CONTRIBUTIONS to SCIENCE, 1 (3): 331-343 (2000) Institut d’Estudis Catalans, Barcelona
Development and testing of galvanic anodes for cathodic protection
Joan Genescà*1 and Julio Juárez2 1 Departamento de Ingeniería Metalúrgica. Facultad de Química. Universidad Nacional Autónoma de México (UNAM) 2 Instituto de Investigación en Materiales. Universidad Nacional Autónoma de México (UNAM)
Abstract Resum
This paper summarizes the results obtained in the research Aquest treball resumeix els resultats obtinguts en un pro- project carried out in our laboratory related to the testing of jecte de recerca dut a terme en el nostre laboratori i rela- sacrificial anodes used in cathodic protection (CP) systems cionat amb l’assaig electroquímic dels ànodes de sacrifici and the development of new In/Hg-free aluminium-alloy an- emprats en protecció catòdica, així com en el desenvolupa- odes. ment de nous ànodes d’alumini -Al- lliures d’indi -In- i de The main contributions are in the following subjects: mercuri -Hg. 1. Corrosion mechanism during electrochemical testing. Les aportacions més importants han estat fetes en els as- 1.1. Al. Electrochemical testing of Al sacrificial anodes. pectes següents: 1.2. Mg. Dissolution mechanism of Mg sacrificial an- 1. Mecanisme de corrosió durant els assaigs electro- odes under electrochemical testing. químics. 1.3. Zn. EIS testing of thermal spray Zn anodes for con- 1.1. Al. Assaig electroquímic d’ànodes de sacrifici de crete applications. Al. 2. Improving the efficiency of Mg sacrificial anodes by mi- 1.2. Mg. Mecanisme de dissolució dels ànodes de sa- crostructure control and heat treatments. crifici de Mg durant l’assaig electroquímic. 3. Design and development of new Al-alloy anodes In/Hg 1.3. Zn. L’impedància electroquímica com a eina d’as- free. saig dels ànodes de sacrifici de Zn aplicats per The evaluation of an Al-Zn-Mg-Li alloy as a potential can- projecció tèrmica sobre el formigó. didate for Al-sacrificial anode was studied. 2. Augment de l’eficiència electroquímica dels ànodes de The Al-Zn-Mg system was particularly selected due to the Mg mitjançant el tractament tèrmic i el control de la presence of precipitates in -Al matrix which are capable of seva microestructura. breaking down passive films whilst presenting good electro- 3. Disseny i desenvolupament de nous ànodes de Al chemical efficiencies. The effect of Li additions on superfi- sense In ni Hg. cial activation of the anode by means of precipitation of AlLi Aquests nous ànodes de Al-Zn-Mg-Li són proposats com type compounds was also examined. una alternativa als actualment en ús. El sistema Al-Mg-Zn ha estat escollit a causa de la presència de precipitats en la matriu de l’alumini, que poden ser capaços de trencar la pel·lícula passiva, però mantenen una alta eficiència elec- troquímica. Keywords: Aluminium, magnesium, zinc, sacrificial anodes, cathodic protection, electrochemical efficiency
Background tion and statement of the principles of the technique were made by Sir Humphrey Davy in 1824. Using small buttons of One means of controlling corrosion is by the employment of zinc, or iron nails, attached to the protective copper sheath- cathodic protection. The first application of cathodic protec- ing installed on the hulls of wooden warships, Davy was able to arrest «the rapid decay of the copper» [1]. Cathodic pro- tection can only by applied when the metal is exposed to an * Author for correspondence: Joan Genescà, Departamento de Ingeniería Metalúrgica, Facultad de Química, Universidad Nacional electrolytically conducting environment. Thus the applica- Autónoma de México (UNAM). México D.F., Mexico tion of the technique is virtually restricted to aqueous envi- 332 Joan Genescà and Julio Juárez
ronments and the control of aqueous corrosion. This is not Electrochemical properties, potential, capacity and corro- so great a restriction as may at first appear, since cathodic sion pattern are the most important references. The potential protection may be employed in moist soils and sands as well is a function of the alloy and the environment. The anode’s as in natural waters, brines and many aqueous process flu- electrochemical capacity represents a figure of the effec- ids. tiveness of the anode alloys. Finally, a third important factor Cathodic protection (CP) is surely the most often used to be assessed is the corrosion pattern and nature of the method to prevent corrosion. The traditional use of cathodic corrosion products. An alloy that under testing produces a protection has been to prevent corrosion of steel in the heavy and dense deposit is likely to be passivated, or to re- ground or water and this is still its most common application. sult in a rough-pitted surface during service. A good anode It is now