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Chromate replacement: what does the future hold?

Abstract

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.

Introduction

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).

Fig. 1
figure1

Compilation of several polarization tests illustrating the cathodic and anodic inhibition of chromates on: a AA2024-T3 in 1 M NaCl exposed to 10−3 M and 10−4 M Na2Cr2O7 (adapted with permission from ref. 19 copyright Electrochemical Society 2001), b 2205 duplex stainless steel in 1 M NaCl and Na2CrO4 varying from 0 to 0.5 M124 (adapted with permission from ref. 124 copyright Springer 2012) and finally c Zinc in acid rain solution exposed to 0, 10, and 50 ppm of SrCrO4 (adapted with permission from ref. 29 copyright Elsevier 2004)

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

Fig. 2
figure2

Schematic illustrations of the different inhibition scenarios associated with Cr(VI) compounds in: a aqueous solution, or when a morphological defect occurs b in the presence of a Sr–Cr-based primer, or c a Cr(VI) conversion coating, with a particular emphasis on the self-healing and barrier properties

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).

Table 1 Summary of primer and conversion coatings systems as well as their characteristics and viability as chromate replacements
Fig. 3
figure3

Several illustrations of the promising inhibition strategies reported in the literature such as a Li-containing coatings, showing that when a morphological defect occurs, Li+ and CO3 ions leach and co-precipitate to form an Al–Li-based hydroxide protective layer; b Smart coatings technologies where self-healing properties are triggered by environmental factors such as pH, moisture, or light, and finally c Mg-rich primers allowing a sacrificial cathodic protection of the underlying metal

Rare-earth-based coatings

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.

Vanadate-based coatings

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.

Lithium-containing coatings

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

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.

References

  1. 1.

    International Agency For Research on Cancer. IARC monographs on the evaluation of carcinogenic risk of chemicals to man: some inorganic and organometallic compounds (1973). http://monographs.iarc.fr/ENG/Monographs/vol1-42/mono2.pdf

  2. 2.

    Pellerin, C. & Booker, S. M. Reflections on hexavalent chromium: health hazards of an industrial heavyweight. Environ. Health Perspect. 108, A402–A407 (2000).

    Google Scholar 

  3. 3.

    Gibb, H. J., Lees, P. S. J., Pinsky, P. F. & Rooney, B. C. Lung cancer among workers in chromium chemical production. Am. J. Ind. Med. 38, 115–126 (2000).

    Article  Google Scholar 

  4. 4.

    Bidstrup, P. & Case, R. Carcinoma of the lung in workmen in the bichromates-producing industry in Great Britain. Br. J. Ind. Med. 13, 260–264 (1956).

    Google Scholar 

  5. 5.

    International Agency For Research On Cancer. IARC monographs on the evaluation of the carcinogenic risk of chemicals to humans: some metals and metallic compounds (1980).

  6. 6.

    National Toxicology Program, U. S. First Annual Report on Carcinogens (1980). ​https://ntrl.ntis.gov/NTRL/

  7. 7.

    MONOGRAPHS, I. Evaluation of carcinogenic risks to humans—chromium, nickel and welding (1990). ​http://monographs.iarc.fr/ENG/Monographs/vol49/mono49.pdf

  8. 8.

    Oliveira, H. Chromium as an environmental pollutant: insights on induced plant toxicity. J. Bot. 2012, 1–8 (2012).

    Article  Google Scholar 

  9. 9.

    Das, A. P. & Mishra, S. Hexavalent chromium (VI): environment pollution and health hazard. J. Environ. Res. Dev. 2, 386–392 (2008).

    Google Scholar 

  10. 10.

    Jacobs, J. & Testa, S. Chromium (VI) handbook 1–22 (2005).

  11. 11.

    Bartlett, R. & James, B. Behavior of chromium in soils: III. Oxidation. J. Environ. Qual. 8, 31 (1979).

    Article  Google Scholar 

  12. 12.

    The European Parliament and the Council. Regulation (EC) No 1907/2006 of the European Parliament and of the Council of 18 December 2006 concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH), Establishing a European Chemicals Agency, amending Directive 199 (2006)..

