Corrosion inhibitors environment conditioning

Most microbial desulfurization studies have been conducted in the laboratory shake-flask type experiments, and the major drawback cited against such a process has been that the rates of pyritic sulfur removal were not high enough to reduce the reactor size to a reasonable capacity (2,6). In this study an attempt has been made to determine the effectiveness of T. ferrooxidans under simulated pipeline conditions for pyritic sulfur removal. Since the microbial desulfurization process is conducted under acidic environment, an attempt has been made to determine the corrosion rates under dynamic conditions using Illinois //6 and Indiana 3 bituminous coals and to investigate the effectiveness of a commercial corrosion inhibitor for controlling the corrosivity. 

Laboratory tests confirmed that in the absence of effective corrosion inhibitors molybdenum disulphide could cause corrosion in humid environments. Kay ° showed in tests with different steels that corrosion was accelerated in the presence of loose molybdenum disulphide powder, especially ball-milled or micronated powder. Her test conditions were realistic, namely 20 C and 90% relative humidity for six days, but the use of loose powder was not representative of practical use, and it subsequently became clear that burnished films were less active in promoting corrosion. Calhoun et al also showed that molybdenum disulphide in a bonded film actively promoted corrosion, although their test conditions were severe, consisting of salt fog and salt spray tests. They found that corrosion was more severe when graphite was present, but that molybdenum disulphide also clearly caused corrosion. 

Simple but pedagogically useful theories of electrode kinetics are presented in Chapter 3. This permits discussion of models for anodic and cathodic reactions at the metal/environment interface and for diffusion of species to and from the interface. Mathematical models of these theories lead to so-called kinetic parameters whose values govern the rate of the interface reaction. The range of values that these parameters can have and some of the variables that can influence the values are emphasized since these will relate to understanding the influence of such factors as surface conditions (roughness, corrosion product films, etc.), corrosion inhibitors and accelerators, and fluid velocity on corrosion rates. This chapter also introduces electrochemical measurements to determine values of the kinetic parameters. 

The vast majority of corrosion inhibitors in neutral environment as well as a number of acid corrosion inhibitors form protective 3D films on the metal surface ( interphase inhibition [4]). These films may consist of adsorbate multilayers, ox-ide/hydroxides, salts, or reaction products formed by interaction of the inhibitor with solution species on or near the corroding metal surface (e.g. dissolved metal ions). The type, structure, and thickness of the inhibiting films are strongly influenced by the environmental conditions. The interphase films act as a physical barrier that blocks or retards transport processes and the kinetics of the corrosion reactions at the metal surface. The inhibitive properties could, in some cases, be correlated with the chemical stability of the corresponding insoluble complexes as well as with the solubihty, adsorbabOity, and hydrophobicity of the inhibitor molecules [35]. Often, other ions from the electrolyte, such as... 

As outlined above, surfactants are added to acids to perform one or more of several needed functions. However, other chemicals are also added to the acid. These additives inclnde corrosion inhibitors [IS], iron control agents [J9, 20], hydrogen sulfide scavengers [21], scale inhibitor [22] and clay stabilizers [23], It is very important to perform compatibility tests of the selected surfactant with the acid formnla, especially in this complex environment. Also, some of the snrfactants are nsed in high temperature and high salinity applications. Therefore, it is necessary to ensure thermal stability of these surfactants nnder these harsh conditions. 

If the pipelines are located in deep water or inaccessible places, maintenance and/or replacement of pipes and clean-up of oil spills is difficult and expensive. The use of expensive stainless steel or other alloys is not a viable solution due to the required capital costs, which are not economically feasible in many cases. Hence the strategy to combat corrosion problems is to use carbon steel pipes, but reduce and, if possible, prevent the rate of material losses. An important method of corrosion reduction practiced in the oil and gas industry is the use of corrosion inhibitors. The inhibitors act by bonding to the metal surface and forming a protective film. The performance of the inhibitor depends on the metal surface, the inhibitor composition, and fluid flow conditions. Some of the inhibitors are not effective because of the multiphase flow conditions in the pipelines. When selecting an inhibitor, it is essential to know its effectiveness in the specific service environment. Detailed knowledge of the metal surface conditions, the operating temperature and pressure, the fluid properties, the solution pH, and the multiphase flow conditions is essential. 

Fatty acids, their esters and triglycerides have lubricating functions. Under higji-pressure conditions, - sulfated fats and oils and sulfonated oils (- sulfated fats and oils) show good performance. Because a drilling hole is sometimes a corrosive environment corrosion inhibitors have to be added to drilling muds. - Fatty acid amino amides, - imidazoline derivatives and - fatty amines are effective. 

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

There are in addition several other factors that accelerate corrosion and must betaken into account these include crevices, galvanic coupling, tensile stress, aeration, presence of impurities, surface finish, etc. If these were also taken into consideration then several million experiments would have to be performed to compile such data. There are many instances where two or more chemicals exert a marked synergistic action such that low dissolution rates obtained in either environment become much greater in the presence of both. Further, the corrosiveness of a chemical will be affected by the presence of certain impurities, which may act as either accelerators or inhibitors. To take all these factors into account would add to an already impossible task and as Evans has remarked, There are not enough trained investigators in the world to obtain the empirical information to cover all combinations of conditions likely to arise . Unfortunately corrosion science has not yet reached the stage where prediction, based on a few well established laws, allows selection of materials to be made without recourse to a vast amount of data. 

This is a prolific field for inhibitors although the main types remain as grouped by Bergmanm (see p. 17). In this application of inhibitors, probably more than in any other, the methods of introducing the inhibitor into the corrosive environment receive as much attention as the nature of the inhibitor. The most severe conditions are those met in acidising treatments, typically with 15-35% hydrochloric acid at high downhole temperatures. 

Inhibitors must be chosen after taking into account the nature and combinations of metals present, the nature of the corrosive environment and the operating conditions in terms of flow, temperature, heat transfer, (see Principles p. 17 14). 

Although the use of inhibitors can mitigate corrosion to some extent in such hostile environments (15) designing systems with appropriate phase behaviour under bottom hole conditions is difficult. To date oil based inhibitor carrier systems have been employed but these require large volumes of fluid circulation and it is often difficult to maintain a continuous oil wet surface on the tubulars. A water-inhibitor system would overcome many of these problems since the sour gas fluid is already saturated with water in the reservoir. Such a system, however, remains to be designed. 

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