Preventing Electrolysis

[rpgs60]

I have just purchased an aluminium centre console 460 Ozcraft, it is fittedwith a 50hp 2 stroke Tohatsu outboard, nav lights, bilge pump etc. My questionis "should I disconnect the battery when the boat is not in use to preventelectrolysis"? Is this necessary and is there other thing I should bedoing? Your learned advice would be greatly appreciated.

Thanks Ross

[chris69]

If you have a battery switch turn that to the off position,if you havent yes disconnect your battery.

[nigelr]

I go to the trouble of disconnecting both mine even though I have an isolator.
Probably stupid, but complete peace of mind is worth it for a minute's effort IMHO.
How do you find the tojo 50?

[TheRealAndy]

Most corrosion in aluminium boats is caused because they are not washed down properly. Salt is hydgroscopic, so it pulls humidity out of the air. When it does this, it forms a local corrosion cell. When this happens, its like a small battery, which then goes on to break down the salt and form an acid. Once the process starts, it keeps going. Result is corrosion and pinholes. This process will occur weather the battery is connected or not. Having the battery connected will not accelerate the problem, so there is no issue with keeping the battery connected.

So the best thing you can do for your boat, is to remove all the salt crystals. Get in there and wash it down with fresh water.

[cehart]

What length of time are we talking about before one would get corrossion issues from not washing ?

[Jarrah Jack]

What length of time are we talking about before one would get corrossion issues from not washing ?

My '79 quinnie as never been washed since I've owned it and isn't showing any problems.

[rpgs60]

Thanks for the advice. Sounds like a thorough clean after each use and for added piece of mind I will disconnect the battery as well!
Nigler, as I have just got the boat home I haven't had a chance to take it out yet but I expect the 50 Tohatsu will do the job fine.
Thanks again

[Axl]

As TheRealAndy has said make sure you give everything (Hull, Trailer, Motor) a good wash after use. This is also a safety thing salt can and will damage most things it is contact with.

As long as your rig is wired up correctly (which I suspect it is) ie: the earth is not diretly on the hull any where you wil have no problems.

I would be more worried when disconecting the battery that if you took the negative side off and it came in contact with the hull you would then have power running around the hull then there may be an issue.

Do yourself a favour and get an isolation switch put in, that is what I have and when it not in use it is switched to the off position. This will also save your battery going flat if something in the boat draws power when its not in use.

[deckie]

If easy enough to get at i would disconnect both terminals for a tinnie after every trip. Not exactly difficult or time consuming. Disconnecting might sound old school but it also sponsors a good practice of checking and cleaning the terminals more often.
The simplest answer to a simple maintenance issue is usually something simple...tho these days we're meant to throw money at everything and/or complicate it.
A little detergent with washdown assists salt removal...once again you can throw money on something different for that also.

[FrogBat]

Make sure all the holes that have been made into the hull are well sealed with paint or a sealant if the hull is painted.
I've noticed most problems with paint start from where a hole has been made for things like rodholders or where screws and bolts are used.
There are speciality products for screws and bolts available but Lanolin grease works well.

If the hull is not painted then a good wash with truck wash and dried well should restrict the electrolysis.

[winston]

rpgs60, basically there are two types of corrosion i.e. electrolytic corrosion and galvanic corrosion, both of which are forms of electrochemical cells. In electrolysis (electrolytic cell) an electrical source is used to facilitate a chemical reaction e.g. electricity is used to electroplate an object. From a boating perspective your battery can act as the power source and your hull as the sacrifical anode, i.e. the hull dissolves. this is easily prevenedt as noted in the responses above.
Galvanic corrosion occurs when a chemical reaction is used to facilitate an electric current i.e. call/battery. All that is necessary to produce this cell is two dissimilar metals and a conducting solution. From a boating perspective this can be salt water and another metal attached to your aluminium hull. Galvanic corrosion is easily prevented by removing the conducting solution e.g. washing off the salt water and/or ensuring that you do not have any metal (excluding the same grade of aluminium) attached directly to the hull e.g. use sikaflex. Cheers Winston.

