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IS 3531 (1997): Glossary of terms relating to corrosion of metals [MTD 24: Corrosion Protection]
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Indian Standard GLOSSARY OF TERMS RELATING TO CORROSION OF METALS ( Second Revision )
ICS 01.040.77; 77.060
(8 BIS 1997 BUREAU OF INDIAN STANDARDS ltdAl’lAKBHAVAN, 9 BAHADUR SHAH ZAFAR MARG NEW DELHI 110002
Corrosion Protection Sectional Committee, MTD 24
FOREWORD
This Indian Standard ( Second Revision ) was adopted by the Bureau of Indian Standards, after the draft finalized by the Corrosion Protection Sectional Committee had been approved by the Metallurgical Engineering Division Council This standard was first published in 1966 and subsequently revised in 1983. The standard is expected to remove any ambiguity and confusion that might arise from individual interpretation of different terms. In the present revision, number of additional definitions have been included and some of the definitions have also been aligned with IS0 8044 : 1989 ‘Corrosion of metals and alloys - Terms and definitions’. Assistance has also been derived from SHREIR ( L L ), ED Corrosion Vo12, 1977, Newness Butterworths, London.
IS 3531 : 1997 Indian Standard GLOSSARY OF TERMS RELATING TO CORROSION OF METALS ( Second Revision ) 1 SCOPE 2.9 Anaerobic Bacteria ( Anaerobes ) This standard covers the terminology used in the Group of bacteria that are unable to multiply in any field of corrosion of metals and alloys, including but a minute trace of oxygen. prevention technology. 2.10 Anarabolic Oxidation 2 DEFINITIONS An oxidation reaction that conforms kinetically to a 2.1 Accelerated Corrosion Test parabolic law so that the rate of oxidation decreases with thickness of the oxide. Corrosion test, carried out under more severe conditions that will yield results in a shorter time than 2.11 Anion in service. A negatively charged ion; it migrates to the anode of 2.2 Activation Overpotential a galvanic or voltaic cell. That part of an over-potential ( polarization ) that exists 2.12 Anode across the electrical double layer at an electorde/ Electrode at which anodic reaction predominates. solution interface and thus directly influences the rate of the electrode process by altering its activation 2.13 Anode Corrosion Efficiency energy. The ratio of the actual corrosion rate of the anode 2.3 Active to the theoretical rate according to Faraday’s law expressed as a percentage. Freely corroding. 2.14 Anode Polarization 2.4 Active-Passive Cell Difference between the potential of an anode Corrosion cell, with anode and cathode formed by passing current and the equilibrium ( or steady-state ) active and passive surface areas of the same metal. potential of the electrode having the same electrode reaction. 2.5 Addition Agent 2.15 Anode Shield A substance, added to an electroplating solution to produce a desired change in the physical properties Protective covering of insulating material, placed on a of the electrodeposit. painted structure in the immediate vicinity of the anode to reduce the cathodic current density in that 2.6 Adhesion area, thus preventing the development of excessive The attractive force that exists between an alkalinity and stripping of the paint (see Saponification, electrodeposit and the substrate. 2.241). 2.7 Alclad 2.16 Anodic Protection Electrochemical protection by increasing the corro- A composite, in which a thin layer of aluminium, or sion potential to a value corresponding to the passive an aluminium alloy of good corrosion resistance, is state. bonded metallurgically to a high-strength aluminium alloy ( of lower corrosion resistance ) to provide a 2.17 Anodic Reaction combination of these two properties. Electrode reaction equivalent to a transfer of positive 2.8 Anaerobic charge from the electronic to the ionic conductor. Air or uncombined oxygen being absent. NOTE - An anodic reaction is an oxidation process. An 1
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exampie common in corrosion is: types :
Me + Me* + ne- a) Drainage Bond - A bond used to effect dram- age; and 2.18 Anodizing b) Resistance Bond - A bond, either incorpo- The formation of oxide films on metals by anodic rating resistors or adequate resistance in it- oxidation of the metal in an electrolyte solution. The self, used for the purpose of limiting current term can be used for thin dielectric films but is flow. more particularly applied to thick porous films on aluminium. 2.28 Breakway Corrosion
2.19 Anolyte A sudden increase in corrosion rate, especially in conditions like high-temperature dry oxidation, etc. The electrolyte solution adjacent to the anode. 2.29 Breakdown Potential 2.20 Anti-Pitting Agent See Critical Pitting Potential (2.79). An addition agent, which is used to prevent the 2.30 Bright Plating formation of pits or large pores in an electrodeposit. Electroplating, under conditions whereby the 2.21 Atmospheric Corrosion electrodeposit has a high degree of specular Corrosion with the earth’s atmosphere at ambient reflectivity. temperature as the corrosive environment. 2.31 Brightener 2.22 Attenuation An addition agent, used specifically to produce an electrodeposit of high specular reflectivity. The decrease in potential and current density along a long buried or immersed structure from the drainage 2.32 Brush Plating point. A method of electrodeposition in which the plating 2.23 Attenuation Curve solution is applied to the article to be plated by means of an absorbent pad or brush which contains the anode. A graph of interfacial electrical potentials between the pipes and surrounding soil in cathodic protection 2.33 Buffer schemes versus the length of the pipe. A substance, or mixture of substances, which when present in an electrolyte solution tends to diminish 2.24 Backfill fluctuations inpH. The material which fills up the space between a buried 2.34 Burnt Deposit anode and the surrounding soil. It should have low resistivity, moisture retaining capacity and be capable A rough, poorly coherent, electrodeposit that results of increasing the effective area of contact between from the application of an excessively high current the anode and the environment. density.
