Chromate-free Coatings Systems for Aerospace and Defence Applications

Abstract

Aluminium alloys used in the aerospace industry provide a combination of good strength to weight ratio and cost but are susceptible to galvanic corrosion. Chromate-based compounds offer excellent anti-corrosive properties and their use has been widespread to provide protection to environmental degradation. Acknowledgement of the dangers to human health and the environment has led to these Cr(VI)-based compounds to be heavily regulated, with a view to phasing out their use. Consequently, more benign alternatives have been extensively investigated, although the total elimination of chromium-based anti-corrosives is still an unlikely proposition for aerospace applications, where the consequences of structural failure as a result of inadequate resistance to corrosion are potentially very serious.

The topic of chrome-free protection of aluminium on aircraft and military land vehicles has been of major interest to manufacturing companies, organisations, and the UK government involved in the aerospace and defence sector for several decades. In the absence of detailed, concrete results, it is important to establish the current state-of-play for chromate-free systems that need to meet the stringent and demanding requirements for use on aircraft (external and internal) bodies and components.
This review identifies the alternatives to hexavalent chromium for corrosion protection in aerospace applications and summarises their suitability, commercial availability and likely widespread adoption.

Introduction

Aluminium alloys are used extensively for aerospace applications due to the combination of low price, light weight and good mechanical properties.  However, these materials are characterised by high chemical activity and potentially poor corrosion resistance and so require protection to prevent environmental degradation.  The most common approach to protection against corrosion is to employ a process of cleaning, surface pre-treatment, and application of organic coating layers.

Traditionally, corrosion prevention coatings have been classified into three types, including conversion coatings, primers, and topcoats.  Chromium-based chemical conversion coating involves immersing the aluminium into a bath containing a mixture of potassium or sodium dichromate and sulphuric acid. This creates a thin, corrosion resistant oxide film and provides the metal with some surface roughness that aids mechanical interlocking, thus allowing greater adhesion to be achieved.  Typically, primer formulations have consisted of chromated pigments dispersed within an epoxy resin.  The organic resin in the epoxy-based primer mediates the global galvanic protection potential as a function of organic polymer resistance [1] and provides barrier protection to the substrate.  The topcoat provides a barrier to environmental exposure, protecting the underlying active corrosion protection layers.

Hexavalent chromium remains the benchmark for corrosion inhibition, providing protection over a wide pH range and electrolyte concentration.  Chromates are both anodic and cathodic inhibitors, restricting the rate of metal dissolution whilst simultaneously reducing the rate of reduction reactions.  In addition, hexavalent chromium imparts a “self-healing” character to the coating during oxidative (corrosive) attack.  Self-healing occurs by the reduction of Cr(VI) in the coating to an insoluble Cr³⁺ compound. Hexavalent chrome coatings also play a critical role in supporting and enhancing the adhesion of the primer coating to the substrate.

The environmental impact of hexavalent chromate, it’s carcinogenicity, and regulations enforcing prohibition of the hazardous substance means chromium compound usage is now restricted unless granted exclusive authorization from the European Chemical Agency (ECHA).  A sunset date of January 2019 was set by the ECHA for the remaining chromium compounds, including strontium chromate.  A number of Authorisation Applications have been submitted for Cr(VI) substances covering aerospace uses, covering both manufacture of formulations and use of the substances and formulations to manufacture & repair aerospace hardware.  The agency has approved Cr(VI) plating treatments for aerospace use up to 2024 and some Cr(VI) additives for aerospace paints up to 2026, although this decision has yet to be approved by the European Commission.

Extensive research into alternative corrosion inhibitors has been underway since the 1980s, but there has been a lack of viable substitutes that promise engineering quality whilst also ensuring user safety.  Commercially available alternatives to chromium (VI)-based treatments are claimed to provide similar performance in terms of corrosion resistance, thermal stability, and adhesion properties for subsequent coating layers.

Alternative Anti-corrosives

Trivalent Chromium

The less hazardous trivalent form of chromium has been employed in anti-corrosion coatings for aluminium alloys.  The use of Cr(III)-based coatings for military applications has been reasonably successful [2], but replacement of Cr(VI) in aerospace is less well advanced.  Trivalent conversion coatings provide high barrier properties but with a significant decrease in the self-healing properties provided by Cr(VI) [3].  A number of trivalent coatings are commercially available, including Alodine T 5900 RTU from Henkel, and Socosurf TCS48 supplied by Socomore.  Generally, hexavalent chromium outperforms the trivalent species; claims that trivalent chromium pretreatment is equal to or better than conventional hexavalent chromates [4] are dependent upon specific applications.

