![]() In legacy coating processes that were the standard for decades, conversion coatings were the undercoat and base for primer paints that went over the aluminum alloy substrate. The path to CHEMEON eTCP has been a long and arduous task, but the payoff is dramatic for aerospace and naval coatings. But it truly was about bringing the best and brightest minds together to pull it off for the next generation of conversion coatings for the aerospace industry.” “For decades, this formulation was something that seemed beyond reach. “It was an intense challenge for everyone involved, almost like cracking a code,” says Ted Ventresca, president and chief operating officer of CHEMEON. In addition, CHEMEON eTCP has excellent corrosion resistance, low electrical resistance and superior paint adhesion. Several attempts came close to an alternative trivalent chromium undercoat that was safer, but they did not provide the obvious color change seen with hexavalent chromium, which could lead to difficulty identifying the presence of coatings for quality control purposes.īut that all changed when CHEMEON Surface Technology in Minden, Nevada, began working with the Naval Air Systems Command (NAVAIR) and the Fleet Readiness Center Southeast (FRCSE) several years ago on a NextGen research project that resulted in CHEMEON eTCP, the first trivalent conversion coating and anodic seal with distinct color - from silver to blue or purple - for visual verification that parts are coated and protected. These coatings are produced by a chemical oxidation-reduction reaction, with the main function to act as an undercoating and a base for organic finishes, as well as provide corrosion resistance and improve adhesion of paint finish systems on aluminum alloysįor decades, the industry searched for a “next generation” enhanced trivalent chromium pretreatment coating (eTCP) that was an environmentally safer alternative to hex chrome, and was also easy to inspect through color change. The result was an iridescent golden color change for ease of inspection, but the chromate was also a known carcinogen and environmental pollutant.Ĭhemical conversion coatings are adherent layers of low solubility oxide, chromate compounds produced on the surface of aluminum that converts the metal surface to a non-metallic inert state. 4 for conditions E and G shows that the trivalent system (Rating = 3) performed better than the hexavalent (Rating = 1).NextGen Path to Enhanced Trivalent Chromium Pretreatmentsįor more than half a century, the finishing industry standard for conversion coating of aluminum aircraft components and surfaces was pretreatments that utilized hexavalent chromium. Analysis of the 336-hr Russian mud data for chloride resistance from Fig. Nonetheless, the CASS corrosion ratings for both samples (Fig. ![]() The corrosion current distribution is less uniform and, in some cases, the active sites were large enough to produce heavier local nickel attack. On the other hand, fewer sites are seen for the hexavalent result (R), with a wider range of pore diameters. The trivalent result (L) shows a relatively uniform dispersal of active corrosion sites, with a narrow range of pore diameters, suggesting the corrosion current is well distributed. ![]() 3, respectively.įigure 6 - Comparison of active sites after 88-hr CASS for (a) trivalent and (b) hexavalent chromium over nickel. Here, 0.3 μm of trivalent chromium over low sulfur nickel is compared with 0.3 μm of hexavalent chromium over microporous nickel, corresponding to the data of samples E and G referenced in Fig. The system is designed to control the corrosion to provide a visually defect-free coating after extended corrosion.Ī comparison of the active sites produced after 88-hr CASS for the systems shown in Fig. The photo shows a small corrosion area on the top with the corrosion proceeding through the bright nickel layer and then spreading out horizontally at the high sulfur layer. ![]() Starting at the copper substrate and working up, there is a semibright nickel layer, a high sulfur layer, a bright nickel layer, a microporous layer (or low sulfur strike) and finally a thin chromium layer (too thin to be visible). The photo compliments the schematic at the left. More active layers will preferentially corrode as compared to more noble layers.įigure 1 relates the STEP test to a modern four-layer nickel system. The importance lies in whether the nickel layer is cathodic or anodic with respect to the reference layer. Measured and reported in millivolts (mV), the values can be either positive or negative. The STEP Test, developed by Harbulak in 1980, 1, 2measures the potential difference between the nickel plating layers, in a decorative multilayer system, e.g., between semi-bright and bright nickel layers in a dual nickel system. ![]()
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