INTRODUCTION
Fasteners for top-side applications on Naval ships have historically been steel fasteners. These fasteners when exposed to marine environments suffer extensive corrosion. Alternatives such as high strength stainless steel, nickel alloy and titanium alloy fasteners are being considered. While these alternative materials offer better corrosion resistance than steel, they can suffer from thread galling when used in the uncoated and/or unlubricated condition. Solid film lubricants such as molybdenum disulfide-impregnated epoxy polyimide have shown anti-galling properties. Additionally, anti-seize compounds such as copper loaded grease have also shown anti-galling properties. In a paper on titanium as alternative fasteners1, K. Faller compares titanium fasteners to steel and high strength alternative materials. In this study, Beta-C titanium fasteners are investigated as an alternative fastener for Naval use. Galvanic interactions and thread galling are evaluated to identify candidate coatings or lubricants so that Beta -C titanium fasteners can be utilized for top-side applications.
Make/break testing was performed on 15 solution treated titanium Beta-C (YS~120 ksi) ½ ”-13UNC hex-cap screws to evaluate anti-galling thread treatments. Each make/break torque test measured torque behavior to reveal any galling or galvanic corrosion tendencies. Titanium Beta-C was supplied by RMI Titanium Company, of Niles, Ohio to Mr. Kurt Faller of B & G Manufacturing of Hatfield, PA, for the production of ½ ” diameter fasteners. Alloy composition, physical and mechanical properties are shown in Tables 1,2 and 3 respectively. Initial make/breaks of some of the fasteners were performed by Mr. Faller. The test fixture with the fasteners was delivered to the Naval Surface Warfare Center Carderock Division (NSWC CD), Corrosion Technology Group, Code 683, Ft. Lauderdale Marine Corrosion Test Site in December 1993.
EXPERIMENTAL PROCEDURE
The test fasteners without washers were tightened with a torque wrench in a steel plate with through hole (Figure 1) to two-thirds of the materials’ yield strength to simulate a stress state of service. Monthly make/break testing was performed on 15 titanium Beta-C ½ ”- 13UNC hex-cap screws. Table 4 lists the three conditions in which the fasteners were tested. The anti-galling materials listed in Table 4 were applied to the fasteners only at the beginning of the test. All of the nuts were untreated.
Test Environment
The fasteners were subjected to periodic seawater wetdown for nine months. The fasteners were attached to seawater wetdown test racks located 30 feet from the Port Everglades channel, 400 feet from the ocean, and approximately 8 feet above sea level. The seawater wetdown system operates on a continuous cycle where the test fixture is sprayed with seawater for a period of 10 minutes and allowed to air dry for 50 minutes. Figure 2 shows the test rack and spray system. This test was designed and developed with close adherence to the American Society for Testing and Materials (ASTM) G332, G443, G504, and G525. Although the seawater spray wetdown is not an ASTM standard, it closely simulates the top side environment of Naval ships.
