Chromate conversion coatings (CCCs) are applied via immersion or spraying onto aluminum and steel substrates. These coatings provide both corrosion inhibition and adhesion promotion between the primer and the pretreatment. Several recent studies have shown that residual hexavalent chromium [Cr(VI)] in CCCs provides corrosion protection via a self-healing mechanism. However, Cr(VI) is a known carcinogen and is highly regulated by the Environmental Protection Agency (EPA) and Occupational Safety and Health Agency (OSHA).

A viable alternative to Cr(VI) coatings must meet or exceed the performance of Cr(VI). In addition, these alternative coatings must be able to passivate the metal surface allowing the corrosion current to shift to the noble metal region. Several studies over the past decade have focused on electroactive polymers (EAPs) as corrosion preventive coatings. Though very little work has been done on alternative pretreatments using EAPs, scientists at the Naval Air Warfare Center Weapons Division (NAWCWD) have successfully synthesized a poly(para-phenylene vinylene) (PPV) derivative that can perform as well as CCCs. This polymer can meet the military pretreatment requirements for alternatives to CCCs. This compound poly(2,5-bis(N-methyl-N-hexylamino)phenylene vinylene (BAM-PPV) has shown corrosion prevention in simulated seawater. Its corrosion prevention properties were also evaluated using accelerated weathering conditions to determine its effectiveness as an alternative pretreatment to CCC. These tests, conducted under SERDP project WP-1148, showed conclusive evidence that BAM-PPV was a promising pretreatment coating as an alternative to CCCs.

This 4-year ESTCP project was undertaken to prove the viability of this new compound and to test other alternative pretreatment coatings as controls. Each service, Wright Patterson Air Force Base (WPAFB), Army Research Laboratory (ARL), and NAWCWD and Naval Air Warfare Center Aircraft Division (NAWCAD) tested BAM-PPV as a pretreatment coating, trivalent chromium pretreatment (TCP), and PreKote. Each service tested these pretreatment coatings according to their military coating specification for alternatives to CCCs. These tests were conducted with and without full military coatings containing Cr(VI) or non-chromium coatings in accelerated weathering chambers, adhesion testing, fluid resistance, and outdoor/marine outdoor exposure testing.

Technology Description

EAPs are composed of conjugated chains containing ππ-electrons delocalized along the polymer backbone. In their neutral form, EAPs are semiconductive polymers that can be doped and converted into electrically conductive forms. The doping process can occur either by oxidative or reductive reactions, though oxidative reactions are more common. The conductivity is electronic in nature and does not involve concurrent ion migration in the solid polymer form. Some doping processes are reversible, with typical conductivities switching between those of insulators to those of metals.

EAPs comprise a broad range of materials that are characterized by conjugated repeat units; this conjugation is responsible for the unique electronic and optical properties of EAPs ranging from low oxidation potential to third order optical nonlinear properties. Researchers have exploited these unique materials to synthesize a variety of EAPs. These polymers exhibit a broad range of conductivities (10-4 to 103 S/cm) in their doped (oxidized) states. There are several classes of EAPs: polyacetylenes, poly(p-phenylene)s, polyheterocycles, poly(phenylene vinylene)s, polyanilines (PANI) and conjugated ladder polymers.

The Los Alamos National Laboratory (LANL) and Kennedy Space Center (KSC) investigators have demonstrated that doped EAPs, such as PANI coatings, inhibited corrosion on carbon steel. Their approach was based on earlier work suggesting that the interfacial contact between the metal and a doped EAP would generate an electric field that would restrict the flow of electrons from the metal to an outside oxidizing species, thus preventing and/or reducing corrosion. The LANL-KSC tests were conducted in a 3.5wt % NaCl/0.1 M HCl environment using 0.005 cm thick films of PANI doped with p-toluenesulfonic acid on carbon steel. The PANI primer was coated with an epoxy topcoat. The PANI/epoxy coating performed significantly better than the epoxy topcoat alone. These initial results were used by researchers at LANL-KSC to develop EAP coatings to resist the corrosive effects of acid vapor generated during space shuttle launches. The ground support equipment and structures at the KSC were susceptible to these severe environmental conditions. Marine outdoor exposure testing over 28 months have shown that PANI/epoxy coatings on carbon steel can survive a severe corrosive environment providing acceptable corrosion protection.

