CORROSION CONTROL OF ABOVE GROUND STORAGE TANK BOTTOM STEEL PLATES USING ALUMINUM MESH ANODE WITH NEWLY DEVELOPED BACKFILL

Transcription

1 CORROSION CONTROL OF ABOVE GROUND STORAGE TANK BOTTOM STEEL PLATES USING ALUMINUM MESH ANODE WITH NEWLY DEVELOPED BACKFILL Miki Funahashi, PE MUI International Co. LLC ABSTRACT Above-ground storage tank bottoms are subject to corrosion. The tank bottoms have been protected from corrosion by several methods, including oil sand, asphalt sand or impressed current cathodic protection. The insulating property of the oil and asphalt supposes to stop corrosion; however, the effectiveness of this approach has been questioned due to corrosion failure resulting from insufficient dielectric protection. Impressed current CP electrochemically stops the corrosion on the tank plates in contact with sand or soil underneath. However, it cannot protect the plate once it loses the contact with the soil. In addition, if the transformer rectifier or the system hardware fails, the cathodic protection is not effective until it repairs or replaced. A new sacrificial anode system was developed using aluminum mesh with a high ph modified sand backfill to overcome the problems raised by the other corrosion control systems. This paper will discuss the new sacrificial anode CP system. Key words: above-ground storage tank, sacrificial anode, cathodic protection, aluminum mesh anode, passive film, backfill material, high ph, INTRODUCTION The bottom plates of above-ground storage tanks are subject to corrosion. In some situations, the tank bottom may be protected from corrosion by oil sand, asphalt sand or impressed current cathodic protection. The protection by oil or asphalt sand takes advantage of the dielectric, non-electrolytic property of the oil and asphalt. Recently, however, it was verbally reported in Japan the effectiveness of this approach has been questioned due to corrosion failure resulting from insufficient dielectric protection. In addition, water or rain intrusion from the edge of the tank bottom plate may accelerate

2 corrosion in those areas. Impressed current cathodic protection generally uses an inert anode with a transformer rectifier. The anode is typically embedded in sand or soil. The cathodic protection current from the anode travels through the sand or soil electrolyte which contacts the steel plate and protects the tank plate from corrosion. So long as the tank plate contacts the sand/soil electrolyte, the impressed current cathodic protection is effective. However, when the volume of the product inside the tank decrease or is emptied, the tank plate rises from the sand or soil, resulting in development of air gaps in some areas. If this occurs, the cathodic protection current cannot reach the tank steel surface located over the air gaps because the air cannot transfer the cathodic protection current (Figures 1 and 2). As a result, the effectiveness of the corrosion protection using an impressed current is lost, and those areas are subject to corrosion. In addition, since cathodic protection is continuously operating system, interruption or malfunction of the transformer rectifier or damage of any cathodic protection hardware stops the protection of the steel plate from corrosion. Sacrificial anode cathodic protection systems using zinc plates embedded in a conventional bentonite/gypsum backfill have been used to protect the tank plates in Japan. However, the disadvantages of this system are: When the zinc plates are embedded in the conventional calcium bentonite and gypsum based backfill, the passivation of the zinc can be minimized. However, since the tank steel plate is also in contact with the backfill, it is now exposed to a highly corrosive electrolyte. Therefore, the steel is subject to corrosion when the current from the zinc anode plates decrease. Aluminum or aluminum alloys are generally used in seawater or brackish water as a sacrificial anode because the high chloride concentration prevents the passivation of the aluminum. The aluminum anode has a high electrical capacity and high utilization factor. In sand or soil environment, however, aluminum or aluminum alloy does not function as a sacrificial anode due to the passivation. Even if an aluminum anode is embedded in the backfill material containing a high chloride concentration, the chloride ions diffuse into the surrounding soil in a short period of time because of the highly mobile tendency of chloride ions. Therefore, the aluminum or aluminum alloys cannot function as a cathodic protection anode in earth burial conditions. To overcome these concerns and problems, a new sacrificial anode system was developed using aluminum mesh in conjunction with specially modified ph buffered backfill sand. The proposed aluminum mesh is Alloy 1100 and the typical chemical composition can be found in Table 1

