The active aluminum-rich primer (AlRP) was invented and developed at NAVAIR to sacrificially protect aluminum alloys and steels from corrosion. The Al pigments (Al-Zn-In) in AlRP were fabricated from a sacrificial anode alloy, which has a lower open-circuit potential than common aluminum alloys. However, initial results indicated that AlRP undergoes severe self-corrosion. Therefore, the Al pigments were pretreated in a trivalent chromium passivation (TCP) bath to reduce the self-corrosion rate. The objectives of this study are to understand the anti-corrosion properties of AlRP on aluminum alloy 2024-T3 substrate and to evaluate the effect of TCP treatment on the Al pigment particles. The polarization curves of AA2024-T3 and active aluminum alloy (Al-Zn-In) show that TCP-treated active aluminum alloy has lower corrosion potential than AA2024-T3 and thus would sacrificially protect it. The AlRPs were exposed in an accelerated exposure test, GMW14872. Exposed samples were then examined using scanning electron microscopy and energy dispersive x-ray spectroscopy to understand the coating degradation process. The AlRP with TCP-treated pigments outperforms a similar coating with untreated pigments.
Metal-rich primers have a long history for corrosion prevention of metals. In addition to barrier protection like other organic coatings, the metallic pigments in a metal-rich primer act as a sacrificial anode and provide cathodic protection to substrates. The driving force is essentially the potential difference between pigments and substrate.1 Zinc-rich primers (ZnRP, also called ZRP) have been used for over 70 years and shown remarkable protective properties on steel.2 Magnesium-rich primers (MgRP or MRP) were invented by Bierwagen, et al., in 2004 to protect aluminum alloys from corrosion.3-4 In 2011, Matzdorf, et al., invented an aluminum-rich coating (AlRP) as an alternative that might be less active than MgRP coatings.5
The pigments in AlRP are fabricated from a commercial Al-Zn-In alloy used for sacrificial anodes in marine applications. The Al-Zn-In alloy has a lower corrosion potential than most Al alloys, such as aluminum alloy 2024-T3 (UNS A92024(1)). Therefore, AlRP should protect Al alloy substrates cathodically. However, the preliminary results from NAVAIR revealed high self-corrosion rates of the Al-Zn-In pigments in the epoxy matrix. To solve this issue, the pigments were pre-treated in a trivalent chromium process (TCP, also called trivalent chromium passivation).6-9
The objectives of this study are to understand the protection and corrosion mechanism of AlRP on AA2024-T3 substrate and to evaluate the effects of TCP treatment on the Al pigment particles using electrochemical assessments, accelerated corrosion testing, and electron microscopy.
NAVAIR (Patuxent River, MD) coated AA2024-T3 panels by spraying four different AlRPs with epoxy as the organic matrix. The AA2024-T3 substrates were pretreated in 20 vol% SurTec 650 chromitAL®† TCP bath for 5 min. The pigment volume concentration of all four AlRPs was about 60%. The pigments were pretreated in 50 vol% TCP bath for 0, 2.5, 5, or 7.5 min, which was the only difference among four coating systems studied. The AlRPs with different pigments are labeled XP1-D, XP1-F2.5, XP1-F5, and XP1-F7.5, accordingly.
Potentiodynamic polarization was performed on a bulk Al-Zn-In alloy and AA2024-T3 using a three-electrode cell. The bulk Al-Zn-In alloy was Galvotec® CW III† purchased from Galvotec Alloys, Inc. with composition given in Table 1. The samples were abraded to 600 grit and then treated in the TCP bath. AA2024-T3 and Al-Zn-In alloy were polarized at a rate of 0.167 mV/s in 0.1 M NaCl solution after stabilization at the open-circuit potential (OCP) for 15 min. Each polarization scan was replicated at least three times. The results in Figure 1 are representative curves.
The cyclic exposure test GMW14872 was adopted for accelerated testing of AlRP-coated panels. The panel edges were covered by black electrical tape and the exposed area of each panel was about 26 cm2. Samples were manually X-scribed through the coating using a carbide scriber. The cross sections of as-received samples and samples exposed to GMW14872 were analyzed using energy dispersive x-ray spectroscopy (EDS) in a scanning electron microscope (SEM).
RESULTS AND DISCUSSION
The OCP of the bulk Al-Zn-In alloy is about −1.1 VSCE, which is more active than common aluminum alloys, such as AA2024-T3 (Figure 1). Thus, primers containing enough loading of Al-Zn-In pigments should provide cathodic protection to Al alloy substrates. Surface treatment by TCP reduced the corrosion rate of the bulk Al-Zn-In alloy by one order of magnitude at the galvanic couple potential if coupled with AA2024-T3 at a 1:1 area ratio and ohmic potential drops are ignored.
Coatings with TCP-treated pigments, XP1-F2.5, XP1-F5, and XP1-F7.5, outperformed XP1-D with untreated pigments in the GMW14872 cyclic corrosion exposure test. After 80 cycles, corrosion attack was evident at scribes of all AlRPs. However, the XP1-D partially peeled off, and the substrate underneath was not protected by XP1-D (Figure 2). In contrast, XP1-F2.5, XP1-F5, and XP1-F7.5 remained relatively intact away from scribes. Among these three AlRPs, self-corrosion was observed at the darkened region of XP1-F2.5. Blisters and corrosion attack were evident along scribes of XP1-F2.5 and XP1-F7.5, while XP1-F5 had corrosion attack only at the scribe.
EDS maps of cross sections at regions of intact coating remote from the scribes and also at the scribes of XP1-D and XP1-F5 are shown in Figures 3 and 4, respectively. Compared with XP1-D containing untreated pigments (Figure 3[a]), the XP1-F5 containing TCP-treated pigments (Figure 4[a]) showed much less oxygen across the coating, which indicates much less self-corrosion. Therefore, the XP1-F5 coating will supply longer barrier protection to the substrate. Both coating edges at the scribe corroded (Figures 3[b] and 4[b]), but the exposed substrate in the scribe close to the coating edge exhibited different extents of cathodic protection. The scribe in the substrate coated with XP1-F5 had less oxide than the one coated with XP1-D, indicating a greater extent of protection. The role of whether this oxide product provides additional protection to the substrate requires further investigation.
Bulk Al-Zn-In alloy is active and protects AA2024-T3.
Surface treatment reduces the corrosion rate of bulk Al-Zn-In alloy. TCP treatment on pigments provides better anti-corrosion performance.
UNS numbers are listed in Metals and Alloys in the Unified Numbering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International.
This project is funded by the U.S. Air Force Academy through the Technical Corrosion Collaboration from the U.S. Department of Defense under the Award No. FA7000-13-2-0019. Special thanks to Victor Rodriguez, Craig A. Matzdorf, and Frank Pepe at NAVAIR, Patuxent River, MD for providing the materials. The authors would also like to thank Nicolas G. Sanchez for preliminary work, and Fan Yang for SEM help.
Recipient of first place in the Harvey Herro Applied Corrosion Technology category in the Student Poster Session at CORROSION 2017, March 2017, New Orleans, Louisiana.