With the demand for a safer alternative, there is no question that there is a need to define and test a process that can readily be industrialized and can replace hexavalent chromium conversion coatings.
The process needs to be able to provide a similar level of corrosion protection to that currently provided by Cr(VI)-containing treatments. However, it also needs to be devoid of the environmental complications associated with Cr(VI). It must comply with the European directives including REACH (Registration, Evaluation, Authorisation of Chemicals) (Official Journal of the European Union, L396/1 30.12.2006), RoHS (Removal of Hazardous Substances) (Official Journal of the European Union, L37/19 13.2.2003), WEEE (Waste Electrical and Electronic Equipment) (Official Journal of the European Union, L197/38 24.7.2012), and ELV (End of Life Vehicles) (Official Journal of the European Union, L269/34 13.2.2003 on end-of life vehicles (ELV)), as well as the new regulations from OSHA (Occupational Safety and Health Administration) in the USA (Federal Registry, 2006).
Not only must the Cr(VI) alternative be able to protect the aluminum substrate, but it also needs to be adaptable to an industrial process. In practice, it is desirable to develop a “drop-in process” that can easily replace the current procedure using hexavalent chromium.
The US Military has developed and rigorously tested a non-toxic and non-restricted replacement for Cr(VI) using a Cr(III) alternative process (Nickerson & Matzdorf, 2012). The Naval Air Systems Command (NAVAIR) patented their trivalent, Cr(III), chromium conversion coating chemistry for aluminum (Matzdorf et al., 2002; Matzdorf & Nickerson, 2003). The NAVAIR trivalent chromium pretreatment (TCP) may also be referred to in the literature as trivalent chromium process. NAVAIR’s TCP complies with the OSHA and European directives, and it has been licensed to a number of manufacturers, including SurTec International GmbH; Metalast International, Inc.; Henkel Surface Technologies; and Luster-On Products Inc. (Price, 2007).
This paper reviews the data presented in the literature. Where possible, it also seeks to confirm published data by locating similar tests carried out by independent groups. It should also be noted that while the commercially available pretreatments from the NAVAIR Licensees may not have identical chemistries (Matzdorf, 2014), one of the inventors of TCP at NAVAIR indicated that they behave in a similar fashion. As a result, this justifies comparing data generated from NAVAIR TCP with equivalent tests carried out on pretreatments from one of the TCP licensees.
As the pretreatment was patented and tested by the US Military, this paper unavoidably covers a number of pretreatments and coatings in use by the military, and the main focus is on AA2024-T3, as it is one of the more difficult alloys to protect. Finally, this paper is focused on the ability of a conversion coating to protect against corrosion and does not consider in detail dry or wet tape adhesion characteristics, or electrical resistance tests, which may be part of a military standard.
2 Pretreatment chemistry
A number of different treatments have been investigated by researchers from various organizations. In general, the non-hexavalent treatment (either Cr(III) or Cr-free) (Palomino et al., 2006; Monetta et al., 2013; Mirovsky et al., 2014; Belsanti et al., 2016) is compared relative to a traditional hexavalent chromium conversion coating treatment. Ideally, the non-chromate treatment should provide protection that is equivalent or better than that provided by the hexavalent conversion coating. The Cr(III) pretreatments covered in this review are certainly not the only available. In fact, some of the first works in this area were done in the early 90s by Pearlstein and Agarwala (1994a,b, 1997). This review focuses primarily, but not exclusively, on the TCP developed by NAVAIR, as well as its commercially available variants. The various pretreatments discussed in this paper have been listed in Table 1.
Together with the Cr(III) pretreatments, two chromium-free pretreatments have also been included in this review, namely, Alodine 5200/5700 and Boegel (AC-131). Although Cr(III) is not currently restricted, it still contains chromium. Conceivably, future restrictions could decrease the safe exposure limit or place stringent demands on Cr(III) disposal protocols. In addition, there is considerable evidence within the published literature to suggest that Cr(III) can be oxidized to Cr(VI) in small quantities (Rochester & Kennedy, 2007; Li et al., 2012a,b; Qi et al., 2015, 2016a).
Although the pretreatment chemistries tend to be proprietary, an indication of the main components has been provided below and summarized in Table 2. Alodine 5900, Metalast TCP-HF, SurTec 650, and Aluminescent should have similar chemistries, as they are the NAVAIR Licensee products. The companies who were given the NAVAIR licenses were required to submit their own versions of the conversion coating within the limitation of the basic chemical formulation (Price, 2007), so the chemistries of the various TCP products are not likely to be identical but are known to behave in a similar fashion to TCP (Matzdorf, 2014).
2.1 Alodine 1200
Alodine 1200 is a hexavalent chromium-based pretreatment from Henkel Corporation. The final solution is mixed from a powder and can be applied by immersion, spray, or wipe. Depending on the substrate alloy, the color ranges from light tan to gold. It has been used as the control standard by many researchers during testing of Cr(VI) alternatives. The primary components are chromic acid, complex fluorides, and ferric compounds (Nickerson & Matzdorf, 2012).
2.2 Alodine 5200/5700
This is a chromium-free pretreatment from Henkel Corporation. Alodine 5700 is the ready-to-use or pre-mixed version of Alodine 5200. It is specifically formulated for treating aluminum and aluminum alloys and is reported to provide an excellent base for organic finishes (Henkel Surface Technologies Technical Process Bulletin No. 1722 2001). It can be applied by spray or immersion. Depending on the substrate alloy, the color ranges from light blue to tan. The main components include an organometallic zirconate complex (Nickerson & Matzdorf, 2012).
