A New Chromium-free, Self-sealing Anodizing Process for DoD Application

Narjes Fredj | NAWCAD

WP22-3165

Objective

The objective of this project is to evaluate and optimize a new corrosion-inhibited, chromium-free, self-sealing anodizing process for aluminum alloys. Such alloys are widely used for military aircraft applications, but they are prone to pitting corrosion when exposed to marine environments, necessitating frequent repair which, in turn, significantly impacts military readiness. Anodizing is commonly used to improve both the corrosion resistance and wear properties of aluminum alloys. Anodize coatings are inherently porous, so they are often subjected to a secondary sealing process. Examples of sealant chemistries include hot deionized water, metal salts and, most commonly, chromates as well as various other proprietary chemistries such as Trivalent Chromium Process (TCP), a more environmentally friendly process. Recent revisions by the American Conference of Governmental Industrial Hygienists substantially reduced the Threshold Limit Value for both trivalent and hexavalent chromium exposure. This highlights the need for a chromium-free strategy to meet new and future environmental requirements. This project will optimize a new chromium-free self-sealing anodizing process using oxalic acid as an additive in sulfuric acid for aluminum alloys. In situ inclusion of nonchromate corrosion inhibitors into the pores will further enhance the inherent corrosion protection properties. This new process has the potential to replace both dichromate and TCP sealers and eliminate the use of chromium–based chemistries from the anodizing process.

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Technical Approach

The approach is to tailor and optimize a chromium-free self-sealing anodizing process for use on aluminum alloys. This process will be improved to offer better corrosion performance by doping the anodizing electrolyte with two cerium based corrosion inhibitors (cerium nitrate and cerium tartrate) and potassium permanganate. It is expected that during anodizing process, the corrosion inhibitors will be embedded inside the tubular-like microstructure of the oxide film, offering improved corrosion protection. The anodizing and sealing will be performed in situ in a single step process. This chromium-free process is anticipated to be faster, cheaper and more efficient than conventional anodizing processes. This project will be comprised of three phases. In a first phase, the project team plans to optimize the chemistry of the anodizing bath by including different ratio of corrosion inhibitors in the bath followed by an optimization of anodizing parameters such as duration and current density. The durability of the oxide films will be evaluated with electrochemical techniques to down select those formulations offering the best corrosion performances. In a second phase, scrap part items from the depot at Fleet Readiness Center Southwest will be selected as part of a depot-level demonstration, and anodized with the new process in a large tank. Finally, in a third phase, the optimized and selected anodized coating will be primed and tested to ensure that the adhesion properties meet the requirement of MIL-A-8625.

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Benefits

The doped self-sealing anodizing process offers two primary benefits. The first of these is the complete elimination of chromium form the anodize process. The second benefit is the potential to anodize and seal the pores of the coating in a single step thus simplifying the process and reducing the process footprint. Secondary benefits include decreases in water and energy usage and reductions in the requirements for the storage, transportation and treatment of chemical waste. This new process offers the potential to make the anodizing process both faster and cheaper.

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Points of Contact

Principal Investigator

Mr. Narjes Fredj

NAWCAD

Phone: 301-342-2718

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