creep and corrosion Flashcards
What is Creep?
- It’s a slow process
- Occurs in components which see a constant static stress applied to them normally in elevated temperature environments. Deformation process
- It’s a diffusion-based process.
- Also occurs in polymer Tg, except isn’t diffusion based there.
What are the stages of Creep?
- To begin (not a stage) a material has a load(force) applied on it that causes instantaneous deformation. Then the stages are:
- Primary: continuously decreasing creep strain hardening
- Secondary (steady state): constant rate (creep strain is linear, epsilon with dot is strain rate), majority of service life is spent in secondary creep
- Tertiary: rate acceleration and failure.
What is the creep rate, ε̊ , equation?
- Rate is the key parameter or life planning.
- The rate is however temperature and stress-dependent (Arrhenius equation with added things, as it is a diffusion-controlled mechanism)
- The rate is influenced by a combination of the applied stress, the temperature and the creep mechanism
What is the difference between oxidation and reduction reactions?
Oxidation (anodic) reactions:
* The element loses or gives up valence electrons
* The oxidation reactions occur at the anode.
Reduction reaction (cathodic):
* The generated electrons are involved in further reactions which vary depending on the solution and the dissolved elements. Reduction of hydrogen in acids.
* The reduction reaction occurs at the cathode.
Why is electric potentials measured for materials?
- Different materials have different electric potentials.
- The standard electrode potential of different metals is ranked based on the measured voltage when the metal is coupled with a standard hydrogen electrode.
What is the standard emf series?
- Shows what is going to be the anode and cathode and what is the magnitude of the driving force of this reaction.
- To see the driving force, compare the different materials standard electrode potential.
What is the Galvanic Series?
- Seawater has cl- ions, changing electrochemistry and charge carrying capability of electrolyte.
- So the placings of some of the materials change in the emf series, shown in the Galvanic series.
What are the 8 types of corrosion?
try define them
- Uniform attack
- Galvanic corrosion
- Crevice corrosion
- Pitting
- Intergranular corrosion
- Selective leaching
- Tribocorrosion
- Stress corrosion.
How does Oxide formation protect agianst corrosion?
- Not all corrosion is bad
- Oxides can form both in aqueous solutions: The oxide that forms becomes a ceramic, that can adhere to the surface, which prevents electron flow a lot more than before.
- Can also occur in air (dry corrosion)
- Oxides provide a barrier to ionic diffusion and electrical conduction (insulative)
What is a passivated metal?
- Where a finishing process provides protection against corrosion, usually by a chemcial treatment. For example:
- Aluminium and chromium develop an oxide layer which adheres well, is coherent (defect-free) and thus protective.
- Steel mixed with 12% chromium- Cr oxidises preferentially on the surface and produces an impervious protective Cr2O3 layer
- CoCrMo alloys used in hips ~26-30% Cr for protective Cr2O3 layer.
- Aluminium can be anodised to increase the thickness of the oxide layer (drive more oxidation electrochemically) - the oxide layer is protective.
How are sacrificial anodes used to protect against corrosion?
Give an example of materials that use it and process names.
- Galvanization: put zinc all over
- Electrode potentials: Zinc will form the anode and steel will be the cathode, and so zinc will corrode while steel stays behind.
What two common ways would you protect a meterial from corrosion with, when underground?
- Imposed protection via impressed current (shown on right diagram)
- Anodic protection: sacrificial anode (shown on left diagram)
What are some ways to prevent corrosion?