2.3 Boegel (AC-131)
Boegel uses Boeing technology but is marketed by Advanced Chemistry and Technology as AC-131. Boegel is currently available through 3M. Boegel is a non-chromated sol-gel treatment. It is included in this review, as it is a product that was developed directly by the aeronautics industry (The Boeing Company). It is currently used by Boeing on both commercial and military aircraft. The coating can be applied by spray, wipe, brush, or dipping. It is dried in place and produces a colorless coating. It is designed to improve the performance of an organic coating by increasing adhesion between the coating and substrate alloy. Boegel does not provide stand-alone corrosion protection so it is not suitable for unpainted parts. Components include organosiloxanes and zirconates (Nickerson & Matzdorf, 2012).
2.4 NAVAIR TCP
Trivalent chromium pretreatment (TCP) can be applied similarly to conventional hexavalent treatments and is designed to improve corrosion resistance and adhesion. Components include Chromium III salt and potassium hexafluorozirconate (Nickerson & Matzdorf, 2012). The following four Cr(III) pretreatments are commercially available versions of the NAVAIR TCP.
2.5 Alodine 5900
Alodine 5900 is the Henkel Surface Technologies’ version of the NAVAIR TCP. Alodine 5900 is a complex trivalent chromium conversion coating formulated specifically for treating aluminum alloys. It can be applied by spray or immersion. The manufacturer indicates that neither the product itself nor the conversion coating developed by the process contains hexavalent chromium. Alodine 5900 contains potassium hexafluorozirconate, sulfate, fluoroboric acid, and Cr(III) salts (Li et al., 2011).
2.6 Metalast TCP-HF
Metalast TCP-HF is the Metalast International, Inc. version of the NAVAIR TCP, and HF is the acronym for hexavalent chromium-free. Metalast TCP-HF is now marketed as Chemeon TCP-HF by Chemeon Surface Technology. It is a trivalent chromium replacement for hexavalent chromium. It contains chromium sulfate, basic solution (Safety Data Sheet, 2016).
2.7 SurTec 650
SurTec 650 is the NAVAIR TCP product from SurTec International GmbH. The manufacturers indicate that it is a hexavalent chromium-free pretreatment that is RoHS and REACH compliant. It can be applied by immersion or spray, and it contains dipotassium hexafluorozirconate as main component (MSDS SurTec650, 2007).
Aluminescent is the NAVAIR TCP product from Luster-on Products, Inc. It is a powdered product that forms a non-hexavalent chromium conversion coating on aluminum. It can be applied by dip, spray, brush, swab, or roll coating, producing a clear, slightly iridescent film with low electrical resistance. Aluminescent is compliant with European RoHS, ELV, and WEEE directives and is listed on the QPL for MIL-DTL-5541F (2006), Type II, Class 1A, and Class 3 (Lane et al., 2012).
3 Formation of trivalent coatings
This section discusses the concepts developed by various researchers in regard to the formation of a Cr(III) coating on aluminum substrates. The researchers mentioned in this section have worked independently and have developed ideas that are not identical.
TCP coatings formed during immersion develop through multiple chemical steps. The formation appears to be associated with a pH increase at the interface. The initial step is believed to be the dissolution of the air-formed (native) oxide layer (Li et al., 2011). It is supposed that the oxygen reduction reaction (ORR) and possibly the hydrogen evolution reaction (HER) result in a local increase in pH at cathodic sites, which induces precipitation from the bath. It is generally assumed that some dissolution of the air-formed oxide film to expose the bare aluminum is an essential precursor during the formation of the TCP coating. The as-formed coating consists primarily of Cr(III) species such as Cr(OH)3, Cr2O3, and CrOOH. Li et al. (2013a) were able to show definitively that there is a significant pH increase during the formation of two different pretreatment conversion coatings – one, a Cr(III) coating (Alodine 5900) and the other, a Non-Cr system (Alodine 5200). A tungsten microelectrode was used to measure the interfacial pH change, and it was reported that the pH value increased from 3.9 to 4.8 in the case of the Cr(III) TCP and from 2.5 to 6.9 for the non-Cr treatment.
Research undertaken by Li et al. (2011) also suggests that the Cr(III) film forms on most areas of the aluminum alloy but not over all the Cu-rich intermetallics. Others (Nickerson & Lipnickas, 2003) have suggested that the high potential difference at Al-Cu in AA2024-T3 could inhibit the deposition of the film at these cathodic sites. Guo and Frankel (2012a) indicated that the Cr(VI) coating on the surface of AA2024-T3 has a non-uniform thickness and that it is known to be thinner above intermetallics. They suggested that further work should be done to evaluate if this is the case for TCP, especially above the Cu-rich intermetallics. Interestingly, Qi et al. (2016b) reported that in the early stages of the coating process, the thickest regions are above the S-phase particles, which undergo rapid dealloying after immersion in the acidic coating bath. Grilli et al. (2011) investigated the corrosion behavior of aluminum alloy 2219 treated with Alodine 1200 (hexavalent), concluding that the chromate coating is thinner and more defective on the second-phase particles.
Although no Cr(VI) species were detected immediately after coating formation (or in the TCP solution) (Li et al., 2011), Cr(VI) was sometimes observed after less than 1 h of air drying and in all coatings after immersion in air saturated NaCl or Na2SO4. Interestingly, they observed that the Cr(III) and Cr(VI) oxide peaks are not distributed uniformly over the entire TCP-coated surface but rather appear to be localized in and around pits (Figure 1) (Li & Swain, 2013). Qi et al. (2016b) reported that the conversion coating on AA2024-T351 initially develops at approximately 0.23–0.27 nms−1 during the first 120 s. However, development subsequently decreases to approximately 0.04–0.05 nms−1.
