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Materials Review for Seawater Cooled Heat Exchangers
Consultant
DAD is a Metallurgical Engineer, Metallurgist, Corrosion Engineering Consultant, Research Scientist, Component Corrosion Failure Consultant with world-class expertise in corrosion performance of materials, metal fatigue, failure analysis, high temperature corrosion, aqueous corrosion, corrosion protection and control, physical, chemical, mechanical properties of metals and alloys, cyclic deformation behavior, stress corrosion cracking, high strength aluminum alloys, aircraft structural materials, and fatigue crack initiation.
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At the client's request, this consultant reviewed the applicability of utilizing either the Sandvik alloy, SAF 2507, the Allegheny Ludlum alloy, AL6XN (now manufactured by others simply as 6XN) or commercially pure titanium, for seawater cooled heat exchangers. It was the understanding that, for this application, the heat exchanger tubes were to be spirally finned on the outside, where they would come in contact with gases, and that the interior of the heat exchanger tubes would generally be filled with flowing seawater, although the seawater might occasionally be stagnant. Heat exchange across the tube walls would result in water temperatures in the 40 - 50ºC range, and the seawater, saturated with air, would be chlorinated at approximately 0.5 – 1 ppm residual chlorine. The seawater would contain ~2000 ppm entrained sand. The heat exchanger tubes were to be rolled into the tube sheets, and might be seal welded. It was also understood that cupronickel alloys had failed due to erosion corrosion in this application.
Background
Alloy SAF 2507
Alloy 2507 is a "super-duplex" stainless steel with a 40% phase distributed in an austenite matrix, containing ~25% Cr and ~4% Mo. The high levels of chromium and moderate levels of molybdenum render the alloy resistant to either chloride pitting attack or crevice corrosion, with a PRE >40. Because of the distributed ferrite phase, the chloride stress corrosion cracking resistance of the alloy is superior to that of most single phase austenitic alloys. According to Sandvik, the PRE of the ferrite phase is equal to that of the austenite phase. As with all stainless steels, erosion corrosion in seawater is generally not considered to be a factor unless fluid flow rates exceed 50m/s, and microbially induced corrosion (MIC) has not been reported. As noted in the Sandvik literature, when temperatures exceed ~40ºC, micro-organism tend to be either not active, or at least are less active. There are a number of thermofilic bacteria, but there is presently no clear evidence that any strains are associated with MIC. Accordingly, at normal operating conditions for the BOC seawater heat exchangers, MIC of 2507 should not be a problem. Additionally, the BOC system will use chlorination as a biocide. Chlorination is not expected to be a problem, since the critical pitting temperature (CPT) is >90ºC at a potential of 600mV SCE; the potential that is observed for 1.0ppm residual chlorine dissolved in artificial seawater. The 2507 is stated to be readily weldable, using weld metal of the same alloy chemistry as the parent alloy, and based on this consultant's experience with other duplex alloys the claim is correct, with the caveat discussed in the next paragraph.
There are several potentially (and hypothetically) detrimental aspects of the 2507 alloy that should be mentioned, although a thorough search of available literature has not confirmed any of them. These include the observation that duplex stainless steels, in general, are prone to form a third phase, known as sigma phase, when heated in certain temperature ranges for specific periods of time. This phase is basically an intermetallic, near stoichiometric, FeCr phase. The phase is brittle, and can lead to general embrittlement of the alloys. Further, because it is ~50% Cr, it lowers the overall Cr content of the other phases of the alloys and accordingly lowers its resistance to pitting and crevice corrosion. While this consultant is not aware of any published literature related to sigma phase formation in 2507, the metallurgy of the alloy is such that there is no scientific rationale that would suggest that it would be more difficult to form than in alloys such as 2205. In fact, with the higher chromium content it should prove to be more readily formed. It should be stated that the phase is only observed when heat treatments have been inappropriate, or upon welding thick sections, where heat transfer is sluggish. In the BOC application it is not likely to form if the tubes are seal welded, because of the rapid heat transfer that can be expected in the thin walls of the tubes. If solid 2507 tube sheets are utilized, there may be a possibility for the sigma phase to form in the tube sheets. Thus the only cause for concern might be thermal stress relief of the heat exchangers after fabrication. To the knowledge of this consultant, time-temperature transformation data have not been published for 2507.