2.25 Base Potential 2.35 Calcareous Scale A scale consisting largely of calcium carbonate and See Potential (2.214) magnesium hydroxide which maybe precipitated from 2.26 Bimetallic Corrosion; Contact Corrosion hard water. (Deprecated ) 2.36 Cathode Electrode at which cathodic reaction predominates. Galvanic corrosion, where the electrodes are formed by dissimilar metals or other electronic conductors. 2.37 Cathodic Area Area on which the cathodic ( Protection ) current is 2.27 Bond picked up from an electrolyte. A metal piece, having very little electrical resistance, 2.38 Cathodic Protection used for connecting two points on the same or Electrochemical protection by decreasing the different structures. Bond can be of following two corrosion potential.
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2.39 Cathodic Reactant from a difference in concentration of the corrosive agent(s) near its electrodes. Species, which is reduced at a cathode. 2.40 Cathodic Reaction 2.52 Concentration ( Diffusion or Transport ) Overpotential Electrode reaction equivalent to a transfer of negative charge from the electronic to the ionic conductor. Change of potential of an electrode, caused by concentration changes near the electrode/solution NOTE -- A cathodic reaction is a reduction process: interface, produced by an electrode reaction. Ox+ne-+Red 2.53 Corrosion 2.41 Catholyte Physicochemical interaction between a metal and its The electrolyte solution adjacent to the cathode. environment which results in changes in the properties 2.42 Cation of the metal and which may often lead to impairment of the function of the metal, the environment or the A positively charged ion; it migrates to the cathode in technical system, of which these form a part. a galvanic or voltaic cell. NOTE - This interaction is usually of an electrochemical 2.43 Caustic Embrittlement nature.
Stress-corrosion cracking of carbon steels caused by 2.54 Corrosion Cell the presence of caustic alkali. Short-circuited galvanic cell in a corrosion system, 2.44 Cavitation Corrosion the corroding metal forming one of its electrodes A process involving conjoint corrosion and cavita- ( see also Galvanic Cell, 2.137 ). tion 2.55 Corrosion Control 2.45 Cavitation Damage Control of the corrosion rate and form of attack of a Erosion of a solid surface caused by the collapse of metal of a given metal/environment system at an vacuum bubbles formed in a fluid. acceptable lever and at an economic cost. 2.46 Cell 2.56 Corrosion Current An electrolytic system, consisting of anode;cathode and an electrolyte. Current due to an electrode reaction, directly causing corrsion. 2.47 Chemical Conversion Coating NOTE - The corrosion current density is equwalent to A protective or decorative coating which is produced the rate of electrochemical corrosion. deliberately on a metal surface by a chemical 2.57 Corrosion Damage environment. 2.48 Chemical Polishing Corrosion effect which is considered detrimental to the function of the metal, the environment or the tech- Improvement in the brightness and levelness of a nical system, of which these form a part. surface finish of a metal by a chemical dissolution reaction. 2.58 Corrosion Depth 2.49 Composite Plate Perpendicular distance between a point on the surface of a metal affected by corrosion and the original An electrodeposit consisting of two or more layers of surface of the metal. metals deposited separately. 2.50 Concentration Cell 2.59 Corrosion Effect A galvanic cell, in which the emfis due to differences Change in any part of the corrosion system caused by in the concentration of one or more electrochemically corrosion. reactive constituents of the electrolyte solution. 2.60 Corrosion-Erosion ( Erosion-Corrosion ) 2.51 Concentration Corrosion Cell Conjoint action of corrosion and erosion resulting Corrosion cell in which the potential difference arises from the abrasive action of a fluid ( solution or gas >,
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IS 3531 : 1997 moving at a high velocity, resulting in continuous corrosion system removal of the protective film from the metal surface. 2.69 Corrosion System 2.61 Corrosion Fatigue System consisting of one or more metals and all parts A process involving conjoint corrosion and alternat- of the environment which influences corrosion. ing straining of the metal. NOTE - Part of the environment may be coatlng, surface NOTE - Corrosion fatigue may occur when a metal is layer, additional electrode, etc. subjected to cyclic straining in a corrosive environment. Corrosion fatigue may lead to cracking. 2.70 Corrosion Test 2.62 Corrosion Inhibitor Test carried out to assess the corrosion behaviour of a metal, the environmental contamination by corrosion Chemical substance which decreases the corrosion products, the effectiveness of corrosion protection rate when present in the corrosion system at a suitable or the corrosivity of an environment. concentration, without significantly changing the concentration of any other corrosive agent. 2.71 Corrosive Agent NOTE - A corrosion inhibitor is generally effective in Substance which when in contact with a given metal a small concentration. In commercial applications additives will react with it. are sometimes named as inhibitors. 2.63 Corrosion Interaction ( Interaction ) 2.72 Corrosive Environment Increase or decrease in the rate of corrosion of a Environment that contains one or more corrosive agents. buried or immersed structure, caused by interception 2.73 Corrosivity of part of the cathodic protection current applied to another buried or immersed structure. Ability of an environment to cause corrosion in a given corrosion system. 2.64 Corrosion Potential 2.74 Couple (Bimetallic, Galvanic ) Electrode potential of a metal in a given corrosion system. Two dissimilar metals in electrical contact. NOTE - The term is used whether or not there is a net 2.75 Covering Power electrical current flowing to or from the metal surface under consideration. The ability of a plating solution to produce an electrodeposit, irrespective of thickness, at low current 2.65 Corrosion Product densities on all significant areas. Substance formed as a result of corrosion. 