Corrosion failures, occurrence

The MTI report (35) listed the corrosion failure modes along with the frequency of the occurrence as follows in Table 3.12. 

TABLE 3.12 Corrosion Failure Modes Along with the Frequency of Occurrence... 

The corrosion failure mode with an average frequency of occurrence is given in Table 4.46. 

The relative frequency of occurrence of various corrosion forms depends on the type of industry and environment. One example from the chemical industry is shown in Table 7.1. In the heading we notice the high proportion of corrosion failures. The percentages of crevice corrosion and galvanic ( ) corrosion are surprisingly low. 

The occurrence of a brittle fracture (absence of plastic deformation) in a normally ductile metal is a strong indication for the presence of EIC. In failure analysis this is used as a diagnostic criterion. Because EIC involves corrosion, failure does not take place immediately, but after a certain time only during which a crack forms and grows to a critical size. This behavior complicates the study of EIC in the laboratory where the phenomena must be accelerated and the results then extrapolated to more realistic conditions. 

TABLE 2— Factors and subfactois controlling the occurrence of a corrosion failure. 

Both time-related failure rates and demand-related failure rates can apply to and be reported for many pieces of equipment. Both types of rates are included in some of the data tables in Chapter 5. If a piece of equipment is in continuous service, such as a transformer, the failure rate is dominated by time-related stresses compared to demand-related stresses. Other failure rates may be dominated by demands. Take a piece of wire and repeatedly bend it. With each bend its probability of catastrophic failure increases. In a relatively short time, if the bending is continued, the wire will fail. On the other hand, the same wire could be installed in a manner that would prevent mechanical bending demands. In this case, the occurrence of catastrophic wire breakage would be remote. In the first instance, the failure rate is dominated by demand stresses and in the second by time-related stresses, such as corrosion. 

The implication of the foregoing equations, that stress-corrosion cracking will occur if a mechanism exists for concentrating the electrochemical energy release rate at the crack tip or if the environment in some way serves to embrittle the metal, is a convenient introduction to a consideration of the mechanistic models of stress corrosion. In so far as the occurrence of stress corrosion in a susceptible material requires the conjoint action of a tensile stress and a dissolution process, it follows that the boundary conditions within which stress corrosion occurs will be those defined by failure... 

The occurrence of stress-corrosion cracking in the martensitic steels is very sensitive to the magnitude of the applied stress. For instance, a 13% chromium martensitic steel tested in boiling 35% magnesium chloride solution (125.5°C) indicated times to failure that decreased abruptly from more than 25(X)h to less than 0.1 h as the applied stress was increased from 620 MPa to about 650 MPa (Fig. 8.25). However, the effects of stress on time to failure are not always so dramatic. For instance, in the same set of experiments times to failure for a 17Cr-2Ni martensitic steel gradually decreased from more than 800 h to about 8 h as the applied stress was increased from 500 MPa to 800 MPa. 

The impetus for further developments was the recognition of the economic significance of corrosion phenomenon during the 19th century that led the British Association for the Advancement of Science to sponsor corrosion testing projects such as the corrosion of cast and wrought iron in river and seawater atmospheres in 1837. Early academic interest in corrosion phenomenon (up to the First World War) was followed by industrial interest due to the occurrence of equipment failures. An example of this is the corrosion-related failure of condenser tubes as reported by the Institute of Metals and the British Non-ferrous Metals Research Association in 1911. This initiative led to the development of new corrosion-resistant alloys, and the corrosion related failure of condenser tubes in the Second World War was an insignificant problem. 

A common problem in boilers is the occurrence of calcium oxide build-up on the heating elements. This is not a corrosion problem in itself, because it is caused by a chemical reaction in the water at high temperatures. However, a scale deposit present on a metal surface may cause corrosion under the deposit. This type of underdeposit corrosion can be aggravated when corrosive species such as sulfides and/or chlorides are present in the water. While scale deposits reduce the thermal conductivity of the steel, and thereby increase energy costs, corrosion of the heating element can lead to a catastrophic tubing failure, which requires costly repairs. 

S.3.7.3 Consequences of Failures There are many consequences of corrosion-based failures ranging from minor failures of equipment, loss of productivity, minor injuries to personnel, and as serious as loss of lives. Failure analysis is the conventional method of relating a failure to its consequences as well as the lessons learned from the failure along with the necessary steps and precautions to be taken to avoid the future occurrence of similar failure. Failures may range from modest cost of replacing a failed component to the possible destruction of a piece of equipment and fatalities. The consequences of a failure determine the priorities of the maintenance or improvements in design to prevent future failures of a similar nature and degree. 

Usually, an accident is caused not by a single event but by the occurrence of several concurrent events, sometimes called Swiss cheese effect, in which corrosion phenomena occur at the microscopic and macroscopic levels and cause strong deterioration of material properties, leading to the failure of a structure. In such situations, the solution to a problem can be the identification of a corrosion barrier that hinders the concatenation of events that would lead to failure. 

The feed control and instrument trips and alarms on the reactor will allow the process to be operated safely. In the event of failure of the control and trip system, the runaway reaction would be safely vented from the reactor. However, since this would result in a serious toxic and corrosive aerosol emission, the reliability of the trip and control system has to be checked. The use of Hazan allows the frequency of the occurrence of the runaway to be determined, in order to decide whether the frequency of emission is acceptable. 

One of the keys to the chemical biocompadbility of stahdess steel and cobalt chromium was the formation of a passivation laya in vivo, dius minimizing the amount of corrosion that occurs to the implant. However, as indicated in Table 13.4, while the strength of these two metals reduced the chance for failure within the implant, their elastic moduli are an order of magnitude higher than that seen in healthy cortical bone. This resulted in the occurrence of stress shielding and concomitant bone loss in many patients with large implants. 

Stress-corrosion cracking of steel was first encountered in a practical way in riveted steam boilers. Stresses at rivets always exceed the elastic Unfit, and boiler waters are normally treated with alkalies to minimize corrosion. Crevices between rivets and boiler plate allow boiler water to concentrate, until the concentration of alkali suffices to induce S.C.C., sometimes accompanied by explosion of the boiler. Because alkalies were recognized as one of the causes, failures of this kind were first called caustic embrittlement. With the advent of welded boilers and with improved boiler-water treatment, S.C.C. of boilers has become less common. Its occurrence has not been eliminated entirely, however, because significant stresses, for example, may be established at welded sections of boilers or in tanks used for storing concentrated alkalies. 

Another potential problem with flash pasteurisers is that of plate failure, when corrosion causes a hole to develop. This can allow unpasteurised beer or coolant to leak into the pasteurised beer. A good maintenance programme will minimise occurrence. Also a booster pump is typically used to ensure pasteurised beer is maintained at the highest pressure in the system (at least 0.5 bar higher than the product) so that in the event of any leakage it will be of pasteurised beer into unpasteurised beer or coolant, ensuring that product is not contaminated (Hyde, 2001). 

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