The aluminum chemistry and corrosion in alkaline solutions

doi:10.1016/j.jnucmat.2008.11.009

Journal of Nuclear Materials

Volume 384, Issue 2, 15 February 2009, Pages 175-189

https://doi.org/10.1016/j.jnucmat.2008.11.009 Get rights and content

Abstract

Aluminum–alkaline solution systems are very common in engineering applications including nuclear engineering. Consequently, a thorough knowledge of the chemistry of aluminum and susceptibility to corrosion in alkaline solutions is reviewed. The aluminum corrosion mechanism and corrosion rate are examined based on current experimental data. A review of the phase transitions with aging time and change of environment is also performed. Particular attention is given to effect of organic and inorganic ions. As an example, the effect of boron is examined in detail because of the application in nuclear reactor power systems. Methods on how to reduce the corrosion rate of aluminum in alkaline solutions are also highlighted.

Introduction

Aluminum and its alloys are widely used in many engineering applications and scientific technologies, such as in aerospace, advanced nuclear reactor, surface coating, metal/air batteries, etc. For example, aluminum–alkaline solution systems are often utilized in the development of metal/air batteries in which the aluminum is used as the anode. These batteries may be used as power sources for electric vehicle propulsion. The battery performance is determined by the electrochemical and corrosion properties of aluminum anodes [1]. Consequently, the aluminum behavior has an import impact on the battery properties. Another common example of aluminum–alkaline solution systems can be found in nuclear water reactors during a loss of coolant accident (LOCA). The chemical environment generated by the injection of coolant into the emergency-core-cooling-system has a pH around 10 [2]. The release of aluminum into solution via corrosion may result in precipitate, which may lead to a system failure.

It is well known that aluminum oxide scale generally can provide better oxidation resistance and yield a lower oxidation rate than other protective oxide layers, for example chrome oxide scale. The aluminum oxide scale is compact and thermodynamically stable in neutral environments, and it also has a good adherence to the substrate. Therefore, the aluminum oxide scale appears to be an ideal protective scale. However, it has been reported that the scale can be dissolved and exists in the solution as the following species: Al⁺³, Al(OH)⁺², $Al (OH)_{2}^{+}$ and $Al (OH)_{4}^{-}$ , in an acidic or alkaline solution.

Because of the extensive applications, the behavior of aluminum and its alloys in various systems have been extensively studied. Critical review articles on the aluminum–water system have been performed by Alwitt [3] and on localized corrosion by Foley [4]. In the present study, we focus on the aluminum–alkaline systems. Available experimental data and theoretical analyses are organized and reviewed. The present studies lead to rethinking of the available data by theoretical and experimental studies.

The rests of the article are organized as following:

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Section 2 presents the aluminum hydroxide phases and the factors that affect the phase transition.
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Section 3 addresses the aluminum solubility in alkaline solution. Methods on how to predict the solubility are presented in this section.
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Section 4 presents the boron effects on aluminum behaviors in alkaline solution including aluminum–boron complex formation and the boron adsorption on aluminum hydroxide.
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Section 5 reviews the particle size distribution measurements of aluminum–alkaline system.
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Section 6 addresses the aluminum corrosion in alkaline solution, including the corrosion mechanism, corrosion rate data, corrosion inhibition and film kinetics.
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Conclusions are summarized in Section 7.

Section snippets

Hydroxide phases

In an aluminum–alkaline solution system, a determination of the dissociation of the aluminum hydroxide solid phase is needed in order to determine the solubility of aluminum in the solution. It has been reported that the dissolution process is a function of the aluminum hydroxide solid phase. The aluminum hydroxide may exist in an amorphous form or as one of three crystalline forms known as gibbsite, bayerite, or nordstrandite. The crystalline polymorphs differ only in the packing arrangement

