Mass and Charge Balance (MCB) Model Simulations of Current, Oxide Growth and Dissolution during Corrosion of Co-Cr Alloy Stellite-6

Abstract

A mass and charge balance (MCB) model that can simulate oxide growth and dissolution kinetics simultaneously during corrosion of an alloy has been recently developed. In this study, the MCB model was applied to the corrosion of the Co-Cr alloy Stellite-6. The construction of the model and the assignment of values to the rate parameters for the model are presented. The model simulation results are then compared with experimental corrosion data collected as a function of pH, temperature and polarization potential. The data include the current during potentiostatic polarization, the corrosion potential under open-circuit conditions, and post-test analyses of the oxide formed and the amount of metal dissolved. Excellent agreement between the model results and experimental data are found. This is evidence that the MCB model is a useful tool for predicting time-dependent corrosion while an oxide film is evolving. Alloys owe their corrosion resistance to protective oxide films formed on their surfaces. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Corrosion involves surface redox reactions (metal oxidation coupled with solution reduction) and interfacial transfer of the charged species involved in the redox reactions. The presence of an oxide film hinders the interfacial charge transfer, slowing the rate of corrosion. Modeling oxide formation and growth as a function of the solution environment is an important component in predicting the corrosion behavior of an alloy.Several corrosion models have been developed for predicting the rate of oxide growth 11,18-31 and/or the rate of metal dissolution in the presence of an oxide film. To obtain the overall rate of corrosion, many of these models focus on solving the transport rate equations for individual charge carriers (interstitial cations and anions, cation and anion vacancies, and electrons and holes) across the oxide film, in addition to the rates of their creation at respective interfaces (i.e. electrochemical redox reaction rates). These models assign rates to the transport of different charge carriers that are difficult to verify. Since the rate of interfacial charge transfer depends on the electric field present at the interface, the electrochemical potentials of the alloy, the oxide and the solution phases are important parameters in determining the corrosion rate.Many of the existing models acknowledge that the driving force for corrosion depends on the potential of the corroding system (the corrosion potential (E corr ) in an open circuit or the applied potential (E app ) during polarization). However, these models do not specifically quantify the driving force as a function of electrode potential and/or other quantifiable potentials such as the equilibrium potential of a redox pair involved in corrosion (E eq rdx ). The electrode potential and the redox pair may change with time as corrosion progresses. The nature of oxide and the oxide layer structure can also change with time as corrosion progresses even under potentiostatic polarization or con...

Inverse Crevice Corrosion of Carbon Steel: Effect of Solution Volume to Surface Area

“…For parallel reactions, the yields of their products are proportional to their relative rates. 42,46,47,58,59 For CS corrosion the oxidation of iron to progressively higher oxidation state cations (Fe II to Fe II /Fe III and to Fe III ) occurs in sequence, while each oxidized iron species can follow two parallel reaction pathways, dissolution into solution or solid oxide formation. Thus, the rates of dissolution and oxide formation of each metal cation (R Diss# and R MO# , respectively) formed from metal oxidation are not independent of each other and are related to the oxidation rate that produces the metal cation (R OX# ):…”

Combined effect of gamma-radiation and pH on corrosion of Ni–Cr–Fe alloy inconel 600

“…3. More details on this quantitative AES depth profile analysis method can be found in references [25,45].…”

“…In the presence of oxide on a corroding surface a net flux of positive charge flows from the metal to the solution phase through the oxide layer. To account for the net cation transfer through a solid oxide lattice, many mechanisms, such as transport of cation and anion vacancies, transport through interstitials and electron hopping (or ion exchanges), have been proposed and incorporated in corrosion kinetic models [12,[25][26][27][28][29][30][31].…”

“…Thus, at any given reaction time the charge flux due to oxidation of metal (Jox (t)) must be the same as the sum of the charge fluxes resulting in growth of oxide MO x (J MOx (t)) and metal ion dissolution (J diss (t)) [12,25]:…”

“…The rate of corrosion is controlled by a number of factors beyond the electrochemical potential of the system. Particularly important factors are the type(s) and the thickness(es) of any oxide layer(s) present on an alloy surface [25][26][27][28][29][30][31]. Since the oxide layer plays an important role in determining the corrosion resistance of an alloy, the type of oxide that can be formed and the rate of its formation and growth are of extreme importance in determining the longer term corrosion rate.…”