Zirconium- and Titanium-based conversion coatings

Conversion coatings based on titanium or zirconium oxides have been implemented in the automotive industry and have displayed good corrosion and wear resistance [5].  These coatings, generally based on hexafluoro compounds, can operate at lower temperature and generate significantly less sludge than phosphate processes.  Chemical dissolution of the oxide layer by the free hexafluoride ions, followed by hydrolysis of the fluorometalates to form precipitated hydrated metal oxide layers and the rapidly generated titanium or zirconium oxides can provide a compact coating [6].  A tendency of Ti/Zr based conversion coatings to inhibit anodic corrosion has been observed [7].

Chemetall GmbH have developed technology for the pretreatment of aluminium surfaces involving application of an aqueous, acid solution that contains Ti and Zr as complex fluorides [8].  Examples of commercial coatings available from Chemetall include Gardobond X4707 H₂TiF₆ + H₂ZrF₆ for AA6060.  Technology developed by Henkel for conversion coatings based on complex fluorides of Ti and Zr [9,10] have led to commercial products (e.g. Alodine 5200 Ti-based organometallic zirconate for pretreatment of AA2024).

Lanthanides

Lanthanide compounds have been studied extensively over the last 20 years as alternatives to chromates for corrosion inhibition [11].  Cerium (Ce) and praseodymium (Pr) salts have been found to provide better inhibition than other rare earth metals, exhibiting larger shifts in the corrosion potential [12,13].  Praseodymium-based inhibitors are commercially available in epoxy-polyamide primers, providing protection by dissolving from the primer and forming precipitates in damaged areas.  An investigation into the mechanisms by which rare-earth compounds provide corrosion protection concluded that Pr is capable of providing corrosion protection on high strength aluminium alloys that is comparable to chromates when a suitable phase is incorporated into an appropriate coating [14].

Cerium is recognised as a good cathodic inhibitor for a range of metals [15] used as a simple salt as well as in cerium-organic compounds.  Cerium inhibits cathodic activity by forming insoluble hydroxides that accumulate at the base of coating pores and/or defects to form an impermeable layer at the coating-metal interface which prevents further chemical attack. Cerium salicylate and cerium dibutylphosphate have shown effective corrosion inhibition applied to aluminium alloy when incorporated into epoxy primer coatings, with cerium dibutyl phosphate providing a greater degree of corrosion protection [16].  Conversion coatings based on cerium salts have been found to be as effective as chromate-based coatings when tested under paint on aluminium alloy AA2024-T3 [17].

Sherwin Williams holds patents for primers and wash primers based on cerium molybdate and on cerium phosphate for corrosion protection of ferrous metals, aluminium and aluminium alloys [18].  It is not known if these technologies have been commercialised.

Electrodeposition of an Al-Ce alloy coating from ionic liquids (ILs) on an AA2024 substrate has been reported [19].  Electrodeposition is an attractive option for applying anti-corrosive coatings as it allows coating of large sizes and complex shapes.  The corrosion resistance of the AA2024 coated with Al–Ce alloy was reported to be significantly higher than that of the bare AA2024 alloy, although it was conceded that the process requires optimisation to achieve higher stability of the electrodeposited coatings as well as higher concentration of cerium in the coating.  Further developments of the process have not been found in the literature.

Rare earth-based corrosion inhibitors, including cerium and praseodymium, were included in the summary of alternatives to Ce(VI) in the Analysis of Alternatives submitted to ECHA in 2015 by the CCST Consortium as their Application for Authorisation [20].  It was concluded then that the rare earth-based systems were so far not sufficiently developed to provide a realistic alternative to Cr(VI)-based basic or bonding primer in aerospace applications.  There is little evidence in the literature to indicate that this situation has changed substantially since.

Silicon-based

Silanes have been extensively investigated as a benign alternative to chromate for corrosion inhibition.  Trialkoxysilane coupling agents have been demonstrated to protect metals efficiently against corrosion by dipping, spraying, or brushing with dilute solutions [21].  It is assumed that upon drying, silane molecules bond tightly to metals through the condensation reaction between silanols from the silane solution and the metal hydroxyls from the metal surface hydroxides, forming covalent metallo-siloxane bonds [22].  The improvement of the corrosion resistance of organosilane coatings can be ascribed to barrier properties.

