Oxygen tubercles

Oxygen Tubercles. Similar to crevice attack, it is encouraged by the deposit of a layer, semipermeable to oxygen (porous layer of iron oxide or hydroxide). 

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. 

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. 

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.)...

Certain conditions, ultimately dictated by economics, make the substitution of more resistant materials a wise choice. Stainless steels (not sensitized) of any grade or composition do not form tubercles in oxygenated water neither do brasses, cupronickels, titanium, or aluminum. However, each of these alloys may suffer other problems that would preclude their use in a specific environment. 

Each tubercle exhibited small clam-shell marks or growth rings (Fig. 3.30). Each ring was formed by fracture at the tubercle base during growth. Ejected internal contents rapidly deposited when contacting oxygenated waters. Tubercles were hollow (Fig. 3.31). Surfaces below the... 

Oxygen corrosion only occurs on metal surfaces exposed to oxygenated waters. Many commonly used industrial alloys react with dissolved oxygen in water, forming a variety of oxides and hydroxides. However, alloys most seriously affected are cast irons, galvanized steel, and non-stainless steels. Attack occurs in locations where tuberculation also occurs (see Chap. 3). Often, oxygen corrosion is a precursor to tubercle development. 

Where feed lines have short pipe runs, where hot wells or FW tanks are of small volume, or when FW is too cold, there often is insufficient time for full DO scavenging to take place, even when using catalyzed scavengers. The inevitable result of this lack of contact time is the formation of oxygen-induced corrosion products, which by various secondary mechanisms may settle out to form permanent deposits within the boiler system. These deposits may develop in several forms (e.g., where DO removal is particularly poor, they often appear as reddish tubercles of hematite covering sites where pitting corrosion is active). Active pitting corrosion combined with the presence of waterside deposits ultimately may lead to tube failure in a boiler or other item of system equipment and result in a system shutdown. 

Manganese and iron oxidation are coupled to cell growth and metabolism of organic carbon. Microbially deposited manganese oxide on stainless and mild steel alters electrochemical properties related to the potential for corrosion. Iron-oxidizing bacteria produce tubercles of iron oxides and hydroxides, creating oxygen-concentration cells that initiate a series of events that individually or collectively are very corrosive. 

Other workers have demonstrated improved lung function (including arterial oxygen level increase [355]) and increased response to bronchodilators [356] after bromhexine. Some evidence of weak activity against tubercle bacillus has appeared [357] this could complicate culture control of anti-tubercular therapy. 

The insoluble corrosion product Fe(OH)2 can help bacterial film to control the diffusion of oxygen to the anodic sites in the pit. This forms a typical tubercle. If chlorides are present in the aqueous solution, the pH of the solution trapped in the tubercle can become very acid due to the autocatalytic propagation mechanism of localized corrosion due to deposit formation and generation of hydrochloric acid. 

Iron-oxidizing bacteria produce orange-red tubercles of iron oxides and hydroxides by oxidizing ferrous ions from the bulk medium or the substratum (Fig. 10). Deposits of cells and metal ions create oxygen concentration cells (Fig. 11) [59] that effectively exclude oxygen from the area immediately under the deposit and initiate a series of events that are individually... 

Formation of concentration cells at the metal surface and in particular oxygen concentration cells. This effect occur when a biofilm or bacterial growth develops heterogeneously on the metal surface. Concentration cells are also associated with the tubercles formed by iron-oxidizing bacteria, such as Gallionella. Certain bacteria may also trap heavy metals sueh as eopper and cadmium within their extracellular polymerie substanee, resulting in the formation of ionie eoncentration cells. 

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