Coating defects, location

Figure 3-28 shows, as an example, results obtained from an intensive measurement of a short section of pipe which could be very strongly polarized to increase the sensitivity of defect location. From Eq. (3-51 a) Tq = 4 mm at = 0.1 V. In the results in Fig. 3-29, rg = 9 cm at AU = 0.1 V. These results are clear indications of water traps resulting from a poor coating [46]. Further examples are shown in Section 3.7. 

In addition to the constraints mentioned above, active areas must be sufficiently long-lived and nearly fixed in position for detection by SVET. This is not normally a problem in the study of coatings where the location of active areas are often fixed by existing or emergent coating defects. However, some pitting phenomena, especially metastable pitting, may not be detected well by this technique. 

PANI/P-PVA (partially phosphorylated poly(vinyl alcohol)) nanoparticles were successfully dispersed in epoxy resin applied to steel [82]. P-PVA is fundamental to obtain a uniform dispersion of PANI nanoparticles, and this fact is responsible for the uniform formation of Fe Oj passive layer at the interface between coating and substrate and therefore for its effectiveness in corrosion protection. PANI was also used in combination with DBSA to be added to epoxy-ester (EPE) system to form a smart anticorrosion coating [73]. DBSA is used as both surfactant and doping agent. By EIS measurements it is deduced that the better anticorrosion performance of PANI (DBSA)/EPE coatings with respect to simple EPE is due to the formation of a second barrier layer by reaction between released DBSA anions and Fe cations at the defective locations of the coating. 

Pearson surveys, named after its inventor, are used to locate coating defects in buried pipelines. Once these defects have been identified, the protection levels provided by the CP system can be investigated at these critical locations in more detail. 

In principle, a Pearson survey can be performed with an impressed CP system still energized. However, sacrificial anodes should be disconnected since the signal from these may otherwise mask actual coating defects. A three-person team is usually required to locate the pipeline, perform the survey measurements, place defect markers into the ground and move the transmitters periodically. 

Fig. 3-26 Location of defects in the pipeline coating with ac by the Pearson method arrangement 1 parallel to the pipeline arrangement 2 at right angles to the pipeline (see caption to Fig. 3-24).

Four classes of LDL receptor mutations have been identified. Class 1 mutations are characterized by the failure of expression of the receptor protein. It is possible, however, that a modified protein is produced but it is not recognized as an LDL receptor protein. Class 2 mutations involve a nonsense mutation (premature termination of protein synthesis Chap. 17), and result in a defect in the transfer of the receptor from the endoplasmic reticulum to the cell membranes. This class of mutation is common in Afrikaners and Lebanese. The Watanabe heritable hyperlipidemic rabbit (WHHL) is an animal model which has a Class 2 defect and has been used extensively for the study of familial hypercholesterolemia. Class 3 mutations result in abnormal binding of LDL. This can be caused by alterations in the amino acid sequence of Domain 1. Class 4 mutations are those with defective internalization due to the receptor s inability to be located in coated pits. This is the result of mutations in the fifth, C-terminal domain. 

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