Anodic Proteccion：阳极proteccion.doc

Anodic Proteccion. BackgroundMeasuring PolarizationMeasurement of corrosion rate is essential for the purpose of materialselection. The compatibility of a metal to its environment is a primerequirement for its reliable performance. Corrosion rate measurement maybecome necessary for the evaluation and selection of materials for a specificenvironment or a given definite application, or for the evaluation of new orold metals or alloys to determine the environments in which they aresuitable. Often the corrosive environment is treated to make it lessaggressive, and corrosion rate measurement of a specific material in theuntreated and treated environments will reflect the efficacy of the treatment.Corrosion rate measurement is also essential in the study of the mechanismsof corrosion.Aqueous corrosion is electrochemical in nature. It is therefore possibleto measure corrosion rate by employing electrochemical techniques. Twomethods based on electrochemical polarization are available: The Tafelextrapolation and linear polarization. Electrochemical methods permit rapidand precise corrosion-rate measurement and may be used to measurecorrosion rate in systems that cannot be visually inspected or subject toweight-loss tests. Measurement of the corrosion current while the corrosionpotential is varied is possible with the apparatus shown in Figure 1.4.Using the example of iron corroding in a hydrochloric acid solution, if theiron sample is maintained at the natural corrosion potential of K 0.2 V, nocurrent will flow through the auxiliary electrode. The plot of this data pointin the study would equate to that of A or C in Figure 1.5. As the potential israised, the current flow will increase and curve AB will approximate thebehavior of the true anodic polarization curve. Alternatively, if the potentialwere lowered below K 0.2 V, current measurements would result in thecurve CD and approximate the nature of the cathodic polarization curve.By using the straight line portions, or Tafel regions, of these curves,an approximation of the corrosion current can be made.Most often, it is the anodic polarization behavior that is useful inunderstanding alloy systems in various environments. Anodic polarizationtests can be conducted with relatively simple equipment and the scansthemselves can be done in a short time. They are extremely useful inAstudying the activepassive behavior that many materials exhibit. As thename suggests, these materials can exhibit both a highly corrosion-resistantbehavior and that of a material that corrodes actively, while in the samecorrodent. Metals that commonly exhibit this type of behavior include iron,titanium, aluminum, chromium, and nickel. Alloys of these materials arealso subject to this type of behavior.Activepassive behavior is dependent on the materialcorrodent combinationand is a function of the anodic or cathodic polarization effects thatoccur in that specific combination. In most situations where activepassivebehavior occurs, there is a thin layer at the metal surface that is moreresistant to the environment than the underlying metal. In stainless steels,this layer is composed of various chromium and/or nickel oxides thatexhibit substantially different electrochemical characteristics than theunderlying alloy. If this resistant, or passive, layer is damaged while in theaggressive environment, active corrosion of the freshly exposed surface willoccur. The damage to this layer can be either mechanical or electrochemicalin nature.The behavior of iron in nitric acid underscores the importance ofrecognizing the nature of passivity. Iron is resistant to corrosion in nitricacid at concentrations around 70%. Once passivated under these conditionsit can also exhibit low rates of corrosion as the nitric acid is diluted.However, if the passive film is disturbed, rapid corrosion will begin andrepassivation will not be possible until the nitric acid concentration is raisedto a sufficient level.PolarizationActivepassive behavior is schematically represented by the anodicpolarization curve shown in Figure 1.6. Starting at the base of the plot, thecurve starts out with a gradually increasing current, as expected. However,at point A, there is a dramatic polarizing effect that drops the current to apoint where corrosion is essentially halted. As the potential is increasedfurther, there is little change in current flow until the next critical stage B,where the breakdown of the passive film occurs and the corrosion currentbegins to increase.Even with an established anodic polarization behavior, the performance ofa material can vary greatly with relatively minor changes in the corrodent.This is also illustrated in Figure 1.7. Frame 1 illustrates the case where theanodic and cathodic polarization curves intersect similar to the behavior ofmaterials with no activepassive behavior. The anode is actively corroding ata high but predictable rate.Frame 2 represents the condition that is often found perplexing whenusing materials that exhibit activepassive behavior. With relatively minorchanges within the system, the corrosion current could be very low when thematerial is in the passive state or very high when active corrosion begins.Frame 3 typifies the condition sought when using materials in the passivestate. In this example, the cathodic polarization curve intersects only inthe passive region, resulting in a stable and low corrosion current. Thistype of system can tolerate moderate upset conditions without the onset ofaccelerated corrosion.The anodic polarization technique is also useful in studying the effectsof variations in the environment and the benefits of alloy conditions. Asillustrated in Figure 1.8, temperature increases can cause a shift of the curve tohigher currents. Increasing chromium contents in steel expands the passiveregion significantly; adding molybdenum raises the potential required for theinitiation of a pitting-type attack. The presence of chloride or other strongoxidizing ions will shrink the passive region.1.4 Other Factors Affecting CorrosionAs has been noted, temperature can have a significant influence on thecorrosion process. This is not surprising because it is an electrochemicalreaction, and reaction rates increase with increasing temperature. There areadditional influences on corrosion other than the corrodent itself.The relative velocities between the component and the media can have adirect effect on the corrosion rate. In some instances, increasing the velocityof the corrodent over the surface of the metal will increase the corrosion rate.When concentration polarization occurs, the increased velocity of the mediawill disperse the concentrating species. However, with passive materials,increasing the velocity can actually result in lower corrosion rates. Thisoccurs because the increasing velocity shifts the cathodic polarization curvesuch that it no longer intersects the anodic polarization curve in the activecorrosion region, as shown in Figure 1.9.The surface finish of the component also has an impact on the mode andseverity of the corrosion that can occur. Rough surfaces or tight crevices canfacilitate the formation of concentration cells. Surface cleanliness can also bean issue with deposits or films acting as initiation sites. Biological growthscan behave as deposits or change the underlying surface chemistry topromote corrosion.Other variations within the metal surface on a microscopic level influencethe corrosion process. Microstructural differences, such as secondary phasesor grain orientation, will affect the way corrosion manifests itself. Forcorrosive environments where grain boundaries are attacked, the grain sizeof the material plays a significant role in how rapidly the materials propertiescan deteriorate. Chemistry va