layer can be merely a trace of innocuous soil or a residue of corrodents or of corrosion products. It also can differ in composition from the bulk metal Corrosion Testing Various types of corrosion-testing techniques are used to investigate corrosion failures and to evaluate the resistance to corrosion of metals and alloys for service in specific applications. They include accelerated tests, simulated-use tests, and electrochemical tests. Other techniques include monitoring performance in pilot-plant operations and in actual service Some accelerated-test methods have been accepted as standard by both the military and industry. To shorten testing time corrosion is accelerated in relation to naturally occurring corrosion, usually by increasing temperature, using a more ggressive environment, or increasing stress(e.g, SCC) Because various factors influence natural corrosion processes and differ widely in time dependence, the results laboratory tests must be interpreted carefully. They can be related to expected actual service behavior only when close correlation with long-term service results has been established Simulated-use tests are used frequently to analyze corrosion behavior of metals and alloys in specific applications. In these tests, either actual parts or test specimens are exposed to a synthetic or natural service environment Electrochemical tests provide data that can establish criteria for passivity or anodic protection against corrosion and determine critical breakdown or pitting potentials. The two general methods of electrochemical corrosion testing are controlled current and controlled potential. For either test method, AstM G 3 provides useful guidance and standardization of the manner of recording and reporting electrochemical measurements In the controlled-current method the current(a measure of the corrosion rate) is controlled and the resulting corrosion potential is measured. Several instruments are available for such tests, in which either logarithmic or linear polarization curves are developed. Both galvanostatic and galvanodynamic polarization measurements are employed to plot anodic and cathodic polarization curves In the controlled-potential method of electrochemical testing, the corrosion potential(oxidizing power) is controlled and the resulting corrosion current is measured. Equipment is available for both constant-potential (potentiostatic) and varying-potential(potentiodynamic)testing to determine overall corrosion- rate profiles for metal-electrolyte systems over a range of potentials For the most part, electrochemical tests are more valuable for evaluating the corrosion resistance of materials per se rather than for direct use as a failure analysis tool Tests for Corrective Action Once the failure cause and mechanism have been established the conclusions and the effectiveness of recommended corrective actions can be confirmed through testing. These tests should simulate as closely as possible the environmental and mechanical conditions to which the failed part was subjected in service. Ideally, the part should be tested in service however, such testing frequently is not feasible because of factors such as length of time to failure or extraneous damage caused by failure of the part. Thus, simulated-service tests may have to be performed in the laboratory. The following parameters should be carefully controlled Environmental factors: Temperature, which may be steady or fluctuating, and also may affect stress Single-phase or two-phase environment, which may involve alternate wetting and drying Environmental composition, including major and minor constituents, concentration and changes thereof, dissolved gases, and pH Electrochemical conditions, which may involve galvanic coupling or applied cathodic protection Mechanical factors Loading, which may be static or cyclic--if cyclic, mean stress may be zero, tensile, or compressive; also, stress- wave shape and period must be defined Surface damage, which may occur by fretting, abrasion, cavitation, or liquid-impingement corrosion Practices in Failure Analysis Thefileisdownloadedfromwww.bzfxw.comlayer can be merely a trace of innocuous soil or a residue of corrodents or of corrosion products. It also can differ in composition from the bulk metal. Corrosion Testing Various types of corrosion-testing techniques are used to investigate corrosion failures and to evaluate the resistance to corrosion of metals and alloys for service in specific applications. They include accelerated tests, simulated-use tests, and electrochemical tests. Other techniques include monitoring performance in pilot-plant operations and in actual service. Some accelerated-test methods have been accepted as standard by both the military and industry. To shorten testing time, corrosion is accelerated in relation to naturally occurring corrosion, usually by increasing temperature, using a more aggressive environment, or increasing stress (e.g., SCC). Because various factors influence natural corrosion processes and differ widely in time dependence, the results of laboratory tests must be interpreted carefully. They can be related to expected actual service behavior only when close correlation with long-term service results has been established. Simulated-use tests are used frequently to analyze corrosion behavior of metals and alloys in specific applications. In these tests, either actual parts or test specimens are exposed to a synthetic or natural service environment. Electrochemical tests provide data that can establish criteria for passivity or anodic protection against corrosion and determine critical breakdown or pitting potentials. The two general methods of electrochemical corrosion testing are controlled current and controlled potential. For either test method, ASTM G 3 provides useful guidance and standardization of the manner of recording and reporting electrochemical measurements. In the controlled-current method, the current (a measure of the corrosion rate) is controlled and the resulting corrosion potential is measured. Several instruments are available for such tests, in which either logarithmic or linear polarization curves are developed. Both galvanostatic and galvanodynamic polarization measurements are employed to plot anodic and cathodic polarization curves. In the controlled-potential method of electrochemical testing, the corrosion potential (oxidizing power) is controlled and the resulting corrosion current is measured. Equipment is available for both constant-potential (potentiostatic) and varying-potential (potentiodynamic) testing to determine overall corrosion-rate profiles for metal-electrolyte systems over a range of potentials. For the most part, electrochemical tests are more valuable for evaluating the corrosion resistance of materials per se rather than for direct use as a failure analysis tool. Tests for Corrective Action Once the failure cause and mechanism have been established, the conclusions and the effectiveness of recommended corrective actions can be confirmed through testing. These tests should simulate as closely as possible the environmental and mechanical conditions to which the failed part was subjected in service. Ideally, the part should be tested in service; however, such testing frequently is not feasible because of factors such as length of time to failure or extraneous damage caused by failure of the part. Thus, simulated-service tests may have to be performed in the laboratory. The following parameters should be carefully controlled: Environmental factors: · Temperature, which may be steady or fluctuating, and also may affect stress · Single-phase or two-phase environment, which may involve alternate wetting and drying · Environmental composition, including major and minor constituents, concentration and changes thereof, dissolved gases, and pH · Electrochemical conditions, which may involve galvanic coupling or applied cathodic protection Mechanical factors: · Loading, which may be static or cyclic—if cyclic, mean stress may be zero, tensile, or compressive; also, stress￾wave shape and period must be defined · Surface damage, which may occur by fretting, abrasion, cavitation, or liquid-impingement corrosion Practices in Failure Analysis The file is downloaded from www.bzfxw.com