Corrosion Flashcards
Oxidation reaction
- When a metal loses electrons + becomes positive - takes place at anode + can be gain of oxygen/loss of H
- Material removed from anode (corrosion)
Reduction reaction
- Element gains electrons + becomes more negative - site of reduction is cathode + can be loss of oxygen/gain of H
- Material added to cathode (electroplating)
Standard half cell
Pure metal electrode immersed in 1M solution of its ions at 25 degrees C
When are potentials calculated for EMF series valid?
For 1M solution at 25 degrees C + requires 0 current
Does Nernst equation require 0 current?
Yuh
Polarization
Change in electrode potential caused by some effect - when electrochemical cell doing work + current flowing through it the potential of each electrode changes due to polarization
Overvoltage
Difference between closed + open circuit voltage
Activation polarization
Potential difference beyond value of equilibrium needed to generate currents depending on energy activation of redox rxn - over voltage is driving force for rxn
Concentration polarization
Part of polarization of electrolytic cell resulting from changes in electrolyte concentration due to passage of current through electrode/solution interface - affects cathode only and when diffusion is rate limiting
Passivity
When thin oxide film forms as barrier to corrosion - occurs only on anode
Passive region
Oxide layer forms on surface which prevents passage of current
Transpassive region
Potential is high enough to break oxide layer
8 types of corrosion
Uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosion-corrosion, stress corrosion
Galvanic corrosion
Occurs when 2 metals w/diff composition are electrically couples while being exposed to electrolyte
How to avoid Galvanic corrosion?
Choose metals close in galvanic series, avoid unfavourable anode-cathode area ratios, electrical insulation + connect to a more anodic metal
Crevice corrosion
Occurs because of concentration differences in ions - occurs in region of low concentration
Pitting
Concentration difference driven like crevice corrosion but corrosion pit forms as deep well driven by gravity
Intergranular corrosion
Occurs at grain boundaries often when metals are heated
Selective leaching
When 1 element of alloy is selectively removed by corrosion process
Erosion corrosion
Occurs from combined action of chemical attack and mechanical abrasion from fluid motion - especially harmful to alloys that normally passivate as it removes protective layer
Stress corrosion
When metals that normally resist certain corrosive environment corrode when stress is applied in addition to corrosive environment - small cracks form + propagate in direction perpendicular to stress
Hydrogen embrittlement
- Reduction in ductility + tensile strength when atomic H penetrates structure of material - failure mode
- May be induced by cathodic protection
General corrosion prevention
- Select material that doesn’t corrode in application environment
- Make environment less corrosive
- Use chemical corrosion inhibitors in environment which chemically bind surface + prevent corrosion
- Make parts of interest a cathode
Cathodic protection
Create electrical connection to more anodic metal which will preferentially corrode - can be forced by applying external potential
Pilling-Bedworth ratio
Evaluates volume of oxide vs volume of metal it’s replacing
P-B ratio < 1
Oxide takes less volume than metal
P-B ratio = 1
Oxide takes same volume as metal
P-B ratio > 1
Oxide takes greater volume than metal
Kinetics of well-adhering films
Oxide growth limited by ionic diffusion described by Fick’s law - growth kinetics for these is parabolic
Kinetics of films that are porous/flake off
Kinetics of very thin oxides growing close to room temperature
Logarithmic growth kinetics
