Electrochemical Studies on the Corrosion Behavior 1 of Conventional and High Alloy Steels in Gulf Seawater

Transcription

1 Electrochemical Studies on the Corrosion Behavior 1 of Conventional and High Alloy Steels in Gulf Seawater Anees. U. Malik and Shahreer Ahmad Research And Development Center, Saline Water Conversion Corporation P. O. Box # 8328, Al-Jubail 31951, Kingdom of Saudi Arabia SUMMARY During the last years, a considerable number of highly alloyed corrosion resistance stainless steels, so called super stainless steels have been developed. More emphasis was put on to increase the localized corrosion resistance, since conventional stainless steels have rather limited resistance to localized corrosion. Typical examples of new super stainless steels are the nitrogen-alloyed 6Mo austenitic and 25Cr duplex steels. The largest application area for these steels has been seawater systems of various kinds. An attempt has been made to determine the corrosion resistance of some typical high alloy stainless steels viz. 316L, 317L, 904L, duplex 2205, 3127hMO, 1925hMO, 254SMO, 654SMO and Remanit 4565, in chlorinated and unchlorinated Arabian Gulf Seawater at ambient and 50 0 C. The stainless steels 316L and 317L have been used as reference alloys. Electrochemical Potentiodynamic cyclic polarization method have been used to determine the E b - passive film break down potential, Eprot - protection potential and Imax - maximum current attained on scan reversal. It was found that at 25 0 C in chlorinated and unchlorinated seawater and at 50 0 C in unchlorinated seawater, the stainless steels 316L and 317L had poor resistance to corrosion, Stainless steels 904L and duplex 2205 at 25 0 C in chlorinated and unchlorinated seawater showed good resistance to corrosion but at 50 0 C these steels failed to resist. Other alloys such as 3127hMO, 1925hMO, 254SMO, 654SMO and 1 Issued as Technical Report No. TR3804/APP96003, September,

2 Remanit 4565 showed better corrosion resistance in all the test conditions. The alloys having a PRE N value of above or equal to 30 at 25 0 C with and without chlorine and PRE N value of 35 at 50 0 C or the sum of Cr+Mo above 25 can be used safely for seawater applications 1. BACKGROUND About 5 years back, the Corrosion Department of SWCC, RDC initiated a major project entitled Crevice Corrosion in Stainless Steels (SWCC R&D Task 37). Most of the aims and objectives of this project were completed by the middle of The results of the studies appeared from time to time in the form of technical reports [ SWCC (RDC) 20, 22, 26 and 28] and papers in corrosion and desalination journals. Last but not the least in importance, is the remaining part of the project dealing with the influence of certain parameters such as temperature and residual chlorine and dominant alloy addition on the localized corrosion behavior of steels as studied by electrochemical means. Under the new guidelines of RDC management, a project proposal was submitted in November, 1995 containing the above-mentioned tasks. The project entitled Electrochemical Studies on the Corrosion Behavior of Conventional and High Alloy steels in Gulf Seawater is of one year duration and was started in January, OBJECTIVES 1. To study the effect of temperature and residual chlorine on the localized corrosion characteristic of conventional and high alloy stainless steels. 2. To study the localized corrosion behavior of conventional and high alloy stainless steels by electrochemical techniques. 3. To determine the influence of dominant alloy additions : Cr, Mo and Ni on the general and localized corrosion of high alloy stainless steels in Gulf seawater environment and chlorinated seawater. 3. INTRODUCTION 1806

3 Since the discovery of corrosion resistance properties of 12% Cr steels in the beginning of this century, stainless steels have been most widely used structural materials after mild steel. Besides convenient availability and moderate cost, stainless steels are easy to produce, cast or fabricate, have versatile mechanical properties and excellent weldability. Over the years there has been continuous innovations in the development of stainless steel technology. Although austenitic stainless steels like 304, 316 or similar alloy have been formidable structural materials since long, development of high alloy stainless steels containing 6% Mo in sixties and the following decades, made possible the applications of stainless steels in most aggressive environments such as seawater handling systems in place of much costiler nickel-base alloys. The applications of conventional and high alloy stainless steels in marine environments particularly pertinent to desalination have been dealt in number of review articles [Hassan and Malik, 1989; Bardal et al., 1993; Malik et al., 1994; Tuthills et al., 1995; Oldfield and Todd, 1995, 1996]. Amongst the family of austenitic stainless steels, 316 L has been the most widely used structural material in industry. It has been the most widely used material for marine installations including desalination plant components which include flash chambers, distiller pipes, pumps and condensers due to its excellent corrosion resistance to chloride attack. In spite of its virtue as an outstanding corrosion and erosion resistance material, 316 L has its shortcoming as vulnerable to local corrosion such as pitting or crevice attack in presence of chloride under static or stagnant conditions [Sedricks, 1979; Smialowska, 1987; Stackle, 1974]. The pitting of stainless steels is the single most frequently occurring corrosion phenomenon in seawater processing plants and has been the subject of numerous studies [Todd 1977, Hodgkeiss, et al. 1987, Moriz. et al. 1986]. It is generally agreed that pit initiation occurs as a result of breaking down of chromia film in presence of chloride, the propagation step in pitting involves an autocatalytic process in which corrosion products are hydrolyzed to provide local encrichment in chloride and acid concentrations. The pits also provide active crevices for a formidable corrosion attack [Fontana, 1986]. In recent years, pitting of metals particularly stainless steel has been the subject of extensive and in depth studies [Oldfield et al., 1980; Smialowska, 1986; Malik et al., 1992]. The 1807