However, an examination of EAPs as an alternative to CCC pretreatments has not been thoroughly studied. The NAWCWD has investigated BAM-PPV as an alternative to CCCs due to PPV’s known redox properties and tertiary (3 ) amine functionalities capable of corrosion inhibition with adhesive properties. BAM-PPV has shown strong adhesion to AA2024-T3 alloy in a simulated seawater immersion test (pH~8). Its ability to inhibit corrosion was demonstrated. Constant current (galvanostatic) and constant potential (potentiostatic) methods were used to investigate the corrosion protection of BAM-PPV on AA2024-T3 coupons. Quantitative evidence was obtained from these results, which showed that corrosion inhibition was obtained without the loss of adhesion and no pH dependency was observed. These tests were run against control samples (non-coated AA2024-T3 coupon), and the BAM-PPV coated coupons outperformed the non-coated coupons. Visual inspection of the coupons showed significant pitting in the non-coated as compared to the coated (BAM-PPV) samples.

Demonstration Results

After 3 years of extensive laboratory testing of these alternatives, a candidate was selected for field testing by the three services. The extensive laboratory testing validated the performance of BAM-PPV as an alternative to CCCs in several specific DoD military laboratory requirements. The performance of BAM-PPV coated with chromated primers and topcoats exceeded the military requirements of 2000 hours neutral salt fog exposure without evidence of corrosion, blistering, or delamination. However, when BAM-PPV was coated with non-chromium primers and topcoats, the performance in most cases was marginal or failure was evident; however, several non-chromium primers with BAM-PV as the pretreatment and topcoats did provide adequate corrosion protection. These samples met the minimum requirement of 2000 hours neutral salt spray exposure. BAM-PPV along with the best performing primers (Cr(VI) and non-chromium) with topcoats was selected for field testing. Toxicology studies have also shown that BAM-PPV is a non-toxic material. Laboratory testing by both WPAFB and ARL showed in several cases that BAM-PPV used as the pretreatment layer in full military coatings (epoxy primer + polyurethane topcoat) showed acceptable corrosion performance, which warranted further evaluation in a field study. Furthermore, outdoor exposure testing was conducted at WPAFB and KSC. The outdoor exposure testing performed by WPAFB using BAM-PPV as the pretreatment layer with primers Cr(VI) and non-Cr(VI), and topcoat, showed comparable performance to a fully chromated system for 24 months of exposure. However, similar testing at the KSC showed that BAM-PPV after 6 months of marine outdoor exposure could not match a fully chromated control system. In this case, the BAM-PPV incorporated as the pretreatment layer with non-Cr(VI) epoxy primer and polyurethane topcoat could not match a fully chromated system (CCC + Cr(VI) epoxy primer and polyurethane topcoat). This test using BAM-PPV as the pretreatment coating was considered a fail. In this type of severe marine outdoor exposure testing, BAM-PPV was not robust enough to survive this environment. A fully chromated coating can survive a 1-year marine outdoor exposure test and this is considered a pass.

Each service conducted field testing on non-critical military hardware using BAM-PPV as the pretreatment coating. The Air Force and Army field testing showed similar performance to a fully chromated system. However, testing by the Navy on support equipment showed that BAM-PPV as the pretreatment coating did not give any improved corrosion or abrasion resistance as compared to the control. This may be due to improper surface preparation of the aluminum alloy prior to coating with BAM-PPV pretreatment. Adhesion of the BAM-PPV onto the Navy aluminum support structures did not yield satisfactory results after 1 year of field testing.

Implementation Issues

Removal of chromates from the pretreatment process would allow for improved compliance with 29 Code of Federal Regulations (CFR) 1910.1026, which calls for a permissible exposure limit (PEL) for Cr(VI) of 5 micrograms per cubic meter (μg/m3) and time weighted average (TWA) with an action level of 2.5 μg/m3. This should not be an issue since the substitution involves trading one EPA/OSHA non-toxic pretreatment for a pretreatment containing a known human carcinogen that has prohibitive PELs and restrictive disposal options. The EAP coatings for the facility will be applied by the appropriate coatings group, and each depot has the appropriate safety permits and EPA reporting requirements to implement this technology.

The only implementation issue that each co-performer identified was the number of passes that BAM-PPV requires for appropriate thickness. Currently CCC, TCP, and Prekote require only:

  • Two passes to produce adequate thickness for pretreatment coatings, and
  • BAM-PPV requires between seven to nine passes for adequate thickness.

This will be a problem for implementation as it will increase time and costs for applying BAM-PPV onto various metal substrates.