2.76 Crevice Corrosion 2.66 Corrosion Protection Corrosion associated with, and taking place in, or Modification of a corrosion system so that corrosion immediately around, a narrow aperture or clearance. damage is mitigated. 2.77 Critical Anode Current Density 2.67 Corrosion Rate Anodic current density that must be exceeded in order Corrosion effect on a metal per unit of time. to produce an active to passive transition; for a given NOTE - The type of corrosion rate be used will depend metal it varies with the nature of the solution, on the technical system and on the type of corrosion temperature, velocity, etc. effect. Thus corrosion rate may be expressed as an increase in corrosion depth per unit of time, or the mass of metal 2.78 Critical Humidity turned into corrosion products per unit area of surface per ‘unit of time, etc. The corrosion effect may vary The value of relative humidity of an atmosphere above with time and may not be the same at all points of the which there is a sharp increase in the corrosion rate of corroding surface. Therefore, reports of corrosion rates a given metal. should be accompanied by information on the type, time dependency and location of the corrosion effect. 2.79 Critical Pitting Potential 2.68 Corrosion Resistance The most negative potential required to initiate pits Ability of a metal to withstand corrosion at a given on the surface of a metal held within the passive region
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IS 3531 : 1997 of potentials, it varies with the nature of solution, tem- 2.90 Dezincification of Brass perature, time, etc. Selective corrosion of brass resulting in preferential 2.80 Crossing Point removal of zinc.
A point where two or more buried or immersed struc- 2.91 Differential Aeration tures cross in a plan. The stimulation of corrosion at a localized area due to 2.81 Current Density the difference in oxygen concentration in the electrolyte solution in contact with the metal surface; Current intensity per unit area of the electrode. the area in contact with the solution of lower oxygen concentration is the anode. 2.82 Current Drainage Survey 2.92 Differential Aeration Cell A survey to determine current requirements for cathodic-protection schemes. A temporary ground Corrosion cell, in which the potential difference arises bed is installed at a suitable location and an impressed from a difference in the concentration of oxygen near current is applied between it and the pipe or any other its electrodes. structure. A survey of potentials along the structure NOTE - In some cases the differential aeration cell produces attenuation curves, and permits fairly may result in an active-passive cell. accurate estimation of initial current requirements. 2.93 Diffusion Coating 2.83 Current Efficiency A coating produced by diffusion at elevated The ratio of the rate ofthe actual electrochemical change temperatures. at the electrode ( anode or cathode) to the theoretical 2.94 Diffusion Layer rate according to Faraday’s law, expressed as percentage. The thin layer of solution adjacent to an electrode 2.84 Deactivation through which transport of species to or from the electrode surface occurs by diffusion rather than by Prior removal of the constituent ( of a liquid ), that is, convection. active in causing corrosion. The term is usually applied to the removal of oxygen by physical and/or chemical 2.95 Drainage methods. Process of ( electric drainage ) draining away or draw- 2.85 Dealloying ing away electric current from a cathodically protected structure back to the source of current through an Selective removal by corrosion of a constituent of an external conductor. alloy. 2.96 Driving emf 2.86 Delayed Failure Driving emf is the open circuit potential difference See Sustained Load Failure ( 2.270 ). between a structure to be protected ( a cathode ) and the system of anodes which protect this. The emf 2.87 Depolarization does not include the voltage drop in the soil or in the Reduction or elimination (by physical or chemical connecting load and is the total voltage available for methods) of the electrode polarization, needed to establishing a protective circuit. The driving emf in produce a specified current. galvanic cells are fixed while those in an impressed current system are variable. 2.88 Deposit Attack 2.97 Earth Localized corrosion ( a form of crevice corrosion ) It can be defined as following: under a metal surface and resulting from a deposit on it. a) The conducting mass of earth or of any conductor in direct electrical connection 2.89 Deposit Corrosion therewith; Corrosion associated with, and taking place under, or b) A connection, whether intentional or unin- immediately around, a deposit of corrosion products tentional, between a conductor and the earth; or other substance. and
IS 3531 : 1997 c) To connect any conductor with the general mass 2.105 Electrode Potential of earth. Voltage measured in the external circuit between an NOTE - Ground is sometimes used in place of earth; electrode and a reference electrode in contact with both terms include expense of natural water. the same ionic conductor.
2.98 Electrical Drainage Protection NOTE -- Ohmic potential drop is subtracted if more than one ionic conductor is included then the measured Electrochemical protection against stray-current value may be only approximate. corrosion by drainage of stray-current from the 2.106 Electrochemical Protection metallic object. Corrosion protection achieved by electrical control of NOTE - The drainage can be obtained, for example, by corrosion potential. connecting the metal to be protected to the negative part of the stray-current source, or by connecting to an 2.107 Electrode Reaction auxiliary electrode. Interfacial reaction equivalent to a transfer of charge 2.99 Electrochemical Cell between electronic and ionic conductors. A cell in which chemical energy is transferred into 2.108 Electrode Reaction Current electrical energy. Current resulting from the anodic and cathodic partial 2.100 Electrolytic Cell current of the electrode reaction.
