CORROSION BEHAVIOR OF SOME CONVENTIONAL AND HIGH ALLOY STAINLESS STEELS IN GULF SEAWATER 1

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1 CORROSION BEHAVIOR OF SOME CONVENTIONAL AND HIGH ALLOY STAINLESS STEELS IN GULF SEAWATER 1 SUMMARY Anees U Malik, Ismail N. Andijani and Nadeem A Siddiqi The corrosion behavior of conventional austenitic and ferritic stainless steels (AISI 304L, 316L, 317L, and 17-4PH) and high alloy stainless steels (904L, SMO 254, 1925 hmo, 3127 hmo, 20 Cb3, Remanit 4565, Monit 44635, Remanit 4575 and Duplex 2205) has been studied at 50oC in Arabian Gulf seawater and Cl - containing aqueous solutions. Corrosion rates of the steels have been determined under severe conditions of salt spray and by the application of electrochemical polarization resistance technique. The pitting behavior of the steel has been evaluated qualitatively by determining E pit pitting potential and ti, induction time for pit initiation in seawater. The dependence of corrosion rate and pitting potential, E pit on steel composition (Cr and Cr + Ni contents), pitting resistance equivalent, PRE N and induction time, ti has been discussed in detail. It has been shown that in crevice/deposit free systems, both conventional stainless steels and high alloy stainless steels behave similarly and can be used without the apprehension of corrosion. INTRODUCTION The conventional metals and alloys which have been used as structural materials are subject to varying degree of corrosion when exposed to marine environments. The material selection is generally a balance between reliability and cost. For example, plain carbon steel which has been the most widely used structural material is abundantly available and is inexpensive, has adequate mechanical properties but has a high general corrosion rate particularly where water velocities are high. The copper-base alloys are widely used in seawater applications due to their naturally occurring and protective oxide film which provides good antifouling characteristics. These alloys are, however, sensitive to ammoniacal and sulfide containing waters and high water velocities and have high strength restrictions. Titanium has excellent corrosion resistance properties in marine environments due to its ability of self passivation in air - the spontaneous formation of a thin and strongly adhering surface oxide. Some nickel-base alloys have 1 Issued as a Technical Report No. SWCC RDC)-20 in September,

2 very good resistance to general corrosion, pitting, general aqueous corrosion and stress corrosion cracking (SCC) in marine environment. However, the relatively high cost of titanium and nickel-base alloys limit their use only to some specialized applications. Stainless steels (containing more than 12% Cr) have been proved to be the most satisfactory materials for marine applications due to their excellent corrosion resistance even at high water velocities, good mechanical properties and fabricability. In addition, they are attractive from a commercial stand point due to their availability and relatively low cost. However, the greatest drawback with conventional stainless steel is susceptibility to localized corrosion in chloride - containing aqueous solutions which limits their use in seawater systems. Because of the superior characteristics of stainless steels to other marine construction materials, efforts have been made to improve the resistance of stainless steels tolocalized corrosion. The alloying element that increases the resistance of stainless steels to pitting and crevice corrosion are chromium and molybdenum, which improve the pitting resistance according to the formula: PRE = % Cr + 3.3% Mo where PRE is the pitting resistance equivalent. Nitrogen also has a strong influence on pitting/crevice corrosion resistance according to the formula: Austenitic Steels PRE N = %Cr x %Mo + 16x%N Conventional austenitic stainless steels are the most frequently employed materials in seawater applications. MO-free alloys such as type L and CF3 have limited use as marine materials due to susceptibility to local attack. Type 304 wire rope is the standard material used for marine wire rope despite its rather short service life, because it has higher strength to weight ratio than more highly alloyed materials. Left wet and spooled on winches, 304 wire rope will be destroyed by crevice corrosion in less than 6 months. Type 304 fasteners give reasonably good service when used to fasten steel or aluminium. The grecinitation hardening; grade (PH) in both cast and wrought forms are widely used in marine equipment wherever their high strengths are required in components. These grades suffer crevice attack to about the same degree as type 304 and require cathodic protection, (C.P.). Less noble materials, such as Al, CS and Ni Resist provide protection to the PH stainless steel component in many assemblies where strength requires the PH grade. Wrought grades 17-4 PH and 15 - PH are the important alloys in this series. 733

