Localized alkaline corrosion of alloy AA5083 in neutral 3.5% NaCl solution

The corrosion behavior of sensitized AA5083 in aerated and deaerated 3.5 wt% NaCl solutions has been studied. The results indicated that an altered surface layer (ASL) with a recrystallized microstructure having no β phase at the grain boundaries formed on sensitized AA5083 during polishing. The ASL prevented the exposure of β phase at the surface and its dissolution at the alloy open circuit potential, even though this potential was above the breakdown potential of bulk β phase. Selective dissolution of β phase was observed when a cathodic reaction caused an increase in localized pH near intermetallic particles or after removal of the ASL by etching to expose the grain boundary β phase. The different corrosion behavior of solutionized and sensitized AA5083 was clarified by testing etched samples. Removal of the ASL is therefore necessary to understand the initiation of intergranular corrosion on sensitized AA5083. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0321506jes] All rights reserved.Manuscript submitted January 22, 2015; revised manuscript received February 16, 2015. Published February 27, 2015 Lightweight materials that provide good formability, high strengthto-weight ratio, weldability and corrosion resistance are desired for use in automotive and marine industrial and military applications. Wrought AA5XXX Al-Mg alloys are well known for these properties, and in recent years these materials have been used to replace heavier materials.1 However, Al-Mg alloys can be susceptible to intergranular corrosion (IGC) attack and intergranular stress corrosion cracking (IGSCC) because of sensitization, which involves the formation of the anodic β phase (Mg 2 Al 3 ) along the grain boundary. 2-4When an Al-Mg alloy containing more than 3 wt% Mg is exposed to room temperature over long periods of time or an elevated temperature (60∼180• C) over a shorter period of time, supersaturated Mg will diffuse to the grain boundary region and form β phase particles.5 Because the breakdown potential (E b ) of β phase (−0.95 V SCE ) 6 is more negative than the open circuit potential (OCP) of the alloy (−0.78 V SCE ), dissolution of β phase along grain boundaries can occur when the alloy is exposed in a corrosive environment. This phenomenon leads to IGC and also IGSCC when a tensile stress is present.1,7-10 Goswami et al. showed that the hardness of AA5083 gradually decreased with aging time, 10 which indicates removal of the strengthening element Mg from solid solution. Searles et al. found that the ductility of sensitized AA5083 during constant extension rate testing increased when a potential below the pitting potential of β phase was applied to the sample compared to testing at the OCP. 1 Jones et al. showed that the crack growth rates of AA5083 were ...

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“…14,17 No clear explanation for the absence of IGC and β phase attack on sensitized AA5083 at its OCP has been provided in the literature.…”

Section: -4mentioning

confidence: 99%

“…7,[14][15][16][17] Many papers assume the dissolution of β phase will occur because its breakdown potential, E b , is lower than the OCP of the alloy.…”

mentioning

confidence: 99%

Influence of the Altered Surface Layer on the Corrosion of AA5083

Seong

Yang

Scheltens

et al. 2015

J. Electrochem. Soc.

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“…When cathodic intermetallic particles enhance the dissolution of the surrounding matrix, it is called alkaline pits. This type of pitting occurs below the pitting potential, while crystallographic pits are formed when an alloy reaches its pitting potential [23].…”

Section: Introductionmentioning

confidence: 99%

The Red Sea as a Corrosive Environment: Corrosion Rates and Corrosion Mechanism of Aluminum Alloys 7075, 2024, and 6061

Al-Moubaraki

Al-Rushud

2018

International Journal of Corrosion

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Corrosion behavior of Al 7075, Al 2024, and Al 6061 in the Red Sea water was studied using weight loss (WL) measurements and potentiodynamic polarization (PDP) technique. The corrosion patterns and corrosion products formed on Al alloys were characterized using optical photography (OP), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The results showed that WL data were consistent with bimodal model rather than the power law function and the corrosion rates exhibit a continuous decrease with exposure time. The increasing order of the Red Sea corrosivity on the studied Al alloys can be given as follows: Al 6061 < Al 2024 < Al 7075. The results of temperature effect revealed that an increase in temperature resulted in an increase in both anodic and cathodic current density and a decrease in corrosion potential. Al 7075 was less influenced by temperature than the other alloys. Pitting corrosion was the predominant corrosion pattern detected on all Al alloy surfaces after prolonged immersion in the Red Sea water. The appearance of S peak in EDS spectra of Al 7075 after corrosion gives an indication of the contribution of bacteria in the corrosion process.

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“…After 54 days of prolonged exposure in 3.5% NaCl solution, it can be observed that the sample has been descaled as a consequence of fracturing of the surface film. The surface oxide film was observed to develop at an early period and increased in area and in thickness with the passage of time [5]. The development of this oxide film did not prevent the formation of cavities and large number of hemispherical micropits during prolonged exposure; these extends over a high proportion of the surface area.…”

Section: Sem and Edx Observationmentioning

confidence: 93%

“…The adsorption of aggressive ions such as Cl − into the faults in the protective film, and their penetration and accumulation in these imperfections is considered one of the triggering factors of the process of nucleation of pitting [3] [4]. Other studies have suggested that pits may develop as a result of a process of hydrolysis which gives rise to a local reduction of the pH which, in turn, impedes the subsequent process of repassivation [5]. In addition, another factor which is associated with the susceptibility of aluminium to pitting corrosion and other forms of localized corrosion is the electrochemical nature of the intermetallic phases [6].…”

Section: Introductionmentioning

confidence: 99%

Corrosion Behaviour of Aluminium Engine Block in 3.5% NaCl Solution

Kaiser¹,

Dutta²

2014

MSCE

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In the present work, the corrosion behavior of aluminium alloy engine block in 3.5% NaCl solution was studied. The work was carried out using conventional gravimetric measurements and complemented by scanning electron microscopy (SEM) and X-ray analyzer (EDX) investigations. The results obtained indicate that the main process the alloy undergoes, under the medium of exposure studied, is related to localized corrosion that takes place as a consequence of the process of alkalinization around the cathodic precipitates existing in the alloy. The alloy suffers a process of corrosion localized to the area surrounding the precipitates of the Al (Si, Mg) and Al-Mg, which resulted in hemispherical pits. This identification was confirmed by SEM and EDX analysis. No evidence was found of the formation of crystallographic pitting for exposure times up to 54 days. Gravimetric analysis confirmed that with varying exposure periods the weight loss of the alloy increases and the corrosion rate decreases with time.

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Localized alkaline corrosion of alloy AA5083 in neutral 3.5% NaCl solution

Cited by 156 publications

References 16 publications

Influence of the Altered Surface Layer on the Corrosion of AA5083

Influence of the Altered Surface Layer on the Corrosion of AA5083

The Red Sea as a Corrosive Environment: Corrosion Rates and Corrosion Mechanism of Aluminum Alloys 7075, 2024, and 6061

Corrosion Behaviour of Aluminium Engine Block in 3.5% NaCl Solution

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