  13. 13.

    The European Commission. Regulations: Commission Implementing Regulation (EU) 2015/864 of 4 June 2015 Amending Regulation (EC) No 340/2008 on the fees and charges payable to the European Chemicals Agency pursuant to Regulation (EC) No 1907/2006 of the European Parliament (2015).​ http://eur-lex.europa.eu/eli/reg_impl/2015/864/oj

  14. 14.

    European Commission. Commission Regulation (EU) No 143/2011 of 17 February 2011 amending Annex XIV to Regulation (EC) No 1907/2006 of the European Parliament and of the Council on the Registration, Evaluation, Authorisation and Restriction of Chemicals (‘REACH’). Off. J. Eur. Union L244/6–L244/9 (2014).

  15. 15.

    Edwards, J. E. Defense Management: DOD needs to monitor and assess corrective actions resulting from its corrosion study of the F-35 Joint Strike Fighter. 84, (2010).

  16. 16.

    F-35 Lightning II. F-35 Lightning II program fact sheet selected acquisition report (SAR) 2015 Cost Data. (2017).

  17. 17.

    Kendig, M. W. & Buchheit, R. G. Corrosion inhibition of aluminum and aluminum alloys by soluble chromates, chromate coatings, and chromate-free coatings. Corrosion 59, 379–400 (2003).

    Article  Google Scholar 

  18. 18.

    Kendig, M. W. & Buchheit, R. G. Corrosion inhibition of aluminum and aluminum alloys by soluble chromates, chromate coatings, and chromate-free coatings. Corrosion 59, 379–400 (2003).

    Article  Google Scholar 

  19. 19.

    Frankel, G. S. & Mccreery, R. L. Inhibition of Al alloy corrosion by chromates. Electrochem. Soc. Interface 10, 34–38 (2001).

    Google Scholar 

  20. 20.

    Ralston, K. D., Chrisanti, S., Young, T. L. & Buchheit, R. G. Corrosion inhibition of aluminum alloy 2024-T3 by aqueous vanadium species. J. Electrochem. Soc. 155, C350 (2008).

    Article  Google Scholar 

  21. 21.

    Guan, H. & Buchheit, R. G. Corrosion protection of aluminum alloy 2024-T3 by vanadate conversion coatings. Corrosion 60, 284–296 (2004).

    Article  Google Scholar 

  22. 22.

    Iannuzzi, M. & Frankel, G. S. Mechanisms of corrosion inhibition of AA2024-T3 by vanadates. Corros. Sci. 49, 2371–2391 (2007).

    Article  Google Scholar 

  23. 23.

    Ramsey, J. D. & McCreery, R. L. In situ Raman microscopy of chromate effects on corrosion pits in aluminum alloy. J. Electrochem. Soc. 146, 4076 (1999).

    Article  Google Scholar 

  24. 24.

    Xia, L. & McCreery, R. Chemistry of a chromate conversion coating on aluminum alloy AA2024‐T3 probed by vibrational spectroscopy. J. Electrochem. Soc. 145, 3083–3089 (1998).

    Article  Google Scholar 

  25. 25.

    Ilevbare, G. O., Scully, J. R., Yuan, J. & Kelly, R. G. Inhibition of pitting corrosion on aluminum alloy 2024-T3: effect of soluble chromate additions vs chromate conversion coating. Corrosion 56, 227–242 (2000).

    Article  Google Scholar 

  26. 26.

    Hurley, B. L. & McCreery, R. L. Raman spectroscopy of monolayers formed from chromate corrosion inhibitor on copper surfaces. J. Electrochem. Soc. 150, B367 (2003).

    Article  Google Scholar 

  27. 27.

    Zhao, J., Frankel, G. & McCreery, R. L. Corrosion protection of untreated AA-2024-T3 in chloride solution by a chromate conversion coating monitored with Raman spectroscopy. J. Electrochem. Soc. 145, 2258–2264 (1998).

    Article  Google Scholar 

  28. 28.

