Metal wastage

The powerful influence of mechanical factors on these phenomena produces distinctive physical features on affected metal surfaces, as well as determining the locations where the damage occurs. Hence, metal wastage influenced by mechanical factors can be sensitive to geometric shapes and topographical features (surface contours). It is, in this sense, location specific. 

Fig. 7.31 Metal wastage of several steels due to oxidation in air (saturated with water at room temperature) for 10 (XX) h at various temperatures, (o) Type 302S3I, (b) type32IS3l, (c) type 3I6S3I and (d) type 3I0S31. T lomi = 5suiract + / ptntiraiion (after Edwards and Nicholson )...

Fig. 7.32 Metal wastage of several steels due to oxidation in steam at various temperatures, (a) Mild steel, (b) Fe-2Cr-0-25Mo, (c) Fe-12Cr -I- Mo -I- V, (d) A.l.S.I. 316 and (e) Fe-18Cr-12Ni-lNb. T,oi,i = Ssutface -I- / penetration (after King el at.)...

While carburisation itself is not a normal corrosion process, in that there is no metal wastage, absorption and diffusion of carbon can lead to significant changes in the mechanical properties of the affected material and in particular to marked embrittlement. Furthermore, initial carburisation can produce an acceleration of the normal oxidation process, a phenomenon that is notable in nickel-chromium alloys. 

Corrosion anywhere within the steam-water circuits results in metal wastage and possible equipment failure. And if the boiler plant is not operated correctly or if the water chemistry is not maintained within certain control parameters, the generated steam may contain contaminants in a cause-and-effect process that ultimately affects the utilization of the steam, reducing quality in a number of areas, and increasing fuel, manufacturing, or maintenance costs. 

Corrosion may also occur under deposits, and the removal of a deposit from a heat transfer surface typically may expose a pit or other form of metal wastage. 

It is at the anode that oxidation takes place, with the anodic metal suffering a loss of negatively charged electrons. The resulting positively charged metal ions dissolve in the water electrolyte and metal wastage occurs. In the corrosion cell, the metal or metal area having the lowest electrical potential becomes the anode. 

Corrosion mechanisms in boiler plant systems take many forms. They always result in metal wastage and usually result in a loss of mechanical or structural strength as well. 

Where air bubbles and other gases are entrained in turbulent FW and an abrupt reduction in pressure takes place, cavitation may occur. The result of the extremely rapid formation and collapse of steam bubbles on the suction side of feed pumps or the discharge side of valves produces erosive microjets that over time may promote severe cavitation-al metal wastage. 

This view of the corrosion process is, however, more often than not too simplified an explanation. First, even when general corrosion take place (as in an idle or wet lay-up boiler), the reaction mechanisms tend to occur at many localized points on the boiler metal surface, typically where cracks and other imperfections in the magnetite film exist. Second, such processes almost always lead to derived forms of localized corrosion, which often result in severe metal wastage through the formation of deep pits. 

The primary difference between these types of metal wastage and oxygen corrosion is that these are all indirect forms of attack, induced by surface shielding (areas of metal surface under deposits or foulants, or cracks and gaps in the metal that are close to a shielding surface). 

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. 

Chlorides in particular present a problem because of their tendency to attack and weaken passive oxide layers and accelerate metal wastage by pitting corrosion and other forms of concentration cell processes. 

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... 

Other forms of concentration-cell corrosion include Caustic gouging Saline corrosion Combination of free caustic and concentrating effect causes severe metal wastage. High chlorides and sulfates, result in corrosion from depolarization and depassivation effects... 

Corrosion of steel by carbonic acid is probably the most common problem in the post-boiler section, producing pipe grooving and general metal wastage, especially in threaded joints. This form of corrosion is not self-regulating and the reaction products can produce more carbon dioxide, thus perpetuating the corrosion problem. Typically, the condensate pH level is depressed to around 5.0 to 5.5. 

Despite the usefulness of this equipment, the devices are not without their difficulties in practical situations. The net result is that there is no single technique that can convincingly, economically, or simply provide all the answers needed to determine the rate of deposit buildup or metal wastage. 

