Paper No.
09520
2009
EFFECT OF CALCAREOUS DEPOSIT FORMATION ON GALVANIC ANODE CATHODIC PROTECTION OF STEEL IN SEAWATER J.L. Solis Activo Cinco Presidentes, PEMEX-PEP Villahermosa, Tabasco, Mexico
[email protected] J. Genesca Departamento Ingenieria Metalurgica. Facultad Quimica. UNAM Ciudad Universitaria. 04510 Mexico D.F.
[email protected] ABSTRACT Durability of metallic structures immersed in natural seawater is often associated to the efficiency of protective systems which consist mainly of cathodic protection. The formation of calcareous deposit depends on the water composition, but specially, on the saturation level respect to CaCO3, and the temperature. The quality of the deposit is significantly influenced by the alkalinity next to the cathode surface, which is defined mainly by current density. The purpose of this study was to determine how calcareous deposits formation in seawater influences the performance of a galvanic cathodic protection system. Measurement of the coupling current between a cylindrical cathode (carbon steel) and a sacrificial Al anode ring was carried out as a function of time. A resistance set between the cathode and the anode was used to simulate the circuit resistance of the cathodic protection system. Additionally, steel cathode potential was registered as a function of immersion time. Calcareous deposits formed during testing were analyzed by X-Ray Diffraction. Keywords: aluminum anode, cathodic protection, monitoring, seawater INTRODUCTION One means of controlling corrosion is by the use of cathodic protection. The first application of cathodic protection and statement of the principles of the technique were made by Sir Humphrey Davy
Copyright ©2009 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE International, Copyright Division, 1440 South creek Drive, Houston, Texas 777084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.
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in 1824. Cathodic protection can only by applied when the metal is exposed to an electrolytically conducting environment. Since 1975, aluminum anode technology in the marine environment has emphasized the indium and mercury-activated alloy chemistry 1. During the last years the predominant alloy has been the generic indium-containing alloy. Activation of the aluminum surface is the process of transition from the naturally passive state to the active state by removal or weakening of the passivating film. Activation may be achieved by cathodic currents, by a reduced substance in the adjacent solution, or by an alloying element added at a low quantity having a suitable negative potential 1. Unalloyed aluminum adopts a relatively noble corrosion potential in saline media as a result of its protective oxide film. The oxide is the cause of polarization when aluminum is placed under anodic load in a cathodic polarization circuit. Numerous alloying combinations have been made to aluminum to reduce anodic polarization traits. With a few exceptions, the alloying approaches used to eliminate passivation, and hence promote surface activation, have been largely empirical. The most widely accepted aluminum anode alloys contain mercury or indium as elements responsible for surface activation. Indium is second to mercury in this application followed by tin, magnesium, cadmium and bismuth, with the latter elements not necessarily listed in their order of importance 1. Aluminum alloys suitable for cathodic protection have been developed in recent years, and the influence of alloying elements such as zinc (Zn), titanium (Ti), mercury (Hg), and indium (In) has been studied by several researchers 2,3. Many works refer to the addition of Hg, gallium (Ga), tin (Sn), and In to aluminum alloys. Each of these elements has been demonstrated to improve aluminum activation in neutral chloride media; however, the good results obtained in this field clash with the increased sensitivity to environmental protection. Particularly the use of Hg (which may be dangerous during the manufacture of the sacrificial anodes) pollutes sea-life, thus giving rise to great concern about the environment. The production of OH- ions via cathodic reactions increases the local pH and change the inorganic carbonic equilibrium in the electrolyte adjacent to the metallic surfaces. As a result of these and in conjunction with the presence of Ca2+ and Mg2+ ions in the solution, CaCO3 and Mg(OH)2 can precipitate on the metallic surfaces and form the so-called