Lecture 12: Erosion Corrosion and Environment-Sensitive Cracking

[Previous Lecture] [Next Lecture] [Course Outline] [General Info] [Study Guide]

Erosion Corrosion

The progressive loss of material from a solid surface due to mechanical interaction between that surface and a fluid.

8.1. EFFECT OF FLOW RATE

(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 !!!

[Back to Top]

IMPINGEMENT ATTACK

Localized erosion corrosion caused by turbulence or impinging flow at certain points.

Examples:

[Back to Top]

8.2 CAVITATION DAMAGE

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

Back to Top]

Environment-Sensitive Cracking

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

Mechanisms

Prevention of SCC

[Back to Top]

Summary

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.

Reading Assignments

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)

[Back to Top]