    Zin, I., Howard, R., Badger, S., Scantlebury, J. & Lyon, S. The mode of action of chromate inhibitor in epoxy primer on galvanized steel. Prog. Org. Coat. 33, 203–210 (1998).

    Article  Google Scholar 

  29. 29.

    Baghni, I. M., Lyon, S. B. & Ding, B. The effect of strontium and chromate ions on the inhibition of zinc. Surf. Coat. Technol. 185, 194–198 (2004).

    Article  Google Scholar 

  30. 30.

    Ketchum, S. J. & Brown, S. R. Chromate conversion coatings of aluminum Alloys. Met. Finish. 74, 37–41 (1975).

    Google Scholar 

  31. 31.

    Hagans, P. L. & Haas, C. M. in Surface Engineering: Dip, Barrier, and Chemical Conversion Coatings, Vol. 5 (eds Cotell, C. M., Sprague, J. A. & Smidt, F. A. Jr.,) 405–411 (ASM International, 1994).

  32. 32.

    Scholes, F. et al. Chromate leaching from inhibited primers part I. Characterisation of leaching. Prog. Org. Coat. 56, 23–32 (2006).

    Article  Google Scholar 

  33. 33.

    Hawkins, J. K. et al. An investigation of chromate inhibitors on aluminium using fluorescence detection of X-ray absorption. Corros. Sci. 27, 391–399 (1987).

    Article  Google Scholar 

  34. 34.

    Cho, K., Shankar Rao, V. & Kwon, H. Microstructure and electrochemical characterization of trivalent chromium based conversion coating on zinc. Electrochim. Acta 52, 4449–4456 (2007).

    Article  Google Scholar 

  35. 35.

    Qi, J. T. et al. Trivalent chromium conversion coating formation on aluminium. Surf. Coat. Technol. 280, 317–329 (2015).

    Article  Google Scholar 

  36. 36.

    Li, L., Desouza, A. L. & Swain, G. M. Effect of deoxidation pretreatment on the corrosion inhibition provided by a trivalent chromium process (TCP) conversion coating on AA2024-T3. J. Electrochem. Soc. 161, 246–253 (2014).

    Article  Google Scholar 

  37. 37.

    Pearlstein, F. & Agarwala, V. S. Trivalent chromium solutions for applying chemical conversion coatings to aluminum alloys or for sealing anodized aluminum. Plat. Surf. Finish. 81, 50–56 (1994).

    Google Scholar 

  38. 38.

    Whitman, B. W., Li, L. & Swain, G. M. Anti-corrosion properties of a TCP pretreatment conversion coating on aluminum alloy 2024-T3 during moist SO2 atmospheric testing: effects of galvanic coupling. J. Electrochem. Soc. 164, 135–147 (2017).

    Article  Google Scholar 

  39. 39.

    Li, L. & Swain, M. G. Formation and structure of trivalent chromium process coatings on aluminum alloys 6061 and 7075. Corrosion 69, 1205–1216 (2013).

    Article  Google Scholar 

  40. 40.

    Qi, J. Trivalent chromium conversion coatings on Al and Al-Cu alloys. (University of Manchester, 2015). ​https://www.escholar.manchester.ac.uk/jrul/item/?pid=uk-ac-man-scw:277523

  41. 41.

    Qi, J. et al. An optimized trivalent chromium conversion coating process for AA2024-T351 alloy. J. Electrochem. Soc. 164, 390–395 (2017).

    Article  Google Scholar 

  42. 42.

    Guo, Y. & Frankel, G. S. Active corrosion inhibition of AA2024-T3 by trivalent chrome process treatment. Corrosion 68, 1–10 (2012).

    Article  Google Scholar 

  43. 43.

    Guo, Y. & Frankel, G. S. Characterization of trivalent chromium process coating on AA2024-T3. Surf. Coat. Technol. 206, 3895–3902 (2012).

    Article  Google Scholar 

  44. 44.

    Li, L., Kim, D. Y. & Swain, G. M. Transient formation of chromate in trivalent chromium process (TCP) coatings on AA2024 as probed by Raman spectroscopy. J. Electrochem. Soc. 159, C326–C333 (2012).