A type of metal wastage caused by the mechanical action abrasion of the metal surface by high velocity steam, air bubbles or solid particles. Often part of a larger erosion-corrosion process. 

High velocity steam or particles striking a metal surface and causing metal wastage by erosion. Also refers to unbumed fuel oil striking a surface and resulting in the formation of carbon deposits and smoke. 

Visible sign of metal wastage in the form of a deep crevice resulting from various forms of localized corrosion. 

Cathodic protection is an electrochemical method of corrosion control that has found widespread application in the protection of carbon steel underground structures such as pipelines and tanks from soil corrosion. The process equipment metal surface is made as the cathode in an electrolytic circuit to prevent metal wastage. 

This type transfers heat from the process liquid to the cooling water across a large number of thinly spaced corrugated plates. They find particular application in food and beverage industry applications or for smaller heat loads in hotels or hospitals. Plate and frame types are widely used because of their compact design and availability of various construction materials. Because localized corrosion and metal wastage can occur within the crevices (crevice... 

This mix of metals may be good for maximizing heat-transfer coefficients and specific applications, but it makes the heat exchanger susceptible to many types of metal wastage and corrosion fatigue failure if vigilance and good water treatment are not practiced. 

Some surface water supplies have only 10 to 50 ppm or more of total calcium and magnesium hardness they are naturally soft waters and may also be described as lean waters, due to dissolved solids from all sources being limited to perhaps only 30 to 60 ppm TDS. Cooling systems using this quality of water as makeup may employ cycles of concentration (COC) of 7 to 1 Ox or more. Control of hardness scales tends not to be an onerous task for modem polymeric scale inhibitors, but lean water formulations must allow for more aggressive conditions and a real risk of some metal wastage. 

Corrosion is an electrochemical process whereby the oxidation of metal(s) or alloys to their (lower energy state) oxides or cations takes place, resulting in loss of mechanical or structural strength and metal wastage. Corrosion takes many forms and includes biocorrosion, which is corrosion taking place at the water-metal interface of a biofilm. 

Fig. 4.1 Corrosion cell in aerated cooling water showing a section of steel with metal wastage and corrosion products at the anode. The corrosion products are Fe(OH)2 (inner layer) and Fe(OH)3 (outer layer)...

The problem with relying solely on anodic area corrosion inhibition is the risk of local film damage, which concentrates the corrosion current flow and permits a highly active anodic cell to be developed and causing accelerated corrosion to take place. This in turn leads to severe metal wastage, often in the form of deep pitting. 

Corrosion in cooling systems is multifarious. The permutations of all the various forms of corrosion and damage possible, associated with the variety of metals commonly used that may suffer metal wastage, is beyond the scope of the practical intent of this volume, but some observations are ... 

It is seldom that a cooling system exhibits only one type of corrosion within its water circuits. Even with well-protected and well-managed systems there are a variety of forces at work in different parts of the system capable of inducing several types of corrosion and mechanical damage simultaneously. Often, in any individual location, more than one corrosion mechanism is present and resultant metal wastage may be a combination of several factors or corrosion mechanisms. 

Under most usual oxygenated and alkaline-operating conditions for cooling water, the oxygen cathode provides the rate-determining step, and depolarization by O2 fthe removal of hydrogen ion) will increase the rate of corrosion and hence metal wastage, as below ... 

This is a general name for a wide variety of corrosion processes that may be actively or passively influenced, or induced, by an even wider variety of microbiological organisms. The electrochemical reactions that occur always result in metal wastage, as with all other forms of corrosion the most active biocorrosion processes primarily involve sessile bacteria rather than planktonic bacteria, algae, or fungi. 

Passive biocorrosion tends primarily to promote general metal wastage under the biodeposit, which can increase the total iron levels in the recirculating cooling water, although pitting corrosion can also occur. 

There may be several different types of bacteria involved, producing a chain of biological pathway reactions that eventually lead to metal wastage. 

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