Sol-gel anti-corrosion coatings are commonly synthesised from silicon alkoxides to prepare an inorganic or hybrid based colloidal suspension (sol phase) to a final solid, dense interpenetrating polymer network (gel phase) through aging and curing processes.  Sol-gels modified with inorganic inhibitors to form a hybrid system have been developed for use as anticorrosive coatings [23].  Incorporation of active species such as binding agents and corrosion inhibitors, which add active protection mechanisms to the system, can improve the protective properties of the hybrid sol-gel coatings.  The salts of the metal inhibitors e.g. cerium, zirconium inhibit cathodic activity by forming insoluble hydroxide film.

Examples of commercially available silane-based pretreatments in the aerospace industry include the Socogel sol-gel conversion coatings, organic-inorganic hybrid compounds manufactured by Socomore that may be applied to a range of metal substrates by dipping, spray or brush.  Socomore state that performance complies with current aeronautical requirements and provide effective alternatives to chromate conversion technologies, and that the adhesion of primers and epoxy-based paints on metallic surfaces are significantly increased.  A water-based, non-chromated, silane sol-gel surface pretreatment coating for aluminium, nickel, and titanium alloy aircraft parts is available from 3M (3M Surface Pre-treatment AC-131).  The sol-gel surface preparation promotes adhesion between metallic surface and organic coating.  The Socogel products and AC-131 are both based on aqueous solutions of zirconium salts, which are activated by an organo-silicon compound, and are already approved by several companies within the aerospace sector.  3M also hold a patent for an anti-corrosion coating for the protection of steel and aluminium alloy substrates, based on tetraalkyl orthosilicate modified with a metal salt from the group consisting of aluminium, strontium, chromium, zirconium salt, and cerium [24].

The CCST’s Analysis of Alternatives [20] concluded that Cr(VI)-free sol-gel systems are technically not equivalent to Cr(VI)-based products and the early research stage for substitution of strontium chromate in basic or bonding primer applications means that it will be at least 15 years before a viable candidate is developed for use in aerospace applications.

Phosphate coatings

Phosphate conversion coatings are generally applied by immersion or spraying and are widely used to protect a variety of metals.  Treating the metal surface with dilute phosphoric acid causes phosphate crystals to form from zinc, manganese or iron phosphate solutions and these thin crystalline layers of phosphate compounds adhere to the surface of the metal.  Anti-corrosive effects of phosphate pigments are due to anodic passivation via precipitation of phosphate complex layers in combination with metal hydroxide barriers.

Although phosphate coatings provide good adhesion and some barrier protection, low phosphate solubility means the phosphate-based film does not have the ability to self-heal.  Phosphates also lack the wide pH stability characteristic of chromate films and cannot serve as a viable system to replace hexavalent chromates, either in conversion coatings or primer coatings.

Recently, soluble phosphates that can leach from the coating and precipitate at the defective sites have been shown to provide a self-healing effect when incorporated into the phosphate coating [25].  Zinc aluminium polyphosphate shows better corrosion inhibition properties compared with conventional zinc phosphate, understood to be due to increased solubility [26].  Improved solubility of strontium aluminium polyphosphates (SAPP) has resulted in further improvements in corrosion inhibition of metal substrates [27].  Rare earth diphenyl phosphates, including cerium diphenyl phosphate, included in an epoxy primer system, have shown promise as corrosion inhibitors of AA2024-T3 alloy in NaCl solutions [28].  In the latter case, corrosion resistance may be a due to disruption of the network formation of the epoxy by the cerium-based inhibitor, resulting in a coating that absorbed more water and allowed greater solubilisation of the corrosion inhibiting compound.

Heubach manufacture a range of phosphate corrosion inhibitors; the polyphosphates are recommended for Cr(VI) replacement in aerospace applications.  This range includes strontium aluminium polyphosphate hydrate, zinc aluminium polyphosphate hydrate (ZAPP), calcium aluminium polyphosphate silicate hydrate (CAPP) for polyurethane and epoxy-based primers.

