Corrosion 4 Flashcards
term used to describe this reduction, denoted as ΔG
Reaction affinity
a quantitative measure of a chemical reaction’s tendency to start, which includes the reaction of a metallic material with its environment.
Gibbs free energy change (ΔG)
- a direct measure of the work capacity or the maximum electric energy possible that is available from a system.
Gibbs free energy change (ΔG)
said to be lost and a spontaneous reaction direction of the system is observed.
negative free energy
change means that the transition indicates an energy increase, and it requires additional energy to be added to the system.
positive value of Gibbs free energy
can be classified as a state function and is independent of the reaction path.
change in free energy
can be defined as a product of charges moved (Q) and the electrochemical potential (E) through which it is moved.
electrical work (w)
crucial part of the corrosion process, can be used to express the tendency of a metal to corrode.
electromotive force (emf)
higher tendency for the overall chemical reaction of the cell to proceed
higher value of E
larger tendency to make the reaction proceed.
More negative value of ΔG
value refers to a reaction that has no tendency to start at all.
positive ΔG°
For a reaction to take place, the driving voltage must be
positive.
ΔG = 0, ΔG > 0
reaction might not occur
system is more likely to change in the other direction to that stated.
when ΔG > 0,
activity coefficient
dissolved species
fugacity coefficient
gaseous species
the tendency of a material to corrode is not a direct measure of reaction rate.
true
The standard conditions for solid, liquid compounds, and even elements is the pure compound or element; in the case of gases, it is set at a pressure of
100 kPa and for solutes, the ideal concentration is 1 M (mol/L).
equilibrium electrodes are also termed
half-cells
”. If these electrodes are maintained at equilibrium conditions, they are now called
standard half-cells
an equation can be derived to determine the emf of a certain cell by virtue of the reactants and products’ concentration.
The Nernst Equation
used to determine the potential of a system wherein the reactants are not in unit activity.
Nernst equation
study of reaction rates at the interface between an electrode and a liquid.
Electrode Kinetics
The deviation from equilibrium potential is called
polarization
measurement of magnitude of polarization with respect to the equilibrium potential of an electrode.
Overvoltage
electrochemical process that is controlled by the reaction sequence at the metal-electrolyte interface.
Activation Polarization
usually is the controlling factor during corrosion in media containing a high concentration of active species.
Activation Polarization
generally predominates when the concentration of the reducible species is small.
Concentration polarization
- electrochemical reactions that are controlled by the diffusion in the electrolyte.
Concentration polarization
polarization can lead to the formation of a protective oxide layer on the metal’ s surface which acts as a barrier, slowing down further corrosion.
Passivation
polarization can also increase corrosion rates by promoting more aggressive electrochemical reactions. This can happen if conditions favor the dissolution of the protective oxide layer.
Increased Corrosion
polarization gradients within the metal surface can lead to the concentration of corrosive agents in specific areas, accelerating corrosion rates locally.
Localized Corrosion
amount of current (I) over a given area (A)
- The total charge that passes through the cell when N moles of the metal M react is Q.
CURRENT DENSITY
a model for the current density of an electrode when the only significant limiting factor is activation.
tafel equation
uniformly over the entire exposed surface.
- does not create a great concern, because the life of equipment can be accurately estimated on the basis of comparatively simple tests.
Uniform Corrosion
Metal deterioration that happens in tight spaces where a stagnant liquid gets trapped.
Crevice Corrosion
- A highly localized form of attack that eats away at the metal, creating small holes or cavities.
Pitting Corrosion
Due to chemical composition difference at the grain boundaries from the bulk of the grain.
two dissimilar metals with potential difference in a conductive solution.
- s a serious threat because it accelerates the deterioration of the less resistant metal.
Galvanic Corrosion
Degradation that occurs when hydrogen penetrates the surface of a metal.
Hydrogen Damage
