Abstract
Research on corrosion protection of aluminum has intensified over the past decades due to environmental concerns regarding chromate-based conversion coatings and also the higher material performance requirements in automotive and aviation industries. Phosphonic acid-based organic and organic-inorganic coatings are increasingly investigated as potential replacements of toxic and inefficient surface treatments for aluminum. In this review, we have briefly summarized recent work (since 2000) on pretreatments or coatings based on various phosphonic acids for aluminum and its alloys. Surface characterization methods, the mechanism of bonding of phosphonic acids to aluminum surface, methods for accessing the corrosion behavior of the treated aluminum, and applications have been discussed. There is a clear trend to develop multifunctional phosphonic acids and to produce hybrid organic-inorganic coatings. In most cases, the phosphonic acids are either assembled as a monolayer on the aluminum or incorporated in a coating matrix on top of aluminum, which is either organic or organic-inorganic in nature. Increased corrosion protection has often been observed. However, much work is still needed in terms of their ecological impact and adaptation to the industrially-feasible process for possible commercial exploitation.
Highlights
Aluminum (Al) and its alloys have been widely used in engineering applications because of their higher strength to weight ratio, ductility, formability, and lower costs
X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are both methods which determine the elemental composition in the first couple of nanometers of a surface and, were often applied to verify the presence of phosphorus, indicating a successful organophosphorus coating chemically-bound to aluminum [6,8,11,16,17,19,20,21,22]
Environmental issues with existing pretreatments for aluminum, together with the need to develop new materials with higher mechanical and chemical performance, is driving the research and industrial communities to develop new pretreatments based on phosphonic acids
Summary
Aluminum (Al) and its alloys have been widely used in engineering applications because of their higher strength to weight ratio, ductility, formability, and lower costs. A variety of phosphonic acids is commonly used to modify the surfaces of metals and their oxides for their corrosion protection, stabilization as nano-particles, adhesion improvement to organic layers, hydrophobization, hydrophilization, etc They are excellent chelating agents and bind very strongly to metals resulting in different 1D to 3D metal organic frameworks (MOFs), called metal phosphonates [3,4]. Phosphonic acid acts as a linker between aluminum and a hydrophobic polymerizable group (pyrrole) [10] or chemically-stable protective material (graphene oxide) [11] Such molecules are applied on the surface of aluminum via dip coatings, spray from aqueous solutions, or are incorporated in a polymeric matrix (adhesives, glues, or paints) which is coated on the metal surface. Thereby, a distinction has been made between the aluminum surface treatment with phosphonic acids or phosphates as a pre-coating and surface coatings dissolved in paints that contain phosphonic acids
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