[TheRealAndy]

rpgs60, basically there are two types of corrosion i.e. electrolytic corrosion and galvanic corrosion, both of which are forms of electrochemical cells. In electrolysis (electrolytic cell) an electrical source is used to facilitate a chemical reaction e.g. electricity is used to electroplate an object. From a boating perspective your battery can act as the power source and your hull as the sacrifical anode, i.e. the hull dissolves. this is easily prevenedt as noted in the responses above.
Galvanic corrosion occurs when a chemical reaction is used to facilitate an electric current i.e. call/battery. All that is necessary to produce this cell is two dissimilar metals and a conducting solution. From a boating perspective this can be salt water and another metal attached to your aluminium hull. Galvanic corrosion is easily prevented by removing the conducting solution e.g. washing off the salt water and/or ensuring that you do not have any metal (excluding the same grade of aluminium) attached directly to the hull e.g. use sikaflex. Cheers Winston.

electrolytic and galvanic corrosion are the same thing.

The correct term is galvanic corrosion. The electrochemical process is exactly the same.

[winston]

TheRealAndy, what I have written is accurate. Please refer to any chemistry text book re oxidation and reduction or google, e.g. http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/electrochem.html
Cheers Winston.

[TheRealAndy]

TheRealAndy, what I have written is accurate. Please refer to any chemistry text book re oxidation and reduction or google, e.g. http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/electrochem.html
Cheers Winston.

Nah, did enough of that at uni when I studied this shit.

[PADDLES]

as far as i understand you're both sort of right, the reaction and the end result are the same but the causes are different, one is caused by an injection of current into the cell caused by a voltage differential and the other actually creates a current and voltage differential, same same only slightly different. some of the interesting end results are the acid formed and in the case of incorrectly set up cathodic protection and alkaline is formed (which then kills your protective coating and promotes further corrosion)

an old yachty mate of mine always told me that the only thing that prevents corrosion is paint we do a bit of marine work and in many cases specify stainless be painted.

[winston]

the reaction and the end result are the same but the causes are different, one is caused by an injection of current into the cell caused by a voltage differential and the other actually creates a current and voltage differential, same same only slightly different.

Paddles, is that not what I clearly explained above? Not sort of right but accurate. Cheers Winston.

[TheRealAndy]

as far as i understand you're both sort of right, the reaction and the end result are the same but the causes are different, one is caused by an injection of current into the cell caused by a voltage differential and the other actually creates a current and voltage differential, same same only slightly different. some of the interesting end results are the acid formed and in the case of incorrectly set up cathodic protection and alkaline is formed (which then kills your protective coating and promotes further corrosion)

Not quite. the cause is the same irrespective of what may influence the rate of corrosion. With galvanic corrosion, the cell is caused by a voltage differential (hence the reason its called a cell! It is actually a little battery!). Does not matter if you increase this by injecting current, it is there, and always will be. To use you example, would also mean that we would also need to coin a new term for galvanic corrosion accelerated by chlorides.


an old yachty mate of mine always told me that the only thing that prevents corrosion is paint we do a bit of marine work and in many cases specify stainless be painted.

Not a truer word has been spoken.

The only other method of corrosion control is cathodic protection, but this is only useful under water, or in moist soil.

[PADDLES]

yeah andy, that's sort of what i was trying to say but didn't do a good job of it, i'm a little bit familiar with cathodic protection ie. active by injecting current or passive by just using a sacrificial anode. this does work exactly the same except you are forcing a galvanic cell to be created that will try to corrode the anode instead of the thing you are trying to protect (the cathode) anodes can be as simple as the zinc blocks you may find in your outboard or as complex as the platinised titanium jobbies that hang from a marine structure. active cathodic protection is a bit of a dark art however and is best left to the experts as the incorrect voltage/current levels can cause the electrolyte to become basic (higher pH) which can destroy your protective treatments. but you are right, for it to work you need both the cathode and anode to be in the electrolyte.