almost universally adopted on ships, oil rigs, and oil material supports a light, obviously porous deposit with a and gas pipelines. Over the last 50 years cathodic protec- smooth clean surface underneath, which is likely to remain tion has advanced from being a black art to something ap- active throughout its service life. Of course, all the electro- proaching a science for these applications. Many excellent chemical properties also depend on the alloy’s composition. references are available which cover the theoretical and There are many ways of testing anodes for electrochemi- practical aspects of CP [2-8]. cal properties. However, the testing should provide informa- Corrosion is an electrochemical process of great eco- tion for: nomic importance estimated to consume 4% of the Gross • production control, National Product (GNP) of United States of America (USA) • anode alloy control, and [9-11[JG1]]. This percentage is likely to be of the same order • anode field-testing. globally. Among the test methods, galvanostatic testing is one of In all low-temperature corrosion reactions, for the reac- the most useful. Galvanostatic testing is common in corro- tions to occur an electrochemical cell is needed. This cell sion research and is carried out by imposing a constant cur- comprises an anode and cathode separated by an elec- rent on a test specimen and reading the resulting potential. trolyte with a metallic conduction. When a metal such as In anode testing this method may also be used to determine steel is used as an electrolyte, a corrosion cell can be the anode’s current capacity. One special galvanostatic test formed. If steel is physically attached (i.e. welded, bolted or is the hydrogen evolution test. Small impurities, which may cast) to a piece of magnesium (Mg) and both are placed in not be detected by chemical analysis alone, can cause local an electrolyte, the Mg will form all the anode and the steel cell action, which leads to lower anode efficiency. Hydrogen will only form the cathode. The resulting corrosion reactions evolution is the primary cathodic reaction taking place at will occur to the Mg which will be consumed. A balancing re- these local cells. Thus, its measurement affords an indirect duction reaction will occur to the steel, which will remain un- determination of the loss of efficiency of the anode. NACE affected by its immersion in the electrolyte. This is the basis TM0190-98 is based in one of these tests [13]. of cathodic protection (CP) [8]. When you cathodically pro- The electrochemical behaviour of sacrificial anode mate- tect steel or any other metal you alter its exclusive action as rials is of vital importance for the reliability and efficiency of a cathode by the imposition of an external anode (sacrificial cathodic protection systems for seawaterexposed struc- or galvanic anode), which will corrode preferentially. tures. From a practical point of view it is necessary to docu- One form of CP is termed «galvanic» (GCP) or «sacrifi- ment service performance and quality level during current cial» anode cathodic protection. With this system, electric production throughout laboratory testing. For quality assur- current is applied by the employment of dissimilar metals, ance (QA) purposes various methods are applied. These with the driving voltage being created by the potential gen- different electrochemical techniques giving either potentio- erated between the steel and the anode in the electrolyte. static or galvanostatic control of test specimens or sponta- The galvanic anodes are alloys of Mg, Zn and Al [1,8], which neous galvanic coupling between the anode specimen and can be applied to protect steel in aqueous and soil environ- a defined cathode («free-running test»). ments. Cathodic protection design reliability depends to a large extent on input parameters used in design calculations. Some of the most important input parameters in these calcu- 1. Corrosion mechanism during electrochemical lations are the electrochemical properties of sacrificial an- testing odes. The electrochemical efficiency of the anode material is used for weight (lifetime) calculations and the anode poten- Introduction tial (driving voltage) is used to calculate anode current out- Testing of sacrificial anodes search for the following para- put. meters [12]: 1. Closedcircuit electrochemical potential. 1.1 Aluminium Anodes. Electrochemical testing of Al 2. Longterm current output characteristics. sacrificial anodes 3. Current capacity per kilogram. The testing of indium-activated, aluminium alloy sacrificial 4. Corrosion characteristics. anode samples, using the procedures of Det Norske Veritas, 5. Anode structure and soundness. DNV, RP B401, Appendix A, [14] is mainly intended to serve Development and testing of galvanic anodes for cathodic protection 333
