Background

One means of controlling corrosion is by the employment ofcathodic protection. The first application of cathodic protec-

tion and statement of the principles of the technique weremade by Sir Humphrey Davy in 1824. Using small buttons ofzinc, or iron nails, attached to the protective copper sheath-ing installed on the hulls of wooden warships, Davy was ableto arrest «the rapid decay of the copper» [1]. Cathodic pro-tection can only by applied when the metal is exposed to anelectrolytically conducting environment. Thus the applica-tion of the technique is virtually restricted to aqueous envi-

CONTRIBUTIONS to SCIENCE, 1 (3): 331-343 (2000)Institut d’Estudis Catalans, Barcelona

Development and testing of galvanic anodes for cathodicprotection

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

This paper summarizes the results obtained in the researchproject carried out in our laboratory related to the testing ofsacrificial anodes used in cathodic protection (CP) systemsand the development of new In/Hg-free aluminium-alloy an-odes.

The main contributions are in the following subjects:1. Corrosion mechanism during electrochemical testing.

1.1. Al. Electrochemical testing of Al sacrificial anodes.1.2. Mg. Dissolution mechanism of Mg sacrificial an-

odes under electrochemical testing.1.3. Zn. EIS testing of thermal spray Zn anodes for con-

crete applications.2. Improving the efficiency of Mg sacrificial anodes by mi-

crostructure control and heat treatments.3. Design and development of new Al-alloy anodes In/Hg

free.The evaluation of an Al-Zn-Mg-Li alloy as a potential can-

didate for Al-sacrificial anode was studied.The Al-Zn-Mg system was particularly selected due to the

presence of precipitates in �-Al matrix which are capable ofbreaking down passive films whilst presenting good electro-chemical efficiencies. The effect of Li additions on superfi-cial activation of the anode by means of precipitation of AlLitype compounds was also examined.

Resum

Aquest treball resumeix els resultats obtinguts en un pro-jecte de recerca dut a terme en el nostre laboratori i rela-cionat amb l’assaig electroquímic dels ànodes de sacrificiemprats en protecció catòdica, així com en el desenvolupa-ment de nous ànodes d’alumini -Al- lliures d’indi -In- i demercuri -Hg.

Les aportacions més importants han estat fetes en els as-pectes següents:

1. Mecanisme de corrosió durant els assaigs electro-químics.1.1. Al. Assaig electroquímic d’ànodes de sacrifici de

Al.1.2. Mg. Mecanisme de dissolució dels ànodes de sa-

crifici de Mg durant l’assaig electroquímic.1.3. Zn. L’impedància electroquímica com a eina d’as-

saig dels ànodes de sacrifici de Zn aplicats perprojecció tèrmica sobre el formigó.

2. Augment de l’eficiència electroquímica dels ànodes deMg mitjançant el tractament tèrmic i el control de laseva microestructura.

3. Disseny i desenvolupament de nous ànodes de Alsense In ni Hg.

Aquests nous ànodes de Al-Zn-Mg-Li són proposats comuna alternativa als actualment en ús. El sistema Al-Mg-Zn haestat escollit a causa de la presència de precipitats en lamatriu de l’alumini, que poden ser capaços de trencar lapel·lícula passiva, però mantenen una alta eficiència elec-troquímica.

Keywords: Aluminium, magnesium, zinc,sacrificial anodes, cathodic protection,electrochemical efficiency

* Author for correspondence: Joan Genescà, Departamento deIngeniería Metalúrgica, Facultad de Química, Universidad NacionalAutónoma de México (UNAM). México D.F., Mexico

ronments and the control of aqueous corrosion. This is notso great a restriction as may at first appear, since cathodicprotection may be employed in moist soils and sands as wellas in natural waters, brines and many aqueous process flu-ids.

Cathodic protection (CP) is surely the most often usedmethod to prevent corrosion. The traditional use of cathodicprotection has been to prevent corrosion of steel in theground or water and this is still its most common application.It is now almost universally adopted on ships, oil rigs, and oiland gas pipelines. Over the last 50 years cathodic protec-tion has advanced from being a black art to something ap-proaching a science for these applications. Many excellentreferences are available which cover the theoretical andpractical aspects of CP [2-8].

Corrosion is an electrochemical process of great eco-nomic importance estimated to consume 4% of the GrossNational Product (GNP) of United States of America (USA)[9-11[JG1]]. This percentage is likely to be of the same orderglobally.

In all low-temperature corrosion reactions, for the reac-tions to occur an electrochemical cell is needed. This cellcomprises an anode and cathode separated by an elec-trolyte with a metallic conduction. When a metal such assteel is used as an electrolyte, a corrosion cell can beformed. If steel is physically attached (i.e. welded, bolted orcast) to a piece of magnesium (Mg) and both are placed inan electrolyte, the Mg will form all the anode and the steelwill only form the cathode. The resulting corrosion reactionswill occur to the Mg which will be consumed. A balancing re-duction reaction will occur to the steel, which will remain un-affected by its immersion in the electrolyte. This is the basisof cathodic protection (CP) [8]. When you cathodically pro-tect steel or any other metal you alter its exclusive action asa cathode by the imposition of an external anode (sacrificialor galvanic anode), which will corrode preferentially.

One form of CP is termed «galvanic» (GCP) or «sacrifi-cial» anode cathodic protection. With this system, electriccurrent is applied by the employment of dissimilar metals,with the driving voltage being created by the potential gen-erated between the steel and the anode in the electrolyte.The galvanic anodes are alloys of Mg, Zn and Al [1,8], whichcan be applied to protect steel in aqueous and soil environ-ments.

1. Corrosion mechanism during electrochemicaltesting

IntroductionTesting of sacrificial anodes search for the following para-meters [12]:

1. Closedcircuit electrochemical potential.2. Longterm current output characteristics.3. Current capacity per kilogram.4. Corrosion characteristics.5. Anode structure and soundness.

Electrochemical properties, potential, capacity and corro-sion pattern are the most important references. The potentialis a function of the alloy and the environment. The anode’selectrochemical capacity represents a figure of the effec-tiveness of the anode alloys. Finally, a third important factorto be assessed is the corrosion pattern and nature of thecorrosion products. An alloy that under testing produces aheavy and dense deposit is likely to be passivated, or to re-sult in a rough-pitted surface during service. A good anodematerial supports a light, obviously porous deposit with asmooth clean surface underneath, which is likely to remainactive throughout its service life. Of course, all the electro-chemical properties also depend on the alloy’s composition.

There are many ways of testing anodes for electrochemi-cal properties. However, the testing should provide informa-tion for:

• production control,• anode alloy control, and• anode field-testing.Among the test methods, galvanostatic testing is one of

the most useful. Galvanostatic testing is common in corro-sion research and is carried out by imposing a constant cur-rent on a test specimen and reading the resulting potential.In anode testing this method may also be used to determinethe anode’s current capacity. One special galvanostatic testis the hydrogen evolution test. Small impurities, which maynot be detected by chemical analysis alone, can cause localcell action, which leads to lower anode efficiency. Hydrogenevolution is the primary cathodic reaction taking place atthese local cells. Thus, its measurement affords an indirectdetermination of the loss of efficiency of the anode. NACETM0190-98 is based in one of these tests [13].

