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Corrosion Types

Of all the complicated subjects one needs to know as a boat owner, this is probably the most difficult, and least understood.

There are many types of corrosion that boat owners have to deal with. Well, actually, there are only two, but there are many different causes with different names. The two basic types are erosion and electro-chemical. 

Erosion 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.

Electro-chemical  corrosion is the primary type of corrosion that boat owners have to deal with. First we need to understand that all corrosion except mechanical erosion is electro-chemical in nature.

Electrolysis People generally do not understand this term, using it as a catch-all to describe any kind of corrosion below the waterline. Electrolysis 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. 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. 

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.

 

We attach pieces of zinc to the underwater metals of boats to protect those metals. What actually happens is that the zinc reverses the normal flow of current between dissimilar metals. The 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 the 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.

Crevice Corrosion  This is the most common form of corrosion found on fiberglass boats, and is the least understood.  As its name implies, crevice corrosion involves water, metals and crevices. For our purpose, a crevice is any cavity that will trap and hold water, while at the same time reducing or eliminating air exposure to the water/metal interface. Crevice corrosion is the same thing as galvanism, only it occurs under different circumstances.

This is also called "closed cell" corrosion by virtue of the fact that little or no air is allowed to get to it. The water/metal interface results in oxidation of the metal which concentrates the hydrogen content of water, and turns the water into an acid. This changes the electrical make up of the affected materials, generating an electrical current that "dissolves" the metal involved.

Stress Corrosion  is yet another form of corrosion, as it's name implies, occurs when a metal is under heavy stress. This is a combination of crevice corrosion cells combined with heavy loading. It most often occurs on sailboat rigging and power boat propeller shafts. 

Propeller shaft breakage has reach almost epidemic proportions these days. That's because builders are opting for low grade stainless shafts made of lesser alloys. All it takes is for a tiny pit to form on a shaft to initiate the crevice/stress corrosion cycle that will ultimately result in fatigue failure. This usually occurs at the stuffing box or keyway cuts, the natural weak points.

Stray Current  The electrical systems on boats have improved sufficiently over the last 20 years that stray current corrosion is much less of a problem. It begins to show up in older boats because of all the jury rigged wiring and systems that get added on over the years. In newer boats, it usually occurs due to ground wiring faults on the dock.

The basic mechanism of stray current corrosion is the same as all other types, only this time the introduction of an outside source of electricity screws up the normal electrical balance of the boats metals, plus adding one other pernicious problem. Energizing all the underwater metals in the boat, that raised electrical potential will seek a path to ground. And rest assured that it will find one. That path will be the piece or pieces of metal below the water line that are the weakest like. It will start with the zincs and once those are destroyed, it will move onto the next lest noble metal. That's usually a poor quality alloy of prop shaft, propeller or through hull fitting. In other words, it's likely to attack one specific piece of metal. As the current leaves the piece of metal on its way to ground, it is carrying bits of the metal with it, as well as the erosion that may occur from part to part.

The conditions created by AC and DC current is not the same, with DC current being the most damaging. The reason for this is that AC current is pulse current that moves in two directions, greatly reducing the corrosion potential for reasons I won't get into here. Technically, AC current requires some kind of diode that converts it into DC current before it causes metallic corrosion. There are lots of naturally occurring diodes in crystalline forms of metallic oxides. Aluminum oxide is VERY good at converting AC to DC   current. The aluminum oxides that form inside aluminum boats!

Detecting Corrosion. If you are getting white and/or green halos around your zincs or underwater metals, you have a stray current leak. Even a very small amount of current will cause a paint reaction, so we have a built-in litmus tester here in our bottom paint. The evidence of corrosion appears in one of three forms. The one that we are all familiar with is the appearance of oxides, the byproduct of a metal that has chemically changed. Copper-based metals like brass and bronze leave green oxides, white for aluminum, and reddish-brown for stainless steels. These are the telltales of ordinary oxidation corrosion.

However, we have these other forms of corrosion to deal with, and these have different affects. Galvanism and stray current are abnormal, the result of something that shouldn't be happening. When the current generated by galvanic action is weak, it will generate the usual corrosion byproduct, the oxide of the metal. Stronger galvanic and stray current will more often result in rapid erosion of the metal, usually to the point where there are no oxides present, but will leave an appearance of bright metal. If you see any part of any underwater metal that is showing bright, regard this as a red flag. It's the indicator of very rapid erosion. Examples of this would be eroded, but shiny surfaces on zincs, or propeller blade tips that are bright yellow and showing a crystalline texture or pattern.

 

The effects of galvanism most often occur very slowly with bronze or brass. Here, a condition known as dezincification occurs. Since copper is alloyed with zinc to make bronze, zinc is the weak link and will leach out of the alloy to leave raw copper. The result is metal that is pinkish in color, is granular in texture and tends to crumble when probed. It is soft and very weak. Any pinkish looking copper-based metal is waving a red (pink) flag.

Very rapid zinc loss that results in bright, shiny metal being exposed is a clear indication of electrical activity, be it galvanic or stray current, usually the later, since galvanism rarely creates enough current to destroy zincs quickly . Bright zinc in association with heavily corroded bottom paint means you have a problem that needs to be addressed immediately. Adding more zinc is NOT the solution.

 

Over zincing, putting too much zinc, on causes the opposite problem. It will reverse the flow of current in the other direction and actually cause corrosion. Over zincing is discernable when you find your props all covered with a coarse layer of zinc oxide. It will feel like sandpaper. This condition will reduce your boat's speed significantly and increase fuel consumption. I'll bet that statement got your attention!!!

Bonding Systems  The purpose of a bonding system is to equalize the electric potential of dissimilar underwater metals by tying them all together with wire or copper straps. The benefits of a bonding system are wide ranging but little perceived. One is that it serves to dissipate stray current leaks. 12 volts of current focused on a small piece of metal will result in rapid destruction. But that same 12 volts spread over a much larger surfaces, causes less damage in proportion to the size of the water exposed surfaces of the metal. Bonding systems can reduce the corrosion potential of metals inside and on the bottom of the boat.

 

 

Source:

www.seasidemarine.com

 

 

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