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Long-Term Pitting Corrosion of 6060 Aluminium Alloy Immersed in Natural Seawater

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Long-Term Pitting Corrosion of 6060 Aluminium Alloy Immersed in Natural Seawater LONG-TERM PITTING CORROSION OF 6060 ALUMINIUM ALLOY IMMERSED IN NATURAL SEAWATER M. X. Liang, I. A. Chaves, R. E. Melchers Centre for Infrastructure Performance and Reliability, The University of Newcastle, NSW2308, Australia SUMMARY: Aluminium alloys are widely used in maritime industries because of their high strength to weight ratio, ease of fabrication and expected corrosion resistance. However, they are susceptible to localised corrosion under specific corrosive environment. Further, information on the long-term corrosion characteristics of aluminium alloys under natural seawater immersed condition is scarce. Hence, this study reports a field investigation on pitting corrosion data of 6060 aluminium alloy immersed for two years in natural seawater with average annual temperature of 20oC. An Optical Microscope was used to examine pit morphology and to measure pit depths. Cross-section microstructure and chemical composition of pits were investigated by means of Scanning Electron Microscopy and Energy Dispersive Spectrometry. Five deepest pits were measured on each face of a sextuplicate set of coupons. The pit depth data was analysed using extreme value statistics. Results show that the depth of the deepest pits progressed in a ‘step-wise’ manner. Pitting severity and the maximum pit depth increased with the depth of immersion. The results support previous findings indicating changes in corrosion mechanism with time. Similar to the corrosion of steels, this is considered to result from the build-up of corrosion products. The reason for this is discussed and further work is outlined. Keywords: Aluminium alloys; pitting corrosion; seawater immersion; microstructure. 1. INTRODUCTION Aluminium alloys are widely used in various fields such as marine infrastructures and aerospace due to their high strength to weight ratio and good corrosion resistance (Perryman 2007, Srinivasa Rao 2004). In recent years, the application of aluminium alloys in structures is increasing, and they became the second prevalent metal alloys used in industry (Vargel 2004). In general, aluminium alloys have both good general corrosion and pitting corrosion resistance in atmospheric environments (de la Fuente et al. 2007). However, they tend to suffer localised corrosion when exposed to the aggressive aqueous environment, such as seawater immersion, for an extended length of time. Pitting corrosion is the most common form of corrosion for aluminium and its alloys (Vargel 2004). It is a long- term hazard for the integrity, safety and serviceability of both new and existing infrastructures (Chaves and Melchers 2014). Pitting corrosion may cause perforation thus pose a potential threat of fracture and other damage modes (Melchers 2015). Up to now, a number of studies have reported on the corrosion of aluminium alloys, such as laboratory-based experiments with simulated seawater as exposure medium (Szklarska- Smialowska 1986) and mathematical based probabilistic modelling for the growth of corrosion pits (Harlow and Wei 1998). Nevertheless, most report the corrosion behaviour of aluminium within a “short-term” (seconds and hours) or “middle-term” (days and weeks) exposure period. This may be somewhat irrelevant to “long-term” (years and decades) practical design considerations. Only limited empirical investigations have considered long- term corrosion loss measurements, for instance, mass loss and pit depth (Southwell et al. 1964, Ailor 1974, Bopinder et al. 1997). It follows that long-term corrosion progressions with time and corresponding mechanisms Corrosion & Prevention 2016 Paper 70 - Page 1 remain unclear. Moreover, when considering the reliability based design of aluminium structures, it is of great importance to have the ability to reliably predict the long-term development of pitting corrosion behaviour (Chaves and Melchers 2014). One way to predict long-term maximum pitting depth with time is to use the most widely-applied power-law function (Szklarska. Smialowska 1986, Vargel 2004): c(t) =At (1) where c is pit depth, t is exposure time and A and B are constants obtained from fitting the function to data. The values of A and B vary and depend on the nature of alloy, temperature and water velocity, etc. (Vargel 2004). Recent studies on corrosion of steels and aluminium alloys have found that the power-law equation is not the best fit for data from long-term exposures in the marine environment (Melchers 2006, Melchers 2010). Instead, a bi-modal model or multi-phase model has been proposed to give a better description for long-term corrosion data (Fig.1) (Melchers 2014b). Previous research has shown that the data for long-term corrosion of mild steels, irrespective of seawater immersion, tidal or marine atmospheric environment, consistently comply with a bi- model trend (Melchers 2014b, Melchers 