Corrosion acceleration mechanisms

Rose, R. M., Schiller, A. L. and Radin, E. L., Corrosion-Accelerated Mechanical Failure of a Vitallium Nail-Plate , Journal of Bone and Joint Surgery, 54A, 854-862 (1972)... 

Corrosion acceleration mechanisms by temperature gradient and fluctuation in dynamic TGTs. 

Influence of Hydrodynamic Conditions Inhibitors are often applied under conditions of liquid flow (e.g. in cooling systems or pipelines) and, consequently, the effect of the flow rate on their performance also has to be considered. First, the hydrodynamic conditions determine the transport of all dissolved species in the solution (inhibitor, H" ", OH , anions, corrosion products) and hence their concentration at the surface of the corroding metal. Since both inhibiting and corrosion-accelerating species are affected, the effect of transport on the inhibition efhciency is not easy to estimate and wiU depend strongly on the inhibition mechanism. Second, the liquid flow may exert mechanical forces on the metal surface, which may promote corrosion and counteract... 

Research has shown thaU in the absence of sulfate, SRB can switch to fermentation and ferment a variety of substances and produce hydrogen, carbon dioxide, and acetate. These products can then be used by other bacteria such as methane-producing bacteria (methanogenes), which, by consuming hydrogen, can accelerate corrosion through mechanisms such as cathodic depolarization. Therefore, fermentative capabilities of SRB allow them to use alternative substances other than sulfate. 

Erosion corrosion may be defined as accelerated corrosion arising from rapid relative movement of the corrosive fluid and the metal surface. At high velocities of the corrosive fluid, mechanical wear or abrasion occurs. An increase in velocity can increase the rate of corrosion by enhancing the supply of O2, CO2, H2S, or other constituents in the medium. 

Welds and mechanical joints can be areas where accelerated corrosion occurs for several reasons. Field coatings applied to joints are sometimes not of the same quality as shop-applied coatings on the remainder of the pipeline. This can lead to the environment penetrating to the metal and the formation of corrosion cells, Mechanical joints and rough welds can lead to crevice corrosion. Weld metallurgy also plays a role in corrosion. Welds containing high levels of sulfide inclusions can lead to localized corrosion of the weld metal. 

Steppan, J., Roth, J., Hall, L., et al., Review of Corrosion Failure Mechanisms During Accelerated Tests, Electrolytic Metal Migration, Journal of the Electrochemical Society. Vol. 134, No. 1, 1987, p. 175. 

Studies of thiourea and various derivatives suggest possible mechanisms for the dual nature of these compounds at a low inhibitor concentrations they are corrosion inhibitors, and at high concentrations they act as corrosion accelerators. Plllai and Narayan showed that indeed the mechanism in 4-12% HCI at room temperature changes in the concentration range of 4-40 /iM. 

While those authors have ruled out the effect of the acid produced by the bacteria on corrosion acceleration, they have suggested that in the presence of an aerobic hetertrophic bacterium, repassivation of pits does not happen but pit growth continues. They nominate pit propagation in the presence of bacteria as the main mechanism for observing the drop in carbon steel s open circuit potential (OCP) and polarisation resistance. 

It is essential to describe the relationship between the two sub-processes. To this aim, a choice was made based on a mechanical reason there is a cause-effect relationship between the corrosion rate and the width of the crack. When the corrosion accelerates (corrosion rate increases), the crack undergoes more stress, which will translate in an acceleration of the cracking. 

Waste corrosion. After canister failure, the waste form will begin to corrode in an environment of stagnant porewater. Vitrified waste dissolves at a very low rate and the corrosion rate is assumed to be constant with time (Grauer 1985). Arguments have been advanced for possible acceleration mechanisms (formation of crystalline secondary products which lower silica concentration and thus accelerate glass corrosion) but long-term (c. 5 year) experiments... 

In concrete, triethanolamine accelerates set time and increases early set strength (41—43). These ate often formulated as admixtures (44), for later addition to the concrete mixtures. Compared to calcium chloride, another common set accelerator, triethanolamine is less corrosive to steel-reinforcing materials, and gives a concrete that is more resistant to creep under stress (45). Triethanolamine can also neutralize any acid in the concrete and forms a salt with chlorides. Improvement of mechanical properties, whiteness, and more even distribution of iron impurities in the mixture of portland cements, can be effected by addition of 2% triethanolamine (46). Triethanolamine bottoms and alkanolamine soaps can also be used in these type appUcations. Waterproofing or sealing concrete can be accompUshed by using formulations containing triethanolamine (47,48). 

Biofilms can promote corrosion of fouled metal surfaces in a variety of ways. This is referred to as microbiaHy influenced corrosion. Microbes act as biological catalysts promoting conventional corrosion mechanisms the simple, passive presence of the biological deposit prevents corrosion inhibitors from reaching and passivating the fouled surface microbial reactions can accelerate ongoing corrosion reactions and microbial by-products can be directly aggressive to the metal. 

Impingement Corrosion This phenomenon is sometimes referred to as erosion-corrosion or velocity-accelerated corrosion. It occurs when damage is accelerated by the mechanical removal of corrosion products (such as oxides) which would otherwise tend to stifle the corrosion reac tion. 

Cavitation Formation of transient voids or vacuum bubbles in a liquid stream passing over a surface is called cavitation. This is often encountered arouna propellers, rudders, and struts and in pumps. When these bubbles collapse on a metal surface, there is a severe impact or explosive effec t that can cause considerable mechanical damage, and corrosion can be greatly accelerated because of the destruction of protective films. Redesign or a more resistant metal is generally required to avoid this problem. 

Two basic mechanisms cause biological corrosion. Biologically produced substances may actively or passively cause attack. Each mechanism either accelerates preexisting corrosion or establishes a new form of metal loss. Often the distinction between active and passive attack is vague. 

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