Steel

• Steel is a ferrous metal. That means it has an iron base. Steel is one of many alloys that begin with iron. The iron goes through a hardening process turning it into steel. Then, quantities of other metallic elements are usually added to create a steel alloy. A few steel alloys, such as stainless steel or or any alloy with a high chromium content, are resistant to rust. The majority of steels --- the kinds most likely to be used for sheet metal applications --- are highly prone to the corrosive power of rust.
  
  

Aluminum

• Aluminum is a non-ferrous metal, meaning iron is not one of its metallic elements. Because it does not contain iron, it is not prone to iron oxide corrosion. However, aluminum goes through it's own form of oxidation. However, oxidization in aluminum isn't as unsightly as it's white --- similar to the color of the base metal. Aluminum is also less prone to oxidation, so the process generally happens much more slowly than in steel.
  
  

Galvanization

• Galvanization provides a coating against rust. Steel is first chemically cleaned, then dipped in a bath of molten zinc-based alloy. Unlike steel, the zinc alloy is highly resistant to corrosion. Because it is applied in a liquid bath, the zinc-alloy covers steel completely, creating a continuous outer sheathing to ward off rust and other corrosive elements. However, galvanization is not impervious to scratches and damage. If a scratch penetrates the galvanization layer, the base-steel will rust quickly and be highly visible. The rust may flow and stain undamaged areas of galvanization.
  
  

Coated and Uncoated Aluminum

• Aluminum can also be coated with protective layers. Coatings include powder coating, spray-on clear polyurethane and anodization. Like galvanization, each of these coatings is thin. The coatings can be scratched through fairly easily. Without a coating, this application would clearly favor aluminum. With a coating, aluminum is still superior to galvanized steel, because the oxidation that occurs when the coating is penetrated is less visible and doesn't spread like iron oxide. Aluminum may be more expensive and in some climates may only offer slight advantages over galvanized steel flashing. In areas where corrosion is accelerated by high volumes of precipitation --- especially mixed with salt water --- aluminum is a better choice.