The electrochemical behaviour of sacrificial anode mate-rials is of vital importance for the reliability and efficiency ofcathodic protection systems for seawaterexposed struc-tures. From a practical point of view it is necessary to docu-ment service performance and quality level during currentproduction throughout laboratory testing. For quality assur-ance (QA) purposes various methods are applied. Thesedifferent electrochemical techniques giving either potentio-static or galvanostatic control of test specimens or sponta-neous galvanic coupling between the anode specimen anda defined cathode («free-running test»).

Cathodic protection design reliability depends to a largeextent on input parameters used in design calculations.Some of the most important input parameters in these calcu-lations are the electrochemical properties of sacrificial an-odes. The electrochemical efficiency of the anode material isused for weight (lifetime) calculations and the anode poten-tial (driving voltage) is used to calculate anode current out-put.

1.1 Aluminium Anodes. Electrochemical testing of Alsacrificial anodesThe testing of indium-activated, aluminium alloy sacrificialanode samples, using the procedures of Det Norske Veritas,DNV, RP B401, Appendix A, [14] is mainly intended to serve

332 Joan Genescà and Julio Juárez

as a QA/QC indicator regarding the electrochemical faradicefficiency and anode potential under cathodic protectionconditions.

The recommended test procedure for quality control pur-poses [14, 15eurocorr99] is carried out galvanostaticallywith four subsequent current density levels each lasting 24hours. Thus, total test duration is 96 hours.

In order to provide information about the behaviour ofthese anodes under DNV experimental conditions , sampleswere held galvanostatically at different current density lev-els, obtaining the impedance diagrams at the beginning andthe end of each period [15].

Several aluminium-based alloys (generally Al-Zn-In type)have been tested [15,16]. Results show that the electro-chemical performance of an Al sacrificial anode alloy isstrongly dependent upon the formation of corrosion productson the surface. An anode in service will typically be coveredwith corrosion products and this influence on the electro-chemical behaviour should be reflected in laboratory testsfor performance documentation. The effect of chemical com-position on electrochemical capacity has also been studied[15,16]. Higher Si, Cu and Fe content seem to be the mainreason the capacity was lower than expected. The recom-mended maximum impurity levels given in RP B401 (Fe max.0,1 wt.% and Si max 0,15wt%)[14] are within the range whereno detrimental effects of these elements can be recorded.

Inhomogenities in the microstructure have been found tobe of importance when indications of abnormal behaviourare indicated through short-term testing. Tendency to passi-vation can be attributed to a strong depletion of Zn in thematrix and a corresponding enrichment at the grain bound-aries. This type of effect, which is probably related to qualitycontrol aspects of the production process, is very efficiently

picked up in the short-term procedure test. Normally, inho-mogenities and variations of anode alloy metallurgy and mi-crostructure are reflected by a large scatter in the efficien-cies recorded between the multiple specimens exposed.EIS has proved an interesting tool to reveal corrosion prod-uct formation on the surface of the anode, affecting electro-chemical efficiency significantly. Nyquist diagrams such asthe one shown in Figure 1, [15] is representative of this be-haviour.

An active closed-circuit potential is desirable because arelatively noble potential could indicate the presence of pas-sivation. Anodes must also possess high faradic efficiencyin order to avoid frequent anode replacement. The NACE[13] and DNV [14] tests specifies that an Al anode shouldhave a closed-circuit potential active to -1.0 V (SCE) and anelectrochemical efficiency, �, between 2300 and 2700 A-h/Kg [15]. Figure 3 indicates that corrosion products’ resis-tance, Rcp, decreases, as current density increases . This isprobably due to the fact that heavier and denser corrosionproducts form at higher current density, which will promotethe self-corrosion activity. This would seem to prove the ef-fect of corrosion product formation of on the self-corrosionrate of the anode. The self-corrosion rate will account for arelatively higher part of the total mass loss at lower anodiccurrent densities. On partly consumed anodes where corro-sion products have settled, the maintenance and develop-ment of the corrosion products will be more efficient at high-er current densities than at lower. Consequently, theself-corrosion rate due to a more corrosive environment be-neath the corrosion product will be correspondingly higherat high anodic current density.

It is evident from the electrochemical impedance datathat the current density condition plays a critical role in theresistive property of the corrosion products, suggesting asignificant change in the corrosion product property formedon the Al anode.

The lower electrochemical activity exhibited by the Al an-ode samples strongly suggests that local indium depletionmay be the fundamental cause of this behaviour as previ-ously proposed by Attanasio et al. [17]. The local In deple-

Development and testing of galvanic anodes for cathodic protection 333

20 25 30 35 40

-15

-10

-5

0

5

Z’’, ohm-cmˆ2 experimentalsimulated

Z’, ohm-cmˆ2

Figure 1. Nyquist plot for indium-activated aluminium alloy anode inASTM seawater at 4 mA/cm2.

0 1 2 3 4 5

160

140

120

100

80

60

40

20

0

Current density, mA/cmˆ2

Figure 2. Electric resistance of corrosion products formed on Al an-ode in ASTM seawater at different current density values.

tion could be the reason that Al anodes underwent pittingcorrosion and did not achieve closed-circuit potentials moreactive than -0.950 V (SCE). In our experiments, the EIS dia-grams obtained at higher current densities show an induc-tive semicircle [15], which could be attributed to pitting cor-rosion. The preferred dissolution morphology is generalattack rather than pitting, since pitting attack has been cor-related to less than optimal performance [18,19]. The exis-tence of pitting attack is also consistent with the notion thatIn depletion is responsible for the observed behaviour, ac-cording to Uruchurtu [20] and Reboul et al. [21].

The anodic polarisation behaviour showed evidence ofboth passivation and pitting. The polarisation test is consis-tent with surface dissolution morphology, as the measuredclosed-circuit potential values were found to be locatedabove the pitting or breakdown potential. The polarisationtest was performed at the end of the short-term electrochem-ical test in order to determine anode behaviour type.

One argument in the past against using the short-termelectrochemical test was that in the two-week [13] or four-day [14] test, only the outer 260 �m is consumed, whereasin the one-year test at least 0.5 cm penetration is realised.Murray et al. [22] have obtained successful results showingthat by evaluating both the ascast material simultaneouslywith cut surfaces (which are representative of the bulk mate-rial) the short-term test appears to be quite representative ofthe cast anode long-term performance.

The primary areas of application for galvanic anodes todate have been in protection of exposed steel in aqueousenvironments, such as seawater, and for protection in damp,low resistivity soils during recent years. However, an in-creasing interest has been dedicated to concrete.

Corrosion of reinforced concrete can be arrested by em-ploying cathodic protection. The California Department ofTransportation pioneered the use of impressed-current ca-thodic protection (ICCP) in the 1970 [23]. Sacrificial anodecathodic protection systems have been employed as part ofa cathodic protection system applied to pilings in LakeMaracaibo, Venezuela �24].

Laboratory evaluations were conducted on Al anodes thatcould be used for sacrificial cathodic protection of rein-forced concrete�16�. Anodes were fabricated into testcoupons and coupled to small lengths of reinforcing steel.The Al-steel couples were placed in simulated concrete en-vironments, which consisted of sealed containers filled withsilica sand treated with a chloride solution. The treatedsands were dried to obtain resistivities of 1600, 4190 and7500 �-cm. The results obtained showed that Al could be analternative for employment as a galvanic anode in reinforcedconcrete in a wide spectrum of resistivity.

1.2 Magnesium anodes. Dissolution mechanism of Mgsacrificial anodes under electrochemical testing [16,25,30].The electrochemical behaviour of magnesium galvanic an-odes under ASTM Test Method G 97-89 [25,26] conditionswas investigated by measuring electrochemical imped-ance. Mg rods machined from a sample of commercial an-ode are used as anodes (working electrode), while a pieceof steel pipe, which served also as the electrochemical cell(cathode test pot) are used as cathodes (counterelec-trodes).

