Characterisation of the corrosion fatigue behaviour of brazed AISI 304L/BNi-2 joints in synthetic exhaust gas condensate

Abstract

Exhaust gas recirculation (EGR) with brazed heat exchangers is commonly used for the reduction of NOx emissions of automotive diesel engines. Since the fatigue loading of brazed EGR coolers during the operation is superimposed by corrosive attack due to aggressive exhaust gases, the corrosion fatigue behaviour of brazed joints is relevant for the component design. In the present study, brazed AISI 304L/BNi-2 joints were cyclically tested in the synthetic exhaust gas condensate K2.2, using a corrosion cell, as well as in air after pre-corrosion acc. to VDA 230–214 with ageing durations of up to 6 weeks. A significant reduction of the fatigue strength at 2 × 106 cycles down to 43% was determined for superimposed and down to 22% for successive corrosion fatigue loading. A galvanostatic anodic polarisation of the specimens tested in situ was applied successfully for an efficient reproduction of the corrosion fatigue behaviour after the long-time and cost-intensive pre-corrosion. In this context, a novel test strategy with local strain measurements, using a newly developed extensometer, as well as electrical and electrochemical measurements was established for a precise corrosion fatigue assessment of the brazed joints. Microstructure-related corrosion fatigue damage mechanisms were evaluated using scanning electron microscopy.