4 studies are mainly directed toward mechanistic aspects which subsequently provide information useful for the development of pitting resistance materials. Electrochemical techniques have been the principal tool for characterizing pitting corrosion. Pitting potential, E pit, protection potential E pp and repassivating potential, E r are the important electrochemical parameters for studying the pitting behavior of materials. Pitting potential, E pit is the potential above which passive alloys are susceptible to pitting corrosion in halide solutions, but below which pits can not be formed, although existing pits can grow if the potential is greater than protection potential. The protection potential (E pp ) is the potential value such that at potential greater or equal to this potential, it can propagate, but below this potential, metal remains passive. E r is called the repassivating potential and represents the potential at which the current falls to the small values recorded during the forward scan. Pitting potential depends on the composition of the bulk solution, and on the surface condition of metal, while protection and repassivating potentials depend upon the composition of the solution contained in pits. Up till recently, determining the pitting potential, E pit was fundamental in evaluating susceptibility of different materials, in different environment to localized corrosion. However, it is accepted widely that due to similarities between crevice and pitting corrosion mechanisms it would be more appropriate to refer localized corrosion potential by a new parameter so called breakdown potential, E b. The latter is defined as a potential where pitting, crevice corrosion or both will initiate or propagate. Logically it is more appropriate to represent localized corrosion by E b rather than referring to E pit or crevice potential separately [Neville et al., 1996]. The use of E b instead of E pit by several authors emphasizes that mechanisms other than pitting can induce the loss of protectivity. However the importance of E pit determined from potentiodynamic polarization curves cannot be undermined. It is a very useful parameter particularly, in comparing the pitting resistance of different materials. In a recent paper, Salvago and Funalalli [1996] determined the distribution of breakdown potential (E b ) of stainless steels in 0.1 NHCl solutions. Representation of E b distribution on the graph of potential (E) vs log [commutative breakdown frequency, F] was found to be particularly informative about localized corrosion resistance. The effect of annealing temperature on the behavior of 25% Cr duplex UNS S32550 in HCl solutions was investigated by determining critical pitting temperature (CPT) and pitting potential, E pit after 1808

5 annealing at different temperatures between 1020 and 1140 o C [Garfias, Sykes and Tuck, 1996]. It was found that the pitting in chloride solution took place preferentially in the ferrite phase rather than in the austenitic phase. Higher annealing temperature (above 1060 o C) increases the ferrite content and dilute the key alloying elements in the ferrite content thus lowering the corrosion resistance of ferrite. Neville and Hodgkiess [1996] investigated the effect of elevated temperature (up to 60 o C) and high velocity impinging flow on the corrosion of stainless steels, Ni- and Co- base alloys in seawater. DC electrochemical experiments demonstrated the clear effect of increased temperature in facilitating premature breakdown of passivating film on of all materials. The effect of high velocity impinging flow was to further shift the passivity breakdown potential to more active values but not necessarily to result in greater depth of attack. With respect to materials comparison, the study has illustrated that under the combined effect of high temperature and high velocity flow, the materials which resist significant attack on a localized level are the Ni- based Inconel 625, the Co- based ultimate and superaustenitic alloys. A substantial amount of work has been carried out on localized corrosion behavior of steels in seawater, however, most of the studies are mainly limited to artificial seawater which is chemically very close to seawater but may not have similar characteristics mainly because of the absence of biological activities. The composition of seawater varies with in wide limits. For example, the dissolved salt content in Baltic Sea amount only 7 g/kg (about 7,000 ppm) it is about 43 g / Kg (about 43,000 ppm) in the Arabian Gulf Water. An approximately 35 g/kg salt content is present in Pacific and Atlantic lie somewhere in the middle [ Homig, 1978]. Seawater temperature also can vary widely from 2 0 C in Arctic to 30 0 C in tropics to 45 0 C in Arabian Gulf. In a study [Agarwal et al 1991] concerning with the performance of platform pumping system, a 2 test spool of alloy 1925hMo was exposed to chlorinated Norwegian seawater (salinity : 20,000 ppm) for period of 4 weeks. The test results showed that in chlorinated seawater at 30 0 C alloy 1925 hmo was free of attack at chlorine level of 0.5 ppm. High chlorine levels and / or elevated temperatures may lead to corrosion attack preferably on the flange and under gasket. Electrochemical testing of some duplex steels in Portland Harbor seawater (about 20,000 ppm Cl -, 2400 ppm SO -- 4 ) was carried out to assess the resistance to crevice 1809

6 corrosion [Carpenter et al 1986]. The results obtained could be correlated with service experience and provide useful information for material selection in seawater handling systems. Lee et al [1984] studied the effect of environmental variables e.g., ph, Cl - and DO on crevice corrosion of stainless steels in Atlantic seawater (18,800 ppm Cl - and 2,500 ppm SO -- 4 ) at Wrghtville Beach, N.C. The uniqueness of natural seawater has been emphasized by the results of propagation studies at elevated temperatures. Substantial reduction in the extent of propagation were noticed on increase in seawater temperature. It appears that reduction at cathode surfaces may be rate controlling. Effect of surface finishing on the crevice corrosion resistance of SS316 in Atlantic seawater (Salinity 31,400-35,100 ppm, Cl - 17,400-19,600 ppm) was studied [Kain, 1991]. Resistance of SS316 in natural seawater appeared to be dependent on crevice geometry. Surface grinding was determined for both anodic dissolution and cathodic polarization of stainless steel. Exposure to natural seawater can result in considerable ennoblement of the corrosion potential over the artificial seawater of comparable salinity. Crevice corrosion propagation studies were carried out on Ni -base alloys 625 and C276 in seawater of Key West, Florida with an avereage slainity of 37.3 ppt and a temperature of C [McCafferty et at, 1997]. Anodic polarization technique was used. Both the alloys underwent crevice corrosion in seawater medium with active crevices were found to contain concentrated amounts of dissolved Ni 2+, Cr 3+, Mo 3+ and F 2+ ions. In another study [Steismo et al, 1997], eleven high alloyed stainless steels were tested for applications in chlorinated Norwegian seawater (Salinity 20,000 ppm). Critical crevice temperature (CCT) was determined using a potentiostatic test method. Results were evaluated in terms of Critical Crevice Index, CCI (CCI = % Cr + 4.1% Mo + 27%N) value of the alloys and compared the results of duplicate specimens in other tests. CCT was found to be in the same range for the 6 austenitic and 5 duplex stainless steels. Repassivation of crevice corrosion was found to occur at far lower temperatures than initiations of crevice corrosion. The effect of temperature on the corrosion of high alloy stainless steel in seawater (North Sea) has been studied [Francis et al 1996]. Super duplex alloys are more resistant to crevice corrosion at elevated temperatures as compared with 6Mo steels. Wallen and Bergqvist 1810