Samples were held galvanostatically at 0,039 mA/cm2 un-til electrochemical impedance spectroscopy (EIS) testingwas completed. All samples were held potentiostaticallyduring the test. Closed-circuit test specimen potential wasobtained daily, before and after EIS testing.

EIS measurements were used to monitor the corrosionprocess on the magnesium daily for the duration of the 14-dayf test. The magnesium anode potential was measureddaily, and was seen to undergo a change from -1,610 mV(sce) to -1,680 mV (sce). Figure 3 gives the characteristicimpedance spectra obtained during immersion of the mag-nesium anode in the solution (for 14 days at 20oC, alwaysshowing two loops). To interpret impedance results, a modelwas established [25] that included the electrolyte resis-tance, the parts of the impedance corresponding to the cor-rosion product film (hydroxide layer) formed, and the imped-ance of the interface. The model was proposed according to

334 Joan Genescà and Julio Juárez

CDC: (RQ)R(RQ)

MeasurementSimulation

3.00

2.00

1.00

0.00

-Zim

x10

3 (�

)

Zr x103 (�)

(a)

0.00 1.00 2.00 3.00 4.00 5.00 6.00

CDC: (RQ)R(RQ)

MeasurementSimulation100 K

10 K

1 K

IZI (

�)

Frequency (Hz)

(b)

1 10 100 1 k 10 k

MeasurementSimulation

45

30

15

0

(alp

ha in

deg

rees

)

Figure 3. Typical impedance diagram: (a) Nyquist and (b) Bode representation of magnesium galvanic anode recorded during ASTM test [25].

the reported behaviour of magnesium ([27,28,29] and wasvalidated by comparing its results to the real condition of themagnesium anode.

According to this model, the electrochemical impedanceof magnesium anode was given by:

The spectra analysis showed the capacitance of a corro-sion porous layer (Cpo) in the high-frequency (HF) regionand double-layer capacitance (Cdl) in the low-frequency (LF)region. The resistive component of the corrosion porous lay-er (Rpo) in the frequency range between 100 Hz and 1000Hz, and the resistance of charge-transfer reaction (Rt) was inthe LF region.

To interpret the impedance diagrams, equivalent circuitmodels were used. The behaviour of the magnesium-elec-trolyte interface could be described by the proposed modelwith the electric equivalent circuit that corresponds to theporous film model or two-layer model [25], which was usedto successfully reproduce all the impedance spectra ob-tained. According to this model, the impedance diagrammight bring into view two semicircles. The HF semicircle isusually attributed to the porous film, whereas the lower fre-quency semicircle corresponds to the charge transfer reac-tions or faradic process.

Cpo decreased with time whereas both Rpo and Rt in-creased with time [25]. This increase in protection with im-mersion time provided evidence for the existence of a pro-tective layer over the magnesium surface, one of Mg(OH)2.

Cpo of the layer could be described by:

Where � is the dielectric constant, �0 is the permitivity infree space (8.854 x 10-12 F/m). C/A is the capacitance perunit area, and d is the thickness. Thus, as the thickness ofthe Mg(OH)2 increases, C is expected to decrease for theCpo value.

Since the resistance of a film is proportional to its resistivi-ty and thickness, an increase in Rpo was interpreted to meanthat passivation of the Mg(OH)2 layer increased with immer-sion time.

Analysis of the corrosion products formed on the magne-sium anode and precipitated in the solution was carried outand the presence of Mg(OH)2 seemed clear for the corro-sion products on the magnesium anode. A visual and micro-scopic inspection of the magnesium electrode did not showany localised corrosion after the test.

In the results obtained, the magnesium anode did not cor-rode locally and charge transfer occurred uniformly under-neath the oxide.

1.3 Zinc Anodes. EIS testing of thermal spray Zn anodes forconcrete applications [31-33 ]

The use of sacrificial zinc anodes for cathodic protection(CP) of steel in reinforced concrete is a subject of increasinginterest, especially regarding potential applications on high-way bridges seriously damaged by chloride contamination.One anode material gaining acceptance is thermallysprayed zinc (TSCP).

The feasibility of using EIS, as a monitoring tool to deter-mine whether CP level is sufficient to mitigate corrosion wasinvestigated for carbon steel in concrete [31,32]. As a firstapproach, this investigation deals with the behaviour of thegalvanic couple steel/zinc under CP conditions in NaCl solu-tion. The EIS diagrams obtained were compared with thosecorresponding to laboratory concrete samples, whose sur-face was thermal sprayed with Zn. [33]

The California Department of Transportation pioneeredthe use of thermal-sprayed Zn anodes for existing structuresdamaged by corrosion in 1982 [34]. This tecnology hassince been employed in a number of large projects such asbridge structures. In Mexico this tecnology is known, but hasnot yet been applied.

In recent years, the cost of rehabilitating and protectingbridges with CP has fallen by as much as one half. Actually,there are more than 500 bridges in North America with in-stalled CP systems [35]. Of the more than 575,000 bridgesin the National Bridge Survey, it is estimated that 44% are inneed of repair [36].

With the object of contributing to a better knowledge ofthe factors afecting these systems’ efficiency and yield, lab-oratory assays have been carried out to gather informationon the behavior and mechanism of Zn/concrete interphaseand especially of the galvanic couple Fe/Zn, which is re-sponsable for galvanic cathodic protection.

Potentials for the Zn anode were measured with respectto the steel cathode. Figure 4 shows results of the potentialof a steel cathode as a function of immersion time. Through-out the experiment, the steel cathode remained at potentialvalues more negative in value than -800 mV (SCE), maintain-ing the steel protected.

CA d

= ee01

1 11

1 11

1 1Z – R

C R C R

s

dl 1 pol po

=

++

+

Development and testing of galvanic anodes for cathodic protection 335

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46

0

-100

-200

-300

-400

-500

-600

-700

-800

-900

-100

-1100

Time, Days

Po

ten

tial

, mV

vs

sce

Figure 4. Potential of cathode (steel) as a function of time during ca-thodic protection of reinforced concrete by thermal spray Zn-anode.

All the Bode impedance diagrams (phase angle vs. fre-quency) obtained during the experimentation time [33] showthe presence of a peak at about 100 - 200 Hz, which couldcorrespond to Zn, and a second peak at lower frequencies,corresponding to Fe. The measured potentials on steel cath-ode corresponding to the same days were -1032 and -896mV (SCE), both of them characteristic of protection of Fe bythe Zn anode.

To confirm these results, EIS experiments were carriedout with the galvanic couple Fe/Zn in the same NaCl solu-tion. The impedance diagrams corresponding to steel, Znand steel-Zn in NaCl solution at the corrosion potential valuewere obtained. The Nyquist diagrams show capacitativesemicircles and for the particular case of the galvanic pairFe- Zn with a ratio area of 1:1, two semicircles. The presenceof two constant times seems to be clear, as verifiable in thecorresponding phase angle diagram. The value of fmax is of 4Hz for steel, 300 Hz for Zn, and for the galvanic couple Fe-Zn, a detailed analysis of which is presented in Figure 5. Thepresence of two peaks at 400 Hz and 30 mHz, that couldcorrespond to Zn and Fe respectively, are clearly shown.

