Sweet corrosion

Tubing corrosion due to FIgS (sour corrosion) or COg (sweet corrosion) may become so severe that the tubing leaks. This would certainly require a workover. Monitoring of the... 

Carbon dioxide dissolves in water to form a weak acid (carbonic acid), which reduces the pH of the solution and, consequently, increases its corrosivity. Corrosion caused by carbon dioxide is generally referred to as sweet corrosion, and results in pitting. The mechanism of carbon dioxide corrosion is as follows [197,198] ... 

Ethoxylated and propoxylated alkylphenol amines, converted into the amides with a fatty acid or similar long chain diacids, are effective in controlling sour and sweet corrosion [1782,1783,1785-1787]. 

The de Waard-Milliams model is a well-known modeH - used in industry (such as subsea pipeline corrosion) to predict corrosion, and it is the cornerstone of commercially available corrosion prediction software packages such as Cassandra. Despite its applicability in industry, a significant disadvantage of this model is that it does not consider MIC. In 2002, a NACE paper was published in which the described models were related to various mechanisms from sweet corrosion, sour corrosion, and organic acid corrosion to oxygen corrosion and MIC. Obviously, it is the model describing MIC that concerns us here. 

Sweet Corrosion. It is caused by the presence of dissolved COj in the prodnced llnids. CO2 reacts with water to form carbonic acid (H2CO3), which dissociates to form hydrogen ions and the carbonate ion. At the anodic sites, the metal atoms give np electrons and dissolve to form metal ions. These electrons are taken up by hydrogen ions at cathodic sites to form atomic hydrogen. Bicarbonate ions, however, react to form a protective iron carbonate film, and the rate of corrosion depends on the stability of this film [2]. The corrosion mechanism can be represented by the sketch shown in Figure 11.7. 

In the case of oil pipelines, the rate of sweet corrosion is significantly high at the 6 o clock position because of the preferential separation of water layers at the bottom of the pipeline. In wet gas handling pipelines, the sweet corrosion may take place in the form of top-of-line corrosion. In this form, due to heat loss to the environment, the pipeline becomes cooler. This causes the condensation of water on the top surface of the pipeline. This prevents the formation of a protective carbonate scale and thereby corrodes the inner top surface of the pipeline. Sweet corrosion initiates at areas where the protective iron carbonate films are unstable. This instability could be due to existing pits on the metal surface or due to presence of surface impurities such as sulfide inclusions. This results in... 

Mesa attacks. They are a form of sweet corrosion. Mesa attacks involve the local removal of the protective iron carbonate film formed during sweet corrosion. This is achieved first by localized corrosion beneath the iron carbonate film and subsequent removal of the film by flow forces. The risk of mesa attacks is reduced considerably at temperatures higher than 100°C, as the rate of precipitation of iron carbonate is very high at such temperatures, leading to improved iron carbonate film protection [8]. 

Martensitic stainless steel (MSS) contains 12-13% chromium and is frequently used in oil and gas applications where sweet corrosion is expected. These steels have been used in Oil Country Tubular Goods (OCTG) applications. They have considerable resistance against sweet corrosion, but they are prone to corrosion even in low levels of H2S. The approximate composition of MSS is given below ... 

Forms of corrosion by CO2 (sweet corrosion) such as ringworm corrosion and mesa corrosion... 

Several oil and gas fields are very active in CO2 corrosion (sweet corrosion). In Holland, the bottom temperature of gas wells was reported to be 97-124°C and the pressure was 0.9-3 bar. The corrosion rate of carbon steel was observed to be maximum at —90°C followed by a gradual decrease above it [28]. The corrosion rate tends to decrease with increased chromium contents. 

Dry COj is non-corrosive up to about 400 C, while it is corrosive when dissolved in water. COj corrosion in the presence of free water is known as sweet corrosion. COj dissolves in the water phase forming carbonic acid, which decreases the water acidity the final pH of the solution will depend on the temperature and COj partial pressure. The corrosivity of COj saturated solutions is much higher than other acid solutions at the same pH, because of the direct action of COj in the corrosion phenomena. 

In the 1950 s, crude oils were either corrosive (sour), or non-corrosive (sweet). Crudes containing more than 6 ppm of dissolved H2S were classed as sour because, beyond this limit, corrosion was observed on the walls of storage tanks by formation of scales of pyrophoric iron sulfides. 

