Carbon dioxide corrosion rates

M. Foss, E. Gulbrandsen, J. Sjoblom 2008. Alteration of wettability of corroding carbon steel surface by carbon dioxide corrosion inhibitors - Effect on carbon dioxide corrosion rate and contact angle. Corrosion 64 (12), 905-919. 

Carbon dioxide corrosion can be controlled by the use of caustic soda and lime and the addition of various inhibitors. Film-forming amine inhibitors are used to reduce the corrosion rates. The control measures will be discussed later. 

The data of Table 10.26 can now be interpreted in terms of mass transport. Above a critical flow velocity, which depends on pipe diameter, the rate of mass transport becomes sufficiently fast to carry away dissolved corrosion products without forming a salt film. In other words, their surface concentration remains below saturation. Under these conditions scales that are formed by precipitation of corrosion products at lower flow velocity can not exist and the corrosion rate therefore will be higher. An example in case is carbon dioxide corrosion of carbon steel that occurs in fluids containing... 

The chemistry of carbon dioxide corrosion in the oil and gas industry is well-known and will not be discussed here. There are extensive research papers available on that subject [see, for example, Nesic (1995)]. The corrosion rate increases with carbon dioxide partial pressure, due to increased dissolution of the gas into the aque-... 

The corrosion of steel by carbon dioxide and other dissolved gases has been studied on a laboratory scale by Watkins and Kincheloe (1958) and Watkins and Wright (1953). These investigators found that the presence of oxygen greatly accelerates the rate of corrosion by carbon dioxide, while hydrogen sulfide in small quantities inhibits carbon dioxide corrosion. G>mparative data taken from their corrosion curves are presented in Table 6-9. 

Because PTFE resins decompose slowly, they may be heated to a high temperature. The toxicity of the pyrolysis products warrants care where exposure of personnel is likely to occur (120). Above 230°C decomposition rates become measurable (0.0001% per hour). Small amounts of toxic perfiuoroisobutylene have been isolated at 400°C and above free fluorine has never been found. Above 690°C the decomposition products bum but do not support combustion if the heat is removed. Combustion products consist primarily of carbon dioxide, carbon tetrafluoride, and small quantities of toxic and corrosive hydrogen fluoride. The PTFE resins are nonflammable and do not propagate flame. 

Union Carbide has developed Amine Guard, which essentially eliminates corrosion in amine systems (32—35). It permits the use of substantially higher amine concentrations and greater carbon dioxide pick-up rates without corrosive attack. This results in an energy requirement comparable to that of the carbonate process and allows the use of smaller equipment for a specific C02-removal appHcation thereby reducing the capital cost. 

The main advantages of the Cosorb process over the older copper ammonium salt process are low corrosion rate, abiHty to work in carbon dioxide atmospheres, and low energy consumption. The active CuAlCl C H CH complex is considerably more stable than the cuprous ammonium salt, and solvent toluene losses are much lower than the ammonia losses of the older process (94). 

Fire Hazards - Flash Point Flammable solid Flammable limits in Air (%) Not pertinent Fire Extinguishing Agents Sand and carbon dioxide Fire Extinguishing Agents Not to be Used Water fecial Hazards of Combustion Products Products of combustion include sulfur dioxide and phosphorus pentoxide, which are irritating, toxic and corrosive Behavior in Fire Not pertinent Ignition Temperature (deg. F) 527 (liquid) Electrical Hazard Not pertinent Burning Rate Not pertinem. 

If calcium or magnesium bicarbonates are present in water, the rise in temperature decomposes them, and subsequent evolution of carbon dioxide will result In a higher corrosion rate, while at the same time calcium and magnesium carbonates may deposit on the metal surface. This scale may be protective, thus slowing the corrosion rate however, it can create concentration cells if it is deposited loosely, exposing parts of the surface. 

Oxygen dissolved in aqueous solutions, even in very low concentrations, is a leading cause of corrosion problems (i.e., pitting) in drilling. Its presence also accelerates the corrosion rate of other corrodents such as hydrogen sulfide and carbon dioxide. Oxygen plays a dual role both as a cathodic depolarizer and an anodic polarizer or passivator. Within a certain range of concentration the... 

Figure 4-463. Effect of carbon dioxide concentration on corrosion rate. (From Ref. [211J.)...

Figure 4-469 shows the effect on corrosion rates of 1020 steel in different water systems with dissolved hydrogen sulfide. The difference in corrosion rates is due to different corrosion products formed in different solutions. In solution I, kansite forms. Kansite is widely protective as the pyrrhotite coats the surface giving slightly more protection until a very protective pyrite scale is formed. In solution II, only kansite scale forms, resulting in continued increase in the corrosion rate. Finally, in solution 111, pyrite scale is formed as in solution I however, continued corrosion may be due to the presence of carbon dioxide. 

Arctic Drilling. Corrosion problems encountered in arctic area drilling are no different from problems faced in other areas of the world. It is a general misconception that during arctic drilling corrosion-related problems are either not very severe or totally absent due to low temperatures. Cool temperatures may slow down the corrosion process. However, they also increase the solubility of oxygen, carbon dioxide and hydrogen sulfide. Therefore, the net result can be an increase in the rate of corrosion. While cold temperatures may cause problems, the temperature fluctuation common in arctic environments can be a more severe source of corrosion-related problems [215]. 

Carbon dioxide produces a solution of carbonic acid (as in boiler condensate, see Section 53.3.2). Carbon steel is often employed but corrosion rates of up to 1 mm/yr can be encountered. Coatings and non-metallic materials may be employed up to their temperature limits (Section 53.5.6). Basic austenitic stainless steels (type 534) are suitable up to their scaling temperatures. 

Corrosive species in the atmospheres include water, salts and gases. Clean atmospheres contain little other than oxygen, nitrogen, water vapor and a small quantity of carbon dioxide. These species are virtually non-corrosive to any of the common constructional materials for plant at normal temperatures. Steel is susceptible to corrosion in even fairly clean air where water can exist as liquid. For plant operating at temperatures up to approximately 100°C coatings are employed to protect steel if required. In clean air corrosion rates are low, and corrosion is primarily a cosmetic problem, although it may be necessary to prevent mst staining of nearby materials. 

Those waters in which the carbon dioxide content is in excess of that required as bicarbonate ion to balance the bases present are among the most aggressive of the fresh waters. Hard waters usually, though not invariably, deposit a carbonate scale and are generally not appreciably corrosive to cast iron, corrosion rates of less than 0-02 mm/y being frequently encountered. Water-softening processes do not increase the corrosivity of the water provided that the process does not result in the development of an excess of dissolved carbon dioxide. 

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