I thought this worth a mention as it's an easy mistake to make:

Just been in the shed going through my box of kit that I use less often than the rest of my SRT gear. I'd chucked in a load of spare krabs and maillons along with my Petzl Rack.

A couple of the bars on the Rack were quite badly corroded on the surface- not just discoloured, but the ali was spalling off in distinct layers. It was lying on top of steel maillons, which must have led to some dissimilar metal corrosion.

Then I emptied out my 'regular' box, and found that the corner of a perfectly good screwgate was pretty much missing. It has more than likely been sitting damp on the steel buckle of my battery belt and corroded away.

So, there you go- keep your aluminium kit away from steel kit if you intend to store it for a prolonged period, or are just a lazy sod like me who leaves stuff wet in the boot of the car for days on end (e.g. Croll on a steel D).

Or, for that matter, if it's likely to get cold whilst in storage, as condensation is enough to make the wet surface join between the two metals.

Wouldn't it be easier to clean and dry your SRT kit after use and store in a drier environment?

underground said:

Or, for that matter, if it's likely to get cold whilst in storage, as condensation is enough to make the wet surface join between the two metals.

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Yes Paul, however having done that, having believed my shed to be dry, I found corrosion to my Rack.

With regard to the car boot storage, using kit regularly has often prevented it being dried and stored between trips- hence my surprise at the krab / belt problem...

I hope that clarifies things for you... :roll:

It's largely acid from peaty water not washed off I think rather than galvanic corrosion...

AndyF said:

It's largely acid from peaty water not washed off I think rather than galvanic corrosion...

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Possibly - salt water can also "eat" alloy.

paul said:

Possibly - salt water can also "eat" alloy.

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I can vouch for that. I do a lot of climbing on sea cliffs and it's frightening the speed at which alloy kit deteriorates, even after washing.

I can agree with the salt water, but my caving kit's never been near the sea.
The reason why I'm sure this is dissimilar metal corrosion is the fact that the kit in this particular box was tored clean and dry- washed in clean water, for that matter before drying.

Then it sounds like some sort of chemical reaction. You haven't got any paints or solvents stored nearby have you? The fumes can affect alloys.

underground said:

I can agree with the salt water, but my caving kit's never been near the sea.
The reason why I'm sure this is dissimilar metal corrosion is the fact that the kit in this particular box was tored clean and dry- washed in clean water, for that matter before drying.

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Fair enough - however I suspect the problem is not simply due to a combination of steel / aluminium alloy and some dampness from condensation as you put before.

I have an old tackle bag of bits I have bought over the years and don't use ranging from a "Curlew Goldlock" U-shaped rack (allow bars/steel frame like most racks) bought in the 70's, a Caving Supplies rack bought in the 80's, old Petzl Stop, 2 old Petzl Crolls, again from the 80's and a mixture of steel krabs and other odd bits. These have been stored in sheds, garages and unheated rooms over the years in that tackle bag for the past 20 plus years. I emptied t out last night and although there's the odd bit of surface rust on the steel bits here and there, the alloy looks perfect.

Makes you wonder if your gear has unknowingly been in contact with something else to accelarate the deterioration of the alloy bits?

Might be relevant.

http://www.corrosion-doctors.org/MatSelect/corraluminalloys.htm#Stress

Plenty of other Google hits if you care to look.

paul said:

underground said:

I can agree with the salt water, but my caving kit's never been near the sea.
The reason why I'm sure this is dissimilar metal corrosion is the fact that the kit in this particular box was tored clean and dry- washed in clean water, for that matter before drying.

Click to expand...

Fair enough - however I suspect the problem is not simply due to a combination of steel / aluminium alloy and some dampness from condensation as you put before.

I have an old tackle bag of bits I have bought over the years and don't use ranging from a "Curlew Goldlock" U-shaped rack (allow bars/steel frame like most racks) bought in the 70's, a Caving Supplies rack bought in the 80's, old Petzl Stop, 2 old Petzl Crolls, again from the 80's and a mixture of steel krabs and other odd bits. These have been stored in sheds, garages and unheated rooms over the years in that tackle bag for the past 20 plus years. I emptied t out last night and although there's the odd bit of surface rust on the steel bits here and there, the alloy looks perfect.

Makes you wonder if your gear has unknowingly been in contact with something else to accelarate the deterioration of the alloy bits?

Click to expand...

Yes, I agree it seems odd- I'm no metallurgist, but it was the closest I could come up with. It seems like a massive coincidence that the Rack had been on top of a bunch of steel maillons, and the krab had been wet, and I assume, in contact with the steel of my belt buckle.

In the case of the Rack it was nothing the Dremel couldn't clear up, but the krab will be relegated to the scrap bin.

