PSI - Issue 66

Johannes L. Otto et al. / Procedia Structural Integrity 66 (2024) 256–264 Johannes L. Otto et al. / Structural Integrity Procedia 00 (2025) 000–000

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2.2. Fatigue and corrosion fatigue characterization As mentioned in the introduction, industrial applications for brazed joints with nickel-based filler metals are often exposed to mechanical, corrosive and thermal stresses. For this purpose, the test setup shown in Figure 3 was developed to perform corrosion fatigue tests at elevated temperatures up to 100°C. The simple design allowed the integration into a servohydraulic testing machine and eliminated the need for an expensive autoclave. The corrosion cell was milled from acid-resistant Polytetrafluoroethylene (PTFE), which is also suitable for elevated temperatures. The hoses and connections are also designed accordingly, while the other components are made of glass. A thermocouple in the cell allows to adjust the temperature using a proportional–integral–derivative (PID) controller. The corrosive medium was stored in a round flask, which is placed in a heating device connected to that PID controller. To prevent vapor formation at elevated temperatures, the round flask was slightly pressurized to p e = 0.5 bar with compressed air. A membrane pump was used to circulate the corrosion medium at a rate of 4 l/h. To obtain information about the corrosion behavior, the passive layer and crack initiation, a three-electrode system with a potentiostat was used to measure the open circuit potential in experiments at room temperature. A carbon electrode was used as the counter electrode, an Ag/AgCl electrode as reference electrode and the specimen as working electrode. The corrosion media used are listed in Table 2. The synthetic exhaust condensate K2.2 was chosen according to automotive industry recommendations, VDA (2018). Unless otherwise stated, the tests were carried out at room temperature without pressurization in an air-conditioned laboratory at 20°C. All experiments were carried out stress controlled at a test frequency of f = 10 Hz and a stress ratio of R = 0.1 in the tension-tension range. This must be considered because, in combination with the ductile material, significant cyclic creep occurred and therefore the tests must correctly be described as creep fatigue or corrosion creep fatigue. A total strain of 15% in the test area (measured with an extensometer at air and converted to the machine strain due to the cell) was used as failure criterion, since the specimens always showed a wide-open crack and high deformation at this point, which only needed a few more cycles to completely fracture. A complete fracture should be avoided for safety reasons due to possible leakage, even if some protection for the testing system was used. At 2 ∙ 10 6 load cycles, which corresponds to a test duration of 55.5 h, the specimens were classed as non-failure, and the test was stopped as well.

Fig. 3. New developed testing setup for corrosion fatigue tests at elevated temperatures (without three electrode setup).

Table 2. Compositions and pH value of the aqueous environments for corrosion fatigue testing.

Name

Abbreviation

NaCl

CH 2 O 2

C 2 H 4 O 2

pH 3.5

Synthetic exhaust gas condensate K2.2 One-molare sodium chloride suspension

EGC NaCl

1.65 g/l 58.44 g/l < 0.01 g/l

0.039 ml/l 0.035 ml/l

– –

– –

7 7

Demineralized water

H 2 O