Clogged up elbows are pretty common. As said by others mainly because of under utilization of the engine's power. You need to push it into 70%-80% of it's power to achieve the high temperatures in the exhaust manifold to burn off the soot. If not, it will start to build-up and become hard.
If your elbow is still looking fairly OK then you can use muriatic/hydrochloric acid to get rid of the soot. Use the same stuff that is used to clean brick-walls. Keep an eye on the process and check the progress regularly. If you see holes starting to show in the injection pipe (that mixes the seawater with the exhaust fumes (look inside the elbow), you will need a new elbow. But you would have needed one anyway as the cast iron corroded completely (not because of the acid but the seawater).
As said by others check for elbows on Ebay. There is a US source that makes stainless elbows for about the same price as the regular Volvo one. I bought one of those a while ago (for a VP D2-40), they are impeccable, other then the color there is no difference in appearance and shape. This will last much longer, although cleaning will be required every few years, depending on usage.