Types Of Marine Corrosion And What To Do About Them?

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By the minute they are fabricated, nearly all metals are working to come back to the original form in which they existed if we dug them out of the floor.


The 3 types of rust boaters typically work to handle are:


  • Simple electrochemical corrosion
  • Galvanic corrosion
  • Electrolytic/stray current corrosion


All three kinds of marine corrosion are the result of an electrochemical process, the difference is how fast the rust happens. The procedure speeds up when multiple alloys are involved (galvanic corrosion) and faster still when unwanted electrical currents are applied (stray current corrosion). While the result of one or more of these kinds of rust is pretty much the same -- exposed metal damage -- there can be many reasons why the corrosion happened at the first place. The challenge we face is identifying the type of corrosion and locating its true reason to minimize future corrosion.

Electrolytic – Stray Current Marine Corrosion

Direct current (DC) stray current corrosion is the corrosion that occurs when current flows through metal and water since it attempts a path back into battery ground. The corrosion rate caused by stray DC current can be 100 times the rate of galvanic corrosion, and it can result in significant damage daily, or even hours. In intense cases of stray current corrosion a lower unit and outdrive casing can be eaten almost completely away, exposing the shafts and gears. While this certainly does occur, there are several other more prevalent instances of stray current corrosion that occur, some which are readily -- but erroneously -- attributed to galvanic corrosion. There is a fairly common ramble present situation which happens on multi-engine ships. Many times, the stainless-steel propeller on a single motor includes a dull surface and the other one is still shiny and clean.


The first clue that this really is a stray current dilemma could be gleaned from such photos. If this was a galvanic corrosion scenario, the stainless-steel propellers wouldn't be affected. The stainless steel would be protected by any busy anodes or, if no anodes are active, by the aluminum housing of the lower units and midsections. In a stray current situation, electrical current is looking for a route back to earth. So as it leaves one metal, regardless of kind, the present will hamper that metal. If the current reaches another metal on its route back to ground, deposits will form on the surface of the metal.


The next clue, in the event the ship operators are observant, is that the matter generally happens while the motors are running, while they are moored. They will clean their props, however, the next time that they return from a day of usage one prop will be coated with buildup. So what could cause this type of stray current corrosion? Frequently, it is due to bad battery cable links. This is an issue because most motors, of multi-engine software, must be at the same electrical potential however, due to charging output variants between running motors, as well as battery condition, they might not be. If they're not, they will attempt to equalize.


Good, clean cable connections and battery wires of the right size are necessary to keep all motors and batteries at precisely the exact same potential. The battery cables that link between the ground terminals of all batteries are especially important to permit equalization. When the connections in the batteries and the motors are poor, stray current can flow from 1 motor to the other through the water in an effort to equalize the potential. 1 common element in this kind of stray current corrosion is lacking or undersized negative battery wires between the beginning and batteries. This condition does not usually cause significant harm in a short time period, but customers will frequently remark on how one prop looks different than another.


There's another similar situation with battery wires that could happen, though, and one which can cause substantial stray present damage in a really short period of time: incorrectly connected battery cables. All negative battery terminals should be connected together. But what happens when a engine's positive battery terminal is connected to some other motor's negative battery terminal?


You may think this would cause an immediate short circuit. Not really. What does occur is 1 motor is going to have a 12V positive capacity on all grounded metals and stray current will flow out of that motor to the other. An incorrect connection can arise when ship owners, an inexperienced apprentice technician, or lawn man is installing new batteries or reinstalling batteries when preparing a ship for use after offseason storage. Watch for these situations, they are not hard to spot and fix once you understand where the proof is directing you.


Electrochemical Marine Corrosion

Electrochemical corrosion, also referred to as self-corrosion, just takes a piece of metal to maintain touch with the electrolyte. In cases like this, the alloy is both the anode and the cathode, as well as the conductive path. Most metals are in reality alloys, meaning that they feature several base metals and impurities. In a metal, one foundation metal acts as the anode, while the other base metal acts as the cathode. The electrolyte can be plain old atmosphere, a liquid, or even a combo of both. Examples we usually see would be the formation of rust on a coating of oxide on bare aluminum. This procedure, though extremely slow, begins when most metals are manufactured and may be the easiest form of rust to slow down. A protective coating, such as the suitable kind of paint, can impede the process way down.


Anodes are set on outboard motors as a sacrifice to the natural process that happens to metals seeking to return to their original form. Aside from a visual check to corrosion, how do we tell whether there are sufficient anodes to protect the motor and other metal components?


