Alloying- and test frequency-dependent fatigue and corrosion fatigue damage mechanisms of AISI 304L joints brazed with BNi-2 and VZ2177

Fatigue and corrosion fatigue tests in laboratories raise the question of whether the test frequency f is representative of real operating conditions, particularly for brazed heat exchangers. To investigate the frequency-dependent effect of the superimposed corrosion loading on the fatigue behavior of brazed stainless steel joints, AISI 304L was brazed using the nickel-based filler metals BNi-2 and VZ2177 in a vacuum furnace. Fatigue tests were performed at test frequencies of 0.1 to 1000 Hz in air and 0.1 to 10 Hz in synthetic exhaust gas condensate K2.2. For AISI 304L/BNi-2, the superimposed corrosion loading at f = 10 Hz reduces the fatigue strength in air to 47%. AISI 304L/VZ2177 reaches only 90% of the fatigue strength of AISI 304L/BNi-2 in air, but approximately 190% in K2.2, with no detectable effect of corrosion loading up to 164 h. The results demonstrate that test frequency must be considered when transferring laboratory data to real applications. Nanoindentation, EDX analyses of FIB lamellae, and AFM investigations show that larger but fewer chromium borides in the diffusion zone result in improved corrosion fatigue strength. A damage model for the frequency-dependent corrosion fatigue behavior has been developed, supporting the reliable and economical design of brazed heat exchangers.