7 [1997] tested piping system, composed of real components made of 2 superaustenitic SS in continuously chlorinated (2 ppm) seawater (Norwegian, Cl - 19,000 ppm) A system consisting of pipes made of 6Mo steel, 254 SMO and flanges made of 7Mo steel 654 SMO was resistant to corrosion provided severe weld defects were absent. Immersion tests in natural and chlorinated North Atlantic seawater show that super duplex and super austenitic 6Mo steels have about the same resistance to initiation of crevice and pitting corrosion [Wallen, 1998]. It has been shown that using superaustenitic 6Mo, superduplex or 7Mo flanges is a possible way to overcome most of crevice corrosion problems. Corrosion behavior of selected materials in Arabian Gulf seawater was studied by investigating the effect of salinity and temperature using electrochemical techniques [Al-Ghamdi et al, 1996]. The results indicate that high salinity alone does not contribute to corrosion trends in seawater, the temperature also plays an important role. There is a steep rise in corrosion rate by a 20 o C increase in temperature. The pitting behavior of AISI 316 L in Gulf seawater was studied electrochemically [Malik and Al-Fozan, 1994]. An increase in temperature caused a decrease in pitting potential and increase in corrosion rate and pitting tendency. Pitting potential and corrosion rates were found to be highly sensitive to surface treatment. Pitting potential was more noble for specimens treated with HNO 3 and lowest for sand blasted specimens. The pitting behavior of some conventional and high alloy austenitic, ferritic and duplex stainless steels has been studied at 50 o C in Gulf seawater [ Malik et al. 1994; 1995]. The pitting potential appeared to be a logarithmic function of Cl - concentration. The conventional austenitic stainless steels (AISI 304, 316 or 317) showed a relatively small shift in pitting potential with variation in Cl - concentration. An increase in Cl - concentration resulted in a shift to more negative (or active) E pit. Pitting potential, E pit was found to be a liner function of Cr, Cr + Ni and PRE N of steels as well as the induction time, t. Although conventional and super stainless steels behave similarly, two separate liner relationship with different slopes are observed. 1811

8 In chloride containing environments where conventional stainless steels are subjected to localized corrosion under certain conditions, the addition of relatively large amounts of Mo ( 6) and chromium provide excellent corrosion resistance against localized corrosion resistance. The synergic effect of chromium and molybdenum in resisting pitting was first shown by Lorenz and Medawar [1969]. The relative effect of Cr, Mo and nitrogen on pitting or crevice corrosion can be assumed qualitatively by pitting resistance equivalent, PRE N which is represented by the empirical equation [Herbsleb, 1982]. PRE N = % Cr x % Mo + 16 X % N A high nitrogen multiple of 30 has been used for selected alloys instead of 16 [Henbner et. al., 1989]. A PRE n above 38 is supposed to provide good resistance to marine corrosion [Glover, 1988]. Crevice corrosion is another form of localized corrosion which is quite frequented in marine structures, pipe lines, pumps or shafts or any location where there is an occurrence of crevice or cavity along with accumulation of stagnant liquid (water) or deposit formation and it is not surprising that attack is localized to shielded areas and rest of the surface remains unaffected. In most cases pitting is precursor of crevice corrosion and there are obvious similarities between crevice corrosion and pitting corrosion mechanisms. The origin of crevice attack stems from the compositional differences developing in the solution, inside and outside of the crevice. These difference lead to the formation of macro corrosion cells with the inner crevice acting as an anode resulting in rapid dissolution of the metal [Oldfield and Sutton, 1978]. The differences between the outside and inside solution can be manifold : differential aeration cell, metal ion concentration, ph, chloride ion concentration, inhibitor concentration, etc. [Ijesseling, 1980]. The effect of crevice corrosion can range from pitting to uniform corrosion of the metal surface within the crevice. Several of the critical factors which affect the crevice corrosion of stainless steel in seawater include the salinity, temperature, dissolved oxygen level, velocity of seawater, the chromium and molybdenum contents of the stainless steel, the electrochemical behavior of the passive film, chemical reaction, transport mechanism and crevice type [Lee and Kain, 1983]. 1812

9 The crevice corrosion manifests itself in many cases [Beavers, 1986] : bolt head to washer, washer to base plate, tube to tube sheet, sleeve to pump shaft, gasket to flange face, teflon to metal, barnacle to metal, valve stem packing, pump shaft packing, etc. Immersion and electrochemical tests were utilized by many authors to study the mechanism of localized corrosion. The behavior of crevice corrosion was investigated electrochemically by many authors for determining parameters such as critical crevice solution, passive current, critical crevice potential, critical crevice temperature and variation of potential with time. The propagation of crevice corrosion has been studied by using different electrochemical techniques such as potentiodynamic, potenitostatic, galvanostatic, remote crevice assembly, compartmentalized cells, etc. [Oldfield et al., 1980, Hack, 1983, Davies Smith et al. 1987, Yashiro et al and Kain et al. 1985]. The effect of crevice former and crevice geometry on the crevice corrosion of 316 L SS in Arabian Gulf seawater was investigated by immersion tests and electrochemical techniques [Malik and Al-Fozan, 1994]. The corrosion potential during induction period was found sensitive to the geometry and type of crevice former used. The effect of dominant alloy additions on the crevice corrosion behavior of some conventional and high alloy stainless steels was studied in seawater at 50 o C [Malik et al., 1995]. The results of the study indicates : (i) Besides superstainless steels, conventional stainless steels containing more than 6% Mo can be used with little risk of corrosion for seawater applications involving crevice forming systems, (ii) Crevice corrosion may initiate in high alloy stainless steel but usually the subsequent propagation step is not followed (iii) CCS ph appears to be a liner function of PRE N showing a strong but negative dependence of critical solution aggresivity on Cr, Mo and N contents of the steels and (iv) At a constant torque, the surface finish (from rough to smoother) and increasing temperature enhance the possibility of corrosion in crevice forming systems. The pitting and crevice corrosion tendencies of stainless pipings in chlorinated seawater were tested electrochemically and by crevice corrosion exposures [Boah and Frazer, 1996]. It has been shown that besides 254 SMO which did not corrode in chlorinated seawater (25,000 to 50,000 ppm Cl - and 0-10 ppm residual chlorine) all other alloys developed crevices and suffered some degree of corrosion. 1813

