The ubiquitous use of chromium and its derivatives as corrosion preventative compounds accelerated rapidly after the second industrial revolution, with such compounds now integral to modern society. However, the detrimental impact of chromium compounds on the environment and human health has prompted the need to revisit the majority of current industrial corrosion protection measures. This review retraces the origins of chromium replacement motivations, introducing the various legislative actions aimed at diminishing the use of chromium compounds, and critically reviews alternative corrosion preventative technologies developed in the recent decades to now. The review, herein, is intended for a broad audience in order to provide a concise update to an increasingly timely issue.
Chromium compounds have been a staple of consumerist society for over a century, with applications ranging from wood treatments to corrosion prevention.1 It has been documented since the 1920’s that hexavalent chromium is carcinogenic in nature, after multiple studies noted an increase in the incidence of nasal and lung cancer amongst industrial workers in direct contact with Cr6+ compounds.2,3,4 A number of studies were collated and reported by the World Health Organization (WHO) in 1972 and 1980, which adjudicated the carcinogenic nature of chromium compounds based upon independent investigations around the world.1,5 Despite the obvious dangers, it was not until 1980 that the toxic nature of hexavalent chromium was officially documented in the annual report on carcinogens established by the US Department of Health and Human Service (HHS).6 Hexavalent chromium is now known to induce irreversible health damage including nose, throat, eye and skin irritation, but most notably, significantly increases an individual’s risk of lung cancer.3,4,7 In addition, chromates have also been documented to induce genotoxicity on aquatic and botanic life through soil and water contamination, owing to poor waste management and disposal maintenance.8,9,10,11
In response the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) was developed in 2006. REACH is a European (E.U.) legislation that obligates companies to identify, manage and register the risks associated with listed dangerous substances when production or consumption levels exceed 1 ton per year.12 Consequently, chromium compound usage is now restricted unless granted exclusive authorization from the European Chemical Agency (ECHA).12 Although companies continue to use hexavalent chromium in numerous industrial formulations, the present period of so-called “respite” has a non-negligible economic impact on companies, as the total cost associated with this authorization is estimated at €1.5 M.13 Furthermore, this authorization is only valid for a limited time, with an imminent sunset date set for January 2019.14 In the wake of legislative shift, albeit no global legislative shift to date, many metal finishing industries are now compelled to seek safe, environmentally friendly alternatives to chromate compounds.
Although extensive research has been underway since the 1980s to date, hexavalent chromium remains the benchmark corrosion preventative compound in essentially all industries. For instance, corrosion protection of metallic components in aeronautical applications continues to rely on hexavalent chromium compounds. In one example that confronts the present industrial status quo, it was stated that environmental and health concerns drove the use of a non-chromated (i.e., non Cr6+ containing) primer on the F-22 aircraft (produced by Lockheed Marin), resulting in inadequate corrosion protection.15 Thereafter the F-22 program switched to a chromated primer. Interestingly, the F-35 (Joint Strike Fighter) under present day development has also opted to use a non-chromated primer that has—interestingly—never been tested on an aircraft in a corrosive operating environment (albeit that the F-35 program cost is estimated at US$1.508 trillion.16 This is just one scenario that highlights the implications of inevitable eradication of chromates from the market, and the lack of viable (demonstrated) substitutes incites the need for unorthodox approaches and for revision of previously disregarded alternatives, in the hope of creating a future that promises engineering quality whilst also ensuring user safety and social responsibility.
Hexavalent chromium: retrospective view
Hexavalent chromium-based coatings are defined by an exceptional versatility, which also includes remarkable corrosion protection and cost-effectiveness.17 The remarkable corrosion resistance of chromates is persistent over a very wide range of pH and electrolyte concentrations;18,19 a feature unable to be matched by alternative inhibiting compounds to date, most having finite operating pH and minimum concentration requirements.20,21,22 The mechanism of corrosion inhibition from chromates, irrespective of chromates being known utilized for many decades prior, is something that has really only been understood in more contemporary times.19,23,24,25 Unlike most inhibiting compounds, chromates are both anodic and cathodic inhibitors, meaning that they can restrict the rate of metal dissolution,19,26,27,28 and simultaneously reduce the rate of reduction reactions (both oxygen reduction and water reduction)19,26,27,29 in many environments (Fig. 1).