4 Structure of trivalent chromium coatings
Early work by Nickerson and Lipnickas (2003) indicated that the NAVAIR TCP is essentially a zirconium and oxygen film with an embedded hydrated trivalent chromium oxide inhibitor species. Zirconium oxide coating alone has very little corrosion protection; however, by adding small amounts of trivalent chromium oxides and hydroxides, a non-toxic, non-carcinogenic conversion coating is formed. A coating model based on zirconium oxide and Cr(III) species was suggested for the trivalent chromium coating. These authors propose a three-layer structure. A simplified version of the TCP structure proposed by Nickerson and Lipnickas (2003) is presented in Figure 2A. It shows an aluminum/coating interface having Al/O/F. The central layer is composed of zirconium and chromium oxides, and the external layer is made up of Zr/Cr/O/F.
Guo and Frankel (2012a) indicated that the TCP (Alodine 5900) coating has a bi-layered structure primarily composed of Zr oxide with a small amount of Cr(III). This is shown schematically in Figure 2B. She observed that there was an aluminum oxide and/or oxyfluoride at the interface between the AA2024-T3 substrate and the TCP coating. Guo and Frankel (2012a,b) report that the thickness of the layer depends on conversion time, with the film being 40–70 nm after 10 min.
The trivalent coating can be seen as a zirconium-based oxide that contains some amount of chromium (Guo & Frankel, 2012a). The coating was also found to display a two-layer structure, with an aluminum oxide or oxyfluoride inner layer and the outer layer being composed of zirconium-chromium oxide (Figures 3 and 4) (Guo & Frankel, 2012a). Dong et al. (2010, 2011) also reported a two-layer structure with a dense layer at the aluminum interface with the TCP coating. Guo and Frankel (2012a) noted that the thickness of TCP was not the same at discrete locations on the heterogeneous microstructure of AA2024-T3.
These findings are confirmed by Qi et al. (2015), who carried out research on high-purity electropolished aluminum treated with SurTec 650. The data indicate a chromium- and zirconium-rich outer layer in combination with an aluminum-rich inner layer (Figure 5) (Qi et al., 2015). They propose that the protective characteristics of the coating are primarily from the inner layer. The absence of diffraction patterns also suggests that the coating is amorphous. Coating thickness was determined from multiple measurements along the length of ultramicrotome cross-sections and found to be about 93 nm after a 600-s immersion. This is in reasonable agreement with the work by Guo and Frankel (2012a). According to Qi et al. (2015), the trivalent chromium conversion coating formed on the high-purity aluminum consists of two main layers. The outer layer, which constitutes most of the coating thickness, consists of AlF3, Al2O3, AlOxF, Cr(OH)3, CrF3, Cr2(SO4)3, ZrO2, and ZrF4 species. The inner layer is aluminum-rich, with the presence of oxide and fluoride species. They believe that the inner coating layer provides the main corrosion protection.
In a subsequent work using TEM and EDS (Qi et al., 2016b), Qi et al. observed a three-layer structure with a relatively thick Zr-Cr-rich layer sandwiched between inner and outer aluminum-rich layers. They also reported Cu enrichment in a thin alloy layer beneath the coating and that local regions of corrosion appear to be associated with the copper layer (Figure 6) (Qi & Thompson, 2016). RBS and NRA confirmed copper enrichment.
Qi et al. (2016b) also reported that the rate of film development decreases as the coating becomes thicker, possibly as a result of reduced transport of species across the thickening coating. The average rate of film development during the first 300 s was determined to be approximately 0.27 nms−1, with a subsequent reduction in the average rate of film development to 0.08 nms−1 for the period of time between 300 and 600 s. Chromium and zirconium appear to be distributed throughout the coating thickness, with a thin, fluorine-rich layer (also containing aluminum and oxygen), at the interface with the substrate. These results are in reasonable agreement with those of Guo and Frankel (2012a), who found a layer of aluminum oxide and/or oxyfluoride at the interface between the TCP coating and the aluminum substrate.
While the various structures proposed are not identical, there is a qualitative agreement. All report an aluminum-fluoride-oxygen inner layer. The outer layer [or outer two layers in the case of Nickerson and Lipnickas (2003)] consists of zirconium/chromium (III)/oxygen. Both Nickerson and Qi et al. (2016b) reported the presence of fluoride in the outer layer, but Guo and Frankel (2012a,b) did not. Qi observed the presence of sulfur throughout the depth of the coating, but neither Nickerson nor Guo reported this. Qi attributed the sulfur to sulfur in the coating bath.
Dardona and Jaworowski (2010) employed in situ spectroscopic ellipsometry to monitor the development of a TCP film in real time. The film was developed on pure (99.998%) aluminum using 15% SurTec 650. The authors were able to confirm that the chemical thinning of the native oxide is a requirement for the initiation of TCP film formation on aluminum. The initial reduction in the thickness of the native oxide would increase the probability of electron tunneling as well as the electric field across the oxide enabling the migration of Al ions. In addition, they were able to identify three distinct periods during film development: (i) an initial induction period of approximately 100 s. During this period, there is no significant growth of the film and the authors assume the chemical thinning of the native oxide occurs during this time; (ii) a linear growth period during which the TCP film develops at a constant rate of 0.4 nms−1 to a thickness of about 50 nm; and finally, (iii) a logarithmic growth period to an ultimate film thickness of 125 nm after 880 s.
5 The effect of pretreatment and post-treatment
Li et al. (2011) hypothesized that TCP coatings may have imperfections such as pores or cracks that could act as pathways similar to the traditional hexavalent counterpart. They assumed that there could be some benefit from aging, as the film dehydrates and the channels partially collapse. They tested their theory by aging TCP-coated specimens in air for 1–4 days. They reported that some, but not all, specimens exhibited a significant improvement in the coatings barrier properties.
