N112-136 TITLE: Environmentally Acceptable Conversion Coating for Un-Coated Aluminum Alloys

TECHNOLOGY AREAS: Materials/Processes

ACQUISITION PROGRAM: PMS 501, Littoral Combat Ship Program, ACAT I

OBJECTIVE: Development of an advanced, environmentally acceptable, conversion coating system that can be inexpensively applied to un-coated, marine-grade, aluminum alloys.

DESCRIPTION: Currently, all ships in the United States Navy are painted with Low Solar Absorbance (LSA), haze gray topcoats. While LSA paints have shown themselves to help reduce the amount of solar energy absorbed by the ship structure, cost and weight constraints have limited the use of these products on new, aluminum ships and structures. For example, the superstructure on the LCS-1, the entire LCS-2, and the HSV freeboard/superstructures are unpainted. This unpainted aluminum has been abrasive blasted to reduce the "shiny" appearance of mill-finished aluminum. However, the blasted surface is rough and could readily trap dirt, exhaust gasses, and rust particles. An un-coated aluminum surface, as compared with an LSA-coated Navy ship will: increase the energy load on the ship's air conditioning plant; increase the rate at which aluminum is aged resulting in decreased toughness as well as increased cracking over time; increase the amount of time or effort that maintenance personnel or sailors have to spend to clean-up the appearance of the ship for events like change or command or port calls. Present methods for the pre-treatment, such as anodizing or the use of chromate conversion coatings, are tank-based and are neither compatible nor realistic or cost-effective to implement in a shipyard/shipbuilding environment. Both these methods also require the shipyard/shipbuilder to employ environmental management plans including the ability to provide waste treatment and require personnel to be outfitted in protective gear during the coating process.

This topic seeks an innovative approach to the development of coating or surface treatment for un-coated, marine-grade, aluminum alloys that will retard corrosion and staining while providing some LSA properties such that coated/treated aluminum would be cooler than untreated aluminum when exposed to sunlight. The primary technical challenge is that conventional paint systems are too expensive and too heavy for aluminum ships and as such the desired corrosion-control and LSA performance normally achieved by over 6-mils of paint must be achieved with a material that is far less costly to apply and that forms a far thinner layer. For the purposes of this proposal, the layer should be less than 1-mil thick. Of particular interest is the method of application and the ability to apply the coating/treatment using current shipyard coating application processes like brushing, spraying, or rolling the material on the surface. Coatings/treatments that can be readily applied by sailors and waterborne maintenance teams would be preferred over systems that have to be applied in a controlled, industrial setting. Specifically, solutions are sought which would:
1. Be compatible with un-coated aluminum alloy surfaces with a minimal level of surface preparation (i.e., the material will not require an abrasive blasted or power-tool cleaned surface).
2. Remove rust staining and create a uniform gray oxide color on otherwise unpainted aluminum surfaces.
3. Leave a residual film on the aluminum surface that shall inhibit corrosion as well as inhibit the absorption of solar radiation. There is no requirement on film thickness and in fact, a thinner, lighter film would be preferred over the 6-9 mil thicknesses for conventional paint systems.
4. Be environmentally acceptable products and shall not contain hazardous heavy metals (e.g., hexavalent chromium, cadmium, lead, etc.), have less than 250 grams/liter of volatile organic compounds, and that could be approved for use by sailors from barges or boats in Navy ports like Pearl Harbor, HI and/or San Diego, CA. As such the product should not be strongly acidic or alkaline, should not include carcinogenic or mutagenic materials in the formula and should not have a flash point below 100F.

PHASE I: Demonstrate the feasibility of an innovative approach to develop a coating system for un-coated, marine-grade, aluminum alloys that will retard corrosion and staining while providing the LSA properties with respect to thermal protection. Establish performance goals and provide a Phase II development approach and schedule that contains discrete milestones for product development.

PHASE II: Develop, demonstrate and fabricate a prototype as identified in Phase I. In a laboratory environment, demonstrate that the prototype product meets the performance goals established during Phase I. Provide a detailed plan for validation testing of the prototype product using established processes for judging the appearance of corrosion on surfaces and measuring radiant heat transfer and method of implementation into a future ship test and/or design environment. Prepare cost estimates, logistics data packages, and interface documents for use in both forward fit and retrofit ship programs.

PHASE III: During Phase III, materials/processes validated in Phase II shall be scaled up to allow field application to a LCS. For example, enough product should be developed to coat a minimum of 50,000 square feet of surface on an actual LCS. The surface treated would be from the boot-top to the top of the superstructure.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The approach would be applicable to all marine applications for aluminum alloys including: Boats & Craft; Masts, booms, and other topside hardware; Aluminum decking and structures used in a seacoast environment. The Commercial industry would benefit from the new technology because industry could leverage the cost and weight savings associated with not having to paint aluminum parts used in the marine environment.

REFERENCES:
1. Stress Corrosion Cracking of Aluminum Alloys - www.key-to-metals.com/Article17.htm

2. Oguocha et al, "Effect of Sensitization Heat Treatment on Properties of Al-Mg Alloy AA5083-H116", Journal of Material Science, DOI 10.1007/s10853-008-2606-1.

3. Bushfield, Harold Sr., et al, "Marine Aluminum Plate ASTM Standard Specification B 928 and the Events Leading to Its Adoption". Presented at the October 2003 Meeting of the Society of Naval Architects and Marine Engineers, San Francisco, California.

4. Bovard, FS, "Sensitization and Environmental Cracking of 5xxx Aluminum Marine Sheet and Plate Alloys," Corrosion in Marine and Saltwater Environments II: Proceedings of the Electrochemical Society, ed. DA Shifler, 2004, pp. 232-243.

KEYWORDS: conversion coating; aluminum; low solar absroption; heat; environmentally acceptable

** TOPIC AUTHOR (TPOC) **

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