Corrosion

What is Corrosion?

A main concern for any boat owner is the corrosion that can and does occur when metal is introduced into a marine environment. Corrosion is of particular interest to Marinette owners because of the basic fact that we have metal hulled boats in a marine environment. Although corrosion does have the potential for disastrous results if left uncontrolled (i.e. turning your boat into a new clubhouse for the fish) understanding the different types of corrosion and what causes it will be your best weapon against it.

There are two types of corrosion that can occur, Galvanic Corrosion and Stray Current Corrosion. Your Marinette is equipped with systems to keep both types of corrosion under control and warn of any potential problems.

Galvanic corrosion is the most prevalent type of corrosion. Fortunately it is generally a slow process because it relies on the electrical potential difference that naturally occurs in dissimilar metals in order to create the necessary current. This current is typically measured in Millivolts. Galvanic Corrosion occurs when two electrically connected dissimilar metals are placed in a conductive fluid (water). A current flows from the more noble (the one with the higher electrical potential or the cathode) of the two metals to the less noble metal (the anode). In an effort to return itself back to its equilibrium point (its original voltage) the less noble metal discharges a current into the conductive fluid which returns to the more noble metal completing the circuit. When the anode discharges the current into the conductive fluid, molecules of the metal leave the anode and deposit on the cathode.

Stray Current Corrosion, on the other hand, can do a lot of damage in a very short period of time because the current is hundreds even thousands of times stronger. The source of the stray current is usually an improperly installed or malfunctioning piece of electrical equipment. Stray Current Corrosion occurs when a current is allowed to escape from its designated wiring and seek its own path back to ground. If the stray current decides to head out into the water through a hull fitting, the fitting will become an anode. No matter where that fitting material is on the nobility chart it will corrode at a very fast rate. While this type of corrosion is not very common its effects can be very destructive and, in extreme cases, cause injury.

When a boat plugs into the marina shore power system it becomes electrically connected to every other boat that is connected to the same snore power system. This electrical connection also includes any metal objects in the marina such as sea walls and docks. If unprotected, your aluminum hull becomes the sacrificial anode for these items and all of the other boats in the marina. At this point you may be asking yourself why every Marinette built has not become a nice aluminum coating on everybody else's props, shafts and metal sea walls?

All is not lost. Marinettes are equipped with several systems that prevent the hull from being eaten away by Galvanic Corrosion. The first line of defense is hull paint that is kept in good condition (I am referring to the base paint and not the antifouling paint) which is a barrier between the conductive fluid (water) and the metal. Your boat should also be equipped with a galvanic isolator. A Galvanic Isolator breaks the electrical connection between your boat and all of the other boats in the marina while providing electric shock protection.

Important note here: The green ground wire from the shore power receptacle must be grounded to your boats electrical system. Cutting or disconnecting this wire can cause serious damage and Injury. Marinettes are also equipped with sacrificial anodes attached to the hull, rudders and trim tabs. As long as these anodes are kept in good condition your hull and underwater hardware will be protected. Since everything that we have talked about so far happens on the microscopic level (and underwater) it is not apparent that there is a problem until the damage has reached a significant level. As a type of early warning system the boats are also equipped with a corrosion monitor that give a reading as to how well the boat is protected against corrosion.

In summary, as long as the prevention systems installed on your boat are in proper working order and you keep an eye on your monitor reading your aluminum hulled Marinette will probably outlast all of the plastic boats docked around you.

The Battle Against Corrosion:

A Possible Way to Reduce the Corrosion Mess Around the Railings

Nearly every Marinette goes through this: corrosion blisters appear on the deck surface near the stainless railing posts. The railings are then removed, the paint sanded to remove the corrosion, the deck repainted, and the railings reinstalled--but it doesn't seem to matter how carefully the repair was done, or if a sealer was used on the screw threads, for the mess will come back in a few years.

A successful approach is to use aluminum rivets to hold the railings in place. The idea is allow the easily-replaced rivets to corrode instead of the deck. The below photo shows the rivets installed in a test piece of scrap aluminum with a Marinette handrailing post attached. The holding strength of the rivets is excellent. A 17/64 inch (just a fraction over 1/4 ") hole is required for them--just the ticket for those already stripped out screw holes.

Note: After 4 years, the rivets have held tightly, and under railing post corrosion has been minimal.

Corrosion Caused by a Lack of Proper Maintenance

Needless to say, my first sight of this mess was a nasty shock! The damage is confined to the starboard prop area--the port side looks pretty good. How this happened is a mystery--perhaps something got jammed in the prop and it flailed away at the bottom paint (that was not properly applied in the "repair")? Or, perhaps some boob used a bronze-shell cutlass bearing in the strut? Either way, I aged a decade or so upon seeing this mess! Much epoxy putty later it's now flat and smooth again, and it has not caused any trouble after 4 years. Alternatively I could have hired someone to MIG weld the area back to solid aluminum. The bottom line: Sandblasting and proper repainting of this 1985 era hull, years ago, would have prevented this horror scene.

Here's an even worse disaster, this time on Keith's 1974, 28 footer. (Keith sandblasted the ENTIRE hull, from gunnel to gunnel, with a 5 hp. portable compressor! That's called dedication.) This was the worst spot in the photos he sent me but, unfortunately, it wasn't the only one. Again, the lack of a complete 'bottom job' (strip to bare metal and redo) after years of usage resulted in the bare metal spots that caused this crevice corrosion mess.

Homemade Galvanic Meter

This portable corrosion meter came with my boat since the Capac probe stopped working long ago. The analog meter that was in the box died when it fell overboard one day (don’t ask!), so I replaced it with a digital meter. The cables, for the probe and ground clip, are soldered to the meter leads and strain-relieved inside the box. If your Capac probe has failed, probes such as this can be purchased for 70 to 80 bucks. The meter cost me $4 at Harbor Freight. Buy a box from Radio Shack, along with some cable and a ground clamp, and you’re in good shape. Accuracy of the readings is better than the original Capac setup (whose needle tends to stick). And you can be a hero at your marina by testing other people's boat (long as your marina guys don't mind, or catch you).

"Hey, what are those funky looking things?"

This 'loaf of bread' is a magnesium anode, and it sure confuses lots of folks. It is bolted to the trim tab such that it does not interfere with its movement via two, 5/16 bolts threaded into the magnesium from below.

Note: This is for FRESH WATER boats only! Magnesium produces way too much voltage in salt water.

"Now, that's a guppy!" (Think of Crocodile Dundee saying this)

This little beauty alone adds 0.15 volts to my galvanic protection system. With a guppy added to mine the readings are too high, but it’s nice to have that option

Protective Zincs in Your Heat Exchangers

Many people don't know this, but there's a zinc element that keeps the tubes of these expensive heat exchangers from rotting out. With replacement costs hovering in the 400 to 600 dollar range, one should replace these zincs on a regular basis. The drain plug in the bottom contains the zinc.