3 ALUMINUM MESH SACRIFICIAL ANODE USING HIGH ph MODIFIED SAND BACKFILL When aluminum or aluminum alloys are embedded in a high ph electrolyte, the formation of the passive film on the aluminum can be prevented as indicated in Figure 3. This allows these metals to function as cathodic protection anodes. Another advantage is that the aluminum oxide products formed on the anode is highly soluble, so that the aluminum oxides do not adhere to the anode surface. When both steel and pure aluminum (anode) are exposed in the same high ph electrolyte environment and electrically connected, the following benefits are accrued: 1. The passivation of aluminum can be prevented. 2. Cathodic protection current requirements are reduced significantly. 3. The consumption rate of the aluminum anode is reduced, increasing its life expectancy. 4. Cathodic reactions occurring on the steel create a more durable passive film over time 5. Due to the strong passive film, the steel still protects for an extended period of time when cathodic protection is removed. The benefit from higher ph backfill is also described by American Petroleum Institute (API), Recommended Practice 651 Cathodic Protection of Aboveground Petroleum Storage Tanks. 1 When steel is in contact with a high ph electrolyte (preferably 10 or greater), the steel passivates and develop a stable protection film on its surface, resulting in no corrosion until the film is disrupted. Furthermore, when cathodic protection is applied to the steel plate, sufficient hydroxyl ions (OH - ) develop on the steel surface as the cathodic reaction products. As a result, the steel plate further passivates. Once the strong and thick passive film is formed on the steel surface, the protection film is maintained for some time even if the tank plate losses contact with the soil or sand underneath (Figures 4 and 5 ). A typical example of passive steel in a high ph electrolyte is the reinforcing steel in chloride free concrete. When steel is embedded in a neutral ph soil or sand electrolyte, typically 10 to 20 ma/m 2 of cathodic protection current density is required to control corrosion. On the other hand, to protect the steel in a high ph electrolyte using cathodic protection, only 0.2 to 2 ma/m 2 is required to control corrosion. 2 This lower cathodic protection current requirement significantly reduces sacrificial anode requirements. By utilizing these advantages, a new sacrificial anode cathodic protection system was developed

4 using aluminum mesh with special buffered high ph buffered modified and a special backfill material. The reasons to select the aluminum mesh are: 1. The wide expanded mesh can be installed under the steel tank bottom plate in order to distribute the cathodic protection current uniformly. 2. The mesh form reduces the amount of aluminum required. This reduces the cost of the anode. 3. The mesh assures a large anode surface area in contact with the backfill. This lowers anode-to-electrolyte resistance and assure uniform consumption of the anode. The new sacrificial anode system has the following advantages over other systems: 1. The capability to use sacrificial aluminum anode for steel plates for cathodic protection of above-ground storage tanks for extended period of time. 2. The mesh form of the sacrificial anode provides uniform corrosion control to the entire tank plate. 3. A small amount of the sacrificial anode is required to protect the steel plate due to the low current demand of the passivated steel plate. 4. Prevention of corrosion can be achieved in areas over air gaps. To achieve the items discussed above, a new sacrificial aluminum mesh sacrificial anode with specially modified ph buffered sand backfill was developed, which have the following properties: 1. It maintains high ph as a buffer backfill material for a long period of time. 2. It holds reasonable amount of moisture for a long period of time to make the sacrificial anode active. 3. It is stable during the welding process of the steel tank bottom plates during construction. EXPERIMENT Two types of backfill materials and commercially available aluminum mesh (Alloy 1100) were prepared: Backfill A: Moist Sand (as control), consists of sand and Zeolite. The ph of this backfill was approximately 7. The water content is approximately 18 percent. Backfill B: High ph modified sand, consists of sand, Zeolite, alkaline buffer chemicals and a stabilizer. The ph of this backfill is 11. The water content is also 18 percent. Approximately 100 mm thick regular sand was laid on the bottom of two plastic containers (approx 1

