Active corrosion current density

These factors can be discussed with reference to the polarization curves for the initial and changing conditions within the occluded region. The combined effects of a potential drop into the pit and the effect of the lowered pH, which raises Epp and increases icrit, are also analyzed by reference to Fig. 7.6 (Ref 20). As previously assumed, the solid anodic curve is taken as representative of a stainless steel in an environment of pH = 1. The dashed extension again represents the anodic polarization behavior in the absence of a passive film. At a potential, Ecorr (or Epot if the potential is maintained potentiostatically), the passive current density would be iCOrr,pass and the active corrosion current density would be iCorr,act- Assume that a small flaw through the passive film is associated with an (IR), drop that lowers the potential in the bottom of the flaw to E,. Since this potential is higher than the passivating potential, Epp, this flaw should immediately repassivate and not propagate. 

A somewhat alternative analysis of pitting attributes pit initiation to the activation of defects in the passive film, defects such as those induced during film growth or those induced mechanically due to scratching or stress. The pit behavior is analyzed in terms of the product, xi, a parameter in which x is the pit or crevice depth (cm), and i is the corrosion current density (A/cm2) at the bottom of the pit (Ref 21). Experimental measurements confirm that, for many metal/environment systems, the active corrosion current density in a pit is of the order of 1 A/cm2. Therefore, numerical values for xi may be visualized as a pit depth in centimeters. A defect becomes a pit if the pH in the pit becomes sufficiently low to prevent maintaining the protective oxide film. Establishing the critical pH, for a specific oxide, will depend on the depth (metal ions trapped by diffiisional constraints), the current density (rate of generation of metal ions) and the external pH. In turn, the current density will be determined by the local electrochemical potential established by corrosion currents to the passive external cathodic surface or by a potentiostat. Once the critical condition for dissolution of the oxide has been reached, the pit becomes deeper and develops a still lower pH by further hydrolysis. 

Applications of Rp techniques have been reported by King et al. in a study of the corrosion behavior of iron pipes in environments containing SRB. In a similar study, Kasahara and Kajiyama" used Rp measurements with compensation of the ohmic drop and reported results for active and inactive SRB. Nivens et al. calculated the corrosion current density from experimental Rp data and Tafel slopes for 304 stainless steel exposed to a seawater medium containing the non-SRB Vibrio mtriegens. 

Figures 16.8 and 16.9 show only the anodic polarization curves for corrosion cells. The important question is, where do these curves intersect with the polarization curves for likely cathodic reactions, such as hydrogen evolution or oxygen absorption The intersection point defines the corrosion current density icorr and hence the corrosion rate per unit surface area. As an example, let us consider the corrosion of titanium (which passivates at negative Eh) by aqueous acid. In Fig. 16.10, the polarization curves for H2 evolution on Ti and for the Ti/Ti3+ couple intersect in the active region of the Ti anode. To make the intersection occur in the passive region (as in Fig. 16.11), we must either move the H+/H2 polarization curve bodily...

It will be shown later that the values of icrit, Epp, and ip, which are the important parameters defining the shape of the active-passive type of polarization curve, are important in understanding the corrosion behavior of the alloy. In particular, low values of icrit enhance the ability to place the alloy in the passive state in many environments. For this reason, the maximum that occurs in the curve at B (Fig. 5.4) is frequently referred to as the active peak current density or, in general discussion, as the active peak. It is the limit of the active dissolution current density occurring along the A region of the polarization curve. 

Fig. 5.45 Schematic polarization curves fortype 304 stainless steel in aerated 1 N H2S04. L (low) and H (high) distinguish the effects that minor composition variables can have on the position of the active peak current density (icrit) in the stainless steel polarization curve. Estimated corrosion potentials and corrosion current densities are shown. In particular, note that corrosion can occur in the active or passive potential range depending on the position of icrit-...

Based on the data presented in Fig. 5.42, for each element/electro-lyte listed below, state whether active or passive corrosion occurs and give the corrosion current density, icorr- In each situation, assume the worst-case condition. 

If the flaw in the passive film is smaller in cross section and greater in depth, then with reference to Fig. 7.6, the resulting increase in resistance can lead to an (IR)2 potential drop that decreases the potential in the bottom of the flaw and/or pit to E2. Then passivity cannot be maintained, and the corrosion current density increases to i2 in the active range. The local corrosion rate is much higher, and a stable pit is initiated at the much higher current density. When the pH of the bulk envi-... 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

Although most corrosion systems can be described by the limiting models presented above, there are instances where control of the corrosion system is a combination of both types, viz., activation controlled anodic partial process with two cathodic partial processes - one under activation control and another under transport control. Examples are iron corrosion in acid solution with inorganic contaminants (, 18) and oxygen ( ). The corrosion current density in such systems is... 

The first question that might be of interest is to determine if the material passivates or undergoes uniform active corrosion in the relevant environment. If the form of corrosion is active corrosion, then the corrosion rate needs to be measured, and a determination can be made if there is sufficient material to survive the lifetime requirements. Corrosion rate, r (units of thickness loss per unit time), is related to a corrosion current density, i corr (A cm ), which is the outcome of most electrochemical tests, by way of Faraday s law ... 

Figure 9.1 Determination of corrosion current density by extrapolation of linear parts of the polarization curves, a) Both the cathodic and the anodic reaction are tmder activation control (the overvoltage curves are Tafel tines), b) The cathodic reaction is diffusion controlled and the anodic reaction activation controlled, c) The cathodic reaction is activation controlled, the anodic curve is irregular, d) The cathodic curve is irregular, the metal is passive, i.e. the corrosion current equals the passive current.

Specific polarization resistance values for metallic materials of 10 Qcm or lower indicate the presence of an active sample surface, while values around 100 x iQs Qcm or higher indicate a passive sample surface. The corrosion current density of the material covered by a passive surface film, ipass, is then calculated as follows ... 

The corrosion current density of the active material, iaa, can now be obtained by substituting rpass with ract in Equation (23) ... 

The second corrosion case (combined activation and concentration polarization for hydrogen reduction and activation polarization for oxidation of metal M) is treated in a like manner. Figure 17.11 shows both polarization curves as earlier, corrosion potential and corrosion current density correspond to the point at which the oxidation and reduction lines intersect. 

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