BS-ISO-17475-2005.pdf

BRITISH STANDARD BS ISO 17475:2005 Incorporating Corrigendum No. 1 Corrosion of metals and alloys Electrochemical test methods Guidelines for conducting potentiostatic and potentiodynamic polarization measurements ICS 77.060 ? Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS ISO 17475:2005 This British Standard was published under the authority of the Standards Policy and Strategy Committee on 10 January 2006 © BSI 2006 ISBN 0 580 47142 X National foreword This British Standard reproduces verbatim ISO 17475:2005, including Corrigendum May 2006 and implements it as the UK national standard. The UK participation in its preparation was entrusted to Technical Committee ISE/NFE/8, Corrosion of metals and alloys, which has the responsibility to: A list of organizations represented on this committee can be obtained on request to its secretary. Cross-references The British Standards which implement international publications referred to in this document may be found in the BSI Catalogue under the section entitled “International Standards Correspondence Index”, or by using the “Search” facility of the BSI Electronic Catalogue or of British Standards Online. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. aid enquirers to understand the text; present to the responsible international/European committee any enquiries on the interpretation, or proposals for change, and keep UK interests informed; monitor related international and European developments and promulgate them in the UK. Summary of pages This document comprises a front cover, an inside front cover, the ISO title page, pages ii to v, a blank page, pages 1 to 13 and a back cover. The BSI copyright notice displayed in this document indicates when the document was last issued. Amendments issued since publication Amd. No. DateComments 1646430 June 2006Addition of missing labels in Figure 1 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI Reference number ISO 17475:2005(E) INTERNATIONAL STANDARD ISO 17475 First edition 2005-10-01 Corrosion of metals and alloys Electrochemical test methods Guidelines for conducting potentiostatic and potentiodynamic polarization measurements Corrosion des métaux et alliages Méthodes d'essais électrochimiques Lignes directrices pour la réalisation de mesures de polarisations potentiostatique et potentiodynamique BS ISO 17475:2005 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI ii Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI iii Contents Page Foreword iv Introduction v 1 Scope . 1 2 Normative references. 1 3 Principle. 1 4 Apparatus 4 5 Procedure 7 6 Test report . 9 Annex A (informative) Method of preventing a crevice attack for certain applications 10 Annex B (informative) Potential of selected reference electrodes at 25 °C with respect to the standard hydrogen electrode (SHE) . 12 Bibliography. 13 BS ISO 17475:2005 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI iv Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO 17475 was prepared by Technical Committee ISO/TC 156, Corrosion of metals and alloys, in collaboration with the Korea Research Institute of Standards and Science. BS ISO 17475:2005 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI v Introduction Corrosion of metals and alloys in aqueous solutions is generally caused by an electrochemical mechanism. Therefore, one can measure or analyse corrosion phenomena, utilizing a variety of electrochemical techniques. This International Standard, based on ASTM G51 and ASTM G1502, defines basic guidelines for potentiostatic potentiodynamic polarization measurements to characterize an electrochemical kinetics of anodic and cathodic reactions. BS ISO 17475:2005 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI blank Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 1 Corrosion of metals and alloys Electrochemical test methods Guidelines for conducting potentiostatic and potentiodynamic polarization measurements 1 Scope This International Standard applies to corrosion of metals and alloys, and describes the procedure for conducting potentiostatic and potentiodynamic polarization measurements. The test method can be used to characterise the electrochemical kinetics of anodic and cathodic reactions, the onset of localised corrosion and the repassivation behaviour of a metal. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 8044:1999, Corrosion of metals and alloys Basic terms and definitions ISO 8407:1991, Corrosion of metals and alloys Removal of corrosion products from corrosion test specimens ISO 9400:1990, Nickel-based alloys Determination of resistance to intergranular corrosion ISO 11463:1995, Corrosion of metals and alloys Evaluation of pitting corrosion ISO 11846:1995, Corrosion of metals and alloys Determination of resistance to intergranular corrosion of solution heat-treatable aluminium alloys 3 Principle 3.1 When a metal is immersed in a solution, the rate of the anodic reaction and that of the cathodic reaction will be in balance at the open-circuit potential (free corrosion potential, Ecor). If the electrode potential is displaced from the open-circuit value, the applied current measured will represent the difference between the anodic-reaction current and the cathodic-reaction current. If the displacement of potential is sufficiently large, the net current will be essentially equal to the anodic or cathodic-reaction kinetics, depending on whether the potential is made respectively more positive or more negative with respect to the open-circuit value as shown for a metal in the active state in acid solutions Figure 1 a) and in neutral aerated solutions Figure 1 b). 