Argentum Solutions, Inc.
Sterling guidance on corrosion and materials degradation
| Potential-pH Diagrams |
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| Intelligent Tools |
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| Corrosion Calculator |
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| Corrosion Economics Estimator |
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| Additional Specialized Tutorials |
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Definitions of Terms |
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artificial neural network – a parallel distributed information processing structure consisting of directed links between points or nodes. The node is called a processing or computing element. Each node contains a transfer function which combines local memory with the inputs from the links to form an output to another node. Each node can have any number of links. The input to the network arrives from the outside world by directed links. The output from the network returns to the outside world by directed links. collective corrosion experience – the quantitative and qualitative knowledge developed independently by a number of corrosion practitioners that can be combined in a logical manner to create a computer-based corrosion prediction resource available to a broad spectrum of users. The knowledge can be structured using artificial intelligence to relate observation to measured variables. It can be made available to the most users externally via the internet or internally via an intranet. corrosion – the chemical or electrochemical reaction between a material and its environment that produces a deterioration of the material and its properties. This term is often used to describe the environmental deterioration of metals. corrosion modeling – this term describes the broad category of tools, often computer based, that can simulate a complete corroding system or portions of a corroding system. These simulations can be thermodynamic, kinetic, or empirical models. They can operate using fundamental equations, empirical correlations, expert systems, artificial neural networks, or combinations of these approaches. corrosion potential – the potential of a corroding surface in an electrolyte relative to a reference electrode measured under open-circuit conditions. This potential is not an equilibrium potential. When measured at steady state and used in conjunction with computer generated potential-pH diagrams or laboratory electrochemical tools, the value of this potential can sometimes provide information on the oxidation state of the surface. crevice corrosion – localized corrosion of a surface, usually a metal, at or immediately adjacent to an area that is shielded from full exposure to the environment because of close proximity between the metal and the surface of another material. Gaskets, joints, weld splash, and deposits can all create the geometrical configuration that can lead to crevice corrosion. electrical resistance probe – this device often used in on-line corrosion monitoring operates on the principle that the electrical resistance of the sensing element made from the alloy of interest increases as its cross sectional area decreases because of corrosion. Changes in resistance are measured indirectly by passing an electric current through the element. When properly calibrated, the change in resistance with time is proportional to the corrosion rate. Abrupt changes in the slope of the plot of resistance versus time can be useful in determining events causing abrupt changes in corrosion. electrochemical impedance – the frequency dependent, complex valued proportionality factor that relates the applied potential and the current response in an electrochemical cell. This factor becomes the impedance when the voltage perturbation and the current response are related linearly and the current response is caused only by the voltage perturbation. The value can often be related to the corrosion rate when the measurement satisfies the above criteria and is made at the corrosion potential. (See the tutorial on electrochemical impedance spectroscopy.) electrochemical noise – fluctuations of potential or current or both originating from uncontrolled variations in a corrosion process. These fluctuations often have to be evaluated statistically to determine if their structure is related to the corrosion process. Under some circumstances the quotient of the electrochemical potential noise divided by the electrochemical current noise is inversely proportional to the corrosion rate. erosion-corrosion – the combination of corrosion and erosion in the presence of a moving corrosive fluid leading to the accelerated loss of material. This term can be used in place of "fluid velocity sensitive corrosion" when the effect accelerates deterioration of the material. field tests - corrosion evaluations performed in the actual real world environment in which corrosion is occurring. Very often, the environment is uncontrolled making direct links between environmental variables and changes in corrosion difficult to establish. The closer the system behavior is to the situation encountered when corrosion occurs, the lower the risk of making an erroneous prediction. fluid velocity sensitive corrosion – a type of corrosion in which the flow of fluid past the surface of a material affects its deterioration. The result can be to accelerate deterioration, for example when the corrosion rate is controlled by mass transfer of a corrosion rate limiting species to or from the surface, by impingement of particles on the surface, or by differences in velocity turbulence across a surface. The result can be to decelerate deterioration, for example when passivation of a surface is controlled by the rate at which a dissolved inhibitor reaches the surface. In this case the amount of inhibitor required can decrease with increasing flow rate. The term "erosion-corrosion" can be used when the effect accelerates deterioration of the material. general corrosion – corrosion that proceeds at approximately the same rate across the entire surface. The term "general corrosion" sometimes is used specifically when regions are present where the corrosion rate may be slightly accelerated or decelerated next to adjacent areas. This term is often used interchangeably with "uniform corrosion" immersion test – a type of corrosion evaluation procedure in which an unused specimen is exposed to an environment for a period of time, appropriately cleaned after removal, and then examined for deterioration occurring during the exposure. The specimen especially an alloy is often referred to as a "coupon". The evaluation can be visual under an optical microscope and, especially in the case of non-metallic materials, mechanical (tensile, hardness, etc.). The