This ROV video clip shows the underwater structure which supports a North Sea oil/gas platform (and a seal).
Notice the 'bracelets' draped around the steel structures and pipes. These are sacrificial Aluminium anodes, they protect the steel from corrosion.
Notice how well they are doing the job, they look pretty well corroded in comparison to the steel.
http://youtu.be/ngP0RIh2EDM
These link to a company that makes such anodes, Houston I believe?...
http://www.galvotec.com/aluminum-anodes.htm
http://www.galvotec.com/
http://www.galvotec.com/pdf/Cat_Aluminum_KT.pdf
Galvanic corrosion has been a well known phenomenon and problem for ships for at least a century. This is why you find "sacrificial zincs" attached to propeller struts on the bottom of ships to prevent corrosion damage to the prop shaft. What you posted pertained to using aluminum instead of the more commonly used zinc for this purpose. Galvanic corrosion is caused by electrical currents set up in two different metals, IN ELECTRICAL CONTACT, that are immersed in seawater. In this case the less "noble" metal will corrode away thus protecting the more "noble" metal. The most noble material is graphite and the least noble is magnesium. Zinc is next to magnesium and aluminum is only a little bit more "noble" than zinc. All three of these metals are less noble than steel so any of them could be used as a "sacrificial anode" to protect a steel structure. Since zinc is less noble than aluminum it is a more efficient anode than aluminum but for large underwater structures, the drill platform, cost becomes the major factor. However, for these to work, they must be attached, with a good electrical bond, to the metal to be protected. Then the sacrificial anode corrodes away at a rate that is the sum of its own corrosion rate and the rate of the protected steel.
My point is, you appear to think that the different amounts of corrosion shown in the video illustrated how fast aluminum corrodes compared to steel if each is immersed in seawater and this is NOT what is shown in the video, The video shows that aluminum acts as an effective sacrificial anode and corrodes rapidly
IF it is in contact with a large steel structure. We see that the steel structure of the Norwich City has not corroded rapidly even without sacrificial anodes. Since the Electra, if it is in the ocean, is not in contact with a large steel structure so it would NOT corrode at the rate depicted in the video so the video provides no support for the idea that the Electra has not been clearly seen due to a high rate of aluminum corrosion in seawater. (Or, maybe, the reason that the NC has not corroded is that the Electra ended up on top of the NC wreckage, in electrical contact, and acted as a sacrificial anode, hmmmmmm.)
gl