N093-208 TITLE: Innovative Coatings for Prevention of Inter-Granular Corrosion in Sensitized 5000 Series Aluminum Alloys

TECHNOLOGY AREAS: Materials/Processes

ACQUISITION PROGRAM: PMS 502, CGX Program, ACAT I

OBJECTIVE: The objective of the project is to develop and implement new innovative approaches and techniques for a more durable coating and application process in order to mitigate inter-granular corrosion in sensitized 5000 series aluminum alloys.

DESCRIPTION: Current marine-grade aluminum alloys (5XXX-series) are commonly used in naval combatants and are known to be susceptible to sensitization.  Once the aluminum material becomes sensitized, it is vulnerable to inter-granular corrosion which manifest as stress corrosion cracking (SCC) and exfoliation corrosion.  SCC is common in sensitized, recrystallized, high-strength, marine, aluminum alloys in naval environments subjected to prolonged tensile stresses.  Exfoliation corrosion affects sensitized, un-recrystallized plates and sheets in marine environments.   Material degradation due to sensitization has been observed in the fleet in the form of SCC cracking problems on the Guided Missile Cruisers (CGs) and exfoliation from the Vietnam-era Swift boats. Currently, ASTM standards and screening tests can ensure that materials at the time of purchase are not sensitized, but are not intended to predict resistance to sensitization in the future.  The marine environment, coupled with elevated temperatures, can lead to sensitization and subsequent degradation from corrosion and conventional navy paints have not been an effective environmental barrier. 

This topic seeks to explore the development of innovative new approaches that will provide a durable, low-cost coating to provide a sensitization barrier as experience has shown that 5000 series aluminum alloys are likely to become sensitized over time.  Applicable processes to apply the proposed coating concept in a shipyard environment are also required. Coating and application process must be environmentally compliant with Navy requirements.  Assessments of coating system through modeling and simulation and/or small-scale prototype demonstrations are encouraged.  The key factors to consider are the effectiveness, moisture and ionic species permeability, adhesion, durability, cost, environmental compatibility, effects from non-skid treatments, compatibility with silicone-alkyd topcoats, and adaptability to the shipyard environment.

PHASE I: Demonstrate the feasibility of a coating and application process in order to mitigate inter-granular corrosion in sensitized 5000 series aluminum alloys by providing a robust environmental barrier for aluminum substrates. Perform bench top experimentation, where applicable, as a means of demonstrating the identified concepts. Establish validation goals and metrics to analyze the feasibility of the proposed solution. Provide a Phase II development approach and schedule that contains discrete milestones for product development.

PHASE II: Formulate, demonstrate, and validate coating and associated processes to implement the concept defined in Phase I. In a laboratory environment, conduct testing, evaluate the coating system for efficacy, moisture and ionic species permeability, adhesion, durability, topcoat compatibility and process repeatability, and finalize the coating system.  Develop a final specification for the coating and application process documentation.

PHASE III: Develop production quality, low cost, and more durable coating and application processes in order to mitigate inter-granular corrosives in sensitized 5000 series aluminum alloys for military and commercial implementation onboard naval platforms.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The coating and application processes developed for the Department of Defense is equally applicable for use in commercial boating industry. Development of an effective coating would have considerable benefits in the commercial industry which must consider the same material degradation effects where 5000 series alloys are used. 

REFERENCES:
1.  Seattle-Post intelligencer, "Aloa Awarded $59.6 million for replacing metal" June 28, 2006

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

3.  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.

4.  Bushfield, Harold Sr., Marc Cruder, Rendall Farley, and Jim Towers, "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.

KEYWORDS: Aluminum; corrosion; crack; coating; alloy; sensitization;

** TOPIC AUTHOR (TPOC) **

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