External corrosion resistance of steel and ferritic stainless steel exhaust systems

The exhaust system has a number of requirements to fulfil. The major ones are to reduce the noise level created by the engine, to channel the waste gases out the rear of the automobile, to minimize the reduction in engine performance, and to have an acceptable service life and cost1. The exhaust system is divided into two main parts, the hot end and the cold end. The components comprising the hot end are the exhaust manifold, the front pipe, and the catalytic converter (if present). The cold end components include the centre pipe, the muffler, and the tail pipe. Typical operating temperatures of the different components are shown in Table I. Of all automobile exhaust failures, about 80% are caused by corrosion while the majority of the remainder are caused by fatigue1. In 1987, an Armco investigation involved the examination of nearly 350 passenger car and light truck failed exhaust systems in the Ohio area. Of the coated carbon steel failures, 70% were failures originating on the interior surface—the remaining 30% were from the exterior surface. The rate of corrosion attack originating from the exterior surface of the exhaust system is strongly influenced by the region in which the automobiles are used. The corrosivity of the external environment is far more severe in cold countries. This is due to the presence of chloride based de-icing salts that are dispersed on the roads to lower the melting point of water, and thereby reduce the amount of ice or snow on the roads. De-icers are selected on the basis that they exhibit a eutectic temperature several degrees lower than the ambient temperature encountered in the ice or snow environment. The eutectic point occurs at the salt concentration where the solution has a minimum melting temperature. For the de-icers to be feasible, they need to be comparatively cheap, must be highly soluble in water at low temperatures, External corrosion resistance of steel and ferritic stainless steel exhaust systems