    Article  Google Scholar 

  45. 45.

    Qi, J. et al. Chromate formed in a trivalent chromium conversion coating on aluminum. J. Electrochem. Soc. 164, C442–C449 (2017).

    Article  Google Scholar 

  46. 46.

    Qi, J., Walton, J., Thompson, G. E., Albu, S. P. & Carr, J. Spectroscopic studies of chromium VI formed in the trivalent chromium conversion coatings on aluminum. J. Electrochem. Soc. 163, C357–C363 (2016).

    Article  Google Scholar 

  47. 47.

    Henkel. Safety data sheet alodine T 5900 RTU. (2008). http://na.henkel-adhesives.com/product-search-1554.htm?nodeid=8797998448641

  48. 48.

    Socomore. Technical data sheet annex application of SOCOSURF TCS/. 1, 1–2. http://www.socomore.com/socosurf-tcs-socosurf-pacs/pr387.html

  49. 49.

    Ely, M., Swiatowska, J., Seyeux, A., Zanna, S. & Marcus, P. Role of post-treatment in improved corrosion behavior of trivalent chromium protection (TCP) coating deposited on aluminum alloy 2024-T3. J. Electrochem. Soc. 164, 276–284 (2017).

    Article  Google Scholar 

  50. 50.

    Zhang, M. et al. Investigating DNA damage in tannery workers occupationally exposed to trivalent chromium using comet assay. Mutat. Res. 654, 45–51 (2008).

    Article  Google Scholar 

  51. 51.

    González Cid, M., Loria, D., Vilensky, M., Miotti, J. & Matos, E. Leather tanning workers: chromosomal aberrations in peripheral lymphocytes and micronuclei in exfoliated cells in urine. Mutat. Res. Toxicol. 259, 197–201 (1991).

    Article  Google Scholar 

  52. 52.

    Ambreen, K., Khan, F. H., Bhadauria, S. & Kumar, S. Genotoxicity and oxidative stress in chromium-exposed tannery workers in North India. Toxicol. Ind. Health 30, 405–414 (2014).

    Article  Google Scholar 

  53. 53.

    Eastmond, D. A., Macgregor, J. T. & Slesinski, R. S. Trivalent chromium: assessing the genotoxic risk of an essential trace element and widely used human and animal nutritional supplement. Crit. Rev. Toxicol. 38, 173–190 (2008).

    Article  Google Scholar 

  54. 54.

    Hinton, B. R. Corrosion prevention and chromates, the end of an era? Met. Finish. 89, 15–20 (1991).

    Google Scholar 

  55. 55.

    Forsyth, M. et al. Effectiveness of rare-earth metal compounds as corrosion inhibitors for steel. Corrosion 58, 953–960 (2002).

    Article  Google Scholar 

  56. 56.

    Wang, C., Jiang, F. & Wang, F. Cerium chemical conversion coating for aluminium alloy 2024-T3 and its corrosion resistance. Corrosion 60, 237–243 (2004).

    Article  Google Scholar 

  57. 57.

    Birbilis, N., Buchheit, R. G., Ho, D. L. & Forsyth, M. Inhibition of AA2024-T3 on a phase-by-phase basis using an environmentally benign inhibitor, cerium dibutyl phosphate. Electrochem. Solid-State Lett. 8, C180–C183 (2005).

    Article  Google Scholar 

  58. 58.

    Campestrini, P., Terryn, H., Hovestad, A. & de Wit, J. H. W. Formation of a cerium-based conversion coating on AA2024: relationship with the microstructure. Surf. Coat. Technol. 176, 365–381 (2004).

    Article  Google Scholar 

  59. 59.

    Hinton, B. R. W. Corrosion inhibition with rare earth metal salts. J. Alloy. Compd. 180, 15–25 (1992).

    Article  Google Scholar 

  60. 60.

    Zhang, S. H., Kong, G., Lu, J. T., Che, C. S. & Liu, L. Y. Growth behavior of lanthanum conversion coating on hot-dip galvanized steel. Surf. Coat. Technol. 259, 654–659 (2014).

