Recognizing Corrosion

The previous chapters provide an introduction to the general science of corrosion processes with some practical applications. In reality, the principles that govern these scientific concepts are rarely of interest to people facing corrosion problems. The main questions generally asked by most people facing a corrosion problem are 

The present chapter will focus on answering the first of these questions and Chap. 7 the last. The following quotation from Fontana and Greene s classic textbook on corrosion engineering originally published in 1967 [1] summarizes a training principle that has been reused extensively by many instructors and that is central in all modern training manuals on the subject. 

It is convenient to classify corrosion by the forms in which it manifests itself, the basis for this classification being the appearance of the corroded metal. Each form can be identified by mere visual observation. In most cases the naked eye is sufficient, but sometimes magnification is helpful or required. Valuable information for the solution of a corrosion problem can often be obtained through careful observation of the corroded test specimens or failed equipment. 

It is now widely accepted that much can be deduced from examination of materials which have failed in service and that it is often possible by visual examination to decide which corrosion mechanisms have been at work and what corrective measures are required. In another widely used NACE document, Paul Dillon and 

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In the absence of any other relevant information, it is essential to obtain this via an internationally recognized corrosion/irritation test before proceeding to a rabbit eye irritation test. This must be conducted in a staged manner. If possible, this should be achieved using a validated, accepted in vitro skin corrosivity assay. If this is not available, then the assessment should be completed using animal tests (see the skin irritation/ corrosion strategy, section 3.2.2). 

An empirical correlation was established between the factors listed in Table 2 and these foiros of corrosion. Fifteen recognized corrosion experts agreed to complete an opinion poll listing the main subfactors and the common forms of corrosion. The responses were then analyzed and represented in a graphical way, such as illustrated in Fig. 2, for... 

Recognize corrosion problems in materials used at the site and make monitoring a normal part of the operation. Sour oil and gas operations are often conducted under high pressure and corrosive conditions. Therefore, in addition to temperature and pressure considerations, system designs for the wellhead, downhole equipment, and pipeUnes must have features to minimize the effects of corrosion and prevent an accidental release of H2S. Corrosion-inhibiting fluids can be used to prevent internal corrosion and cathodic protection can be used to prevent external corrosion. Also, during extended periods of shut-in and injection into pipelines, inhibitor applications may be beneficial. 

The value of this scheme, extended to other corrosion modes and forms, should be apparent. It is considered to be extremely useful for analyzing corrosion failures and for reporting and storing information and data in a complete and systematic manner. An empirical correlation was established between the factors listed in Table 5.2 and the forms of corrosion described earlier (Fig. 5.1). Several recognized corrosion experts were asked to complete an opinion poll listing the main subfactors and the common forms of corrosion as illustrated in the example shown in Fig. 5.21. Background information on the factors and forms of corrosion was attached to the survey. The responses were then analyzed and represented in the graphical way illustrated in Fig. 5.22. 

Use and Uimitations of Electrochemical Techniques A major caution must be noted as to the general, indiscriminate use of all electrochemical tests, especially the use of AC and EIS test techniques, for the study of corrosion systems. AC and EIS techniques are apphcable for the evaluation of very thin films or deposits that are uniform, constant, and stable—for example, thin-film protective coatings. Sometimes, researchers do not recognize the dynamic nature of some passive films, corrosion produc ts, or deposits from other sources nor do they even consider the possibility of a change in the surface conditions during the course of their experiment. As an example, it is note-... 

Underdeposit corrosion is not so much a single corrosion mechanism as it is a generic description of wastage beneath deposits. Attack may appear much the same beneath silt, precipitates, metal oxides, and debris. Differential oxygen concentration cell corrosion may appear much the same beneath all kinds of deposits. However, when deposits tend to directly interact with metal surfaces, attack is easier to recognize. 

Fresh acid attack is recognized by the absence of corrosion product in wasted areas and the sharpness of attack. Oxide layers are usually easily stripped by a test drop of hydrochloric acid in freshly corroded areas. Deposits are almost always absent. Edges of attacked areas are sharp and angular, as intervening corrosion has not recently occurred. In stainless steels such distinctions blur, as corrosion in intervening periods is usually slight. 

Impediments to water flow resulting from inadequate equipment design or lodgement of foreign objects in the tubes can exercise a dramatic effect on the erosion-corrosion process. Much of this influence is linked to the creation of turbulence and the simple increase in fluid velocity past obstructions. The importance of these factors is quickly recognized when the phenomenon of threshold velocity is considered. 

Galvanic corrosion typically involves two or more dissimilar metals. It should be recognized, however, that sufficient variation in environmental and physical parameters such as fluid chemistry, temperature (see Case History 16.3), flow velocity, and even variations in degrees of metal cold work can induce a flow of corrosion current even within the same metal. 

The fall in reduction of area and the occurrence of internal cracks are a measure of the corrosion damage. There exists a clear correlation with cathodic current density in which a slight inhibition due to O2 and stimulation by CO2 can be recognized. The susceptibility is very high in the range of cathodic overprotection and is independent of the composition of the medium. 

The action of effects in the environment and cathodic current densities on ac corrosion requires even more careful investigation. It is important to recognize that ac current densities above 50 A m can lead to damage even when the dc potential is formally fulfilling the protection criterion [40]. 

Compared with cathodic blisters, which can be recognized by their alkali content, anodic blisters can be easily overlooked. Intact blisters can be recognized by the slightly lower pH value of the hydrolyzed corrosion product. The pitted surface at a damaged blister cannot be distinguished from that formed at pores. 

The surfaces to be protected should be the total surface, including inserts, spars and pipes. The upper 1.5 m of the side walls and the covers should be provided with a coating of recognized quality [10] to protect against corrosion. 

There are now two distinct forms of hot corrosion recognized by the industry, although the end result is the same. These two types are high-temperature (Type 1) and low-temperature (Type 2) hot corrosion. 

Gases that tend to polymerize with temperature must be recognized and, as mentioned for corrosion, temperature limits must be imposed at the application phase to prevent problems later in operation. The oniv compressor, of the types covered, that is not as sensitive to the presence of polymers is the helical-lobe compressor. It should be recognized ihat even this machine does have limits. 

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