In subsequent work, Li and Swain (2013) studied the effect of aging temperature and time on the physical structure and corrosion protection of TCP coatings on AA2024-T3. They used Alodine 5900 to form the coating and found that there was a beneficial effect to aging the TCP coating overnight at elevated temperatures up to approximately 100°C. The coating was found to undergo dehydration, to condense, and to become more hydrophobic – in combination, these changes contribute to an improvement in corrosion resistance. Aging for longer periods of time (7 days) at ambient temperatures also improved the corrosion resistance; however, there is a deleterious effect to aging the coating at a higher temperature (150°C). They observed that the coating suffered severe cracking and even detachment in some cases.
A number of water immersion post-treatment protocols were also evaluated by Qi et al. (2016a). They tested post-treatment water baths with (i) no water treatment, (ii) water treatment at 20°C, and (iii) water treatment at 40°C. They report that the specimen that did not receive post-treatment immersion displayed detachment at the base of the coating. The specimen that was treated at 20°C exhibited enhanced corrosion at the base of the coating. The authors attribute this to fluorine enrichment at the base of the coating. The specimen that received an immersion treatment in deionized water at 40°C had an intact cross-section and no indication of localized corrosion. In addition, the authors report that the 40°C bath resulted in a coating that was thicker than the other treatments.
6 Electrochemical behavior of TCP-coated aluminum
A number of researchers have assessed the corrosion behavior of various TCP-coated aluminum alloys using electrochemical techniques.
Dong et al. (2011) coupled results from both electrochemistry and neutron reflectivity (NR) to explain the behavior of the TCP film when exposed to a chloride-containing aqueous environment. To accomplish this, they employed a split cell design where the working electrode was a Si wafer coated with a thin layer (about 1000 Angstroms) of aluminum. The counter electrode was a Si wafer coated with a thin layer of gold. A reference electrode (SCE) was positioned between the Al and Au layers, and the cell was filled with a NaCl-D2O electrolyte.
They measured the OCP of the TCP-treated aluminum by means of a potentiodynamic scan and determined that the OCP was approximately −1.000 V vs. SCE. Potentiostatic testing in conjunction with NR was carried out at various anodic potentials between −0.975 V and −0.725 V. They observed that the TCP film was stable and protective in the aqueous chloride-containing environment at anodic potentials up to approximately −0.800 V (200 mV anodic of the OCP). They attributed the passivity to the inhibition of ionic transport across the TCP film. At a potential of −0.800 V, an increase in current fluctuation in the potentiostatic curve was observed, and at −0.775 V, the TCP film swelled and the underlying aluminum layer dissolved.
Li et al. (2011) studied the formation of the TCP coating on AA2024-T3. They presented open circuit potential (OCP) data during the formation of the coating. They noted that within the initial 50–75 s, the OCP displayed a negative shift of more than 400 mV and related this to the dissolution of the air-formed aluminum oxide. After a minimum, the OCP increased slightly to achieve a quasi-steady state. They repeated the test with a thicker oxide and noted that the cathodic shift in OCP was slower, but the final OCP values for the thick surface oxide and the thinner surface oxide were similar. They concluded that, once the oxide is dissolved, the TCP formation likely proceeds in an identical manner in both cases.
Qi et al. (2015) reported similar behavior of the OCP in naturally aerated solutions for the SurTec 650. Initially, a rapid decrease in the OCP was observed, followed by a gradual rise to a relatively stable value. Qi also proposed that the initial fall in potential is due to thinning of the native oxide film. In Qi’s case, the initial rapid decrease in potential was 350 mV. This is comparable to that reported by Li et al. (2011) (400 mV), despite the fact that the research by Li is on AA2024-T3 and Qi’s data are for a high-purity electropolished aluminum; in addition, the research was carried out with two different TCP solutions – Qi used SurTec 650 and Li used Alodine 5900.
Potentiodynamic curves (Li et al., 2011) for TCP-coated and TCP-uncoated specimens carried out in 0.5 m Na2SO4 revealed that both the anodic and cathodic currents were substantially attenuated for the TCP-coated specimen. The Rp value was inversely proportional to the corrosion rate, so as the Rp increases, the corrosion rate decreases. The Rp values found for TCP-coated specimens were approximately 10 times that of the non-TCP coated specimen – indicating that the TCP is providing some protection to the alloy.
Potentiodynamic polarization curves for AA2024-T3 in aerated dilute Harrison’s solution [0.35 wt.% (NH4)2SO4 and 0.05 wt.% NaCl in distilled water] reported by Guo and Frankel (2012a) indicated that the breakdown potential (Eb) for the surface treated with TCP was 200 mV higher than the non-coated specimen; however, similar behavior was observed for a non-TCP-coated specimen that had undergone desmutting (acid treatment). They suggest that this indicates desmutting, which is a critical step for improving corrosion performance. This observation would seem to agree with findings that indicate that the corrosion protection provided by TCP is sensitive to the pretreatment process.
The cathodic polarization curves indicate that the ORR on the AA2024-T3 substrate is suppressed by the presence of the TCP coating. This is in agreement with early work that was carried out by Nickerson and Lipnickas (2003), who observed that the trivalent chromium coating inhibits the initiation of corrosion through suppression of the ORR.
The electrochemical properties of AA6061 and AA7075 coated with TCP (Alodine 5900) were studied by Liangliang Li et al. (2013b). Both alloys were assessed in 0.5 m Na2SO4 and 0.5 m Na2SO4 plus 0.05 m NaCl at ambient temperature. Specimens without TCP were tested immediately after they were prepared, while TCP-coated specimens were permitted to age overnight in ambient laboratory atmosphere.