The ability to function as potent corrosion inhibitors in a wide range of pH conditions, has also meant that chromates have been (widely and successfully) implemented into corrosion protection primers and coatings. For example, when a morphological failure in a chromated coating or primer occurs, the chromate ion (CrO42−) leaches into the electrolyte (and to so called “active sites”) and undergoes cathodic reduction (coupled to anodic metal dissolution) to form a protective chromium oxide (Cr2O3) film (Fig. 2).30,31,32,33
The Cr2O3 film can serve as a barrier to corrosion, given its relative insolubility in aqueous environments that span the environmental pH spectrum, and its adherence to the underlying metal.18 A similar “universal” anti-corrosion compound is yet to be identified for implementation in coatings and primers, such that the need for chromate replacements spans many industries and forms of corrosion protection deployment. The need for chromate replacements in primers is a field in which only minor advances have been occurring and will be an area where significant translation will be required in the near term.
In light of the performance and simplicity of chromates for corrosion protection, it is clear that replacements are not only needed, but will also ideally be required to meet acceptable anti-corrosion performance (which is now industrially expected and will also be demanded by consumers who have become accustomed to specific durability or warranties), and cost expectations as previously met by hexavalent chromium.
Trivalent chromium: the unsuccessful alternative
The substitution of the hexavalent chromium with the less toxic trivalent chromium, was initiated in the early 1990s for chemical conversion coatings on Al alloys.34,35,36,37,38,39 The trivalent conversion coating (TCC) typically contains fluoride salts (NaF), trivalent chromium salts (Cr(OH)3 or Cr2(SO4)3) and hexafluorozirconate (ZrF62−).40,41,42,43 Surface film formation takes place in an acidic environment where the co-precipitation of hydrated zirconia and trivalent chromium hydroxides occurs, hence providing high barrier properties.40 However, due to the absence of hexavalent chromium within the coating, a significant decrease in the so-called self-healing properties associated with trivalent chromium protection strategies was noted.37
Perhaps, most critically however, recent work has indicated that during the coating growth in the case of trivalent conversion coating for alloys, trivalent chromium may be oxidized by hydrogen peroxide (generated by oxygen reduction or deliberately added to the coating environment), forming hexavalent chromium. In fact, it was argued that the fluorides species present in the bath (usually added to accelerate film growth and native oxide dissolution) were promoting hydrogen peroxide formation, subsequently oxidizing Cr3+ species to Cr6+.35,39,44,45,46
Regardless of the recent findings on TCC chemistry, trivalent chromium-based formulations remain the most common Cr6+ replacement to date for chemical conversion coatings on aluminum or zinc alloys and are currently commercialized by several coatings suppliers (such as Alodine T 5900 RTU from Henkel,47 or Socosurf TCS48 supplied by Socomore). This is often justified by the relative low toxicity of TCC formulations in regards to Cr6+, as the proportion of Cr6+ present in the coating was purported to not exceed the 0.1 wt% set by REACH regulation,49 although not yet met by all current industrial formulations.35
In addition, the trivalent chromium (Cr3+) alternative still involves the need of chromium resources, extraction and disposal; and in light of recent studies, trivalent chromium may not be considered as a drop-in replacement to Cr6+ - owing to some recent and emerging findings. In one study, suspicions regarding the eventual carcinogenicity of trivalent chromium amongst tannery workers have been raised,50,51,52 following some evidence of genotoxicity risk associated with several Cr3+ compounds53 thereby rending Cr3+ problematic in a future applications.
A critical review of alternatives to chromium technology
This section will discuss the different alternatives systems designed since the 1980s, and will particularly focus on their main advantages and limitations. The mechanistic aspects involved in each system are summarized in Table 1. The most promising technologies and their associated corrosion protection mechanisms (namely Li-containing coatings, MgR primers and nanocomposites coatings are illustrated in details in Fig. 3).
