SECTION 7 

 TITANIUM ALLOYS 



Titanium and titanium alloys owe their corrosion 

 resistance to a protective oxide film. This film resists 

 attack by oxidizing solutions, in particular those con- 

 taining chloride ions. It has outstanding resistance to 

 corrosion and pitting in marine environments and 

 other chloride salt solutions. 



The chemical compositions of the titanium alloys 

 are given in Table 81, their corrosion rates and types 

 of corrosion in Table 82, their susceptibility to stress 

 corrosion in Table 83, and the effects of exposure on 

 their mechanical properties in Table 84. 



7.1. CORROSION 



The corrosion rates and type of corrosion of the 

 titanium alloys are given in Table 82. 



Except for two alloys, there was no corrosion of 

 any of the titanium alloys during exposures in surface 

 seawater or at depths of 2,500 and 6,000 feet. 

 Reference 15 reported a corrosion rate of 0.19 mpy 

 for unalloyed titanium and of 0.18 mpy for 6A1-4V 

 after 123 days of exposure at the 6,000-foot depth, 

 but no corrosion of these same alloys after 75 1 days 

 of exposure at the 6,000-foot depth. Also, no visible 

 corrosion was reported. For practical purposes these 

 values are considered to be inconsequential. 

 DeLuccia, Reference 17, reported cracking in the 

 heat-affected zone parallel to the weld bead in alloy 

 6A1-4V after 197 days of exposure at the 2,500-foot 

 depth. Investigation of the weldments showed that 

 the welds had been made under improper conditions 

 and were contaminated with oxygen which made 

 them brittle. 



Alloys 75A, 0.15Pd, 5Al-2.5Sn, 6A1-4V, 

 7Al-2Cb-lTa, 6Al-2Cb-lTa-lMo, and 13V-llCr-3Al 

 were both unwelded and welded. They were fusion- 

 welded by the inert-gas shielded arc, nonconsumable 

 tungsten electrode process (TIG). There were trans- 

 verse butt welds across the 6-inch dimension of the 

 specimens and 3-inch-diameter ring welds in the 

 centers of 6 x 12-inch specimens. The welded speci- 

 mens were intentionally not stress relieved in order to 



simulate the conditions present in a welded structure, 

 i.e., to retain the maximum residual internal welding 

 stresses. The process of placing a circular weld in a 

 specimen imposes very high residual stresses in the 

 specimen. Such circular welds simulate multiaxial 

 stresses imposed in structures or parts fabricated by 

 welding. There was no visible corrosion of these 

 welded alloys except for stress corrosion cracking of 

 alloy 13V-llCr-3Al. This will be discussed under 7.2. 

 Alloy 6A1-4V was also exposed as: 



(l)Wire, 0.020- 0.045- 

 diameter. 



and 0.063-inch 



(2)Cables, 1/16-inch (1x19), 1/4-inch 

 (6x19), 1/4-inch (6x19) with Type 304 

 stainless steel swaged ends, and 1/4-inch 

 (6 x 19) with ends tied with mild steel wire. 



(3) Flash-welded tube. 



(4) Flash-welded sphere. 



(5) Piece from broken sphere. 



(6) Welded rings 9.625-inch OD x 1.125-inch 

 wide x 8.75-inch ID. One ring was unstressed 

 and the others were stressed up to a maxi- 

 mum of 60,000 psi. 



There was no visible corrosion on any of the above 

 specimens except for the AISI Type 304 swaged 

 fittings and the mild steel wire. The faying surfaces of 

 the Type 304 stainless steel fittings were severely 

 attacked by crevice corrosion. The rate of this crevice 

 corrosion was probably increased by the galvanic 

 couple formed by the two dissimilar metals, with the 

 stainless steel being the anode of the couple. The mild 

 steel wire used to tie the end of one titanium cable 

 was corroded almost through by galvanic corrosion; 

 the mild steel wire was anodic to the titanium cable. 



7.2. STRESS CORROSION 



Specimens of the alloys were stressed in various 

 ways and to values equivalent to 30, 35, 50, and 75% 



225 