calcareous deposits. The formation of calcareous deposits influences the required protection current for long-term protection of metallic structures immersed in seawater. Deposition involves the nucleation and growth of minerals on a solid surface from the interfacial solution which is supersaturated with respect to certain compounds 4. The current density required to protect steel surfaces in seawater is greatly reduced by the formation of these calcareous deposit film. These films form because a cathodic current in seawater environment produces an increased pH resulting from the production of OH- ions. The deposit layer acts as a physical barrier which can polarize the interface as a result of oxygen diffusion limitation, thereby protecting the surface 5. The dominant cathodic half reaction is the oxygen reduction reaction, ORR O2 + 2 H2O + 4 e- → 4 OH-
(1)
This electrochemical reaction occurs, depending on the potential, by a two-step mechanism as follows 6: O2 + 2 H2O + 2 e- → H2O2 + 2OHH2O2 + 2 e- → 2 OH-
(2) (3)
2
Since pH of seawater is controlled by the carbonate system, the above reaction shifts the carbonate balance according to OH- + HCO3- → CO32- + H2O
(4)
allowing CaCO3 precipitation Ca2+ + CO32- → CaCO3↓
(5)
As can be seen the hydroxyl ions produced tend to be consumed by the precipitation of CaCO3. At more cathodic potentials, at about -1.0V (sce), hydrogen evolution reaction, HER, can occur on steel substrates. 2 H2O + 2 e - → H2 + 2 OH-
(6)
At higher pH values magnesium hydroxide also precipitates. The production of hydroxyl ions, OHby (2), (3) and (6) reactions allows the magnesium hydroxide precipitation, provided that the interfacial pH reaches the critical pH value of 9.3 6 Mg2+ + OH- → Mg(OH)2↓
(7)
The kinetics of this process is complicated. Calcium carbonate commonly exists in two crystal structures, calcite and aragonite. Chemical analysis of such scales has often shown that the calcareous deposits are basically composed of Ca rich phases such as aragonite and calcite (two allotropic forms of CaCO3) including magnesium species and brucite (Mg(OH2)) for high cathodic potentials 7-9. The formation of mineral scaling on engineering components can occur as a result of cathodic protection (CP). The consequences of such scale formation have been generally considered to be beneficial owing to the reduced current requirement and hence reduced costs of CP. This paper reports the result of a study focusing on the type of scale compound precipitated during CP, and its relation to applied potential and current levels. EXPERIMENTAL PROCEDURE Experiments were carried out with the electrochemical cell shown in Figure 1, based on Wang et al. and Festy and Le Flour 11 work, which consists of a cylindrical cathode (3 inch diameter). This design allows recording the coupling current between the cathode and a galvanic anode and the cathode and anode potential as a function of time. 10
The cathode was machined from a commercial pipeline carbon steel of 3 in. of diameter, which chemical composition is given in Table 1.
3
b c
2
2 cm
d
a
e
26 cm
2
2
FIGURE 1 - Electrochemical test cell. a: carbon steel tube of 3” diameter; b, d and e: Teflon as electric isolator; c: Al galvanic anode TABLE 1 CHEMICAL COMPOSITION OF THE CATHODE TEST MATERIAL (wt %) C
Mn
0.060
0.54
P
S
Si
Cu
Cr
0.018 0.008 0.013 0.020 0.020
Ni
Mo
V
0.030
0.006
0.004
Aluminum, Al, galvanic anode was machined from a commercial aluminum-zinc-indium, Al-Zn-In, anode to a rectangular cross-section ring geometry with outside diameter of 3 inch. Table 2 provides the chemical composition for this electrode. The potential of the Al anode is -1.10 V (Ag/AgCl) which was obtained by the method of sacrificial anode testing described in the NACE Standard TM0190-98 12. Al anode was threaded, Figure 2, to the steel cathode, as shown in Figure 3. A resistance set between the cathode and the anode simulates the natural circuit resistance of the cathodic protection system, structure plus electrolyte. This resistor provides a means for current measurements and to limit the magnitude of polarization for the cathode. The measurements have been performed in synthetic seawater prepared according ASTM D 1141 13 standard. A silver/silver chloride, Ag/AgCl/synthetic seawater was used as the reference electrode. The experimental set-up is shown in Figure 4. TABLE 2 CHEMICAL COMPOSITION OF AL-ALLOY AS DETERMINED BY EMISSION SPECTROSCOPY ANALYSIS (wt %)
Al
Zn
95.47
4.74
In
Cu
Mn
Mg
Si
Ti
Pb
