Time dependence effects corrosion-rate measurements

The effect of changing system parameters (e.g., corrosion potential, Tafel slopes, etc.) has been treated qualitatively. The effect of double-layer charging time has also been dis-cussed. " In most cases, the charging time is negligible compared to the usual measurement times, but unreasonably long measurement times, on the order of hours, may be required for systems with very low conductivity solutions and for low corrosion rates (that is, when the rc time constant of the double layer is large). For transient experimental techniques, the time-dependent effects manifest themselves as frequency or scan-rate dependence of the results. - -... 

This last effect may be an indication of adsorption of a small impurity in the electrolyte. The inhibited corrosion rates decrease with time and become essentially constant after about two hours. These slopes are not dependent on scan rate or on corrosion rate. The most interesting effect is observed when the inhibited hydrochloric acid solution is aerated the anodic Tafel slope increases while the cathodic Tafel slope decreases dramatically. As would have been expected from the resistance probe measurement the corrosion rate in the aerated inhibitor solution increases. 

Both the H2S concentration over the range of 0-1.0 v/o of the CGA gas and the temperature controlled the measured corrosion rates. Figure 1 illustrates the effect of temperature on corrosion rates of several alloys and coatings in the CGA gas containing 1 v/o H2S. Alloys AISI 309, AISI 310, and IN-800 demonstrate a clear temperature dependence of total oxidation-corrosion in 1000 hr. The 309 alloy had a scatter band of 5 to 125 mils total metal loss for four specimens at 1650°F. This is typical of borderline alloys that undergo time-dependent transitions to accelerated corrosion rates. Total corrosion of aluminized 310 and 800 was relatively unaffected by temperature over the range of 1500°-1800°F for 1000 hr exposures. 

Electrochemical corrosion techniques are essential in predicting the service hfe ofmetal-hc components used in chemical and construction industries. They measure the corrosion rates, the oxidizing power of the environment, and evaluate the effectiveness of corrosion protection strategies. The following direct current dc electrochemical methods are discussed in this chapter linear polarization technique, Tafel extrapolation, and open circuit potential (OCP) vs. time measurements. Electrochemical impedance spectroscopy (EIS) is introduced as an alternating current technique ac. This technique uses alternating current to measure frequency-dependent processes in corrosion and estimates the change of polarization resistance. 

Different transient techniques have also been suggested for the measurement of corrosion rate. Pulse techniques can be used to eliminate from the polarization data the effects of uncompensated solution resistance and mass transport, or to minimize the effect of time-dependent phenomena. However, these techniques must be used with caution because the classical electrode kinetic theory can be used in the data evaluation only if /corrA/<0.9. The square-wave techniqueand ac impedance techniquehave also been used to measure the polarization resistance. The linear potential scan (potentiodynamic) technique has been used to obtain the polarization curve or the polarization resistance (small-amplitude cyclic voltammetry and exponential scan techniques were also proposed to determine the polarization curve. 

The corrosion test is designed to measure the rate of corrosion of a very thin copper wire. It is critical that the copper wire be thin because the test depends upon the difference in resistance of the wire over time. Therefore, no solder is used in preparing the test coupons—only flux, solder paste flux, or cored-wire flux should be applied to the copper anode and heated to soldering temperatures. This test provides complementary information to the SIR test that measures the insulating characteristics of the laminate. SIR readings, however, reflect the combined effect of (a) residues that are corrosive to the conductor tracks and (b) residues that interact with the laminate. 

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