Electrochemical impedance aiding the selection of organic coatings for very aggressive conditions

• Six coatings of recognized brands were evaluated for harsh conditions in the oil primary processing.
• Experimental conditions included high salinity, H2S/CO2 and high temperatures.
• Evaluation of coatings applied on super duplex and carbon steels suggested more adhesion problems for super duplex.
• All coatings failed after one year testing, pointing to the need of R&D in this area.
• Arrhenius-type plot of impedance modulus can aid to rate coatings performance for high temperature service.

The super duplex steel has been used in equipment for oil primary processing. Despite being assigned to this type of steel excellent anticorrosive properties, inadequate working conditions may impair its performance. In general, when this happens, metallic or organic coatings are applied aiming at prolonging the equipment lifetime. However, there is no information about the behavior of coated super duplex. This paper compares the performance of commercial organic coatings applied to carbon steel and super duplex, when subjected to harsh conditions present in the primary processing of oil, namely: high salinity, high temperature, presence of H2S and CO2. The adopted experimental approach allows discussion about: (i) organic coatings state-of-art for this industrial field; (ii) influence of super duplex on the coatings performance and (iii) development of methodology to aid on the organic coatings selection for such aggressive conditions. The experimental consisted of immersion testing at 150 °C and 180 °C in autoclave during one year, with performances monitored by electrochemical impedance, visual inspection and adhesion measurements. At the end of this test all coatings had some kind of deterioration, pointing to the need of research and development in this area. There were cases where the super duplex adhesion losses were more significant than with carbon steel, suggesting that special attention in super duplex surface treatment is important. The glass transition temperatures (Tg) of the coatings were determined by dynamic mechanical analysis. Permeability to water vapor and impedances were measured with free-films of the coatings at temperatures above and below Tg. These last results confirm the importance of the thermal behavior characterization of the coatings, particularly for conditions involving exposure to temperatures above Tg. Marked differences on Arrhenius-type plots of free-films impedance modulus at low frequency range were associated with performance and appears as a tool that can assist the selection of coatings for high temperature service.