To me, metal was always intimidating. I would open my car's engine bay and be welcomed by all sort of different surfaces. Some would be gray, some would be shinny, some would be porous while other would be slick, some would have a color while other was covered by some whitish dust. There are various metals and metal compounds used in making of your vehicles. To demystify, here is a list of what all those metals mean and how to best use and protect them.
Steel
Much of your car is made of steel sheets shaped into various surfaces. Steel is strong and flexible, relatively easy to work with and weld but also prone to rust. The moment it's exposed to moisture or worse, salt and moisture, it starts corroding. There is no way around it. Bare steel will start developing rust in as few as a few hours. The only way to avoid the "brown cancer" of rust is to separate the steel from the environment. There are numerous ways to introduce such a barrier. You can galvanize, paint, powder coat or plate the steel surface. There are other, less permanent metal protectants such as waxes or oils. Each method has its pluses and drawbacks but any is better then just leaving the bare metal exposed to environment.
Painted steel
One of the easiest methods to distance the bare metal from the environment, thus providing an attractive protection from rust, is paint. The paint must be properly applied to avoid peeling. Though it's a great way to protect the steel, its drawback is that paint is prone to mechanical damage. Scrapes and scratches loose the protective properties on the exposed metal. Even small pin-point damage lets moisture under the paint surface aggravating the damage (rust bubbles from underneath). That said, any paint is better than leaving metal bare. To weld, the paint must be removed from the metal surfaces that are to be bonded together. The area where the paint was removed (thus exposing the metal to possibility of rust), you can use weld-through primer to provide minimal rust protection. Most of the weld-through primer will melt away during welding so some other way of rust preventative must be considered in addition to the weld-through primer.
Galvanized steel
One of the best methods of protecting steel materials against corrosion is through galvanization. Galvanized steel is a clean steel with zinc metallurgically bonded to the surface, completely covering the steel with a protective coating. The most common method is hot-dip in which the parts are coated by submerging them in a bath of hot, molten zinc. This protects the underlying steel from abrasion and exposure to atmospheric conditions for many years. The corrosion of zinc is very slow, which gives it an extended life while it protects the base metal. Unlike other coatings (such as paint), small damaged areas need no touch up as small scratches are self-healing. The durability of galvanized steel provides excellent corrosion protection, one of the best currently available on the market. It provides up to 10 times the abrasion resistance of paint coatings and has superior resistance to mechanical damage. Unfortunately for auto body repairs, galvanized metal is not easy to work with. While zinc protective layer keeps metal from corroding, it also makes welding difficult. If you’re planning on welding, you are almost always going to need to grind off the galvanized layer and weld with adequate ventilation because zinc produces toxic fumes. Moreover, it's often hard to paint over galvanized metal as paint flaking car occur. Most paints will not be able to adhere to galvanized steel without the correct preparation of the substrate.
In the picture above, the sure sign of steel galvanization is the whitish powder on its surface. Notice how where the pipes were welded and zinc was striped away, heavy corrosion sets in.
Powder Coating
In powder coating, the paint dust is electrostatically attached to the part then cured under heat. Powder coating a metal object allows for a dense sturdy finish, more durable than conventional paints.
Stainless steel
Stainless steel is an alloy of iron that resists rusting and corrosion. As a result, stainless steel is ideal for applications where corrosion resistance is essential. It generally, stainless steel is more aesthetically pleasing than regular steel because it has a bright finish that won’t fade over time. Because stainless steel contains chromium, this makes it more challenging to weld. Welding regular steel presents far fewer problems than welding stainless steels so in car restoration the stainless steel is generally reserved to use in fasteners. Car enthusiasts replace many regular steel nuts and bolts to provide long term rust protection. The limiting factor is the price as stainless steel tends to be much more expensive than regular steel.
Looking at the picture above you can see stainless steel nuts and bolts still looking "new" many years after this pillar was installed at a train station right next to a river (thus exposed to atmosphere with lots of moisture). Notice the washers rusting away - clear sign that the installers ran out of stainless steel ones and use regular steel washer.
Cast iron
Many "solid" parts such as engine block or break calipers on your car are cast from iron. Just like steel, those parts can corrode very quickly if not protected. On engine parts even just a layer of oil is sufficient. The parts that shouldn't be oily, can be painted or powder coated. Cast iron is notoriously hard to weld. Welding cast iron can be problematic because of its high carbon content. During the welding process, this carbon migrates into the weld metal and/or the heat affected zone adjacent to the weld metal, causing elevated hardness/brittleness. This is how Cast Iron gets its reputation for post weld cracking.
Aluminum
Another popular metals used in cars is aluminum. Aluminum does not rust, but it does corrodes. It's actually very prone to corrosion, however, aluminum corrosion is aluminum oxide (its surface turns to whitish powder), a very hard material that protects the aluminum from further deterioration. Unfortunately, aluminum is not easy to weld for a hobbyist car re-builder because of difference in thermal conductivity, aluminum requires much higher heat inputs than steel during welding.
Plated metal/steel
Plating is a finishing process in which a metal is deposited on a surface. Plating is used for corrosion inhibition, to decorate parts, to improve solderability, to harden, to reduce friction and to improve wearability. Many parts of the engine and many bolts were plated. Many plated parts gain a strong, durable outer layer greatly hampering rust.
You have a bew basic plating process in the car rebuilding:
Zinc plating - one of the easies processes that prvides a decent rust protection. Parts turn out often shinny silver.
Yellow Zinc - somehow similar to zinc plating but much stronger to mechanical damage and around 3x more rust preventive.
Nickel plating - produces a very string, rust resitant surface.
Chrome plating - a finish treatment adding a thin decorative layer over underlaying metal to make a shinny and very strong surface finish.
Brass
Brass is the generic term for a range of copper-zinc alloys with differing combinations of properties, including strength, machinability, ductility, wear-resistance, hardness, color, electrical and thermal conductivity, hygiene and corrosion resistance. Because brass is softer than regular metal it tends to be used in bushings and other "wear" applications.
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