Instanteous corrosion rate

Atmospheric corrosion involves a series of processes with periods of high corrosion rates interrupted by periods of negligible corrosion rate. For a deeper understanding of atmospheric corrosion, there is a need for more sophisticated techniques for measuring instant corrosion rates coupled with monitoring the deposition of the most important corrosion stimulants. Activities in these and other areas are presently being carried out, and it is anticipated that atmospheric corrosion will continue to develop into a multidisciplinary scientific field. 

Measurements of polarization resistance Rp, given by LPR probes, can lead to measurement of the corrosion rate at a specific instant, since values of Rp are instantaneous. 

Corrosion likelihood describes the expected corrosion rates or the expected extent of corrosion effects over a planned useful life [14]. Accurate predictions of corrosion rates are not possible, due to the incomplete knowledge of the parameters of the system and, most of all, to the stochastic nature of local corrosion. Figure 4-3 gives schematic information on the different states of corrosion of extended objects (e.g., buried pipelines) according to the concepts in Ref. 15. The arrows represent the current densities of the anode and cathode partial reactions at a particular instant. It must be assumed that two narrowly separated arrows interchange with each other periodically in such a way that they exist at both fracture locations for the same amount of time. The result is a continuous corrosion attack along the surface. 

Linear polarization instruments provide an instantaneous corrosion-rate data, by utilizing polarization phenomena. These instruments are commercially available as two-electrode Corrater and three electrode Pairmeter (Figure 4-472). The instruments are portable, with probes that can be utilized at several locations in the drilling fluid circulatory systems. In both Corrater and Pairmeter, the technique involves monitoring electrical potential of one of the electrodes with respect to one of the other electrodes as a small electrical current is applied. The amount of applied current necessary to change potential (no more than 10 to 20 mV) is proportional to corrosion intensity. The electronic meter converts the amount of current to read out a number that represents the corrosion rate in mpy. Before recording the data, sufficient time should be allowed for the electrodes to reach equilibrium with the environment. The corrosion-rate reading obtained by these instruments is due to corrosion of the probe element at that instant [184]. 

Lpp is often approximately linear with potential within 5-10 mV of Ecorr" as shown for AISI 430 stainless steel in H2SO4 (Fig. 3). The slope of this plot, AE/Ai, when determined at E orr as shown in Fig. 3 defines the polarization resistance, which is inversely proportional to corrosion rate [26,27]. The surface area of the working electrode must be known. Knowledge of Rp, Pa, and p permits direct determination of the corrosion rate at any instant in time using Eq 26 [24,25,27,28]. 

Several measurements can be made after a coupon-type corrosion sensor has been attached to a cathodically protected pipeline. on potentials measured on the coupon are in principle more accurate than those measured on a buried pipe, if a suitable reference electrode is installed in close proximity to the coupon. The potentials recorded with a coupon sensor may still contain a significant IR drop error, but this error is lower than that of surface on potential measurements. Instant-OFF potentials can be measured conveniently by interrupting the coupon bond wire at a test post. Similarly, longer-term depolarization measurements can be performed on the coupon without depolarizing the entire buried structure. Measurement of current flow to or from the coupon and its direction can also be determined, for example, by using a shunt resistor in the bond wire. Importantly, it is also possible to determine corrosion rates from the coupon. Electrical resistance sensors provide an option for in situ corrosion rate measurements as an alternative to weight loss coupons. 

Hydrogen cyanide is moderately lipid-soluble, which, along with its small size, allows it to rapidly cross mucous membranes, to be taken up instantly after inhalation, and to penetrate the epidermis. In addition, some cyanide compounds, such as potassium cyanide, have a corrosive effect on the skin that can increase the rate of percutaneous absorption (NIOSH 1976). Information regarding dermal absorption in animals and evidence that cyanide can be absorbed through the skin of humans is provided in Sections 2.3.1.3 and 2.2.3, respectively. 

It is not possible to use Equation 10.39 to estimate the rate of corrosion at an instant of time because x is generally unknown. 

Epelboin, L Keddam, M. Takenouti, H. (1972) Use of Impedance Measurements for the Determination of the Instant Rate of Metal Corrosion. Journal of Applied Electrochemistry, Vol. 2, pp. 71-79, ISSN 0021-891X... 

The atmospheric corrosion of zinc starts with the instant formation of a thin film of zinc hydroxide, which seems to occur in different crystal structures, and the subsequent formation of a protective layer of basic zinc carbonate, Zn5(C03)2(0H)g. The pH of the aqueous layer controls the stability of initial corrosion products and results in the dissolution of Zn +. From the HSAB principle one expects Zn, classified as an intermediate acid, to coordinate with a number of different bases. In accordance with this, the prolonged exposure of zinc can proceed along a variety of different paths of reaction sequences depending on the actual deposition rates of atmospheric constituents. Among these Cl and SO2 seem to be the most important. 

For instant, the above relationship can be used to establish the equivalent of corrosion current of x A/cm with the rate of corrosion for iron in mpy as shown below... 

I. Epelboin, M. Keddam and H. Takenouti, Use of impedance measurements for the determination of the instant rate of metal corrosion, J. Appl. Electrochem., 1972,2, pp. 71-79. 

---