3 Type 316 or 316L are the workhorse alloys for marine and marine related services. Readily available worldwide and general familiarity facilitate their usage in all waters where 304 or 304L have experienced substantial corrosion and in new applications where the Mo-free grades are expected to perform poorly. Seawater related applications include horizontal piping, tubing, demisters, troughs, flash chambers, vents etc in multi stage flash (MSF) desalination plants and high pressure tubing and membrane module in seawater reverse osmosis (SWRO) plants. However, the alloy is prone to corrosion under high chloride and/or stagnant conditions usually experienced during shut down or standby periods. Type 316 is not suitable for vertically tubed chemical industry condensers unless uncondensables are completely vented through tube sheets. 317 L containing about 4% Mo has better corrosion resistance than 316L and has been used in SWRO tubings. Allov 20 CB-3 and CN7M are not usually considered for seawater service, however under the right conditions these alloys have given outstanding performance, as in the navy pumps. Crevice attack will occur in prolonged standby service, like O rings and gaskets. The 4.5% Mo containing type 904L has demonstrated sufficient resistance to corrosion in seawater to perform reasonably well as tubing. The alloy appears to be a promising candidate for brine service. 6% Mo Steels Mo-containing conventional austenitic steels are nearly ideal construction materials for process equipments handling seawater system providing excellent long term performance. In most applications, a small degree of crevice attack is tolerated- as long as the resulting maintenance is within limits, the user considers it acceptable. Until 15 years ago, if the corrosion resistance beyond that of conventional steel was not acceptable the usual practice was either to bear high maintenance where stainless steels were used or to face high investment costs by using high priced nickel base alloys or other exotic materials. In the late seventies, however, the gap between the conventional SS or more exotic materials was started filling by the introduction of austenitic steels alloyed with approximately 6% Mo and or 0.2% Nitrogen. The new 6 MO austenitic steels have not only much better resistance to pitting or crevice corrosion in chloride - containing environments than the conventional steels but also have good resistance to general corrosion and SCC. 734

4 During its service life of more than 7 years 254 SMO has exhibited resistance to crevice corrosion in seawater as condenser tubing and as a piping in a variety of seawater applications such as SWRO high pressure tubing. 254 SMO is a suitable material for eject condensers in MSF plants as reported from 7 years service experience. It has also been used for tube and shell heat exchangers when seawater is used as a coolant. It is not recommended for plate heat exchangers handling full strength seawater. Nitrogen alloyed super austenitic Remanit 4565 S features both high strength and excellent corrosion resistance in a wide range of corrosion media. It was originally developed for applications in chloride containing solutions and especially seawater. The high nitrogen content ensures high strength and outstanding pitting and crevice corrosion resistance even in as welded condition. Nicrofer 3127 hmo is an austenitic low carbon Fe-Ni-Cr-Mo alloy developed by VDM. Due to high Cr and Mo contents and extra low carbon content, the alloy has excellent resistance to pitting, crevice and intergranular corrosion and SCC, similar to high nickel alloys. It has good workability and weldability. Cronifer 1925 hmo is of very similar composition to alloy 904L but with Mo content increased to 6.5%, giving substantially better resistance to crevice corrosion and pitting. The resistance to SCC induced by pitting is also improved. Due to good resistance in seawater the steel is used for offshore plant equipments. Ferritic Steels Ferritic stainless steels are used when exceptional resistance to SCC along with good pitting/crevice corrosion resistance is required in aggressive environments. Sunerferrit Remanit 4575 is a highly alloyed 28/2 Cr-Mo steel containing about 4% Ni and is stabilized with Nb. The steel has outstanding corrosion resistance in chloride containing waters and was developed for applications in hot chloride solution systems such as those encountered in desalination plants. Monit has good performance in condenser and auxiliary cooler applications, its industrial service experience though is limited. It has been considered a good candidate material for salinewater service. But it may suffer some crevice corrosionunder deposits. Duplex Allovs Duplex steels are available both as wrought and cast products, have a microstructure consisting of a mixture of austenite and ferrite (about a ratio), and combine the near immunity of the ferritic stainless steels towards chloride induced-scc along with the toughness and ease of fabrication of the austenitic stainless steels. 735