10 The effect of chlorination on some high alloy grade stainless steels (254 SMO, Monit and Sandvik Sanicro 28) in North Seawater was studied [Wallen and Henrikson, 1989]. The investigations showed that continuously chlorinated seawater was considerably more aggressive than unchlorinated seawater or intermittently chlorinated seawater and that a high temperature increased the risk for localized corrosion at the same chlorine concentration. The highest alloyed steel grades were very resistant to crevice corrosion even in continuously chlorinated seawater- Contrary to crevice corrosion, the risk of galvanic corrosion decreased considerably if the seawater was chlorinated. Hack [1983] studied the crevice corrosion behavior of austenitic and ferritic stainless steels at 30 o C and showed that some high alloy steels are not only more resistant than AISI 316 but also exhibit resistance equivalent to more costly Ni-base alloys. Austenitic stainless steel required 8% Mo to prevent crevice corrosion whereas ferritic steels required 25% Cr and about 3.5% Mo. Using an accelerated technique, Oldfield [1990] studied the influence of alloying element addition on the crevice corrosion behavior of a number of commercial steels in a marine environment. Amongst the commercial steels, 254 SMO provided the best resistance. Corrosion resistance increases if the Mn level is less than 0.5% or there is a decrease in S level. Oldfield [1988] reported the effect of temperature and surface roughness` on the seawater crevice corrosion resistance of commercial and model stainless steels including 316 and 20 Cr-6Mo types containing 2 ranges of nickel : 10 to 30% and 20 to 40%. The following conclusions were drawn from the study regarding resistance to crevice corrosion initiation : (i) better resistance with rough surfaces in comparison to ground to polished surfaces (ii) resistance increases as the temperature decreases from 70 to 50 o C and (iii) little influence of variation in nickel contents. For resistance to corrosion propagation : (i) Ni has been found extremely beneficial in improving the resistance of 316 type steels and (ii) Ni above 20% did not have any significant effect. 4. EXPERIMENTAL 4.1 Materials 1814

11 Nine different commercial grade stainless steels, 316L, 317L, 904L, 3127hMo, 1925hMO, 254SMO, Duplex 2205, Remanit 4565 and 654SMO were used for this study. Table 1 lists the composition of these alloys. The seawater and chlorinated (0.2 ppm residual chlorine) seawater mentioned in this report is considered as Arabian Gulf raw seawater unless otherwise mentioned. The composition of the seawater is given in Table Sample Preparation For potentiodynamic cyclic polarization (PCP) experiments, round samples of ~15mm dia. and 3-5 mm thick were machined from the sheets and were abraded sequentially on 120, 180 and 320 grit SiC paper to simulate service condition. The abraded samples were cleaned in ethanol by ultrasonic cleaner. All the specimens were abraded only one hour before each experiment. Rectangular coupons of the size 100x20x3-5 mm were cut from the sheets and were grinned by surface grinder for open circuit potential measurements (OCP). The surface finish was ~ 0.12 µm rms. The grinned samples were cleaned with ethanol in ultrasonic cleaner. The samples were prepared few days before the experiment and were kept in a dessicator. 4.3 Open Circuit Potential Measurements Open circuit potential measurements of nine different alloys (listed in Table 1) were carried out in chlorinated ( ppm residual chlorine) seawater at 25 0 C and raw seawater at 50 0 C. In the experiments with chlorine dosing the residual chlorine was monitored every day except weekends. A special test cell was designed and fabricated in the lab with the help of instrumentation section and workshop of the RDC (Figure 1). Two multipen chart recorders of 6 and 3 pens were electrically connected to the specimens. The change in voltage, against the SCE, used as reference electrode, was plotted Vs time. Everyday the data were transferred in to the computer and a combined graph was drawn to study the change in potential for each of the alloys. 4.4 Potentiodynamic Cyclic Polarization Experiments (PCP) 1815

12 These experiments were carried out in seawater at room temperature (25 0 C) with and without dosing of sodium hypochlorite (NaOCl) solution and in raw seawater at 50 0 C. The residual chlorine was measured by HACH DREL/1C portable kit. All potentiodynamic cyclic polarization (PCP) experiments were carried out using the corrosion cell from EG&G model K0047 (Figure 2) with saturated calomel electrode (SCE) as reference and graphite rods as counter electrode. Flat sample holder from EG&G model K0105 (Figure 3) which provides 1 cm2 exposed surface area of the sample was used for all the PCP experiments. The flat sample holder (K0105) made up of high density polyethylene (HDPE) supplied with the instrument could not be used for samples having a thickness of more than mm. Since the thickness of outer wall adjacent to the washer was about 1.0 mm, on applying force by tightening the screw, the outer wall was used to bulge out. As a result of bulging, the electrolyte use to reach inside the holder during the experiments and consequently, the experiment had to be aborted. Sometimes its affects had been noticed only after finishing the experiment. Therefore, a new sample holder, which could be capable of holding samples up to 4.0 mm thick and with an increased outer wall thickness was designed and fabricated in the workshop (Figure 3). The new holder overcome all the shortcomings observed in the old one. This holder was used for carrying out all electrochemical experiments in chlorinated and unchlorinated seawater. The PCP experiments were carried out on flat round ~15mm dia. specimens in seawater with and without chlorine (residual chlorine 0.2 ppm) at room temperature. The samples were immersed in the test solution 15 minutes before starting the experiment. The potentiodynamic tests were carried out using a computer controlled EG&G model 273 potentiostat. The experiment was programmed to change the specimen potential relative to reference (SCE) electrode from its free corrosion potential or open circuit potential (OCP) in both the directions i.e. cathodic and anodic. The current corresponding to each potential was recorded and displayed on the monitor continuously. The experiment was programmed to start the reverse scan after attaining a current density of 100 µa/cm 2 to zero Volts Vs SCE. Each 1816

