the contained service fluid initially contacts onlyone polymer surface.Permeation and degradation are unrelated properties, but may interact; for instance, the com-ponents of the permeation process (solvation and diffusion) give hostile chemicals a route into thepolymer bulk, where they may (or may not) interact in a degradative way with the material. If animpermeable polymer existed, any degradative chemical reaction would be confined to the surface.While there is only one permeation phenomenon, other physical features of polymers can affect thepermeation rate. The rate depends on a balance of factors; polymer and fluid type, fluid concentration(may apply as a pressure differential), temperature, exposed surface area and thickness. For anyone fluid, permeation is an inherent material property, that is, different polymers display differentpermeation behavior in a particular liquid.5J. Permeation Defined Permeation is a two-stage process involving, first, adsorption/evapora-tion of the fluid by the polymer surfaces (quantified by surface solubility coefficient, (s) and,second, diffusion (quantified by coefficient D) of the fluid through the polymer bulk. (A relatedtotal—immersion phenomenon—absorption—possesses no evaporation stage.) Both D and s areinherent material properties and the permeation coefficient, Q, is the product of these other twocoefficients, that is,Q = DsAt constant conditions, different fluids will permeate at different rates through a given polymer,with the rate raised by increasing the exposed area and/or decreasing specimen thickness. As alreadynoted, the concentration gradient is the main driving force for permeation of a single fluid through apolymer membrane. For a gas, this is used most conveniently by considering the difference betweenits partial pressures on either side of the membrane: if the gas is the only permeant and there is norestriction to evaporation when through the membrane, this difference will equal its applied pressure.With the units in commonplace use (cm2/s/atm), Q is defined as the permeation rate for a fluidpassing right through a polymer cube of dimension 1 cm, driven by a pressure of 1 atm. Similarly, Dis the diffusion rate across the cube and s is the fluid concentration in the polymer surface, both atatmospheric pressure. Hence, for a given fluid, all these coefficients can be measured from a singlepermeation experiment. The temperature dependencies of <2, D, and s are each given by Arrheniusrelations of the form, (for Q)lnG = Aexp^/^where A is a constant and Ea is the activation energy associated with permeation. By performingaccelerated testing at elevated temperatures, a plot of In Q (or D or s) versus reciprocal temperature,followed by extrapolation to the service temperature, allows service permeation, and so on,characteristics to be estimated. It should be noted that at high pressures, these coefficients canbecome pressure dependent. Permeation can occur simultaneously in opposite directions if, forexample, a polymer membrane separates two different fluids. It is also worth stressing thatpermeation characteristics can change over time if other factors that affect the polymer are altered.The parameter Q provides insight on how much fluid passes right through a polymer at steady-state conditions, whereas D is used to estimate the time to breakthrough and s governs the amount offluid initially available for migration. Predictions stemming from Q should be applied to the correctservice geometry. In general, permeation rates for thermoplastics are an order of magnitude belowthose typical of elastomers, reflecting the different molecular architecture prevalent in each materialclass. Most elastomers are amorphous with relatively high free volume content (i.e., space fordiffusing small molecules to occupy) and exist above Tg at room temperature. Thermoplasticspossess crystalline zones which, as already described, do not admit diffusing small molecules; thepath of diffusing molecules is thus distorted and lengthened.5.2. Permeation Measurement As far as testing is concerned, permeation (\"transmission\") testsare best for gases (and perhaps water [21]), while absorption (\"total immersion\") tests are moresuitable for liquids. A variety of arrangements exist to measure gas permeation at high pressures andtemperatures. One setup, developed by MERL [22], can perform at pressures up to 15000 psi andtemperatures in excess of 20O0C, and is shown schematically in Figure 2. The centrepiece is apermeation cell in which the polymer membrane is supported by a smooth, porous, stainless steelsinter; a gauge is incorporated to measure sample thickness changes while under pressure. BandHigh pressure gasAdapted valve bodyPermeationcellLeakageportPolymersampleLow pressure gasFIGURE 2. Schematic of high-pressure permeation test cell assembly showing location of elastomer sampleand the developed in situ thickness gauge.heaters and good insulation provide the required test temperature. After permeation through thepolymer sample, gas on the lower pressure side is collected in a fixed volume reservoir. Allpressures and temperatures are continually monitored, using pressure transducers and thermo-couples, respectively, and logged by a computer. The increase in low pressure with time isconverted to a rate of gas permeation at standard conditions.With oriented thermoplastics (e.g., some extruded polymers), anisotropic effects have led to thesuggestion that the rate of permeation through the exposed ends of the article, say a pipe, can greatlyexceed that of the same fluid through the curved surface. The passage of migrating fluid moleculesbetween crystallites along the length of the pipe is often less impeded than that of molecules