as a QA/QC indicator regarding the electrochemical faradic picked up in the short-term procedure test. Normally, inho- efficiency and anode potential under cathodic protection mogenities and variations of anode alloy metallurgy and mi- conditions. crostructure are reflected by a large scatter in the efficien- The recommended test procedure for quality control pur- cies recorded between the multiple specimens exposed. poses [14, 15eurocorr99] is carried out galvanostatically EIS has proved an interesting tool to reveal corrosion prod- with four subsequent current density levels each lasting 24 uct formation on the surface of the anode, affecting electro- hours. Thus, total test duration is 96 hours. chemical efficiency significantly. Nyquist diagrams such as In order to provide information about the behaviour of the one shown in Figure 1, [15] is representative of this be- these anodes under DNV experimental conditions , samples haviour. were held galvanostatically at different current density lev- An active closed-circuit potential is desirable because a els, obtaining the impedance diagrams at the beginning and relatively noble potential could indicate the presence of pas- the end of each period [15]. sivation. Anodes must also possess high faradic efficiency Several aluminium-based alloys (generally Al-Zn-In type) in order to avoid frequent anode replacement. The NACE have been tested [15,16]. Results show that the electro- [13] and DNV [14] tests specifies that an Al anode should chemical performance of an Al sacrificial anode alloy is have a closed-circuit potential active to -1.0 V (SCE) and an strongly dependent upon the formation of corrosion products electrochemical efficiency, , between 2300 and 2700 A- on the surface. An anode in service will typically be covered h/Kg [15]. Figure 3 indicates that corrosion products’ resis- with corrosion products and this influence on the electro- tance, Rcp, decreases, as current density increases . This is chemical behaviour should be reflected in laboratory tests probably due to the fact that heavier and denser corrosion for performance documentation. The effect of chemical com- products form at higher current density, which will promote position on electrochemical capacity has also been studied the self-corrosion activity. This would seem to prove the ef- [15,16]. Higher Si, Cu and Fe content seem to be the main fect of corrosion product formation of on the self-corrosion reason the capacity was lower than expected. The recom- rate of the anode. The self-corrosion rate will account for a mended maximum impurity levels given in RP B401 (Fe max. relatively higher part of the total mass loss at lower anodic 0,1 wt.% and Si max 0,15wt%)[14] are within the range where current densities. On partly consumed anodes where corro- no detrimental effects of these elements can be recorded. sion products have settled, the maintenance and develop- Inhomogenities in the microstructure have been found to ment of the corrosion products will be more efficient at high- be of importance when indications of abnormal behaviour er current densities than at lower. Consequently, the are indicated through short-term testing. Tendency to passi- self-corrosion rate due to a more corrosive environment be- vation can be attributed to a strong depletion of Zn in the neath the corrosion product will be correspondingly higher matrix and a corresponding enrichment at the grain bound- at high anodic current density. aries. This type of effect, which is probably related to quality It is evident from the electrochemical impedance data control aspects of the production process, is very efficiently that the current density condition plays a critical role in the resistive property of the corrosion products, suggesting a significant change in the corrosion product property formed on the Al anode. -15 Z’’, ohm-cmˆ2 experimental simulated The lower electrochemical activity exhibited by the Al an- ode samples strongly suggests that local indium depletion may be the fundamental cause of this behaviour as previ- ously proposed by Attanasio et al. [17]. The local In deple- -10
160 140 -5 120 100 80 0 60 40 20
5 0 20 25 30 35 40 0 1 2 3 4 5 Z’, ohm-cmˆ2 Current density, mA/cmˆ2
Figure 1. Nyquist plot for indium-activated aluminium alloy anode in Figure 2. Electric resistance of corrosion products formed on Al an- ASTM seawater at 4 mA/cm2. ode in ASTM seawater at different current density values. 334 Joan Genescà and Julio Juárez
CDC: (RQ)R(RQ) CDC: (RQ)R(RQ) 45 Measurement Measurement Measurement Simulation 100 K Simulation Simulation 3.00
) 30 ) ( 3 2.00 10 K IZI ( x10 im 15 -Z
1.00 (alpha in degrees) 1 K