13 experiment was repeated 4-6 times and average of nearest three experiments test data has been reported in this report. 4.5 Residual Chlorine Degradation Measurements The degradation of different level of residual chlorine ( ppm) with time was monitored at 25 0 C in open and closed systems. Initially the residual chlorine level was raised to a predetermined value, then the degradation with time was monitored at short intervals during the first few hours and later on at an increased intervals. Figure 4 shows the change in the concentration of residual chlorine in an open and closed system with time. It is evident that initially the degradation of chlorine was very fast and later on it gets slowed down. Some trial experiments were carried out to find out the degradation of chlorine in presence of some alloys. Figure 5 shows the variation in OCP with time in presence of SS316L and 0.23 and 0.74 ppm of residual chlorine at room temperature. It was found that the maximum effect of chlorine dosing on open circuit potential of the alloy was observed after 15 minutes of chlorine dosing. The corrosion potential becomes more noble and reaches to a maximum value after ~15 minutes of chlorine dosing. After minute of chlorine addition the potential starts decreasing and reaches to same potential as was before chlorine dosing after more than 10 hours. The total potential scanning range for alloys under test was Volts. For SS316L the total scan range is about 1.5 Volts while for other alloys is about 2.5 Volts. Therefore, the duration required with conventional scan speed (20mV/min.) for an average scan range of 2.5 Volts will be about 2.0 hours. However it has been observed that the maximum effect of chlorine dosing appears after 15 minutes of dosing which persist up to ~1.0 (Figure 5) and then hour starts decreasing very rapidly. Therefore, on the basis of trial experiments of chlorine degradation with time in presence and absence of alloy the potentiodynamic cyclic polarization experiments were set up in such a way so that the effect of chlorine on alloys could be studied. Therefore a scan rate of 60 mv/min. after 15 minutes of sample immersion was selected for all the experiments. Although this scan rate is 3 times higher than the conventional scan rate. 1817

14 Recently Latha and Rageswari [1997] reported the results for stainless steels at same scan rate. For the materials showing passive behavior in static seawater on increasing the potential from the free corrosion potential value E corr in anodic direction, low currents were observed until a potential, E b is reached. E b represents the potential at which the current starts to rise significantly indicating a loss of passivity or film breakdown. The anodic polarization scan was reversed once the current reached 100 µa/cm 2. The current I max represents the maximum current attained should it not begin to fall immediately after scan reversal. E r or E prot is called repassivation potential at which the current falls to the small values recorded during the forward scan [Neville and Hodgkiess 1996]. Figure 6 represents the various points on a Potentiodynamic cyclic polarization curve showing E b, E r or E prot and I max. 5. RESULTS AND DISCUSSION When stainless steel is exposed to seawater its corrosion potential quickly rises in the noble direction. At the start of the experiment, low potential is observed but reaches seemingly to a constant level after few weeks of exposure [Mollica and Tervis 1976, Holth et. al., 1988; Gallagher and Malpas, 1989]. The natural seawater contains living organisms, which form a slime layer on the metal surfaces immersed in natural seawater. The biofilm formed on the surface has a strong catalytic effect on the cathodic reaction of corrosion process, i.e. reduction of oxygen [Mollica and Tervis 1976 and Johsen 1983]. This results in more noble corrosion potential than in sterile chloride solutions and an increased cathodic efficiency at cathodic polarization. For this reason a natural seawater is more corrosive than artificial seawater or sodium chloride solutions. The noble potential shown by the stainless steels in natural seawater increases the risk of galvanic corrosion on less noble materials. Amaya and Miykui [1994] also reported noble corrosion potential of stainless steel type 316L and 304L in natural seawater. The cathodic polarization behavior indicated the existence of some higher redox potential substance than oxygen. They identified this substance as hydrogen 1818

15 peroxide, which could be generated by reduction of oxygen in presence of enzyme (Oxidaze), in the natural biofilm. The biological activity in seawater may cause practical problems leading to costly production disturbances. For example, a high degree of fouling creates the friction losses in a pipeline and may induce erosion in certain metals. Furthermore, even slight fouling lowers the thermal efficiency of heat exchanger surface. The conventional way to prevent biofouling or to reduce its effects is to chlorinate the water. This is normally done by adding a strong hypochlorite solution or by electrolyzing the water, continuously or intermittently [Wallen, 1990]. Chlorine is a strong oxidant which displaces the corrosion potential of stainless steels to more noble values [Malpas et. al., 1986]. In a continuos chlorination system the rate of increase in potential increases with increase in chlorination but always faster than in unchorinated water [Gundersen, 1988]. The final potentials are also considerably more positive than those measured in unchlorinated water. Therefore increasing the risk of pitting and crevice corrosion. The activity of the biofilm is considerably reduced if the seawater is heated to a temperature above ambient temperature, normally to C. The other factor that might have an effect on biofilm is the flow rate of seawater. Mollica and Tervis [1976], reported that no active biofilm develops if the flow rate exceeds 2m/Sec. While Johsen and Bradar [1986] founded that even 4.5 m/sec. does not reduce its activity. 5.1 Potentiodynamic Cyclic Polarization Studies (PCP) Figures 7-33 show the potnetiodynamic scan plot for all nine alloys under study. The values of breakdown potential (E b ), protection potential (E prot ) and maximum current density attained after the start of reverse scan ( at 100 µa/cm 2 ) at 25 0 C with and without chlorination and at 50 0 C in seawater are given in tables 3-5. The breakdown potential of different alloys at 25 0 C in seawater follow the sequence: 316L<317L< L