passingthrough the pipe in a direction orthogonal to the extrusion axis. Thus the permeation rate obtained bytesting an unsealed pipe section may be misleading (too high) if such orientation effects are present;after all, the exposed pipe cross-section is not a surface relevant to pipe service conditions. A sealededge/unsealed combination of tests is required. For aqueous acids, permeation tests detect acid bytitration or electrochemical means; results should be validated by a series of absorption tests.It is recommended that laboratory permeation tests be performed on samples removed fromactual structures. Failing that, samples should be manufactured under conditions that mimic actualproduction as closely as possible; this is less important for elastomers.A special case is that of poly(tetrafluoroethylene) (PTFE) linings and barriers. As a result of thespecial processing techniques required for its fabrication, a consequence of the lack of melt-processability due to the extremely high molecular weight of commercial grades, this perfluoro-plastic invariably contains ultrafine porosity within its bulk. These special techniques (e.g., ramextrusion) involve the fusion of partially molten PTFE particles, after the required shape has beenformed, by sintering; pores remain where the process is incomplete. If PTFE contains open pores,then testing with a liquid at several pressures should resolve the issue; with open pores, the\"permeation\" rate will be high, and pressure dependent. If the pores are closed the rate will be lowand generally pressure independent. The presence of pores in PTFE can lead to applications in filtersystems.5.3. Other Factors that Influence Permeation The permeation rates characteristic of elastomersare generally an order of magnitude greater than those measured for thermoplastics [6]. Hydrophilicresidues in polymers (e.g., residual protein in natural rubber (NR) lead to higher than expectedamounts of water being absorbed. Increasing temperature permits greater thermal motion ofpolymer chains, thereby easing the passage of diffusants. If a polymer surface is degraded bycontact with hostile chemicals, access to the interior may be facilitated; the permeation character-istics change accordingly. Application of mechanical stresses can, by altering chemical potentialfactors, also affect permeation rates.D. TEST METHODSIn this section, available test methods for measuring the properties outlined in Section C arediscussed, with emphasis on the flexible pipe example [3]; the methods described below are notrelevant to composites [23]. In evaluating polymeric materials for barrier applications, test piecesshould ideally be cut/machined from actual liner or pipe, or from articles fabricated under similarprocessing conditions.1. Tensile PropertiesMechanical properties can be determined by tensile testing (e.g., ASTM D638) using standard testpieces, at a series of service-related temperatures, to obtain values for Young's modulus, tensilestress at yield, percent elongation at yield, tensile (ultimate) strength at break and percent elongation(strain) at break. Such measurements should be repeated using (standard) test pieces that have beensubjected to relevant chemical ageing procedures, in order to determine changes after specified timesof exposure to chemicals of interest. Limitations of tensile strength include its rather strongdependence on the method of testpiece preparation, and its failure to test notch sensitivity.2. Fracture ToughnessThe basis of this test is given in a draft European Structural Integrity Society (ESIS) protocol [24]. Itprovides a measure of fracture toughness (J) by relating the total work done, in displacing thecompact tension testpiece (Fig. 3) by a preselected amount, to the depth of the crack that results.After the test, the fracture surfaces are exposed and the crack depth measured. It is necessary forcompact tension test pieces (described in the protocol) to be accurately machined, and for thick(6 mm minimum) samples to be used to ensure that only plane strain conditions apply during testing.The test piece has a lateral slot at the centre of one side, into which a sharp crack tip is added with arazor blade just before testing. A useful form in which to present data is as a plot of fracturetoughness (resistance) versus crack growth, constructed using the results from several separate testsin which different levels of deflection are employed. Fracture toughness, 7, is calculated from anappropriate equation for the compact tension testpiece. In summary, the test measures the intrinsicfracture toughness of the polymer immediately prior to crack growth, that is, the capacity of thepolymer to withstand crack propagation. This is a more fundamental property than tensile strengthand can also determine notch sensitivity.FIGURE 3. Side view of compact tension testpiece, showing the slot where the crack tip is added, and the holesfor loading the testpiece; all dimensions in millimeters (mm).3. Crack Growth FatigueThis test extends the fracture toughness concept by providing stress-strain cycles on the sametestpiece type to illustrate the fatigue behavior of thermoplastic materials. Dynamic fatigue occursunder cyclic loading where a small amount of crack growth appears during each load cycle, initiallyfrom the crack introduced into the material. Dynamic fatigue resistance is measured using a compacttension test piece that contains a crack of defined dimension. The material property required is therelation between crack growth rate and the energy input level as determined, for example, by the so-called J integral. The rate of crack growth under cyclic conditions is measured over an appropriaterange of J values. The test uses the