Anodes above the waterline

[alexhibbert]

Given that galvanic corrosion can occur above the waterline, in humid/salty air, does the logic follow that zinc anodes can also work above the waterline? If not, for a specific reason, and you must mix metals (galv and 316 SS for example), how can you protect the metal, apart from paint?

The theory and practice seem to conflict slightly. Many sources say immersion is needed for galvanic corrosion. But obviously people know that it also happens in damp air, above the surface. Likewise, anode manufacturers say they only work underwater. Also only half true?

Thanks.

 

[Poey50]

A galvanic cell needs an electrolyte whether above or below the waterline and sacrificial anode material will work in the same way although will look different. As an example above water, I've needed to use copper on aluminium as part of my lithium battery build. That could be subject to damp (the electrolyte leading to galvanic corrosion). I use Ox-gard between the surfaces. This protects from moisture (denies an electrolyte) and oxygen but is formulated with zinc which is also a conductor.

 

[alexhibbert]

Thanks. So what I'm trying to work out is: if there are interior areas where stainless 316 and galv steel must be mechanically connected (much more galv than stainless), in an environment that might be damp but not immersed, would it help to attach small zinc anodes near the interaction points?

 

[Poey50]

Thanks. So what I'm trying to work out is: if there are interior areas where stainless 316 and galv steel must be mechanically connected (much more galv than stainless), in an environment that might be damp but not immersed, would it help to attach small zinc anodes near the interaction points?

I think there would need to be more liquid than damp to make small anodes a workable solution.

 

[Tranona]

Just isolate the two metals either with a paste like Duralac or physically by for example a plastic washer between the two if bolted together. Galvanised steel to 316 is not a common combination whereas aluminium and 316 is for example on masts where either of these techniques is used. If the contact is intermittent such as a stainless shackle or connector and galvanised chain then erosion of the zinc is usually very slow and chopping the link off if it gets rusty is the usual way of dealing with it. If you are using stainless fastenings into galvanised or the other way round either Duralac or a flexible sealant should be enough to isolate the two.

 

[alexhibbert]

I think there would need to be more liquid than damp to make small anodes a workable solution.

By the same logic, if it's not damp enough to benefit from an anode, is it too dry to worry about galvanic corrosion?

 

[alexhibbert]

Just isolate the two metals either with a paste like Duralac or physically by for example a plastic washer between the two if bolted together. Galvanised steel to 316 is not a common combination whereas aluminium and 316 is for example on masts where either of these techniques is used. If the contact is intermittent such as a stainless shackle or connector and galvanised chain then erosion of the zinc is usually very slow and chopping the link off if it gets rusty is the usual way of dealing with it. If you are using stainless fastenings into galvanised or the other way round either Duralac or a flexible sealant should be enough to isolate the two.

These are high strength bolted structures, so I don't want to introduce plastic into the mix. I'll have a look at Duralac, since at some point I do need to release the bolts slightly to change the spring washers for Nord locks, and I could do this at that time.

 

[TernVI]

Galvanising is basically just very local zinc anodes for the underlying steel isn't it?
So yes, it still works above the water line.
But it's working in small local electrolyte areas?

 

[alexhibbert]

Galvanising is basically just very local zinc anodes for the underlying steel isn't it?
So yes, it still works above the water line.
But it's working in small local electrolyte areas?

Yes that's a reasonable logic. I'm trying for a low-hassle solution. There are about a dozen areas where stainless brackets and bolts meet large galv box section, and it would take no more than an hour to bond small zinc anodes (with electrical contact to the galv, of course) next to each. They are £2 each. If they work, it would also mean I can easily visually monitor any anode corrosion.

 

[pyrojames]

Yes that's a reasonable logic. I'm trying for a low-hassle solution. There are about a dozen areas where stainless brackets and bolts meet large galv box section, and it would take no more than an hour to bond small zinc anodes (with electrical contact to the galv, of course) next to each. They are £2 each. If they work, it would also mean I can easily visually monitor any anode corrosion.

The galvanizing is an all over zinc anode, why bond more zine to it? It's called galvanizing, because it forms a galvanic cell.

 

[alexhibbert]

The galvanizing is an all over zinc anode, why bond more zine to it? It's called galvanizing, because it forms a galvanic cell.

Because, if (this is the whole point of the thread) the anode corrodes sacrificially, in preference to the galvanising layer, it will be easy to swap over and monitor. Re-coating in-situ, corroded box section would be a nightmare.

 

[Poey50]

By the same logic, if it's not damp enough to benefit from an anode, is it too dry to worry about galvanic corrosion?

-

 

[pyrojames]

Because, if (this is the whole point of the thread) the anode corrodes sacrificially, in preference to the galvanising layer, it will be easy to swap over and monitor. Re-coating in-situ, corroded box section would be a nightmare.

You will only get a galvanic cell if there is exposed steel. Corrosion will be limited by the zinc I that area. By fastening an anode to the steel, you are more likely to expose some steel to moisture and create a potential for corrosion. This will make the situation worse rather than better.

 

[alexhibbert]

You will only get a galvanic cell if there is exposed steel. Corrosion will be limited by the zinc I that area. By fastening an anode to the steel, you are more likely to expose some steel to moisture and create a potential for corrosion. This will make the situation worse rather than better.

Ok that's helpful, and changes the parameters somewhat. The only exposed carbon steel will be where I drilled and cut the box section - the drill holes will be hard to protect, but the cut ends can easily be painted/coated.

 

[pyrojames]

That's more challenging. I have used cold spray on galvanizing to seal cut bolt ends or iron floors. That was best done within seconds of the cut when the steel was fresh and hot. If you can get a grinder or sander to clean the exposed surface, and heat it a bit, spray on galvanizing would help. It might be worth spraying the holes and running in the bolts while the spray is still wet

 

[Spirit (of Glenans)]

The galvanizing is an all over zinc anode, why bond more zine to it? It's called galvanizing, because it forms a galvanic cell.

I beg to differ. Isn't the zinc deposited on the steel by a galvanic process?

 

[VicS]

I beg to differ. Isn't the zinc deposited on the steel by a galvanic process?

Galvanising usually refers to hot dip galvanising in which the stel is dipped into molten zinc

There are other processes including electroplating but with hot dip galvanising the zinc is partially alloyed with the steel surface

 

[Spirit (of Glenans)]

As you say, it's an inexpensive and easy exercise. Why not just do it and monitor the effects, i.e. 'suck it and see'. If you did, you could resurrect this thread in a year or three and let us know the results.

 

[Spirit (of Glenans)]

Galvanising is done by dipping the steel into molten zinc.

Sorry, yes I was confusing it with anodising.

 

[Tranona]

Because, if (this is the whole point of the thread) the anode corrodes sacrificially, in preference to the galvanising layer, it will be easy to swap over and monitor. Re-coating in-situ, corroded box section would be a nightmare.

But it won't - the zinc on the galvanising will erode first because it is in direct contact with the stainless. Also your anodes will not be on the electrolyte as that will be the moisture between the stainless and the box section (if there is any). I think you are worrying about nothing - simply isolate the stainless from the box section. Not sure why you don't like the idea of a thin plastic film between the two, but just using a sealant between them and around the bolt shanks will give a permanent seal. seal round the holes in the box section as well to protect the bare steel edges from drilling the holes.