Rare-earth-based inhibitors were first introduced in the late 1980s as promising candidates for Cr (VI) replacement systems on aluminum alloys.54 The rare-earths complexes (such as cerium-nitrate, cerium-chloride (CeCl3), cerium-sulfate (CeSO4)2, cerium-dibutyl-phosphate ([Ce(dbp)3]), cerium-salycilate (Ce(sal)3), cerium-anthranilate ([Ce(C7H6NO2)3]), cerium glycolate ([Ce(C2H3O3)]),55,56,57,58 or lanthanum nitrate (La(NO3)3)59,60) tend to form Cerium or Lanthanum oxides/hydroxides at a pH > 10, and therefore could inhibit cathodic reactions on corroding surfaces. Indeed, rare-earth salts were able to inhibit the corrosion of several systems such as Zn, galvanized steel, Mg, or Al alloys and successfully integrated into primers or conversion coatings.56,57,58,61,62,63,64 However, the rare-earth-based oxides could dissolve in an acidic environment and therefore making this rare-earth-based surface layer less stable or durable, compared to the Cr2O3 layer form using hexavalent chromium. In addition, several studies performed on a coated Al alloy AA2024-T3 reported minor improvement in pitting potential and corrosion current, calling into question the efficacy of Ce-based conversion coatings.56,65 Although rare-earth ores are abundant in earth’s crust, their extraction and production on an industrial scale has proven expensive, and as a result, only began in the late 1950s.64 Despite progression within the industry, production is currently threatened by the near-monopolization of the rare-earth market by China coupled with their export restrictions, as the majority of such substances lie within their soil.66,67 These non-technical factors could impact the global supply-demand chains and price fluctuations (i.e., Ce varied in price from US$4.35/kg to US$100/kg over the period of 2001–201267,68) of rare-earth compounds, thus introducing a level of uncertainty on the cost of rare-earth-based anti-corrosive coating systems.
The chemistry of vanadium compounds in the context of corrosion inhibition was first introduced in the 1960s.69 Bienstock and Field introduced sodium metavanadate (NaVO3) as a potential inhibitor for the corrosion of steel in hot carbonate systems.69 A few decades later, promising vanadate-based conversion coatings on Al alloys 2024 were successfully developed, as they were shown to adsorb on the cathodic sites, hence significantly inhibiting the oxygen reduction rate (ORR).20,21,22,70 However, concerns have been raised towards the carcinogenicity and mutagenicity of vanadium and its compounds,71 in the context of human administration for diabetes treatments,72 but also with clear evidence of lung tumor in rat and mice after V2O5 inhalation.73 Although vanadium compounds display interesting properties, and not only in the materials engineering field, the carcinogenicity of these compounds is now proven by numerous studies74,75 making this alternative no longer viable.
The unique properties of Li were originally investigated in the context of Al alloys for weight reduction purposes.76,77,78,79,80 However, studies progressively reported the inhibitive character of Li when exposed to Al, particularly in alkaline environments.81,82,83 It was suggested that Li was readily intercalated into the Al hydroxide matrix,84,85 hence stabilizing the passive film when exposed to an aggressive environment.81,86 On the other hand, Buchheit et al. reported the formation of Al–Li layered double hydroxide (LDH) during the anodic dissolution of an Al–Cu–Li containing alloy.83 These natural mixed metal oxides have the following composition: MII(1−x) MIII(x)(OH)2[An-]x/n and consist of alternate layers of positively charged brucite sheets and water-anion layers, electrostatically bound to each other. Based on this idea, the concept of Li-containing coating emerged in 1996, when Buchheit et al. published work on the chemistry of Li-based conversion coatings for Al alloys.87,88,89 This compact polycrystalline layer was able to impart significant corrosion resistance to Al alloys and pass a 168-hour salt spray test.18 More recently, the utilization of Li salts loaded in a polyurethane organic coating as a replacement for SrCrO4 primers became of interest, demonstrating effective corrosion inhibition after a neutral salt spray test (ASTM B-117) exposure as reported by Visser et al.90,91 Nowadays, the self-healing and barrier properties of Li coatings are extensively documented90,91,92,93 and are illustrated in Fig. 2a. Although this technology was only applied on Al alloys and is still not industrialized, the early stages are suggesting promising outcomes both in corrosion resistance and self-healing properties similar to that of hexavalent chromium.
Organic coatings and nanocomposites
Organic systems that are used to impart corrosion protection to metal systems include organic inhibitors, hybrid sol–gel and polymer-based coatings.61,94,95 The polymeric primers on their own cannot impart sufficient corrosion protection to alloys and anti-corrosives such as SrCrO4 are interspersed into the primer, to impart active corrosion protection of the metal substrate. Herein, instead of a deep focusing on the mechanisms involved, we briefly cover the main aspect of these technologies.