A cell in which electrical energy is used to bring about NOTE - In the steady state the sum of the electrode electrode reactions and is thus converted into reaction currents is equal to the net electrical current. chemical energy. 2.109 Electrogalvanizing
NOTE - The term ‘Electrochemical Cell’ is frequently Galvanizing by electroplating. used to describe both types of cells. 2.110 Electroless Plating 2.101 Electrochemical Corrosion Formation of a metallic coating by chemical reduc- Corrosion involving at least one electrode reaction. tion, catalyzed by the metal deposited.
2.102 Electrochemical Equivalent 2.111 Electrolysis Decomposition by the passage of electric current. The theoretical number of moles of electrochemical change resulting from the passage of 1 Faraday of 2.112 Electrolyte electrical change; if M is the molar mass ( kg ) and z A substance which in solution gives rise to ions; the the number of electrons required for- one act of the term is also used for a solution of an electrolyte or for reaction then the electrochemical equivalent isM/z kg. a molten ionic salt. 2.103 Electrode 2.113 Electrolyte Solution Electronic conductor in contact with an ionic A solution in which the co bduction of electric current conductor. occurs by the passage of dissolved ions. NOTE - In the electrochemical sense the electrode is 2.114 Electrolytic Cleaning in fact restricted to a narrow region on both sides of the interface of this system. The electrode is often named Cleaning obtained at cathode or anode of a cell after the electrode reaction taking place, for example, containing a suitable solution. oxygen electrode. 2.115 Electrode Acceptor 2.104 Electrode Polarization A species in solution, that accepts one or more Change of electrode potential with respect to a refer- electrons from a cathode for each act of the cathodic ence value. reaction and is thus reduced to a lower valence state.
NOTE - Often the corrosion potential is used as the 2.116 Electra-osmosis reference value. The change may be caused, for example, by the application of an external electrical current or by Passage of a liquid through a porous medium ( such the addition of an oxidant or reductant. as soil ) under the influence of a potential difference.
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2.117 ElectrophoreticPlating 2.127 Fatigue Production of a layer or deposit by discharge of Failure of metal under conditions of repeated colloidal particles in a solution on an electrode. alternating stress. 2.118 Electropolishing 2.128 Filiform Corrosion Surface finishing of a metal by making it the anode Corrosion in the form of hair or filaments progressing in an appropriate solution, whereby a bright and level across a metal surface. surface showing specular reflectivity is obtained. 2.129 Film 2.119 EMF Series A thin coating of material, not necessarily thick enough A table of the standard equilibrium electrode potentials to be visible. of systems of the type Mz+ ( aq ) + ze = M, relative to the standard hydrogen electrode, and arranged in 2.130 Flade Potential order of sign and magnitude. The potential at which a metal which is passive 2.120 Epitaxy becomes active ( see Passivation Potential, 2.197 ). The phenomenon whereby a deposit or coating takes 2.131 Flood Coat up the lattice habit and orientation of the substrate. Application of coal-tar or bituminous coatings to pipe- 2.121 Equilibrium Potential lines by flooding the heated coating materials over the pipe surface; wrappings and slings are used to The electrode potential of an unpolarized reversible pull the coat underneath the pipe. electrode. 2.132 Fogging 2.122 Erosion-Corrosion Reduction of the lustre of a metal by a film or particulate A process involving conjoint corrosion and erosion. layer of corrosion product, for example, the dulling of NOTE - Erosion-corrosion may occur in, for example, bright nickel surfaces.
- Pipes with high fluid flow velocity, 2.133 Fouling and pipelines carrying fluid - Pumps Deposition of flora and fauna on metals exposed tp containing abrasive particles in suspension. natural waters, like sea water. 2.123 Evans - Diagram 2.134 Free Corrosion Potential Diagram in which the potential versus current relationships for the cathodic and anodic reactions Corrosion potential in the absence of net electrical of a corrosion reaction aredrawn as straight lines current flowing to or from the metal surface. intersecting at the corrosion potential, thus indicating 2.135 Fretting Corrosion the corrosion current associated with the reaction. A process involving conjoint corrosion and oscilla- 2.124 Exchange Current Density ( i,) tory slip between two surfaces in contact.
The rate of exchange of electrons, expressed as an NOTE -- Fretting corrosion may occur, for example. at electric current per unit area, between the two species mechanical joints in vibrating structures. concerned in a reversible electrode process at the equilibrium potential. 2.136 Galvanic Anode 2.125 Exfoliation The electrode in a galvanic couple formed by two dissimilar metals, as applied to cathodic protection Loss of material in the form of layers or leaves from a in which the galvanic current is flowing from this solid metal or alloy. electrode into the electrolyte. The galvanic anodes corrode and are designated as sacrificial anodes. 2.126 Faraday 2.137 Galvanic Cell Quantity of electric charge required to bring about a change of one electrochemical equivalent ( 1F = Combination of different electrodes connected in 965OOC). series with an ionic conductor.