0.00 0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 1 10 100 1 k 10 k 3 Frequency (Hz) Zr x10 ( )
(a) (b)
Figure 3. Typical impedance diagram: (a) Nyquist and (b) Bode representation of magnesium galvanic anode recorded during ASTM test [25]. tion could be the reason that Al anodes underwent pitting Laboratory evaluations were conducted on Al anodes that corrosion and did not achieve closed-circuit potentials more could be used for sacrificial cathodic protection of rein- active than -0.950 V (SCE). In our experiments, the EIS dia- forced concrete 16 . Anodes were fabricated into test grams obtained at higher current densities show an induc- coupons and coupled to small lengths of reinforcing steel. tive semicircle [15], which could be attributed to pitting cor- The Al-steel couples were placed in simulated concrete en- rosion. The preferred dissolution morphology is general vironments, which consisted of sealed containers filled with attack rather than pitting, since pitting attack has been cor- silica sand treated with a chloride solution. The treated related to less than optimal performance [18,19]. The exis- sands were dried to obtain resistivities of 1600, 4190 and tence of pitting attack is also consistent with the notion that 7500 -cm. The results obtained showed that Al could be an In depletion is responsible for the observed behaviour, ac- alternative for employment as a galvanic anode in reinforced cording to Uruchurtu [20] and Reboul et al. [21]. concrete in a wide spectrum of resistivity. The anodic polarisation behaviour showed evidence of both passivation and pitting. The polarisation test is consis- 1.2 Magnesium anodes. Dissolution mechanism of Mg tent with surface dissolution morphology, as the measured sacrificial anodes under electrochemical testing [16,25,30]. closed-circuit potential values were found to be located The electrochemical behaviour of magnesium galvanic an- above the pitting or breakdown potential. The polarisation odes under ASTM Test Method G 97-89 [25,26] conditions test was performed at the end of the short-term electrochem- was investigated by measuring electrochemical imped- ical test in order to determine anode behaviour type. ance. Mg rods machined from a sample of commercial an- One argument in the past against using the short-term ode are used as anodes (working electrode), while a piece electrochemical test was that in the two-week [13] or four- of steel pipe, which served also as the electrochemical cell day [14] test, only the outer 260 m is consumed, whereas (cathode test pot) are used as cathodes (counterelec- in the one-year test at least 0.5 cm penetration is realised. trodes). Murray et al. [22] have obtained successful results showing Samples were held galvanostatically at 0,039 mA/cm2 un- that by evaluating both the ascast material simultaneously til electrochemical impedance spectroscopy (EIS) testing with cut surfaces (which are representative of the bulk mate- was completed. All samples were held potentiostatically rial) the short-term test appears to be quite representative of during the test. Closed-circuit test specimen potential was the cast anode long-term performance. obtained daily, before and after EIS testing. The primary areas of application for galvanic anodes to EIS measurements were used to monitor the corrosion date have been in protection of exposed steel in aqueous process on the magnesium daily for the duration of the 14- environments, such as seawater, and for protection in damp, dayf test. The magnesium anode potential was measured low resistivity soils during recent years. However, an in- daily, and was seen to undergo a change from -1,610 mV creasing interest has been dedicated to concrete. (sce) to -1,680 mV (sce). Figure 3 gives the characteristic Corrosion of reinforced concrete can be arrested by em- impedance spectra obtained during immersion of the mag- ploying cathodic protection. The California Department of nesium anode in the solution (for 14 days at 20oC, always Transportation pioneered the use of impressed-current ca- showing two loops). To interpret impedance results, a model thodic protection (ICCP) in the 1970 [23]. Sacrificial anode was established [25] that included the electrolyte resis- cathodic protection systems have been employed as part of tance, the parts of the impedance corresponding to the cor- a cathodic protection system applied to pilings in Lake rosion product film (hydroxide layer) formed, and the imped- Maracaibo, Venezuela 24]. ance of the interface. The model was proposed according to Development and testing of galvanic anodes for cathodic protection 335
