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.
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