Magnesium rich primers (MgRP)

Primers loaded with magnesium particles protect a metallic substrate by galvanic means, imparting cathodic protection when in electrical contact with the underlying metal.  This sort of sacrificial galvanic coating requires a high loading of pigment to provide the necessary conductivity between Mg metal particles, as well as between the particles and the substrate.  Optimum PVC of 45% has been found to provide a balance between cathodic protection, long term barrier protection, and the beneficial characteristics of preserved, isolated clusters of Mg pigment available for the future protection of defects [29].  In addition to the corrosion protection afforded by sacrificial epoxy MgRPs, good adhesion, compatibility with the substrate, and chemical resistance has been reported in the CCST Analysis of Alternatives [20].

For MgRP, electrical resistance imparted by the pretreatment may limit or delay sacrificial anode-based cathodic protection. Conversion coatings and anodized coatings have been found to cause delayed galvanic coupling between MgRP and aluminium alloy substrate [30], dependent upon the thickness and chemistry of pretreatments.

A magnesium rich primer has been reported that can match, and even exceed the neutral salt spray and filiform resistance of chromate-based primers that depend on leachability of the chromate-based inhibitor, when all of the primers are topcoated [31].  MgRPs are available commercially from AkzoNobel as Aerodur 2100 MgRP, corrosion inhibiting epoxy-modified polyamide primer that is claimed to “outperform chromated systems under certain conditions”.  This primer is apparently based on lithium carbonate (and possibly calcium phosphate) – a leachable corrosion inhibitor and potential replacement for hexavalent chromium in organic coatings – modified with an electrochemical active magnesium compound.

Although the use of Mg-rich primer displays promising results for Al alloys in terms of corrosion resistance, this technology may not provide the performance and versatility of Cr(VI)-based products.  It was reported in the CCST authorisation application [20] that further development of Mg-based systems was unlikely, since other alternatives are showing greater promise.

Zinc-based

Zinc has been used in anti-corrosive coatings for steel for many years.  Zinc-rich primers, containing more than 80% w/w zinc particles, impart cathodic protection in contact with the underlying steel substrate.  Inorganic zinc silicates contain metallic zinc particles in combination with a zinc silicate binder; organic zinc paints are usually based on epoxy binders, although urethanes are also used.  The relative position of zinc in the electropotential series that provides steel with galvanic protection, makes zinc-rich primers less suitable for corrosion protection of aluminium alloys due to the similar electrochemical potential of zinc and aluminium and the presence of (significantly more noble) copper in the aluminium alloy.

Generally, zinc-based coatings provide inadequate corrosion protection for aluminium substrates for use in aerospace applications, as reported in the CCST Analysis of Alternatives [20].  It seems unlikely that further research effort will be dedicated to such coatings systems as other substances are reported to show more promise as anti-corrosives for corrosion protection of aluminium alloys.

The Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) has reportedly developed corrosion protection coatings for aluminium alloys based on primers containing intermetallic magnesium-zinc phases, providing an enhanced long-term protective effect, as demonstrated by salt spray test of epoxy-based coatings applied to pickled AA 2024 substrate [32].

Zinc-based pigments are regarded as hazardous to the aquatic environment.  Since the implementation of the CLP (Classification, Labelling and Packaging) regulation in 2015, zinc-based compounds need to be labelled accordingly as “very toxic to aquatic life with long lasting effects”.

Lithium-containing coatings

Lithium salts have been found to provide effective corrosion protection for aluminium alloys used in aerospace applications [33].  Active corrosion inhibition is provided by the migration of lithium ions from the coating matrix to the region of corrosion, forming a protective layer comprised of three distinct regions: a dense layer near the alloy surface that provides the corrosion protection; a porous middle layer; and a columnar outer layer [34].  Protective coatings based on lithium carbonate have been shown to provide fast, effective and irreversible corrosion inhibition, providing the essential characteristics needed for effective active corrosion protection [35].

The active corrosion inhibition provided by lithium salts may provide the aerospace industry with an effective alternative to hexavalent chromium for the corrosion protection of aluminium alloys.  Protective primers containing lithium carbonate are available commercially from AkzoNobel as Aerodur 2111 and Aerodur 2400 MgRP, the latter also incorporating magnesium particles.

Vanadates

Vanadium compounds have shown promise as alternatives to chromates as inhibitors to protect aluminium alloys such as AA2024-T3 as electrochemical inhibitors, and as self-healing corrosion inhibition when applied as surface treatments and pigmented organic coatings [36].  However, concerns regarding the carcinogenicity and mutagenicity of vanadium and its compounds have made this alternative unattractive as a benign chromate replacement.

