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The goal of this project is to develop a chromium-free sealer to be applied after plating and anodizing surface finishing processes. This sealer needs to fill in pores and defects created during the plating or anodizing processes and provide the exceptional corrosion resistance that is currently achieved with hexavalent or trivalent chrome products.
It is no small feat to match the inhibiting performance of chromates. To do this, TDA Research, Inc. has developed a multi-pronged corrosion inhibitor approach, where the project team will use the driving force of the corrosion itself to generate a corrosion inhibiting protective layer. The project team has demonstrated this approach can outperform the hexavalent chrome anodizing pretreatment (AMS-M3171) on magnesium alloys, one of the most susceptible metals to corrosion. In this project, the objective is to show that this novel inhibitor package in a sol-gel coating can be a highly effective, chromate-free posttreatment sealer. The new inhibitor package attacks corrosion on multiple fronts by containing anodic corrosion inhibitors that form a polymerized film on the surface when corrosion occurs and additional organic inhibitors that complex with metal cations to form passivating insoluble complexes, both of which shut down the corrosion process. This powerful new inhibitor package will be incorporated into an in-house developed, low temperature process sol-gel (most commercial sol-gels are not hydrophilic enough for our alternative corrosion inhibiting formulations) so that the inhibitors are present at the metal surface in high concentrations (high concentrations provide an extra driving force to form the corrosion inhibiting films). The objective is to show that the sol-gel sealant provides a high level of corrosion protection, even when abraded, through the use of these active corrosion inhibitors. It will also be demonstrated to have excellent adhesion to organic coatings.
Because of the way the corrosion process itself triggers the anodic corrosion inhibitors to passivate the surface (through the use of oxidizable aromatic thiols), the project team will demonstrate that the approach is widely applicable and can be readily used across a wide range of other metals and metal alloys as a conversion or sealer coating. In addition to Zn and Zn/Ni plated substrates which were the substrates of highest concern in the Statement of Need, the project team will also test the sealant on Cd plated and anodized (MIL-A8625F [Type II, III]) AA2024.