Then I emptied out my 'regular' box, and found that the corner of a perfectly good screwgate was pretty much missing. It has more than likely been sitting damp on the steel buckle of my battery belt and corroded away.

Click to expand...

If you take two disimilar metals such as iron and copper for definite (and I am fairly sure this holds true for iron and aluminium) and put them in an electrolyte you have a battery.
Try it with a 6" nail, a piece of 15mm copper pipe and a potato.
put a voltmeter between the metl bits, you'll be surprised!

There may have been enough dirt in the fibres of your belt and on the kit (even after cleaning) to dissolve into any water and make it an electrolyte.
I'm not talking large or even necessarily visible amounts here.
If the two components were nicely snugged onto the belt and also touching at their other extremity then you have a circuit which will encourage all sorts of chemical reactions.

I'm surprised at the degree though, what I describe is unlikely to make as much difference as that unless the conditions are especially good.

You didn't mention it - was the battery attached?
Could any current have ben supplied? - this would make a BIG difference.

With regard to the car boot storage, using kit regularly has often prevented it being dried and stored between trips- hence my surprise at the krab / belt problem...

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Hmm, that sounds familiar, and my boot is permanently damp from all the muddy caving kit that gets chucked in there.
That's got me thinking... thanks.

Bit of a trawl from the past here, but I was referring to this post earlier and spotted Dep's question about the battery. No, it's a battery belt with no battery attached (or in the box)...

see here http://ukcaving.com/board/index.php/topic,4138.0.html

Its a well known problem on Landrovers too!

underground said:

paul said:

underground said:

I can agree with the salt water, but my caving kit's never been near the sea.
The reason why I'm sure this is dissimilar metal corrosion is the fact that the kit in this particular box was tored clean and dry- washed in clean water, for that matter before drying.

Click to expand...

Fair enough - however I suspect the problem is not simply due to a combination of steel / aluminium alloy and some dampness from condensation as you put before.

I have an old tackle bag of bits I have bought over the years and don't use ranging from a "Curlew Goldlock" U-shaped rack (allow bars/steel frame like most racks) bought in the 70's, a Caving Supplies rack bought in the 80's, old Petzl Stop, 2 old Petzl Crolls, again from the 80's and a mixture of steel krabs and other odd bits. These have been stored in sheds, garages and unheated rooms over the years in that tackle bag for the past 20 plus years. I emptied t out last night and although there's the odd bit of surface rust on the steel bits here and there, the alloy looks perfect.

Makes you wonder if your gear has unknowingly been in contact with something else to accelarate the deterioration of the alloy bits?

Click to expand...

Yes, I agree it seems odd- I'm no metallurgist, but it was the closest I could come up with. It seems like a massive coincidence that the Rack had been on top of a bunch of steel maillons, and the krab had been wet, and I assume, in contact with the steel of my belt buckle.

In the case of the Rack it was nothing the Dremel couldn't clear up, but the krab will be relegated to the scrap bin.

Click to expand...

Don't forget that "steel" is a word covering many different alloys, with very different properties. I suspect that at least some of the things listed above are made of stainless steel, whereas a belt backle might not be.

Mark

Long time since my materials lectures, but I still keep a bit current with corrosion issues through work.  Essentially any dissimilar metals with any dampness is bad news.  Virtually all common corrosion of metails is down to some form or other of galvanic action.  The more dissimilar the metals (in terms of their relative electric potential) the more rapid the effects.

The chemical story is that metal ions are exchanged through the fluid they are in contact with, obviously if the fluid contains any elctrolyte (such as salt for instance) this helps the process, but this can even happen with relatively pure fresh water.  Once corrosion starts, things go downhill quickly.  Any fluid trap (such as a developing pit on the metal surface or small gap between two bits of metal) will tend to lead to local concentration of the electrolyte in the fluid, leading to accelerated corrosion.

These effects can be dramatic to say the least, and frighteningly rapid in the right circumstances.

Things that improve the situation are those which remove the source of the cell which is formed by the metals and fluid.  Namely keeping dissimilar metals apart and dry.  Also surface coatings (such as anodising of aluminium equipment) can help, but as soon as the coating is breached then you form a cell, with the corrosion concentrated at the point where the coating is breached.

As has already been stated, terms such as "steel" and "Aluminium Alloy" actually cover a wide range of chemical make up, therefore dissimilar metal corrosion can occur between two bits of steel or aluminium.  Also do not be fooled into thinking that Stainless steels are corrosion proof, they are not.  They are resistant to corrosion in some environments, but just as apt to reduce to a pile of dust in others.

In short, the best way to make sure that your metalwork remains serviceable is to wash it off in fresh water after each trip, and to hang it up somewhere with a good airflow to dry out as soon as possible.  Leaving wet metalwork in any form of bag is bad news.