I typically specify that painted aluminum receive a coating of alkali resistant paint when touching cementitious materials, however if the painted aluminum meets AAMA 2605, the requirement is optional. One of the major coating manufacturers has reached out to me and advised that if there is not 100% coverage, no paint coating will protect the aluminum from potential corrosion.
Any comments from my peers?
I’ve been specifying a polyamide epoxy for aluminum in contact with masonry, concrete, or other cementitious materials. Look at Sherwin-Williams Macropoxy 646-100 as an example.
Are they subtly suggesting 100% coverage is not required by spec? 2605 coatings are typically limited to visible/exposed surfaces, with an abrupt cut-off point. On coil coated sheet, I suspect 50% coverage might be standard; worth a few phone calls.
Coatings can be scratched and do not protect edges. If practical, consider a more robust barrier such as strips or sheets of roofing or waterproofing membrane. Or, use stainless steel and paint to match the aluminum.
Good idea, Michael Chusid. We use the same principle inside the building with steel studs at outside walls and on slabs. Isolating metals is a good Division 01 Execution Requirements provision if not a general note on the drawings.
I believe that powder-coating with 2605 PVDF and FEVE products coats both sides but, as Michael points out, does not protect at scratches or cut edges.
I avoid using sleeves in concrete, preferring instead to cast pipe and tube directly into the concrete as this tends to trap less water longterm so painting with an HPC is a good idea. The hard part always seems to be avoiding placement at joints in the concrete.
Bigger question is how to isolate dissimilar metals like aluminum from galvanized steel. Bituminous coatings are no longer suggested. I like James’s idea of a polyamide epoxy but I’m not sure it will block galvanic action. Where possible, I usually go with Michael’s suggestion using a neoprene or other rubber isolator, preferably at least 60 mils thick as this works well as a gasketing material. Obviously this generates a lot of negative feedback from Contractors.
Ken - curious why you suspect epoxy would not block galvanic action?
Good summary of aluminum/concrete interaction at http://www.theconcreteproducer.com/how-to/concrete-production/how-does-contact-with-aluminum-affect-concrete_o.
Questions that may help you find solution:
Is concrete cured before aluminum is installed?
Is moisture or another electrolyte present?
Do you care what the aluminum looks like or how it performs when it corrodes?
Is there room for products of corrosion to expand without cracking concrete?
Is aluminum in vicinity of iron, copper, or other dissimilar metal?
Can you reduce the alkalinity of concrete by using pozzolans?
Brett, I’ve never had a chemist tell me that it does. I presume that it would, just as the old zinc chromate paints did, but I’ve never heard anything definitive one way or the other.
Have you? I’d appreciate knowing. Even better if there is also something available that can also stand up to UV.
Michael; good list of questions, but I have to wonder when you would have concrete that does not have some moisture content, or have I just been working too long in Houston.
Peter -
Perhaps it is I that has been baking too long in the hot Los Angeles sun.
ken - at face value, I’d expect most epoxies to be non conductive, which ought to break the circuit. Not sure how durable to UV, water, salts, etc. and there are a ton of different epoxies.
In general epoxies alone are not very uV resistant that is one reason many epoxy based systems in exterior applications have a urethane top coat. This is taken directly from the PPG web site:
“Epoxy coatings provide excellent resistance to severe corrosiveness such as chemicals and alkali. Many epoxy coatings, not necessarily all, offer excellent barrier protection for both atmospheric and immersion environments. When epoxy is used on exterior surfaces where it is exposed to uV rays, it is recommended to also apply a layer of topcoat, such as urethane, to prevent the epoxy from yellowing or chalking.”