## 3 Results and Discussion
### Influence of Alumina Coating on the Corrosion Fatigue Behavior of 1.4542  
Earlier the authors presented the influence of corrosive media on the mechanical behavior of stainless steel AISI 630 (X5 CrNiCuNb16-4, 1.4542) during carbon capture and storage as well as in geothermal energy production [13; 21]. Earlier results do not change with alumina coatings indicating that the coating has no influence on the corrosion fatigue behavior. This includes S-N curve with low coefficient of correlation (r\({}^{2}\) = 0.33) and a large scattering range (TN = 1:34.4), corrosion fatigue strength 60% below the endurance limit measured in air (620 MPa), reaching a maximum of 10 \(\times\) 10\({}^{7}\) cycles at a stress amplitude of 150 MPa (Wohler-exponent of k = 3.59), unusual corrosion pattern and failure related to statistical crack initiation due to either pitting or the formation of micro cracks depending on the stress amplitude [13].  
The influence of ethanol based alumina coatings of the fatigue behavior is demonstrated in Figure 5. The coating is distributed homogeneously over the test area of the specimen. Four specimen were tested, one failed due to machine failure but three reached 10\({}^{7}\) cycles at stress amplitude of 200 MPa. However, these endurance limits were only reached because of the low stress amplitude and are not related to the protection of the coating. The alumina coating was almost completely detached leaving the bare surface in contact with the corrosive environment resulting in severe pitting (Figure 5, bottom). An important finding is that this type of coating (first developed for high temperature alloys (use temperature > 550 \({}^{\circ}\)C) and nickel-base superalloys) does not perform in saline-water based aquifer environments with process gasses such as CO\({}_{2}\). Also the considerable low drying and heating temperatures do not guarantee the adhesion of the coating.  
Figure 5: Fatigue specimen and surface image after coating with ethanol based alumina sol prior (**top**) and after (**bottom**) to exposure to CCS environment.  
When a consistent protective coating is present on the metal surface, fatigue cracks are observed to initiate at alumina particles embedded in the substrate steel matrix during blasting, rather than at corrosion pits formed during testing, as highlighted by [PERSON] et al. [48]. The researchers argue that the corrosion fatigue strength of a coated substrate is governed by the same mechanism that influences the fatigue behavior of the material in the air.  
In the case of the detached ethanol-based alumina sol-gel coating on X5 CrNiCuNb16-4, pits precipitate early, causing crack initiation comparable to the non-coated specimen within a corrosive environment (CO\({}_{2}\)-saturated saline water), according to [PERSON] et al. [48]. [PERSON] et al. [46] suggest that shot peening prior to coating provides an excellent alternative to increase the fatigue strength of AISI 4340 steel. In AISI 630, X5 CrNiCuNb16-4, the residual stresses in the coating and substrate may change from tensile near the coating surface to compressive inside the coating center. These compressive stresses may then enable the coating to remain attached to the metal surface during fatigue testing.