This article is intended to give you a fundamental understanding of the causes and effects of corrosion, as well as how to identify problems and correct them before they become severely damaging. Boat owners have to deal with many types of corrosion. Actually, there are only two, but there are many different causes with different names. The two basic types are erosion and electro-chemical.

Erosion
It is a strictly mechanical form of corrosion that is caused by friction. This can be mechanical corrosion, such as that of sandy water flowing around a bend in a pipe, which acts just like sand paper. Then there is another type of erosion, which is caused by high speed water flow. The pitting one sees on rudder blades behind propellers is an example of non-abrasive erosion. In fact, many people mistake this condition for electrolysis. This is caused by the stream of bubbles from the propeller hitting the rudder. High speed flow corrosion is rarely found in boats, other than this instance. The most frequent occurrence is within the cooling systems of engines.

Electro-chemical corrosion
It is the primary type of corrosion that boat owners have to deal with. All corrosion except mechanical erosion is electro-chemical in nature. This is just as true of a drop of water on a piece of raw steel, as it is of a stray current leak going through a bronze propeller. There is no need to understand this phenomenon completely, but a brief description will help.

All particles of basic elements or compounds have electrical charges, be they positive or negative. If two different materials have the very same electrical charge, nothing will happen. These materials or substances are, we say, "compatible" as in joining certain types of stainless steel and bronze together. If two materials have a sufficient different charge, then a flow of current (electrons) will occur. This is the principle that makes a dry cell battery work. Dry cells use carbon and another metal to generate an electrical current flow between the negatively charged carbon, and a highly charged metal.

Electrolysis
It is simply the result of stray current, and nothing else. Galvanism and electrolysis produce similar results, only they have different causes. We would be better off using the term "stray current corrosion" because this identifies the cause.

Galvanism
This is the term applied to the flow of electrons when two dissimilar metals are mated together. Basically, there will be very little flow when two metals are mated together dry. But add water to the join and suddenly corrosion blossoms. That's because water is a conductor and becomes the facilitator of the current flow. This is why mating dissimilar metals is much less of a problem inside your house than it is on your boat. All forms of galvanism involve metals, but all metals don't look like metals. Carbon is a metal that is used in making rubber, and so carbon rubber when mated to stainless steel can produce quite a reaction.

Galvanism is a very complex issue. Boats, of course, have a lot of different metals in them, including those below the water line.This is complicated by the fact that all bronzes, brasses and all stainless steels are not the same. There is a very wide range of alloys, meaning the mixing of different metals to achieve specific metallurgical properties, between what we usually think of as basic metals. This accounts for why there is such a wide range of performance of these metals, and sometimes why they corrode when they shouldn't. If the right alloys aren't used, we have a problem.

We attach pieces of zinc to the underwater metals of boats to protect those metals. Actually, zinc reverses the normal flow of current between dissimilar metals. zinc will emit current that raises and equalizes the electrical potential of all the metals in the system. It does this by releasing electrons, which are positively charged ions of the metal itself. This causes zinc to erode and disappear. These ions will attach themselves to the other metals, which explains why your props and other metals may end up with a rough, scaly surface; they've become covered with zinc oxide.

From
A Boatman's Primer on the Essentials
by David Pascoe, Marine Surveyor