Crevice corrosion inhibitors

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. 

Chromates are particularly effective inhibitors, and there appear to be several components to inhibition. Chromate in solution inhibits metal dissolution and oxygen reduction reactions. It also slows metastable pitting, the transition to stable pitting, and, when present in sufficient concentration, the growth stage of pitting and crevice corrosion. 

Production of sulfides. This may involve the production of FeS, Fe (OH)2 etc. and an aggressive chemical agent such as hydrogen sulfide (H2S) or acidity. Micro-organisms may also consume chemical species that are important in corrosion reactions (e.g., oxygen or nitrite inhibitors). Alternatively, their physical presence may form a slime or poultice, which leads to differential aeration cell attack or crevice corrosion. They may also break down the desirable physical properties of lubricating oils or protective coatings. (Stott)5... 

In fact, when testing corrosion inhibitors, the major amount of corrosion may occur under the mounting washers as fresh inhibitor cannot reach such a confined space and, in addition, the low oxygen concentration leads to corrosion. As in the section above, ferric ions may seep from within the crevice and deposit around the fixture, thus giving rise to under-deposit corrosion. 

Crevice corrosion A1015 with rubber O ring in OAM NaCl CeCl3 inhibitor [6]... 

To combat the crevice corrosion one should seal gaps and clearances by metal materials not provoking corrosion of the basic material and apply electrochemical protection and corrosion inhibitors [13,20]. 

Although the design aspects provide most of the corrosion protection for airplanes, corrosion inhibitors are widely used to provide additional protection when used periodically in service. Corrosion inhibitors are applied in areas such as the lobe of the fuselage and most of the aluminum parts. The corrosion inhibitors are petroleum-based compounds that either displace the water or serve as a coating. The water-displacing inhibitors are sprayed onto a structure to penetrate faying surfaces and keep water away from crevices. The application of these inhibitors must be repeated at intervals of every few years. The more viscous heavy-duty inhibitors are also sprayed, which form a much thicker film and have a lesser penetrating ability. These thicker inhibitors are applied on airplane parts that are most prone to corrosion. 

Transport Phenomena Memy corrosion inhibitors are higher molecular compounds, and are usually present in small concentrations. If the objective is localized corrosion inhibition (crevice corrosion, imder-dep>osit corrosion, etc.), the ability of the inhibitor to migrate where it is needed becomes a major concern that needs to be included in the design of the test methodology. 

Suppliers usually provide the laboratory evaluation (using conventional test methods) of corrosion inhibitors, scale inhibitors, bactericides, and oxygen scavengers. User companies generally provide evaluation by field monitoring with corrosion specimens. Crevices may be incorporated in specimens to evaluate the possibility of localized corrosion, such as that related to oxygen contamination. 

Local pH drop is not the single cause of crevice corrosion. For example, local depletion of passivating species may be the origin of crevice corrosion as is the case for steels in cooling water with anodic inhibitors such as nitrite or chromate ions. ... 

I Note that cathodic inhibitors do not cause such severe crevice corrosion because their local depletion does not significantly increase the cathodic current available in a deaerated crevice. 

This section deals with the corrosion behavior of duplex stainless steel, some high austenitic stainless steels, and a nickel-based alloy in 28% HCl acidizing solutions, either in inhibited or uninhibited conditions, at 130 °C. Weight loss, crevice corrosion, and stress corrosion cracking tests were carried out for 6 and 24 hours, with amine-based commercial corrosion inhibitors, originally formulated for carbon steel. 

Crevice corrosion tests revealed susceptibility to corrosion attack in the presence of corrosion inhibitors, while in their absence no preferential attack in crevice sites was observed, see, in the presence of commercial inhibitors, did not occur for any tested materials in the absence of corrosion inhibitors, transgranular microcracks were present at the bottom of pits on high austenitic stainless steel. 

This form of corrosion can result because of a deficiency of oxygen in the crevice, acidity changes in the crevice, buildup of ions in the crevice, or depletion of an inhibitor. 

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. 

The chromic acid process is preferred where the eleciroiyie is likely to be entrapped in crevices as it is an inhibitor for aluminium whereas sulphuric acid is corrosive. 

Presence of crevices, dead-ends, etc. Effective protection by inhibitors relies on the continued access of inhibitor to all parts of the metal surface Aeration and Movement of the Liquid). It frequently happens that this condition is difficult to achieve due to the presence of crevices at joints, deadends in pipes, gas pockets, deposits of corrosion products, etc. Corrosion will then occur at these sites even though the rest of the system remains adequately protected. 

When metal parts make contact, or when metals contact a nonmetallic part such as a gasket, the crevice formed at the point of contact can become a corrosion site. Corrosive attack at this site is usually more severe than the surrounding area due to factors such as oxygen deficiency, acidic changes, or inhibitor depletion. Using... 

The application of a mixture of inhibitors for the corrosion control may be useful. Capillary condensation can cause the formation of a liquid in the crevices at lower values of relative humidity. It would be prudent to fill crevices with corrosion-inhibited putty. 

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