Is galvanic corrosion of Whites Galvanised Steel Retain It sleeper upright posts a thing?

[aussiblue]

My sleeper garden walls I built 40 years ago are succumbing to dry rot despite all the Creosote and Copper Arsenic Paint that was applied at the time. After 40 years I guess I can't complain. I have investigated reconstituted limestone and concrete wall alternatives but, I guess due to the current high demand levels in the building industry, I couldn't get any contractor suppliers to respond to my requests for quotes.  Therefore I have decided to just replace these sleeper walls with new sleepers (these https://www.bunnings.com.au/200-x-50mm-2-4m-treated-pine-sleeper-h4_p8032702 )  and use Whites Joiner Posts (these https://www.bunnings.com.au/50-x-1500mm-joiner-post-galvanised-steel-retain-it-sleeper-upright_p1101... ) instead of the 100mm upright sleepers I used last time.  I figure if they also last 40 years they'll probably outlive me and/or my residency here. The sleepers have already been delivered, I have the quick set concrete for the posts and i have bought the retaining posts so I thought I was all ready to go. However, while we were loading up my ute with the Whites Joiner Posts, a  person claiming to be a tradie who installed  garden walls in Perth hills areas,  advised my wife while she was going through the Bunnings checkout  that I should paint the galvanised posts and the end of the sleepers with bituminous paint to prevent the corrosion of the galvanised posts.  The claim is that the copper arsenic used in the H4 treatment causes a galvanic reaction that corrodes the steel in spite of it being galvanised and if unprotected the foot of the posts will rust away within a few years,.  Sources like: https://www.researchgate.net/publication/225782648_Corrosion_of_ferrous-_and_zinc-based_materials_in... and https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiSzdKIu4D6AhVh9zgGHZ3xA... and https://8pointshome.com/modern-pressure-treated-lumber-corrosive/   do suggest that CCA treated pine might cause corrosion and here http://www.bluescopesteel.com.au/howto/avoid-corrosion-caused-by-contact-with-timber Bluescope Steel advises:

"Avoid corrosion caused by contact with timber:

Green timber or timber treated with copper chromium arsenic (CCA) should not come into direct contact with galvanized steel, ZINCALUME® steel or COLORBOND® steel because acidic substances from these timbers have a corrosive effect on the metallic coating. The time of wetness of the metal surface is a factor in the overall corrosion mechanism (the longer the product is wet, the greater the corrosion).

Even run off from CCA treated timber can contain corrosive leachate which will cause corrosion if it contacts galvanized steel, ZINCALUME® steel or COLORBOND® steel. BlueScope Steel recommend against the use of CCA treated timbers for high moisture situations such as roof and ceiling batons, composite fence constructions and built up flooring or external decking which may utilise a metallic coated steel structural support system. If CCA treated timbers must be used, the timber and/or steelwork must be sealed by fully painting the material prior to installation."

a) how are the H4 Pine sleepers treated; is it CCA?

b) is corrosion of the galavanised White's sleeper joiners a real world risk? If so,  why does Whites or Bunnings not provide a caution when supplying glavanised post that will be very likely used with CCA treated sleepers?

c) if the risk is real will painting the bottom of the posts or/and the end of the sleepers with Crommelin Expertior Grade  Brushable Waterproofer (i.e., https://www.bunnings.com.au/crommelin-4l-exterior-grade-brushable-waterproofer_p0961599 ) solve the problem?

d) would a much more easily applied automotive bitumous under-body spray paint such as Rust-Oleum Undecoat Pro or similar work equally as well?

 

[KatieC]

Hi @aussiblue,

Thank you for asking about replacing your current garden walls with new sleepers and joiners. @EricL will be on later this afternoon to further assist but let me tag @Adam_W and @TedBear in the meantime to see if they have some advice they can give. It would be great to see what your new garden walls look like when you are done, please share your journey and tag me.

Katie

[EricL]

Hi @aussiblue

Thank you for sharing your question about galvanic corrosion when galvanized posts are in contact with CCA-treated timber.

To answer your first question, H4-Treated Pine means that it is treated to the H4 hazard class in accordance with the Australian Standard AS 1610. That means the timber can be used in direct contact with the soil and is treated to withstand fungal decay and termite attack. Depending on who the supplier is, the timber can either be treated with CCA or MCA micronized copper azole. The MCA-treated one is safe for use in garden beds. However, timber treated with CCA is generally not recommended for use in the garden if you are planning on growing food.

Many thanks for your feedback regarding the Whites galvanised posts. We are forwarding it to our safety team for their consideration.

Adding an extra coat of bitumen to the timber ends and the post itself will help prevent any chemicals from making direct contact with the galvanized post. Another suggestion is to use sleepers for the posts and build them exactly the way the original was built. It should last you another 40 years.    

Please keep us updated with your progress. We look forward to seeing your garden walls refreshed.

Eric

[aussiblue]

Given the originals were jarrah soaked in Creosote and then painted with copper arsenic and the replacements are pine I doubt they will get forty years and I'll think I''l be lucky to get 20 years out of them. Going on 70, I hope I really am still around in 20 or better forty years to find out though.

[Noyade]

Just thinking about longevity.

I installed/positioned these sleepers around 21 years ago. Bunnings wasn't in town then.

Are they Iron Bark??

I remember slapping sump oil on them. Probably not a good idea? One or two have split at the top but structurally sound. But I'm still a long way from 40 years @aussiblue 

[aussiblue]

or Karri perhaps?

[EricL]

Hi @Noyade

My grandfather used sump oil on our backyard fence posts and I use to be able to smell them coming home from school. But they sure lasted a long time as they outlived both my grandparents.  

Eric