5 Alloy2205 has become somewhat of a general purpose alloy providing general corrosion and pitting/crevice corrosion resistance superior to that of conventional austenitic grades such as 316L and 317L. This is primarily due to higher Cr + Mo contents in the duplex alloy. Some representative applications include seawater piping systems and air-cooler tubing on offshore oil platforms. The alloy is, however, subject to corrosion similar to the standard stainless steel grades when water is allowed to stand for extended periods in pipelines. This report contains the results of a systematic study carried out to investigate the corrosion behavior of conventional and non-conventional high alloy stainless steels in Arabian Gulf seawater. The main aim of the investigations is to assess the influence of the dominant alloying elements e.g., Cr, Ni and Mo on the general corrosion and pitting characteristics of the stainless steels in marine environments. The study encompasses all the austenitic, ferritic and duplex stainless steels referred to the above sections. A subsequent report would present the results of crevice corrosion behavior of these steels. EXPERIMENTAL Materials Conventional stainless steels (410, 304L, 316L, 317L, 904L, 17-4PH) and high alloy stainless steels (254 SMO, 1925 hmo, 3127 hmo, 20 Cb3, Remanit 4565, Remanit 4575, Monit and Duplex 2205) were obtained commercially in sheet and/or rod forms. The flat, circular flat or cylindrical specimens were used during the experiments without any further heat treatment. The chemical composition of the alloys is given in Table 1. Gulf seawater was collected in bulk amounts from intake point of Al Jubail desalination plant and was stored in large polyethylene containers. The chemical composition of the seawater and its important characteristics are given in Table 2. Chloride - containing aqueous solutions of varying concentration (300, 500, 1000 and 5000 ppm) were prepared by dissolving analytical grade NaCl in distilled water and adjusting the ph to 7.8 by the addition of NaOH. Techniaues and Procedure Salt Spray Tests were carried out in a salt spray fog chamber (Figure 1) following ASTM B Test coupons of 12 cm 2 were cut from the sheets and abraded to 180 grit Sic papers. The abraded specimens were cleaned in an ultrasonic cleaner followed by degreasing with acetone and drying in a hot air blower. The dried specimens were weighed before putting in the salt spray chamber. A 5% NaCl solution (ph = 6.5) was used for salt spray test. The coupons were exposed to salt spray for time periods varying from 3000 to 5000 hours. 736

6 ForElectrochemical Polarization Experiments circular, flat or cylindrical test specimens were used. The exposed area of the test specimens which was screwed in the sample holder was 1 cm 2. Potentiodynamic Polarization Experimets were carried out on an EG & G model Soft Corr Measurement System. The system is consisted of Model 273 potentiostat/galvanostat, Model 342 Soft Corr Software and Model 30 IBM PS/2. All the experiments were carried out using a corrosion cell with saturated calomel as reference and graphite as counter electrodes. Open Circuit Corrosion Potential (OCP) measurements were carried out in a separate cell with alloy coupon as a working electrode and saturated calomel (SC) as reference electrode. OCP of all the alloys were recorded in natural seawater under static conditions at 50 C. It took about 4 to 40 hrs to achieve a constant potential corresponding to open circuit corrosion potential. Potentiodynamic Polarization Measurements were carried out using a scan rate of 0.2 mv/s commencing at a potential above 250 mv more active than the stable OCP. Before starting the polarization scan, the specimen in the sample holder (WE) was cathodically polarized at mv for 5 minutes followed by stabilization for about 1 hr for attaining a steady state which was shown by a constant potential. Polarization Resistance Measurements were conducted at a scan rate of 0.1 mv/s with starting and final potentials corresponding to -20 mv to + 20 mv vs OCP, respectively. The maximum current range was 0.1µA. Before starting the experiments, the specimens (W.E) were left for about 1 hour for attaining a steady state which was shown by a constant potential. RESULTS Salt Spray Tests The alloy specimens were taken out from salt spray chamber after exposition to 3024 and 4824 hrs, respectively and were weighed subsequently. Weight changes in the range of 10 to 50 µg/cm 2 were noted indicating very nominal changes during salt spray exposures. None of the specimens showed any sign of scaling or corrosion product deposition. The corrosion rate values computed from weight losses have the range of 0.00l to mpy. The results of salt spray tests are summarized in Table 3. Open Circuit Corrosion Potential Measurements Fig. 2 and Fig. 3 show some typical time vs open circuit potential plots for different steels immersed in seawater at 50 o C. The induction time, ti for pit initiation has been determined from the plots and the values are listed in Table 4. High alloy stainless 737