The behavior of the galvanic couple can be explained bymeans of the equivalent electrical circuit. In this electric cir-cuit, the solution resistance will be in series with two parallelRC circuits. The results obtained modeling with this circuit[33] show a good agreement with experimental results. Thephase angle diagram as a function of frequency permits cor-roboration that the pair Fe-Zn initially shows a behavior char-acteristic of steel. But from the third day the behavior ismixed (Zn dissolution to protect Fe) and for the fourth daythe predominance of Zn is clearly shown.

2. Improving electrochemical efficiency of Mganodes [37-43]

Mg anodes currently have limited offshore use. Alloys of Mgare particularly suited for high resistivity environments wheretheir inherent negative potential and high current output perunit weight is considerable. The theoretical half-cell potentialof Mg is -2.37 V (NHE) or -2.12 V (SCE). However, the practi-cal measurement of magnesium anode potential is consider-ably more noble. The AZ63A anode (nominal 6% Al and 3%Zn) has a potential of -1.48 V (SCE), while the proprietaryGalvomag anode containing high purity magnesium with0.9 - 1.2 % added Mn and controlled impurities has a solu-tion potential of -1.68 V (SCE).

Aside from the large difference observed between theo-retical and measured solution potentials, magnesium an-odes generally have a current efficiency less than other gal-vanic anodes. The theoretical current yield of a magnesiumanode is approximately 2200 A-h/kg. The practically ob-served current efficiency rarely exceeds 50%.At least threeareas are known to contribute to the factor responsible forthe low current efficiency and more-noble-than theoreticalsolution potential of the magnesium anode [25,44].

• Anolyte chemistry. Changes in anion and cation con-centration, which occur close to the dissolving magnesiumsurface and the effect that these ions have on behaviour.

• Anode alloying elements and structure. Alloying ele-ments and thermal story attendant with casting affect thestructure, which are in turn related to filming characteristicsand local-cell action.

• Anodic electrochemistry.Studies on factors responsible for the low current efficien-

cy in magnesium anodes, including changes in anion and

336 Joan Genescà and Julio Juárez

-1.000

-750

-500

-250

0

Z’’

Z’

0 250 500 750 1.000

Frequency (Hz)

10-3 10-2 10-1 100 101 102 103 104 105

-50

-40

-30

-20

-10

0th

eta

Frequency (Hz)

10-3 10-2 10-1 100 101 102 103 104 105

103

102

101

Z

Fe y Zn Relacion (1:1)Fe y Zn Relacion (1:1)

Figure 5. Impedance diagrams of the galvanic couple Fe-Zn in 3% NaCl [33].

cation concentration which occur close to the dissolvingmagnesium surface and the anodic electrochemistry, havebeen carried out in the past [45,46].

The composition of the magnesium anode is known to af-fect its current capacity, solution potential and efficiency.Many alloying elements have been used in order to improvethe properties of magnesium anodes [43,44].

As has been pointed out in the past, the metallurgical fea-tures of Mg sacrificial anodes such as casting conditionsand heat treatment are closely related to anode operationpotential and also to their own efficiency. In the magnesiumtype of anodes, contrary to the requirements for anode ma-terials, corrosion occurs by pitting rather than by uniformcorrosion. This corrosion characteristic shifts the potential tomore electronegative values [46]. For instance, magnesiumsuffers pitting when it is exposed to chloride solutions innonoxidizing media at open circuit potentials [47]. The dele-terious effects of Ni, Fe and Cu in Mg anodes are well-docu-mented [48].

In order to investigate the main reasons for low current ef-ficiency of the commercial magnesium sacrificial anodes, astudy was carried out with the main objective of correlatingthe microstructure characteristics and the alloying elementsof the magnesium anodes with their efficiency. Heat treat-

ment effects on the current efficiency of commercial Mg an-odes was also studied [38,43].

Figure 6 shows a micrograph of the structure obtained bygravity casting of commercial magnesium into a coppermould. As can be observed, the microstructure in all the in-gots was of the cellular type. Cell size in ingots varied be-tween 5 �m in the centre of the ingot to above 10 �m in theouter region of the ingot. The microstructure mainly consistsof cells of �-Mg, but few precipitates were observed at cellboundaries. Because the magnesium ingots were producedin order to assess their behaviour as magnesium sacrificialanodes, a heat treatment was performed at 300 oC for eighthours. Figure 7 shows the microstructure observed in all thespecimens, which consists mainly of equiaxed grains. Insidethese grains a cellular structure was observed and thesecells were nearly free of precipitates.

Microanalyses were carried out in order to quantify howthe microstructure was modified by the chill casting tech-nique and the heat treatment of specimens, before perform-ing the evaluation of magnesium sacrificial anode test, interms of the nature of impurities like copper, iron and nickel.

In the as-cast magnesium ingots and especially in re-gions where the precipitates were present, the microanaly-sis system detected precipitates rich in iron (shown in Figure6) and a uniform concentration of copper, iron and nickel insolid solution with some major concentration of iron at grainboundaries.

Development and testing of galvanic anodes for cathodic protection 337

Figure 6. Cellular microstructure obtained by gravity casting com-mercial magnesium into a cylindrical copper mould. Second phaseparticles were detected as Fe/rich particles [43].

Figure 7. Equiaxed grain structure of a magnesium sacrificial anodeafter heat treatment (300 oC, 8 hrs). Inside the equiaxed grains, acellular microstructure is observed with nearly free precipitates [43].

Microanalyses of the magnesium ingots with heat treat-ment revealed small iron-rich precipitates and a layer of alu-minium-rich second phase of about 0,05 �m in thicknesswas detected at grain boundaries.

The efficiency values for sacrificial anode specimens inthe ascast condition are superior to 50% and with heat treat-ment, the efficiency values are close to 70% [43].

After evaluation of the magnesium sacrificial anode, thesurface of specimens was observed. Figure 8 shows the mi-crostructure of magnesium in the as-cast condition afterevaluation of its efficiency. As can be seen, corrosion ofmagnesium occurs preferentially in regions rich in secondphase Fe-rich precipitates, which were located at cellboundaries. Corrosion in these regions is by pitting, forminga step-like pattern.

Figure 9 shows the observed microstructure of the mag-nesium sacrificial anode with heat treatment after the test.The corrosion features are more uniformly distributed com-pared to specimens without heat treatment, and corrosionwas of the uniform type.

Heat treatment effect on commercial magnesium anodeswas also studied [38,42,43]. By using specific heat treat-

ments [42] the efficiency of the Mg anodes was improved,which made it possible to control the amount and size of theprecipitates and of the second phase particles localised atgrain boundaries and inside the matrix.

Lower anode efficiency was related to the presence ofMnrich second phase particles where corrosion occursalong a narrow region in the grain boundaries. In order toproduce high efficiency of Mg-anodes it is necessary to con-trol the impurities that are uniformly distributed in solid solu-tion, and other microstructural parameters such as grainsize. The solution could be to use alternative castingprocesses [37] and heat treatments [39,41].

