galvanic corrosion

MARINE GALVANIC CORROSION​

HELP, My Zinks Are Dissapearing

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DISCLAIMER: This is a very controversial and technical subject.  I have always questioned everything.  it helps me fully understand and ensure things are done correctly, safely, and most efficiently.  This helps me as an owner of Maritec Marine, to always do what is beat for the customer. On the subject of galvanic corrosion i don't agree with some of the recomendations because frankly it makes no sense.  some of the practices do not help and can even accelerate galvanic activity.

​Galvanic Corrosion Basics

Most know that their underwater equipment and hardware is susceptible to galvanic corrosion.  Often time it is incorrectly called electrolysis, but we all know what we are talking about.  The concern is that your underwater equipment and hardware will deteriorate to a point of needing replacement or worse, disappearing to a point of sinking your boat.  Zinks are used as a stop gap to save your expensive hardware but only works if the correct size, location, and properly connected.

Fear sets in when a boat owner notices or a diver informs them that their zincs are disappearing but you should also be concerned if zincs are not deteriorating.  What is the reason for fast or accelerated corrosion, there are several.

How Galvanic Corrosion Works (my version)

When you have 2 disimlar metals they will have a different quantity and adhesion stregth of thier electrons.  this difference will allow the electrons to move from one of the metals to the other.  when this occurs the metal loosing electrons gets smaller, develops crevices, and erodes as the electrons leave. 

I am not a schooled chemist but my imperfect analogy for explanation a very thin strip of plastic maybe 6 inches by 2 inches and 2 small round magnets, one stronger than the other.  connect or  attrach the magnets to each other on opposite sides of this plastic strip, now add another 20 magnet sets to that strip of plastic and hold one end of it in the air so the magnets are dangling down.  now take another plastic strip and attach much stronger magnets only on one side using glue.  Now pass the strips of plastic past each other, theoretically a few of the magnets will jump over to the single ones and the 2nd magnet will fall on the floor, this is a very simple and not really scientifically correct example of how the galvanic reaction occurs.

Requirements

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For galvanic corrosion to occur three things need to be in place. 2 disimalar metals such as a bronze propeller and a stainless steel prop shaft.  Second: these two disimlar metals need to both be in the same electrolyte like saltwater enabling the electrons to transfer bewteen the disimalr metals.  And third:  physical contact of the 2 metals.

Anode Cathode

the disimalar metals are refer to as an anode and cathode, the anode is the one giving up the electrons and can be any metal, even a stainless steel propshaft connected to a stainless steel propeler are techinally not the same exact metal so there is a differential in the atomic structure of each one.

Why Stainless Steel Rusts

stainless steel deck hardware can rust because it is actualy a mix of metal.  if water (electrolyte) gets traped against the stainless it completes the requirements of galvanig activity resulting in rust. 

Nobility

the differential of a metals galvanic reaction is refered to as nobility.  metals of very close nobility like two different stainless steels has a very low nobil difference resulting in a very slow reaction, while zinc and stainless steel have a large noblity differential causing an accelerated galvanic reaction. 

Why Stainless Steel Rusts

stainless steel deck hardware can rust because it is actualy a mix of metals.  if water (electrolyte) gets traped against the stainless it completes the requirements of galvanig activity resulting in rust. 

Prevention 

using what we learned, to redirect the galvanic transfer of electrons we need a larger nobility difference that the boats anode.  that what zinc is, it protects your bronze anode because it is more noble and gives up it electrons easier which is why it zincs are referd to as "sacraficial".  Zinc is not the only scraficial metal used.  for example aluminum is recomended in certain electrolytes and hull materials.  although extremly ligh fo its strength, carbon fiber has metal in it and vey suseptable to galvanic activity and so carbon fiber boats need protection as well.

Bonding is Bad

This is where the controversy stsrts.  Different metals, electrolite, and connection is needed for galvanic activity, so why do we connect (bond) a thruhull?  if not bonded it eliminates the required connection thus preventing any galvanic activity.

Cannons

How does a brass cannon last for 300 years on the sea floor yet your thru hulls are at risk?  Because the cannons have no connection to a disimilar metal.

Explanations

I have gotten into heated arguments with ABYC certified galvanic prevention "experts", ABYC consultants on the subject, and specilaist selling only zincs and other scarficial metals and none of them could succesfully explain why we bond boats.  most eventually reply with "because ABYC says so".

ALWAYS PROTECT

Prop shafts are physiclly connected to the propeller, and aluminum engine blocks are connected to steel pistons.  any connected metals MUST ALWAYS HAVE GALVANIC PROTECTION.

Not Protected

A lot of 50 years old plus vessels and many revent European boats have never been bonded.  50 years is a long time for ganlvanic activity to occur.  Inspect your bonding wires, most are so corroded they aren't bonded to the system.  even somee Yanmar diesels do not have galvanic plugs which confuses me, but they last just fine.

Carbon FIber

cabon fiber is conductive and so galvinic hulls complete the connection requirement for any metals in the water so everything must be somhow bonded to a sacrificial metal.  Racing boats are not typically in the water long and arent protected.

accekerated gakvanic activity