Pitting corrosion in the wet section (rear section) of the automotive exhaust systems

Inside the system, condensation of combustion gases produces sulphurous acid, sulphuric acid and low levels of hydrochloric acid, creating critical conditions with acidic pH-values. These condensates, combined with an accumulation of chloride ions and deposits of electrochemically active soot particles, result in a substantial wet corrosive impact on the inner parts of the components.Various stainless steel grades like ferritic, austenitic and manganese containing austenitic steel are used to achieve an optimal combination of properties in the rear section. The resistance of different steel grades to pitting corrosion can generally be compared on the basis of their alloy composition. For pitting corrosion resistance of a material it is important that the dissolution rate is to be low and the material should possess the ability to repassivate quickly during the idle periods.

The hot front section of the exhaust system (manifold pipes, catalytic converter) requires steels with a high scaling resistance, an ability to resist oscillating stresses due to vibration, optimal elevated-temperature and creep strength, minimum susceptibility to embrittlement and a low coefficient of thermal expansion [4].

In the center section of the exhaust system (center muffler, connecting pipes) resistance to both high temperatures and wet corrosion are needed. Depending on running conditions, either hot conditions prevail (full throttle) or wet corrosion loading dominates (short-distance driving) [4].

In the rear section (rear muffler) wet corrosion becomes the main factor. Inside the system, condensation of combustion gases produces sulphurous acid, sulphuric acid and low levels of hydrochloric acid, creating critical conditions. These condensates, combined with an accumulation of chloride ions, some acidic pH values and deposits of electrochemically active soot particles, can result in substantial wet corrosive loading on the inner surfaces of the components. Compared with this, the external corrosive loads through rainwater, road dust, slush and de-icing salts are almost negligible.

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