3. Design and development of new Al-alloy anodesIn/Hg free.

3.1 IntroductionPresently, the most commonly employed sacrificial metalsfor cathodic protection systems are alloys of magnesium(Mg), zinc (Zn) and aluminium (Al). The Al-anodes are close-ly related to alloy chemistry and to environmental applica-tion. Aluminium has attained considerable merit as the basisfor a galvanic anode mainly due to its low density, largeelectrochemical equivalent, availability and reasonablecost. The low electrode potentials of Al-anodes are readilyadaptable to a variety of saline environments such as sea-water, marine muds and brackish waters. Unalloyed-Aladopts a relatively noble solution potential in saline media asa result of its protective oxide film. The oxide is the cause ofrapid polarization when aluminium is placed under a corro-sion load in a cathodic protection circuit. Nevertheless, thesuccess of the Al-anode depends upon the alloying of cer-tain metals whose surface role is to ultimately prevent theformation of a continuous adherent and protective oxide filmon the alloy, thus permitting continuous galvanic activity ofthe Aluminium. Research carried out towards the develop-ment of Al-alloys appropriate for cathodic protection hasconsidered the influence of alloying elements such as Zn, ti-tanium (Ti), mercury (Hg), and indium (In) (see, for exampleRef [49,50]). The employment of each of those elements hasshown an improvement of Al-activation in neutral chloridemedia. However, the seemingly good results obtained in thisfield are in contrast with the increased sensitivity to environ-mental protection. Particularly the use of In alone or coupledwith Hg in Al-alloys during dissolution results in sea life pollu-tion and gives rise to great environmental issues. In order toavoid sea life pollution due to elements such as Hg and In,and at the same time providing an Al-alloy adequate for ca-thodic protection application, the Al-Zn-Mg system has beeninvestigated in terms of distribution of intermetallics in the � -Al matrix capable of breaking down passive films as well aspresenting good electrochemical efficiencies [51].

For instance, reports have been made [52], in the as-castcondition, of the existence of a microstructure consisting of� -Al solid solution with precipitation of the -phase and aneutectic consisting of a fine dispersion of the � + segregat-

338 Joan Genescà and Julio Juárez

Figure 8. Microstructure of magnesium in the as-cast condition afterthe evaluation test, in which pitting of the corrosion pattern can beobserved [43].

Figure 9. Microstructure of magnesium in the as-heat treated condi-tion after the evaluation test. Corrosion is more uniform as comparedto the as-cast magnesium [43].

ed at grain boundaries. Further dispersion of the phase inthe matrix was increased by means of thermal treatmentsapplied to as-cast ingots, by taking advantage of the fast ki-netic reactions occurring in solid state at 400°C, giving as aresult Al-anodes with electrochemical efficiencies up to 78%[53].

The main purpose of this research is to identify the possi-bility of the substitution of Al-Zn-In and Al-Zn-In-Hg sacrifi-cial anodes, by alloys of the Al-Zn-Mg type, in order to avoidsea-life pollution without decreasing current efficiency of theresulting anodes. The first part of this research was focusedon the identification and distribution of precipitates in the Al-alloy, in order to achieve two goals; the first one is to obtain agood surface activation of the anode; the second, to yieldcorrosion products similar to those found in sea water in or-der to avoid pollution of sea life. As a first step reference wasmade of the work of Barbucci et al. [53] producing Al-Zn-Mgalloys but with additions of Li. The resulting microstructurewas then characterized, with particular attention to identifi-cation of precipitates in the � -Al matrix and eutectics in in-terdendritic regions, in both as-cast ingot and aged sam-ples. The research was also directed towards the effect of Liadditions on superficial activation of the anode by means ofprecipitation of the � -AlLi intermetallic at grain boundariesand/or matrix, and taking advantage that the Zn decreasesthe solid solubility of Li in the � -Al phase [54].

3.2 Experimental ProcedureAn Al-5 at. % Zn-5 at. % Mg-0.1 at. % Li alloy was preparedwith commercially available Al, Zn and Mg with purities of99.98 %. Li was used as a wire of 3.2 mm in diameter and99.9% of purity with 4.5 mg/cm of Sodium (Na). Due to previ-ous experiences during melting of this kind of alloys and inorder to avoid losses of Mg, Zn and Li, these elements wereplaced in Al-capsules. Initially, the Al was placed in an alu-mina/graphite-coated crucible and melted in a resistancefurnace under an argon atmosphere. Once the Al was melt-ed, the liquid bath was overheated 150ºC and the Al-cap-sules containing Zn and Mg were added. The bath wasstirred with argon for 10 minutes in order to achieve uniformdistribution of Zn and Mg. Immediately after this operation,the Al-capsule containing Li was added to the liquid bath,which was stirred with a flux of argon for another 5 minutesafter which the liquid alloy was poured into a copper mold ofdimensions 8x8x50 cm.

In order to perform the characterization of the resultingmicrostructure, the ingots were sectioned transversally tothe heat flow, ground, polished and etched in Keller’sreagent to reveal the different phases, precipitates and/or in-termetallic compounds present in the ingot. Aged treat-ments were performed in the as-cast ingot in order to en-hance precipitation, following to aging steps: 1) aging at400°C for 5 hours, and 2) aging at 400°C for 5 hours with anadditional heating of 160°C for 2 hours. The resulting mi-crostructure was characterized using a Stereoscan 440scanning electron microscope (SEM) and a 2100 Jeol scan-ning transmission electron microscope (STEM). Both elec-

tron microscopes were equipped with WDS-microanalysesfacilities. X-ray diffractometry on aluminium samples in allconditions using a Siemens 5000 X-ray diffractometer usinga Cu K� radiation, a Ni filter and a scan velocity of 2°/min.

3. 3 Results and discussionA representative microstructure observed in the as-cast in-got as shown in Figure 10, consisted of � -Al dendrites withsizes between 130 to 150 � m. In the interdendritic regions,the presence of a eutectic and black spherical particles wasobserved. The eutectic showed a white color with a maxi-mum width of 10 �m, always following the contour of thedendritic arms. This eutectic, instead of presenting plateletmorphology, like that reported by Barbucci et al. [53]showed the presence of rows formed by gray spherical par-ticles.

Figure 11 shows the microstructure observed in samplesaged at 400°C (5 hr). The dendritic structure was modified,giving rise to the coarsening of primary and secondaryarms. The white eutectic (with a maximum with of 7 � m)started to migrate towards future grain boundaries, leavingtraces of the interdendritic species in the � -Al matrix, which

Development and testing of galvanic anodes for cathodic protection 339

Figure 10. As-cast microstructure of the Al-Zn-Mg ingot.

Figure 11. Microstructure observed in samples aged at 400°C (5 hr).

takes the morphology of spherical particles. Also, it was ob-served that the spherical particles which were present asrows inside the white eutectic, started to growth. The blackspherical particles located at secondary dendritic arm spac-ing did not show any change at this stage.

In order to evaluate the effect of a secondary aging treat-ment, the samples aged at 400 ºC for 5 hours received anadditional aging treatment a 160 ºC for 2 more hours. Theobserved microstructure under this aging condition is shownin Figure 12, where there appears to be an increase in thequantity of black spherical particles following the contours ofthe secondary dendritic arms. The width of the space occu-pied by the black spherical particles increased from 2 � m(in the as-cast ingot) to � 6 � m (in this aging stage).

In order to qualitatively identify the species present in theas-cast ingot and in the aged specimens, X-ray diffractome-try was applied and from the collected data, seven peakswere detected in each condition. As expected, the mainpeaks corresponded to the � -Al phase. Also, the presenceof binary precipitates of MgZn, Mg4Zn7, Mg7Zn3, MgZn2,AlMg, Al3Mg2, Mg17Al12, Al4Li9, LiZn, AlLi were detected;ternary precipitates of AlMg4Zn11, Al2MgLi, LiMgZn,Al2Mg3Zn3 and quaternary precipitates of Al0.9Li34.3Mg64.5Zn

and Al0.9Li34.3Mg64.5Zn also appeared. The kind of precipi-tates and their respective d-spacing are shown in Table 1.

