PiT measurement

A characteristic of the corrosion on buried ferrous metals is that the attack is usually mostly in the form of pitting, especially with the cast irons. This raises a problem in measuring the extent of corrosion in burial trials. Usually both the weight loss, measuring the average loss of section, and the deepest pit, measuring the maximum loss of section, are reported. For assessing the severity of the attack on buried pipes, the second parameter is clearly the most important. 

Figure 4.35 illustrates the effect of temperature on the rate of development of pitting, measured as a corrosion current in an acidic solution containing Cl it is seen that quite small increments in temperature have large effects. The influence of temperature is of considerable significance when metals and alloys act as heat transfer surfaces and are hotter than the corrosive environment with which they are in contact. In these circumstances. 

Figure 16. Electrode for pitting measurements with cylindrical specimens using the Avesta cell principle. The modification of the specimen holder is described in ASTM GS. Designed by Mathiesen and Maahn. (Reprinted from Ref. 73 by permission of the authors.)...

In general, limited-dispersion conditions arc realized by reducing as much as possible the distance be tween injector and detector, slowing the flow rate, and increasing the sample volume. Thus, for the pit measurements just described, the length of0.5-mfn tubing was only ID cm and the sample size was pi.,... 

The depth of field defines the distance to the focal plane an object can have to still appear sharp. For example, in PIV/PIT measurements, particles that are not within the depth of field appear blurred and with a bigger diameter. The depth of field depends on the wavelength of the illuminating light, the optics, and the surrounding medium (e.g., air). 

In uncontaminated seawater in the tropical environment of Panama, commercially pure aluminum (1100) or 0.6% Si, 0.8% Mg, 0.2% Cu-Al alloy (6061-T) corroded at decreasing rates with time. After 16 years of exposure, the total weight losses were 67 and 63g/m, and the corresponding observed deepest pits were 0.84 and 2.0mm (0.033 and 0.079in.), respectively, for smaU-size test panels [5]. At the same location in fresh water, probably contaminated with heavy metals, the corresponding weight losses for 16 years were higher—347 and 103 g/m — and the deepest pits measured 2.8 mm (0.11 in.) each. 

The coupon racks were visually observed every 30-40 days. Photographs were also taken periodically. Some coupon assemblies were periodically removed from the basin for pit measurements, i.e. of pitting density and pit depth. Pit measurements were performed using an optical metallurgical microscope at a magnification of 200-500 x. The coupons from rack 1 were washed in 5% phosphoric acid to remove the oxide layer prior to carrying out the measurements. The coupons from rack 2 were taken for pit examination without removing the oxide layer. 

The crevice couples were stuck together at the first examination carried out after 40 days. Other coupons showed normal corrosion. ITie corrosion process was found to be much slower after the first visual examination. However, in February 1997, one crevice couple was removed, and the pH of the solution inside the crevice was determined and found to be 5.5. After about four months, a few nodules were visually detected on some coupons. After six months of exposure, three coupons were removed for destructive examination. They were rinsed in water and acetone, dried, reweighed and then cleaned with 50% phosphoric add. They were again rinsed with water and acetone, dried and reweighed. Weight loss was insignificant even after removal of the oxide. Coupons were then taken for pit measurement. The whole rack was removed on 2000-10-12 and the coupons were photographed, as shown in Fig. 13.1. 

Fig. IS (a, b) Use of a special cuff with photo sensors for PIT measurement...

Parkinson and Sherman [24] suggested that PIT measurements can be used to predict emulsion stability. They found a relationship between PIT and emulsion stability, the PIT being inversely proportional to the rate of droplet coalescence. They ran the experiments with Nujol emulsions stabilized with Spans and Tweens materials. 

The PIT is obviously linked with the so-called cloud point [24], at which the dehydration of the polyethoxylated chain reaches such a point that the surfactant is no longer soluble in water and a surfactant phase separates from the original aqueous solution (in the absence of oil phase). It is known that the presence of a small amount of dissolved hydrocarbon can change both the critical micelle concentration and cloud point, but the effect is not documented to a point that it can be quantified. On the contrary, the PIT measurement is quite reliable, and in its last version, it takes into account not only the surfactant type but also the aqueous- and oil-phase nature (i.e., all three formulation variables). 

Table 10.4 Corrosion rate (mm/yr) based on the maximum pit depth and based on the mean of the pit depths that were calculated from pit measurements on SRpCT tomograms. The pitting factors demonstrate that pitting is the prominent corrosion form in MgF2 coated LAE442 after 6 and 12 weeks of implantation. With kind permission from Elsevier (Witte et a ., 2010).

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