16 The E b for all the alloys except 316L and 317L are in the range of mv SCE. A similar trend was observed in chlorinated seawater also. In chlorinated seawater the values of E b were slightly lower as compared to raw seawater. At 50 0 C, the E b of all alloys follow the following sequence: 316L<317L< L The lower E b value indicates more chances of easily breakdown of passive film and early start of pitting and crevice corrosion. The lowest E b values were found at 50 0 C followed by corresponding values in chlorinated and unchlorinated seawater at room temperature. Comparatively the E b values of all alloys except Duplex 2205 at 50 0 C are high. The E b value of 904L in seawater at 25 0 C also falls in the same range. A similar trend for E prot was also observed (Figure 35). The maximum Imax value recorded for 316L at 500C followed by Duplex 2205 and 904L. At room temperature, in chlorinated and unchlorinated seawater, 316L and 317L show slightly higher current densities as compared to rest of the alloys under test. An increase in I max indicates the possibility of general corrosion under the test condition. At room temperature in both chlorinated and unchlorinated condition, the current density I max of all the alloys except 316L and 317L is in the same range. At 50 0 C the I max follow the sequence: 316L< L 317L< < A sharp decrease in E b of all the alloys was observed, It is interesting to note that E b of Duplex 2205 steel was reduced drastically from 1010 mv SCE at 25 0 C to 354 mv SCE at 50 0 C. A negligible difference was found in the I max values of 3127HMO, 1225HMO, 254SMO, Remanit 4565 and 654SMO alloys in all the experimental conditions. Duplex 2205 steel showed maximum difference in I max at 50 0 C as compared to both room temperature condition followed by 904L, 316Land 317L steels. Low decrease in E b was observed for 3127HMO, 1925HMO, Remanit 4565 and 654SMO alloys. Therefore these alloys have less chance of crevice and pitting corrosion in seawater and chlorinated seawater at 25 0 C and in seawater at 50 0 C. Stainless steels 316L and 317L at 1820

17 25 0 C, and 904L and Duplex 2205 at 50 0 C can suffer from general as well as crevice and pitting corrosion due to low E b and high I max values. Figures show graph of E b Vs PRE N value (Table 1) and sum of Cr+Mo concentrations for all the nine alloys at 25 0 C with and without chlorine (0.2 ppm) and at 50 0 C. At 25 0 C with and without chlorine an increase in E b value is observed above a PRE N value of 30 and 50 0 C above 35 an increase in PREN value can be seen (Figure 39). The sum of Cr+Mo concentrations in the alloys above 25 (Figures 40-42)show an increase in the Eb value for all the alloys under all test conditions. Therefore it can be concluded that the alloys having a PRE N value of above or equal to 30 at 25 0 C with and without chlorine and PRE N value of 35 at 50 0 C or the sum of Cr+Mo above 25 can be used safely for seawater applications. The results of potentiostatic studies indicate excellent corrosion resistance by 6Mo steels in natural seawater (25 0 C and 50 0 C) and chlorinated seawater (25 0 C). The findings are in confirmetry with the results reported by Agarwal et al [1991], Francis et al [1996], Wallen and Bergqvist [1997] and Steismo et al [1997] from similar studies on these alloys in Atlantic and Norwegian seawaters. 5.2 Open Circuit Potential (OCP) Studies The variation in open circuit potential (OCP) of all the alloys at 25 0 C in chlorinated and at 50 0 C in unchlorinated seawater can be seen in figures and respectively. A significant enoblement in OCP at different concentrations of residual chlorine was observed in all the alloys (Figures 43-50). The OCP increases with increase in residual chlorine at 250C (Figure 51). At 500C, OCP of 1925HMO, 254SMO, 654SMO, 3127HMO and Remanit 4565 reached to zero value with in 10 hours after the immersion. While 316L, 317L and 904L take about 15 hours to reach this value. In case of Duplex 2205, a frequent change in potential from anodic to cathodic and vice versa was observed (Figure 58). This indicates that some corrosion phenomenon is occurring at the metal/solution interface. On examination under scanning electron microscope (SEM) Duplex 2205 showed some pits (Figures 61-62) at the water/air interface. None of the other alloy showed any pit formation during the 1600 hours immersion. 1821

18 The use of stainless steels 316L, 317L at 25 0 C in seawater applications is unsafe. Because of their very low E b and high I max values, a further decrease in E b values in chlorinated seawater increases the risk of their use. Because the OCP at high level of residual chlorine is very near to the Eb, especially for 316L and 317L at 25 0 C and duplex 2205 at 50 0 C in seawater (Table 4-5), a sharp decrease in E b value from 1095 mv SCE at 18 0 C to 417 at 30 0 C was reported [Neville and Hodgkiess, 1996]. Frequent change in OCP, pit formation and sharp decrease in E b value provide strong evidence that the duplex 2205 steel is not a suitable alloys for high temperature seawater applications. 6. CONCLUSIONS 6.1 Specific Conclusions Following conclusions can be drawn from the studies : 1. All the stainless steels under test show an increase in open circuit potential in presence of residual chlorine. 2. The enoblement in open circuit potential increases with increase in the quantity of residual chlorine. 3. The E b values of stainless steels 316L, 317L and 904L at 25 0 C in seawater follow the sequence : 25 0 C > 25 0 C (Chlorinated) > 50 0 C. 4. Duplex 2205 steel shows a drastic reduction in E b from 1010 mv at 25 0 C to 325 mv at 50 0 C therefore its use should be restricted to low temperature applications. 5. The E b values of 3127HMO, 9125HMO, 254SMO, 654SMO and Remanit 4565 under all the test conditions except duplex 2205 steel at 50 0 C does not show any significant variations. 6. A remarkably large difference in E b and E prot values of stainless steels 904L and duplex 2205 at 50 0 C was observed, which show the susceptibility to general and crevice or pitting corrosion for these alloys at this temperature. 7. At 50 0 C all the alloys other than 316L, 317L, 904L and duplex 2205 can be used safely. 1822

