Anode life and sea temperature

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ChrisE

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Does anyone know if the rate of decay of a anode is affected by sea temp? Boat now in Norway where the sea temp hasn't risen above 12C, is currently 8C and expected to be 3-5C any day now. As our antifoul looks OK I'm loath to lift the boat if the anodes will survive another season.

TIA

Chris
 
Simple rule of chemistry Q10=2 - in other words increase the temperature by 10degrees centigrade and the reaction rate doubles. So if the average of the Norwegian year is say 10 degrees and say Spain is 20 degrees, the anode will decay twice as fast in Spain
 
I'm with MACD,I'm not certain that temperature is a major factor in electrolytic reactions.
At some stage a proper Physical Chemist will come on the scene and enlighten us!
If water freezes around anodes I'm certain that you wont get any corrosion but thats because of the change of state not temperature directly.
 
I have my own views on this but I will keep them to myself cos always some idiot comes along and tells me I don't know what I am talking about.

However, to help the more sensible among you draw some conclusions you may like to think about the well documented information as to how temperature affects the performance of some other types of cells, for example those in the batteries we buy to give us electricity.

John
 
That's the bit of chemistry I had at the back of my mind and if I read Ships_Cat aright that's what he thinks as well. I started from the proposition that cold batteries aren't as lively as warm ones.
 
I'm no chemist either, but would have thought that temperature affects electrical activity as cold elements are less active at an atomic level, and more active when heated...... basic physics/chemistry surely?... stress again.. I'm no chemist.... but do have a physics/electronics background....
 
Might well be wrong; initial hesitancy was cos I couldn't find any "evidence" which is usually not a problem with something as fundamental as this.
This site provides some clues http://www.esru.strath.ac.uk/EandE/Web_sites/99-00/hybrid_PV_FC/electrointro.html and provides direct correlation between temp and activity.Would delve some more but it's bed time.

Not helped by the fact that electrolysis in google seems to be obsessed with hair removal.
 
Re: Interestingly

I think that the metallurgists might be wrong then /forums/images/graemlins/smile.gif, as physicists and chemists think otherwise.

The Nernst Equation determines the potentials of each of the reduction and oxidation half cells (a search on Nernst Equation will bring it up) but this link, for example, http://www.mcnallyinstitute.com/CDweb/g-html/g002.htm explains what happens in a cell in simple galvanic corrosion terms - the second bullet point towards the bottom of the page gives the answer regarding temperature increase. Note that when they say increases the rate of corrosion, in terms of our zinc anode it is the corrosion of that which would be meant.

Frankly, while I have never checked, I suspect that the differences in water temperature differences experienced by a boat make very little difference to the activity of the cell with the boats zinc anode as one half (ie makes little difference to the rate that the zinc is lost). Temperature in Nernst's Equation is in degrees Kelvin (273K = 0C) so you need a big temperature change to change the cell voltage by much.

Time to run for cover from the battery maniacs!

John
 
Re: Interestingly

[ QUOTE ]
electrolytic corrosion increases as you get closer to 4C.

Ask any professional metallurgist

[/ QUOTE ]Beat me to it - in certain specific cases, however in general and considering the inferred application in this case....

In addition to the chemical properties of water, physical characteristics will influence corrosion. The most important of these physical characteristics are temperature and velocity of flow.

Temperature speeds up the rate of corrosion just as it does most other reactions. However, the effect of temperature on corrosion can be more complex. A high water temperature reduces the solubility of calcium carbonate in water, which promotes scale formation and slows corrosion. Temperature also alters the form of corrosion. Pits and tubercles tend to form in cold water while hot water promotes uniform corrosion. Uniform corrosion spreading across the entire surface of a material is far less problematic than tuberculation, so high temperatures can actually seem to slow the corrosive process.

The influence of flow velocity on corrosion is also rather complex. Moderate flow rates are the most beneficial since they promote the formation of scale without breaking loose tubercles. At low flow velocities, corrosion is increased and tends to be in the form of tuberculation due to the prevalence of oxygen concentration cell corrosion. At very high flow velocities, abrasion of the water against the pipe tends to wear the pipe away in a very different form of corrosion. High flow velocities also remove protective scale and tubercles and increase the contact of the material with oxygen, all of which will increase the rate of corrosion.

Are all the physicists and chemists even more confused now? or is it starting to make some sense?

Salinity must also be taken into account so really there can be no hard and fast answer to the question unless many more parameters than just temp are taken into account - ask any professional metallurgist who specialises in corrosion:o /forums/images/graemlins/wink.gif
As an example take the Scottish Sea Lochs - during the winter they are colder than the summer ( to the point of freezing in some locations) but they can be more acidic due to fresh water runoff from the hills surrounding them, so while the temp is lower anode depletion rates can be higher than in summer when the surface temp is higher but less acidic. Conversly, salinity tends to be higher during the summer months (if there is such a thing as summer in Scotland).

I would suggest a wee dive under the boat and check how much of the anode is remaining before deciding to replace it or not. There again that is only my opinion as a professional metallurgist and is worth precisely what you paid for it - nothing!
--------------------
hammer.thumb.gif
"Artificial intelligence is no match for natural stupidity"
 
Re: Interestingly

Cliff,

Thanks for the detailed reply and your practical suggestion of a peek underneath is very sensible.

However, the thought of a wee dive makes me think I ought to buy one of those underwater camera thingies!

Chris
 
Re: Interestingly

I worked for a corrosion company for >10 years, so am pretty sure my answer is correct /forums/images/graemlins/smile.gif
 
Re: Interestingly

You could try to buy a silver/silver chloride half cell (http://www.buckleys.co.uk/mrcell.htm), you can then measure the potential (voltage) and current used by the anode, this will tell you if it is working or needs replacing

I have no idea how much they cost though, probably more than a camera /forums/images/graemlins/smile.gif
 
Re: Interestingly

But was not the question about temperature and its effect on the anode? Whereas you talk in the main about the corrosion of other remote metal objects eg the internals of a pipe, or about the effects of other things in the boat's environment which are all entirely at large and may apply anywhere, not just because of a warm to cold water repositioning of the boat.

Do you wish to start me on those? /forums/images/graemlins/smile.gif

But in the end, I do not know of any professional operators that worry about what seawater port, sea or latitude they are in as to how much faster or slower their anodes are eroding. None of the ones I work with seem to and I do not concern myself either with my own boat.

Possibly, if the poster's boat is a well built frp or timber boat, it may even be quite happy without any anodes /forums/images/graemlins/smile.gif.

John
 
Re: Interestingly

As a temporary safety measure you could allways dangle another temporary anode over the side of your boat so you don't have to rely just on your fixed anode
 
Re: Interestingly

John,
I wish that were true, I lost a prop some years ago 'cos the anode hadn't been wired up and as I 've just bought an expensive autoprop I didn't want to see it disappear as well.
Chris
 
Re: Interestingly

The corrosion rate doubles for every 10degreeC increase in temperature, also, higher temperatures (>20C) encourage SRB bacteria which have a serious effect on corrosion (obviously not that much of a problem if it is pretty cold)
 
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