The presence of the TCP coating on AA6061 in 0.5 m Na2SO4 resulted in a 500-mV shift in the negative direction for Ecorr. In close proximity to Ecorr, the cathodic current was attenuated by approximately 10 times and the anodic current was attenuated by about five times with regard to the uncoated specimen. A much smaller difference in the change in Ecorr was observed for the AA7075 specimen. Nevertheless, both the anodic and cathodic currents were attenuated by about 10 times for the TCP-coated specimen with respect to the uncoated specimen. Results in the Cl− containing electrolyte reveal that both the anodic and cathodic currents are suppressed at potentials close to Ecorr. In addition, these alloys are susceptible to pitting in Cl− containing electrolytes. The bare AA6061 and AA7075 alloys exhibit pitting potentials (Epit) of −0.343 V and −0.269 V (Ag/AgCl), respectively, and pitting is initiated at potentials slightly positive to Epit for both alloys. Conversely, the specimens treated with TCP reveal an Epit value that is shifted in the positive direction by about 400 mV compared to the bare specimens. In the case of AA6061, the TCP specimen did not show any sign of pitting up to the anodic limit of the test of 0.5 V (1.2 V vs. OCP).
In a recent paper, Li et al. (2016) investigated the corrosion of uncoated AA2024-T3 and compared it to that seen for comparable specimens treated with either the non-chromated process (NCP) (a non-chromium, Zr and Zn based coating) or the trivalent chromium process (TCP). They carried out a series of tests, which included full immersion, salt spray, beach exposure, and electrochemistry. While the electrochemical data suggested that TCP and NCP provided a similar reduction in corrosion, the other tests (immersion, salt spray, and beach exposure) all revealed that TCP outperformed both the NCP and the uncoated specimens. NCP was seen to reduce the corrosion by a factor of 2, and TCP was observed to provide a factor of 10 improvement. The disparity in the results emphasizes the need for corroborating results from multiple techniques during research.
7 The presence of Cr(VI) in trivalent chromium coatings
Iyer et al. (2010) characterized the trivalent chromium process (TCP) developed by NAVAIR concentrating on the determination of the presence and level of Cr(VI). They performed tests on unpainted aluminum substrates, as-deposited coated aluminum panels, corroded panels, and both new and depleted coating solutions. The test methods included Auger electron spectroscopy, diphenyl carbazide, and UV-visible spectroscopy. Their tests revealed no detectable Cr(VI) on TCP-coated panels or in TCP solutions. However, they did detect the likely presence of Cr(VI) on TCP-coated AA2024 substrates when specimens were exposed to very severe acids. They suggest that this may be a transient form of Cr(VI).
A comparative study was carried out by Suib et al. (2009) to determine hexavalent chromium levels on AA2024-T3 coated with TCP. The experiments used only commercially licensed TCP from SurTec International and Henkel Surface Technologies. Coated TCP specimens were exposed to accelerated neutral salt spray following ASTM B117 (2011) or salt spray with periodic infusion of SO2 gas in accordance with ASTM G85-11 (2011). Tests were run for up to 744 h. Analysis for Cr(VI) in the TCP coating before and after exposure revealed no detectable Cr(VI). Analysis was done by leaching the specimens in boiling water and detecting the Cr(VI) in the leachate by the diphenyl carbazide reaction. In addition, they tested the TCP process solution but were unable to detect any Cr(VI). Chen et al. (2012) and Powers (2008) were also not able to detect the presence of Cr(VI) in their Cr(III) coatings.
This is not the case with a number of other authors. As early as 2007, Rochester and Kennedy (2007) reported observing Cr(VI) formation on Cr(III) coatings for specimens exposed to salt spray testing, indicating that a salt spray environment was oxidizing enough to oxidize Cr(III)–Cr(VI). Liangliang Li et al. (2012a,b) used Raman spectroscopy to study the transient formation of Cr(VI) for a TCP coating on AA2024-T3. They report that formation of Cr(VI) occurs when the coated alloy is immersed in air-saturated electrolyte solutions. They also report that Cr(VI) can form after exposure to humid air. They present evidence to support a two-step process in which oxygen diffuses to the copper intermetallic sites where it forms hydrogen peroxide. H2O2 is a strong oxidant and is able to oxidize Cr(III) to Cr(VI). For specimens exposed to air, Cr(VI) is localized near intermetallics. For specimens exposed to a solution, hydrogen peroxide can diffuse to other sites and oxidize Cr(III). They suggested that Cr(VI) may then be reduced to Cr(III) to form a passive layer.
Li et al. (2012a,b) also suggested that the presence of trace hexavalent chromium on trivalent chromium conversion coatings may develop during exposure to moist air. In addition, they propose that higher bath pH and immersion time may accelerate the development of hexavalent chromium. Qi et al. (2015) suggest the presence of Cr(VI) following Raman spectroscopy. While the research by Li et al. (2012a,b) suggests the formation of hydrogen peroxide generated by the reduction of oxygen at copper-rich particles, the study by Qi provides evidence for the formation of Cr(VI) species in the absence of copper-rich, second-phase particles in the substrate. Another paper by Li et al. (2012a,b) reported the finding of some Cr(VI) oxide species. These Cr(VI) species apparently do not form immediately but were found to be in some coatings after an hour of air drying and in all specimens after exposure to air-saturated Na2SO4 or NaCl. Qi et al. (2016a) also assessed the amount of Cr(VI) in fresh water baths as well as in water baths that had been used to provide post-treatment at 20°C and 40°C of specimens treated with Surtec 650 chromitAl. They were not able to locate any evidence of Cr(VI) in the water baths using 1,5-diaminonaphthalene, which turns a violet color when Cr(VI) is present.