Molybdates

Molybdate and molybdate-based compounds have in the past been used to replace chromate as corrosion inhibitors for water treatment systems and have shown promising corrosion protection behaviour in coatings, providing anodic corrosion passivation for a variety of metals and alloys.  The protective action of molybdate pigments is based on their slight solubility in water; when a coating film containing molybdate corrosion inhibitors is exposed to water, molybdate ions are released into the film.  Interaction with the metallic substrate promotes the formation of a passivating oxide layer.  Molybdate can also provide a synergistic inhibition effect in combination with other compounds, effectively enhancing corrosion protection.  This synergistic effect between molybdate and phosphate has been reported for copper and carbon steel [37] and is likely to have similar benefits for aluminium alloys.

The incorporation of zinc molybdenum phosphate (ZMP) into epoxy-amide primer coatings has been studied for corrosion inhibition on steel [38].  ZMP consists of zinc phosphate modified with up to 1% zinc molybdate (ZnMoO3); the Mo anion acts to re-passivate pits formed on the substrate.  It was found that ZnO addition enhanced the anti-corrosion performance when used at an optimum ratio with ZMP, forming a ZnO-Mo barrier layer as long as pH <9 is maintained.

A number of molybdate-containing primers are commercially available in the form of zinc-molybdate based corrosion inhibiting primers (e.g.TT-P-645 for marine; AMS 3117 for military) for use on steel and aluminium surfaces.  It is claimed (by Heuback) that zinc molybdenum orthophosphate hydrate (HEUCOPHOS ZMP) shows excellent anti-corrosive performance when used in water-based coating systems using 1-part polyurethanes.  Molybdate-containing inhibitors exhibit an immediate, positive effect for steel corrosion, but an incubation period may be required for aluminium before the effect of a given inhibitor can be determined [39].  The CCST consortium concluded in the Analysis of Alternatives [20] that molybdate-based primers showed insufficient corrosion prevention for use as a strontium chromate replacement for the aerospace sector.

Organic coatings

Several organic compounds have demonstrated corrosion inhibition on aluminium alloys; examples include amines [40], imidazole derivatives [41], and thiourea derivatives [42].  Benzotriazole (BTA) derivatives are reported to be under investigation as corrosive inhibitors for use in epoxy basic primers for AA2024 within the aerospace industry [43].  The inhibition effect of BTA is related to the formation of a thin layer of inhibitor molecules on top of the aluminium oxide surface [44], hindering the corrosion activity by both anodic and cathodic inhibition and providing self-healing ability.

The benzotriozole derivative 5-Methyl-1H-benzotriazole (MeBT) is an active component of aircraft de-icing and anti-icing fluid and has been used to inhibit corrosion of metal components for a variety of industrial applications since the 1980s.  MeBT has been shown to prevent the corrosion of aluminium in hydrochloric acid (HCl) solutions [45] and to prevent the corrosion of steel in 0.1 M sulfuric acid solution [46] and in 3.4% chloride [47].  Experimental evidence indicates that combining BTA with inorganic inhibitors such as silica sol-gel epoxy coatings can be used to enhance corrosion protection [48].

Development of corrosion protection primers based on organic inhibitors are the subject of extensive R&D efforts in several industry sectors.  There is no clear evidence that these are currently able to provide a viable alternative to chromate-based inhibitors for the aerospace sector.

Calcium-based

Like rare earth metals, calcium precipitates to insoluble hydroxide film in very alkaline conditions, potentially characteristic of a cathodic inhibitor.  The role of calcium as a corrosion inhibitor is not clearly demonstrated in the literature, however.  Investigation into characterisation of any inhibiting effect of CaSO4 on AA7075-T6 concluded that calcium did not appear to contribute to inhibition in any of the evaluations performed [49].

There are a number of calcium-based corrosion inhibitors available.   It is claimed (by ICL) that Halox calcium borosilicate-based corrosion inhibitor can match the long-term protection of zinc chromate.  Protection is provided by anodic passivation and can be combined with zinc phosphate-based inhibitors for long-term corrosion protection.  Formulations based on medium and long-oil alkyds, epoxy esters, alkyds, 1K polyurethanes, 2K polyurethanes and moisture cure systems are recommended.  Calcium-modified silica gel is available from Heubach (HEUCOSIL CTF) which, it is claimed can provide effective corrosion protection of pre-treated steel and aluminium.

