Corrosion Flashcards
What is an electrochemical reaction
A reaction which involves charge transfer (electro) and formation of a new compound from chemical elements
What is required for charge transfer to occur and what functions to fill this requirement
Charge transfers requires a charge carrier
+ve → ions
-ve → electrons
(T/F) metal ions do not act as a charge carrier for positive charge
F
(T/F) In an electrochemical cell, something losses change and something gains charge
T
Parts of an electro chemical cell
1) electrodes → anode, cathode
2) electrolyte: solution where electrical conduction is carried out by ions
3) electrical contact→ wire
Anode____ electrons, cathode ___ electrons
Losses, gains
Anode = ___. Cathode=____
Oxidization, reduction
What can be an electrolyte
Any thing that allows the flow of ions
What can be an electrical contact
Any thing that allows electrons to flow (current)
How much of an electrochemical cell is submerged
Every thing but the wire/electrical contact
Which way do electrons flow and why
From anode to cathode
Cathode has higher electron affinity
What does current meassure
Charge
What does voltage measure
Electrical potential difference
(T/f) since there is a current, there is a voltage
T
Spontaneous def
The direction a reaction proceeds in naturally
What is required to proceed with an unspontaneous reaction
Induced charge
As electrons more from ___ to ___ through ___ , ions move from ___ to ___ through ___.
Anode
Cathode
Electrical contact
Anode
Cathode
Electrolyte
What is a half-cell reaction
A reaction which describes a element of intent in relation to a standard cell, allows us to compare elements of interest
How to tell anode vs cathode on an EMF table
More positive = cathode = wants electrons
What form is the EMF series in
Reduction form, typically based on a standard hydrogen electrode
What is a standard electrode
An electrode that is used as a reference to compare reactions or elements against
Describe SHE
Standard hydrogen electrode
Electrolyte of I mol/L [ ] of metal ions
298 k
Saturated with 1 atm of hydrogen gas
Platinum electrode
E not = 0.0 v
(T/f) liquid is compressible
F
Does potential change if conditions change from standard conditions? How do we account for this?
Yes, nerst’s eg
Examples of standard electrodes
Standard hydrogen electrode
Silver-silver chloride ( agci)
Calomel +0.244 V
Ag Cl standard electrode vs SHE
+ 0.222 V
What does the degree symbol mean?
Standard conditions
What are standard conditions?
298.15 K→ 25 C
I mol/ L cathode ions near cathode and same for anode
1 atm pressure
Voltage units
V= j/c
E meaning and relation to spontaneous
E = electrical potential
Positive = spontaneous
Delta g?
Gibs free energy, energy change in a system
>0 = cathode = absorbs energy
<0= anode = release energy
What is n
N = # of electrons transferred
F
Fairiday’s constant
Total energy change per mol of elections
What does the sign of delta G depend on
E
Corrosion def
Gradual deterioration of materials over time due to chemical or electrochemical reactions with surrounding environment leads to dissolution or formation of non-metallic scale or film
Results in deterioration of properties optical, physicals anything that effects function)
Occurs in metals and ceramics
Why is corrosion bad?
Leads to premature failure
Degradation of mechanical properties
Decrease of material integrity and thus structural strength
Costs money to fix things when they break
Ex of corrosion
Rust (iron)
Patina ( copper)
Oxidization
Chemical attacks
Anodizing
Can corrosion ever be stopped?
No, there will always be current, it can only be greatly slowed down
High level, how to slow down corrosion
Proper design, material selection and processing
Nickname for corrosion
Environmental attack
Degradation def
Corrosion in polymers due to internation with it’s environment
Requirements for corrosion (an electrochemical cell) to work
1) materials with different desire for elections (E not cell not equal to 0)
2) path for electron transfer
3) path for ion transfer
Concentration of solid species
Always 1 since they do not change
Can nearst eq be used for half-cell reactions
Yes be careful using it in reduction form
What are the two main ways corrosion can be categorized
By uniformity
By mechanism
Categories of uniformity of corrosion
Uniform corrosion → slow (slow ro corrosion), easy to spot (large area)
Localized corrosion → fast (fast ro corrosion), difficult to spot (small area)
Mechanisms of corrosion
Electro chemical cell
Galvanic Cell
Multiphase / impurity cell
Intergranular corrosion
Grain-grain boundary cell
* Stress-driven corrosion
Concentration cell
Crevice corrosion
Pitting corrosion
* microbial corrosion
What is galvanic corrosion
Two metals or alloys with different E in electrical contact with each other in a corrosive electrolyte
Is essentially an electrochemical cell
What is the most common electrolyte
Water
(T/f) dropping water on a part of a cell would not start concentration cell corrosion
F
Why does measure electrical potential between two electrodes sometimes not and up to the theoretical value
Water tends to be very active as an electrode and can react to beach an electrode, replacing ore of the existing electrodes
What ions can water be split into
H+ and OH-
What happens when water reacts instead of the intended electrodes
Instead of plating the other electrode, the anode its will react with H+ or OH- to create a gas
What determines which water ion interferes with a reaction
The presence of oxygen
Oxygen-free = H+
Aerated = OH-
What is the reaction quotient for half-cell reactions
1/ [ ion]
Are high ph substances corrosive? What about basic?
Ph = power of hydrogen = # of hydrogen ions
High ph = more hydrogen ions = more corrosive since more hydrogen electrode to react
Same premise for basic on an OH- scale
What is a multi-phasic or impurity cell
A electrochemical cell is created between different phases of an alloy
This creates an area of fast localized corrosion
Common in alloys since more alloys rely on multiplphases
Localized corrosion takes over and the alloy is no longer protected
*electrochemical potential difference created due phases with different potentials
What is sanitization
Multi-phasic cell corrosion in stainless steel
Usually occurs due to poor heat treatment
Anode = ferrite (2+) or austensite
Cathode = cemented (OH-)
What is grain-grain boundary cell corrosion
- potential difference created by pot diff that exists between grains and GB
It is very small but is enough to cause corrosion
Ex: etching