8 Active healing characteristics of TCP
One of the more important benefits of hexavalent chromium conversion coatings is the phenomenon of active healing. A hexavalent conversion coating is mainly composed of Cr(III) and Cr(VI) species. The coatings contain a reservoir of soluble Cr(VI), which can migrate to damaged areas and then reduce to an insoluble Cr(III) species and inhibit corrosion.
In an early work, Nickerson and Lipnickas (2003) indicated that there was much less self-healing characteristic for the trivalent coating than there was for a hexavalent control specimen. Certainly, it would seem that the reservoir of soluble Cr(VI) available for self-healing would be less in a TCP, but there does appear to be some self-healing.
Guo and Frankel (2012a,b) were able to demonstrate active healing of TCP through research carried out using an artificial scratch cell. Their goal was to employ an artificial scratch technique to assess and quantify the self-healing properties of TCP on AA2024-T3. The cell design included exposing two parallel AA2024-T3 specimens separated by an o-ring containing a corrosive electrolyte. One of the AA2024-T3 specimens was treated with TCP, and the other was untreated. Although the treated and untreated specimens are not in physical contact, results reveal the presence of Cr species on the untreated specimen, showing that the TCP coating can release chromium, which can then transport to an adjacent untreated region. In addition, the authors were also able to measure the polarization resistance (Rp) for an untreated surface near a TCP-coated surface. Rp is inversely proportional to the corrosion rate, and the authors report that Rp was approximately twice that of a specimen exposed to the same solution but not in proximity to a TCP-coated surface. This strongly suggests that the Cr released from the TCP-coated surface can not only be transported to a bare region but also reduce the corrosion in that region.
In the case of non-hexavalent chromium coatings, the level of Cr(VI) detected by the various researchers is low and, even when it is observed, it is believed to be a transient effect. However, there may be a benefit to the possibility of Cr(VI) formation in this way. As a result of the transient formation of Cr(VI), it is believed that the TCP coating provides some active corrosion protection. The soluble Cr(VI) appears to migrate to damaged areas of the coating and to then be reduced to an insoluble Cr(III) species, which may provide corrosion protection to that area. In other words, the Cr(III) coatings may provide some self-healing similar to that observed for a hexavalent coating.
9 Technological standards and procedures for preparation and testing
A typical military coating system will include a pretreatment (conversion coating), primer, and a topcoat. The primer may, or may not, contain hexavalent chromium, depending on the requirements of the military standard used. As a consequence of the development of the various non-hexavalent pretreatments, a number of Military Standards have been updated to include a Type II, non-hexavalent, treatment.
Table 3 lists some military designations for primers and topcoats and gives their main components. Primers include high solids epoxy (with Cr(VI)) (MIL-PRF-23377J, 2006), waterborne epoxy (with Cr(VI)) (MIL-PRF-85582C, 2012), waterborne epoxy (no Cr(VI)) (MIL-PRF-85582N, 2012), water-reducible epoxy (MIL-P-53030, 1992), and epoxy (MIL-DTL-53022, 2010). The topcoats include high solids polyurethane (MIL-PRF-85285E, 2012) and single component polyurethane that is chemical agent resistant (MIL-DTL-53039, 2013).
A number of US Military Standards have been adopted by civilian organizations. The two US Military Standards that are of most interest for this paper deal with the corrosion resistance of aluminum alloys: MIL-DTL-81706 (2004) and MIL-DTL-5541F (2006).
9.1 MIL-DTL-81706 (March 2006)
This specification covers chemical conversion coatings on aluminum and aluminum alloys. Before a chemical conversion coating product can be used to treat parts according to the MIL-DTL-81706 (2004), first, it must be inserted on the Qualified Products List (QPL). To be placed on the QPL, a material must undergo rigorous testing. This standard covers a number of required performance characteristics, including but not limited to (i) corrosion resistance, (ii) paint adhesion (wet tape), (iii) wet tape after repair, (iv) coating weight (Class 1A only), (v) low electrical resistance (Class 3 only), and (vi) storage life. Concerning corrosion resistance testing for Class 1A, the unpainted specimens being tested must be able to pass 336 h of salt spray testing in accordance with ASTM B117 (2011), with the significant surface will be inclined 6 degrees from the vertical without showing any evidence of corrosion at the naked eye.
9.2 MIL-DTL-5541F (July 2006)
This standard covers the requirements for qualified chemical conversion coatings applied to the surface of components made of aluminum alloys. The requirement for maximum corrosion resistance Class 1A is that five panels of unpainted AA2024-T3 must be exposed to 168 h of salt spray in accordance with ASTM B117 (2011). At the end of the test, they must have no more than five corrosion sites per panel and less than 15 in total per five panels (MIL-DTL-5541F, 2006).
10 Testing of unpainted specimens
10.1 Testing of unpainted specimens by NAVAIR and concurrent technologies corporation (CTC)
Between April 2000 and March 2012, a series of tests were performed by NAVAIR on a number of aluminum alloys to assess the effectiveness of NAVAIR TCP. The results are presented in three reports (Phase I, Phase II Interim, and Phase II Final), all of which are available in one document (Nickerson & Matzdorf, 2012). Concurrent Technologies Corporation (CTC) also carried out a series of tests to assess potential Cr(VI) replacement chemistries. Some of the results generated by CTC are in common with and can be compared to test results from NAVAIR.
10.2 Phase I report July 2003 project
The effectiveness of selected Cr (IV) alternative treatments was assessed for unpainted alloys. Specimens were exposed to Neutral Salt Fog Exposure for up to 2 weeks, and the results are an average of five test coupons. Table 5 presents the results for Alodine 5200 and TCP compared to Alodine 1200S [Cr(VI) conversion coating].
