Galvanised structure rusting in fresh water - advice re anodes pls

[sarabande]

Not exactly boaty but the principles will be well-known to the forum.

The hydro-electric station I look after has a fairly substantial screen made from galvanised steel . The river water is understood to be 'towards the acidic', and I have noticed some small 5 - 15mm spots of corrosion taking place on the underwater structure.

What anti-corrosion devices might prevent ongoing damage please ?

 

[tico]

Think that Vic will be the guru on this , but it would appear that the water (peaty i presume) has removed the Zinc. allowing rust to form.

a few hefty zinc anodes hanging in the water and in low resistance circuit should improve matters. What about using a load of part used ones from the local boatyard?

 

[lw395]

How long has it been in there?

Is the zinc simply all used?
What is the dark surface between the rust spots? Dark iron compound or vegetation?

I think, like an anode on a boat, this kind of thing is a lifed item. You could take it out and re-galvanise it, or just start saving up for a new one.

A check that it's not earth-bonded to anything might be worthwhile?

 

[vyv_cox]

Anodes are your best hope. Galvanising is effectively an anodic layer but it does not last for ever, pinholes such as you have may occur if the job was not well done in the first place. What is the size of the screen?

 

[sarabande]

Screen gap between verticals in Pic 1 is ~= 110mm. Width of the strut in Pic 2 is ~= 125mm.


The screen is well bedded (earthed) into the river bed and abutments, and I am pretty sure that we don't have any stray electrical currents escaping from the alternator (75kW) otherwise Western Power Distribution, Environment Agency and Natural England would have jumped on us.

Water temp ranges from - something or other in winter to nearly 30 in low summer flows, The latter may therefore preclude aluminium anodes, so perhaps magnesium is a better choice ?

pH values fluctuate depending on leaf fall and run-off from the high peaty moorlands; I don't have the figures to hand but can find out. Probably not critical as if anode replacement should be easy.

 

[ghostlymoron]

How old is the screen? I've installed a lot of semi submerged equipment but that kind of corrosion on hot dipped galvanising is new to me.

 

[vyv_cox]

Looks very typical pitting corrosion. Not normal for mild steel unless the coating breaks down. Very common with chromium plating of course. Maybe your acidic waters are responsible. I assume hot dip galvanising, which should normally be fault free.

 

[sarabande]

Galvanised structure was built by local agricultural steel fabricators (been around for 25 years , so must be reasonably good) and galvanised poss in Plymouth ? It has been installed 3.5 years.

I am not sure what the dark bits next to the rust points are, but will give it a scrape tomorrow. The rest of galvanised steel in the air seems fine.

 

[MM5AHO]

We galvanize quite a few of these intake screens for hydro plants in Scotland and the expected life is often 20 -30 years. Except in peaty waters! There the same as you show happens, and I've assumed in the past that this is due to the low pH water; some was measured at about pH5.
I'm not sure why the corrosion is preferential like that in spots, but one I examined a few years back, was just like this and had no zinc coating in the spots, only rust, yet the thickness of zinc between them was good at typically 80u or more, and that after 5 years. (It was probably 100 or 120u initially).
For submerged steel even in "fresh" water, a good idea is to shotblast before galvanizing to re-profile the surface and so get a thicker zinc coating, typically >150u. Life is generally proportional to coating thickness.
The only cure we could recommend was an organic coating over the top. They chose polyester applied as powder. (sometimes called "powdercoating", though that term defines the application method not the polymer).

 

[ffiill]

For what its worth one year I moored my boat on Loch Ness using a galvanised chain.Not only was it ungalvanised bright steel after a year it had lost about 20% of its thickness!