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NOTE - The galvanic cell is an electrochemical source to the positive terminal of an independent source of of electrical current and will produce current when the direct current, in order to lead to earth the current electrodes are connected by an external conductor. used for the cathodic protection of a buried or immersed 2.138 Galvanic Corrosion metallic structure. Corrosion due to the action of a corrosion cell. 2.148 Half-Cell
NOTE - The term has often been restricted to the One of the electrodes, and its immediate environment, action of bimetallic corrosion cells, that is, bimetallic in an electrochemical or electrolytic cell; particularly, corrosion. an electrode and environment arranged for the pas- sage of current to another electrode or especially for 2.139 Galvanic Current the measurement of its electrode potential. A dc current passing into or out of a structure due to 2.149 Holiday a galvanic couple being established, in which the struc- ture forms one of the electrodes. By using less noble A break or pinhole in an otherwise uniform protective metals artificial galvanic couples are formed in such coating. a way, that the galvanic current protects a desired structure. 2.150 Holiday Detector 2.140 Galvanostatic Polarization (Intentiostatic) A high-voltage low-current output generator with a built-in spark gap used for detecting pinholes Polarization of an electrode during which the current ( holidays ) in pipeline coatings. density is main& :,l at a predetermined constant value. 2.151 Hydrogen Electrode 2.141 Galvanic Protection An electrode at which the equilibrium H’ ( aq ) +e = Electrochemical protection in which the protecting % H, is established. By definition, at unit activity of current is obtained form a corrosion cell formed by hydrogen ions and unit fugacity of hydrogen gas connecting an auxiliary electrode to the metal to be the potential of the standard hydrogen electrode protected. E’H+/ % H,=O.OOV. 2.142 Galvanic Series 2.152 Hydrogen Embrittlement A list of metals and alloys based on their relative A process resulting in a decrease of the toughness or potentials in a given specified environment, usually ductility of a metal due to absorption of hydrogen. sea water. NOTE --- Hydrogen embrittlement often accompanies 2.143 Galvanizing Hot-Dip hydrogen formation, for example, by corrosion or electrolysis, and may lead to cracking. Coating of iron and steel with zinc using a bath of molten zinc. 2.153 Hydrogen Overpotential (Overvoltage) 2.144 Gaseous Corrosion The displacement of the equilibrium or steady-state electrode potential of a cathode required for the dis- Corrosion with gas as the only corrosive agent and charge of hydrogen ions at a given rate per unit area without any aqueous phase on the surface of the metal. of electrode. 2.145 Graphitic Corrosion 2.154 Immunity ( Thermodynamic ) Selective corrosion of grey cast iron, resulting A condition whereby the equilibrium activity of metal in preferential removal of metallic constituents, ions ( simple or complexed ) in solution is prevented leaving graphite. from exceeding some arbitrary value by making the potential of the metal sufficiently negative to that of 2.146 Green Rot the solution. Pourbaix has defined this arbitrary Carburization and oxidation of certain nickel alloys activity at 10e6ion/ 1. at around 1 OOO’C, resulting in a green corrosion 2.155 Impingement Attack product. Localized corrosion resulting from the action of 2.147 Ground Bed corrosion and/or erosion ( separately or conjoint ) A system of buried or submerged electrodes connected when liquids impinge on a surface.
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2.156 Impressed Current corrodent as co-ordinates and curves ( IS0 corrosion curves ) of various specified constant corrosion rates Cathodic current supplied by a dc source to a structure of the metal. in order to lower the potential to the protective poten- tial for cathodic protection. 2.167 Knife-Line Attack
2.157 Impressed Current Protection Severe highly localized attack ( resembling a sharp Electrochemical protection in which the protecting cut into the metal ) extending only a few grains away current is supplied by an external source of electric from the fusion line of a weld in a stabilized austenitic energy. stainless steel, which occurs when the metal comes into contact with hot nitric acid and is due to the 2.158 Inhibitor precipitation of chromium carbides.
A substance added to an environment in small 2.168 Knife-Line Corrosion concentrations to reduce the corrosion rate. Corrosion resulting in a narrow slit in or adjacent to 2.159 Inseparable Anodes and Cathodes the filler/parent boundary of a welded or brazed joint. Anodes and cathodes which cannot be distinguished 2.169 Layer Corrosion experimentally although their existence is postulated by theory. Corrosion of internal layers of wrought metal, occasionally resulting in exfoliation, that is, detach- 2.160 Insulating Flanges ment of unattached layers. Flanges which permit mechanical continuity but break the electrical continuity on a pipe. The flange faces NOTE - Exfoliation is generally oriented in the direction of rolling, extrusion or principal deformation. have insulating gaskets in between and the nuts and bolts are again insulated from the substantial flanged 2.170 Leveller faces. A substance which is added to a plating bath to 2.161 Interaction produce a levelling or smoothing action.