    Article  Google Scholar 

  61. 61.

    Zheludkevich, M. L. et al. Nanostructured sol-gel coatings doped with cerium nitrate as pre-treatments for AA2024-T3 Corrosion protection performance. Electrochim. Acta 51, 208–217 (2005).

    Article  Google Scholar 

  62. 62.

    Montemor, M. F. et al. Evaluation of self-healing ability in protective coatings modified with combinations of layered double hydroxides and cerium molybdate nanocontainers filled with corrosion inhibitors. Electrochim. Acta 60, 31–40 (2012).

    Article  Google Scholar 

  63. 63.

    Hughes, A. E. et al. Development of cerium-based conversion coatings on 2024-T3 Al alloy after rare-earth desmutting. Surf. Interface Anal. 36, 290–303 (2004).

    Article  Google Scholar 

  64. 64.

    Behrsing, T., Deacon, G. B. & Junk, P. C. in Rare Earth-Based Corrosion Inhibitors (ed. Ltd, E.) 1–37 (Woodhead Publishing Limited, 2014).

  65. 65.

    Kolics, A., Besing, A. S., Baradlai, P. & Wieckowski, A. Cerium deposition on aluminum alloy 2024-T3 in acidic NaCl solutions. J. Electrochem. Soc. 150, B512–B516 (2003).

    Article  Google Scholar 

  66. 66.

    Humphries, M. Rare-earth elements: the global supply chain. (2013).

  67. 67.

    Haque, N., Hughes, A. E., Lim, S. & Vernon, C. Rare-earth elements: overview of mining, mineralogy, uses, sustainability and environmental impact. Resources 3, 614–635 (2014).

    Article  Google Scholar 

  68. 68.

    Brumme, A. in Wind Energy Deployment and the Relevance of Rare-Earths (Springer, Fachmedien Wiesbaden, 2014).

  69. 69.

    Bienstock, D. & Field, J. Corrosion inhibitors for hot-carbonate systems. Corrosion 17, 571t–574t (1961).

    Article  Google Scholar 

  70. 70.

    Li, J., Hurley, B. & Buchheit, R. Inhibition performance study of vanadate on AA2024-T3 at high temperature by SEM, FIB, Raman and XPS. J. Electrochem. Soc. 162, C219–C227 (2015).

    Article  Google Scholar 

  71. 71.

    Leonard, A. & Gerber, G. B. Mutagenicity, carcinogenicity and tertogenicity of vanadium compounds. Mutat. Res. 317, 81–88 (1994).

    Article  Google Scholar 

  72. 72.

    Domingo, J. L. Vanadium and diabetes. What vanadium Toxic.? Mol. Cell. Biochem. 203, 185–187 (2000).

    Google Scholar 

  73. 73.

    Ress, N. B. et al. Carcinogenicity of inhaled vanadium pentoxide in F344/N rats and B6C3F1 mice. Toxicol. Sci. 74, 287–296 (2003).

    Article  Google Scholar 

  74. 74.

    Ehrlich, V. A. et al. Inhalative exposure to vanadium pentoxide causes DNA damage in workers: results of a multiple end point study. Environ. Health Perspect. 116, 1689–1693 (2008).

    Article  Google Scholar 

  75. 75.

    Assem, F. L. & Levy, L. S. A review of current toxicological concerns on vanadium pentoxide and other vanadium compounds: gaps in knowledge and directions for future research. J. Toxicol. Environ. Health Part B 12, 289–306 (2009).

    Article  Google Scholar 

  76. 76.

    Martin, J. W. Aluminum-lithium alloys. Annu. Rev. Mater. Sci. 18, 101–119 (1988).

    Article  Google Scholar 

  77. 77.

    Lequeu, P., Smith, K. P. & Daniélou, A. Aluminum-copper-lithium alloy 2050 developed for medium to thick plate. J. Mater. Eng. Perform. 19, 841–847 (2010).

    Article  Google Scholar 

  78. 78.

