Lecture 12: Erosion Corrosion and
                      Environment - Sensitive Cracking

• Erosion Corrosion
• Effect of Flow Rate
• Impingement Attack
• Cavitation Damage
• Environment-Sensitive Cracking
• Mechanism
• Prevention
• Summary
• Reading Assignments

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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 !!!

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IMPINGEMENT ATTACK

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

Examples:

• a sudden change in the bore diameter or direction of a pipe;
• a badly fitting gasket or joint which introduces a discontinuity in the otherwise smooth metal surface;
• a crevice which allows liquid flow outside the main body of fluid;
• presence of deposit which may disturb the laminar flow.

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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

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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

Mechanisms

• Pre-existing active paths model. e.g.sensitized stainless steel, intergranular SCC
• Strain-generated active paths model. e.g. for passive metals stress ruptures film locally setting up active-passive cell
• Adsorption-related mechanism damaging species adsorbs on the crack surfaces, lowering the fracture stress

Prevention of SCC

• Select a material not known to crack in the specific env involved; ensure that the microstructure is not susceptible to intergranular corrosion cracking.
• Lower internal stresses on the metal and use stress relief anneal
• Design to minimize tensile stresses and stress raisers.Eliminate the specific species (Cl^-, NH₃, etc) from the environment.

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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)

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