ESIS standard protocol [24] for the determination of / values.This relation is different at different temperatures and so a sequence of tests needs to be carried out atservice temperatures of interest. A convenient way of representing the results is empirically to definea term \"crack growth resistance\" as the / value for a crack growth rate of 10 nm/cycle. It cannot beassumed that materials with high fracture toughness will also have low rates of crack growth at lowenergies.4. Stress RelaxationStress relaxation provides a measure of the rate of decrease in stress during a constant state of strain.A linearity of applied stress with log time is the norm for viscoelastic materials, providing that nochemical (air) ageing occurs during the test. For semicrystalline and/or glassy polymers, withless viscous flow, linearity of stress with another function of time may apply. Any changes causedby oxidation are only likely to occur (if at all) at high test temperatures. If no such chemicalageing occurs during service, the same relationship can be extrapolated to indicate stress retentionduring service life. If measurements with suitably aged samples should indicate a chemicallyinduced change in relaxation rate, appropriate corrections to the simple extrapolation might beestimated.The retention of stress is important in end-fitting regions of flexible pipe. At the end of the pipe,the pressure sheath is terminated by a swaging process involving metal elements, to isolate the borefrom the outer armour layers. This ring supports the entire weight of the internal sheath duringperiods when the production schedule requires the pipe to be decompressed. Hence, this test isnecessary to measure the rate of loss of applied stress when a polymeric material is compressed to afixed deformation. If the rate of stress relaxation is high then it is possible that the loss of sealingforce could lead to leakage, or, in the extreme, the end-fitting could loosen.5. High-Pressure Gas PermeationAs described in Section C.5, this test involves applying gas at a predetermined pressure to a suitablydesigned and sealed testpiece mounted in an appropriate cell. The gas permeation coefficient (Q),diffusion coefficient (D), solubility coefficient (s), and concentration (c) are all determined from thetest: c is the product of s and applied pressure (Henry's law). If the test is repeated for a range ofpressures and temperatures a predictive model of these coefficients, based on Arrhenius plots, can beestablished.6. Rapid Gas DecompressionIt is a crucial requirement for any polymeric lining subject to prolonged exposure to high-pressuregas (> 5000 psi) that rapid decompression of the gas should not cause internal fracturing/blisteringof the polymer [25-28]. This can be caused by dissolved gas no longer being in equilibrium, fromHenry's law. A full characterization of resistance to rapid decompression should include the effect ofappropriate mechanical constraints and also the effect of temperature, pressure, and relevant gastype. The test consists of exposing samples, preferably cut from pipe/liner, to the test gas forsufficiently long as to allow them to become saturated with gas. Knowledge of permeation behavior(Section C.5) is highly relevant here, involving D and c, allowing the calculation of the soak periodand the amount of gas absorbed. The pressure is then released at controlled rate (e.g., 1000 psi/min).A representative number of repeat cycles are performed and, after a suitable degassing period,samples are removed and inspected for damage. Ideally, thermoplastic materials likely to best resistrupturing will be characterised by low gas solubility in the relevant gas, and a high diffusioncoefficient; the latter is essential if the gas is to exit the polymer rapidly after/during thedecompression.7. Liquid CompatibilityWhen evaluating the compatibility of any polymeric lining or barrier with the liquid it is to contactduring service, testing must be undertaken at realistic temperatures and pressures, using samples cutfrom actual liner, or similarly processed material. In the case of the flexible pipe pressure sheaths,which carry a variety of different crude oils, depending on the field location, model reference oils canbe developed that have the same solubility parameter as the crude, and a similar ratio of aliphatic/naphthenic/aromatic constituents [19]. Other solvents can be included to represent oilfield produc-tion additives, for example, corrosion inhibitors. Measurements made during exposures can lead tothe equilibrium mass uptake and volume swell for the polymer/liquid system, and provide diffusionand permeation data. If tensile testpieces are used, the change in tensile properties as a function oftime in the fluid of interest can be monitored.8. Chemical Deterioration/AgeingThe ageing effects of hostile service fluids are assessed in a similar way; test vessels used tend to bepressure cells. Chemicals that may be hostile to the polymeric flexible pipe inner sheath areconstituents of injected fluid mixtures such as biocides, corrosion inhibitors, and scale inhibitors;these include quaternary ammonium compounds, lower alcohols and glycols. Hydrogen sulfide ispresent at low levels in many oil wells (sour wells) and steel surfaces are vulnerable to attack by thiscorrosive gas. When including H2S in laboratory ageing fluids, special precautions are necessary tosafely contain, monitor and dispose of liquids containing this dangerous chemical. Some flexiblepipes are employed in fields requiring that they be exposed for short periods to strong acids and neatalcohols; resistance should be assessed. In total, there could be as many as a dozen different fluids towhich the polymer is exposed, long