CORROSION OF ALUMINIUM
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In one mode of attack whereis the intermetallic is classified as a cathode, the surrounding matrix tends to corrode leaving a ring shape around the particle or also called trenching. There is still some uncertainty on whether or not the trench itself is a result of microgalvanic coupling alone, or if the major contributor is local pH elevation, however a good treatise of this topic was given in a multi-part series of papers by the group at Virginia Ilevbare, Schneider et al. In some cases damage may propagate to the base of the particle and eventually lead to particle fall out Buchheit, Grant et al.
The other mode of attack is when the intermetallic acts as anode and matrix as cathode; whereby the intermetallic will corrode leaving a cavity on the surface. Finally, in some microstructurally complex systems with ternary and above alloying additions, another type of attack found to not follow the traditional way of determining anode and cathode is incongruent dissolution commonly found in 2xxx and 7xxx due to the presence of S phase Al 2 CuMg Buchheit, Grant et al. This intermetallic contains Cu and Mg, whereby Blanc, Lavelle et al. Although the more detailed study of Boag et al.
How to Protect Aluminum From Corrosion - Monroe
This may have been due to a combination of the types of aluminium hydroxyl-chloro complexes that form and the partial switching of the areas of S-phase to Cu remnants where cathodic reactions lead to the formation of insoluble Mg OH 2 Boag, Hughes et al. After 15 minutes the Mg is removed as well and no chloride was detected on these particles. The attack then continues with the dissolution of Al matrix Buchheit, Grant et al.
Localized attack of the intermetallic also influenced by the chloride content and pH of the electrolyte. Higher chloride content is widely reported to have higher pitting occurrence due to passive layer disruption of chloride ions Seri ; Blanc, Lavelle et al. There exists a dedicated monograph on this topic Muster, Hughes et al.
Protecting Aluminum From Corrosion
It has also been noted that the intermetallic Mg 2 Si can undergo similar incongruent dissolution, whereby Si enrichment occurs at the expense of dissolving Mg Birbilis and Buchheit ; Jain, ; Eckermann, Suter et al. The revelation of a large number of microstructure vs. Instead, a demonstration is given here. Figure 6 shows the micron scale microstructure for T3 and H before and after corrosion exposure in 0.
These relatively low magnification images do not reveal the precipitate structure in AAT3, instead showing the coarse intermetallics that exist in the alloys.
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What is observed is that a higher intermetallic density gives more possible sites for localised attack. In contrast to AA, the alloy H with a smaller number of alloying elements, and Cu free, has a lower constituent number density.
These are comparative images to show the extent of damage following immersion. This section covers general approaches to protection of aluminium alloys in view of recent advances in the understanding of alloy microstructure. It includes an overview of pretreatment processes such as anodising, conversion coating and organic coatings barrier and inhibitor combinations.
It will examine recent advances in inhibitor design such as building in multifunctionality and touch upon self-healing coating systems. Approaches using multifunctionality can target anodic and cathodic reactions more effectively than using individual monofunctional inhibitors. Standard metal finishing processes, which have been used for many years, are likely to continue to be used into the future unless they contain chemicals that are targeted for replacement such as chromium. The function of these coatings is primarily to provide better adhesion properties for paint coatings and a secondary role is to provide corrosion protection.
The general approach for applying these coatings relies on metal finishing treatments treatment prior to painting involving immersion in acidic and alkaline baths with the objective of reducing the heterogeneous nature of the metal surface such as removing the NSDL and second phase particles Muster This is achieved in multistep treatment processes for metal protection Twite and Bierwagen ; Buchheit ; Muster as for instance:. On aluminium, most anodised coating processes produce an outer oxide with a cellular structure on top of a thin barrier layer that provides some protection against corrosion.
Inhibitors can be incorporated into the outer porous layer of the anodized layer during formation or as a seal after formation to offer some extra protection upon damage. Chromic acid anodizing is one of a number of processes that are available for electrochemical growth of surface protective oxides. More environmentally friendly alternatives to chromic acid anodizing such as sulfuric, sulfuric-boric, sulphuric-tataric and phosphoric based processes have been available for a long time.
There have been a number of recent advances in reducing the energy consumption of anodizing processes as well as improving coating properties.
Aluminium Corrosion Removal
These advances are based on an improved understanding of the alloy microstructure described above and involve selective removal of second phase particles as part of the anodising process. An alternative approach to anodizing is to precipitate a coating on a surface through chemical means called conversion coatings. For high strength Al-alloys such as 2xxx and 7xxx series chromate conversion coating CrCC is still the preferred process.
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These processes are widely developed for chemically pretreated surfaces that have nearly all the IM particles removed by chemical pretreatment and are not specifically designed to address electrochemical and compositional variations found for a heterogeneous surface such as when the IM phases are present.
Work like that in Buchheit and Birbilis depicting the reaction rate variation across the surface, however, opens an avenue to start designing inhibitors where the initial reaction rate distribution across a surface can be significantly reduced to limit the overall activity of the surface. In this context reaction of inhibitive phases with manufactured IM compounds as well as IM particles within the alloy have been studied for a number of systems Juffs, Hughes et al. Once the anodised or conversion coating is applied, the surface is ready to receive the organic coating.