    Wanhill, R. J. H. in Aluminum-Lithium Alloys: Processing, Properties, and Applications 503–535 (Elsevier Inc., 2013). https://doi.org/10.1016/B978-0-12-401698-9.00015-X

  79. 79.

    Meriç, C. Physical and mechanical properties of cast under vacuum aluminum alloy 2024 containing lithium additions. Mater. Res. Bull. 35, 1479–1494 (2000).

    Article  Google Scholar 

  80. 80.

    Prasad, K. S., Prasad, N. E. & Gokhale, A. A. Microstructure and Precipitate Characteristics of Aluminum-Lithium Alloys. Aluminum-Lithium Alloys: Processing, Properties, and Applications (Elsevier Inc. 2013).

  81. 81.

    Rangel, C. M. & Travassos, M. A. The passivation of aluminium in lithium carbonate / bicarbonate solutions. Corros. Sci. 33, 327–343 (1992).

    Article  Google Scholar 

  82. 82.

    Gui, J. & Devine, T. M. Influence of lithium on the corrosion of aluminum. Scr. Metall. 21, 853–857 (1987).

    Article  Google Scholar 

  83. 83.

    Buchheit, R. G., Wall, F. D., Stoner, G. E. & Moran, J. P. Anodic dissolution-based mechanism for the rapid cracking, preexposure phenomenon demonstrated by aluminum-lithium-copper alloys. Corrosion 51, 417–428 (1995).

    Article  Google Scholar 

  84. 84.

    Williams, G. R. & O’Hare, D. A kinetic study of the intercalation of lithium salts into Al(OH)3. J. Phys. Chem. B 110, 10619–10629 (2006).

    Article  Google Scholar 

  85. 85.

    Isupov, V. P. Intercalation compounds of aluminum hydroxide. J. Struct. Chem. 40, 672–685 (1999).

    Article  Google Scholar 

  86. 86.

    Rangel, C. M. & Travassos, M. A. Li-based conversion coatings on aluminium: an electrochemical study of coating formation and growth. Surf. Coat. Technol. 200, 5823–5828 (2006).

    Article  Google Scholar 

  87. 87.

    Drewien, C. A., Eatough, M. O., Tallant, D. R., Hills, C. R. & Buchheit, R. G. Lithium-aluminum-carbonate-hydroxide hydrate coatings on aluminum alloys: composition, structure, and processing bath chemistry. J. Mater. Res. 11, 1507–1513 (1996).

    Article  Google Scholar 

  88. 88.

    Buchheit, R. G., Bode, M. D. & Stoner, G. E. Corrosion-resistant, chromate-free talc coatings for aluminum. Corros. Sci. 50, 205–214 (1993).

    Article  Google Scholar 

  89. 89.

    Buchheit, R. G. & Stoner, G. E. Method for increasing the corrosion resistance of aluminum and aluminum alloys. 1–5 (1993).

  90. 90.

    Visser, P., Liu, Y., Terryn, H. & Mol, J. M. C. Lithium salts as leachable corrosion inhibitors and potential replacement for hexavalent chromium in organic coatings for the protection of aluminum alloys. J. Coat. Technol. Res. 13, 557–566 (2016).

    Article  Google Scholar 

  91. 91.

    Visser, P. et al. The corrosion protection of AA2024-T3 aluminium alloy by leaching of lithium-containing salts from organic coatings. Faraday Discuss. 180, 1–16 (2015).

    Article  Google Scholar 

  92. 92.

    Liu, Y. et al. Protective film formation on AA2024-T3 aluminum alloy by leaching of lithium carbonate from an organic coating. J. Electrochem. Soc. 163, C45–C53 (2016).

    Article  Google Scholar 

  93. 93.

    Hughes, A. et al. Particle characterisation and depletion of Li2CO3 inhibitor in a polyurethane coating. Coatings 7, 106 (2017).

    Article  Google Scholar 

  94. 94.

    Montemor, M. F. Functional and smart coatings for corrosion protection: a review of recent advances. Surf. Coat. Technol. 258, 17–37 (2014).

    Article  Google Scholar 

  95. 95.