As evident from Table 5, Alodine 1200S and TCP coatings exposed for 1-week (168 h) and 2-week (336 h) Neutral Salt Spray Exposure following ASTM B117 (2011) exhibited no corrosion for AA2024-T3, AA7075-T6, and AA5083-H131. AA2024-T3 and AA7075-T6 specimens treated with Alodine 5200 exhibited surface corrosion within 48 h, whereas AA5083-H131 specimens displayed corrosion after 2 weeks of exposure. On the AA2219-T87 alloy, which is known to be prone to corrosion, NAVAIR TCP performed substantially better than the hexavalent conversion coating or Alodine 5200.
10.3 Comparison of results from NAVAIR and concurrent technologies corporation
Similar testing was performed by Concurrent Technologies Corporation. Table 6 presents the results generated during a 168-h Neutral Salt Spray Exposure on AA2024-T3 (Concurrent Technologies Corporation, 2007). For ease of comparison, results from the ESTCP (Nickerson & Matzdorf, 2012) and CTC Report (Concurrent Technologies Corporation, 2007) for unpainted specimens have been presented together in Table 6. The two sets of results have the following in common:
Both reports assessed AA2024-T3.
Each tested Alodine 1200S and Alodine 5200.
NAVAIR tested TCP.
CTC tested the products marketed by the NAVAIR Licensee Companies.
NAVAIR reports that there was no corrosion on their specimens treated with NAVAIR TCP after the 168-h salt spray tests – they rate the TCP-coated specimens the same as the Alodine 1200S specimens that they tested. Conversely, CTC reports pitting corrosion on all of the specimens they tested except for the Alodine 1200S, which exhibited no pitting corrosion. CTC failed all of the TCP-based conversion coatings except Alodine 5900, as shown in Table 6.
10.4 Comments on the NAVAIR and CTC test results
It could be anticipated that the Navy TCP and the products from the TCP Licensees should generate similar data, but this is not the case for the 168-h salt spray tests conducted by NAVAIR and CTC. The only TCP product that was able to pass the MIL-DTL-5541F (2006) standard used by CTC was Alodine 5900. The Metalast and SurTec products behaved in a similar fashion to each other but failed the test as they developed a significant number of pits. Aluminescent TCP revealed excessive pitting. Alodine 5200 (Cr-Free) also exhibited excessive pitting corrosion.
10.5 Testing of unpainted specimens by Boeing
Boeing tested five different TCP conversion coatings from four different suppliers as listed below (Gaydos, 2008). All of the products are on the QPL-81706 for Type II, Class 1A, and Class 3. Type II refers to products that do not contain hexavalent chromium. Class 1A is for maximum corrosion resistance, and Class 3 is for corrosion resistance where electrical conductivity is also required.
Henkel: Alodine T-5900
SurTec: SurTec 650 ChromitAl TCP
Each individual vendor prepared test specimens using their preferred process. Specimens were tested for 168 h using salt spray. One of the requirements for the specimens was that they must be able to pass MIL-DTL-5541F. The authors report that the products from two of the four vendors could not pass the test associated with MIL-DTL-5541F. The other two products were able to pass, but only if the aluminum was just given an alkaline clean or was cleaned and then given a mild deoxidizing treatment. The report did not reveal the details of which products were not able to pass the tests. In a subsequent phase, all five QPL-81706 TCP formulas were applied with a mild Boeing cleaner and mild deoxidizing treatment (no etch). Four out of the five could not consistently pass corrosion tests with AA2024-T3; however, all were able to pass corrosion testing with AA7075 and AA6061.
11 Testing of painted specimens
In general, TCP has been shown to perform as well as hexavalent conversion coatings for coating systems that include a chromated primer. However, replacing the hexavalent conversion coating with a trivalent alternative while keeping a chromated primer will only reduce the presence of Cr(VI) – not eliminate it. TCP does not typically exhibit the equivalent corrosion resistance for salt fog, SO2 salt fog, cyclic corrosion, and beachfront exposure on hard-to-protect alloys like AA2024-T3, AA7075-T6, and AA2219-T87 (Matzdorf et al., 2005). AA2219 contains very high levels of copper (5.8–6.8%).
11.1 Comparison of army and navy salt spray results
In Phase I of their report, NAVAIR carried out a 3000-h salt spray testing to evaluate the protection offered by various non-hexavalent chromium treatments for specific primer/topcoat pairs frequently used by the US Military. Many of the pretreatments NAVAIR tested were not able to sustain acceptable performance and were terminated before the end of the test. According to the authors, the most consistent performance was seen for Alodine 1200S, Alodine 5200, Boegel, and NAVAIR TCP (Nickerson & Matzdorf, 2012). The Army Research Laboratory (ARL) (Placzankis et al., 2003) also performed salt spray testing to evaluate the protection offered by a number of non-hexavalent treatments. They evaluated specimens after a number of exposure times to a maximum of 3000 h. For these tests, a thickened version of TCP (TCP 10) was used.
Tables 7–9 compare test results generated by the Navy to equivalent tests performed by the Army. The data have been restricted to 3000 h of exposure, as this was the only salt spray data common to both the ARL (Placzankis et al., 2003) and NAVAIR (Nickerson & Matzdorf, 2012). Tests were carried out on specific pairs of coatings (primer and topcoat) that are typically used by the US military. Prior to painting, all of the panels were cleaned and pretreatments were applied following each of the pretreatment manufacturer’s specifications. Coating was carried out at the NAVAIR facilities.