5 m L x 0.5 m W x 0.7 m D) as shown in Figure 6. Backfills A and B were placed in separated containers containing an aluminum mesh anode. The height of each backfill material was approximately 100 mm, and the aluminum mesh (0.22 m x 0.22 m x 4 mm, 55 gram) was positioned at the mid depth of the backfill after they were weighted (Figure 7). Two steel plates (0.28 m x 0.14 m x 2 mm each) were laid on the top of the backfill and welded each other (Figure 8). The steel surface area in contact with the backfill was m 2. The steel plate and the aluminum mesh were connected through a 0.1 ohm shunt resistor and a switch to measure the current output and instant-off potentials. The potentials were measured using a portable copper/copper sulfate reference electrode. The plastic containers containing Backfills A and B were covered, and the effectiveness of the cathodic protection was monitored for 329 days using the 100 mv Depolarization Criterion. The 100 mv Depolarization Criterion is only applicable to this condition due to the high ph. (Figure 9) After the test, the steel plates and the aluminum mesh were removed from the backfill material and left in a plastic container for about one month. The containers were stored at approximately 30ºC with a relative humidity greater than 90%. The steel plates and the aluminum meshes were removed for visual inspection at the end of one month. To determine the aluminum mesh electrochemical capacity, electrochemical testing was performed using an impressed current of 1 ma/cm 2. The schematic test set up is shown in Figure 10 and is similar to ASTMG97 ( Test Method for Laboratory Evaluation of Magnesium Sacrificial Anode Specimens for Undergrounds Applications ). The test was conducted in the high ph modified sand with 18 percent of moisture content. A piece of the pure aluminum mesh containing 3 grids (7 cm x 3cm opening) was used for this testing. The surface area was approximately 30 cm 2 and the initial weight was grams. After the exposure test of 240 hours, the aluminum mesh was removed from the sand for the weight loss measurement. RESULTS Tables 2 to 4 show the data collected during the 329 days test period, and Figures 11 and 12 show the performance of the sacrificial anode system, The results are summarized as follows. Backfill A

6 Table 2 shows that the steel plate on Backfill A was never cathodically protected by the aluminum anode using either the 100 mv depolarization or the -850 mv criterion. The initial current density was 3 ma/m 2 but dropped to a much lower current density in approximately 60 days. The static potential of the aluminum plate indicated that it passivated after 60 days of the test period. (Table 3) The aluminum mesh did not show any corrosion stains, and the surface was completely smooth, indicating the passivation of the aluminum mesh (Figure 11). The static potential of the steel plate indicated that it did not passivate (Table 4). The steel plate showed significant corrosion as evidenced by brown rust stains on the entire surface (Figure 12). The ph on the steel and the aluminum mesh surfaces were approximately 6 and 8, respectively. The ph at the aluminum-backfill interface coincided with the passive potential of the aluminum. Backfill B Backfill B enhanced the aluminum anode activity, and the aluminum plate cathodically protected the steel plates based on the 100 mv depolarization criterion (Table 2). The criterion was readily achieved by initial current density of 1 ma/m 2 which fell to 0.2 ma/m 2 after 84 days. The static potentials of the aluminum plate indicated that it did not passivate in the backfill. (Table 3) The aluminum mesh showed corrosion stains on the entire surface. Small corrosion pits were also detected. The material loss due to corrosion was negligible. (Figure 13) The static potential of the steel plate indicated passivation within 3 days from the start of testing. The steel plated showed a uniform black passive film on the entire surface. The surface was completely smooth. No corrosion loss or rust stains were observed (Figure 14). The ph on the steel and the aluminum plate surfaces were approximately 12 and 11, respectively. These potentials coincided with the conditions of the aluminum and the steel plate in the backfill. The steel condition exposed to air for one month indicated that the steel plate over moist sand showed further corrosion (Figure 15), but there was no evidence of corrosion on the steel plate which had been in contact with Backfill B (Figure 16). Determination of the aluminum mesh life After the specimen was exposed to 1 ma/cm 2 (10 A/m 2 ) for 240 hours, the weight loss was 4.92