See Corrosion Interaction ( 2.63 ). 2.171 Levelling Action 2.162 Interaction Testing The ability of a plating bath to produce a surface which Routine investigation carried out when installing is smoother than that of the substrate. cathodic protection schemes on pipelines. 2.172 Limiting Current Density NOTE - The accepted criterion in the United Kingdom is that when the secondary structure potential has been The current density at which change of polarization moved more than 0.02 V in a positive direction, step produces little or no change of current density. must be taken to eliminate the interaction. 2.173 Line Wrapping 2.163 Intergranular Corrosion The technique of wrapping a pipeline over the pipeline Corrosion in or adjacent to the grain boundaries of a trench; applied only to welded steel pipelines. It is metal. carried out by a line-wrapping machine which travels 2.164 Internal Oxidation ( Subsurface Corrosion, along the pipe, cleaning and priming it, pulling in the Subscaie Formation ) reinforcement, and if necessary, applying the outer wrap. Formation of isolated particles of corrosion product ( or products ) beneath the metal surface, which may 2.174 Linear Polarization be coated with a uniform film of corrosion product. The linear relationship between overpotential and cur- 2.165 Ion rent density, that is. considered to prevail at An electrically charged atom or complex or atoms. potentials very close to the corrosion potential. 2.166 IS0 Corrosion Chart 2.175 Local Anodes and Cathodes A chart having temperature and concentration of the See Inseparable Anodes and Cathodes ( 2.159 )
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IS 3531 : 1997 2.176 Localized Corrosion 2.188 Normal Electrode Potential Corrosion at discrete sites of the metal surface See Electrode Potential ( 2.105 ). exposed to the corrosive environment. 2.189 Occluded Cell 2.177 Locating and Bonding A corrosion cell of a geometry that prevents When cathodic protection is applied to an existing intermingling of the anodic reaction products jointed pipeline, all joints must be located without ( anolyte ) with the bulk solution, resulting in a decrease digging up the pipe. On location, each joint is exposed in pH of the anolyte; shielded areas, pits, crevices and an electrically conducting bond, usually galvanized and cracks in the surface of the metal are the examples steel strip or copper cable, is welded into position. of an occluded cell.
2.178 Matt Surface 2.190 Open Circuit Potential A surface of low specular reflectivity. The potential of an electrode from which no signifi- 2.179 Metal Distribution Ratio cant current is flowing. The ratio of the thicknesses of metal produced during 2.191 Overpotential (Overvoltage, Polarization) electroplating on two specified parts of the cathode. The displacement of the equilibrium (or steady-state) 2.180 Metal Spraying electrode potential, required to cause a reaction to Application of a metal coating to a surface ( metallic proceed at a given rate. or non-metallic ) by means of a spray of metal parti- 2.192 Oxidation cles. The metal particles may be produced by ‘Atom- izing’ a metal wire in a flame-gun or by introducing Loss of electrons by a species during chemical or metal power into a similar gun. electrochemical reaction; addition of oxygen or removal of hydrogen from a substance. 2.181 Microbial Corrosion 2.193 Oxidizing Agent Corrosion associated with the action of micro- organisms present in the corrosion system. A substance that causes oxidation of another species.
2.182 Migration of Ions 2.194 Partial Reactions
Movement of ions towards the anode and cathode Anodic reaction ( reactions) and cathodic reaction under the influence of an electrical potential gradients. ( reactions ) constituting a single exchange process 2.183 Millscale or a corrosion reaction Thick oxide layer on metals produced during fabrica- 2.195 Partial Current ( Current Densities ) tion by hot rolling; most of the oxidation occurs as The currents ( current densities ) corresponding with the metal passes from one rolling mill to the next. each of the partial reactions. 2.184 Negative Potential 2.196 Parting See Potential ( 2.214 ). Selective dissolution of one metal, usually the most 2.185 Nearest Equation electro-reactive, from an alloy, leaving a residue of the The thermodynamic relationship between the revers- less reactive constituent. ible potential of the reaction ( or half-reaction ) and 2.197 Passivation Potential the activities of the species involved in the reaction. A corrosion potential, at which the corrosron current 2.186 Noble Potential has a peak value, and above which there is a range of See Potential ( 2.214 ). potential, where the metal is rn a passive state. 2.187 Non-electrochemical Corrosion, Chemical 2.198 Passivation Corrosion (Deprecated) Decrease of corrosion rate by the formation of a Corrosion not involving electrode reaction. corrosion product on the metal surface.
