Processing-Related Corrosion

The top section of a crude unit can be subjected to a variety of corrosive agents. Hydrochloric acid formed from the hydrolysis of calcium and magnesium salts is the principal strong acid responsible for corrosion in the crude unit top section. Carbon dioxide is released from crudes typically produced in C02-flooded fields and crudes that contain large amounts of naphthenic acid. 

Acids such as formic, acetic, propionic, and butanoic are released from crudes with a high amount of naphthenic acid. Hydrogen sulfide, released from sour crudes, significantly increases the corrosion of the cmde unit top section. Both sulfuric and sulfurous acids formed by either oxidation of hydrogen sulfide or direct condensation of SO2 and SO3 increase corrosion. 

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Bias potentials provide corrosion mechanisms due to both anodic reaction and cathodic reaction. These reactions include the formation of acidic or alkaline electrolytes, and ion migration. Pore corrosion and corrosion product creep, galvanic corrosion, and firetting corrosion in connectors are other important mechanisms with electronics. Many of these mechanisms occur due to processing related corrosive residues. 

With respect to material selection, it is important to know about the typical process-related corrosion mechanisms. In the field of ammonia production, one should keep in mind the following ... 

Naphthenic acid corrosion has been a problem ia petroleum-refining operations siace the early 1900s. Naphthenic acid corrosion data have been reported for various materials of constmction (16), and correlations have been found relating corrosion rates to temperature and total acid number (17). Refineries processing highly naphthenic cmdes must use steel alloys 316 stainless steel [11107-04-3] is the material of choice. Conversely, naphthenic acid derivatives find use as corrosion inhibitors ia oil-weU and petroleum refinery appHcations. 

This type of stress-related corrosion process may result in boiler failure through a sudden and violent rupturing of the boiler tube metal. Austenitic stainless steels also are corroded by SCC mechanisms in the presence of concentrated chlorides (chloride-induced SCC). 

Anaerobic conditions in wastewater of sewer systems may give rise to formation of volatile substances that are typically identified by a number of problems, like malodors, health risks and corrosion. The conditions and processes related to such problems are dealt with in Section 3.2.2. Further information on measurement, modeling and control of odors can be found in Stuetz and Frechen (2001). 

Anaerobic conditions are traditionally a major concern when dealing with microbial-induced transformations of wastewater in a collection system. The problem is primarily associated with the risk of hydrogen sulfide and odorous organic compounds. The corresponding problems appear as concrete and metal corrosion, health-related impacts and malodors. Such in-sewer process-related problems have been reported as early as over 50 years ago (Parker, 1945a, 1945b Pomeroy and Bowlus, 1946). 

Electrochemical Characterization Technloues. Since corrosion Is an electrochemical process, It Is not surprising that a considerable amount of work has been reported over the years on electrical and electrochemical techniques for the study of the corrosion process. Leldhelser Ql.) and Szauer (12.> 11) have provided good reviews of the principal techniques. Walter has recently provided a review of DC electrochemical tests for painted metals (14). Both AC and DC methods have been employed to study a variety of Issues related to corrosion and corrosion protection. DC techniques are especially useful for studying substrate processes, while AC impedance techniques are most useful for studying processes relating to coated substrates and the performance of coatings. 

In this paragraph comparatively much attention will be paid to the curve in which tensile stress is plotted out in relation to relative elongation, because important properties can be inferred from this curve. One of these is the elastic modulus, a material property which was briefly discussed in chapter 9. This E-modulus often depends on the temperature and this relationship is represented in a log E-T curve. Next properties of the three groups of materials are compared in a table and finally some attention will be paid to processing and corrosion . 

Losses of phosphoric acid mists or phosphorus pentoxide fumes through a corrosion-induced rupture of ductwork or a vessel, or from a process-related parameter, represent the most commonly experienced emission control problems with furnace acid plants. When this occurs, a white plume, or ghost, of aerosol droplets or particles remains visible downwind long after the steam component has dissipated. Corrosion problems are now better understood so that more exposed components are constructed of stainless steel. If high temperatures are also involved, the equipment is lined with graphite [28]. 

If materials selection depends on corrosion control by process-related measures (such as chemical treatment), these should be indicated on the MSD. Indicate the intended injection points and the type of chemical to be injected. Examples include corrosion inhibitors, scale inhibitors, biocides, pH control chemicals, wash water, etc. Also indicate the location of proposed corrosion monitoring and sampling sites. If anodic or cathodic protection is to be part of the corrosion control design, the MSD or its Notes section should indicate the piping and/or equipment to be protected. 

For each, the generally required environmental condition (aerobic or anaerobic), the primary metabolic processes related to MIC, and the resultant chemical species that can increase corrosion rates are summarized in Table 7.4. These characteristics are discussed in the following paragraphs. 

Microorganism Generally required environmental condition l Primary metabolic process related to MIC Resultant chemical species that can increase corrosion rate... 

B. H. Cartledge Oak Ridge National Lab.) While I appreciate Huddle and Anderson s (Lecture 41) sympathetic reference to any work on inhibition by the pertechnetate ion, I must record any objection the interpretation of any views which they present. The processes of corrosion and inhibition are usually too complex to be related uniquely to single anodic and cathodic reactions. Thus, in aerated aqueous systems containing one of the inhibitors of the X04 type, there are at least three possible cathodic processes namely, reduction of oxygen, of hydrogen ions, or of the inhibitor anion, if it is reducible. The kinetics, as well as the thermodynamics, of the several reactions, determines which of them will predominate in any particular case. 

The assumption underpinning the incident pyramid is that the causes for all types of event are the same. In fact, this assumption is only partially correct because the root causes of minor events are different from those that lead to process safety events. Therefore, improving day-to-day safety will not necessarily reduce the number of serious incidents. Minor events are typically caused by occupational problems such as trips and falls, lack of proper PPE, and improper use of machinery. Major events, however, are more often caused by process safety problems such as incorrect instrument settings, corrosion, or mixing of incompatible chemicals. Hence a program that leads to improvements in occupational safety will not necessarily help reduce the frequency of process-related events. Indeed, improvements in the occupational safety record may induce a false sense of confidence regarding the potential for a major event. (It is probable, however, that a poor performance in occupational safety will correlate positively with a poor performance in process safety.)... 

Companies in the process industries habimally handle large quantities of toxic, flammable, and explosive materials—often at high temperamre and pressure. Such processes are inherently hazardous and have the potential to cause loss of life, serious injuries, and severe pollution. Since these processes are often quite complex and sophisticated and have many recycle streams, it means that the identification of hazards is not an easy process. Therefore, a wide range of PHA techniques has been developed and used over the years. The key to almost all of these techniques is that a team of experts analyzes the process in question to determine how major failures—often involving a very unlikely sequence of events—could occur. A PHA is not concerned with occupational hazards such as trips, falls, and the use of lock-out/tag-out rules. Instead it focuses on process-related issues such as overpressuring a vessel or damage caused by corrosion. 

The closely related process of corrosion is also amenable to study by the EQCM. Since the species transferred across the interface (metal ions) are generally relatively heavy, a gravimetric sensor is very sensitive. An excellent example of this... 

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