Another consideration for the use of the 2507 alloy involves the high strength of the alloy, which will require additional force to roll the tubes into the tube sheets (see Sandvik literature). Also to be considered is the availability of the alloy for manufacture of the tube sheets. In our conversation you had suggested that the tube sheets would be clad with the 2507 alloy rather than utilizing solid 2507 tube sheets. If the alloy is metallurgically clad to an underlying alloy, e.g. roll bonded, the possibility of sigma phase formation in the cladding also becomes a possibility.
Finally, although there is currently no record of MIC in the 2507 alloy, it must be admitted that the alloy has experienced only limited use. As has been mentioned, at operating temperatures, with chlorination, MIC should not be a concern. During stagnant periods, however, this consultant assumed that the temperature of the water will approach ambient, and continuous chlorination may not be applied. In two phase stainless steel weldments, admittedly of much lower alloy content, we have, on several occasions, observed selective and extensive MIC of the ferrite phase. In one case involving, a conventional two phase weldment in 304 stainless steel, perforation of 0.75 inches occurred in a one year period. The high chromium and molybdenum contents of the 2507 alloy render this possibility unlikely, but as indicated, there has not been extensive use of the alloy in situations where aggressive microbes are available. It is this consultant's opinion that the possibility of MIC should not be a concern, and offers these observations to be complete.
Alloy 6XN
Alloy 6XN (AL6XN) is a single phase "super-austenitic" alloy, with ~20-22% Cr and 6- 7% Mo. The austenite phase is stabilized by adding more nickel than in the 300 series of alloys and also adding nitrogen to further stabilize the austenite. The high levels of Mo provide a PRN >40, and the additional nickel also provides some resistance to chloride stress corrosion cracking. Erosion corrosion is not considered to be a factor in the use of this alloy although, in comparative tests, alloy 2507 showed somewhat superior resistance to erosion corrosion (~20%). In this consultant's opinion, the improved erosion corrosion that was observed is primarily due to erosion alone, rather than to a corrosion component. For example, alloy 2205, which has approximately the same strength level as 2507, but inferior corrosion resistance showed equivalent erosion corrosion behavior to 2507. Resistance to erosion in the absence of corrosion scales with the strength of a given alloy.
MIC of the 6XN alloy has not been observed and, given the temperature and chlorination conditions for the BOC heat exchangers should not be a consideration. Chlorination of the seawater is not a concern for the 6XN alloy. In experiments performed in this consultant's laboratory on a 6XN alloy in ozonated seawater, only very mild corrosion was observed under severe crevices, and no general pitting was observed. The ozone levels used in our experiments elevate the potential of the steel by ~800mV SCE vs. ~600mV SCE in water chlorinated to 1ppm. Thus the driving force to initiate crevice and pitting corrosion is much more severe in ozonated solutions than in chlorinated solutions.
Stress corrosion cracking (SCC) of 6XN alloy in boiling MgCl2 tests has been reported at stresses above the yield stress, although a specific threshold stress for 6XN in boiling MgCl2 has not been reported. A threshold stress for the 2507 alloy in boiling MgCl2 has been reported to be ~50% of the yield stress. Perhaps more importantly for the BOC application, no SCC failures were observed in a boiling 25% NaCl solution, even when stressed above the yield strength. In the boiling 25% NaCl solution 2205 and Ferralium 255 (a duplex 25% Cr alloy containing 2-4%Mo) either passed this SCC test or failed, depending on heat treatment. The Sandvik literature indicates that the 2507 alloy does not crack in either neutral or acidified chloride solutions, but considering the data for the other duplex alloys, it is unknown if the 2507 alloy will also exhibit susceptibility dependent on microstructural considerations. In any event a boiling chloride solution is considerably more aggressive than the BOC application, and it is this consultant's opinion that neither alloy will suffer SCC in seawater heat exchangers.
The 6XN alloy is readily weldable and, although it is generally recommended that overmatched fillers be used (e.g. alloy 625), we have successfully used matched alloys for highly alloyed stainlesses in paper mill operations (specifically alloy 904L). There is a possibility that a chi phase (Cr2N) can form in heavy section welds and can induce some reduction in ductility. However, formation of the chi phase will not affect corrosion resistance. In the BOC application, however, heavy section welding is not anticipated.
The down side of 6XN vs. 2507 is primarily its lower yield strength (380MPa vs. 550MPa), although specific strength should not be a factor in materials selection for the BOC application.
