Can Someone explain Electolysis please?

Hello,
Having been involved with dinghies most of my life I have never concerned myself with Electrolysis, and how it affects boats more permanently in the water.
Would anyone be kind enough to explain it, and all it's associated problems to me? Along with different considerations for different hull materials?
All help greatly appreciated.

 

Matthew,
This is a good article in pdf form that helped me to understand how dissimilar metals interact. This type of corrosion can happen with other dissimilar materials as well.

http://www.kastenmarine.com/mbqMetRef.pdf

At least it’s a place to start.

Gary

 

Matthew

The reference given by duluthboats is excellent and covers a wide range topics associated with electrolysis except the very basic items as the document assumes you understand something about it.

So here are a few things that the document skips or only covers briefly.

When metals are connected, are in water and close to each other they generate a small electric current and act as a battery, the terminology of +/– and cathode/anode is the same. When this starts, the anodic material slowly strips off and ultimately leaves a hole. The rate of corrosion depends on various things

• The difference in voltage caused by the metals (more detail below) and take care that you choose the correct type of stainless steel. The wrong choice is very expensive especially if the hull is copper sheathed or of a copper rich alloy. The standard rules-of-thumb don’t always apply, often it is the opposite.
• The area of the metals interacting – that is the proportion of one to the other (more detail below)
• The salinity of the water – salt water is worse than fresh water, more action takes place. But fresh water in rivers downstream of industrial/chemical complexes and big cities can contain high levels of pollution that can cause corrosion just as bad as salt water. It is safer to protect a hull using salt water standards unless you are sure that the water is drinkable where you sail.
• Temperature – worse with high temperature.
• Metal condition. Although you might have a steel hull the steel is not the same throughout and hammering and mishandling can change the properties to cause increases in local corrosion. So does welding.

Voltage difference.
Go to page 20 of duluthboats reference, the table headed Average Voltage in Seawater.

On the right the block heading is Most Noble Cathodic. The sixth item is silver; gold and platinum are the best and they’d be at the head of the metals – the moral being that you should build a platinum boat But this explains why such tables refer to “noble” materials.

The difference between the voltage of any two metals is the voltage that would be generated between them. The higher the voltage the worse the corrosion and the recommendation of 0.2 volts is a good guide – generally.

Area of metals.
Just as important as the voltage difference is the ratio of areas. Say you had a bare steel hull (the anode) and a small brass (or any other copper alloy) fitting in the hull you have a very large cathode and a small anode. This would not cause a problem as it is the steel that corrodes, but there is so much of it you would hardly notice.

Even if the hull was properly coated with a suitable high quality epoxy or something equally good you could have a serious problem if damage occurs. Suppose there was a minor scratch but it went through the coating and exposed the metal underneath. Now you have a tiny area compared to the fitting which is now comparatively huge. You now have a case of galloping corrosion causing a serious pitting in the hull.

But say you had a wooden hull and copper sheathed it and you had a cast iron fitting you have the opposite effect. Now the hull is the anode and the fitting corrodes, not only that but the anode is huge in comparison and your fitting could disappear in a matter of weeks.

Potential hot spots
These are places generally on power driven boats where there could be a fairly high group of metals such as bronze forming propellers, bearings and the like so there is a concentration of cathodes attacking the localised hull. In these cases it is customary to attach anodes, usually zinc but magnesium is better if you can afford it. These anodes corrode and so protect the hull. They come in all sorts of shapes and sizes and you can protect your craft for different life-spans. Not enough and you must attach new anodes every few months, too many and they may last your lifetime but they are heavy and they add to the hulls resistance so more power is needed.

The anodes must be well attached to the hull. They have a steel bar at the ends and they are either welded or bolted to the hull depending on which type and size are selected. The manufacturers give detailled instructions on attaching them.

Summary/wrap up
There is much information in the reference paper on the properties and qualities of metals and the lower part of page 17, [*]Preventing Galvanic Corrosion[/b], gives excellent advice.

I hope my notes have helped and maybe you can now search for better explanations knowing what to look for.

Michael

 

Ya, what Mike said. Sorry I posted the wrong link. The one below should be read first. I had them both and mixed them up. Mike D explained it about as well as can be in a small space. Anyway here is the link to the other article by Michael Kaston.

http://www.kastenmarine.com/mbqCref.pdf

Gary

 

Thanks guys.