Mounds

Figure Bl.19.19. Examples of inaccessible features in AFM imaging. L corresponds to the AFM tip. The dotted curves show the image that is recorded in the case of (a) depressions on the underside of an object and (b) mounds on the top surface of an object. M- L andMoi correspond to convolutions of the surface features with the tip shape. (Taken from [ ], figure 2.)...

Several striking examples demonstrating the atomically precise control exercised by the STM have been reported. A "quantum corral" of Fe atoms has been fabricated by placing 48 atoms in a circle on a flat Cu(lll) surface at 4K (Fig. 4) (94). Both STM (under ultrahigh vacuum) and atomic force microscopy (AFM, under ambient conditions) have been employed to fabricate nanoscale magnetic mounds of Fe, Co, Ni, and CoCr on metal and insulator substrates (95). The AFM has also been used to deposit organic material, such as octadecanethiol onto the surface of mica (96). New appHcations of this type of nanofabrication ate being reported at an ever-faster rate (97—99). 

Deposits which are forming are frequentiy characterized by venting streams of hot (300°C) mineralized fluid known as smokers. These result in the local formation of metalliferous mud, rock chimneys, or mounds rich in sulfides. In the upper fractured zone or deep in the rock mass beneath the vents, vein or massive sulfide deposits may be formed by the ckculating fluids and preserved as the cmstal plates move across the oceans. These off-axis deposits are potentially the most significant resources of hydrothermal deposits, even though none has yet been located. 

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. 

Floor. A localized corroded region is always present beneath the tubercular mound. The depression is usually much broader than it is... 

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. 

Figure 3.29 As in Fig. 3.28, but with tubercles removed to show pitlike depressions beneath each mound. (Magnification 7.5x.) (Courtesy of National Association of Corrosion Engineers, Corrosion 89 Paper No. 197 by H. M. Herro.)... 

Figure 4.4 Corrosion product mounds covering localized areas of metal loss on an aluminum heat exchanger tube. Attack initiated beneath a thin deposit layer.

Figure 4.17 Corroded copper tubes from a heating coil. Cooling water contacted the visible surface. The mounds consist of copper carbonate (see Fig. 4.18).

Figure 4.18 As in Fig. 4.17 but with carbonate mound removed to reveal sparkling lavender cuprous oxide crystals. 

The condenser was plagued by rapid attack on waterside surfaces. The entire internal surface was fouled with silt and other deposits, beneath which a cuprous oxide layer was present (Fig. 4.23). Localized areas of metal loss were present beneath mounds of corrosion product. Some of these localized areas were deep enough to threaten tube integrity. 

Ferrous sulfate was added to the condenser in hopes of retarding attack. A thick, tan deposit layer rapidly formed on tubes near inlets (Fig. 4.24). Corrosion continued unabated. Underdeposit corrosion caused localized areas of metal loss (Fig. 4.25). Corrosion product mounds contained up to 10% chloride. 

Figure 4.23 Thick, red cuprous oxide layer covering internal surface of 90 10 cupronickel condenser tube. Note the green mounds marking sites of localized wastage.

Figure 4.25 As in Figs. 4.23 and 4.24. Corrosion product mound broken open to reveal sparkling cuprous oxide crystals.

Close visual examination of internal surfaces using a low-power stereomicroscope revealed a coating of reddish iron oxides on the internal surface. A population of small, knoblike mounds of corrosion product resembling tubercles was present on the surface (Fig. 5.16). 

Small organisms frequently become embedded within corrosion products and deposits. The organisms may make up a sizable fraction of the deposit and corrosion product. Seed hairs and other small fibers often blow into cooling towers, where they are transported into heat exchangers. The fibers stick to surfaces, acting like sieves by straining particulate matter from the water. Deposit mounds form, reinforced by the fibers (see Case History 11.5). 

Figure 6,4 Small pits on a 316 stainless steel plate. A light pita, marking ghost images of deposit mounds.

Figure 6.9 Irregular deposit and corrosion-product mounds containing concentrations of sulfate-reducing bacteria on the internal surface of a 316 stainless steel transfer line carrying a starch-clay mixture used to coat paper material. Attack only occurred along incompletely closed weld seams, with many perforations. Note the heat tint, partially obscured by the deposit mounds, along the circumferential weld.

Stainless steels attacked by sulfate reducers show well-defined pits containing relatively little deposit and corrosion product. On freshly corroded surfaces, however, black metal sulfides are present within pits. Rust stains may surround pits or form streaks running in the direction of gravity or flow from attack sites. Carbon steel pits are usually capped with voluminous, brown friable rust mounds, sometimes containing black iron sulfide plugs fFig. 6.10). 

Figure 6.X7A A corrosion-product and deposit mound on a mild steel service water pipe honeycombed by small tubelike organisms. Each hole is approximately 0.01 in. (0.025 cml in diameter.

Internal surfaces were covered with brown and tan deposits and corrosion products pits were present beneath small mounds of reddish-... 

After only 4 months of service, the main condenser at a large fossil utility began to perforate. Initial perforations were due to erosion-corrosion (see Case History 11.5). Small clumps of seed hairs entering the condenser after being blown into the cooling tower were caught on surfaces. The entrapped seed hairs acted as sieves, filtering out small silt and sand particles to form lumps of deposit (Fig. 6.24A and B). Immediately downstream from each deposit mound, an erosion-corrosion pit was found. 

Figure 6.24 Small mounds of deposit entrapped by seed hairs that stuck to surfaces.

A few months later, a second set of tubes was submitted for examination. These tubes had not been cleaned. Close examination revealed arrowhead-shaped mounds of fibrous debris lodged on the tube wall (Fig. 11.20). Dislodgement of these mounds revealed an arrowheadshaped region of shallow corrosion containing sparkling crystals of cuprous oxide, essentially identical to that described above. 

The fibrous strands that composed the mounds adhering to the tube wall were examined and identified as seed hairs from grass. Subsequently, fibrous material was collected from the cooling tower basin and seed pods from grass growing in a bog located adjacent to and upwind of the cooling towers. This material was also examined and found to be identical to the seed hairs that composed the mounds adhering to the tube wall. 

These spots then became localized collection sites for seed hairs, silt, and corrosion products, forming a mound that was shaped into an arrowhead by water flow. 

The mound shielded the tube wall, causing underdeposit corrosion (see Chap. 4, Underdeposit Corrosion ). 

When a critical mound size was attained, turbulence was created immediately downstream of the mound, resulting in erosion and eventual perforation of the tube wall. 

Figure 11.20 Elongated mounds of fibrous debris attached to the internal surface. (Magnification 7.5x.)...

Figure 17.10 shows metal loss on the throat of the pump housing. External pump housing surfaces were also affected (Fig. 17.11). Note the large tubercles. (Tubercles are knoblike mounds of corrosion products. They typically have a hard, black outer shell enclosing porous reddish-brown or black iron oxides) (see Chap. 3, Tuberculation ). The metal surface beneath these tubercles had sustained graphitic corrosion, in some cases to a depth of Vi in. (0.6 cm) (Fig. 17.12).

Tuberculation occurs in aqueous solutions. Mounds form over metal surfaces providing for concentration differences, favorable environments for biological growth, and an increase in acidity leading to hydrogen formation. 

In some countries, mounding is used instead of conventional insulation. The tank is completely covered with clean sand or other clean material. Portions of the covering must be removed from time to time so that the outside of the tank can be inspected. 

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