Corrosion

A Boatman's Primer on the Essentials

by David Pascoe, Marine Surveyor

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

This essay 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.

Introduction

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.  The following descriptions have been simplified for clarity since most corrosion mechanisms are very complex. This is intended for laymen so if you are a scientist, kindly cut me some slack here.

Erosion

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. In fact, many people mistake this condition for electrolysis, a subject that I'll get to in a moment. 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

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. 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  

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, as was described above.. 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. 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.

Scale of Nobility

Metals are rated on what is called a Scale of Nobility, which simply means the materials ability to resist this kind of corrosion. There is also a chart called  the "galvanic series"   which shows the electrical potential of metals in seawater.  A more noble metal is one that has a neutral or negative electrical potential. It will not generate a flow of positive ions, and is called "noble." The reverse of this is the least noble metal, which has a high positive charge, and which will generate an electrical current. These include such metals as zinc, unalloyed aluminum and copper, iron and steel. Graphite and carbon bottom out the list, being the most highly charged metals.

Crevice Corrosion

This is the most common form of corrosion found on fiberglass boats, and is the least understood. Electrical currents are generated anytime there is a change in chemical composition. That's why powerful explosives can be made of such ordinary things like plastic. 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.

Crevice corrosion on screws. The one on the right shows the typical wasting of the shank right under the screw head. The one on left was exposed to water both under the head, and on the inside of the hull where it has thinned at both locations. This is probably what the bolt in the photo below would look like when removed.

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. These crevices or closed cells can become dynamic, meaning that the process can perpetuate itself for a long time -- either until the acidic water is exhausted or an oxygen source is created that lowers the acidity of the water and stops the corrosion. If no oxygen source is introduced, the corrosion process continues until the metal is completely gone.

This is is one of the telltales of crevice corrosion in through hull bolts. The fact that this is Taiwan stainless only makes the problem worse.

This is what copper-based paint looks like when stray current is involved with these through hull bolts. The copper-based paint has reacted. You can't get a better indicator of a problem than this.

To illustrate this phenomenon, consider that you could hang a stainless steel bolt over the side on a string. It would hang there forever and nothing would happen to it. But put that faster into the bottom of a hull and watch what happens (see photos below). Water gets into the screw or bolt hole where there is no free-flow of water, so that the small amount of water in the screw crevice turns acidic and creates a galvanic cell. This usually occurs right under the screw or bolt head, eroding the shank of the screw or bolt until it becomes loose. Once it does become loose, then a better flow of Ph balanced water is introduced, and the corrosion stops because the water is no longer acidic. Virtually the same thing will occur with stainless fasteners into an aluminum mast. But in this case, the corrosion stops as soon as the water evaporates from the crevice. In the case of an aluminum fuel tank, installed in such a way as that water gets trapped against the tank, like a foamed in place tank, or a tank sitting on a plywood deck, the very same thing happens. Which tends to leave us mystified why you could throw your aluminum parts over the side and they'd sit there forever without corroding, while the seeming protected parts on your boat corrode badly. Crevice corrosion always occurs in places you can't see, though it usually leaves telltale evidence.

Stress Corrosion

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. Old style swage fittings on sail boat rigging combines both stress and corrosion cells from entrapped water within the swaged cable. It also occurs at mast rigging attachments where water is entrapped between the mast and bolt-on parts, or even getting under welded parts. See photo below.

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. Good propeller shafts don't break because they don't corrode. If you have this problem, it's ultimately a question of how many new shafts do you want to buy before you replace them with better quality.

Highly polished stainless steel is more corrosion resistant than those without a mirror finish. The reason is that unpolished metal has machine marks on it that serve as crevices for corrosion to start. Polishing smoothes these crevices over. However, high polishing won't help much for lesser grades of stainless.

Micro photograph of stress corrosion cracking. Initially invisible to the eye, water gets into these fissures and hastens the destruction of the part.

It's All the Same

All forms of corrosion are the same electro-chemical process caused by different circumstances. That there are so many different circumstances is why we have so much trouble understanding the nature of our corrosion problems. Every combination of metals in different locations on our boats corrodes for different reasons. And if we don't understand why, then there's little hope of preventing it, either by the boat builder, or the boat owner. 

Related Article: Corrosion in Marinas

First posted January 4,  1999 at www.yachtsurvey.com.
Page design changed for this site.

Posted July 12, 1999

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