Active-passive materials

As discussed in detail in Chapter 2, the corrosion potential is determined by the intersection of the sum of the anodic Evans lines and the sum of the cathodic Evans lines. For active-passive materials, the only new wrinkle is the increased complexity of the anodic line. Since the anodic line is not single-valued with respect to current density, three distinct cases can be considered. In all cases, the condition E /a = X Ic determines the position of the corrosion potential, and the condition im = z a - ic determines the appearance of the polarization curve... 

The cathodic reaction kinetics thus play an important role in determining the corrosion state for an active-passive material. The introduction of additional cathodic reactions to an environment or the change in the kinetics of one already present can dramatically affect the state of the material s surface. Figure 11 shows... 

Figure 15 Applied current densities required for different applied potentials for an active-passive material in acid. If there is a dissolution rate of 1 pA/cm2, cathodic protection to Ec would require an applied current density of 10,000 pA/cm2, while anodic protection to E4 would require only 1 pA/cm2. (After Ref. 21.)...

F ie 7.9 Schematic representation of a cyclic anodic polarization curve of an active-passive material in a chloride-containing environment pitting potential ( pu) and protection potential ( p ) are identified [1]... 

Active-Passive Material A metal or alloy that corrodes to an extent and then passivates due to an oxide film formation on its surface. 

The ease with which stainless steels can passivate then increases with the level of chromium within the alloy and so materials with higher chromium content are more passive (i.e. conduct a lower passive current density) and passivate more readily (i.e. the critical current density is lower and the active/passive transition is lower in potential). They are also passive in more aggressive solutions the pitting potential is higher. 

Pressurised water nuclear reactors require metals that will have a high degree of corrosion resistance to pure water at around 300°C. Laboratory testing of materials for this application have included potentiostatic polarisation experiments designed to clarify the active-passive behaviour of alloys as well as to establish corrosion rates. Since pressure vessels are used for this work, it is necessary to provide sealed insulated leads through the autoclave head . 

Honeywell has also been active in developing a combined active-passive oxygen barrier system for polyamide-6 materials [201]. Passive barrier characteristics are provided by nanoclay particles incorporated via melt processing techniques, while active contribution comes from an oxygen-scavenging ingredient (undisclosed). Oxygen transmission results reveal substantial... 

Water and electrolytes. Each day in an average adult, about 5.51 of food and fluids move from the stomach to the small intestine as chyme. An additional 3.5 1 of pancreatic and intestinal secretions produce a total of 9 1 of material in the lumen. Most of this (>7.5 1) is absorbed from the small intestine. The absorption of nutrient molecules, which takes place primarily in the duodenum and jejunum, creates an osmotic gradient for the passive absorption of water. Sodium may be absorbed passively or actively. Passive absorption occurs when the electrochemical gradient favors the movement of Na+ between the absorptive cells through "leaky" tight junctions. Sodium is actively absorbed by way of transporters in the absorptive cell membrane. One type of transporter carries a Na+ ion and a Cl ion into the cell. Another carries a Na+ ion, a K+ ion, and two Cl ions into the cell. 

The second approach is an adaptation of the voltammetry technique to the working environment of electrolytes in an operational electrochemical device. Therefore, neat electrolyte solutions are used and the working electrodes are made of active electrode materials that would be used in an actual electrochemical device. The stability limits thus determined should more reliably describe the actual electrochemical behavior of the investigated electrolytes in real life operations, because the possible extension or contraction of the stability window, due to either various passivation processes of the electrode surface by electrolyte components or electrochemical decomposition of these components catalyzed by the electrode surfaces, would have been... 

As a compromise between the above two approaches, the third approach adopts nonactive (inert) materials as working electrodes with neat electrolyte solutions and is the most widely used voltammetry technique for the characterization of electrolytes for batteries, capacitors, and fuel cells. Its advantage is the absence of the reversible redox processes and passivations that occur with active electrode materials, and therefore, a well-defined onset or threshold current can usually be determined. However, there is still a certain arbitrariness involved in this approach in the definition of onset of decomposition, and disparities often occur for a given electrolyte system when reported by different authors Therefore, caution should be taken when electrochemical stability data from different sources are compared. 

