Why do marine-grade aluminium gutters have corrosion holes?

[aussiblue]

In Autumn 2015 we replaced all my gutters around my house with marine grade aluminum gutters. Today I noticed white corrosion marks on the outside of the gutter and when i touched them (well pressed hard on them with my finger) they turned into round 0.22 bullet sized holes. These gutters were meant to last 25 years and outlive me but they are not yet 8 years  old.  I'll post some pics tomorrow when it daylight and easier to do so.  From my evening inspection it (hopefully) seems confined to a section of just less than 3 meters (or just 0.5 of a meter greater than the spare guttering I have on hand).

Does anyone have idea of the cause? I clean the gutters each year before winter with my Karcher pressure washer and it's drain cleaning attachment so I think my gutters are well cared for.  The only two things that set this section of gutter apart are that a) its below my roof mounted evaporative air conditioner  and collects it's runoff about 10 meters upstream (the runoff from the overflow is now leaking out my holey gutters)  and b) it's almost immediately below but about half a meter downstream not upstream of   a square of lead flashing that surround the soil vent pipe for the kitchen sink drain.  A bit of "googling"suggest that run off from lead flashing may cause galvanic corrosion of both aluminum and colorbond gutters (see https://gartalk.garlandco.com/2012/09/19/understanding-galvanic-corrosion-and-how-to-avoid-it-in-met... ), So is this the likely cause?

I will have to check in daylight but perhaps the fancy flashing around the dropper on the roof is also lead. Just found the brochure and it a DEKS  SDA55 rubber and aluminium product (this: https://deks.com.au/product/dektite-evap-blk-550x550-1040x1290mm-gry/ )  so that's not the cause.

I see that the business I purchased from closed up shop in August 2021. So I guess the claimed 25 year guarantee is out the window. I guess if  a rooftop inspection tomorrow confirms the corrosion is confined to this one section I will have to cut it out and fit a new section of aluminium guttering.  Now that business is no longer operating is there somewhere else I can buy a replacements section. Obviously I cannot use steel or colorbond guttering as it will cause corrosion if fitted between aluminium guttering.

The Weatherflash  fully annealed, 99 percent pure, uncoated aluminium flashing would seem best for replacing the existing lead flashing square but Bunnings don't seem to sell it in 450 mm squares like they do the lead flashing; so is there another alternative? OK checking out my Decktite evap flashing suggests that something like this from Bunnings may be the way to go https://www.bunnings.com.au/deks-150-300mm-red-silicone-dektite-premium-roof-flashing_p4829521.

[aussiblue]

Thanks, but don't worry while I am very confident about working on the roof and do it often I would never get up there while dizzy. I also have a roofer's helmet that I am getting better at remember to wear although it makes the head a little hot in summer and a Linq fall arrest harness system for when I am doing things where I think the risk of falling off the roof is greater than normal (i.e leaning over the edge to attend to barge boards.  The worse I have ever done on the roof is have a tile break under my foot so my foot cracked the ceiling (but no damage to  me other than my pride). In this case the ladder was very low and I was only a foot or so off ground level, and my wife and son were standing by to catch me. I am just getting occasional dizziness ( amd haven't actually faited or fallen over ...yet) and it was not happening while on this ladder.

[aussiblue]

Finally got back to this today and got up on scaffold and had a good look.  A close examination made it very clear it was caused by the ZINCALUME® steel Fielders watergate leafguard (this stuff https://www.bunnings.com.au/fielders-watergate-10m-leaf-guard_p3040239 ) reacting with the lead flashing  and then the corroded leafguard reacting with the aluminium gutter  where it was friction fitting into the aluminium gutters. The leafguard section involved was completely brown with rust and falling apart in some places. There was also lots more brown rust where the leafguard was contacting the gutter and making rust holes.  There were more larger hidden rust holes on the inner side of the gutter where there was more contact between the the gutter and the leafguard. In fairness Bluescope warn that Zincalume should not be left in contact with uncoated lead or runoff from uncoated lead. See http://www.bluescopesteel.com.au/howto/avoid-incompatible-metals and  https://www.ridgewayroof.com.au/corrosion-chart.html "Due to phenomenon known as galvanic or bi-metallic corrosion, some commonly used metals can cause accelerated corrosion when used with ZINCALUME® steel, zinc/aluminum alloy coated and COLORBOND® prepainted steel."  Perhaps the Bunnings website should also not misleadingly  state that Fielders Watergate Leafguard is made of perforated Zinc; it is actually  not Zinc but is magnetic steel albeit ZINCALUME® or a aluminium/ Zinc/Magnesium alloy coated steel per https://steel.com.au/products/zincalume-steel   

See  https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiQ_dzQ9O...

That information about what the gutter mesh was made from would not have changed my purchase decision at the time I bought the leafguard but it would have made diagnosis of the cause of the holes in my gutter easier and it may be useful for other to know what it is actually made of.  The aluminium gutter mesh now available from Bunnings would be a better option for my aluminium gutters but Bunnings did not carry this line at the time I installed my gutters.

Fortunately only one full  section of the leafguard and the gutter beneath it was affected.

So as well as replacing the rusted out leafguard  and patching the gutter with Commelin's Exterior Grade Waterproofing and Crommelin's Reinforcing Fabric, I will be  replacing the lead flashing with a Dekite rubber and aluminium flashing.

So I guess I at least have learnt that lead flashing (particularly uncoated lead flashing ) used with Zincalume gutter mesh and aluminum gutters is a no-no.  So it is clear why the lead flashing caused corrosion of the Zicalume but I haven't completely worked out why that corrosion of the Zincalume then also caused a corrosive  reaction between it and the aluminium gutters. In addition to the friction fit contact areas at the rear of the gutter it also appeared drops of the oxidation substance also fell through or dripped down and caused corrosion rain drop sized holes holes  in the face of the gutter (or perhaps the oxidising substance dripped down from the contact area on the front lip of the gutter where the leafguard clipped in or perhaps it was the flakes of rusted leafquard I found in the gutter). Perhaps Noyade's metallurgist brother can shed some more light on this too.

 

 

[EricL]

Hello @aussiblue 

Thank you so much for posting the results of your investigation. Who knew uncoated lead flashing would have that effect on aluminium gutters. I'm sure this information will come in handy to our members who have the same type of guttering. 

Again, thanks for sharing that information.

Eric