Corrosion Flashcards
How does energy change through a corrosion process?
Starts as an ore, increase energy to make it into a steel beam, decrease energy and add air and moisture and goes back to rust which is the ore.
What environments cause various materials to corrode?
Metals - water and high temperature oxidation
Ceramics - sea, water, microbial
Glass - high temperature oxidation
Polymers - oxidation, perishing
Timber - water swelling, fungal activity
Textiles - microbio attack by sweat
What are the two charges on grains through a metal and which corrode?
Anodes - positive (they corrode)
Cathodes - negative
How does corrosion work in anodes?
Layer of water on surface, oxygen above
In anodic regions, positive metal ions escape to water (dissolution)
Metal loses electrons and oxidates
Current (electrons) flow from anode to cathode, which come to the surface and produce OH- ions with the water (reduction)
What are the essentials for electrochemical corrosion reactions?
1) Anodic site - oxidation or electron loss occurs
2) Catholic site - reduction or electron gain occurs
3) Electrolytic conductor - contact medium that allows ions to flow between anode and cathode like water
4) Electronic conductor - connection between anode and cathode which will allow flow of electrons (most metals)
What is Electrode Potential?
Not all electrons oxidise and give metals with equal ease.
When a perfect ideal metal is placed in an electrolyte, an electrode potential is developed which is a measure of tendency of metal to give up electrons (oxidation). This is offset by an equal opposite force for the metal to gain electrons (reduction) so no net reaction occurs.
Rate at which electrons leave compared to when they rejoin the metal is measured as a potential.
What are the Standard Electrode Potentials?
Potentials given for each elements.
Tendency for the forward reaction (reduction)
More positive = more likely to get reduction
More negative = more likely to get oxidation
Most negative are the most reactive
What is the Gibbs Energy change of a cell reaction?
Change in G0 = -n F E0cell
N - number of electrons involved
F - faraday constant
E0cell - equilibrium cell potential (given)
If change in G0 is positive, reaction is spontaneous
If change in G0 is negative, reaction is not spontaneous (wont occur unless electrons are provided by external source)
What is pitting and what can cause it?
Localised corrosion which attacks small areas on a surface where the passive surface has broken down. The passive film keeps this in one place to stop it spreading. These pits can vary in depth and can lead to cracking and failure.
Chemically, what usually causes pitting?
An aggressive negative ion in the electrolyte (typically chloride - found in seawater)
These chloride ions are small and diffuse through the solution easily, sometimes even migrating through and braking down the oxide film.
In pitting corrosion, what is essentially the anode and cathode?
Bottom of pit is the anode and the surface is the cathode so oxidation keeps occurring in the pit.
How can pitting corrosion become autocatalytic?
Positively charged ions are released by the anode at the bottom of the pit. Hydrolysis reaction takes place which reduces the OH- ions so negatively charged Cl ions are attracted to neutralise the charge. This makes the electrolyte in the pit highly acidic and concentrated with chloride ions so it speeds up the pitting corrosion.
What is bi-metallic or galvanic corrosion?
When two dissimilar metals are coupled in an aqueous environment (occurs like the standard anode cathode corrosion)
What surface area of anode and cathode are favourable?
Anode is undergoing corrosion so large anode area and small cathode will minimise damage as its spread over a larger area.
How would Cu with Fe rivets corrode differently from Fe with Cu rivets?
Fe is more anodic and Cu is more cathodic so if you used Fe rivets, there would be a lot of corrosion as the anodic area is small, compared to Cu rivets on an Fe base which is a large anodic area so less corrosion.
What is creviced corrosion and where does it occur?
Localised corrosion that occurs due to a part of the metal surface being in a shielded environment compared to the rest of the metal which is in a larger volume of electrolyte (occurs on any but mainly passive metals)
Occurs in small areas with stagnant solutions like in holes, lap joints, under rivet heads etc plus under sand, barnacles etc in seawater.
Gap must be wide enough to allow solution in but narrow enough to keep it stagnant (microns to sub mm range)
How does crevice corrosion take place chemically?
Initiation is due to a difference in oxygen concentration (differential oxygen cell)
Reduction occurs on bulk surface (cathode) and initially the same in the crevice.
Initially, reduction and dissolution occur uniformly across the surface and counteract each other.
Eventually, oxygen in crevice area depletes so reduction in crevice ceases which creates the differential oxygen cell
In crevice, lower oxygen area has a lower potential and becomes anodic compared to the bigger cathodic surface
Like pitting, eventually positive metal ions build up in crevice.
A hydrolysis reaction takes place and causes reduced OH- ions so Cl- ions come in to neutralise the charge and this acidifies and furthers the corrosion.
