Deposit corrosion

A particularly insidious failure mechanism that is commonly found in carbon-steel tubing is under-deposit corrosion. In many cases, corrosion products fomi a scab that can mask the presence of the pitting, making it difficult to quantitatively assess using conventional NDT methods. However, by combining proper cleaning procedures with laser-based inspection methods, the internal surface of the tubing can be accurately characterized and the presence of under-deposit corrosion can be confirmed and quantified. 

Favored locations for erosion-corrosion are areas exposed to high-flow velocities or turbulence. Tees, bends, elbows (Fig. 11.5), pumps, valves (Fig. 11.6), and inlet and outlet tube ends of heat exchangers (Fig. 11.7) can be affected. Turbulence may be created downstream of crevices, ledges (Fig. 11.8), abrupt cross-section changes, deposits, corrosion products, and other obstructions that change laminar flow to turbulent flow. 

Ammonia sulfide is not corrosive, but it can precipitate. Un deposit corrosion and pitting can occur. 

Electrochemical noise. Fluctuations in potential or current from baseline values during electrochemical measurements are particularly prominent during active/passive transitions. This so-called electrochemical noise is of particular value in monitoring localised corrosion, i.e. pitting, crevice and deposit corrosion and stress-corrosion cracking . 

To maintain all boiler surfaces and other waterside surfaces in the HW and steam-system cycles in a structurally sound and clean condition, properly protected against the operational and economic problems associated with deposition, corrosion, fouling, and contamination... 

Under higher waterside pressure conditions, consideration of bulk water turbulent flow, the thickness of the steam-water laminar flow sublayer film at the heat transfer surface, and the general waterside physicochemical operating conditions that exist are important issues in reviewing the potential risks of deposition, corrosion, and other problems that may occur within an operating boiler. 

Thus, the proper control of deposition, corrosion, and fouling and boiler structural integrity are interdependent functions, and all these phenomena are directly related to boiler design and real-time operating conditions. 

Almost all larger FT and HP boiler plants employ some form of external capital equipment for MU water and FW treatment. The rationale for installing capital equipment is to eliminate (or at least minimize) the level of mineral impurities, process contaminants, and noncondensable gases entering the boiler via the FW system to reduce the potential for the development of waterside deposition, corrosion, steam contamination, and other waterside problems. 

Under the sludge considerable orange-red tuberculation corrosion deposits may develop. In cause-and-effect fashion, the tubercles grow cause fouling, permit under-deposit corrosion to persist, and generally act as a binding agent for carbonates, silicates, and other precipitates. 

Deposition commonly reflects a combination of physicochemical processes and localized effects. It may occur through fouling as a result of contamination by process materials, perhaps plus scaling from the supersaturation of dissolved salts, and coupled with some active under-deposit corrosion. As a consequence, deposits forming within a boiler are almost never single mineral scales but typically consist of a variable mix of scale and corrosion debris, chemical treatment residuals, process contaminants, and the like. 

Tuberculation, crevice corrosion, and under-deposit corrosion are... 

Under-Deposit Corrosion In the same way that oxygen becomes depleted in a crevice, and a differential-oxygen concentration cell is established, leading to localized corrosion of the oxygen-starved anodic area, so the same phenomenon readily occurs in dirty boilers under deposits, sludge, and other foulants. 

The rate of metal wastage of this indirect form of corrosion may be increased by the presence of other direct corrosion influences in the deposit or foulant. Also (and similar to crevice corrosion), there may be general oxygen corrosion occurring at the same time or perhaps acting as an initiator to the under-deposit corrosion process. 

This type of corrosion is liable to occur in any part of a boiler where silt, muds, scales, precipitants, or foulants exist and is by no means limited to ferrous metals. Stainless steels, brasses, and cupronickels are all subject to under-deposit corrosion and deep pitting. 

Localized, concentration-cell corrosion (differential aeration corrosion), occurring as Tuberculation corrosion Crevice corrosion Under-deposit corrosion Pitting corrosion All forms of localized, concentration-cell corrosion are indirect attack type corrosion mechanisms. They result in severe metal wastage and can also induce other corrosion mechanisms, e.g. Stress corrosion Corrosion fatigue... 

Blowdown and heat recovery system (BDHR) flash tanks and heat exchangers are potential candidates for sludging, leading to restrictions in the drain line or heat transfer surfaces. Deposits in the BDHR heat exchanger may lead to under-deposit corrosion and leaks. 

Heat exchange surfaces must be kept clean deposits reduce heat transfer efficiency and promote various forms of under-deposit corrosion. It also is easier to keep a clean system clean than to prevent a dirty system from getting dirtier, so measurement of the dirt loading or deposit loading on a heat transfer surface is an important part of determining when a boiler needs cleaning. 