An interesting feature of these X-ray diffractograms wasan increase in the relative intensity (I/I0) of peaks II, III and VIIfor both aged conditions, indicating, from a qualitative pointof view, the precipitation of particles containing Li.

In addition, WDS-microanalyses were carried out in spec-imens in both as-cast and as-aged conditions (see Table 2).For example, in the as-cast specimens, it was possible to re-tain 4,7 at.% Zn and 4,2 at. % Mg in � -Al solid solution. Liwas not detected due to the characteristics of the detector.

In the first as-aged condition (400°C, 5 hr), the amount ofZn and Mg present in the � -Al solid solution decreased. Thisdecay in both elements was attributed to the coarsening ofthe eutectic located in interdendritic regions. Regardingcomposition of this eutectic, the amount of Zn detected wasranged from between 32 to 33 at. % and the amount of Mgwas in the range between 27 to 28 at. %, the remaining be-ing Al. As mentioned before, the black spherical particlesobserved in the interdendritic regions did not present anychange, and their composition was almost constant, corre-sponding to precipitates of Mg7Zn3. In the second agingstage an increase in the quantity of black particles in the in-terdendritic region was observed, with an almost constantcomposition. The only detected change in composition cor-

340 Joan Genescà and Julio Juárez

Figure 12. Microstructure observed in samples aged at 400ºC (5 hr)with an additional heating of 160 °C (2 hr).

Peak As-cast Aged*1 Aged*2 Phasesd(Å) I/Io d(Å) I/Io d(Å) I/Io

I 2.340 100 2.334 100 2.344 100 � -Al, MgZn, Mg4Zn7, Mg7Zn3, AlMg, Al3Mg2, Al4Li9, AlMg4Zn11

II 2.028 19 2.023 53 2.028 35 � -Al, AlMg4Zn11, Al2MgLi

III 1.434 44 1.432 49 1.435 54 � -Al, MgZn, MgZn2, Mg17Al12, LiZn, LiMgZn, Al3Mg2

IV 1.224 38 1.223 38 1.224 14 � -Al, AlMg, AlLi

V 1.172 5 1.171 4 1.172 4 Al0.9Li34.3Mg64.5Zn

VI 0.932 6 0.932 12 0.932 5 � -Al, Al0.7Zn0.3, Al2Mg3Zn3

VII 0.908 4 0.906 19 0.906 12 AlLi, Al0.7Zn0.3, Al0.9Li34Mg64Zn, Al2Mg3Zn3

Table 1 Phases and compounds identified by X-ray diffraction

*1 aged at 400ºC, 5 hr, *2 aged at 400ºC with an additional heating of 160 °C for 2 hr.

Condition (Æ) As-cast Aged*1 Aged*2

Microstructure (Ø) (at. %) (at. %) (at. %)

� -Al dendrites Al 91.00 ± 2.0 --- ---Mg 4.20 ± 0.50 3.50 ± 0.50 3.50 ± 0.10Zn 4.70 ± 0.20 3.22 ± 0.65 4.00 ± 0.15

White eutectic Al 41.00 ± 3.0 39.00 ± 4.0 41.00± 3.0Mg 32.00 ± 1.0 33.00 ± 2.0 32.00± 2.5Zn 27.00 ± 3.0 28.00 ± 1.5 27.00± 3.6

Black particles Al --- --- ---Mg 71.00 ± 2.0 72.00 ± 2.5 70.00 ± 4.00Zn 29.00 ± 6.0 28.00 ± 4.0 29.00 ± 2.00

Table 2. WDS results of microanalyses of as-cast and aged speci-mens (in at. %)

*1 aged at 400ºC, 5 hr, *2 aged at 400ºC with an additional heating of160 °C for 2 hr.

responded to the transition of the eutectic to a dendrite-likeprecipitate, whose composition corresponded to Al-16,6 at.% Zn-13,25 at. % Mg with a contamination of 3,3 at. % Fe.

TEM observations performed in the specimens with andwithout heat treatment identified the presence of a platelet-like precipitate of about 1,800 nm in length and sphericalprecipitates (40 to 200 nm. Selected area diffraction pat-terns taken in those precipitates identified them as the inter-metallic -Al2Zn3Mg3, Mg7Zn3 and � -AlLi (80 nm).

Regarding the electrochemical behavior in terms of effi-ciency of the as-cast ingot and aged samples, it can be saidthat the efficiency on the as-cast ingot showed an averagevalue of 62%, while the efficiency in aged samples at 400ºC(5 hr) showed an average value of 67%, and the aged sam-ple at 400°C for 5 hours with an additional heating of 160°Cfor 2 hours, showed an average value of 65%.

Recent research directed towards the development ofaluminium sacrificial anodes of the Al-Mg-Zn type, reported�53� values of electrochemical efficiency between 63 to78% (-1,082 mV; SCE). These results were attributed to agood dispersion of the -phase [55] in the � -Al matrix,which was reached by a long-term aging treatment (400ºC,24 hr). The intermetallic compound was shown to be respon-sible for the breakdown of the passive film and at the sametime to lead to a quite generalized dissolution. When addi-tions of In, gallium (Ga) and calcium (Ca) were made into theAl-Mg-Zn alloy �56�, and the resulting alloy was thermallytreated at 500ºC (4 hr), the Al-anodes reached efficienciesup to 95.6% (-1090 mV; SCE). This excellent efficiency valuewas attributed to a homogeneous distribution of Ga and aprecipitation of In and Ca. Therefore, research has shifted tothe production of Al-alloys which can show high electro-chemical efficiencies. To attain that goal, during the presentresearch it was observed that the as-cast microstructuremust be improved in order to increase the electrochemicalefficiency of Al-anodes by means of decreasing or eliminat-ing the presence of Mg7Zn3 precipitates in interdendritic re-gions. Moving in this direction is due to the fact that duringdissolution of the Al-anode, the Mg7Zn3 particles did not dis-solve. This provoked the isolation of some � -Al dendrites,giving place to a localized pitting corrosion mechanism andat the same time decreasing the electrochemical efficiencyof the Al-anode. On the other hand, it was detected that pre-cipitates of the -Al2Zn3Mg3 and � -AlLi type, played an im-portant role in terms of breaking down the aluminium oxidepassive film, simultaneously permitting a continued galvanicactivity and an increase the electrochemical efficiency of theAl-anode.

3.4 Conclusions1. The resulting Al-Zn-Mg-Li alloy showed two kinds of

species in the interdendritic spacing. These corresponded,to a eutectic of Al2Zn3Mg3 and precipitates of Mg7Zn3.

2. By means of TEM observations, the presence of the -Al2Mg3Zn3 intermetallic compound, precipitates of Mg7Zn3

and � -AlLi precipitation in the � -Al matrix were identified.The presence of those species for the activation of the alu-

minium electrode were found relevant by means of passivefilm breakdown which can lead to a quite generalized disso-lution of the Al-anode.

3. In order to improve the electrochemical efficiency of theAl-anode, it was apparent that research must be focused onthe role played by the -Al2Zn3Mg3, Mg7Zn3 and � -AlLi com-pounds in the � -Al matrix, and on the effect of the decay ofeutectic and particles in interdendritic regions. This will re-sult in the prevention of the formation of a continuous, adher-ent and protective oxide film by particle precipitation, lead-ing to a uniform dissolution of the Al-anode.

