This paper details the findings of a comprehensive study conducted to determine if a spray applied two-component inorganic industrial coating can be used to mitigate the soil-side corrosion of aboveground carbon steel storage tank bottoms. The coating is applied on the soil-side which is on the back side of the welded sections of the tank bottom. The study included microstructural characterization of the weld zone and surrounding areas of welded steel test panels. which were coated on the back side. The coating material as well as the passive layer formed by the coating on the steel surface were analyzed using XRD, SEM and EDS for their morphological and structural characterization before and after welding. Surface characteristics of the passive layer formed on the coated carbon steel samples were investigated with the help of electrochemical techniques consisting of Ecorr, LPR and CPP studies. Performance tests on coated panels were conducted under simulated field environments using Cyclic Aging Resistance and Immersion tests.
The electrochemical data indicated that the high heat input during the welding process affects the nature and quality of the passive layer on the carbon steel surface which implies that the steel surface in the weld and heat affected zone (HAZ) areas may be less corrosion resistant compared to the steel surface in non-welded areas. However, the coating showed resistance1 to seawater immersion and cyclic aging tests demonstrating its effectiveness in the protection of tank bottom plates from corrosion induced by underground water and soil when applied on the backside of welded steel plates, as claimed by the manufacturers.
Protection of aboveground water storage tank bottom plates against soil side corrosion is an important concern for the oil and gas industry, particularly when considering long-term continuous operation requirements for these assets. Corrosion of the tank bottom plates is mainly due to the formation of galvanic corrosion cells on the soil side. This galvanic corrosion cell formation is more pronounced if the tank content is stored at elevated temperature and/or located in aggressive soil. Galvanic cells on a metallic surface may occur due to different reasons such as, differences in surface potential in different areas of the metallic surface, differences in O2 concentration, differences in the pH level of the soil, differences in the metal ion concentration, microstructural differences, etc. Such differences result in the formation of anodic and cathodic areas and enhanced corrosion occurs at the anodic sites. Effective corrosion control measures must be taken to ensure bottom plate integrity for continuous, safe and economical operation of storage tanks. The most commonly used corrosion protection method is the use of barrier protection systems such as sand-bitumen foundation pad or soil side coating with or without cathodic protection (CP). However, field experience has indicated that these protective methods are not effective to prevent long term soil side corrosion attack of the tank plates due to the following:
• During tank construction, individual bottom plates are first coated on soil side and then welded together to complete the entire tank bottom. Near the weld joints, the heat of welding destroys the coating, which cannot be repaired, being on the soil side. Thus, the tank bottom inherently has unprotected areas in contact with soil.
• The heat of welding may result in differences in microstructure of the steel plates adjacent to weld joints causing preferential corrosion at the heat affected zone (HAZ).
• The coating on the soil side of the bottom plates is also subjected to normal degradation over time.
• The performance of the CP system can be influenced by variation in soil conditions and frequent monitoring is generally required to verify the proper operation.
• During loading and unloading of the tank, the bottom plate is subjected to flexure and settlement which can damage the sand-bitumen pad and allow water ingress and cause premature corrosion failure of the bottom plates.
