Navy

New electrodeposition process for improved conversion coatings

An electrolytic process to control the deposition of chemical conversion coatings like TCP on metal surfaces that yields a thicker coating and improved corrosion resistance

Materials

U.S. Navy scientists and engineers have invented and demonstrated at scale an electrolytic process that passes current through a conversion coating enabling “induced current” deposition in less than an hour, resulting in greater coating thickness and performance on metal substrates, without sacrificing adhesion.

electro deposition electrodeposition

Two TCP-coated, AA2024-T3 aluminum samples are shown in this side-by-side comparison. The left sample shows corrosion after six weeks of neutral salt fog testing. The right sample, immersion coated with the new electrodeposition process for five minutes, shows none, the best performance ever achieved by the Navy. (Navy photo)

These electrodeposited coatings meet ASTM D3359 standards for paint adhesion and MIL-DTL-81706 for military coatings, including tough-to-achieve ASTM B117 (neutral salt fog exposure) without pitting for a duration of four to six weeks on AA2024-T3 panels. This performance is 2-3 times better than the Navy’s standard TCP and significantly outperforms “next-generation TCP” (PAX 267).

This novel electrodeposition process may also enable improved performance of many non-chrome conversion coatings for MIL-DTL-81706. While the Navy has optimized the process for use with TCP on aircraft-grade aluminum, different coating-substrate pairs are possible.

Electrodeposition processes are not new to metal finishing, witness anodic coatings for aluminum per MIL-A-8625. However, impressed current, in particular, is new to conversion coatings and uses a lower current deposition process than generic anodic coatings. This process allows for scalable coating weights between 2-7 times the average coating weight for typical diffusion-controlled immersion processes, improving stability, salt-fog protection, and without creating a powdery appearance.

The Navy’s new electrolytic process overcomes previous drawbacks without requiring new processing tanks and other expensive equipment. Modification to immersion tanks includes installing a parts rack made of suitable conductive material, and a source of current under 3 amperes per square foot.

Navy personnel have scaled their testing to 60-gallon immersion tanks on flat panels. They do not anticipate challenges in increasing the scale or in immersing shaped parts and pieces for commercial application. The Navy is also developing a handheld device for portable electrodeposition of conversion coatings for field maintenance and repairs.

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