    Yasakau, K. A., Tedim, J., Zheludkevich, M. L. & Ferreira, M. G. S. in Handbook of Smart Coatings for Materials Protection 224–274 (Woodhead Publishing Limited, 2014).

  96. 96.

    Bierwagen, G. P. & Tallman, D. E. Choice and measurement of crucial aircraft coatings system properties. Prog. Org. Coat. 41, 201–216 (2001).

    Article  Google Scholar 

  97. 97.

    Glover, C. F., Richards, C., Baker, J., Williams, G. & McMurray, H. N. In-coating graphene nano-platelets for environmentally-friendly corrosion protection of iron. Corros. Sci. 114, 169–172 (2017).

    Article  Google Scholar 

  98. 98.

    Pourhashem, S., Vaezi, M. R., Rashidi, A. & Bagherzadeh, M. R. Exploring corrosion protection properties of solvent based epoxy-graphene oxide nanocomposite coatings on mild steel. Corros. Sci. 115, 78–92 (2017).

    Article  Google Scholar 

  99. 99.

    Wolf, E. L. in Applications of Graphene 19–39 (springerBriens in Materials, 2014). https://doi.org/10.1007/978-3-319-03946-6

  100. 100.

    Deloitte. Graphene: research now, reap next decade. (2016). https://www2.deloitte.com/global/en/pages/technology-media-and-telecommunications/articles/tmt-pred16-tech-graphene-research-now-reap-next-decade.html

  101. 101.

    Montemor, M. F. Functional and smart coatings for corrosion protection: a review of recent advances. Surf. Coat. Technol. 258, 17–37 (2014).

    Article  Google Scholar 

  102. 102.

    Khramov, A. N., Voevodin, N. N., Balbyshev, V. N. & Mantz, R. A. Sol-gel-derived corrosion-protective coatings with controllable release of incorporated organic corrosion inhibitors. Thin Solid Films 483, 191–196 (2005).

    Article  Google Scholar 

  103. 103.

    Buchheit, R. G., Guan, H., Mahajanam, S. & Wong, F. Active corrosion protection and corrosion sensing in chromate-free organic coatings. Prog. Org. Coat. 47, 174–182 (2003).

    Article  Google Scholar 

  104. 104.

    Andreeva, D. V. & Skorb, E. in Handbook of Smart Coatings for Materials Protection 307–327 (Woodhead Publishing Limited, 2014).

  105. 105.

    Andreeva, D. V., SkorbE. V. & ShchukinD. G. Layer-by-layer polyelectrolyte/inhibitor nanostructures for metal corrosion protection. ACS Appl. Mater. Interfaces 2, 1954–1962 (2010).

    Article  Google Scholar 

  106. 106.

    AkzoNobel Aerospace Coatings. Material safety data sheet - 10P2013, High Solids Epoxy Primer (2013). ​https://www.e-aircraftsupply.com/MSDS/19243210P20-13%20MSDS.pdf

  107. 107.

    Bender, H., Dale Cheever, G. & Wojtkowiak, J. J. Zinc phosphate treatment of metals. Prog. Org. Coat. 8, 241–274 (1980).

    Article  Google Scholar 

  108. 108.

    Ogle, K., Tomandl, A., Meddahi, N. & Wolpers, M. The alkaline stability of phosphate coatings I: ICP atomic emission spectroelectrochemistry. Corros. Sci. 46, 979–995 (2004).

    Article  Google Scholar 

  109. 109.

    Jiang, L., Wolpers, M., Volovitch, P. & Ogle, K. The degradation of phosphate conversion coatings by electrochemically generated hydroxide. Corros. Sci. 55, 76–89 (2012).

    Article  Google Scholar 

  110. 110.

    Chen, X., Zhou, X., Abbott, T. B., Easton, M. A. & Birbilis, N. Double-layered manganese phosphate conversion coating on magnesium alloy AZ91D: Insights into coating formation, growth and corrosion resistance. Surf. Coat. Technol. 217, 147–155 (2013).

    Article  Google Scholar 

  111. 111.

    Ke, C. et al. Protective strontium phosphate coatings for magnesium biomaterials. Mater. Sci. Technol. 836, 521–526 (2017).

