# recorded lecture

# live lecture
pre recorded lectures are about electrohemistry
there is polarisation and passivation

typically, the anode corrodes 

polarisation means that the rate of corrosion is highest at the start and then it slows down as the metal polarises


uniform corrosion occurs everywhere at once

## localised corrosion
localised corrosion can be a lot faster than uniform corroion
the bigger the cathode, the bigger the surface area
the bigger the anode, the wider area the corrosion occurs over
the bigger the electrochemmical driving force, the fast er the corrosion

zinc can be put on top of steel to protect it as a sacraficial anode

if there is a pit in the microstructire, the surface can be the cathode and the pit the anode because of the different species. This will lead to fast corrosion

## more kinds of corrosion
![[Pasted image 20230213141327.png]]
crevice corrosion is localised and rapid
this could be from under a bolt perchance
the electrochemistry is driven by a difference in oxygen concentration. The covered area loses oxygen so it is the anode

![[Pasted image 20230213141527.png]]

usually hidden, if the pit gets clogged, it will accelerate as no more oxygen gets in

### grain boundaries
![[Pasted image 20230213141632.png]]
grain boundaries are high energy areas, but they also have different species in them
if stainless is welded, the chromium in it will precipitate on the grain boundaries, making an anode
the precipitated chromium in the grain boundary is the anode, and the grains are the cathode

to prevent this, welds should be cooled down quickly
carbon content should be minimised so carbides cant form as easily
something could be alloyed in that will form a carbide preferentially over chromium

## cavitation

combines corrosion and erosion to eat away at coatings
this can be particularly bad if there are small particles in the fluid that are brought along with the fluid

it is dependant on the angle of the impact by the jet. If it is at 90 degress it is plastic or brittle failure
closer to parallel is abrasion

## stress corrosion cracking
attacks at grain boundaries
little warning, can happen at $0.01K_{1c}$
needs to be in a corrosive environment

## corrosion protection
can connect a steel pipe to magnesium underground to protect the pipe
this is **anodic protection**

can force a current into an anode to protect a tank/pipe
this is **cathodic protection**

a system can be sealed so oxygen cannot get in. This means that there will be a bit of corrosion at first to use up the oxygen, and then there is no corrosion because there is no oxygen

welds are better than bolts for corrosion prevention

using polymers means no corrosion will occur, becuase it cant

paints can be used to seal 

## wear

2 body abrasion is when there are two rough surfaces moving past another and the peaks that touch have high stresses, pressures and temperatures
3 body abrasion is when there are 2 objects moving past each other and a hard particle in the middle of them

adhesive wear is when the surfaces weld to eachother