Materials.Business Newsletter ⚙️ Oct 17th, 2022
Podcast Guests: Mariano Kappes PhD
Localized corrosion of UNS A95052 aluminum alloy for application in multi-effect desalinator plants Dannisa R. Chalfoun,*,** , Mariano A. Kappes,*, **,***, ‡ Mauricio Chocrón,*** and Raul B. Rebak**** ARTICLE INFO Article history: Received Day Month Year (This style is Article History and Keywords) Accepted Day Month Year Available Day Month Year Keywords: aluminum alloys, crevice corrosion, pitting corrosion, repassivation potential. * Instituto Sabato, UNSAM/CNEA, Av. Gral. Paz 1499, San Martín, Buenos Aires, B1650KNA, Argentina ** National Scientific and Technical Research Council, Godoy Cruz 2290, Autonomous City of Buenos Aires, C1425FQB, Argentina *** National Commission of Atomic Energy of Argentina, Av. Gral. Paz 1499, San Martín, Buenos Aires, B1650KNA, Argentina. **** GE Global Research, 1 Research Circle, Niskayuna, NY 12309, USA ‡Corresponding author: +54-11-6772-7353. Email: firstname.lastname@example.org ABSTRACT Aluminum alloy UNS A95052 (AA 5052) results very attractive for desalination applications due to its good corrosion resistance in seawater at temperatures up to 125ºC, low cost, good thermal conductivity, and non-toxicity of its corrosion products. The pitting corrosion potential, Epit, and the pit repassivation potential, Er, pit, of AA 5052 were measured in deaerated 65,000 ppm sodium chloride (NaCl) solutions at 30, 60 and 85ºC. Epit decreased with temperature, in accord with literature results. Er, pit was a function of anodic charge passed during pit growth stage. A complete evaluation of suitability of this alloy from a corrosion perspective requires also studies of crevice corrosion at different temperatures, considering that multi-plate designs of desalinators have metal plates in contact with rubber gaskets and seals. Cyclic potentiodynamic polarization were used to estimate crevice repassivation potentials, Er,crev, at 30, 60 and 85ºC, in specimens with an attached rubber O-ring as a crevice former. This crevice former simulated the partially occluded geometry expected in desalination plants. Stable crevice corrosion potentials, Ecrev, were similar to Epit, and, when polarized to a similar anodic charge density, Er,crev were similar to Er, pit. Based on this result, from a corrosion perspective, the presence of crevices in the desalination plant is not expected to present an additional risk during operation of the plant. Electrochemical tests were also performed in saturated AlCl3 solutions to explain the results using Galvele’s localized acidification model.
Seawater desalination using waste heat from nuclear or fossil power plants is an attractive alternative for the production of potable water.1 Multi-effect desalination process (MED) relies on the evaporation of water from a thin film of seawater.2 The heat released during condensation of distilled water on the metallic surface is transported through the thickness of the plate and used for the evaporation of water from a subsequent film of seawater, thus allowing an efficient production of distillate. Materials for use in such plants must have an adequate corrosion resistance in seawater at 70°C or higher, 1,2 depending on the design and type of plant. The formation of a continuous film of seawater and condensed water is critical to minimize precipitation of deposits and hot spots. Aluminum alloys (AA), in particular UNS A95052(1) (referred hereafter as AA 5052), possess several advantages for this application, including a good thermal conductivity, 3,4 high seawater corrosion resistance4 , low cost when compared to copper or titanium alloys, 5 non-toxicity of the alloy or its corrosion products, 6 and good wettability in seawater at high temperature. 3 Therefore, AA are often the chosen materials for MED applications. 1–3,6–10 Among wrought AA alloys, non-heat-treatable 5xxx series have the highest resistance to seawater corrosion, due in part to the absence of copper as an alloying element. 4,11. Magnesium (Mg) is the main alloying element in this series, and its equilibrium solubility at room temperature in the aluminum matrix is below 1wt% 12. In alloys with higher content of magnesium phase (Mg2Al3) precipitation is thermodynamically possible. However, a supersaturation of Mg is required due to kinetics constraints, and a threshold content of 3wt% of Mg is often quoted for phase precipitation 13–15. The threshold value might depend on details of the microstructure including amount of cold work, grain size and concentration of other alloying elements 16 . phase is anodic with respect to the matrix, 13,17,18 and its grain boundary precipitation can lead to intergranular corrosion (IGC) and stress corrosion cracking (SCC) problems 13,14,16,19–21. Those problems are typically observed in alloys with magnesium content above about 4 wt.%. Below this threshold value in Mg concentration, specimens subjected to a sensitizing heat treatment (150°C for seven days) exhibited a negligible weight loss in the concentrated nitric acid immersion test21 . Hence, for AA 5052 with a Mg content around 2.5 wt.%, absence of or a scarce amount of phase in AA5052 is expected, which results in its high resistance to intergranular corrosion and intergranular SCC 12 . Specimens of AA 5052 exposed to 70°C for 30 months exhibited an extremely low corrosion rate of 4 mg/cm2 in concentrated nitric acid 22 , which is more than one order of magnitude lower that for alloy 5083 (UNS A95083, with 4.85 wt.