What fixings to use to prevent electrolysis

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barnzy
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What fixings to use to prevent electrolysis

Post by barnzy »

Hi guys. I put a post up a while back about how to stop electrolysis. Well the boats painted now. So I'm currently fitting it back out. What's fixings should I use? Some people say aluminium rivits, and others say 316 stainless. I can't seem to find anything on marine grade pop rivits.
What do you's think?


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grumpy
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Re: What fixings to use to prevent electrolysis

Post by grumpy »

Me thinks anything you use that is not aluminium into aluminium will eventually cause electrolysis.

I use stainless rivets above the waterline and they last a fair while before they start to show signs in the alloy. My floor is screwed off with 316 stainless screws and you can see the white powdery action from the electrolysis after about 6 mths. Just means every now and then I have to shift the odd screw.

It does help if you can get a layer of silicone between the stainless pop rivets.

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dannett
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Re: What fixings to use to prevent electrolysis

Post by dannett »

Just quickly to clear up confusion theissue is not electrolysis but galvanic corrosion. Unfortunately, unless the metals are identical (including the alloy) it is impossible to prevent through metal to metal contact. And salt water acts as an electrolyte which increases the reaction speed, this is where the electrolysis confusion comes from.

Basically, every metal is considered an anode and a cathode and depending on where it lies on the chart has a positive or negative voltage assigned to it.

To reduce the amount of galvanic corrosion you need to select metals that are close to each other on the galvanic index (refer chart below) to reduce the potential difference (voltage) aiming for difference under 1 volt and in harsh environments less than 0.5 volts is preferred. The other thing to consider is the overall mass of both objects. Because the stainless steel screws are relatively small they have a lesser impact upon the aluminium. If the masses were reduced the aluminium would corrode rather quickly. Also in our favour is that aluminium tends to form a protective oxide coating which reduces the reaction speed.
galvanic-series.gif
So 304 stainless screws are a good choice in marine grade aluminium hulls because there is only a voltage difference around 0.7 volts. (304 is about -0.1 volt and marine grade aluminium is around -0.8 volts).

Heaps of resources around on the subject if you're keen to know more. http://www.ssina.com/corrosion/galvanic.html

Dan
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jabsy
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Re: What fixings to use to prevent electrolysis

Post by jabsy »

A severe drowning in lanolin grease between the 2 dissimilar metals as well as the associated fixings also will help.
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Re: What fixings to use to prevent electrolysis

Post by ronje »

The problem with stainless steel fittings into aluminium is that you don't really know what you are dealing with in terms of its galvanic corrosion properties.

For example 304 and 316 stainless aren't far apart from each other on Dannet's galvanic table. Look at the chart and you'll see that they're both around the -.1 to -.2 depending on which type. Look at the little black boxes opposite each type.

If you look at the the little clear boxes for each type, you'll see that both are significantly different.

These are differences between active and passive stainless 316 and 304. .4 volts difference for 304 and similar for 316.

However, both of them are quite different to the aluminium galvanic voltages of -.8 to -1 volt.

Differences of .7 to .9 volts with both types of stainless if both are passive and .4 to .6 volts if both are active types. Those differences depend on the film surface conditions of the stainless screws.

In addition, the aluminium varies in galvanic volts depending on which type of marine aluminium is used. To change strengths of aluminum, manufacturers muck around with the alloys used.

The main one used in that process is magnesium which is one of the "out-there" materials to drag the aluminium galvanic voltages even higher.

And there-in lies the main problem.

We don't know what type of aluminuim is used in forming the hull. Probably one of the 5000 series but which one? The magnesium % is different for different types and so the galvanic voltages are different.

We can't stop galvanic corrosion. All we can do is try to minimise it.

Not being metallurgists how can we fit out a boat then to minimize galvanic corrosion?

Assume the worst and fit "barrier protection" pastes.

I don't know about lanolin for barrier treatment. Jabsy may well be right.

304 stainless is a bit closer to aluminium on the table so I'd go for that type with Tefgel or similar
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Re: What fixings to use to prevent electrolysis

Post by dannett »

ronje wrote:We can't stop galvanic corrosion. All we can do is try to minimise it.
This is the main point I was trying to make.
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Re: What fixings to use to prevent electrolysis

Post by ronje »

And you did indeed. I was just agreeing with you.

The different types of aluminium alloys are a bit of a problem. Particularly those from China where most manufacturers seem to have the hulls fabricated. Plenty of well known manufacturers in Aus do it.

Trying to accommodate those differing types of alloys with differing fittings is a bit like trying to poke a marshmellow up a wild tiger's bum.

To be honest, I'd actually go for aluminium screws etc to get the minimum galvanic voltage difference but they are a bit harder to find.

There's also the mindset in Aus that "if it ain't 316 stainless its no b......dy good in salt water".
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Re: What fixings to use to prevent electrolysis

Post by dannett »

ronje wrote:There's also the mindset in Aus that "if it ain't 316 stainless its no b......dy good in salt water".
Isn't that the truth.

You are right to point out the variance in the galvanic index, I expected that "marine" grade aluminium (one of the 5000 series) is at the noble end of the range given. But I went looking and it turns out this is not so. At the lower end (more noble) is the 2xxx series which is copper alloys used in the aerospace grade industry. Great corrosion resistance, but not welding friendly.

I couldn't find any data sheets that actually listed the galvanic index value of the particular alloy. There were a few tables which listed the 5xxx series at the higher end (less noble) but these didn't identify many of the alloys and possible not the ones we are interested in. You are right, you would need to be a metallurgist to really know the best compatibility.

Aluminium screws would be the go but like you say difficult to find. Easier would be good quality aluminium rivets (2000 series?). These should do the trick for a lot of applications provided that you can eliminate movement stop the rivets being pulled out.
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Re: What fixings to use to prevent electrolysis

Post by Till »

Use duralac with the screws
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Re: What fixings to use to prevent electrolysis

Post by barnzy »

Thanks for all the feed back, it is a bit of a pain, but I think it's worth the muck around. Do it right and it will weather better
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Re: What fixings to use to prevent electrolysis

Post by grumpy »

Thanks for the input guys a well informed and researched post.

GALVANIC CORROSION! I need to remember that. :mrgreen: :mrgreen:

Cheers
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Re: What fixings to use to prevent electrolysis

Post by jersy »

Fark google! Way over my head but I get your drift. Excellent info again fellas. 8) :applause:
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