## 3 Results and Discussion
### Influence of Alumina Coating on Static Corrosion Behavior of 1.4021  
Figure 2 shows the sol gel coated (water based as well as ethanol based, binder: polyvinylbutyl) coupons of X20 Cr13 before and after exposure to CO\({}_{2}\) saturated saline aquifer water at 60 \({}^{\circ}\)C and ambient pressure. The macroscopic degradation of the coating is demonstrated by lateral detachment of the coating and dissolution of the alumina sol (Figure 3). Figure 1: Schematic set-up of operating corrosion chamber for in-situ corrosion fatigue testing (vertical resonant testing machine (A), clamping socket and plate (B), corrosion chamber (C), gear pump (D), aquifer reservoir with heating (E), pH-measuring and mixing unit (F), camera (G), thermocouple (H) and union nut (I)). Regions of the coupon surfaces with degraded coating are highly susceptible towards CO2 corrosion and show typical corrosion products and patterns of uncoated samples as demonstrated earlier [13; 21; 56]. Surface corrosion layers and pits reveal both, FeCO3 and FeOOH as the main precipitation phases with no dependence on the coating. The lack of coating elements within the corrosion products indicate that the coating is detached before surface reaction take place at the newly exposed metal surface. Pit formation is driven by the formation of carbonic acid and existence of HCO3 as well as a transient ferrous hydroxide phase Fe(OH)2 reacting to corrosion products -mainly siderite- on the pits as detected on the surface elsewhere. The corrosion kinetics of 1.4021 have been extensively studied [13; 21] indicating that exposure at ambient pressure serves as a worst-case scenario simulation. The corrosion rates of coated coupons align with earlier findings for X20 Cr13 at ambient pressure remained approximately 0.1 mm/year after 1000 h of exposure. Locally, regions fully coated showed very good corrosion resistance and low corrosion rates in the liquid phase (where coupons  
Figure 2: Sample surfaces of water and ethanol based alumina coated martensitic stainless steel X20 Cr13 after 1000 h of exposure to CO2 saturated saline aquifer water at 60 \({}^{\circ}\)C and ambient pressure. Top: coated, upper row: front side, lower row: back side. Bottom: corroded after exposure: upper row: front side, lower row: back side. are fully immersed in the aquifer) are low, indicating that the CO\({}_{2}\) partial pressure is not sufficient to initiate the corrosive reactions described by [PERSON] et al. and [PERSON] et al. [10,13]. Figures 3 and 4 clearly show that ethanol based alumina sol does not coat the entire surface after exposure. Regions with locally destroyed coating reveal corrosion products according to the reactions and products stated earlier. There are three possible reasons for early failure of the coating (note that all samples were wetted perfectly prior to exposure to CCS environment):  
1. Due to the surface tension the coating is thinner towards the edges of the coupons than in the center. Therefore, stress causes micro cracking at the edges with the capillary forces driving the saline aquifer brine underneath the coating wetting the base metal surface. 2. The high porosity may also be cause for the instability of the coating. The brine may have contact to the base metal surface through pores that connect surface and environment. Early corrosion reactions cause the coating to detach then in lateral direction. 3. The reliability of the coating may also be reason for failure, because the method of coating directly influences the continuity and thickness of the coating. Dip coating leads to an increasing thickness of the coating towards the bottom of the samples because gravity forces the sol to flow before its gel status. Due to the \"flowing\" of the sol a bulge precipitates that has higher micro stress than the remaining coating. Therefor micro cracking of the coating is initialized within this bulge.