7 grams, The anode capacity of the aluminum mesh was calculated by the following equation: Anode Capacity (Amp-Hr/kg) = Total current (Amp-Hr)/ Weight Loss (kg) The total current used for 10 days was 7.2 Amp-hr and the weight loss was 4.92 grams. Therefore, the electrical capacity of this aluminum mesh was approximately 1,463 A-hr/kg. It is assumed that the average current density for the steel plate to protect is 0.3 ma/m 2 or A/m 2 because this current density met the 100 mv Depolarization Criterion for the steel plate with this backfill. 1,463 A-hr/kg / A/m 2 = 4,878,049 Hr-m 2 /kg 557 year-m 2 /kg Since the aluminum mesh used with Backfill B was kg/m 2, the anode life is 557 year/kg-m2 x kg/m2 = 644 years This assumed that the backfill is stable, and corrosion is uniform overtime with 100% utilization. 30 years is a typical cathodic protection design life for an above ground storage tank bottom. Therefore, a significantly lighter aluminum mesh, such as 0.1 kg/m 2 should be sufficient for 30 years life. CONCLUSIONS 1. Exploiting the electrochemical characteristics of aluminum or its alloys in a high ph electrolyte, as well as the electrochemical characteristics of steel in a high ph electrolyte, the commercially available aluminum mesh in the high ph modified sand backfill provides cathodic protection system for the tank bottom steel plates. 2. After 329 days of cathodic protection application, the steel plate was completely passivated and showed protection passive film on the steel surface. 3. The high ph modified sand significantly reduced the cathodic protection current requirement for the steel plate with time. 4. This passive film on the steel plate was maintained in air for a one month period at approximately 30º C and in 90% relative humidity without cathodic protection. This indicates that this system can continuously provide corrosion protection to the tank plate over air gaps for at least one month. REFERENCES

8 1. API Recommended Practice 651, Third Edition, Cathodic Protection of Aboveground Petroleum Storage Tanks, January European Standard, Cathodic Protection of Steel in Concrete, EN12969, March, Table 1. Alloy 1100 Chemical Composition Element Percent (%) Aluminum min. Copper Silicon + Iron 0.95 max. Manganese 0.05 max. Zinc 0.10 max. Others, Each 0.05 max. Others, Total 0.15 max. Table 2. Results of Cathodic Protection. Backfill A Backfill B Test duration (days) Cathodic protection current density Amount of depolarization (mv) Cathodic protection current density Amount of depolarization (mv) (ma/m 2 ) (ma/m 2 )

9 Table 3. Static Potentials of the Aluminum Anodes Test Duration (days) Static Potential of Aluminum Mesh in Backfill A Static Potential of Aluminum Mesh in Backfill B Table 4. Static Potential of the Steel Plates Test Duration (days) Static Potential of Steel Plate in Backfill A Static Potential of Steel Plate in Backfill B Figure 1. Ideal Condition for Cathodic Protection for Steel Plate Figure 2. Steel Plate Over Air Gaps Cannot be Protected by CP

10 Corrosion Rate of Al (mg/dm2/hr) New Backfill ph Figure 3. Aluminum Corrosion Condition in Variuos ph Environments From Shatalov, Dokl. Akad. Nauk, 86, 775 (1952), as reproduced by Deltombe and Puorbaix, Corrosion, 14, 498 (1958) and Figure 4. Cathodic Protection Current Distribution Figure 5. Steel Plate Passivates and is Protected Over Air Gaps Figure 6. Shows Regular Sand Before Install New Figure 7. Aluminum Mesh Being Weighted Before Sacrificial Anode System Installation

11 Figure 8. After Steel Plates Positioned on New Sacrificial CP System Figure 9. Experiment Conditions DC Power Supply - + A Steel Can Aluminum Mesh New Backfill Figure 10. Weight Loss Test to Determine the Aluminum Mesh Electrical Capacity

12 Figure 11. No Sigh of Corrosion on Aluminum Figure 12. Shows Sever Corrosion of Steel Mesh in Backfill A on Backfill A Figure 13. Shows Corrosion on Aluminum Figure14. Shows No Corrosion on Mesh in Backfill B Steel Plate in Backfill B Figure 15. Shows Further Corrosion After One Month Figure 16. Shows No Corrosion of Steel Plate without CP (Backfill A) after One Month Without CP (Backfill B)