Hi guys out of interest , i was reading a thread on electrolicis but my mind turned to thinking dont we all use shackles and winch hooks permanantly attaching our boats to the trailers ,so why do we make such a big deal with dissimilar metals touching the boat . the only thing i can think of is it causeing a problem while on the water . Can some one explain it ..............matt
A bad days fishing has got to be better than any day at work......
matty, the salt water is the key. it is an electrolyte (just like the acid in a lead/acid battery) so starts the reaction. basically the two different metals form the anode and the cathode, if the hull or aluminium fuel tank becomes the anode then you're in trouble.
I can add a little about the metal difference ....
Metals have a place on what is called the "Nobility Scale" ... all fall somewhere on the scale.
Some are more noble than others ... or regal or kingly if you like ... GOLD is the most noble with mercury a close second (from memory).
Less noble are things like aluminium, zinc and chromium.
Here's a thing as well .... LESS noble metals are ANODIC or good as anodes (outboard fins etc - zinc)
More noble metals are Cathodic and good as cathodes = copper and lead (for batteries etc).
Here's another thing .... the closer the metals are on the scale the less reaction they have between them when it comes to Galvanic Action.
As an example ... Aluminium is close to zinc and further from stainless so it reacts little with zinc but much more with stainless.
And also ..... in the case of aluminium and stainless the aluminium is low nobility verses the stainless so it is ANODIC and sacrifices itself to the process. Thats why the ally disappears ..just like our zinc anodes on our boats disappear.
The time it takes is proportional to the surface area and the electrolyte availability and composition (salt V fresh etc).
Regards
PWCDad
G"day mate , electrolysis is as pwc described two differant metals in a solution with a stray current . galvanic corrosion is more two metals placed together with a medium such as water to form an unconfortable alliance. [ makes me think of my last relationship]
Thanks guys i think that clears that up ,i was just wondering because i pulled the floor of my son-in-laws boat a few weeks ago to fix some faulty electrics and i found all sorts of things in the hull sinkers swivels and old screws and i was surprised how these things only left a small mark in the hull and they look like they had been there for years ,its an old clarke and he has had it 4 yrs and it was second hand then ...thanks matt
A bad days fishing has got to be better than any day at work......
Hi Matt,
There is more to Electrolysis Corrosion than meets the eye. It actually effects all boats irrespective of size and in most cases because it is the main cause of corrosion to boats can be a costly problem to just simply ignore and hope that it goes away. There are ways to minimise or slow the process down however, I believe finally there is a product available that guarantees to eliminate electrolysis and prevent corrosion altogether if you want more info on electrolysis go to www.seabis.net and i am sure you will find all you need if not ask one of their engineers or boat technicians- it is free
Originally Posted by Angelika koop
Not mine! I've got a Poly Boat. Viva La Revolution!!
So should plate boat owners be disconnectiing recovery winch hooks,and turn buckle tie downs,when in storage?
David
When modifying/customising my tinny I used "Duralac" (an anit-corrosive joining compound that inhibits elctrolytic corrosion between dissimilar metals) between ALL metal on metal contacts, especially on stainless to aluminium. All screws, rivets etc got a coat before joining. A tube is intially expensive at around $40. BUT the protection it offers is worth a lot more.
Andrew
Not mine! I've got a Poly Boat. Viva La Revolution!!
What sort of Poly boat..? tell me why I should buy one rather than a 5.0M CC alloy with a 75 for coastal reef and bay work..? genuine question...all the people who are getting ready with all their tupperware jokes and other rubbish please get back to work.
I looked at one about 2 yrs ago and it just looked a bit uneven and "flexy" if you know what I mean...so interested to hear..this will test your objectivity
nah dave just give ya boat a hose down before storage to wash the salt away
Thats not really necessary. I doubt that you would get much water trapped between hooks and turnbuckles etc.
BTW, Electrolosis is actuallly current flowing through an electrolyte (salt water) and Galvanic corrosion is a result of current flowing from one metal to another (in an electrolyte), resulting in the less noble metal being corroded.
There is many ways galvanic corrosion occurs. It can occur on the grain boundaries of metal, resulting in the surface corrosion (like surface rust on a car) and it can occur between different metals, such as zinc and aluminium. The problem for most boaties is when the aluminium is immersed in electolyte (salt water) and this is when you get 99% of your problems. The best way to deal with this is to use sacrifcial anodes, such as those found on your outboard. IF the boat is trailered, then the anodes on your outboard will probably suffice, so long as your outboard is bonded to your hull.
Stainless steel bolted to aluminium can also cause problems becase the electrolyte becomes trapped between the two metals. Once the corrosion sets in the alumiuim oxide will also help retain electrolyte. This is where duralac has its place. Duralac will isolate the 2 metals so that there is no current flow between the two metals. No current flow means no galvanic corrosion.
Sometimes you can get localised corrosion that will happen when you leave a piece of stainless in your bilge, and there is always bilge water in the boat. Hosing out with fresh water helps a lot because fresh water is a poor electrolyte. Better solution is not to leave stuff in the bilge.
Mindi, I have a polyboat too. Not the best looking but definately one of the most rugged boats I have ever owned.
Interesting subject.
Question - Should we be cleaning the sacrificial anodes on our outboards? I noticed mine are in pretty good shape (probably not a good thing when I think about it?).
The should be fine, but the definately work better if they surface between the anode and the cathode(boat/outboard) is clean.
im not a sparky or metalurgist but in lay mans terms is this true that if you run wiring in a alloy boat you should always run earth back to the battery not the hull otherwise you would speed up the electrolisis process
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