Prediction of the aluminum solubility

The aluminum solubility in alkaline solution is determined by dissolution reaction of the aluminum hydroxide, whose equilibrium is a function of aluminum hydroxide phase: ${Al(OH)}_{3} = {Al}^{3 +} + 3 {OH}^{-},$ with the thermodynamic solubility product ( $K_{sp}$ ) as $K_{sp} = [{Al}^{3 +}] [{OH}^{-}]^{3},$ where [ ] represents the species concentration. So the concentration of aluminum can be obtained: $[{Al}^{3 +}] = K_{sp} [H^{+}]^{3} / K_{w},$ where $K_{w}$ is the equilibrium constant. In alkaline or acidic solution, aluminum can exist as the following forms: Al³⁺, $Al (OH)_{4}^{-}$ ,

Boron behavior in the solution

In a pressurized-water-reactor (PWR), boric acid is present in the reactor core to balance the pH of the primary coolant. During a LOCA, the coolant with a pH of approximately 10 is injected into the emergency-core-cooling-system (ECCS). The interactions between the boron and the aluminum corrosion products can result in precipitates that affect the efficiency of ECCS. Therefore, the boron–aluminum interactions are of paramount importance during post-LOCA in a PWR containment environment.

Particle size distribution

The aluminum behaviors such as the solubility in alkaline solution are functions of the particle size distribution that is affected by the aluminum hydroxide phase and the organic elements.

Previous investigations into the particle size distribution of aluminum have been made using dynamic light scatting (DLS), Small-angle X-ray scattering (SAXS), and acoustic techniques [6], [54], [55]. A typical particle size distribution at pH 11 for different aging time is shown in Fig. 9 [56]. Clearly, the

Corrosion mechanism

It is well known that aluminum metal is very inert in neutralized solution, while pure aluminum is too reactive to be used in concentrated alkaline solutions [57]. Experimental results show that there are two competing processes at the aluminum metal surface: direct dissolution of the aluminum metal and electrochemical formation/dissolution of the aluminum hydroxide films. The first process is very intense, which leads to a high corrosion rate. With elapsed time a film forms on the metal

Summaries and conclusions

The current status of aluminum behavior in alkaline solutions, including the hydroxide equilibrium solid phases and the phase transition, prediction of aluminum solubility, organic and inorganic effects, aluminum corrosion and corrosion inhibition has been reviewed. This review should serve to aid the community engaged in the application of aluminum in engineering applications in the general knowledge of aluminum chemistry and corrosion and remind us to rethink some previous conclusions.

The

Disclaimer

This paper was prepared as an account of work sponsored by the US Nuclear Regulatory Commission, an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party’s use, or the results of such use, of any information, apparatus, product, or process disclosed in this report, or represents that its use by such third party

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ReviewThe aluminum chemistry and corrosion in alkaline solutions

Abstract

Introduction

Section snippets

Hydroxide phases

Prediction of the aluminum solubility

Boron behavior in the solution

Particle size distribution

Corrosion mechanism

Summaries and conclusions

Disclaimer

J. Cryst. Growth

Geoderma

J. Colloid Interf. Sci.

J. Colloid Interf. Sci.

Prog. Cryst. Growth Char. Mater.

J. Colloid Interf. Sci.

J. Pharm. Sci.

J. Pharm. Sci.

J. Cryst. Growth

J. Mol. Spectrosc.

J. Pharm. Sci.

J. Colloid Interf. Sci.

Polyhedron

Mar. Chem.

Geochim. Cosmochim. Acta

Solid State Sci.

J. Catal.

Coordin. Chem. Rev.

Electrochem. Acta.

Electrochim Acta

Corrosion Sci.

Corros. Sci.

Corros. Sci.

Electrochim. Acta

Surface Technology

Electrochimica Acta

Thin Solid Films

J. Solid State Electrochem.

Corrosion

Clays Clay Miner.

Clays Clay Miner.

Kolloid-Zeitschr. Zeitschr. Polym.

J. Appl. Chem. USSR

Crystallisation

Review
The aluminum chemistry and corrosion in alkaline solutions