How The Corrosion is Affected by the Current

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 Factors That Affect the Corrosion

It is clear that the corrosion process of metallic elements on the boat depends on whether the metal/alloy is in direct contact with the seawater or if the metal is not in direct contact with the saltwater.
In the first case (Direct Contact) the following factors can affect the corrosion process:

  • Acidic and Alkalinity level of the water
  • Stray Currents 
  • Cathodic protection of the metal
  • Permanent contact between different metals and Saltwater (Galvanism)
  • Bonding system quality

In the second case (Indirect Contact) the following factors can affect the corrosion process:

  • The humidity level of the environment
  • Stray Currents 
  • Pollutants level on the air
  • Bonding system quality
  • Coating protection quality
  • Inappropriate selection of materials

What Type of Current causes corrosion?

Both types of current (AC and DC) can accelerate the corrosion process.
The more common causes of AC/DC stray-current corrosion are:

  • Extension cords dropped into the bilge
  • Improper wiring on the boat
  • Improper wiring on the dock
  • Reverse polarity
  • Defective Battery chargers
  • Defective Galvanic Isolator
  • Neutral and Ground tied on-board
  • Shorts to ground
  • Inappropriate selection of materials
  • Non-Ignition protected devices such as Starters, Alternators, Ignition Coils, and Battery Chargers

Why a Grounding System?


Any electrical device has a power unit. For example, the Dishwasher has an AC induction motor. When the motor is running, doesn't matter if is ignition protected, some electrons will migrate to the appliance case producing static electricity. If the dishwasher metallic case is not grounded, the static electricity will be there until someone touches the appliance; at that moment the store ed current will be drained through the person's body.
If nobody touches the unit then the static electricity will accelerate the corrosion process in between the dissimilar metals of the unit. Copper rivets will corrode aluminum cases etc, etc.
All the units should be grounded properly, then the grounding conductor should be connected to the external Copper bar buried in the back yard of your home. or to the sacrificial anodes located in the transom of your boat. This is the function of the grounding system in your home and your boat.




Stray Current Sources

Stray currents which cause corrosion may originate from Direct-current distribution lines, substations, or street railway systems.
Alternating currents may cause corrosion especially if the grounding system is defective.
The corrosion resulting from stray currents (external sources) is similar to that from galvanic cells (which generate their own current) but different remedial measures may be indicated.
However, stray current strengths may be much higher than those produced by galvanic cells and, as a consequence, corrosion may be much more rapid.


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Factors that affect the Corrosion Process

Definitely, it is clear that there are six main factors that accelerate the corrosion process:
1.- Permanent leaks of current
2.- Stored static electricity
3.- Bad Bonding and Grounding Systems
4.- Improper selection of materials
5.- Absence or inappropriate installation of protective devices such as the Galvanic Isolator
6.- A Reverse polarity condition

   1.- Permanent leaks of current. There are AC and DC leaks of currents, both of them contribute to the corrosion process. Those leaks can be classified into Intermittent and Permanents.
Sporadic Current leaks occur by bad wiring. Unwanted open wires are the most common source. When those wires touch accidentally metallic surfaces, a short to ground occurs, then if the unit is not Bonded to the system a  static amount of electricity will be stored in the metallic case of the unit, starting a slow process of oxidation in between different couples of dissimilar metals.
Sources: Engines and Generator harnesses are a common source of current leaks. The excessive engine vibration and high temperatures, melt and break some wires producing intermittent and permanent leaks of current.
Loose terminals, corroded terminals, and bad crimping terminals are other sources of current leaks.

  2.- Stored static electricity: It is a silent source of small DC currents. Due to the absence of good grounding conductors, those small currents, in presence of environmental humidity accelerate the corrosion process in between dissimilar metals. All the metallic elements in a boat should be bonded into the main central bonding conductor Does no matter if the element is located over or below the waterline level or if the element is or is not in permanent contact with the seawater.
Sources: The use of non-ignition-protected devices is the main source of static currents. The second source is bad wiring to the main grounding conductor.

  3.- Bad Bonding and Grounding Systems. The bonding network and the grounding connection are the heart of the system. As you can see in the annex video-Clip, the life of your vessel depends on the quality of the bonding connections and the proper connection to sacrificial anodes. It is too simple if you follow the recommendations to select the appropriate sacrificial anodes according to the hull material and type of water (See the annex Poster)



                                                            Buy This Poster


Learn more in the following video



 6.- A Reverse polarity condition: This is another concept difficult to relate with the corrosion process for Engineers and Technicians.
The reverse polarity indicator is a LED light with two legs. One leg with a resistor connected to the Ground busbar and the other one connected to the Neutral bus bar at the AC panel. In normal conditions with good panel wiring, this light should be Off because no current should be flowing to the Neutral conductor. Remember that the number recommendation for good boat wiring is always to keep the Ground and Neutrals bus bars isolated from each other. ( They should be connected together ONLY at the power source).
It is important to refresh also that both, Negative DC bus bar and the Ground AC bus bar should be connected together at the main bonding conductor.
Now, suppose that for some reason somebody switches the AC HOT line with the Neutral line before the main breaker at the AC panel, then right now the Neutral bus bar will be a HOT bus bar and of course, the Hot bar will be neutral. And now suppose that somebody connects accidentally the Neutral with Ground (A common residential error).
Well, this is the worse scenery, in this moment the AC ground bus bar and also the Neutral bus bar are HOT bars, then all the metallic elements previously bonded into the main grounding conductor will be receiving a permanent flow of AC current, accelerating the corrosion process in those elements. If this scenario is not fixed quickly some metallic through hulls will be eroded in a couple of days with catastrophic consequences.




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