|
[Previous Lecture] [Next Lecture] [Course Outline] [General Info] [Study Guide]
The progressive loss of material from a solid surface due to mechanical interaction between that surface and a fluid.
(1) disturbs equilibrium, increases dissolution;
(2) supplies oxygen, minimizes differential aeration cells.
(3) supplies aggressive ions, increases dosage of additives (inhibitors etc).
(4) carries solid particles, scours away protective layers, enhances corrosion.
(5) sufficient flow to prevent deposition of dirt, minimizes differential-aeration cells.
The effects of flow rate are unpredictable !!!
Localized erosion corrosion caused by turbulence or impinging flow at certain points.
Examples:
A particular form of erosion corrosion caused by the formation and collapse of bubbles of vapor on metal surfaces.
In cavitation
components moving at high speed through a stationary fluid such as propellers, impellers, hydraulic turbine gear.
In erosion corrosion
fluid flows across stationary metal surfaces such as pipes, tanks
Season cracking (1921): cracking of brass cartridges in the monsoon season
residual stress from manufacturing process + ammonia from horse urine => cracking of brass
stress corrosion cracking, environment-sensitive,
caustic embrittlement => explosion of boilers
mild steel + deposit of caustics (NaOH added to boiler water to prevent scaling)
Fig10.2
A particular metal/environment in the presence of tensile stress => environment-sensitive cracking
table 10.1
In erosion corrosion , cavitation and impingement, the mechanical factor (flow rate) plays a more important role than the electrochemical processes. In environment-sensitive cracking, three essential factors (tensile stress, specific chemical species and a susceptible material) are combined to cause a cracking.
To reinforce learnings in this lecture read pages 191-197, 214-228 (textbook)
To prepare yourself for the next lecture
read pages 256-273, 279-300 (textbook)