Trichloroethylene [79-01-6J, trichloroethene, CHCL=CCL2, commonly called "tri," is a colorless, sweet smelling (chloroformlike odor), volatile Hquid and a powerhil solvent for a large number of natural and synthetic substances. It is nonflammable under conditions of recommended use. In the absence of stabilizers, it is slowly decomposed (autoxidized) by air. The oxidation products are acidic and corrosive. Stabilizers are added to all commercial grades. Trichloroethylene is moderately toxic and has narcotic properties. 

Natural gas with H2S or other sulfur compounds present is called sour gas, while gas with only CO2 is called sweet. Both H2S and CO2 are undesirable, as they cause corrosion and reduce the heating value and thus the sales value of the gas. In addition, H2S may be lethal in very small quantities. Table 7-1 shows physiological effects of H2S concentrations in air. 

Organic materials Corrosive vapours are sometimes emitted by organic materials used either in packaging or in the manufactured article, and may be troublesome in confined spaces. Some woods, particularly unseasoned oak and sweet chestnut, produce acetic acid (see Section 18.10), and certain polymers used in paints, adhesives and plastics may liberate such corrosive vapours as formic acid and hydrogen sulphide . It may be necessary to carry out exposure trials, particularly where materials capable of liberating formaldehyde or formic acid are involved. Most corrosion problems of this kind can be prevented by using desiccants, and in many cases they are confined to imperfectly cured materials. For an excellent review see Reference 9. 

Sweet Crude—Crude oil that (1) is not corrosive when heated. (2) does not evolve significant amounts of hydrogen sulfide on distillation, and (3) produces light fractions which do not require sweetening. Examples are offshore Louisiana, Libyan, and Nigerian crudes. 

In consideration of some operational difficulties, like biofouling and corrosion, directly related to the feed-water source, more reliable water sources have been evaluated by the Evides Industriewater that owns and operates this plant, like anaerobic groundwater and sweet tertiary wastewater. 

Perchloryl fluoride is a non corrosive gas with a sweet taste. It has a melting point of -148 Celsius and a boiling point of -47 Celsius. Perchlorly fluoride is a very stable gas at room temperature. Explosions will result if perchloryl fluoride is treated with reducing agents such as amides, powdered metals, hydrides, sulfites, phosphites, phosphides, or hydrazines or when treated with alcohols. Avoid skin contact and inhalation. 

WATER — Sweet Water — So called because it has the power to dissolve Gold and Silver without corrosion. 

TURBITH MINERALS - is Mercury without any Corrosive, precipitated in a sweet state. 

When the sulfur content of the crude oil is low (usually less than one percent by weight), the crude oil is known as a sweet crude, while crude oil with higher concentrations of sulfur is called sour crude. Removal of sulfur and other impurities form part of the treating processes and sulfur itself can form a valuable by-product in a refinery as an input into the chemical industry. Other impurities include nitrogen, oxygen, and salt, as well as small quantities of metals such as vanadium and nickel that are common in certain of the heavier crude oils. As well as extraction processes to purify oil and its products of impurities, specific additives are also used to react with corrosive or odiferous constituents to produce harmless and odorless substances. Such processes are generally termed sweetening processes. 

Alcohol, one and a half ounce tincture of steel, one and a half ounce corrosive sublimate, one and a half ounce sweet spirits of nitre, ono and a half ounce blue vitriol, one ounce nitric acid, three-quarters of an ounce. Mix and dissolve in one quart of warm water. Keep in glass bottles. 

Browning Becipes /or Tufist and Laminated Barreie. i, Sweet spirit of nitre, one balf ounce tincture of steel, one-quarter ounce corrosive soldi mate, f>ne-haJf ounce aqua foi tis, sixty drops nitiate of silver, four grains a small lump of cb and one pint of rain water. 

Corrosive, colorless gas. Somewhat sweet, suffocating odor- mp —76.34. bp 11.75", Extremely reactive. Glass wool and organic matter burst into flames on contact even with dil vapors. Violently hydrolyzed by water. Attacks quartz if traces of moisture are present. Liquid is yellow-green in color and solid is white. 

Monochlorobenzene and 1,2-dichlorobenzene are both colorless, flammable hquids with a sweet, penetrating odor. These are soluble in benzene and alcohols, but are insoluble in water. The vapors are heavier than air and may travel a considerable distance from the source of evaporation. Ignition and flashback to the source may lead to a fire, releasing in toxic and corrosive gases. Monochlorobenzene is a process solvent for methylene diisocyanate, the latter being used as a solvent in adhesives, polishes, waxes, pharmaceutical products, and natural rubber. 

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