The required protection may be obtained by active, passive, or a combination of both protection systems. For example, steel support located in a fire exposed area within process unit battery limits may be protected by either a fixed water spray system or the application of fire resistant insulating material to the steelwork or possibly both. Note Passive protection is generally the preferable method for protecting structural steel. 

Active or passive material transfer, with a low degree of induced shear... 

For this study by Thompson et al (42), ion implantation and RBS were combined with more traditional electrochemical measurements to help establish the corrosion mechanisms of alloys in which a noble metal (Pt) was combined with an active/passive base metal (Ti). The alloys were created by ion implantation of Pt into pure Ti and were not of uniform bulk composition. Such surface alloys offer the possibilities of using a very small amount of a noble material to create a corrosion resistant coating on an otherwise chemically unstable but inexpensive metal or alloy. 

Figure 4 Schematic Evans diagram for a material that undergoes an active-passive transition. Important parameters that characterize this behavior are indicated.

In order to determine the corrosion state of an active-passive system, the position of the corrosion potential relative to pp must be determined. According to Fig. 4, if Econ is below Ew, the material will undergo uniform dissolution under film-free conditions. If EC0II is above Epp but below Et, the material will be passive and will dissolve at its passive current density, which is often on the order of 0.01 mpy. Corrosion-resistant alloys are designed to operate under such conditions. For situations in which Ec0II is above Et, the material will dissolve transpas-sively, i.e., uniformly. 

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

Figure 6 Schematic Evans diagram and resulting potential-controlled polarization curve for a material that undergoes an active-passive transition and is in an oxidizing solution. The heavy line represents the applied currents required to polarize the sample. If the sample did not undergo an active-passive transition, it would corrode at a much higher rate in this solution, as is indicated by the intersection of the dotted line and the cathodic curve.

Singbeil and Garner (10) showed that the use of anodic protection can prevent stress-corrosion cracking in the pressure vessel steels exposed to alkaline solutions used in digesters in the pulp and paper industry, as shown in Fig. 17. The 200 mV anodic polarization placed the material above the active-passive transition where cracking had been shown to occur (10). 

Localized corrosion is the direct result of the breakdown of passivity at discrete sites on the material surface. As was stated above, once passivity is established on a surface, one might expect either that it would remain passive or that a complete activation of the surface would occur. However, what is often observed in practice is the appearance of discrete areas of attack that begin to corrode actively while the vast majority of the surface remains passive. These isolated regions of attack are more than mere annoyances the local penetration rates can be on the order of 10 mpy or higher, leading to rapid perforation of any reasonably sized container. Since the original intent in using passive materials (e.g., CRAs) in any application is to exploit their low dissolution rates, localized corrosion can be a major operational problem. 

Figure 40 Double loop EPR data for Type 304 stainless steel heated at 600°C for 100 h (solid line) and 1 h (dotted line). The extremely sharp active-passive transition at —0.5 V(SCE) is due to ohmic drop effects. Note the much larger i, for the sensitized (100 h) material. (Data courtesy of M. A. Gaudett, University of Virginia.)...

Electrochemical testing and determination of polarization characteristics of every component are recommended. If one of the metals has active-passive behavior, the state of the contact material should be considered for the expected active and passive states. Both Pourbaix pH diagrams and the potential of the passive metal or alloy can be helpful for this purpose. Bacterial corrosion in case of intended media and conditions should be investigated. 

For materials that exhibit classical active-passive behavior, passivation is more conducive under static rather than dynamic conditions. For the latter, the frequency of cyclic loading is often one of the critical factors that influences CF in corrosive environments. Cathodic protection generally mitigates CF and SCC, but increases the probability of HEC of susceptible materials. 

The second part of the book consists of two chapters namely the forms of corrosion and practical solutions. The chapter, Forms of Corrosion consists of a discussion of corrosion reactions, corrosion media, active and active-passive corrosion behavior, the forms of corrosion, namely, general corrosion, localized corrosion, metallurgically influenced corrosion, microbiologically influenced corrosion, mechanically assisted corrosion and environmentally induced cracking, the types and modes of corrosion, the morphology of corroded materials along with some published literature on corrosion. 

---