to the four exostructure mounting lugs and 

 at the hatch hinge mounting. 



Galvanic Corrosion 



To prevent corrosion caused by the electro- 

 chemical reaction between two dissimilar 

 metals in electrical contact with one another, 

 a great deal of effort was made to minimize 

 the area of exposed surfaces by painting 

 them. To prevent galvanic attack on the 

 pressure hull, electrical insulation between 

 it and adjacent titanium alloy structural 

 members was provided in the design. Small 

 lead weights (dropped individually to attain 

 positive buoyancy) are held by steel hooks 

 cast in the top of each weight. To prevent 

 galvanic attack between hook and weight, a 

 plastic sleeve covers each hook. 



Crevice Corrosion 



To minimize crevice corrosion, non-drain- 

 ing crevices are kept to a minimum and a 

 thorough fresh water wash down after each 

 dive is specified. Protection of areas impossi- 

 ble to wash is called for as follows: a) Be- 

 cause contact between fairing and metallic 

 exostructure members would be too snug to 

 permit washing, such members would be fab- 

 ricated from a titanium alloy (Ti-6%, Al-4% 

 V) which is immune from corrosion under 

 such conditions, b) The 0-ring groove in an 

 aluminum propulsion controller housing 

 forms a perfect crevice in a corrosion-suscep- 

 tible material. Hard coat anodizing and scal- 

 ing of the aluminum and a coating of silicone 

 grease is specified, based on experience with 

 DEEPSTAR 4000. 



Stress Corrosion 



Components of DEEPSTAR 20000 which 

 could be stressed under tension were de- 

 signed so that failure would not occur from 

 stress corrosion crack propagation or corro- 

 sion fatigue. Fracture mechanics methods 

 were applied to safeguard against such envi- 

 ronmental effects. Fracture mechanics anal- 

 ysis determines if stress corrosion will occur 

 at flaws, e.g., welds, and if such flaws will 

 grow under cyclic loading to a point where 

 stress corrosion will occur. The method 

 works in the following manner: A defect of a 

 particular maximum size is assumed in the 

 component (the limitation of this maximum 

 size is attained from non-destructive testing 



of the component) and if, through cyclic load- 

 ing, this defect will grow to a size where 

 stress corrosion can occur, then the compo- 

 nent is unacceptable. DEEPSTAR 20000's 

 variable ballast tanks are composed of tita- 

 nium (in which crack propagation can pro- 

 ceed at rates of inches/hour), and fracture 

 mechanics showed that 4,000 cycles of load- 

 ing were required before cracks would grow 

 to a critical depth at which stress corrosion 

 would occur. Similar calculations were made 

 on the pressure hull weldments; they, too, 

 show acceptable limits. 



The corrosion control program on DEEP- 

 STAR 20000 was based primarily on the fact 

 that the vehicle would be taken out of the 

 water following each dive, thus permitting 

 easy field maintenance and repair to chipped 

 paint (the first line of defense against corro- 

 sion), etc. Components considered most sus- 

 ceptible to corrosion, and least protectable 

 were designed for easy removal, and spares 

 would be carried for replacement. Such was 

 the case with the cast aluminum alloy pro- 

 peller blades. 



Protective painting, a thorough fresh 

 water washdown, inspection and an onboard 

 inventory of replacement parts constitute 

 the major corrosion control program in sub- 

 mersibles today. 



In large, complex vehicles where all compo- 

 nents are not situated for easy routine main- 

 tenance and repair, considerable effort must 

 be expended to combat corrosion. Rynewicz 

 (35) outlines the corrosion control methods in 

 the DSRV and the results of test programs 

 leading to these methods. A number of these 

 methods were gained from operating experi- 

 ence (45 dives) with DEEP QUEST. 



For a thorough and rigorous treatment of 

 the entire scope of materials for ocean engi- 

 neering, the reader is referred to the work of 

 Koichi Masubuchi (36) who has left no stone 

 unturned in treating materials, fabrication, 

 selection, testing and protection of pressure 

 hulls and associated structures. 



REFERENCES 



1. Shankman, A. D. 1968 Materials for 

 pressure hulls — present and future. Na- 

 val Eng. Jour., v. 80, n. 6, p. 972-979. 



2. Link, M. C. 1973 Windows In The Sea. 



Smithsonian Inst., Wash., D.C. 



276 



