In the first video of the series you'll see examples of "electrolysis". This occurs when two different metals are in contact with each other, such as a bronze propeller on a stainless steel shaft. The key here is that they are submerged in salt water which is a good conductor of electricity. Different metals have their own balance of positive and negative electrons. Stainless steel is higher on the scale of nobility than bronze, a mix of different metals in itself so it is "stronger". The propeller connected to the shaft when submerged in salt water cause the electrons seek a new balance. Think of two columns of water, one higher and than the other. Connect them and the water levels equalize. One column loosing water and the other gaining water. In the case of the metals, the weaker metal looses electrons. Electrons represent mass. The loss of mass is what we see as corrosion.
The second video in the series shows the effects of crevice corrosion. Metals including stainless steel can corrode even if they are not in contact with other metal. Crevice corrosion occurs when a small amount of water becomes trapped and does not exchange. Over time the PH balance of the small amount of water changes to the point of it becoming acidic, much like battery acid. A screw securing a plastic part to the hull can have this happen. Typically the head of the screw breaks off which can be problematic. Another scenario is a plastic propeller on a stainless steel shaft. There is often trapped water between the shaft and the inside of the propeller hub causing corrosion to the propeller shaft.
The remaining videos in the series examine the preventative measure of installed anodes, often commonly referred to as "zincs". To be clear a "zinc" is an anode made primarily with the metal zinc. Another type of anode that may be used on your boat is made of an aluminum and magnesium blend. Whether zinc or aluminum, these anodes when attached to another metal become the weakest link thus sacrificing themselves to protect the various metal components of a boat. Experienced divers watch for changes in the behavior of a boats anodes. If an anode stops corroding, its the same as not having one installed at all. Why would an anode suddenly stop working and what can be done to correct the issue? Sometimes an anode suddenly goes into "hyper burn", this is a big red flag to a diver that something is wrong. What has changed? Should a marine electrician be contacted to investigate? Ultimately your experienced diver is often the first one to recognize what can turn out to be a serious issue with your boat. Knowledgeable divers take preventative measures concerning anodes as part of their responsibility of maintaining not just bottom paint, but the entire hull including the running gear below the waterline