Corrosion curve-fitting techniques

Haynie and Upham (1970) compiled data for the atmospheric sulfur dioxide level, relative humidity, and zinc corrosion rate for eight U.S. cities. Using regression and curve-fitting techniques, they developed the equation ... 

Several techniques have been suggested that are intermediate between the three-point technique and the curve-fitting technique. They allow the determination of the corrosion current density and the Tafel slopes from measurements taken near the corrosion potential by using, usually, a graphical approach. " Performance that is superior to the three-point technique is generally claimed. 

Figure 8. Absolute value of the error in corrosion current density due to the neglect of mass transport for curve-fitting technique. (61 points between -30 and -l-30mV). ...

Corrosion scientists seldom have enough replicate data to analyze distributions using such techniques as histograms and tests of fit. Sometimes, however, enough data are available to order and plot as a function of linearized forms of the different kinds of probability distributions. Also, nonlinear least squares curve fitting may be used. The best approach is to choose the type of distribution that cleeirly produces the best approximation to a straight line. If an extreme value distribution is only slightly better than a normal distribution, a normal distribution may be assumed because the statistical techniques are easier to use and more universally understood. 

Until the last few years the most common technique for measuring reaction rates under pressure (and it is one that is still frequently used) has been to confine a sample of the mixture in a tube fitted with a movable piston and place it in a pressure vessel. When the vessel is pressurized the piston moves and transmits the pressure to the mixture. Many reaction mixtures corrode materials used for pressure vessels, and the corrosion products might infiuence the rate of the reaction. The reaction mixture and the pressure vessel are therefore not usually in contact. For the same reason the reaction mixture is not usually in contact with the hydraulic fiuid. After a known time, the pressure is reduced, and the sample is removed and analyzed. Other samples are put into the vessel and left for different times, and so a series of points on the rate curve can be determined. When liquids are compressed adiabatically their temperature increases according to the relation... 

Equation (41) is identical in form to Eqs. (18 and 24). The curve is centered around Ecorr rather than and the current density at zero overpotential is icorr instead of io- This expression, along with the theory for mixed potentials, was derived by Wagner and Traud, and therefore will be referred to as the Wagner-Traud equation. As described in the Chapter 7.3.1.2 on experimental techniques, the Wagner-Traud equation is used in software analysis packages that accompany modem computer-controlled potentiostats. A nonlinear least squares fit of this equation to the experimental data provides values of corr. corr. ha. and he vvith the assumption that perfect Tafel behavior is observed for both the anodic and cathodic reactions, and that the extrapolations of the Tafel portions of the curves both intersect at the corrosion potential. 

An extension of the ECT method described above that has been used to evaluate alloys for service on North Sea oil platforms [97,98] htis been referred to as cyclic ther-mammetry [99]. The technique involves exposing a coupion fitted with an MCA to a test solution under eui applied potential that simulates a process environment. The current density of the crevice specimen is monitored eis the temperature of the test solution is slowly chemged (e.g., 4°C/24 h) in a cychc mEinner up to a critical vcJue smd then back towEird room temperature. The test results Eire in the form of a curve (Fig. 5) that looks very much like a conventional Emodic polEirization curve [97] with the exception that temperature rather than applied potential is the independent variable. VEdues for a critical crevice corrosion... 

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