[TheRealAndy]

yeah andy, that's sort of what i was trying to say but didn't do a good job of it, i'm a little bit familiar with cathodic protection ie. active by injecting current or passive by just using a sacrificial anode. this does work exactly the same except you are forcing a galvanic cell to be created that will try to corrode the anode instead of the thing you are trying to protect (the cathode) anodes can be as simple as the zinc blocks you may find in your outboard or as complex as the platinised titanium jobbies that hang from a marine structure. active cathodic protection is a bit of a dark art however and is best left to the experts as the incorrect voltage/current levels can cause the electrolyte to become basic (higher pH) which can destroy your protective treatments. but you are right, for it to work you need both the cathode and anode to be in the electrolyte.

When they use a power source coupled with cathodic protection its called impressed current cathodic protection.

[cormorant]

I'm "impressed". Didn't know that term. Learn something every day.

Now for the can of worms. Do the 4wd antirust systems actually work and how??? Back under a rock now.

As a side issue there are different types of Sika and other brands designed specifically for different jobs with different materials. They have different cures ( acid, water ,solvent , 2 pack etc) and end up with different cured PH level and are compatable with different chemicals,fuel,alloy, plastics saltwater etc. Using the "one Sika, silicon, mastic solves all" has cost a lot of people money and sinking boats ,- may take years . Use the right ones for the right purpose and take the effort to read the spec sheet not just follow the nong in the shop or a mates "no"idea.

[PADDLES]

good on ya cormorant for asking the 64 million dollar question ................... "Do the 4wd antirust systems actually work and how???"

i'm stuffed if i know how really ............

[TheRealAndy]

I'm "impressed". Didn't know that term. Learn something every day.

Now for the can of worms. Do the 4wd antirust systems actually work and how??? Back under a rock now.



No they don't work. The theory is not possible. For impressed current cathodic protection you need an electrolyte, something you don't have with a car (unless its submerged).

Best prevention for rust on cars is per Paddles suggestion, paint (or some sort of coating). Remove oxygen from the equation and you don't get oxidisation!

[john b ward]

I Was always of the belief that lost hooks and sinkers in a alloy boat wouldincrease the electrolysis with the dis-similar metals and the sightly acidicsalt water making a alloy tinny much like a Battey , hence the use of thespecial paint always used when screwing stainless screws into the hull ? ....

[TheRealAndy]

I Was always of the belief that lost hooks and sinkers in a alloy boat wouldincrease the electrolysis with the dis-similar metals and the sightly acidicsalt water making a alloy tinny much like a Battey , hence the use of thespecial paint always used when screwing stainless screws into the hull ? ....


Aluminium is less noble than lead, so lead will cause the aluminium to corrode. The paint used (or one of) when screwing stainless is duralac, and its sole purpose is to electrically isolate the 2 metals to prevent galvanic coupling.

[john b ward]

Aluminium is less noble than lead, so lead will cause the aluminium to corrode. The paint used (or one of) when screwing stainless is duralac, and its sole purpose is to electrically isolate the 2 metals to prevent galvanic coupling.

So i was right in my thinking mate?
that duralac wont come off my concrete its tuff stuff !
if i can be so ignorant what is less noble ?

[TheRealAndy]

So i was right in my thinking mate?
that duralac wont come off my concrete its tuff stuff !
if i can be so ignorant what is less noble ?

In a nutshell, less noble means less resistant to corrosion. So when a less noble material is electrically connected to a more noble material (in an electrolyte) the less noble material will always corrode first. Thats how sacrificial anodes work on your outboard. The anodes are made made from zinc or magnesium which is less noble than aluminium, so it corrodes first.