I had got away from working on cars for a while. During that time I had worked for a company that did a lot of historical buildings. Many of these had semi exotic metals. Lead, zinc, aluminum, stainless steel and copper. Had to learn the different ways to work with them. Each had their own characteristics. Some would work well with others. Some not.
Aluminum is really an easily corrosive metal by itself. But it is it's oxidation that makes it durable. Aluminum oxide is the second hardest naturally occurring substance, second to diamond. The oxide is so thin we can see through it. When you clean it off, the aluminum reacts so fast to oxygen, it takes very little time for the oxidation to reform. Mercury reacts with aluminum in a way which keeps the oxide from reforming and aluminum under those conditions, will "rott" away quickly.
Zinc and copper are very similar, with their oxides, but takes much longer(several years) The oxides are what makes them very resistant to water. Unlike ferrous metal's oxides, water does not penetrate into them. Cleaning these oxides reduces the thickness of the metals a little each time. So not only for the "look" the patinas are left on to lenthen their service life. The patinas will last decades longer than clear coats.
Most of the copper cladded building we worked on had the copper replaced not because they failed, but mostly due to damage. We had did the Naval Academy(Mother B) and most of the over 100 year old roof panels were replaced due to the many hurricane induced damage they had received or improper care. Most of the unreplacable ornimental metal work was cleaned, repaired and reinstalled.
Copper and zinc are mixed as alloys to add desired hardness. Both brass and bronze are copper/zinc (sometimes tin) alloys.
On the Supreme Court building we worked with brass, copper and lead. Government and historical buildings are the only ones EPA allows to have lead reinstalled.
On the Hippodrome Theater we worked with zinc on the marque. And the bank next door had copper ballisters and brass rails for snow/ice protection.
Edited note: one of the other biggest issues with roof panel corrosion also was from seagull and pigeon crap. The Maryland state highway building in Ocean city had to have the ridge cap replaced and bird spikes installed due to all the seagulls landing on it. (YUMMY)
[This message has been edited by cmechmann (edited 05-04-2014).]
took what, 30years to happen?