Chemical treatment programs based on the direct addition of chemicals to FW or BW in order to prevent subsequent deposition, corrosion, or other problems from occurring. With precipitating types of internal treatments, the boiler waterside space is employed as a reaction vessel and, where a particular boiler design is unsuitable, inadvertent problems of fouling may occur. 

A great number of measurements have been reported for articles electroplated with zinc. The various aims have been evaluation of the corrosion rate of zinc that had been plated in a number of commercial cyanide-free zinc baths," comparison of the corrosion rate of a composite material (zinc with codeposits of various oxides) and of pure zinc deposits," corrosion testing of various alloyed zinc platings (Zn-Ni, Zn-Co, Zn-Fe), with or without subsequent post-treatment. Most of the work in the last category was only recorded in internal reports. The published work consists of an examination of the corrosion behavior of a ctoomated Zn-Fe... 

Exploration of the scope of NPS in electrochemical science and engineering has so far been rather limited. The estimation of confidence intervals of population mean and median, permutation-based approaches and elementary explorations of trends and association involving metal deposition, corrosion inhibition, transition time in electrolytic metal deposition processes, current efficiency, etc.[8] provides a general framework for basic applications. Two-by-two contingency tables [9], and the analysis of variance via the NPS approach [10] illustrate two specific areas of potential interest to electrochemical process analysts. 

The cooling tower, which is an efficient air scrubber can easily become a catchall for contaminants resulting from the location of the tower or from the industrial process. In arid areas, ingress of sand contributes to fouling, which reduces efficiency and contributes to biofilm and under-deposit corrosion. In coastal areas, sand laden with chlorides can cause corrosion of stainless steel components and impair chemical corrosion inhibitor performance. Heavy industries, such as steel or aluminum manufacture, produce severely contaminated cooling water resulting from direct contact with metal slags and lubricants. 

This demand will reduce the amount of chlorine available for microbiological control and lead to slime growth, especially in the tower basin and water distribution system, with biofilms and under-deposit corrosion being common effects of this problem. 

As discussed above, in high chloride content cooling water, when foulants or deposits are present, the risk of under-deposit corrosion increases, which may be accompanied by a high local chloride concentration. However, it is also likely that a local lowering of pH will occur due to the presence of H+ ions. 

Crevice corrosion, under-deposit corrosion, and tuberculation are all forms of concentration cell corrosion, and all involve oxygen to a greater or lesser extent. 

Pitting corrosion is a general term that can be considered a visible sign of the results of concentration cell corrosion and of further induced-corrosion processes such as when chloride attack occurs. Although pits can also occur with acid corrosion, etc., under-deposit corrosion, of course, can also involve direct metal surface attack, from, say, biologically induced corrosion (but that is discussed separately). 

Galvanic corrosion rates may diminish as corrosion debris begins to act as an electric current insulator (although under-deposit corrosion processes may then take over). 

Because of the ability of glucoheptonates to chelate calcium and iron and dissolve rust films without attacking the bare metal, they are very useful as metal surface cleaners. They are often considered a ferrous corrosion inhibitor, but the real function of these chelants is their ability to dissolve iron- and calcium-rich deposits on the metal surface, within a pH range of 5 to 9, and to provide clean metal surfaces. Thus they permit access by other true corrosion inhibitors and help to minimize differential aeration and under-deposit corrosion mechanisms. 

Chlorine also reacts with common cooling water contaminants, such as hydrogen sulfide, sulfur dioxide, and ammonia. This produces an increase in hydrogen ions (which also tend to lower the system pH) and an increase in total chlorides (which can further increase the risks of corrosion, especially under-deposit corrosion). 

Is the cooling system designed for a large, continuous process application, such as oil, steel, pulp and paper, or petrochemical Is there a risk of oil, ammonia, or sulfur compounds leaking into the system Are there problems of black slime and under-deposit corrosion Who makes the water treatment decisions on-site Is it the laboratory, utilities, production, or all departments collectively Will the customer look for training support ... 

Portable deposit/corrosion monitors are typically housed in an enclosure of perhaps 30 in. H x 20 in. W x 15 in. D. Components include inlet flow controller, strainer, adjustable electric heater, (outer) see-through glass housing, (inner) heated specimen tube or block, hot/cold temperature readout, corrosion rack, plus thermal overload, low-flow cut-off, and other safety devices. The specimen tubes or blocks are available in different metals (as are the corrosion coupons) and can usually be replaced in a matter of minutes. Unlike test heat exchangers, the cooling water in this type of monitor flows on the shell side of the specimen tube. 

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