Tubercles

D-Arabinose is found in the glycoside bar-baloin and in the polysaccharides of the tubercle bacillus. 

Tuberculocidal Test. The tubercle bacillus is resistant to disinfectants because the cells are protected with a waxy coating that is not readily penetrated. The tuberculocidal test is a use dilution practical type test that employs porcelain cylinders. The bacteria are different from those in the use dilution method (Table 10), the incubation time is longer, and the details of the procedure are different. For example, in the tuberculocidal test the test is divided into two parts, a presumptive test and a confirmatory test. The former employs Mycobacterium smegmatis and the latter employs Mycobacterium bovis (BCG). For the presumptive test the incubation time is 12 days, as against 48 hours for other bacteria used in the use-dilution method. For the confirmatory test the incubation time is 60 days, with an additional 30 days in case there is no growth. As shown in Table 10, the concentrations of the phenol standard are higher than used with other bacteria. 

Tubercles are mounds of corrosion product and deposit that cap localized regions of metal loss. Tubercles can choke pipes, leading to diminished flow and increased pumping costs (Fig. 3.1). Tubercles form on steel and cast iron when surfaces are exposed to oxygenated waters. Soft waters with high bicarbonate alkalinity stimulate tubercle formation, as do high concentrations of sulfate, chloride, and other aggressive anions. 

Tubercles are much more than amorphous lumps of corrosion product and deposit. They are highly structured. Structure and growth are interrelated in complex ways. 

As rust accumulates, oxygen migration is reduced through the corrosion product layer. Regions below the rust layer become oxygen depleted. An oxygen concentration cell then develops. Corrosion naturally becomes concentrated into small regions beneath the rust, and tubercles are born. 

All tubercles have five structural features in common ... 

Outer crust. A friable outer crust forms atop the tubercle. The crust is composed of ferric hydroxide (hematite), carbonates, silicates, other precipitates, settled particulate, and detritus. Ferrous ion and ferrous hydroxide generated within the tubercle diffuse outward through fis-... 

Figure 3.3 Well-developed tubercle shows chemical compounds and structure. (Courtesy of National Association of Corrosion Engineers, Corrosion 91 Paper No. 84 by H. M. Herro.)...

Figure 3.5 Schematic pH eind oxygen concentration profiles in an active tubercle. Below the magnetite shell oxygen concentration decreased sharply. pH rises above the magnetite shell due to cathodic hydroxyl-ion generation emd falls below the shell due to concentration of acidic anion, (Courtesy of National Association of Corrosion Engineers, Corrosion 91 Paper No. 84 by H. M. Herro.)...

Cavity. A fluid-filled cavity is sometimes present beneath the core (Fig. 3.3). The cavity may be huge as in Fig. 3.8 or small as in Fig. 3.9. The cavity may result, in part, from acidic conditions internally. Acid conditions may prevent precipitation of oxides and hydroxides inside the tubercle. 

Tubercles grow on nonstainless steels and some cast irons. Sensitized stainless steel and a few other alloys are rarely affected. Surfaces must... 

Deposits stimulate tubercle formation. Hence, regions where foreign material accumulates are common tubercle breeding grounds. Stagnant, low-flow areas also promote tubercle growth. 

The relationship between flow and tubercle growth is dichotomous. Low flow may stimulate growth, but zero flow (so that water contacting... 

Figure 3.8 Large tubercle broken open to reveal the internal cavity. The cavity is filled with fiuid in service.

Tubercles form under high- and low-flow conditions. Flow directly influences tubercle morphology. When flow is great, tubercles elongate... 

Figure 3.9 Small tubercle cross section. The steel surface is at the bottom, and the red material is an epoxy mounting medium. Note the small, shallow central cavity. (Magnification 7.5x.)...

Figure 3.11 Tubercles elongated by flow in a mill water supply line.

As bulk water pH falls, tubercle numbers and size tend to increase. At sufficiently low pH, however, precipitates and oxides cannot form (i.e., they are dissolved) and tubercular structures cannot exist. 

The presence of tubercles is usually obvious. Friable brown and orange nodular encrustations on mild steel and cast iron cooling water components are almost always tubercles (Figs. 3.12 through 3.14). The presence of a crust, shell, core, cavity, and corroded floor are definitive (Fig. 3.3). Careful analysis can provide considerable information concerning growth, chemical composition, and associated metal loss. 

The outer crust is composed of rust (hematite), precipitate, and settled particulate. Treatment chemicals may also deposit preferentially atop tubercles in response to associated corrosion. It is common to find several percent of zinc and phosphorus compounds in tubercles that grow in zinc- and phosphate-treated waters. Silicates also can be found in... 

Figure 3.14 Small, hard tubercles on an essential service water system pipe in a nuclear utility.

Figure 3.15 Knife dislodging a dry tubercle from a steel pipe surface.

Table 3.1 gives the local elemental composition of three different tubercles from three different systems formed under different chemical treatments. At the floor of each tubercle, the concentration of chlorine and sulfur is higher than in the crust. The concentration of most crust elements, except that of iron, also decreases near the tubercle floor. The crust contains traces of treatment chemicals including zinc, phosphorus, and silicon. Tubercle 1 contains up to 40% silicon in the crust, which strongly suggests accumulation of silt by settling of particulate. 

Figure 3.17 Intact tubercle showing physical integrity after re-movi from surface in Figs. 3.15 and 3.16.

Figure 3.18 Multiple magnetite shells in a small tubercle. Multiple shells form due to successive fracture during growth. (Magnification 2x.)...

Tubercle compositions were measured using energy dispersive x-ray analysis) ... 

As a result of the concentration of acidic species, such as chloride and sulfate, material scraped from the inside of tubercles is virtually always acidic when mixed with water. Acidity varies not only from tubercle to tubercle but also from place to place in a given tubercle. Acidity is greatest near the corroded metal surface. The size of the fluid-filled cavity can indicate acidity. The larger the cavity, the more acidic the internal environment. 

The corroded tubercle floor is almost always a dish-shaped depression, much wider than it is deep (Fig. 3.23). Undercutting is very rare. The metal-loss width almost exactly matches the tubercular mound width. Corrosion rates exceeding 50 mil per year are rare, except when tubercles are young. Average local corrosion rates are usually 20 mil per year or less. 

If pH is unusually low within the tubercle, the floor may be heavily striated. Small, shallow parallel grooves will appear in the depressions beneath each tubercle (Fig. 3.24). The striations are caused by preferential corrosion along microstructural defects such as deformed metal. 

Figure 3.19 Tubercle only a few minutes after removal from a water-filled pipe.

Figure 3.20 As in Fig. 3.19, with tubercle opened to show the dark interior. 

---