# Why did the Soviet Alfa-class submarines have a titanium hull?

My limited knowledge in engineering tells me:

• Titanium is expensive, stronger and heavier than aluminium, lighter and weaker than high yield steel.

• The most significant advantage of titanium is the good strength to weight ratio.

• In case of submarines, weight is not really a big concern, as long as the weight is manageable (less than the displacement and bigger than displacement - maximum ballast mass).

The mentioned points exclude the use of titanium. Since the designers decided otherwise, and surely had some good reasons, I wonder where my misunderstanding is?

• Does titanium rust / corrode in seawater? Sep 15, 2018 at 21:45
• Wikipedia told me that titanium is virtually corrosion-free. Otoh traditional steel hulls seem to cope with seawater, so I am inclined to believe that corrosion resistance isn't the deciding factor. Sep 15, 2018 at 21:53
• Your point two is a reason for titanium, not against. Point 1 is fairly irrelevant, since it compares density but ignores strength - on that measure, wood would be a much better (and cheaper) material than any metal! Sep 15, 2018 at 22:18
• Engineers would choose titanium if they can afford it. So basically the flowchart goes like this. Can you afford titanium? Yes, do. No then you will have to look into alternatives. Sep 15, 2018 at 23:52
• Weight IS a problem. High strength hulls are required to go the desired depths; High strength > high weight ( too much weight and they don't come back up). That is why steel hulls are high strength steel . . They were HY 80 ( high yield 80,000 psi yield) ; The Navy was working on HY 100 last I knew. These higher strength steels bring problems with welding and toughness that I know of and likely more problems = . But that is what is required to go deep. High strength titanium has similar problems , I doubt corrosion resistance was a significant factor. Sep 16, 2018 at 0:40