PREMATURE FAILURE OF REPAINTED EPOXY ON THE BOTTOM PLATE OF A MAIN FUEL OIL TANK - A CASE STUDY 1

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1 PREMATURE FAILURE OF REPAINTED EPOXY ON THE BOTTOM PLATE OF A MAIN FUEL OIL TANK - A CASE STUDY 1 M. Mobin and A.U. Malik Saline Water Desalination Research Institute Saline Water Conversion Corporation P.O. Box 8328, Al- Jubail Kingdom of Saudi Arabia rdc@swcc.gov.sa ABSTRACT This article describes the premature failure of repainted epoxy coating applied over the inner side of the bottom plate of a fuel oil tank leading to perforation of the bottom. Some cavities and minute pits were already present on the inner surface of the plate when the coating was reapplied but the painters overlooked them. After about 2 months of service following repainting, penetration of the bottom plate of the tank occurred. The coating failure is attributed to chloride contamination of the steel surface prior to the coating application. INTRODUCTION Failures of coatings applied over the surfaces having soluble salt contaminations have occurred extensively. In general, the failures are in the form of blisters, rust, tubercles or loss of adhesion in areas where steel was previously exposed to corrosive condition. The cause of failure is generally attributed to the retention of minute amount of contaminant or corrosion product along grain boundaries of steel surfaces, even though blasted to white metal. The chloride contamination, which is most prevalent in marine environments, is the single most damaging anion because of its property to migrate under coating film. The chloride containing solutions have a high osmotic pressure contributing to moisture penetration, loss of adhesion and blistering. A number of studies concerning with the level of chloride contamination on clean steel surfaces and premature failure of coatings applied over such surfaces have been reported in the literature [1-5]. Various correlations between the level of chloride and 1 This Article published in the Journal Materials Performance in February 2005, 2, pp

2 premature failure of coatings were also deduced. Unfortunately, chloride contamination on steel structures is not only a surface contaminant but also it resides in the rust and micropits and remains in significant amount even after blast cleaning. Neal and Whitehurst [6] studied the effect of chloride contamination of line pipe on the performance of fusion bonded epoxy (FBE) coating by contaminating pipes with various level of chloride that remained in the micropits after sand blasting of steel surface. It was found that chloride contamination could cause serious loss of performance in FBE coating in hot cathodic disbonding and hot water tests. For under ground coatings and other immersion coatings in critical applications a maximum level 2 µg/cm 2 of chloride contamination was suggested. Further, the chloride contamination that remained in micropits after blast cleaning can only be satisfactorily removed by phosphoric acid treatment. The present investigation concerns with the premature failure of a repainted epoxy coating applied over a pitted steel surface leading to its perforation. After about 14 years of service, as a result of large amount of seawater ingress in the tank, the tank was emptied, sand blasted and coating was reapplied without carrying out chloride decontamination. The other details of the tank are listed in Table 1. VISUAL EXAMINATION The photograph of the bottom plate sample in as received condition is shown in Figure 1(a and b). The visual examination of the plate shows two holes with marked cavities (average depth and diameter of the cavities: ~5.1 and 15.0 mm, respectively) besides some pits on the inner surface (working side) of the plate. Figure 2 shows closer view of the plate showing large perforation (hole) in the plate sample. ANALYSIS OF PLATE MATERIAL AND CORROSION PRODUCTS The chemical composition of the plate material as analyzed by Inductively Coupled Plasma (ICP) technique and C and S Analyzer is listed in Table 2. The chemical composition matches with the specified composition of the plate material ASTM A36, a carbon steel. The EDX studies of the corrosion products on the surface of the pit, beneath the epoxy lining, shows strong peaks of Cl, Si, Fe, O, Al, Mg, S, P, K, Ca and Ti (Figure 3). This suggests the presence of chloride contamination and other