the reported behaviour of magnesium ([27,28,29] and was 1.3 Zinc Anodes. EIS testing of thermal spray Zn anodes for validated by comparing its results to the real condition of the concrete applications [31-33 ] magnesium anode. The use of sacrificial zinc anodes for cathodic protection According to this model, the electrochemical impedance (CP) of steel in reinforced concrete is a subject of increasing of magnesium anode was given by: interest, especially regarding potential applications on high- way bridges seriously damaged by chloride contamination. 11 One anode material gaining acceptance is thermally = 1 1 Z–Rs sprayed zinc (TSCP). 11+ 11 The feasibility of using EIS, as a monitoring tool to deter- CRC+ + R dl 1 pol po mine whether CP level is sufficient to mitigate corrosion was investigated for carbon steel in concrete [31,32]. As a first The spectra analysis showed the capacitance of a corro- approach, this investigation deals with the behaviour of the sion porous layer (Cpo) in the high-frequency (HF) region galvanic couple steel/zinc under CP conditions in NaCl solu- and double-layer capacitance (Cdl) in the low-frequency (LF) tion. The EIS diagrams obtained were compared with those region. The resistive component of the corrosion porous lay- corresponding to laboratory concrete samples, whose sur- er (Rpo) in the frequency range between 100 Hz and 1000 face was thermal sprayed with Zn. [33]
Hz, and the resistance of charge-transfer reaction (Rt) was in The California Department of Transportation pioneered the LF region. the use of thermal-sprayed Zn anodes for existing structures To interpret the impedance diagrams, equivalent circuit damaged by corrosion in 1982 [34]. This tecnology has models were used. The behaviour of the magnesium-elec- since been employed in a number of large projects such as trolyte interface could be described by the proposed model bridge structures. In Mexico this tecnology is known, but has with the electric equivalent circuit that corresponds to the not yet been applied. porous film model or two-layer model [25], which was used In recent years, the cost of rehabilitating and protecting to successfully reproduce all the impedance spectra ob- bridges with CP has fallen by as much as one half. Actually, tained. According to this model, the impedance diagram there are more than 500 bridges in North America with in- might bring into view two semicircles. The HF semicircle is stalled CP systems [35]. Of the more than 575,000 bridges usually attributed to the porous film, whereas the lower fre- in the National Bridge Survey, it is estimated that 44% are in quency semicircle corresponds to the charge transfer reac- need of repair [36]. tions or faradic process. With the object of contributing to a better knowledge of
Cpo decreased with time whereas both Rpo and Rt in- the factors afecting these systems’ efficiency and yield, lab- creased with time [25]. This increase in protection with im- oratory assays have been carried out to gather information mersion time provided evidence for the existence of a pro- on the behavior and mechanism of Zn/concrete interphase tective layer over the magnesium surface, one of Mg(OH)2. and especially of the galvanic couple Fe/Zn, which is re-
Cpo of the layer could be described by: sponsable for galvanic cathodic protection. Potentials for the Zn anode were measured with respect C 1 to the steel cathode. Figure 4 shows results of the potential = ee0 Ad of a steel cathode as a function of immersion time. Through- out the experiment, the steel cathode remained at potential Where is the dielectric constant, 0 is the permitivity in values more negative in value than -800 mV (SCE), maintain- free space (8.854 x 10-12 F/m). C/A is the capacitance per ing the steel protected. unit area, and d is the thickness. Thus, as the thickness of the Mg(OH)2 increases, C is expected to decrease for the C value. po 0 Since the resistance of a film is proportional to its resistivi- -100 ty and thickness, an increase in Rpo was interpreted to mean -200 that passivation of the Mg(OH)2 layer increased with immer- -300 sion time. -400 Analysis of the corrosion products formed on the magne- -500 sium anode and precipitated in the solution was carried out -600 -700 and the presence of Mg(OH) seemed clear for the corro- 2 -800 Potential, mV vs sce Potential, sion products on the magnesium anode. A visual and micro- -900 scopic inspection of the magnesium electrode did not show -100 any localised corrosion after the test. -1100 In the results obtained, the magnesium anode did not cor- 0 2 4 6 8 10121416182022242628303234363840424446 Time, Days rode locally and charge transfer occurred uniformly under- neath the oxide. Figure 4. Potential of cathode (steel) as a function of time during ca- thodic protection of reinforced concrete by thermal spray Zn-anode. 336 Joan Genescà and Julio Juárez
-1.000 103 Fe y Zn Relacion (1:1) Fe y Zn Relacion (1:1)