test can be conducted in the laboratory or in the field. A related test, the sequential immersion test, in which a specimen is periodically removed from the environment and weighed so as to estimate diffusion into and out of the material during exposure can be used for non-metallic materials such as elastomers. inhibition - the reduction (not necessarily elimination) of corrosion by certain components in an environment. Chemical species are not the only components that can decrease corrosion. Other properties such as temperature, pressure, applied electrical potential, and fluid motion can act to decrease corrosion. inhibitor – a chemical species or combination of species that when present in the appropriate concentration or form reduce but not necessarily eliminate corrosion. These species can range from chemical species that chemically adsorb onto a surface to those that react with the surface to change its structure to a more inert state. This term is used as a descriptor for the chemical species that can inhibit corrosion. intelligent internet accessible tools – computer based engineering tools useful for corrosion prediction that can be accessed over the internet by a broad spectrum of users in diverse locations. These tools may contain artificial intelligence in the form of artificial neural networks and expert systems. They are useful for harvesting and updating the collective corrosion experience and making that information widely available. laboratory tests – corrosion evaluations performed in a controlled environment set up to predict corrosion occurring in the real situation. Such evaluations include those that accelerate corrosion behavior and those that model as closely as possible to actual corrosion behavior. The closer the laboratory system is to simulating the real situation that caused the corrosion, the lower the risk of making an erroneous prediction. localized corrosion - corrosion at discrete sites such as crevice corrosion, pitting corrosion, and stress corrosion cracking in which large parts of the adjacent surface are left virtually undamaged. Corrosion rates in this region are often orders of magnitude faster than on the rest of the surface. materials degradation – the chemical or electrochemical reaction between a material and its environment that produces a deterioration of the material and its properties. Though this terminology can be used to describe the environmental deterioration of all materials, it usually refers to deterioration of nonmetals such as plastics, resins, and elastomers. metallic materials – class of opaque, fusible, relatively ductile when solid, and often lustrous materials that are good electrical conductors because of non-localized electrons, good conductors of heat, and yield oxide and hydroxide compounds when they react with water. non-metallic materials – when referred to in corrosion terminology, this group of materials usually include all types of materials that are not metals. The classification includes elastomers, plastics, resins, glass, wood, and ceramics. Sometimes a fine line exists between non-metals and metals because non-metals may have some of the properties that are found in metals. (See the tutorial on the sequential immersion test for chemical compatibility of non-metallic materials.) pitting corrosion – localized corrosion of a surface, usually a metal, confined to a point or small area that is in the form of a cavity. Very often, these cavities take on a hemispherical configuration. They can become elongated in flow fields when corrosion is sensitive to fluid flow. polarization resistance – the slope at the corrosion potential of the potential versus current density curve. When the technique used to determine the slope is applied appropriately, this slope is inversely proportional to the corrosion rate (corrosion current density). The technique is useful for estimating corrosion rates of alloys in fluids supporting an electric current. polarization scan - an experimental procedure in which the potential (or sometimes current) is ramped at a constant slow sweep rate first in the anodic or noble direction until a certain current or potential is reached. Then the scan direction is reversed. The reverse scan is usually continued beyond the open circuit potential. The relationship between the voltage and the applied current is used to predict the propensity of the alloy to suffer pitting corrosion, crevice corrosion, or general corrosion. Sometimes, corrosion rates can be estimated from the relationship. (See the tutorial on polarization scans.) potential-pH diagrams – thermodynamic diagrams often computer generated that plot the lowest free energy metal-containing species in a metal-environment system as a function of the hydrogen ion activity and the electrochemical potential. The environment is often aqueous with appropriate ionic constituents. Ionic complexing agents can be included. When tailored to a specific system, these diagrams can provide a practical corrosion road-map with an ability to predict what type of corrosion products might be produced. Kinetic measurements are required to demonstrate what actual corrosion would occur. reference electrode - an electrode whose potential is stable and reproducible. The potential can be created by a steady state reaction on the surface or by means of an electrochemical equilbrium between the surface and the environment. Reference electrodes can be used directly immersed in the environment or isolated from the environment by means of a connection with limited diffusion. rotating cylinder electrode – this laboratory device is a cylinder within a cylinder in which one of the cylinders (usually the inner cylinder) rotates and the other is stationery. The apparatus operates under well-defined turbulent flow conditions to low rotation rates. The device is especially useful for simulating and predicting mass transfer affected corrosion that can occur in a number of field geometries, such as a pipe, flow expansion, non-circular duct, impinging jet, and annulus. (See the tutorial on the rotating cylinder electrode .) uniform corrosion – a type of corrosion (deterioration) distributed evenly over the entire surface. The term "uniform corrosion" is sometimes used specifically when every site on a metal surface can act simultaneously as an anode or cathode. This term is often used interchangeably with the term "general corrosion". A number of the definitions were adapted from (1) "Standard Terminology Relating to Corrosion and Corrosion Testing", ASTM Standard G 15-02, Vol. 03.02, Annual Book of ASTM Standards and (2) "Glossary of Terms", "Corrosion: Fundamentals, Testing and Protection", Vol. 13A, ASM Handbook, 2003. |
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David C. Silverman, Ph.D. - Primary Consultant |