    Google Scholar 

  112. 112.

    Jokiel, P., Uebleis, A., Boehni, H. & Weng, D. Corrosion and protection characteristics of zinc and manganese phosphate coatings. Surf. Coat. Technol. 88, 147–156 (1996).

  113. 113.

    Thomas, S., Birbilis, N., Venkatraman, M. S. & Cole, I. S. Self-repairing oxides to protect zinc: review, discussion and prospects. Corros. Sci. 69, 11–22 (2013).

    Article  Google Scholar 

  114. 114.

    King, A. D., Kannan, B. & Scully, J. R. Environmental degradation of a mg-rich primer in selected field and laboratory environments: Part 1-without a topcoat. Corrosion 70, 512–535 (2014).

    Article  Google Scholar 

  115. 115.

    Twite, R. L. & Bierwagen, G. P. Review of alternatives to chromate for corrosion protection of aluminum aerospace alloys. Prog. Org. Coat. 33, 91–100 (1998).

    Article  Google Scholar 

  116. 116.

    Nanna, M. E. & Bierwagen, G. P. Mg-rich coatings: a new paradigm for Cr-free corrosion protection of Al aerospace alloys. J. Coat. Technol. Res. 1, 69–80 (2004).

    Article  Google Scholar 

  117. 117.

    Maier, B. & Frankel, G. S. Behavior of magnesium-rich primers on AA2024-T3. Corrosion 67, 55001-1-55001–15 (2011).

  118. 118.

    King, A. D. & Scully, J. R. Sacrificial anode-based galvanic and barrier corrosion protection of 2024-T351 by a Mg-rich primer and development of test methods for remaining life assessment. Corrosion 67, 1–22 (2011).

    Article  Google Scholar 

  119. 119.

    King, A. D., Kannan, B. & Scully, J. R. Environmental degradation of a mg-rich primer in selected field and laboratory environments: part 1-without a topcoat. Corrosion 70, 512–535 (2014).

    Article  Google Scholar 

  120. 120.

    Bierwagen, G., Battocchi, D., Simões, A., Stamness, A. & Tallman, D. The use of multiple electrochemical techniques to characterize Mg-rich primers for Al alloys. Prog. Org. Coat. 59, 172–178 (2007).

    Article  Google Scholar 

  121. 121.

    Abreu, C. M., Izquierdo, M., Merino, P., Nóvoa, X. R. & Pérez, C. A new approach to the determination of the cathodic protection period in zinc-rich paints. Corros. Eng. Sect. 55, 1173–1181 (1999).

    Article  Google Scholar 

  122. 122.

    Battocchi, D., Simões, A. M., Tallman, D. E. & Bierwagen, G. P. Electrochemical behaviour of a Mg-rich primer in the protection of Al alloys. Corros. Sci. 48, 1292–1306 (2006).

    Article  Google Scholar 

  123. 123.

    King, A. D., Kannan, B. & Scully, J. R. Environmental degradation of a mg-rich primer in selected field and laboratory environments: part 2-Primer and Topcoat. Corrosion 70, 536–557 (2014).

    Article  Google Scholar 

  124. 124.

    Luo, H., Dong, C. F., Cheng, X. Q., Xiao, K. & Li, X. G. Electrochemical behavior of 2205 duplex stainless steel in NaCl solution with different chromate contents. J. Mater. Eng. Perform. 21, 1283–1291 (2012).

    Article  Google Scholar 

  125. 125.

    Visser, P. & Hayes, S. A. Anti-corrosive coating composition WO 2010112605 A1. (2009).

  126. 126.

    Henckens, M. L. C. M., van Ierland, E. C., Driessen, P. P. J. & Worrell, E. Mineral resources: geological scarcity, market price trends, and future generations. Resour. Policy 49, 102–111 (2016).

    Article  Google Scholar 

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All authors contributed to the manuscript, O.G. drafted the paper. S.T., provided a review on organic coatings and edited the manuscript. C.S. reviewed the manuscript and N.B. reviewed, edited and modified the manuscript.

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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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