19 8. Alloys having PREN value grater than 35 showed Eb values in a close range e.g., (at 25 0 C), (at C, residual chlorine ppm) and (at 50 0 C). 6.2 General Conclusions Electrochemical studies carried out on high alloy stainless steels provide useful information regarding the general and localized corrosion behavior of 316L, 317L, 904L, duplex 2205, 3127HMO, 1925HMO, 254SMO, 654SMO and Remanit 4565 alloys in normal and chlorinated seawater. Chlorination level should be maintained to such values so as to prevent the formation of biofilm but not enough to increase the open circuit potential. High alloy stainless steels such as 3127hMo, 1925 hmo, 254 SMO, 654 SMO and Remanit 4565 have very little tendency to crevice and pitting corrosion in normal seawater at 25 0 C and 50 0 C and chlorinated seawater at 25 0 C. 7. RECOMMENDATIONS The electrochemical data generated during this study provide useful information about the corrosion resistance of conventional and exotic construction materials in chlorinated and normal seawater. This information together with the information available about the corrosion behavior of materials from previous studies (SWCC (RDC) Reports 20, 22, 26 and 28 ) could be complied and may be utilized for the material selection in existing or future desalination or seawater handling system. The corrosion data of the materials in Gulf Seawater computed from aforementioned studies should be compared with data, if available from similar studies carried out in waters of different oceans. 8. FUTURE WORK PLAN 1. Long term crevice corrosion studies of these alloys under flowing seawater condition and at different level of intermittent and continuos dosing of chlorine and temperature should be studied in a test loop or pilot plant to ascertain their resistance. 2. To prevent the formation of biofilm but not enough to increase the open circuit potential, threshold level of residual chlorine should be determined. 1823

20 REFERENCES 1. Agrwal, D.C., Jesner, M.R. and Rockel, M.B., (1991), 6% Mo Austenitic SS selection for Offshore Applications, 23 rd Offshore Technology Conference (OTC), Houston, Texas. 2. Al-Ghamdi, A.M. Nusair, Cocks, F.H. (1996) Corrosion Behavior of Selected Metals in Arabian Gulf Seawater" Proceeding of the 7 th Middle East Corrosion Conference, Bahrain Amaya, H and Miyuki, H,, (1994), Proceedings International Symposium, Techno-Ocean 94, 1, Beavers, J.A. Kpock, and Berry, W.E., (1986), Corrosion of Metals in Marine Environment, Metals and Ceramic Information Center Report, MCIC Boah, J.K. and Frazer, P., (1996), Localized Corrosion Tendencies of Piping Materials Used in Chlorinated Seawater, Proceeding of the 7 th Middle East Corrosion Conference, Bahrain, Bordal, E., Drougli, J.M., and Cartland, P.L., (1993) Corrosion Science 35, Carpenter, M.D., (1986), British Corrosion J., 21(1), Davies- Smith, L.R., Lane, J.D. and Riley, T., (1987), Br. Corrosion J., 22(2), Fontana, M.G., (1986), Corrosion Engineering McGraw Hill, New York. 10. Francis, R., Hughes, S. and Byrne, G., (1991), The Effect of Temperature on the Corrosion of Higher Alloy SS in Seawater, Proc. VII Middle East Conference, NACE, Bahrain, Gallagher, P. and Malpas R.E., (1989), Corrosion/89, paper no Garfias - Mesias, L.F., Sykes, J.M. and Tuck., C.D.S, (1996), Materials Performance 27, Glover, T.J., (1988), Materials Performance, 27(1), Gundersen,R., (1988), Proceedings UK Corrosion/88, Brighten, UK, Oct.3-5, Hack H.P., (1983), Materials Performance 22, Hassan, M.A., and Malik, A.U., (1989), Desalination, 74, Henbner, U., Rockel, M. and Wallis, E., (1989), Eerstoffe Korrosion, 40,

21 18. Herbsleb, G., (1982), Werkstoffe and Korrosion, 33, Hodgkeiss T., and Ary, N.G., (1985), Desalination 55, Hodgkeiss, T., Maciver, A., and Chong. P.Y., (1987), Desalination 66, 147. Holthe R, Gartland, P.O. and Bardal, E., (1988), Proceedings 7 th International Congress on Marine Corrosion and Fouling, Valencia, Spain. 21. HÖmig, H.E., Seawater and Sewater Distillation, (1978), Vulkan - Verlag, Essen. 22. Ijerseling, F.P., (1980), Br. Corrosion J., 15, Johnsen R, Bardal, E. and Durgli M., (1983), Proceedings 9 th Scandinavian Corrosion Congress, Copenhagen, Denmark, Sept.12-14, 1, p Johnsen, R. and Olsson, (1986), Proceedings 10 th Scandinavian Corrosion Congress, Stockholm, Sweden, June 2-4, Kain, R.M., and Sleep, T., (1985), ASTM, STP 866, Kain, RM., (1991), Effect of Surface Finish on the Crevice Corrosion Resistance of SS in seawater and Related Environment, Corrosion 91, paper Lee. T.S., and Kain, R.M., (1983), Corrosion '83, NACE, paper Lee. T.S., Kain, R.M. and Oldfield, J.W., (1984), Materials Performance, 23(7), Latha, G. and Rageswari, S., (1997), Corrosion protection & control, Feb., Lornz, K., and Medawar, G., (1969), Thyssen Forschung, 1(3), Malik, A.U., and Al-Fozan, S., (1994), Desalination, 97, Malik, A.U., Kutty, P.C.M., Siddiqi, N.A., Andijani, N. and Ahamd, S., (1992), Corrosion Science 33(11), Malik, A.U., Kutty, P.C.M., Siddiqi, N.A., Andijani,I. N. and Al-Fozan, A., S. (1994), Desalination 97, Malik, A.U., Siddiqi, N.A., Ahamd, S. and Andijani, N., (1995), Corrosion Science, 37(10), Malik, A.U., Siddiqui, N.A., and Andijani, I.N., (1994), Desalination, 97, Malpas R.E., Gallagher P. and Shone E.B., (1986), Proceedings Chlorination of Seawater systems and its Effect on Corrosion, Birmingham, UK, Mar.4, Paper TRCP 2919 R, Soc. of Chem. Ind. 1825