7 steels show relatively large induction times (20-40 hrs) indicated by a constancy in potential whereas conventional steels show lower induction times (4-14 hrs). Some of the plots show regimes of high or low constant potentials (plateaus) emerged during sudden rise or fall in potential. Polarization Resistance Measurements Polarization resistance measurements were carried out using alloy coupons immersed in natural seawater or in aqueous solutions containing different concentrations of chloride at 50oC. Typical polarization resistance plots are shown in Figure 4 and Table 5 lists the corrosion rate values computed from the linear polarization resistance plots. These values are based on the cathodic and anodic tafel values which were obtained from previously carried out tafel plot runs. The polarization resistance data provide the following information regarding the behavior of the alloys in Cl - containing solutions and seawater. i The variation in corrosion rates of an alloy with change in chloride concentration usually does not follow a regular pattern. (ii) (iii) (iv) (iv> As expected, the corrosion rates of high alloyed stainless steels are usually lower than the conventional austenitic steels. The corrosion rates vary from 0.02 to 0.12 mpy depending upon the alloy system. The corrosion rate of an alloy in seawater is not necessarily higher than that in aqueous solutions containing chloride of much lower concentrations. Amongst the different types of steels studied the corrosion rates in seawater at 50 C follow the sequence: 3127 hmo < 904L < 254 SMO < Remanit 4565 < 304L< Remanit 4575 < 1925 hmo < Duplex 2205 < Monit < 17-4PH < 316L < 317L< 20 Cb3 In 1000 ppm Cl - containing aqueous solutions, the following sequence is noted: 3127 hmo<904l<20 Cb3<254 SMO<Monit 44635~1925 hmo<17-4ph<remanit 4575 <Duplex 2205<304L<316L<317L. Potentiodynamic Polarization Measurements Some representative potentiodynamic polarization curves for the steels in seawater and Cl - containing solutions are shown in Figure 5 and Table 5 lists the values of corrosion potential, E corr and pitting potential, E pit for different steels in both the media. The pitting potential, E pit is assessed as that potential at which a consistent increase in current 738

8 density occurs, indicating the initiation of non-passivating pits. In general, E Pit is shifted to more negative (or active) value with increasing chloride concentration. High alloy steels appear to have significantly higher pitting potential than conventional stainless steels. In seawater, the pitting potential, E pit for different steels at 50 o C follows the sequence: 1925 hmo > Remanit 4565 > 3127 hmo > Remanit 4575 > 254 SMO > Monit > Duplex 2205 > 20 Cb3 > 904L> 317L> 17-4PH > 316L> 304L. The pitting potential, E pit for different steels in 1000 ppm Cl - solutions at 50 C follows the sequence: 3127 hmo > Remanit 4575 > Monit > Remanit 4565 > Duplex 2205 > 254 SMO > 20 Cb3> 1925 hmo > 317L> 904L> 316L> 17-4PH > 304L. DISCUSSION The salt spray tests, which represent the testing of materials under severe conditions of salt environments, carried out on conventional as well as superaustenitic, superferritic and duplex steels, show extremely low corrosion rates even after exposures of about 5,000 hrs (Table 3). These results indicate virtual immunity of the alloys toward salt environments comprising of salt and salt-aerosol atmosphere. In relative sense, the corrosion rates of high alloy stainless steels (SMO254,1925 hmo, Remanit 4575, Monit and Duplex 2205) are significantly lower than the conventional austenitic stainless steels (304L, 316L and 904L). The corrosion rates of conventional and superstainless steels in seawater as well as chloride - containing aqueous solutions as determined by polarization resistance technique are invariably one order of magnitude higher than the corrosion rates from salt spray tests. During salt spray tests there is sufficient time for the oxide film to grow and form stable and protective film whereas in electrochemical polarization technique, the alloy has been subjected to polarization almost immediately after the formation of protective oxide film - the aforesaid appears to be a plausible explanation for the higher corrosion rates from polarization resistance technique. The corrosion rate data for stainless steels in seawater indicate that the rates are greatly dependent on the composition of the steels particularly on Cr, Mo and Ni contents. A plot of Cr + Ni content vs corrosion rate for the various steels indicate that apart from a few exceptions, a linear relationship exists (Figure 6). Considering the pitting resistance equivalent PRE N of the various steels studied it has been found that the super stainless steels which customary have high PREN show lower corrosion rates, a plot of PREN and corrosion rates for steels in seawater exhibits this trend (Figure 7). The bar diagrams (Fig.8 and 9) show the dependency of the corrosion rates on (Cr + Ni) contents and 739

