TECHNOLOGY AREAS: Materials/Processes, Weapons

ACQUISITION PROGRAM: PMS404 TORPEDO MK 48 Mod 7 CBASS ACAT III

OBJECTIVE: Reduce cost and efforts related to corrosion damage and associated maintenance activities.

DESCRIPTION: A certain portion of Heavy Weight Torpedoes in the Fleet is exercised routinely for training purposes. After such exercises, they must return to Intermediate Maintenance Activities (IMA) for turn-around before sea-water and exhaust products cause extensive corrosion damage. Besides the expected inspections and refueling, the turn-around process also includes the flushing of the fuel tank and engine, disassembly and rebuild of engines and engine accessory components. It is roughly estimated that 1/4 of work involve corrosion mitigation, namely by disassembly, cleaning, and the subsequent rebuild. If the corrosion can be eliminated or substantially decelerated the significant portion of this work may be reduced or eliminated.

The torpedo�s components include a substantial amount of aluminum, which is low in the galvanic series for corrosion purposes. Many components are also made of Stainless Steel, and many interfaces are sealed by preformed packing (i.e. o-rings), which tend to form trap space or crevices for cooling/pressure-compensating sea-water and exhaust products. Corrosion problems are especially severe in/near these sealing features.

The fuel used in these torpedoes is OTTO Fuel II, which is comprised of Propylene glycol dinitrate, 2-Nitrodiphenylmine, Di-n-butyl sebacate, with trace amount of sodium. Detail of this fuel can be found in MIL-O-82672A, Military Specification for OTTO FUEL II. Fuel Tanks are one of the major corrosion concerns. Because fuel tanks are back-filled with sea-water for pressure compensation, these 7000 series aluminum fuel tanks with anodic and epoxy coating experience severe corrosion, and cost of repair and refurbishment is increasingly undesirable. Technical Report NAVSWC TR 91-136 noted that OTTO Fuel II reacts with sea-water w/ dissolved oxygen and form an acidic localized environment near the fuel/water interfaces and attacks the coatings and the aluminum substrate. One of the products of the reaction, ammonia, is found to be accelerating the corrosion and damage rate of the fuel tanks.

The Afterbody / Tailcone sections, which house the engine and associated accessories, contain many aluminum and stainless steel components. After a torpedo in-water run, many of these component surfaces are wetted with sea-water and exhaust products that adhere to surfaces and crevices of these components. The corrosion is especially severe under the black deposit of the exhaust, which is believed to be carbon. Technical Report NUSC 81-3, Properties of OTTO Fuel Combustion Products, details the composition of the exhaust from Otto Fuel II (Components include: elemental carbon, CO [toxic], CO2, H2, H2O, N2, CH4, NH3, HCN [toxic], etc�) . It should be noted that sea-water is mixed into the exhaust after cooling other components. The presence of sea-water in the components also causes galvanic, crevice, pitting, and general surface corrosion.

Currently, the method of corrosion prevention is primarily by the means of coating (anodic, passivation, and/or epoxy/other coating), and cleaning during disassembly. It is found that coating effectiveness suffers from wear and application defects, and cost and efforts of disassembly cleaning is high. Means of neutralizing/suppressing the corrosive environments/agents is believed to be more desirable.

PHASE I: Perform research and theoretical modeling for corrosion reduction and compatibility with subsystems within the torpedo. Identify potential chemical/methods for corrosion control. Propose application methods and address compatibility with torpedo components (i.e., preformed packaging, fuel, lubricant, etc.). Perform small scale testing in environments similar to what maybe encountered in the torpedoes. Provide preliminary material safety and disposal/discharge limitation information.

PHASE II: Demonstrate compatibility and effectiveness of the chosen method with torpedo subsystems. Provide details of application requirements into the torpedo subsystems. Address material safety and disposal/discharge requirement.

PHASE III: Work with acquisition program to incorporate the corrosion mitigation system into the work flow or components of the torpedo subsystems.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: It is unlikely that corrosion control methods any are tested for compatibility with Otto Fuel and/or Otto Fuel exhaust by-product, which is rich in compounds of nitrogen and carbon. While immediate area of commercial application cannot be identified, such applications are likely.

REFERENCES:
1. MIL-O-82672A: Military Specification for OTTO FUEL II

2. NAVSWC TR 91-136: Synergistic Effects of Otto Fuel II and Synthetic Seawater on the Corrosion Behavior of Aluminum Alloys

3. Corrosion Inhibitors, An Industrial Guide, by Flick, 1993

4. Uhlig�s Corrosion Handbook, 2nd ed, Edited by R. Winston Review

KEYWORDS: Torpedoes; Corrosion; Aluminum; Marine; Chemical; Seawater

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