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Electrochemical study of polyaniline/silicon dioxide containing coatings in geothermal solution
(2018)
Polyaniline/Silicon Dioxide containing coatings were electrochemically investigated in a saline geothermal solution.
With the increase of exposure time, impedance values of coated specimens decreased at low frequency, which could be caused by the decrease of pore resistance due to electrolyte or water uptake.
Coating system needs further optimization work.
Geothermal brine is considered to be an aggressive environment as it contains various dissolved salts. Even though carbon steel is commonly used as a construction material due to its machinability and economical reason, it is susceptible to uniform and localized corrosion in a high temperature and high pressure system. Therefore, a coating system is introduced to protect the carbon steel against corrosion in such environment.
-It is necessary to find the optimum composition of SiO2 addition in the organic coatings to enhance the material performance, i.e. coatings adhesion, thickness optimization, thermal resistance.
Current project aims at the addition of Polyaniline (PANi) as the active agent to improve the corrosion resistance of materials against a high saline medium at elevated temperatures.
1. Introduction
Alumina coatings are one possibility to increase the corrosion resistance, lifetime and application range of thermally loaded steel components, e.g. in modern power plants where the use of the Oxy-fuel technology corrosive fuel gas (H2O-CO2-O2-SO2 at 650 °C) affects the steel parts. In previous investigations the efficacy of protective alumina coatings on steel P 92 under those conditions was demonstrated. A shutdown and re-start of power plants or parts of them causes thermal stresses of the components which can cause detrimental effects like microstructural changes in the steel itself, changes in its oxidation behavior, delamination or microstructural changes in the coating. All those effects can lead to failure of the components, resulting in lifetime reduction.
2. Objectives
As a first step, we concentrate on the influence of thermal cycling tests and observe the impact on the microstructure of the coating and the interface in laboratory air. These investigations will help understanding the processes which occur, show directions of potentially necessary changes of the coating due to improved thermal stress behaviour.
3. Materials & methods
P 92 is a ferritic-martensitic steel, containing 9% Cr which forms protective Cr-oxide-rich scales in dry environments and non-protective ones in water-containing environments. Coupons of P 92, having ground surfaces, were dip-coated via a sol-gel process and subjected to thermal cycling for 500 h (1000 cycles) in laboratory air in a temperature range between room temperature and 660° C. The resulting mass loss was determined by weighing.
Samples for TEM investigations were produced as cross sections normal to the sample surface by FIB preparation (Quanta 3D, (FEI)). The TEM/STEM investigations were performed using a JEM2200FS (JEOL) operated at 200 kV. The microstructure of the coating and the interface after cycling tests was characterized via TEM, HREM, and STEM images, electron diffraction as well as EDX and EFTEM methods.
4. Results
At steep edges in the surface profile the coating was imperfect and cracks have formed during the thermal cycling. Flat surface regions are well-covered. The whole interface region between the steel and the coating shows a dense Cr-oxide-rich zone, which can form protective regions in case of local failure. The Cr-oxide zone is followed by a region of mixed oxides, containing Cr, Mn, Fe, and Al in variable composition, to which a porous δ-Al2O3 zone is joined.
5. Conclusions
• Alumina coatings promote the formation of dense, Cr-rich zones at the interface, which makes the system self-healing.
• These zones are stable during thermal stresses, even in regions with cracked coatings.
• They cause reduction of outward diffusion and mass loss during thermal cycling.