9 PRE N values for the various steels in seawater at 50 C. Similar relationships appear to exist between corrosion rate vs Cr + Ni and corrosion rate vs PRE N for the steels in chloride containing aqueous solutions (Figure10 and 11). Considering the influence of chromium alone, corrosion rates normally decrease with increasing chromium contents (Figure12). In general, with increasing induction time, the corrosion rate decreases or in other words, more is the time for pitting initiation, greater is the resistance of metal towards corrosion in seawater (Figure13). The pitting potential E p shifts to a more negative value with increasing chloride concentration. The plots of logarithm of [Cl - ] and E p (Figure14) follow a linear relationship in most of the alloy systems. The conventional austenitic steels have much higher slopes than the superstainless steels indicating a relatively small shift in pitting potential with variation in Cl - concentration in case of high alloy stainless steels. Considering the influence of alloying elements on the pitting behavior of the steels, interestingly, Epir vs Cr + Ni plots for different steels exhibit a linear relationship (Figure15) but on the basis of this behavior they can be classified into 2 categories (i) conventional austenitic steels: 304L, 316L, 317L, 904L and (ii) high alloy stainless steels: 1925 hmo, 254SM0, Monit etc. A similar behavior has been shown in the plot of E pir vs Cr contents (Figure16) in which both conventional and superstainless steels show linear behavior but slopes are different. The superstainless show much lower increase in pitting potential with increasing Cr content in comparison to conventional austenitic steels which show much higher shift in pitting potentials. Two different linear plots of E pit vs PRE N are obtained for conventional and super stainless steels, respectively (Figure 17). As magnitude of PREN increases the value of E pit increases. This behavior is the natural consequence of the dependence of pitting resistance on the PRE N of the steels. Materials having greater PRE N have lower tendency to pitting or require much higher potential to initiate pitting in the alloys. The open circuit corrosion potential measurements of the various stainless steels in seawater at 50 o C, provide potential vs time plots from which the values of ti, the induction time for pit initiation have been computed (Table 4). The influence of ti on alloy composition, pitting resistance equivalent, PRE N and pitting potential, E pit has been studied and some interesting information has emerged. The Cr+Ni contents of the steels and PRE N appear to be the linear functions of induction time, ti Fig.18 and Fig.19. High alloy stainless steels with high PRE N values have large induction times for pit initiation. In high alloy stainless steels a large increase in induction time, ti is observed with relatively small increase in Cr + Ni contents or PRE N. However, an opposite behavior is observed in conventional stainless steels e.g., 304L, 316L, 317L or 904L where induction time decreases with increasing Cr +Ni content or PRE N This behavior indicates that conventional steels undergo pitting in seawater in much shorter times and presence of Mo smaller than 6% might not play a significant role in enhancing the pitting initiation time. High alloy stainless steels and conventional austenitic steels follow 740

10 separately linear relationship between E pit and t i with nearly equal slopes (Figure 20). AS expected, the pitting initiation time ti is a functions of the potential E pit at which initiation of non passivating pit occurs. CONCLUSIONS (i) (ii) (iii) (iv) (v) (vi) Corrosion rates of conventional austenitic stainless steels and superaustenitic, superferritic and duplex stainless steels in seawater at 50 C are extremely low but the latter have significantly higher corrosion rates than the conventional steels. Corrosion rates determined from long duration salt spray tests are about one order of magnitude lower than those determined from electrochemical polarization techniques. This has been attributed to the time lag for stabilization of protective oxide layer in the two methods. Corrosion rates of stainless steels in seawater are the functions of Cr + Ni content, pitting resistance equivalent, PRE N and induction time, ti. While the presence of Cr and Ni provides a stable protective oxide film and overall strength to the alloy, PRE N is the measure of resistance to pitting and corrosion rate decreases linearly with increase in PRE N values. Pitting potential, E pit is a logarithmic function of Cl - concentration. The conventional austenitic steels show a relatively small shift in pitting potential with variation in Cl - concentration. An increase in Cl - concentration results in a shift to more negative (or active) E pit. Pitting potential, E pit is a linear function of Cr, Cr + Ni, and PRE N of the steels as well as the induction time, ti. Although conventional and superstainless steels behave similarly, two separate linear relationships with different slopes are observed. Both conventional austenitic stainless steels as well as superstainless steels containing > 6 Mo can be used without suspicion of corrosion for seawater applications involving crevice/deposit free systems. REFERENCE 1. D.C.Agarwal, M.R.Jasncr and M.B., Rockel, 6% Mo Austenitic Stainless Steels for Offshore Applications, Proc. 23rd Offshore Technology Conference, Houston, Texas, May 6-9,1991, pp Edwar J. Kubel Jr, Curbing Corrosion in Marine Environments, Advanced Materials and Processes, Nov

11 3. Arthur H. Tuthill, Usage and Performance of Nickel - Containing Stainless Steels in Both Saline and Natural Waters and Brines, Materials performance, 27,47-50 (1988). 4. B.Todd and J. Oldfield, Reverse Osmosis - Which Stainless Steel to Use,Acom. (Avesta). 1-2, l-4 (1991). 742

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