Organic-based coatings traditionally used to protect metal systems such as Al alloys comprise of multiple layers (i.e., primer and top coat).96 However, such multi-layered polymeric coatings are yet permeable to air and moisture,97 resulting in coating degradation and ultimately substrate corrosion. The protectiveness of the organic coating itself can be significantly enhanced if its barrier characteristics are improved. For instance, graphene-based nanocomposite coatings have been found to serve as excellent barriers upon surfaces such as carbon steel98 and pure iron.97 It has been hypothesized that the presence of graphene in nano-dimensions, within the organic coating tends to increase the path for the conducting agent to reach the metal/coating interface.98 In addition, Glover et al.97 observed a reduction of oxygen diffusion and exceptional resistance to cathodic delamination when graphene nanoplatelets were added into polyvinyl butyral (PVB) coating. However, one of the major limitation of this technology is the cost of graphene, which may exceed AUD$130/kg, rendering this technology non-viable for a short term alternative.99,100
Today, the design of “smart” organic coatings for controlled inhibitor release, wherein, the coating itself could respond to an external stimulus such as moisture ingress or pH variations to release inhibitors is becoming rather widespread95,101 (Fig. 2b). Such smart systems include pH permeable polyelectrolyte-based coatings, conjugated (or conducting) polymers and organic coatings coupled with encapsulated inhibitors encapsulated in nano/micro containers.102 Khramov et al. immobilized organic inhibitors in cyclodextrin cavities, within sol–gel coatings that were applied onto AA2024-T3 substrates and were found to offer better protection, than those systems in which the inhibitors were directly dispersed into the sol–gel matrix.95 Buchheit et al.103 used Zn–Al layered double hydroxides (LDHs) as containers for inhibiting decavanadate ions (V10O286-), within epoxy coatings upon AA2024-T3. It was observed that firstly, decavanadate ions were controllably released from the LDH containers and also that the aggressive chloride ions were also entrapped within such containers. Organic inhibitors which have shown potential in substituting chromate-based inhibitors, especially in hybrid sol-gel coatings upon Al alloys include benzotriazole, tolyltriazole, 2-mercaptobenzothiazole, mercaptobenzimidazole, 8-hydroxyquinoline, salicylaldoxime, quinaldic acid.95,101 Andreeva et al.104,105 developed pH sensitive polyethyleneimine, polystyrene sulfonate and 8-hydroxyquinoline sandwich structures upon AA2024, following a layer by layer deposition approach. It was proposed that the polyelectrolyte layers served to impart corrosion protection by three mechanisms, (i) pH buffering (ii) controlled inhibitor release, and (iii) tendency to seal/eliminate any mechanical cracks in the coating.105
It is evident that several novel methodologies have been trialed to achieve chromium-free corrosion protection, however a key limitation for such advanced systems remains the cost towards implementation on a commercial scale. Smart coatings technologies remain at an experimental stage, and do not comply with anti-corrosion standards required for industrial implementation.101 Ideally, the targeted average price of future coatings should remain cost effective, which means not exceeding the current cost of SrCrO4-based primers for aerospace application-estimated to be around AUD$0.04/m2. 106
Phosphate coatings are widely used to protect metals such as carbon steels, zinc and galvanized steels, and Mg alloys.107,108,109,110,111,112 Usually, a metal phosphate surface layer is precipitated onto the metal from a phosphoric acid solution bath to form the conversion coating. In spite of its proven biocompatibility (for Mg-implant applications), the phosphate compounds (such as Zn or Mn phosphates) that form upon the surface generally prone to chemical dissolution in alkaline environment and depends on the nature of the phosphate cation.108,109 This is due to the fact phosphates such as Zn3(PO4)2 have a limited pH stability in aqueous solutions (ranging between pH 5 and 10), unlike Cr2O3 which has a wider pH stability ranging from pH 2 to 12.113 Interestingly, strontium phosphate (SrP)-based conversion coatings have been found to successfully impart corrosion resistance to active metals such as pure Mg.111 A protective surface film was found to form upon pure Mg after about 3 min of exposure in the coating bath, with an inner magnesium oxide layer and an outer SrP layer.111 Overall, phosphate-based surface layers may serve as a protective barrier upon metals under near neutral pH conditions. Nonetheless, it has been proven that the phosphate-based film does not have the ability to self-heal nor does it have the wide pH stability characteristic of the Cr2O3 film.113 Phosphates therefore cannot serve as viable systems to replace hexavalent chromates.