term, during service, and hence should be so during realistictesting. These tend to be longer term (typically one year) tests, designed to evaluate the effect onrelevant polymer properties (see above) of exposure to service fluids. The effects of ageing are oftenvisible after exposures: changes in color, changes in volume, presence of surface fractures, or,complete disintegration in extreme cases.9. Environmental Stress CrackingWhen mechanical stress is combined with a hostile chemical environment, faster rates of failure canoccur then would occur due to the stress or the environment alone. The cause can be physico-chemical (i.e., high local swelling in solvents with certain solubility parameters) and/or trulychemical (e.g., ozone cracking of white-walled tyres in very sunny climates). The stress required toinduce cracking is invariably lower than it would be in air, hence the need to assess relevant polymer/fluid combinations. Temperature, stress level, time and the nature of the fluid all influence theprocess. In general, the extent of the damage incurred increases with stress level and temperature.The effect of dilution is less clear cut. For polymers, increasing molecular weight can reducesusceptibility to this problem. Testing is straightforward: samples are held strained at the appropriatelevel in rigs, and these are immersed in the fluid of interest. MERL has developed special testmethods for characterising the combined effects of material stress and aggressive chemicalenvironment, including high fluid pressure.E. PREDICTION OF POLYMER SERVICE LIFEIf service temperatures are low (e.g., 230C) the permeation rate, especially for thermoplastics, isoften very slow. In testing, therefore, an element of acceleration (usually higher temperature) is oftenintroduced. As already noted in Section C.5.1, because diffusion follows Arrhenius-type kinetics,testing at (typically) three different higher temperatures allows Arrhenius plots to be constructedwith extrapolation giving an estimate of the permeation rate at the service temperature. If servicetemperature is high, property measurements should be made at this temperature.Predictions are also possible for the consequences of chemical ageing using a similar technique tothat for permeation. For these, it is necessary to identify a property change or property level thatmarks the limit of being acceptable. Then, by accelerated testing at elevated temperatures, in eachcase the ageing time roc necessary to bring about the defined changes (or reach the defined level) isrecorded. Because reciprocal time is a rate, from the original Arrhenius concept, In (1/roc) versus l/Tplots will be linear (T in kelvin). Once again, extrapolation will lead to the time to reach the limitingpoint at service temperature. Practically, for partially accelerated tests at slightly elevatedtemperatures, an extrapolation of property versus time plots may be necessary to obtain a value forfa: a knowledge of the chemical kinetics (i.e., first order, second order) can improve the quality ofthis extrapolation.F. CONCLUSIONSIt should be apparent from the preceding discussion that the largest single factor in using a polymerto restrict, or even prevent, a potentially corrosive fluid reaching a metal surface is permeation.Hence, the focus on this topic and its measurement in polymeric materials. The challenge inqualifying polymers for corrosion control is to apply (or if necessary develop) relevant tests tomeasure appropriate permeation characteristics for the particular fluid in question.However, other factors also apply. A flexible pipe example has been used to illustrate thecomplexity and breadth of testing required to validate a polymeric material for duty in hostiledynamic service conditions. Properties besides permeation requiring measurement are selected asappropriate from: mechanical, fracture toughness, crack growth fatigue, stress relaxation, rapid gasdecompression, environmental stress cracking, and liquid compatibility and chemical ageing.Although extreme in the number of different property/fluid combinations requiring assessment,the flexible pipe case does highlight the need to realistically test polymers for critical applications.Clearly, in relatively simple static applications (e.g., a tank liner operating at low temperature andpressure, where the polymer coating is expected to protect the tank surface from a single fluid type),less testing is required. Compatibility with, and ageing in, the relevant service fluid must beappraised, as well as permeation issues and general strength properties, but dynamic testing, rapidgas decompression, stress relaxation and crack growth fatigue are not relevant for such an end use. Inthe case of widely used barrier materials, such as fluoroplastics, extensive technical information isavailable from suppliers and the open literature. However, this is no substitute for properly targetedtesting if the slightest doubt exists about the suitability of any polymer/fluid combination underservice conditions.G. REFERENCES1. F. Grealish, G. McGuinness, and P. O'Brien, Proceedings of the Conference on Oilfield Engineering withPolymers, London, Oct. 28-29, 1996, MERL, Hertford, UK, 1996, pp. 47-55.2. R. Campion, M. Samulak, A. Stevenson, and G. Morgan, Proceedings of the Conference on OilfieldEngineering with Polymers, London, Oct. 28-29, 1996, MERL, Hertford, UK, 1996, pp. 60-80.3. P. Painter and M. Coleman, Fundamentals of Polymer Science, Technomic, Lancaster, UK, 1994, Chapters 1,7,8.4. G. Odian, Principles of Polymerization, Wiley, New York, 1991, pp. 1-37.5. K. Saunders, Organic Polymer Chemistry, Chapman and Hall, London, UK, 1988, pp. 1-44.6. J. 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