PIT Process

Figure 4.34 illustrates, by means of potential/anodic current density curves, the influence of pH and Cl ions on the pitting of nickel The tendency to pit is associated with the potential at which a sudden increase in anodic current density is observed within the normally passive range ( b on Curve 1 in Fig. 4.34). It can be seen that in neutral 0-05 M Na2S04 containing 0-02m Cl" (Curve 1) has a value of approximately 0-4 V h- When pitting develops, the solution in the pits becomes acidic owing to hydrolysis of the corrosion product (see Section 1.6) and when this occurs the anodic current density increases by at least two orders of magnitude and tends to follow the curve obtained in 0 05 m H2SO4-t-0-02 m NaCl (Curve 2). Comparison of Curves 2 and 3 illustrates the influence of Cl" ions on the pitting process.

As part of the pitting process, tuberculation tends to develop especially when flow rates are low (as they often are in HW heating boilers). The pitting corrosion initiator may be shielding rust or deposits and foulants swept along to the area of lower flow. 

Three basic mechanisms of pit initiation have been advocated in the literature (see, e.g., Strehblow117) as applying to pitting processes at any passive metal. They are shown schematically in Fig. 23. 

The methods in the second group leading to the potential-independent CPT are practically independent of the surface condition and therefore more likely reflect the nature of the propagation part of the pitting process and characterize the steel s bulk material. 

Besides W/O and O/W emulsions there are so-called multiple emulsions of the W/O/W type. These emulsions can be produced in a one-stage modified PIT process [32] or by emulsification of a primary W/O emulsion in an outer water phase. These systems are an approach to protect sensitive active substances such as vitamins or enzymes in a formulation. 

There are essentially two ways to prepare nanoemulsions. These are the phase inversion temperature (PIT) process and the high-pressure homogenization (HPH) process. 

Unfortunately, the droplet size distribution of a nanoemulsion prepared by the PIT process is relatively large. Due to the high Laplace pressure, Ostwald ripening takes place rapidly, limiting the lifetime of the nanoemulsions to a few minutes to a few days. The addition of a water-insoluble component can significantly reduce the breakdown kinetics however, long-term stability is rarely achieved with this process. 

Nano wax dispersions which are prepared by a PIT process can be added to hair shampoo formulations [67]. The formulations improve the combability of wet hair and accelerate the hair drying after the wash. Wax dispersions can also be used to carry fragrance-active components. These additives can be used to give a long-lasting fragrance impression for cosmetic product applications. 

An explanation of the effect of convective mass transfer on the growth of a single pit was provided by Beck and Chan [112]. They suggested that the current due to the pitting process could be limited by... 

Figure 3.3, while the main pitting process is shown in Figure 2.4. The effective recycling of chloride ions makes chloride attack more difficnlt to remedy as chlorides are therefore harder to eHminate.

Valve Pits - Process Piping Plan - Sections and Details Process Water Pipe Tunnel - Support Details and Piping... 

FIGURE 8.1.12 Cross-sections of PIT processed Bi-2223 (upper Sumitomo Electric) tape and (lower Hitachi Cable) Bi-2212 round wire in which 18 mulufilamentary tapes are assembled. 

FIGURE 8.1.13 Jc-H curves of PIT processed Bi-2223 and Bi-2212 superconducting tapes at temperatures of 4.2, 20, and 77 K. Jc-H curves ofconventionalNb-Ti and NbsSn wires are also included for comparison. 

Stockert, L. and Boehni, H., Metastahle Pitting Processes and Crevice Corrosion on Stainless Steels, Advances in Localized... 

An additional unique feature of the in vivo corrosion environment is the existence of bioelectric effects. These are potentials and ionic currents of physiological origin resulting from nerve and muscle activity, heart and brain function, stresses applied to skeletal tissues, etc. Since their magnitudes are small, usually they cannot be expected to have much influence on in vivo corrosion processes. In cases of border-line passivity, however, it is possible that these potentials could polarize portions of implant surfaces sufficiently to exacerbate pitting processes [74]. 

The strongly anisotropic shape of the crystallites favors the development of a texture parallel to the surface of the Ag tube in powder-in-tube (PIT) processing, and this is highly desirable for the superconducting properties. The Ag content also serves as a source of pinning centers that hinder the flux line motion. 

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