22 37. McCarfferty, E., Bogar, F.D., Thomas II, E.D., Creegan, C.A., Lucas, K.E. and Kaznoff, A.I., (1997), Corrosion, 53(10), Mollica A, Tervis A., (1976), Proceedings 4 th International Congress on Marine Corrosion and Fouling, Antibes, France, June 14-16, Moniz, B.J. and Pollocks, W.J., eds., (1986), 'Process Industries Corrosion' NACE, Houston. 40. Neville, A. and Hodkiess, T., (1996), Corrosion Science, 38, Oldfield, J.W., (1990), Corrosion 46, Oldfield, J.W., and Sutton, W.H., (1980), Br. Corrosion, J., 15, Oldfield, J.W., and Todd, B., (1995), A Review of Materials and Corrosion Desalination - Key Factors for Plant Reliability - Award Winning Treatise. IDA World Congress on Desalination and Water Science, Abu Dhabi, November Oldfield, J.W., and Todd, B., (1996), Desalination 108, Salvago, G., and Fumagalli, (1996), Corrosion, 52, Sedricks, A.J., (1979), 'Corrosion of Stainless Steels' Wiley, New York. 47. Smialowska, Z.S., (1986), 'Pitting Corrosion of Metals' NACE, Houston. 48. Smialowska, Z.S., (1987), 'Industrial Problems Treatment and Control Techniques' Pergamon Press, Oxford. 49. Stackle, R.W., Brown, B., Kruger, J. and Agrawal, A., (eds.), (1974), 'Localized Corrosion' 3 rd Edition, NACE Houston. 50. Steismo, U. and Drugli, J.M., (1997), Corrosion, 53(11), Todd, B., (1977), 'Materials for Seawater System' Chemistry and Industry, July 2, Tuthills, A.H., Moller, G.E., Todd, B. and Hornburg, D., (1995), 'The suitability of 6% Mo Austenitic SS for Desalination Plant Services', Proc. IDA World Conference on Desalination and Water Sciences, Abu Dhabi, November VII p Wallen, B, (1990), acom, No Wallen, B., and Henrikson, S., (1989), Werkstoffe and Korrosion 40, Wallen, B. and Bergqvist, A., (1997), The seawater Resistance of a Superaustenitic 7Mo Stainless Steel, Acom,

23 56. Wallen, B., (1998), Corrosion of Duplex Stainless Steels in Seawater, Acom, Yashiro, H.Y. Tarnno, K., Hanayama, H. and Mirura, A., (1990), Corrosion 46,

24 Table 1. Composition of different alloys (wt.%), balance in all the alloys is Iron and PRE N values. S.No Alloy Cr Mo Ni C Cu Mn N PRE N 1. SS 316-L SS 317-L SS 904-L hmo hmo N SMO N Duplex N Remanit N SMO N 57 Table 2. Composition of Arabian Gulf seawater at AL-Jubail. Constituents Arabian Gulf Seawater,Al-Jubal Cations (ppm) Sodium, Na + Potassium, K + Calcium, Ca 2+ Magnesium, Mg 2+ Copper, Cu 2+ Iron, Fe 3+ Strontium, Sr 2+ Boron, B 3+ Anions (ppm) Chloride, Cl - Sulfate, SO4 = Bicarbonate, HCO3 - Carbonate, CO3 = Bromide, Br - Fluoride, F - Silica, SiO 2 Other Parameters Conductivity, (µs) ph Dissolved Oxygen (ppm) Carbon di Oxide (ppm) Total suspended solids (ppm) Total Dissolved Solids (ppm) Temperature Range ( 0 C)

25 Table 3. Breakdown Potential (E b ), and protection Potential (E prot ) verses Saturated Calomel Electrode (SCE) and maximum current after scan reversal (I max ) of nine different alloys in Arabian Gulf water at 25 0 C. S.No Alloys E b (mv) (mv (µa/cm 2 ) L L L hMO hMO SMO Duplex Remanit SMO E prot I max Table 4. Breakdown Potential (E b ), and protection Potential (E prot ) verses Saturated Calomel Electrode (SCE) and maximum current after scan reversal (I max ) of nine different alloys in chlorinated ( ppm) in Arabian Gulf water at 25 0 C. S.No Alloys E b (mv) E prot (mv I max (µa/cm 2 ) L L L hMO hMO SMO Duplex Remanit SMO Table 5. Breakdown Potential (E b ), and protection Potential (E prot ) verses Saturated Calomel Electrode (SCE) and maximum current after scan reversal (I max ) of nine different alloys in natural seawater at 50 0 C. S.No Alloys E b (mv) (mv (µa/cm 2 ) L L L hMO hMO SMO Duplex Remanit SMO E prot I max 1829

26 Figure 1. Photographs showing the Open Circuit Potential measurement system Figure 2. Photograph of K0047 corrosion measurement cell kit measurement 1830