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2.199 Passivator into oxide during a high temperature oxidation reaction. A substance which in solution causes passivity. 2.206 Pits (In Electroplating) 2 200 Passive State, Passivity Macroscopic channels in an electrodeposit which may State of a corrosion system resulting from passivation. or may not extend to the substrate. NOTE - The passive state usually refers to the metal. 2.207 Pitting Corrosion 2.201 Passivity Corrosion resulting in pits, that is, cavities ektending A metal or alloy which is thermodynamically unstable from the surface into the metal. in a given electrolyte solution is said to be passive when it remains visibly unchanged for a prolonged 2.208 Pitting Initiation Potential period. The following should be noted: Lowest value of the electrode potential at which pit a) During passivation the appearance may change initiation is possible on a passive surface. if the passivating film is sufficiently thick, for NOTE - In some countries the pitting initiation potential example, interference films on titanium; has beencalled ‘Pitting potential’, whereas in other countries The electrode potential of a passive metal is the latter term refers to the potential achieved after b) pitting has become established. always appreciably more noble than its potential in the active state; and 2.209 Pitting Potential cl Passivity is an anodic phenomenon, and Potential at which a metal pit ( see also Critical Pitting control of corrosion by decreasing cathodic Potential, 2.79 ). reactivity, for example, amalgamated zinc in sulphuric acid or by cathodic protection is not 2.210 Polarization (Overpotential, Overvoltage) passivity. Difference of the potential of an electrode from its An alternative definition of passivity is that a metal in equilibrium or steady-state potential. a given solution is said to become passive when there 2.211 Potential@ Equilibrium Diagram is a significant decrease in the corrosion rate on rais- ( Pourbaix Diagram ) ing the potential of the metal above a certain critical value by an external emf or by adding an oxidant to Diagram with equilibrium potential and pH as the solution. co-ordinates, that show the phases that are at equilibrium when a metal reacts with water or an aqueous 2.202 Patina solution of a specified electrolyte. A green coating of corrosion products of copper 2.212 Pores, Porosity ( In Coating) ( basic sulphate, carbonate and chloride ) which forms on copper or copper alloys after prolonged atmospheric Microscopic channels in coatings ( metallic or non- exposure. metallic ) which extend to the substrate. 2.203 pH 2.213 Positive Potential A measure of the hydrogen ion activity defined See Potential ( 2.214 ). by pH = - log a”+, where aH is the activity of the hydrogen ion. 2.214 Potential 2.204 Pickel ( Pickling ) Potential difference at an electrode defined with reference to another ‘specified electrode. A solution ( usually acidic ) used to remove or loosen corrosion products from the surface of a metal. Anodic a) Base Potential - A potential towards the and cathodic pickling are forms of electrolytic pickling negative end of a scale of electrode potentials. in which the metal is anodically or cathodically polarized The potential of an electrode which is made in the pickel. cathodic is said to become ‘more base’; or ‘more negative’ (preferred term). 2.205 Pitting-Bedworth Ratio b) Negative Potential - A potential more Ratio of volume of oxide to volume of metal converted
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IS 3531 : 1997 negative than the potential of the standard 2.223 Protective Current hydrogen electrode. It is the total current to be picked up on the structure c) Noble Potential - A potential towards the so that it reaches protective potential. positive end of the scale of electrode potentials. 2.224 Protective Current Densrty The potential of an electrode which is made anodic is said to become more noble or more Density of the current, to or from the surface. that is, positive ( Preferred term ). required to maintain a potential in a protective poten- tial range. d) Positive Potential - A potential more posi- tive than the potential of the standard hydrogen 2.225 Protective Potential electrode. The potential of a structure with reference to a speci- 2.215 Potential/Current Density Curve; Polariza- fied standard reference half cell at and below which tion ( Curve Deprecated ) the structure stops corroding due to any electro- Diagram relating electrode potential to current lytic action. density. 2.226 Rack NOTE - The dependent variable can be either the elec- A frame for suspending and carrying current to article trode potential or the current density. during plating and related operations, 2.216 Polarization Resistance 2.227 Rate-Determining Step ( rds ) The quotient of electrode potential increment and current increment. The step in the sequence of steps of a reaction that has the highest activation energy and is thus rate NOTE - Usually the polarization resistance is measured controlling. in the vicinity of the free corrosion potential. 2.228 Reactive (Active Sacrificial ) Anode 2.217 Potentiostatic Polarization A mass of metal ( Mg, Zn and Al ) which buried or Polarization of an electrode during which the poten- immersed and connected to a metallic structure which tial is maintained at a predetermined constant value is to be protected, forms a cell with that structure and by means of a potentiostat. has the effect of making it more negative with respect 2.218 Primary Structure to the surrounding environment.
The basic structure to which cathodic protection is to 2.229 Redox Potential be applied, as distinct from a secondary structure, on The electrode potential of a reversible oxidation-re- which interference takes place. duction system like Cuz+ / Cu, Fe3+/Fe2, MnOJ Mn?, 2.219 Protective Coating Cl&l -.
Layer(s)of materialapplied to a metal surfaceto provide 2.230 Redox System corrosion protection, A reverisble oxidation-reduction system. 2.220 Protective Potential 2.231 Reducing Agent
The threshold value of the corrosion potential which A substance that causes reduction of another species. has to be reached to enter a protective potential range. 2.232 Reduction 2.221 Protection Potential for Pitting A chemical or electrochemical reaction in which a species Potential below which neither new pits can be gains electrons; the removal of oxygen or the addition initiated nor preexisting pits continue to propagate. of hydrogen.
