Origin of Corrosion

The earliest published accounts of the causes of corrosion are the two publications by Robert Boyle (1627-1691) entitled Of the Mechanical Origin of Corrosiveness and... 

Experience has shown that a few design details are frequently the cause of failures, which will be further discussed in Chapter 12. Let it suffice here that the origin of corrosion can often be traced to simple errors that could have been avoided without any appreciable increase in cost... 

Traces of chlorine that remain at the surface of the sihcon wafers after the etching process, when exposed to ambient air, are the major origin of corrosion phenomena. This typically leads to bubble-type outgrowths on almninum conductors (Fig. 9-72). With an increased copper content in aluminum, this problem is aggravated because this system then acts like a galvanic cell, which is characterized by HCl-catalyzed chemical reactions. Two different strategies are feasible... 

Corrosion is known commonly as rust, an undesirable phenomena which destroys the luster and beauty of objects and shortens their life. A Roman philosopher, Pliny (AD 23-79) wrote about the destruction of iron in his essay Ferrum Corrumpitar. Corrosion since ancient times has affected not only the quaUty of daily Hves of people, but also their technical progress. There is a historical record of observation of corrosion by several writers, philosophers and scientists, but there was Httle curiosity regarding the causes and mechanism of corrosion imtil Robert Boyle wrote his Mechanical Origin of Corrosiveness. ... 

Understanding fundamental origins of corrosion, the electrochemical basis for much of it as well as how and why standard tests are designed is critical. Encyclopedic knowledge of facts available in databases is of less importance. 

Often, analysis of corrosion products is requested because there is a tendency, rightly, to consider that they may allow tracing the origin of corrosion in service. 

The presence of sulphates, especially at high concentrations, can hardly tell more than the atmospheric origin of corrosion (see Section C.2.6), and the presence of carbonates indicates corrosion in water [50],... 

In general, the qualitative and quantitative analysis of corrosion products does not yield information that could be useful for determining the origin of corrosion under service conditions. 

Corrosion is a complex phenomenon that depends on a number of parameters, related both to the environment and the metal. Sometimes, corrosion damage is difficult to explain, either because the origin of corrosion has not been identified or because the theoretical foundations are inadequate for giving a convincing answer. 

Gas leaving the converter is normally cooled to 180—250°C using boiler feedwater in an "economizer." This increases overall plant energy recovery and improves SO absorption by lowering the process gas temperature entering the absorption tower. The process gas is not cooled to a lower temperature to avoid the possibiUty of corrosion from condensing sulfuric acid originating from trace water in the gas stream. In some cases, a gas cooler is used instead of an economizer. 

Sulfide inclusions have been identified as the origins of pits and stress-corrosion cracks in stainless steel stmctures under some conditions (see... 

Stress-corrosion cracks tend to branch along the metal surfaces. Typically, evidence of corrosion, such as accumulations of corrosion products, is not observed, although stains in the cracked region may be apparent. Stress-corrosion cracks tend to originate at physical discontinuities, such as pits, notches, and corners. Areas that may possess high-residual stresses, such as welds or arc strikes, are also susceptible. 

Microscopic examinations revealed tight, unbranched, transgranular cracks originating on the external surface. Many of these cracks originated at shallow pockets of corrosion. 

Graphitic corrosion has two distinct features that are useful in distinguishing it from other forms of corrosion. First, it affects an unusually limited number of metals the only metals commonly affected are gray cast iron and nodular cast iron. Second, metal that has experienced graphitic corrosion may retain its original appearance and dimensions. Consequently, graphitic corrosion frequently escapes detection. 

Metallographic examination of other areas revealed similar degrees of corrosion on the two blades. At no point on the coated blade had the corrosion penetrated to the base metal, although in the two areas on the coated blade about 0.002 inch of the original 0.003 inch coating had been oxidized. 

The presence of active sulphate-reducing bacteria usually results in graphitic corrosion and this has led to a useful method of diagnosing this cause of corrosion. The leaching out of iron from the graphitic residue which is responsible for the characteristic appearance of this type of corrosion leads to an enriched carbon, silicon and phosphorus content in the residue as compared with the original content of these elements in the cast iron. Sulphur is usually lost to some extent but when active sulphate-reducing bacteria are present, this loss is offset by the accumulation of ferrous sulphide in the residue with a consequent increase in the sulphur content of the residue out... 

Acid concentration (%) Temperature (°C) Corrosion rate (mm/y) Origin of data... 

The absence of corrosion, coupled with the fact that scale and other deposits appear to be dislocated by thermal cycling, result in a finish on tantalum heating surfaces that is as good as the original, even after 20 or 30 years in service, and also ensure that good heat-transfer properties are maintained throughout the life of the equipment. The use of tantalum for process equipment also ensures freedom from contaminations of the product. 

Rates of corrosion rarely remain constant with time. More often than not, rates of attack tend to diminish as a result of the formation of adherent insoluble corrosion products or other protective films originating in the environment (Fig. 19.1). Therefore, extrapolation of results of tests that are too short is more likely to indicate a tower resistance to attack than will... 

Economizers are not usually designed to generate steam, and any deposits found in them therefore are not likely to be a result of carbonic acid corrosion or contamination from steam. Rather, the transport and buildup of corrosion debris within an economizer tends to originate from corrosion processes occurring either in the economizer itself or in some upstream part of the pre-boiler system. Economizer deposits typically develop in the presence of oxygen and possess a high iron content. 

Where corrosion takes place, the origins of the metal oxides and salts formed from corroded boiler system metals should be traced in a systematic fashion to establish cause and effect and avoid misclassify-ing the fundamental waterside problem. Occasionally however, it is difficult to positively confirm the starting point of a corrosion problem because it is common for corrosion products to be transported from their point of origin and deposited elsewhere in the steam-water circuit, or alternatively to act as binders and contribute to fouling and contamination of the overall boiler plant system. 

Additionally, as a result of the turbulent nature of the flow of FW as it passes through the system, both new and old debris also may continue to pass through and eventually lodge in the boiler. As a consequence, the transport of corrosion products and debris from various parts of the overall boiler plant system through the pre-boiler section (where additional debris may originate) and on to the boiler continues to be a... 

Ferrous bicarbonate may be transported to a point where little or no amine is available, which then provides the source for various secondary corrosion reactions. Corrosion mechanisms and deposition formation often take place at points in the system well downstream of the original points of steam condensation and initiation of corrosion. 

Nickel is sometimes found in boiler deposits as nickel oxide, either in the gray-black nickel (-ous, -ic) oxide Ni304 form, or the black nickel sequioxide Ni203. Where present in the boiler, it usually originates from corrosion of condenser or preheater tubes. 

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