% Mg). However, some unusual cases of intergranular attack of alloy 5052 were reported 4 , and they are probably related to presence of cold work. In particular, in desalination service, IGC was observed in the area of tubes rolled into the tube sheet, at a service temperature above 104°C 23 . While resistance to IGC of alloy 5052 is expected to be high under most conditions, the presence of chlorides (Cl- ) in feed seawater can lead to pitting corrosion of the AA 5052 alloy. 6,9,24,25 A common measure of resistance to pitting corrosion is the pitting potential, Epit, 26–29 defined as the potential above which corrosion pits are stable. 26 Epit of aluminum alloys in sodium chloride solutions decreases as the (1)UNS numbers are listed in Metals and Alloys in the Unified Numbering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International. temperature 6,30,31 and chloride concentration increases. 28,32 During the operation of the desalinator, the material can experience exposure to environments with chloride concentrations above that of seawater (~20,000 ppm Cl- ) at temperatures of 70°C or higher, therefore promoting pit growth. Stable corrosion pits can continue to grow even if polarized below Epit, 29,32 and eventually they can repassivate if polarized below the repassivation potential or protection potential, Er,pit. 26,33 For potential values between the Er,pit and Epit,, metastable pits can nucleate and stable pits can grow, therefore, a conservative criteria for preventing pitting damage is that the steady state corrosion potential, Ecorr, fulfills Ecorr 100 mV. This difference guarantees that in case of pit initiation due to possible excursions of Ecorr (for example, due to ingress of oxygen), any corrosion pit nucleated during the oxygen ingress transient will repassivate once the system returns to normal steady state conditions. In other words, based on this data, pitting corrosion should not be a problem during steady state deaerated operation of the desalination plant. On the other hand, in presence of heavy metal cations like Cu++ 24,28 Ecorr could increase and stabilize at Epit value of aluminum, 28,35 and this caused pitting problems in service in some aluminum desalination plants. 6,36 It is recommended11,36,37 to avoid the use of copper alloys upstream the aluminum desalinator to prevent pitting corrosion and/or galvanic corrosion. While pitting corrosion can be controlled by removing oxygen from the seawater feed and by minimizing the presence of heavy ions that shifts Ecorr to nobler values, the possibility of crevice corrosion has to be evaluated19. Desalination plants, and in particular those of the multieffect type, usually have polymeric seals or gaskets in contact with the aluminum alloy plates or tubes, 2 which could act as initiation sites for crevice corrosion. Crevice corrosion of AA 5052 was studied in seawater with artificial crevice formers at room temperature.35 For a 3.0 wt% NaCl solution, the crevice repassivation potential, Er,crev, was -0.960 VSCE, 35 which is below Ecorr of aluminum in synthetic seawater at room temperature. No studies of the dependence of Er,crev of AA 5052 with temperature were found in the literature; however, it is likely that the Er,crev should decrease with increasing temperature, in a similar fashion as Er,pit and Er,crev of UNS A9110034 or Er,pit of AA 5052. 6 Crevice corrosion was observed during potentiostatic laboratory tests, where UNS A95082 was polarized to a potential slightly above Er,crev measured in 3.0 wt% NaCl. 35 In laboratory tests, the potentiostat can supply all the current necessary for the crevice to grow. On the other hand, the growth of crevices under open circuit conditions in seawater are limited by the rate at which oxygen38 and water can reduce on the free surface, and therefore can control the dissolution of Al. Despite predictions of laboratory tests reported by Furuya and Soga35 , some authors have stated that the crevice corrosion of aluminum alloys in neutral chloride solutions at room temperature is “not a concern for industrial applications”, 39 is “not as destructive and common as crevice corrosion of steels and titanium”40 and is an effect that weakens with full immersion in seawater, because pitting in the boldly exposed surface becomes comparable to the attack in the crevice. 41 In this regard, field exposures of UNS A91050A, UNS A95083 and UNS A96082 aluminum alloys with artificial crevice formers to aerated seawater did not show evidence of crevice corrosion. 42 Similar results were obtained for Al-Cr, Al-Mg and Al-Mg-Cr alloys. 43 However, all those results addressed corrosion resistance in chlorides solutions at room temperature, while the possibility of crevice corrosion occurrence at higher temperatures was not studied. The crevice corrosion of alloy AA 5052 has not been studied at temperatures higher than room temperature. Those studies are critical for multi-plate multi-effect desalinator (MP-MED) plants based on multi-plate geometry, where elastomeric seals required for preventing leaks and oxygen ingress from the environment create artificial occluded regions or crevices in contact with the metal surface. Furthermore, according to a recent study30 the structure of passive films on pure aluminum changes at a temperature of 40°C and above, and the effect this oxide film change might have on crevice corrosion resistance of AA was not studied until now. Therefore, the main objective of this work is to measure the Ecrev and Er,crev of AA5052 at temperatures and chloride concentration relevant to desalination service , in order to predict resistance of the alloy to crevice corrosion.
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