The exhaust gas emitted from automobiles is one cause of air pollution, and various efforts have been made to prevent this problem. One of the preventive measures is the system shown in the figure. The exhaust gas from a gasoline engine forms condensed water containing the ions of ammonia (NH4-), sulfuric acid (SO42-), chlorine (Cl-), nitric acid (NO3-), carbonic acid (CO32-), etc. when it is cooled. These ions are very corrosive and create a very severe environment for materials. The whole external surface of an exhaust system must also have corrosion resistance to the deicing salts used in cold districts and to atmospheric salt in coastal districts.

The components downstream from the muffler of an exhaust system must have sufficient corrosion resistance to withstand the internal attack of condensed water containing these corrosive ions and external attack of these salts. The materials mainly used for these components are low-carbon ferritic stainless steel incorporating not less than 11% chromium. The components nearer to the engine in front of the muffler need high oxidation resistance, because they are heated to about 773K (500) during operation. The exhaust manifold, which is heated to the highest temperature, must possess both high-temperature strength and resistance to thermal fatigue. High-chromium stainless steel is now used mainly for these upstream components. As a result, the life of the exhaust system has been substantially extended.

In addition to these efforts to improve corrosion resistance, it is also necessary to gain economic improvements by reducing the weight of the complete exhaust system. Material development is being carried out at present to achieve this result.