References

[1] Ashworth, V. (1986). The theory of cathodic protectionand its relation to the electrochemical theory of corro-sion in Cathodic Protection: Theory and Practice, V.Ashworth and C.J.L. Booker Editors, Ellis HorwoodLtd., Chichester, England, p. 13-30.

[2] Peabody, A.W. (1967). Control of Pipeline Corrosion,NACE, Houston.

[3] Parker, M.E. and Peattie, E.G. (1984). Pipeline Corro-sion and Cathodic Protection, Third Edition, Gulf[JG2][JG3]Publishing Co., Houston.

[4] Morgan, J.H. (1987). Cathodic Protection, Second Edi-tion, Nace International, Houston.

[5] von Baeckmann W., Schwenk, W. and Prinz, W. Edi-tors, (1997), Handbook of Cathodic Corrosion Protec-tion. Theory and Practice of Electrochemical ProtectionProcesses, Third Edititon, Gulf [JG4][JG5][JG6]Pub-lishing Co., Houston.

[6] Code of Practice CP1021, Cathodic Protection,BSI[JG7], London (1973),

[7] NACE RP 01-69, Control of External Corrosion on Un-derground or Submerged Metallic Piping Systems,NACE, Houston [JG8](1969),

[8] Avila, J. and Genescà, J. (1991). Mas Alla de la her-rumbre II. La lucha contra la corrosión, FCE, MéxicoD.F[JG9].Genescà, J. (1993) «Anodos galvánicos en proteccióncatódica. Factores que afectan su eficiencia compor-tamiento» Memoria del Seminario Internacional Agen-cia Internacional de Cooperacion del Japon (JICA) In-stituto Politecnico Nacional. (IPN), Villahermosa, 11Marzo.Bertancourt, L., Rodriguez, C., Genescà, J. (1993),«Ensayos normalizados para ánodos galvánicos demagnesio». Memorias del XIX Congreso AcademiaNacional Ingeniería, Acapulco, 24 Septiembre. pág.163-167.Genescà, J. (1994), «Cathodic Protection Criteria forBuried Steel Pipeline». Memorias de las conferenciasdel III Seminario México-Japón´94. Materiales deacero para tuberías para la industria petrolera. IPN-JICA. 19-21 Enero. México D.F.. pag. 6.1 a 6.12.

[9] Hoar, T.P. (1968)

Development and testing of galvanic anodes for cathodic protection 341

[10] Avila, J. and Genescà, J. (1989), Mas alla de la her-rumbre, Col. La Ciencia desde México, Fondo CulturaEconómica, México D.F.

[11] Bennett, L. H[JG10]. (1978), Economic Effects ofMetallic Corrosion in the US, NBS Special Publication511-1, Washington DC.

[12] Jensen, F. (1979). Testing of Sacrificial Anodes - Ne-cessity and Experience, Trans. I. Mar. E (C), Vol. 91.Conference No. 1, paper C14, 86-100.

[13] Standard Test Method «Impressed Current LaboratoryTesting of Aluminum Alloy Anodes», NACE StandardTM0190-98. NACE International, Houston, 1998.

[14] DNV Recommended Practice RP B401 (1993): «Ca-thodic Protection Design», Det Norske Veritas IndustryAS, Hovik 1993.

[15] Suárez, M., Rodriguez, C., Rodriguez, F.J., Juárez, J.and Genescà, J., (1999), EIS testing of an indium acti-vated aluminium alloy sacrificial anode using DNV RPB401, EUROCORR’99. Simposium: Marine Corrosion.Dechema, Frankfurt, p.1-9.

[16] Genescà, J., Betancourt, L., Jerade, L., Rodriguez, C.and Rodriguez, F.J. (1998), «Electrochemical Testingof Galvanic Anodes» in Electrochemical Methods inCorrosion Research VI. Ed. Pier Luigi Bonora and F.Deflorian, Materials Science Forum, Vols. 289-292, pp.1275-1288, ISBN # 0-87849-819-2, Trans Tech Publi-cations Ltd, Uetikon-Zuerich, 1998.Espelid, B., Schei, B. and Sydberger, T. (1996). Char-acterisation of sacrificial anode materials through labo-ratory testing, Corrosion ‘96. Nace International, Hous-ton.

[17] Attanasio, S.T., Murray, J.N. and Hays, R.A., (1996),Non-Uniform Electrochemical Behavior of Indium-Acti-vated Aluminum Alloy Anodes, Corrosion/96, NACE In-ternational, Houston , Paper No. 546.

[18] Hine, R.A. and Wei, M.W., (1964), Mat. Prot. 3, 11, 49.[19] Lye, R.E., (1990), Mat. Perf. 29, 5, 13.[20] Uruchurtu, J. (1991), Electrochemical Investigations of

the Activation Mechanism of Aluminum, Corrosion 47,6, 472-479.

[21] Reboul, M.C., Gimenez, P. and Rameau, J.J., (1983), AProposed Activation Mechanism for Al-Zn-In Anodes,CORROSION/83, NACE, Houston , Paper No. 214.

[22] Murray, J.N., Hays, R.A. and Lucas, K.E., (1993), Test-ing Indium Activated, Aluminum Alloys Using NACETM0190-90 and Long Term Exposures, CORRO-SION/93, NACE, Houston , Paper No. 534.

[23] Whiting, D.A., Nagi, M.A. and Broomfield, J.P. (1996),Laboratory evaluation of sacrificial anode materials forcathodic protection of reinforced concrete bridges,Corrosion (52) 6, 472-479.

[24] Rincon, O.T., Carruyo, A.R., Romero, D. and Cuica, E.(1992), Evaluation of the Effect of Oxidation products ofAluminum Sacrificial anodes in Reinforced ConcreteStructures, Corrosion 48 (11) 960-966.

[25] Genescà, J., Betancourt, L. and Rodriguez, C., (1996)«Electrochemical Behavior of a Magnesium Galvanic

Anode under ASTM Test Method G 97-89» Corrosion(NACE) 52 (7) 502-507.

[26] ASTM Test method G 97-89, (1989), «Standard TestMethod for Laboratory Evaluation of Magnesium Sacri-ficial Anode Test Specimens for Underground Applica-tions» 1989 Annual Book of ASTM Standards, vol.03.02, West Conshohocken, PA. ASTM. P. 377-380.

[27] Casey, E.J. and Bergeron, R.E., (1953), Can. J. Chem.31, 849.

[28] Perrault, G.G., (1970), J. Electroanal. Chem. 27, 47.[29] Straumanis, M.E. and Bathia, B.K., (1963), J. Elec-

trochem. Soc. 110, 357.[30] Betancourt, L., Lee, J.C., Rodriguez, C. y Genescà, J.,

(1995), «Comportamiento electroquímico del Magne-sio como ánodo galvánico». AFINIDAD 52 (458) 258-266.

[31] Talavera, M.A., Pérez, J.T., Pérez, T. y Genescà, J.(1999), «Seguimiento por impedancia electroquímicade la protección catódica del acero por ánodosgalvánicos de zinc», Memorias Congreso SociedadMexicana Electroquímica, SME´99, trabajo # 13, pag.13/1 a 13/11. Mérida.

[32] Pérez, J.T., Talavera, M.A., Castro, P., Genescà, J.,(1999), «Protección catódica del acero de refuerzo enconcreto mediante zinc aplicado por rociado térmi-co»., Memorias V Congreso Iberoamericano de Pa-tología de las Construcciones, CONPAT´99, Montev-ideo. Tomo III. Pag. 1533-1540. ISBN 9974-39-193-8.