There are many different types of organic coatings, however because of the focus on 2xxx and 7xxx alloy used in the aerospace industry this section will only deal with that application area.
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The organic coating system usually consists of a primer and a topcoat. The primer is the main protective layer including corrosion inhibitors that can be released when corrosive species or water reach the metal. From the perspective of providing protection for the underlying aluminium alloy, the inhibitor needs to be available during a corrosion event at a concentration higher than the minimum concentration at which the inhibitor stops corrosion critical concentration.
While this sounds obvious, the critical inhibitor concentration needs to be maintained over many years for structures such as airframes, where maintenance may not be possible in parts of the aircraft because of poor access. The chromate systems itself provide continuous protection and repair to the surface for as long as the dose of chromate remains above the critical concentration.
This mechanism of inhibitor release and metal protection is recognized as a self-healing mechanism, since the release of the active species recovers the protective layer on top of the metal. The search for green inhibitors as replacements for chromate has been driven by legislative imperatives for a number of years.
Needless to say, replacement inhibitors do not have the same intrinsic inhibitive power at low solubility as chromate. Thus solubility, inhibitive power and transport within the primer system which consists of a number of inorganic phases as well as the epoxy ultimately mean that finding a replacement for chromate is difficult. This means that alternatives must be present at higher concentration leading to the use of more soluble compounds and consequently encapsulation as a method of regulating the response to external or internal triggers emerges as a prospective way to achieve this objective.
Many current inhibitors are water soluble salts and thus ionic. Consequently, they exist as either anions or cation in solution and perform the single function of anodic or cathodic inhibition. So the simplest improvement to inhibitor design is to increase the functionality by finding compounds which play both a cationic and anionic inhibitive role. Anions with dual functionality, such as some of the transition metal oxyanions which are both oxidants and anions, have been investigated extensively.
The oxidizing oyxanions or some organophosphates have some degree of bio-inhibition required for some applications. Substitution of different organophosphates into rare earth-based inhibitors provide versatility in designing inhibitors for specific applications Birbilis, Buchheit et al. However, good bio-inhibition usually means that there are increased environmental and health risks.
Obviously the number of cathodic and anodic inhibitors means that there are an enormous number of possible combinations, particularly if ternary and quaternary combinations are considered. Hence high-throughput techniques are being used to assess new inhibitor. As pointed out above, the kinetics of inhibitor release are of the utmost importance since the inhibitor should be available at levels above the critical inhibitor concentration.
Optimization of the release kinetics by novel delivery systems becomes integral to incorporation of new inhibitors. There are a number of different mechanisms investigated for release of healing agents or corrosion inhibitors which can be incorporated into organic coatings.
Both mechanical damage and water are triggers for inhibitor release. In the former case mechanical damage breaks capsules containing water soluble inhibitors.
In the latter case water dissolves inhibitor directly incorporated in the primer. Droplet formation within defects such as scratches means that the inhibitor is only released when required i. Thus it has been demonstrated that water can trigger cerium dibuthylphosphate Ce dbp 3 release into an epoxy matrix resulting in improved adhesion and resistance to filiform corrosion attack through interfacial modification Mardel, Garcia et al. In terms of delivery systems, hard capsules, which have been used in polymer healing Dry ; White, Sottos et al.
In polymer applications, capsules up to a few hundred microns can be accommodated Yin, Rong et al. The concept of encapsulation has already been successfully applied to protective organic coatings under different concepts: i liquids filling completely the void created by the damage by adopting a bi-component systems where one component is encapsulated and the other distributed in the matrix Cho, White et al. One adaption for capsules is to increase the volume of self-healing material by manufacturing rods instead of spheres. Rods with the same cross-sections as spheres can deliver larger volumes of material Bon, Mookhoek et al.
For inhibitors, their role is to prevent a surface reaction corrosion and therefore, the volume of material required is much smaller than that required to actually fill the defect. Water is the most obvious trigger since it can permeate most polymers. The presence of chloride ions and other anions within the coating can be used as specific triggers for the release of corrosion inhibitors and uptake of corrodents using anion exchange materials, such as layered double hydroxides e. In this context hydrotalcites have been loaded with vanadate, chromate, nitrate and carbonate which exchange for chloride ions and prevent interfacial damage Bohm, McMurray et al.
The incorporation of Mg particles into paint act as sacrificial anodes to protect Al alloys and steels Battocchi, Simoes et al. The search for new multifunctional inhibitors has led to the development of high throughput and combinatorial assessment of new combination of inhibitors. These include multielectrode techniques, and high throughput versions of standard corrosion tests. Since corrosion of aluminium alloys tends to be dominated by electrochemical processes, most of the techniques employed for the evaluation of corrosion and protection are based on electrochemical approaches.
Furthermore, combining electrochemical techniques with other microscopic, analytical and spectroscopic techniques enables the identification of corrosion products in solution such as inductive coupled plasma ICP and UV-Vis. The increasing number of corrosion inhibitor alternatives to chromates has boosted interest in developing high-throughput techniques and combinatorial assessment of new corrosion inhibitors in aqueous solution.
At the same time, the traditional techniques accelerated or not employed in the evaluation of the performance of organic coatings require long evaluation periods and are relatively expensive to run, and only offer qualitative or semi-quantitative information at best e.