3.2 In certain metal-environment combinations, the metal may be in the passive state (Figure 2). If an aggressive anion is present and the potential is increased (made more positive) with respect to the open- circuit potential, localised breakdown of passivity (e.g. pitting, crevice corrosion, intergranular attack) can result with an attendant increase in the applied current (Figure 2). The potential corresponding to the increase in current can be used as a measure of the resistance of a metal to localised corrosion. BS ISO 17475:2005 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 2 BS ISO 17475:2005 a) Corrosion rate is under diffusion control b) Example by diffusion of oxygen in water Key X potential Y log current density 1 cathodic 2 anodic Ecor corrosion potential cor corrosion current density Er reversible electrode potential o exchange current density d limiting diffusion current density, which corresponds to the maximum diffusion rate of oxygen in the solution Figure 1 Schematic anotic and cathodic polarization curves for metals corroding in a system where the cathodic reaction is reduction of protons Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 3 a) Metals with active-passive transition b) Metals without active-passive transition Key X potential Y log current density Ecor corrosion potential icor corrosion current density Epp passivation potential icrit critical current density for passivation ip passive current density Ef flade potential Eb breakdown potential Esp secondary passivation potential a Active. b Passive. c Transpassive. Figure 2 Schematic anodic polarization curves BS ISO 17475:2005 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 4 3.3 If the potential is subsequently decreased after the onset of localised corrosion, the potential at which repassivation occurs (when the applied current returns to approximately the same value as the passive current) can be considered to be indicative of the resistance of the metal to propagation of localised corrosion; the more noble the potential, the greater the resistance. 3.4 The displacement in potential can be stepwise, with the magnitude of the potential step and the time at a specific potential selected according to the application and purpose of the experiment. This type of testing is known as potentiostatic. 3.5 When the potential is displaced in a continuous mode at a controlled scan (displacement) rate, the test is described as potentiodynamic. 3.6 The kinetics of the electrochemical processes occurring on the surface can be time dependent, for example due to film formation, and hence the time that the potential is held at a specific potential in potentiostatic testing or the potential scan rate in potentiodynamic testing can be critical. For example, too high a rate of change may lead to overestimation of the breakdown potential for localised attack. For this reason, the interpretation of polarization data shall be considered carefully, particularly when applied to service conditions. 3.7 The measurement of the electrode potential can be influenced by ohmic drop in the solution. For solutions of low conductivity, a correction shall be made. 4 Apparatus 4.1 Potentiostat The potentiostat should be capable of controlling the electrode potential to within ± 1 mV of a preset value. A scanning potentiostat is used for potentiodynamic measurements. For such measurements, the potentiostat shall be capable of automatically scanning the potential at a constant rate between preset potentials. 4.2 Electrode potential-measuring instruments The instrument should have a high input impedance of the order of 1011 to 1014 , to minimize current drawn from the system during measurement. The sensitivity and accuracy of the instrument should be sufficient to detect a change of 1,0 mV. 4.3 Current-measuring instruments Use appropriate current-measuring instruments with maximum error 0,5 %. 4.4 Test cell 4.4.1 The test cell should contain the working electrode (the metal to be polarized), a reference electrode for measuring the electrode potential, and one or two auxiliary electrodes. The test cell should incorporate inlet and outlet gas ports and a port for insertion of a temperature-measuring device. NOTE The term auxiliary electrode is synonymous with counter electrode. 4.4.2 The detailed construction of the test cell depends on the application. Examples commonly used are shown in Figure 3. The important distinction of Figure 3 b) is that the auxiliary electrodes are separated from the main cell of the working electrode by a fritted disc, in order to limit contamination of the main cell by reaction products generated at the auxiliary electrodes. 4.4.3 The auxiliary electrodes should be positioned so that the current distribution about the specimen is symmetrical. BS ISO 17475:2005 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 5 a) Auxiliary electrode included in the main cell b) Auxiliary electrode separated from the main cell Key 1 specimen 6 fritted disc 2 reference electrode 7 thermometer 3 auxiliary electrodes 8 probe here corresponds to Luggin capillary 4 reference electrode 9 salt-bridge connection to the reference electrode (not shown) 5 gas inlet Figure 3 Schematic diagram of electrochemical polarization cells with auxiliary electrode BS ISO 17475:2005 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 11:42:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 6 4.4.4 The reference electrode may be inserted directly into the main cell. Precautions should be taken to ensure that it is maintained in the proper condition. To avoid mutual contamination, a double-junction reference electrode may be used or the reference electrode located in a separate chamber and linked to the main cell by a salt bridge. To minimise the potential drop between the reference electrode and the working electrode, a Luggin capillary should be used as shown in Figure 3 a) and Figure 3 b). The tip of the capillary probe shall be positioned so that it is at a distance from the working electrode of about, but not closer than, 2 times the diameter of the tip. 4.4.5 The test cell should be constructed of a material which is inert in the environment at the temperature of testing. 4.4.6 The volume of solution in the test cell shall be such as to reduce to insignificance any change in the solution chemistry as a consequence of the reaction processes. NOTE In most cases, a solution volume greater than 100 ml/cm2 of specimen surface is considered to be adequate. 4.4.7 To assess the effect of flow on the electrode kinetics, a magnetic stirrer may be used but, where more defined control is required, the use of a rotating disc or rotating-cylinder assembly is recommended. 4.5 Electrode holder The auxiliary and working electrodes shall be mounted in such a way that the holder and mounting material have no influence on the measurement. An example of an electrode-mounting assembly is shown in Figure 4. For s