The best way is to utilize a silver/silver chloride reference electrode to assess the strand possible. The instructions that come with the silver/silver chloride reference electrode will guide you through a procedure. The general overview of the procedure is to connect with the voltmeter's negative lead into the reference electrode.


Then place the reference electrode in the water close to the submersed metals. With the voltmeter's positive lead on the ship's DC ground or bonding system, you may observe an extremely low voltage reading between the reference electrode and the boat. The voltage, typically from the -750 into -1100 mV range, is the strand possible. You will find established ranges for various hull materials and push systems, which you should be within when the correct amount of anode protection exists. Anytime you're working with a rapid corrosion dilemma, the hull potential has to be assessed for into the root of the problem. Replacing corroded parts without solving the true problem will only result in continued corrosion issues.


Galvanic corrosion normally involves two dissimilar alloys, aluminum and stainless steel. These metals are bonded together either by direct contact or from the electric system and are submerged in an electrolyte -- the water the ship is in. The assortment of those elements basically becomes a big wet cell battery.


What occurs in a battery if a link is made between the positive and negative posts? Electrons flow between the battery's plates, that can be anodes and cathodes. On a ship with no corrosion protection, aluminum, being the most active alloy, will be the anode and the stainless steel, a less active metal, will be the cathode. Electrons will flow from the anode to the cathode, resulting in a loss of anode material, visible as corrosion on the aluminum parts.

Galvanic Marine Corrosion

Galvanic corrosion typically appears as paint blisters using a white powdery residue on the surface that is exposed. Corners and edges of components, such as the leading edge of the unit and the surfaces of this anti-ventilation plate, will usually be the first areas affected. Galvanic corrosion is a lot more destructive than electrochemical corrosion but can be controlled and reduced if you comprehend the corrosion process.


Make sure you check for deficiencies and employ sound preventative measures. The best first line of protection is to employ a layer of fantastic paint to insulate the metals out of the electrolyte. Another system is to introduce another alloy, yet another active than aluminum, into the system. The secret is installing the proper number of sacrificial anode material to safeguard all the aluminum and stainless steel, joined either by direct contact or link to the ship's electrical system.


Sacrificial Anodes Facts

The factory-installed anodes have to be from the water to provide protection. Trim tab anodes might not be in the water if the reduced unit is tilted upward. Transom bracket anodes may not be in the water on engines installed in applications requiring extremely large mounting.


Added anodes are needed when other metallic parts are in the machine: aluminum hulls, jack plates, trim tabs*, trolling motors, etc.. The engine's anodes not just will be unable to guard the other elements, but will also be not able to defend the motor. They'll also erode exceptionally quickly. Additional anodes may be required on a engine when used in applications other than what it was originally designed for. Case in point: SHO® models, primarily a freshwater bass boat engine, may require more anodes if used in saltwater, brackish water, as well as when frequently moored in fresh water.


Anodes have to be clean and free of paint to operate properly. Anodes has to be electrically conductive and connected to the boat and motor's grounding system. Conductivity through the anode material and between the anode and the boat's electrical system deteriorates with time and exposure.


By design, anodes deteriorate as they provide security and have to be replaced occasionally. Replace anodes when they have eroded to 2/3 their initial size. Beware, looks can be deceiving. Anodes may get rid of mass (weight) without noticeably looking smaller. Can you ever find one which seemed like it had been full of worm holes? If the electrons in the anode do not make it to the cathode, the anode will shed even more electrons in a quicker rate in its valiant effort to safeguard the cathode.


Boats connected to shore power might be electrically connected to other boats and dock structures throughout the frequent grounding wire (green) in the AC grid, forming a single huge system. Anodes on a single boat will try to protect other boats or submerged dock structures that do not have sufficient anode protection of their own. A galvanic isolator installed close to the ship's shore power receptacle from the AC grounding (green) wire blocks the flow of galvanic current between boats and dock structures attached to a shore power system.


Not only is too little anode security a issue, too much defense is also not great. Too many anodes or the wrong kind of anode material may produce a various electrochemical reaction that generates hydrogen on the metal surface. Metal components electrically isolated from the remainder of the boat aren't bonded. These components may need their own standalone anode protection to lessen electrochemical corrosion. Zinc, magnesium, and aluminum anodes are available for many resources. How can aluminum shield aluminum? Magnesium is a very good anode material for freshwater, but should never be utilized in saltwater or brackish water as sodium will overprotect and deteriorate very rapidly when used in saltwater.


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