Model corrosion protection mechanisms

Most accelerated corrosion tests cannot accurately predict the corrosion rate or part corrosion life. These accelerated tests should be used to provide qualitative insight into how materials will react in a corrosive environment, instead of being used as a quantitative prediction of part performance. Specific analytical method, test, and modeling are often applied for individual cases due to the complexity of these subjects. ASM Handbooks provide a comprehensive discussion on corrosion in a set of three volumes 13A (ASM, 2003), 13B (ASM, 2005), and 13C (ASM, 2006). Volume 13A introduces the fimdamental principles of corrosion mechanisms, their testing methods, and corrosion protection. Volumes 13B and 13C focus on specific materials, environments, and industries. 

Requirements specified in this way are deemed-to-satisfy rules. Such rules cannot be used to quantify the performance of the structure in general, specific effects of additional measures (for instance increasing the cover to the steel), or the consequences of sub-standard practice (for example using a higher w/c). In this respect it is important to note that EN 206 also allows the use of alternative performance-related design methods with respect to durability that consider in a quantitative way each relevant deterioration mechanism, the service life of the element or structure, and the criteria that define the end of the service life. Such methods should draw a picture of the characteristics that the concrete must possess to protect the reinforcement for the service life requested from a predictive model of the corrosion attack. These refined methods (as opposed to standard methods) may be based on long-term experience with local practices in local environments, on data from an established performance test method for the relevant mechanism, or on the use of proven predictive models. 

The durability of a glass is a function of both its kinetic rate of approach to equilibrium and its final thermodynamic equilibrium state in an aqueous environment [8,9]. Time-dependent corrosion of many glasses has been subjected to kinetic models [5,10-12]. Ion exchange, diffusion, and protective layer formation mechanisms are explained by these models. 

Corrosion is an irreversible surface modification of a material due to chemical reaction with the environment that results in the formation of metal ions dissolved in the liquid (material loss) and, in the case of passive metals, of surface oxide films. A preliminary attempt to include particle flow in tribocorrosion was already proposed by Stemp [11] and Mischler et al [9] to explain the discrepancy mentioned above between first body degradation and mechanical wear. This paper is aimed at developing a phenomenological model of tribocorrosion by combining electrochemical corrosion effects with the third body concept of wear. The approach is applied to three electrochemically controlled wear situations, i.e. wear under cathodic protection (absence of corrosion), wear in presence of passive films and wear combined with metal dissolution. The proposed concepts are compared to already published results concerning carbon steel and stainless steels and their merits are discussed. 

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