2008). Most recently, the long-term corrosion data for aluminium pit depth and mass loss also have been shown to follow bi-model trends (Melchers 2014a, Melchers 2015). Figure 1 Schematic bi-modal model for long-term corrosion loss and pit depth in marine environments (Melchers 2014a) The theory underpinning the model is as follows: as pitting increases, the build-up corrosion products increase. The topographically non-uniformity of corrosion products facilitates the formation of anoxic conditions in the pits, which, as a result, promote the transformation of corrosion mechanism from corrosion rate controlled by oxygen reduction (mode 1 in Fig.1) to hydrogen reduction dominated (mode 2 in Fig.1) (Melchers 2014b). Due to the lack of suitable long-term field data and insufficient understanding of the long-term corrosion progressions, especially associated with the point t in Fig.1, this paper reports data for long-term pitting corrosion of 6060 aluminium exposed in natural seawater for two years. Pits morphology and microstructure are discussed and the maximum depths of pits are analysed by applying extreme value statistics. 2. EXPERIMENTAL DETAILS AND PROCEDURES 2.1 Test site The natural seawater test site for the experimental work reported herein is at the NSW Fisheries Research Station at Taylors Beach, Port Stephens, NSW, Australia (32.33S, 151.03E). The parameters measured at this site have been reported before (Jeffrey and Melchers 2002). Located 17 km from the open sea the inlet continues a further 2 km inland and disperses onto a region of mangrove covered mud flats. The seawater characteristics at the test site are summarised in Table 1. Table 1. Exposure and nutrient conditions at Taylors Beach test site. Data from Jeffery and Melchers (2002). Corrosion & Prevention 2016 Paper 70 - Page 2 acteristic Value Minimum water depth, m 1.2 Maximum tidal movement, m 2.0 Water temperature, ℃ 10.4-30.2 Exposure Water velocity (peak), m/s 0.06 Dissolved oxygen, % saturation 95-100 PH 8.1-8.2 Salinity, % 2.01-3.48 Nitrates, mg/L as N 0.02 Nitrites, mg/L as N <0.01 Ammonia, mg/L as N 0.03 Nutrient Sulphate, mg/L 1700 Ortho phosphorous, Mg/L as P <0.005 Total phosphorous, mg/L as P 0.022 Calcium, mg/L 419 It is important to note that the temperature of the water varies during the day and in different seasons. Nonetheless, since all the test specimen strips are located very close to one another and under the same condition, there is a very low possibility that the difference of corrosion behaviour between specimens is a result of temperature changes. Thus only annual average temperature (around 20℃) is considered in this report. Also, the water velocity at Taylors Beach is slow. It was reported that the peak velocity at the surface was around 0.05m/s and it would reach about 0.06m/s as the water depth increases by 0.5 ~ 1.0 meter (Jeffrey and Melchers 2002). A previous study has shown that at low water velocity (<1.0 m/s) the water flow effects corrosion only for a short period after first exposure (typically 30 days) (Melchers and Jeffrey 2004). Moreover, it has little direct further influence on the long-term corrosion rate of metal since the corrosion products and marine growth form on the surface of the metal and provide protection against the impingement of water. Hence in this report, it is assumed that the water velocity has no effects on the pitting corrosion of aluminium alloys. 2.2 Test materials The test materials were commercial Al 6060 extrusions at T5-temper state. The chemical composition is summarised in Table 2. A total of six specimen strips (750mm × 40mm × 6mm) were exposed parallel to the water surface. Four strips (strip A, strip B, strip C and strip D) were immersed 640mm under the low tide water level and two other strips (strip E and strip F) were immersed 160mm deep below the low tide water level. These are nominal immersion depths. Some strips were parallel to each other and others were perpendicular to these, as shown in Fig. 2. This was done to assess the effect of orientation on corrosion losses. Table 2. Chemical composition of testing materials (weight %) Si Fe Cu Mn Mg Pb Ni Zn Ti Sn Cr Al Corrosion & Prevention 2016 Paper 70 - Page 3 0.44 0.18 0.01 0.06 0.4 <0.01 <0.01 <0.01 0.01 <0.01 <0.01 Balance All aluminium strips were retrieved after two years’ exposure. Following standardised procedure (ASTM 2011a), the strips were cleaned by mechanically scraping loose debris off the surface, cleaning using nylon cloths under flowing tap water followed by ultrasonic cleaning and then air drying. Given the soft nature of aluminium, care was taken to avoid damage to the surface during all stages of cleaning. Once cleaned each strip was cut into six 60mm × 40mm × 6mm coupons, which were marked with indentations for identification. Figure 2 Schematic view of the arrangement of test specimens. 2.3 Pit depth measurement After an initial visual inspection of pit morphology, the coupons were observed under an upright white light optical microscope (ZEISS AXIO Imager A1).
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