Corrosion in Exhaust System Vol-2
Aluminum-plated or Aluminized steel
Aluminum-plated or aluminized steel has been used in the exhaust system of vehicles, which exhibits a corrosion resistance higher than that of carbon steel in the exhaust gas condensate environment in automotive muffler and shows excellent resistance to oxidation as well for application in high temperature environments. Various aluminizing processes have been developed, such as laser aluminizing, electro spark depositing, vacuum plasma spraying and gas phase aluminizing. Hot dip aluminizing is one of the most economical techniques for aluminizing steel surfaces [5].
Commercial 1020 carbon steel (C: 0.18�0.23 wt. %; Si: 0.15�0.35 wt. %; Mn: 0.30�0.60 wt.%; P: 0.03 wt.%; S: 0.035 wt.%) is dipped in a pure molten aluminum pool at 8500C for 30 s
Fig-1 A cross-sectional SEM image of a nano crystallized aluminum coat on carbon steel and line auger spectrometer analysis: Al and Fe distributions across the Al coat� and the steel substrate.
Fig. 1 presents a cross-sectional SEM image and Al and Fe distribution profiles of a sandblasted and recovered aluminum coated carbon steel specimen. The chemical analysis using an auger spectrometer indicates that there was only a small quantity of Fe element diffused into the aluminum coating in the vicinity of Al/steel interface. Usually, har and brittle inter metallic compounds, such as FeAl2 and/or Fe2Al5, form at the aluminum/steel interface, the diffusion layer�s thickness and the content of inter metallic compounds can be varied by changing the annealing or diffusion temperature and duration. Since this work is focused on the out most layer in which little inter metallic compounds are present, we therefore set the hot dipping time as only 30 s.
Surface crystallization of aluminized mild steel
An aluminum layer is coated on carbon steel by dipping the steel sample in a molten Al pool at 8500C for a 30 s. The aluminum coating was then surface nano crystallized by sandblasting and recovery heat treatment. The former introduced plastic deformation with dislocations network or cells to the surface layer, while the subsequent recovery treatment turned the dislocation cells into nano-sized grains.
Fig-3 illustrates variations in hardness from the top surface to inside the coatings. The hardness of all nano crystalline aluminized specimens reaches 60�90Hv at the top surface layer, decreases to30�50Hv with increasing the depth to 30 �m, and then becomes stable with respect to the distance. The significant increase in hardness by nano crystallization is attributed to the high-density grain boundaries which block the dislocation movement and thus results in increased hardness.
Among all the specimens the nano crystalline aluminum coating surface experienced annealing at 300 0C for 60 min exhibits the highest corrosion potential and the lowest corrosion current density, followed by that annealed at 4000C. Annealing at 200 oC did not result in marked improvement, which could be attributed to possibly incomplete nano crystallization when annealed at the relatively lower temperature during which dislocation cells might not be fully turned into nano-sized grains, bearing in mind that dislocations make electrons more active and promote electrochemical reactions. Compared to the untreated Al coating, the nano crystallized Al coatings experienced recovery treatment at 300 and 400 0C showed higher corrosion potential sand lower corrosion currents in the 0.5M H2SO4 solution. The higher corrosion potential of the nano crystallized Al coating reflect sits increased inertness to corrosion and the lower corrosion current is directly related to a lower corrosion rate of the coating in the solution. The improved corrosion resistance is attributed to the nano crystallized surface layer in which the high-density grain boundaries promote atomic migration and thus accelerate passivationor the formation of a protective Al oxide film to block further corrosion reaction.
