There are a number of reasons.
It is not painted, or covered by a corrosion resistant material, as the height temperatures it is exposed to would tend to burn-off or other wise remove such coatings.
The exhaust gases passing thought it contain corrosive substances such as sulphur oxides (Sulphuric acid) Nitrogen Oxides (Nitric acid) to name a few. Even the use of corrosion resistant coating may not be enough to protect against these, given that chemical reactions are improved with heat (IE the hotter it is, the faster it goes), and exhausts are hot.
Also, one of the products of combustion is water, and water rusts iron/steal, even in the absence of acids.
Other metals could be used, but the additional costs of using stainless steel, or perhaps aluminum, are (one must assume) currently not seen as beneficial by purchasers.
It should also be noted that over time the "sound absorbent" material within the exhaust, normally glass fibre, becomes impregnated with carbon. This means that an exhaust that wound last for ever would still need to be replaced eventually, as is would lose it's sound absorbing properties.
One reason why car exhausts rust faster than the car body of the car is because they are often at a different temperature. The exhaust can be very hot and very cold so this creates a reaction rate which inturn makes the exhaust rust faster. hope that helps :)
If the vehicle has a stainless steel exhaust system it will last the life of the car. If it does not and you drive in roads treated with salt it will not last as long. Surface rust does not mean the exhaust in need of repair. Surface rust is normal on a steel exhaust system.
When the rust weakens important structural joints or opens areas where exhaust fumes can enter the cabin area, yes it can be harmful.
The reason being is that the exhaust not only releases gases and vapors but humidity as well. That is the reason why at times u notice it sweating.
Serious rust can compromise the strength of structural joints. Serious rust can also allow unwanted exhaust fumes enter the cabin area.
A car exhaust can rust due to constant exposure to moisture, salt, and corrosive elements found in the environment and within the exhaust gases. When the exhaust system operates, it produces water vapor as a byproduct, which can condense and accumulate, especially during short trips where the system doesn't reach optimal temperatures to evaporate moisture. Additionally, road salt used in winter can accelerate the corrosion process. Over time, these factors contribute to the deterioration of the metal, leading to rust formation.
It is part of the exhaust system.
It rusts because of the oxygen present in atmosphere. Rust is iron oxide - where the iron in the steel has combined with oxygen. Preventing oxygen reaching the metal is the primary way to avoid rust, and this is done primarily with paints and other coatings.
it is called the Exhaust pipe
It is part of the emmission system and is mounted in the exhaust pipe near the engine.
Surface rust isn't dangerous, cancerous rust on the frame can be dangerous because the car could snap in half depending on the extent of the rust. The best solution usually is just to get a new car because the frame usually isn't worth fixing. In my state they consider it dangerous because if you get stuck in snow they fear exhaust entering the cabin and killing its occupants. Rust holes of any kind will fail your inspection.
Americans commonly refer to a car's exhaust pipe simply as the "exhaust pipe." It is part of the exhaust system, which directs harmful gases away from the engine and out of the vehicle. Some might also colloquially refer to it as the "tailpipe," particularly when discussing its visible end at the rear of the car.