Corrosion Flashcards
What does corrosion consist of?
A chemical reaction with a transfer of electrons from one chemical species to another.
-Metal atoms characteristically give up electrons. OXIDATION
Where does oxidation occur?
Oxidation occurs at the anode, Oxidation is an anodic reaction
What is the other chemical reaction involved in corrosion?
Electrons generates from each metal atom must be transferred and become part of a second chemical species. This is termed a REDUCTION reaction.
Where does reduction occur?
The location at which reduction occurs is called the cathode.
What is the REDOX reaction for the corrosion of zinc in an acidic solution?
The oxidation reaction is Zn → Zn 2 + +2 e-
The reduction reaction i 2H + +2 e- → H2
What is electrode potential?
Not all metals oxidise with the same ease. A current will pass. This current can be termed the electrode potential of an electrochemical cell
What is a galvanic couple?
Arrangement is called a galvanic couple. Two metals electrically connected in a liquid electrolyte when one corrodes (anode) while the other acts as a cathode
What is the emf series
The electromotive force (emf) series is generated by coupling standard hydrogen electrode and ranking according to voltage
Explain the emf series
Metals higher on the emf series are more cathodic meaning they are least likely to react (corrode) and metals lower down on the series are more anodic which means that they react more easily. When standard half cells are coupled, the metal lowest in the emf series will corrode.
What are the terms for the following; CPR=KW/⍴At?
CPR is the corrosion penetration rate
W is the weight loss after time t (mg)
⍴ is the density of the material (g/cm^3)
A is the exposed area (cm^2)
K is a constant, for mm/year this value is 87.6
What is the galvanic series?
A series which represents the relative reactivities of metals/alloys in seawater. No voltages are provided with this series.
What are the means of corrosion prevention?
-Materials selection
-Lower temperature
-Apply physical barriers
-Add inhibitors
-Cathodic (sacrificial) protection
Explain material selection for corrosion prevention
-Use metals that are relatively unreactive in the corrosion environment
-Use metals that passivate (form a thin adhering oxide layer) which slows corrosion.
Explain lowering temperature for corrosion prevention
Lowering the temperature reduces the rate of oxidation and reaction. Practically very hard to implement
Explain applying physical barriers for corrosion prevention
We can add films/coatings which will protect the material from corrosion as the material will not be in contact with the corrosive enrivonment
Explain adding inhibitors for corrosion prevention
These are substances that decreases the reactivity of the electrolyte solution. Usually used in a closed system such as radiators and boilers etc.
Explain cathodic (sacrificial) protection for corrosion prevention
Involves supplying electrons from an external source to the metal to be protected making it a cathode. Corrosion reaction is forced to occur in the reverse (reduction) direction.
There are two methods for this:
- Attach a more anodic material to the one to be protected
-Use a sacrificial anode
What are the forms of corrosion?
-Uniform attack
-Stress corrosion
-Erosion-corrosion
-Pitting
-Crevice
-Galvanic
-Intergranular
-Selective leaching
Explain a uniform attack
Oxidation & reduction reactions occur uniformly over surfaces
Explain a galvanic attack
Dissimilar metals are physically joined in the presence of an electrolyte – the more anodic metal corrodes.
Explain a crevice attack
Corrosion occurring in narrow and confined spaces
Explain pitting
The downward propagation of small pits and holes
Explain intergranular corrosion
This is corrosion that occurs along grain boundaries
Explain Erosion-Corrosion
Arises from the combined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion. Can be identified by surface grooves and waves having contours that are characteristic of the flow of the fluid
Explain stress corrosion
Results from the combined action of an applied tensile stress and a corrosive environment. Some materials that are inert in a particular corrosive medium become susceptible to stress corrosion when a stress is applied. Small cracks form and then propagate in a direction perpendicular to the stress applied.
Explain selective leaching corrosion
This attack can be found in solid solution alloys. It occurs when one constituent is removed as a consequence of the corrosion process.