Electrocoat

Electrophoretic deposition is widely used in the automobile industry to coat car bodies, although it is extensively used in a range of industrial coating sectors.  Following pretreatment, the vehicle body or part is immersed in the electrodeposition tank that contains primer typically comprised of approximately 20% paint solids (resin and pigments) dispersed in water, with less than 2% organic solvent.  Upon application of an electric potential, charged particles are deposited on the oppositely charged metal surface.  During deposition, the exterior surfaces of the workpiece are coated first until the thickness of the applied film introduces resistance; remote inner surfaces then draw increasing current; the process continues until all exterior and interior surfaces have been coated; parts are then rinsed and thermally cured. Cathodic electrocoat dominates the automotive industry due to improved corrosion protection and chemical resistance compared to anodic treatments.  The high temperature cure requirements typical of the cathodic process (generally >150°C) are unsuitable for aerospace aluminium and anodic systems have been developed for aerospace applications that allow for lower temperature (~100°C) [50].

Traditionally, electrocoats have not met the rigorous anti-corrosion and chemical resistance performance demands of the aviation sector.  The Aerocron anodic epoxy electrocoat primers, developed by PPG are intended to be applied over a variety of chromate and chrome-free pretreated substrates.  PPG, in partnership with Air Tractor, have announced the first electrocoat primer system for full-scale original-equipment aircraft parts manufacturing in the aerospace industry.  A purpose-built facility to house the electrocoat system has been constructed at the Air Tractor agricultural manufacturing facilities in Olney, Texas.

A major drawback of the electrocoat process is that it is a dip coating process and can be applied only to OEM parts and parts that can be removed from the aircraft during overhaul, and not to assembled aircraft.  Additionally, electrocoated parts can only be repaired using conventional chromate anti-corrosive primers.  Corrosion inhibition is provided by the formation of an interfacial oxide layer at the surface of the aluminium alloy during anodic paint electrodeposition [50]; this process does not provide active corrosion inhibition.

Nanocomposites

Graphene-based nanocomposite coatings have been found to serve as excellent barriers upon surfaces such as carbon steel [51] and aluminium alloy [52], providing a barrier to the permeation of ions through the coating by means of a tortuous path to the metal/coating interface.  Graphene does not provide self-healing active corrosion inhibition; mechanical damage will render the coating ineffective in the damaged area.  However, substantial improvements in strength and durability of epoxy coatings, compared to control anticorrosive coatings applied to steel have been reported [53], providing additional protection to damage of the coating.  It has been proposed that the conductivity provided to the coating by dispersed graphene platelets may provide an alternate path for the electrons, released at the anode, thereby retarding the overall corrosion reaction [54].  It is important to note that, given graphite’s position in the galvanic series, graphene is likely to be cathodic to most metals, including steel and aluminium.  Consequently, coating defects that affect the barrier function of the graphene may lead to accelerated local corrosion of the underlying, more anodic substrate that can seriously weaken the coated metal.  This effect may be mitigated by addition of a more anodic material to graphene-polymer coatings, or by developing graphene-based coatings that can self-heal and so withstand minor damage [55].  Graphene suppliers currently actively involved in developing anti-corrosion coatings include Talga (Talphene range) and Applied Graphene Materials (Genable range); Greece-based industrial paint producer Hydroton has launched a new two-part zinc epoxy paint enhanced with graphene developed by The Sixth Element Materials.

Incorporation of nanoparticles such as silica, ceria, zirconia, alumina, titania, and zeolite can provide improved mechanical properties, increased thickness, and lower crack sensitivity, resulting in enhanced corrosion protection of the underlying substrate [56].  Hybrid organic–inorganic sol–gel-matrices, with up to 20 wt.% incorporated ceria nanoparticles, have been investigated as coatings for an AA2024-T3 aluminium alloy [57].  CeO₂ nanoparticles (5 nm) were dispersed throughout the cross-linked silane-based sol-gel coating which was applied to aluminium alloy panels by dip coating.  It was found that 7% CeO₂ addition significantly enhanced the barrier properties of the sol-gel and prolonged the lifetime of the coating.