Table 7 compares the results for AA2024-T3 (3.8–4.9% Cu) specimens. The four pretreatments (Alodine 1200S, Alodine 5200, Boegel, and NAVAIR TCP) behaved well and exhibited comparable performance when a Cr(VI) primer was employed. The results are similar for all pretreatments, and in addition, comparison of the results from the two groups of researchers exhibited a good match, which improved the level of confidence in the data.
For systems where the primer did not contain any Cr(VI), results are less encouraging. In all cases, the rating is lower and, in general, the TCP did not do as well as the Alodine 1200, Alodine 5200, or Boegel. Results for the two groups are reasonably consistent, again suggesting that the ratings presented by both groups are accurate. Table 8 compares the results for AA7075 (1.2–2.0% Cu) specimens. Again the four pretreatments coated with a chromate primer all exhibit very good protection, and results are very similar for both the Navy and Army. For the non-chromated primer systems, results between the two research groups are consistent and indicate that TCP is providing a similar but slightly lower level of protection relative to the hexavalent conversion coating, except in the case of 53022/53039 (epoxy/single component polyurethane) where both research groups show TCP outperforming all other pretreatments including the hexavalent chromium conversion coating. Table 9 compares the results for AA5083 (0.1% Cu) specimens. The four pretreatments provide good protection for the systems that include chromated primers, and the results are similar for the two groups. As was the case for the other alloys, the non-chromated primer systems do not perform as well. Results from the two groups are very good as well as consistent for the coating system 85582N/85285. For the system 53030/53039, results are similar between the two groups, except for Boegel. Boegel gave poor results for the Army tests but reasonable results for the Navy. The results for the final coating system (53022/53039) are very different for the two groups. The Army reports good performance by all pretreatments except TCP, which had a low rating. On the other hand, the Navy report poor performance for both 1200S and Boegel and reasonably good performance for the TCP.
12 Concluding remarks
TCP coatings formed during immersion develop through multiple chemical steps. The formation appears to be associated with a pH increase at the interface. The initial step is believed to be the dissolution of the air-formed (native) oxide layer. A trivalent coating can be viewed as a zirconium oxide that contains some Cr(III). They appear to include a chromium- and zirconium-rich outer layer in combination with an aluminum-rich inner layer, which may provide most of the corrosion resistance. The layer thickness depends on the conversion time, and the rate of film development decreases as the film develops. The thickness of the layer may also depend on post-treatment in a water bath.
A number of authors have examined the possibility of Cr(III) oxidizing to Cr(VI) in various trivalent processes. In some cases, the presence of Cr(VI) has not been observed, but other researchers have found hexavalent chromium. There may be a correlation between exposure to moist air or an aqueous environment and the presence of small quantities of Cr(VI). It has also been suggested that higher bath pH and immersion time may accelerate the development of hexavalent chromium. The presence of Cr(VI) from TCP coatings has been corroborated by numerous researchers. If there is any level of environmental concern, it would presumably depend on the amount of Cr(VI) that was developed during oxidation of the Cr(III). The amount is considered to be both very limited and probably transient. One possible benefit to the limited amount of soluble Cr(VI) is that it may provide some active healing. As a result of the transient formation of Cr(VI), it is believed that the TCP coating provides some active corrosion protection. The soluble Cr(VI) appears to migrate to weak or damaged areas of the coating (such as a scratch). The Cr(VI) species is then reduced to an insoluble Cr(III) species, which may provide corrosion protection to that area. It also appears likely that Cr(VI) does not exist in either the as-prepared or used bath.
A number of different pretreatments have been investigated by researchers from various organizations in order to confirm the effectiveness of TCP treatments in terms of corrosion protection. In general, the non-hexavalent pretreatment is ranked relative to a traditional hexavalent conversion coating treatment. Ideally, the non-chromate treatment will provide protection that is equivalent to, or better than, that provided by the hexavalent conversion coating. TCP is currently available commercially and has undergone numerous tests by multiple military and civilian organizations and universities. It appears that the performance of the TCP coatings is relatively variable, depending on pretreatment, supplier, and testing conditions, and it is difficult to draw a final conclusion on the suitability of presently available TCP coating as a reliable replacement for Cr(IV) coatings in technological applications. As discussed earlier, a comparison of results for non-coated specimens tested by NAVAIR and from Concurrent Technologies Corporation (CTC) revealed very different data. This could be due to the sensitivity of TCP chemistries, which are susceptible to differences in the cleaning and deoxidizing steps as well as the actual operating parameters of the TCP solution, which might not as well understood and controlled as for the traditional hexavalent chromium treatments. There does appear to be a direct correlation between the amount of copper in the composition and the difficulty in providing corrosion protection by conversion coating with either TCP or a hexavalent conversion coating. In this context, the cleaning and deoxidizing step, which controls the alloy surface chemistry and microstructure prior to the formation of the conversion coating, might be critical in determining the film deposition kinetics and ultimately its properties. This aspect has not yet received the deserved attention in the technical literature and might be worthy of further consideration.
Finally, it should be noted that on occasions, TCP outperformed the hexavalent counterpart, and this could be a clear indication of their potential for success once the effects of the process parameters that control coating quality are well understood. Future research should focus on understanding the individual contribution of each processing step, and a parameter such as a holistic and knowledge-based approach is developed for TCP coating. Based on the results presently available, it is not unrealistic to imagine that such approach would enable to exploit the full potential of TCP coating and obtain protective film with properties comparable, if not superior to those currently provided by hexavalent chromium-based chemistries.
The authors wish to thank the STEPFAR Programme Grant (PON 03PE-00129-1).
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About the article
Published Online: 2017-08-12
Published in Print: 2017-12-20
Conflict of interest statement: The authors declare that they have no conflicts of interest regarding the publication of this article.