As for galvanic corrosion, zinc is not compatible with copper or iron, so zinc is usually installed with a decent slope in order to drain water that might contain trace copper or iron residue. Obviously it is important not to allow contact between zinc and any metals that are at the opposite end of the galvanic chart.
According to VM ZINC, Zinc is highly corrosion resistant in marine environments as well as inland. Regardless of location, erosion rates of the metal are relatively similar. In marine environments, zinc combines with salt in the air mainly to form zinc oxychloride (Zn2 O Cl2). Water soluble and not very adhesive, the white zinc chloride washes off the surface easily, and then the zinc patina develops and maintains the blue-gray color. However, unless washed away by rainfall, direct saltwater or sea spray contact ultimately reduces the lifespan of the material in dry, low rainfall maritime environments when the white chloride (essentially dried salt) stays on the surface.
Depending on atmospheric conditions, corrosion rates are low and vary between 0.5m and 1.0m per year. About 60% of the worn material continually washes off the surface while the remainder stays to form the patina layer. The highest wear rates are found in heavily polluted industrial areas, where sulfur dioxide concentration is high. In general, the service life of zinc as a roofing material in a marine atmosphere also ranges between 80 and 100 years, depending on exposure, roof geometry and installation quality.
Corrosion due to condensation trapped on the underside:
Several sources indicate that there are problems with corrosion from the underside of zinc roofing if moisture is allowed to condense there.
If the roofing system is designed such that warm moist air from the inside can travel up through the framing, insulation, and sheathing, as it gets colder, water vapor is likely to fall out of the air and be trapped. If the conditions are not right for patina formation (no air) or if the water sits on the metal for a long time, the zinc is likely to corrode. The manufacturers of zinc roofing all recommend either venting the roof, using zinc panels with a protective coating on the underside, or using a drainage layer directly under the roofing to allow moisture to escape.
According to an article by Janet Zaso in the AIA Continuing Education Center,
"Zinc roofing systems can be successfully installed on warm and cold roof applications. The term cold roof used here refers to one with a ventilated substrate rather than a reflecting cool roof, although some zinc roofing may also have this characteristic. Cold roofs are typically seen in residential homes with gable roofs and involve a roof ventilation system in which air is introduced at the eaves and ventilated at the ridge. Typically the layers of a warm roof include a metal deck, a vapor barrier and a layer of rigid insulation thats on top the rafters rather than between or below them. Because a warm zinc roof doesnt have the same air ventilation system as a cold one, and because zinc metal will corrode if moisture isnt wicked from it, proper detailing will avoid roofing failures. Correct installation of a zinc roofing system is essential for protecting the metal against corrosion. Corrosion can be seen visually as a white chalkiness on the metal surface. Typically, the causes of zinc corrosion are water from condensation adhering to the back of the panel or standing water on the surface due to a lack of slope."
"Generally, one of three approaches is used to prevent zinc corrosion from moisture. Skip sheathing is the oldest method and involves placing pine boards beneath the zinc roofing. The boards are spaced apart so that when condensation on the back side of the metal drips off, it will drip behind the pine boards where it can dry from air flow behind it essentially a cold roof. This approach is still followed in Europe in some places and contributes to the longevity of some European buildings. Another method for protecting against condensation is installing a drainage mat beneath the zinc roofing. The newest protection against corrosion is a special backside polymer coating. The coating is often between 30 microns and 60 microns thick and protects the zinc metal from coming into contact with moisture from condensation."
According to Contrarian Metal Resources, Zinc panel systems must be back ventilated in order to prevent corrosion from the interior surfaces caused by trapped moisture. More about the ventilation requirements of ZINC can be found in the INFORMATION TOPICS section. For additional information regarding the properties of zinc, Alltrista Zinc Products is an excellent resource."
A competitor to zinc in the premier roofing material market, states that their product is more durable because: Ice dams in valleys and along eaves are a common winter occurrence. Water trapped behind these dams can (and does) penetrate locks and seams. With zinc roofs, this can lead to underside corrosion. Moisture trapped on the underside of copper, on the other hand, has no negative effects, "making FreedomGray the better choice for long-term durability."
According to an Old House Web article on flashing:
"The underside of zinc flashing requires ventilation to allow protective patination to develop. If the underside is allowed to stay damp, white rust and corrosion can reduce service life severely." The underside must be protected by "bituminous sheet material" against alkaline materials such as fresh concrete or mortar; acid reacting antifreeze agents; wood preservative, or contact with copper.