Metal rich primers
Primers, with a thickness of ~25–30 µm,114,115 loaded with active metals such as Zn or Mg primarily serve to protect steel and Al alloys.116,117 The Zn or Mg particulates, in either spherical or flake form, are able to impart cathodic protection when in electrical contact with the underlying metal and are illustrated in Fig. 2c.116,118,119 In the case of Zn rich primers, the cathodic protection range is estimated to be at or below ~−0.78 VSCE for steel.120,121 On the other hand, in the case of Mg-rich primers, the corrosion potential of the underlying metal (such as an Al alloy ~−0.64 VSCE in 3% NaCl) could also be maintained well below its pitting potential during contact with Mg-rich particles in the primer (~−0.93 VSCE in 3%NaCl).120,122 The oxides/hydroxides of the active metal particles (with an approximate size of 30–40 µm122) could also serve to provide long-term protection to the underlying metal substrate as reported by Bierwangen et al. Indeed, the Mg-rich primer system was reported to provide protection after a 3000 h Prohesion exposure and up to 6000 h under ASTM B117 salt spray testing when a top coat was applied.120
However, the “self-corrosion” of active metal particles within the primer itself has proven to be a major drawback of metal-rich primer systems. The high solubility of some of the Zn or Mg salts could also result in high osmotic pressure within the primer, causing blistering of such coatings. Moreover, the corrosion of these metals is accompanied with local pH variations (particularly a local alkalization in the case of Mg), which could cause delamination of the coatings. These aspects need to be clarified in future work in this particular area. The addition of a top-coat upon metal rich primers was suggested to significantly improve the protective capabilities of metal-rich primer systems.123 Although the use of Mg-rich primer displays promising results for Al alloys in terms of corrosion resistance, this technology is not as versatile as SrCrO4 and may not be suitable for more reactive substrates such as galvanized steel or Mg alloys. As reported by Bierwangen et al., only Mg-rich coated AA2024-T3 showing OCPs <−0.90 VSCE showed cathodic protection and the requirement for sacrificial protection is for the metal rich coating to be more electrochemically active than the substrate.120
Challenges and opportunities
This concise review has provided a factual background and emphasized the challenges that remain to be overcome to find a sustainable, cost-effective, and versatile chromate replacement. To date, trivalent chromium was considered by many industrials as the most promising alternative and its use in coating and pretreatment formulation is becoming more and more envisaged. However, recent studies outlined the presence of hexavalent chromium within the coating, in concentrations that could be not compliant with the existing REACH legislation (>0.1 wt), including the suspected carcinogenicity of trivalent chromium reported by several studies.
Nonetheless, the emergence of several alternatives, such as the particularly cost attractive lithium-based technology, has shown promising results (self-healing and barrier properties) on AA2024-T3. Further studies on various substrates, (Mg alloys, or galvanized steel) to explore the potential versatility of this technology is certainly a path that needs further exploring. On the other hand, Mg-rich primers have shown excellent corrosion performances, however, this technology remains limited to Al alloys and cannot be transposed to more reactive substrates such as magnesium alloys or galvanized steel.
It appears that the majority of the explored technologies are to date, not universal and many technical constraints, such as upscaling and cost still need to be adjusted. One solution would be reconsidering the cost of future coating system with the prospect of finding a sustainable alternative.
On the other hand, the present research on coating design has significantly progressed with the use of methodologies allowing a fundamental understanding of the microstructural and environmental factors triggering the corrosion. For instance, the knowledge of dissolution mechanisms during open circuit potential when the sample is exposed to a corrosive environment is essential and should be routinely integrated during coating design, to anticipate and tailor the anti-corrosion performance of every coating system.
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Gharbi, O., Thomas, S., Smith, C. et al. Chromate replacement: what does the future hold?. npj Mater Degrad 2, 12 (2018). https://doi.org/10.1038/s41529-018-0034-5
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