A Study on the Impact of Corrosion under Insulation (CUI) on the Acoustic Performance of an Absorptive Muffler of Automobiles

Automobile exhaust systems help to attenuate the engine combustion noise as well as the high frequency flow noises which are generated as the gas expands and contracts through various ducts and orifices of muffler system. One of the solutions to mitigate the noise generated due to the latter is by means of an absorptive muffler, comprising a fibrous acoustic medium which helps to absorb noise of certain frequencies which are sensitive to the human ear. Typically, the construction of such a system consists of the fibrous acoustic medium encompassing a perforated inner pipe on the inside and enclosed by an outer metal case on the outside. The temperature limitations of the acoustic medium sometimes necessitate the placement of the fibrous acoustic system away from the engine source in order to prevent any damage to the fibers upon direct contact with the flue gas. However, this results in a potential for condensation of engine out gas in and around the inner pipe – fibrous system interface leading to a corrosive environment. This eventually leads to the formation of rust on the inner pipe a condition commonly known as corrosion under insulation (CUI). The corrosion over a period of time causes deposits to generate on the perforations causing them to partially or fully close which subsequently results in the deterioration of the acoustic performance of the absorptive muffler.

This paper deals with the various factors that promote CUI conditions in a typical absorptive muffler of an automobile by means of a market case study. It also dwells on various guidelines for mitigating the impact of rust formation which can lead to noise deterioration, from both design and manufacturing perspectives.