Smart coatings for controlled inhibitor release, wherein inhibitors are immobilised within a matrix dispersed within a coating react to environmental changes within the protective coating/substrate interface.  Sol-gel coatings containing organic inhibitors encapsulated in cyclodextrin cages have been found to provide enhanced corrosion inhibition to AA2024-T3, compared to systems in which the inhibitors were directly dispersed into the sol–gel matrix [58].  A cross-linked divinyl benzene ion exchange resin, similar to those used for removal of chromate from drinking water, was added to a model organic polyvinyl butyral (PVB) coating.  Enhanced corrosion inhibition for hot dip galvanised steel was observed when organic benzotriazole anions [59] and inorganic cations were exchanged into the resin with ammonium functionality or with a sulphonated functional group, respectively, to be released only when the coating encounters a corrosive electrolyte, via ion exchange mechanisms.  This system was originally designed to replace current technologies used in the architectural steel coatings market, and is expected to be applicable across marine, automotive, and aerospace sectors.

Conclusions

Hexavalent chromium-based compounds that have been used for several decades as corrosion protection additives for metals have been identified as substances of very high concern (SVHC) and are now subject to strict controls over their use.  Their status as SVHC is due to their carcinogenicity and environmental impact profile.  Extensive research since the 1980s has identified a number of promising alternatives, particularly the lithium-based technology that provides self-healing and barrier properties and that has been commercialised by AkzoNobel.  Rare earth-based technology, particularly cerium- and praseodymium-based, have been extensively investigated as a chromium replacement and shown considerable promise in a number of technologies.  Titanium/zirconium-based conversion coatings have high corrosion resistance and are already applied in industry.  It has been reported that Ti-Zr can significantly improve the corrosion resistance of aluminium alloys and even provide anticorrosion performance and adhesion properties superior to that of chromate conversion coatings, although these coatings have so far not been proven to meet the stringent requirements of the aircraft industry.  Silicon and its related species have been the subject of detailed investigation.  Silanes are commonly used as coupling agents and may offer the best opportunity to generate an environmentally benign alternative to Cr(VI).  Sol-gel coatings prepared using silane-based chemistry, incorporating corrosion inhibitors and nano-particles have received a lot of attention but, although much research has been conducted into the technology, realistic alternatives to chromate coatings have yet to be identified.  Phosphate coatings are used extensively to protect a variety of metals by a barrier process.  Additional self-healing property can be realised with the use of polyphosphates and these products are finding application in the aerospace industry.  Primers loaded with particulates of active metal zinc or magnesium are used to impart cathodic protection to steel and aluminium alloys.  High volume fraction of metal particles is required to maintain electrical contact with the metal substrate; only temporary corrosion protection is provided.  Longer-term protection is provided by the oxides and hydroxides of the metal, although versatility and efficacy are insufficient to replace current Cr-based inhibitors for aerospace.  Vanadates may be able to provide self-healing anti-corrosion systems but there are concerns about potential carcinogenicity.  Molybdates provide active corrosion protection to aluminium alloys, although they are currently considered to provide insufficient corrosion protection for aerospace.  Organic inhibitors such as BTA have been used as corrosion inhibitors for aluminium alloys; incorporation into smart, controlled release matrices has been shown to improve performance efficacy.  PPG has developed an electrocoat process to be applicable for protection of aerospace-grade aluminium alloys but is limited by a non-active protection of the substrate and problems with applying the coating to assembled components.  Graphene-based nanocomposites can provide an impermeable barrier function to corroding ions for protection of metal substrates.  This is not an active protective system and pinholes or mechanical damage inflicted on the coating may accelerate corrosion due to graphene’s chemical inertness relative to most metals.

Hexavalent chromium remains the benchmark corrosion protection for metals.  The lack of suitable alternatives that have been conclusively proven to provide adequate protection in safety-critical applications such as aerospace means that Cr(VI) compounds have been authorised for continued use beyond the sunset dates set by ECHA; this is in spite of the compelling evidence of their threat to human health and the environment.  It is vital that an update in the evaluation of the performance of the alternatives to Cr(VI) is carried out to determine the success of recent developments of more benign coating systems to allow for the elimination of chromate-based coatings, as intended within REACH.

Acknowledgements

his review article is part of an ongoing research project supported by Pera International investigating chromate-free coatings for application in the aerospace and defence sectors.

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