EGR (Exhaust Gas Recirculation) is a method for reducing NOx emissions for heavy-duty diesel engines. EGR works by introducing part of the exhaust gases back to the engine cylinders. Exhaust gases consists mainly of CO2, NOx, SO2 and H2O. As the temperature decreases, these gases form a corrosive condensate.
The EGR components which are exposed to the condensate environment must therefore be of corrosion resistant materials. The objective of this study is to investigate suitable materials for use in exhaust condensate environment.
The goal is to evaluate the pitting corrosion resistance for eight different commercial stainless steels and two commercial aluminium alloys in exhaust gas condensate environment. Furthermore, nitriding surface treatments on one martensitic stainless steel and anodising treatments on one aluminium alloy, were also included in this study.
Five different exhaust gas condensates with different concentrations of sulfuric acid, nitric acid and chloride were chosen to perform electrochemical measurements. Two pH values 2.5 and 1.5; three chloride concentrations, 32 ppm, 200 ppm, 3300 ppm were included in the environmental parameters.
The testing temperature was 60 oC, since it is the temperature which can still be expected to produce substantial amount of exhaust gas condensate in the EGR system. The electrochemical method used, was anodic polarisation measurements. This is a useful method to evaluate the pitting resistance for stainless steels in chloride containing solutions.
The results show that the two aluminium alloys and the martensitic stainless steel were subjected to both general and pitting corrosion in a normal condensate solution at pH 2.5. The anodised film on the aluminium surface was not stable in condensate environments with low pH value. After twelve hours of exposure to a condensate at pH 2.5 at 60 oC, the protective effect of the lm became negligible.
The austenitic, ferritic and duplex stainless steels show, however good resistance against both corrosion types. Increasing condensate acidity from pH 2.5 to 1.5 could not be observed to increase risk of pitting corrosion for the austenitic, ferric and duplex steel stainless steels.
High concentrations of sulphuric acid, low pH value, but low chloride content (200 ppm) do not increase the risk for pitting corrosion for austenitic steels 1.4404 and 1.4301, duplex 2304 and ferritic 1.4521. However, chloride concentration of 3300 ppm, signficantly increased risk of pitting corrosion, especially for the austenitic stainless steels. Duplex stainless steel show better pitting resistance in high chloride environments, in addition to the good general corrosion resistance in low pH value environments.
There is no dierence in corrosion resistance between the nitride coated 1.4112 steel and the steel without coatings. No dierences can be observed between the plasma and gas nitrided samples. Further investigation in less corrosive environment is recommended, since anodic polarisation is not a suitable method to study general corrosion behavior.
The pitting corrosion resistance in condensates with high chloride concentrations at 60 oC follows the sequence 1.4301<1.4521<1.4404<duplex 2304<duplex LDX2404<duplex 2205. Clearly, duplex stainless steels have better pitting corrosion resistance in low pH environment when chloride concentration is increased. Considering the operating conditions of the EGR components, the element prices, it is probably more benecial to consider the duplex stainless steels for use in the EGR system.
Source: Umeå University
Author: Olofsson, Andreas