External corrosion of tank bottoms is a significant problem for tank owners for both economic and environmental reasons. Corrosion Protection of Above Ground Storage Tanks is typically provided by a impressed current cathodic protection (CP) with or without coatings. Two primary Impressed current CP configurations are used worldwide in tanks. The field-erected mixed metal oxide (MMO) anode grid configuration was an early application of MMO anodes for tank bottoms and has a large global installed base. A newer design to protect tank bottoms is a system of parallel or concentric ring MMO linear anodes with factory connected tail cables.This paper will review the design of “grid” type and “parallel/concentric” linear anode type tank CP designs provide an economic assessment assign a qualitative risk assessment matrix and discuss the implementation impact based on a large grass root tank farm project in Africa. Where the owner/engineers elected to use linear anodes.
Assessment of the external coating condition of underground pipelines depend on the use of approved external corrosion direct assessment (ECDA) indirect inspection techniques such as alternating current voltage gradient (ACVG) alternating current attenuation (ACCA) and direct current voltage gradient (DCVG). Efficient application of these techniques (especially ACVG and DCVG) to ensure optimal detection and prioritization of coating anomalies largely depend on a myriad of factors including: depth of cover probe spacing current level and soil resistivity.Research and industry experience have shown that normalization of key factors such as current level probe spacing and depth of cover led to accurate detection and prioritization of coating anomalies for ECDA and coating assessment projects. However there have been scenarios where known coating faults have not been detected with DCVG and ACVG even when key factors were considered during the indirect inspection.This paper will present case studies that showcase instances where the optimization of coating anomaly detection and prioritization methodologies led to a successful execution on ECDA projects and prevented unnecessary pipe bell-hole excavations. In addition lessons learned from scenarios where known coating faults were not detected with voltage gradient surveys will be presented.Key Words: Pipeline Integrity ACVG DCVG ACCA Cathodic Protection Indirect Inspection ECDA Pipeline Integrity Management Program.
Examples of experimental protocols and competitive analyses for coating selection for transmission and distribution (T&D) structures are provided in this paper. The tests were conducted in different electrolytes with painted galvanized and non-galvanized steel including solutions with high chloride soil solutions with different salt concentrations and aqueous solutions using deionized water sea water and corrosivewater etc. The results provide an indication of coating quality and corrosion resistance. Coating evaluation using various combinations of electrochemical impedance spectroscopy (EIS) and other tier testing techniques including cathodic disbondment and cathodic protection shielding is also considered. Finally methods of EIS-based technologies for rapid evaluation of organic coatings are reviewed and case histories in T&D utility industry is presented.Key words: coating selection coating evaluation transmission and distribution structures electrochemical impedance spectroscopy case studies
Concrete is the most widely used construction material and can be durable for hundreds of years; however the largest cause of concrete deterioration is due to corrosion of reinforcing steel.Steel is thermodynamically unstable and will eventually oxidize to a lower energy state.The rate of deterioration in existing structures can be modeled to predict the service life of a given structure based on the service environment concrete quality chloride ingress carbonation depth corrosion rate measurements and other factors to various degrees of success. Service life is defined as the remaining useful time for a structure based on the current deterioration rate without repairs.This paper will provide an overview of many of the service life models in use and provide an evaluation protocol for their application.
This paper describes the corrosion risk assessment and corrosion mitigation of bottom plates on aboveground tanks used for the storage. The standard method of determining the corrosiveness or the effectiveness of cathodic protection on a tank bottom is the tank-to-soil potential measurement. One of the problems associated with monitoring cathodic protection systems on tank bottoms is the inability to assess empty space under the tank or to place a reference electrode in close proximity to the underside of the tank resulting in measurements that may not represent the tank-to-soil potential at specific areas or at the center of the tank bottom. When utilizing earthen foundations soil analysis is useful to help determine whether the potential corrosion activity will be high enough to make cathodic protection necessary and whether cathodic protection will be a practical application to prevent corrosion. The advantages and disadvantages of each will be discussed. Determination of aggressive ions such as chlorides and sulfates along with measurement of moisture pH and resistivity at shallow and deep burial are helpful for further corrosion analysis. Predictive modeling based on soil corrosivity data will provide life expectancy or remaining life with moderate to high confidence levels. Corrosion mitigation techniques such as cathodic protection concrete foundations and VCI will be discussed briefly.In this paper case histories of corrosion perforation will be provided. On-site visual inspection magnetic flux leakage (MFL) inspection ultrasonic testing (UT) and evaluation of the external cathodic protection (CP) system will be considered in the failure analysis. The corrosion products were analyzed using energy-dispersive X-ray analysis (EDAX) to determine the possible corrosion mechanisms.
The corrosion behavior of austenitic stainless steels (UNS S30400/S30403 S30800) with machined electropolished or temper filmed surfaces was tested in deaerated pH adjusted water at 500F 550F and 620F for up to 10000 hours. Test specimens were subsequently analyzed by X-Ray Diffraction Auger Electron Spectroscopy and Electron Spectroscopy for Chemical Analysis to determine oxide composition thickness and structure. The resultant corrosion rate of the machined specimens exhibited an inverse temperature dependency with the rates decreasing by an order of magnitude from the lowest to the highest test temperature. Corrosion of electropolished specimens was only slightly sensitive to temperature effects and was similar to machined specimens at the highest temperature. Temper filmed materials had distinct phases of slow or rapid corrosion with different durations depending on the test temperature. Corrosion films on all specimens consisted of varying degrees of iron-rich spinel crystals covering a smooth layer of chromium-rich oxide most likely Fe3O4 and/or FeCr2O4 with some (CrFe)2O3. The temper films consisted of Cr2O3 which persisted throughout the test. The elemental composition of the films particularly the nickel content varied with increasing temperature.Key words: Corrosion austenitic stainless steel electropolish temper film oxide
Survey of sacrificial anodes mounted on complex structures sometimes includes field gradient measurements for example made by stabbing an anode using a twin-cell probe.Typical practise is for survey companies to convert the twin-measurements of potential into an estimated anode output but there is a risk that estimates may be inaccurate if the method used does not appropriately take into account factors such as:The stab is made away from the middle of the anodeThe probe is not oriented perpendicular to the surface of the anodeThe structure around the anode leads to distortion of the electric fields for example because there is a coating defect to one side of the anode or the structure is not symmetric around the anodeThrough the use of examples this paper investigates the effect of such factors on measured field gradient. It goes on to show how field gradient measurements made with probe orientations which do not correspond to electric-field flux-lines or which are not central on an anode can nevertheless be used to estimate anode output.The paper seeks to provide reference material which can be used to simplify estimation of anode output from measured field gradients.
A geothermic system faced serious injection obstruction problems two-and-a-half months after start-up. The obstruction was so severe that the operation was suspended and research was done in order to determine the cause. To solve the obstruction problem the system was treated downhole using acid and biocide. During this treatment dangerous amounts of H2S were released. This study was initiated to understand the reason of this problematic and give options for treatment and prevention. The results show high amount of bacteria in the obstruction material indicating that microorganisms caused the obstruction. The detected species were typical thermophilic species with optimal growth temperature at 40-60 ºC. The change in temperature in combination with the used oxygen scavenger containing nutrientsnecessary for microbial growth and activityis suspected to have caused the obstruction.The H2S formation was likely caused by a combination of biological FeS formation and the release of H2S by the addition of acid during cleaning. To prevent this type of issues biocide treatment is recommended not only during but also before operation is started. Alternatives for biocides have been investigated as well however more research is needed to understand the practical feasibility of those alternatives.