3 impurities, which remained on the pitted surface in significant amount prior to the application of the coating. DISCUSSION Though crude oil itself may not be so corrosive the contact of crude oil with seawater or marine atmosphere can make it extremely corrosive. The cavities and minute pits already present on the inner surface of the plate are due to local corrosion as a result of long exposure of epoxy coating to the crude oil. The seawater in contact with crude oil slowly penetrated through the epoxy lining and initiated localized corrosion attack at the coating/metal interface and led to the breakdown of the coating. The breakdown of the epoxy coating is followed by the formation of an electrolytic cell. The anode of this cell is the minute exposed area of the metal and the cathode is the large coated area. The large potential difference characteristic of this passive-active cell accounted for considerable flow of current with rapid corrosion at the small anode. This resulted in the formation of small pits. The corrosion resistant epoxy lining surrounding the anode and the activating property of corrosion products within the pits accounted for the tendency of corrosion to penetrate the metal rather than spread along the surface. Further pit growth is controlled by rate of depolarization at the cathode area. Since the ph of the crude oil is around 5, the pit growth is expected to be controlled by the amount and availability of dissolved oxygen. The propagation of pit involves the dissolution of metal and maintenance of a high degree of acidity at the bottom of the pit by the hydrolysis of dissolved metal ions. The anodic metal dissolution reaction at the bottom of the pit is : Fe Fe 2+ +2e This reaction is balanced by the cathodic reaction on the adjacent surface ½ O 2 +H 2 O +2e 2OH - The increased concentration of Fe ++ within the pits resulted in the migration of chloride ions (already present as contaminant in the crude oil) to maintain neutrality. The iron chloride formed is hydrolyzed by water to give iron hydroxide and free acid FeCl 2 + 2H 2 O Fe (OH) 2 +2H + + 2Cl -

4 The generation of acid lowers the ph values at the bottom of the pit and accelerates the dissolution of metal whereas the ph of the bulk solution remains neutral. The EDX profile of the corrosion products at the surface of the pit beneath the epoxy lining shows strong peaks of Cl, Si, Fe, O, Al, Mg, S, P, K, Ca and Ti suggesting the presence of significant amount of chloride contamination and other impurities in the corrosion product, which remained in the pits prior to application of the coating. The chloride in contact with steel forms ferrous chloride, which in presence of air further oxidized to ferric chloride. Ferric chloride being hygroscopic salt drew moisture from humid air creating a concentrated ferric chloride solution on the steel surface. The epoxy coating applied over such a surface failed prematurely by iron salt drawing water through coating by osmosis and creating an area of poor adhesion and blistering. A fragile epoxy coating applied over the already existing cavities further contributed in the premature failure of the coating. After failure of the epoxy coating at the existing cavities, an ideal site for pitting corrosion was created. The corrosion proceeded with much faster rate and ultimately led to the perforation of remaining plate thickness in about two months. CONCLUSIONS 1. The cavities and the minute pits already present on the inner side of the bottom plate of the tank are due to local corrosion as a result of the failure of the epoxy coating on its long exposure to the crude oil. 2. The present failure of the coating and subsequent perforation of the plate is due to: (a) The presence of chloride contamination and other impurities, which remained in the existing cavities and pits in significant amount prior to the application of the coating. The chances of retention of chloride in the cavities are high in absence of a chemical cleaning operation after sand blasting. (b) The presence of a fragile coating applied over the existing cavities.

5 RECOMMENDATIONS The following preventive measures are recommended to overcome this type of failure. 1. To avoid premature failure of such coatings chloride decontamination should be considered as an essential step after sand blasting. The chloride decontamination may be achieved by phosphoric acid treatment when an insoluble iron phosphate is preferentially formed to iron chloride. 2. The surface pits should be repaired before applying the coating. REFERENCES 1. B.R. Appleman, JPCL, October, (1987): p V.E. Helvig, Metal Finishing, July, (1980): p D.G. Weldon, A. Bochan and M. Schlieden, JPCL, June, (1987): p S. Flores and T.L. Starr, JPCL, March, (1994): p B.P. Alblas and A.M. Van Londen, Protective Coating Europe, Feb., (1997): pp D. Neal and T. Whitehurst, MP, 34 (2), (1995): p. 47 Table 1. Information data about the MFOT Plant Equipment # MFOT # 3 12 PA 52B 103 Type Floating roof Capacity cubic meters Diameter 46.5 meters Height meters Fluid stored Crude oil Bottom & shell plate material ASTM A36 Bottom plate thickness 7.5 mm Annular plate thickness 12.0 mm Fluid temperature 35 to 40 o C Internal paint DFT 240 µ Type of cathodic protection Impressed current (external) ph of stored fluid ~5

6 Table 2. Chemical Composition of Sample Plate as Analyzed by ICP Technique and C and S Analyzer Element Weight % C 0.17 Mn 0.47 S Cu 0.12 Cr 0.05 Ni 0.03 Silica (soluble) <0.03 Fe 98 A B Figure 1. Photographs: (a) inner surface of plate sample (b) outer surface of plate sample - both in as received condition Figure 2. Photograph showing a closer view of the hole in the existing cavity

7 cps 15 Si 10 Fe O Cl 5 Al S P 0 C Mg Na K Ca Ti Ba Fe Au Energy (kev) Figure 3. EDX profile of corrosion products collected from the surface of the pits beneath the epoxy lining on the plate