2.222 Protective Potential Range 2.233 Reference Electrode
A range of corrosion potential values in which an Electrode, having a stable and reproducible potential, acceptable corrosion resistance is achieved for a which may be used in the measurement of other elec- particular purpose. trode potentials. 12
IS 3531 : 1997
2.234 Relative Humidity corrosion and stress, which is usually confined to the stress-corrosion cracking of brass in ammoniacal The ratio of the amount of water vapour present in the environments. atmosphere at a given temperature to the amount required for saturation at the same temperature, 2.245 Secondary Structure expressed as a percentage. A structure not in view when anodic protection is 2.235 Remedial Bond originally planned, but enters the picture as a result of interference. A bond established between a primary and secondary structure in order to eliminate or reduce corrosion 2.246 Selective Corrosion interaction. Corrosion of an alloy whereby the components react 2.236 Reversible Electrode in proportions which differ from their proportions in the alloy. An electrode in which a small increase or decrease in potential can reverse the direction of the electrode 2.247 Selective Leaching reaction. See Parting ( 2.196 ). 2.237 Reversible Potential 2.248 Sensitization See EquilibriumPotential ( 2.121). Suseptibility of intergranular attack in a corrosive 2.238 Rust environment resulting from heating a stainless steel at a temperature and time that results in precipitation Visible corrosion products consisting mainly of of chromium carbides at grain boundaries. hydrated iron oxides. 2.249 Service Corrosion Test 2.239 Rusting ( Rust ) Corrosion test conducted in service. Corrosion of iron or ferrous alloys resulting in a corrosion product which consists largely of hydrous 2.250 Sherardizing ferric oxide. The coating of iron and steel with zinc by heating in 2.240 Sacrificial Anode contact with zinc powder at a temperature below the See Reactive Anode ( 2.228 ). melting point of zinc. 2.241 Saponification 2.251 Shield A non-conducting medium which is used for altering Deterioration by softening of paint film caused by action of aqueous alkali, resulting from cathodic the current distribution on a cathode or an anode. protection at excessively high current densities, on 2.252 Simulated Corrosion Test the fatty-acid constituents of the film. Corrosion test conducted under simulated service 2.242 Saturation Index conditions. An index which shows if a water of given composition 2.253 Spalling andpH is at equilibrium, supemammted orunsaturated with respect to calcium carbonate or magnesium The break-up of a surface through the operation of hydroxide. internal stresses, often caused by differential heating or cooling. 2.243 Scale 2.254 Standard Electrode Potential Solid layer of corrosion products formed on a metal at high temperature. See Electrode Potential ( 2.105 ). NOTE - The term ‘Scale’ is also used in some countries 2.255 Steady-State Potential for deposits of hardness salt from water. The potential ( not varying with time ) of an electrode, 2.244 Season Cracking that is, operating under steady-state conditions of Cracking resulting from the combined effect of zero or constant current density. 13
IS 3531 : 1997 2.256 Stoichiometric Number 2.269 Sulphate-Reducing Bacteria ( SRB ) Number of times the rate-determining step must Species of anaerobic bacteria Desulphovibno, that occur for one act of the overall reaction. is, capable of causing rapid corrosion of iron and steel in near-neutral solutions in the absence of 2.257 Stray-Current dissolved oxygen. Current flowing through paths other than the intended 2.270 Sustained Load Failure circuits. Delayed failure due to the presence of hydrogen In 2.258 Stray-Current Corrosion stressed high tensile steels. Corrosion due to stray-current. 2.271 Symmetry Factor 2.259 Stress Intensity Factor ( Kisco ) Ratio of distance across the double layer to summit of energy barrier/distance across the whole double layer. A fracture toughness parameter used for evaluating susceptibility to stress-corrosion cracking. 2.272 Tafel Equation The linear relationship between the overpotential and 2.260 Stress Corrosion the logarithm of the current density for an electrode A process involving conjoint corrosion and straining reaction in which charge transfer is rate determining. of the metal due to residual or applied stresses. 2.273 Tarnish 2.261 Stress Corrosion Cracking Dulling, straining or discoloration of metals due to the formation of thin films of corrosion products. The Cracking due to stress corrosion. term can also be applied to thin transparent film which may give rise to interference colours. 2.262 Strike (n ) 2.274 Temporary Protection An electrolyte solution used to deposit a thin film of metal at and initial current density lower than what is Corrosion protection intended to last only for a normally used. limited time. 2.275 Thief 2.263 Strike ( vb ) An auxiliary cathode which is placed in a position To electroplate for a short time at a higher initial relative to the article to be electroplated so that the current density than is normally used. current density on certain areas is reduced. 2.264 Stripping 2.276 Throwing Power Removal of a metal coating by means of an electrolyte A measure of the ability of an ele&plating solution solution or an electrolyte solution and external emf. to give a uniform plate on a irregularly shaped cathode. 2.265 Structure to Electrolyte Potential 2.277 Transfer Coefficient (cc) Potential of a structure with reference to a standard The transfer coefficient of a cathodic process is the reference electrode located as close to the structure fraction of the electrical energy difference that assists as is practically permissible in the electrolyte or soil. the transfer of an ion through the double layer towards 2.266 Subscaie Formation the electrode and inhibits its transfer in the reverse direction; frequently experimental values of cl: are See Internal Oxidation ( 2.164 ). approximately0.5. 2.267 Substrate 2.278 Transpassivity
The basis metal on which a coating is present. Active behaviour of a metal at potentials more positive than those leading to passivity. 2.268 Subsurface Corrosion 2.279 Transpassive State See Internal Oxidation ( 2.164 ). State of anodically passivated metal characterized b!
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IS 3531 : 1997 a considerable increase of the corrosion current, in 2.283 Uniform Corrosion the absence of pitting, when the potential is increased. Corrosion proceeding at almost the same rate over the whole surface of the metal exposed to the corrosive 2.280 Transport (Transference) environment.
The term ‘transport’ is frequently used to include dif- 2.284 Voltaic Cell fusion as well as migration of an ion to an electrode, A term sometimes used for an electrochemical cell, it ( see Migration of Ions, 2.182 ). is sometimes used to refer a cell in which chemical changes are caused by the application of an external 2.281 Transport Number en-if
The proportion of the current carried by a particular 2.285 Water-Link Corrosion ion, it is also known as transfer number. Corrosion along, and as a consequence of the pres- ence of, a gas/liquid boundary. 2.282 Tubereulation 2.286 Weld Corrosion Localized attack in which the corrosion products form Corrosion associated with the presence of a welded warlike mounds over the corroded areas. joint and taking place in the weld or its vicinity.
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This Indian Standard hasbeen developed from Dot : No. MTD 24 ( 3558 )
Amendments Issued Since Publication
Amend No. Date of Issue Text Affected
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