Titanium Alloys
For most heat exchanger applications, commercially "pure" (unalloyed) titanium is generally used. The terms "pure" and "unalloyed" are misnomers, however, since the strength of the alloys is controlled by the addition of small amounts of iron and oxygen. For grades 1- 4, Fe varies from 0.2 to 0.5%, O from 0.18 to 0.4%, resulting in yield strengths of 170MPa to 480MPa. Ti Grade 7 is also considered to be an "unalloyed" titanium, but contains 0.2% Pd for corrosion resistance in especially aggressive solutions. The alloys are all single phase and are not susceptible to the precipitation of second phases from either heat treatment or welding. The alloys are considered to be weldable but require more rigorous techniques than the stainless steels typically do. There are some instances of hydrogen pick-up and subsequent embrittlement from welding performed in moist atmospheres. However, the problem is easily avoided with standard welding techniques.
There are no reported incidences of MIC, and the unalloyed titanium is entirely resistant to chloride stress corrosion cracking except in molten chloride salts. As mentioned, they can experience embrittlement by hydrogen, but not under general corrosion conditions in seawater solutions (in the absence of severe crevices). They are virtually immune to erosion corrosion and their intrinsic erosion resistance will scale with alloy strength. There is no evidence of a corrosion contribution to solid particle erosion of any of the titanium alloys. A PRE number cannot be assigned, but these alloys are far more resistant to pitting or crevice corrosion than are the stainless steels with a critical pitting potential >5 Volts in chloride solutions. Crevice corrosion has been observed for titanium in hot (>70ºC) concentrated, acid, chloride solutions with particularly aggressive crevice geometries, but is still superior to the crevice corrosion resistance of any of the stainless steels. In the BOC application, if seal welding of the tubes is not performed, there is a possibility for crevice corrosion to occur between the rolled in tubes and the tube sheets. Note that this consideration is also important for the stainless steels.
A potential disadvantage for the titanium alloys, as with alloy 6XN, is their lower strengths, although it does not appear that strength is a primary consideration for the BOC application, except for the possibility of solid particle erosion. A more important disadvantage appears to be the difficulty in attaching the finned structure to the outer surfaces of the titanium tubes.
Discussion
Based on the background presented here, it is apparent that any of the materials under consideration for the seawater heat exchangers (2507, 6XN, Ti) should provide adequate service. This is especially true if the primary mode of failure to be considered is erosion corrosion. Most commercial alloys depend on surface films for protection from corrosion. On copper alloys, the film is generally a form of copper oxide, often containg chlorides and/or sulfates, is generally hydrated, and has poor mechanical properties. Additionally the copper reaction products are relatively slow to form, and eroding particles continuously remove films that are formed, leaving bare metal to be exposed to the corroding solutions. Chlorination increases the corrosion rates by raising the potential on the alloy surface, and thus increasing the thermodynamic driving force for active corrosion. Additionally the soft copper alloys suffer intrinsically from solid particle impingement once the protective corrosion product is removed.
On stainless steels, however, as well as titanium, the protective films are only a few molecules thick, but form a chemical bond with the alloy surface. They are tenacious and strong and, if damaged, will generally reform in neutral solutions in micro-seconds or even in nano-seconds. Thus these alloys, which are considered to be "passive", are highly resistant, or even immune, to the corrosion component of erosion corrosion. Chlorination in this case can even be beneficial by increasing the driving force for repair of the passive films if they are damaged by erosive processes.
To summarize, it is this consultant's opinion that all three alloys under consideration will provide adequate service from strictly a corrosion point of view. All are immune to SCC in seawater environments, all are highly resistant to erosion corrosion, all will provide satisfactory pitting resistance, and all are probably immune to MIC, with the possible (but not probable) exception of the ferrite phase in the 2507 alloy. Crevice corrosion susceptibility can be virtually eliminated by seal welding the tubes to the tube sheets.
Thus, the choice of alloy for BOC's application should be determined based on such considerations as manufacturability, availability, cost and erosion resistance. From those points of view, this consultant considers the 2507 alloy to be an excellent choice. It has outstanding localized corrosion resistance, including pitting and crevice corrosion resistance, is immune to SCC in aerated seawater at the application temperatures, and will provide superior erosion resistance compared to AL6X, or titanium alloys. As has been stated in this report, erosion resistance of corrosion resistant alloys depends on alloy hardness, and 2507 is clearly superior to AL6X in that respect. Alloy 2507 has a Rockwell C hardness as high as 32 (Brinell hardness of ~300, while AL6X and the titanium alloys exhibit Rockwell B hardnesses of only ~70-100 (Brinell hardness of. 125-240).
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