[33] Talavera, M.A., Perez, T., Castro, P. and Genescà, J.,(2000),»EIS Measurements on Cathodically ProtectedSteel in Concrete», NACE Corrosion’2000. Houston,Paper # 794. Pag. 794/1-794/9.

[34]. «The First Zinc-Anode CP System - 11 Years Later «,Concrete Repair Digest (1994): p.292.

[35] Jackson, D. «Cathodic Bridge Protection is More Af-fordable than Ever», Focus, September (1997): p.5.

[36] Burke, P.A. Materials Performance 33,6(1994):p.48.[37] Juárez, J., Martinez, L. and Genescà, J. (1993). «Solid-

ification Techniques and Electrochemical Properties ofMagnesium Base Anodes» Corrosion’93. Paper No.536. NACE, Houston.

[38] Juárez, J., Genescà, J and Pérez, R., (1993), «Improv-ing the Efficiency of Magnesium Sacrificial Anodes»,Journal of the Minerals, Metals and Materials Society45 (9) 42-44.

[39] Salas, G., Noguez, M.E. and Genescà, J., (1994),«Cooling rate effect in the ascast magnesium sacrifi-cial anode structure and its corrosion evaluation».Primer Congreso de Corrosión NACE- Región Lati-noamericana. Maracaibo, Venezuela. Pag. 94064/1-94064/12.

[40] Garcia, G., Campillo, B., Genescà, J., Juárez, J., Ro-dríguez, C., and Martínez, L., (1994), «Influencia de lostratamientos térmicos en un ánodo de magnesio evalu-ado bajo ASTM G-97-89». Primer Congreso de Cor-rosión NACE- Región Latinoamericana. Maracaibo,Venezuela. Pag. 94056/1-94056/10.

342 Joan Genescà and Julio Juárez

Development and testing of galvanic anodes for cathodic protection 343

[41] Campillo, B., Rodriguez, C., Juárez, J., Genescà, J. andMartinez, L., (1996), «An Improvement of the Anodic Ef-ficiency of Commercial Mg Anodes». NACE Corro-sion’96. Houston. Paper # 201. Pag. 201/1-201/18.

[42] Campillo, B., Rodriguez, C., Genescà, J., Juárez-Islas,J., Flores, O. and Martínez, L. (1997) «Effect of HeatTreatment on the Efficiency of Mg Anodes», Journal ofMaterials Engineering and Performance 6, 449-453.

[43] Genescà, J., Rodriguez, C., Juárez, J., Campillo, B.and Martinez, L., (1998), «Assesing and ImprovingCurrent Efficiency in Magnesium Base Sacrificial An-odes by Microstructure Control», Corrosion Reviews16 (1-2) 95-125.

[44] Schreiber, C.F. (1986), Sacrificial Anodes in CathodicProtection: Theory and Practice, V. Ashworth andC.J.L. Booker Editors, Ellis Horwood Ltd., Chichester,England. P. 78-93.

[45] Schrieber, C., (1991[JG11]), in Ashworth, V.,Hanawalt, J.D., Nelson, C.E. and Peloubet, J.A., Corro-sion Studies of Magnesium Alloys, Metals Technology,September.

[46] Bessone, J.B., Suarez, R.A., De Micheli, S.M., (1981),Sea Water Testing of Al-Zn, Al-Zn-Sn and Al-Zn-In Sac-rificial Anodes, Corrosion 37 (9) 533-539.

[47] Tunold, R. (1991), Corrosion Science 17, 533.[48] Jones, H. (1984), Journal of Materials Science 19,

1043.[49] Despic, A.R., Drazic, D.M., Purenovic, M.M. and

Cikovic, N., (1976), Electrochemical properties of Al Al-loys Containing In, Ga and Tl, Journal of Applied Elec-trochemistry 6, 527.

[50] Salleh, M., (1978), Ph. D Thesis, UMIST, Manchester,U. K.

[51] Clark, J.B., Aronson, H. and Domian, H.A., (1961), Ret-rograde movement of alpha:beta brass boundaries,Trans. Am. Soc. Met., 53, 295.

[52] Kuznetsov, G.M., Barsukov, A.D., Krivosheeva, G.B.and Dieva, E.G., (1986), A study of phase equilibriumsin Al-Zn-Mg alloys, Akad, Nayk, SSSR Met. 4, 198.

[53] Barbucci, A., Cerisola, G., Bruzzone, G. and Saccone,A., (1977), Activation of aluminium anodes by the pres-ence of intermetallic compounds, Electrochimica Acta,42 (15) 2369.

[54] Kilmer, R.J. and Stonere, G.E., (1991), The effect of Znaddition on the enviromental stability of Al-Li alloys,Light Weight Alloys for Aerospace Applications II,TMS, 3.

[55] Petrov, D.A., (1993), in Ternary AlloysA comprehensivecompendium of evaluated comstitutional data andphase diagrams, eds G. Petzow and G. E. Effemberg,VCH Verkgsgesellschaft 7, 57.

[56] Zhand, X.,Wang, Y. and Huo, S (1995), Study on elec-trochemical properties of Al-Zn-Mg-In-Ca alloy sacrifi-cial anode, Corrosion Science and Protection Tech-nique , 7 (1) 53.

Acknowledgements

The authors would like to thank other staff and students ofthe Universidad Nacional Autónoma de México, UNAM, whohave contributed to our understanding of the subject. Wewould like to recognize the contributions of our studentsJuan C. Lee, Luis Betancourt, Liliana Jerade, Marco Talav-era, José T. Pérez, Mario Suarez and Socorro Valdez, andcolleagues Bernardo Campillo, Lorenzo Martinez, RamiroPerez and Carlos Rodriguez.

One of the authors, JG, wishes to thank Consejo Nacionalde Ciencia y Tecnologia, CONACYT, and Dr. Luis XimenezCaballero for financial support during sabbatical leave.

This research continues to receive support from DGAPA-UNAM and CONACYT.

About the authors

Joan Genescà i Llongueras born in Terrassa, Catalonia, in 1949. Doctor in Chemical Engineering (1980) from the InstitutQuimic de Sarriá, IQS, since 1982 he has been full professor of Electrochemistry and Corrosion Engineering at the Departmentof Metallurgical Engineering, School of Chemistry, UNAM. Co-author of the series of books «Mas allá de la herrumbre», (Be-yond Rust), edited by Fondo de Cultura Económica, México, he has published another book about atmospheric corrosion andone textbook on the thermodynamic and kinetic principles of corrosion. His research is on the various aspects of corrosion andcorrosion engineering, specially cathodic protection and electrochemical methods in corrosion research. He is a member ofthe Societat Catalana de Química (IEC), National Association of Corrosion Engineers (NACE), the Electrochemical Society andthe Academia Nacional de Ingeniería (México).

Julio Juárez-Islas received his bachelor degree in Metallurgy from the Department of Metallurgy, Faculty of Chemistry-UNAM in 1980. In 1982, he obtained his Diploma in Metallurgy and, in 1987, his PhD. Degree in Metallurgy, from the School inMaterials at Sheffield University. From 1986 to 1999 was invited to collaborate with the Royal Airforce Establishment in the fieldof rapid solidification of light alloys. Currently, he is working at the National University of Mexico as a full professor in the field ofDevelopment of New Materials. His research has been awarded three national prizes and one international prize in the field ofDevelopment of Technologies.