I saw a documentary YouTube video recently and it said that there is a small amount of deterioration on the boat deck near the bridge. As for the ship itself as it is covered in coral the steel is not deteriorating as badly as on Titanic. I expect it remain intact for many decades to come. The boilers and engines will eventually collapse inside as they are still holding in place on a ship that's on it's side. This will probably cause massive damage and cause the ship to collapse quicker.
I have recently read that during a dive on Britannic, 2006, John Chatterton found some Rusticles, near the fireman's passage, if I recall correctly. They were hard, and "dripping with spikes." They cut into his diving suit, and he soon developed a fever. He was out of it in 48 hours.
Some testing platforms were retrieved from Britannic, placed on the A-Deck promenade, just barely behind the Fourth Window forward. What they found shocked them.
There was a 29% steel loss. They found out 5 different bacteria were working together. These Rusticles were also hard. Scientists predicted that had these Rusticles been here from the beginning, Britannic would have collapsed by 1960. Testing Platforms from Titanic had a 1.7 loss of iron per year. Britannic's had almost a 5% loss per year. (so, the platforms were placed there in 2002)
Now, these Rusticles don't populate and attack weak and bent metal like Titanic's do. They're slower to populate. These special Rusticles are mostly populated in "hotspots", so they aren't all over the wreck.
(Unfortunately, Carl Spencer was the one who retrieved these testing platforms, on the 2008. May he rest in peace)