Page 213 - Corrosion Engineering Principles and Practice

188 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 189

Mechanism of Metal Loss Velocity Exponent, n
Corrosion
• Liquid-phase mass transfer control 0.8–1
• Charge-transfer (activation) control 0
• Mixed (charge/mass transfer) control 0–1
• Activation/repassivation (passive films) 1
Erosion
• Solid-particle impingement 2–3
• Liquid droplet impingement in high-speed gas flow 5–8
• Cavitation attack 5–8

TABLE 6.2 Flow Velocity as a Diagnostic Tool for Erosion–Corrosion Rates Following
Damage to the Protective Film

Sources of various mechanical forces involved in the erosion of
protective films and underlying metal are listed here and illustrated in
Fig. 6.39 [29]:
• Turbulent flow, fluctuating shear stresses, and pressure impacts
• Impact of suspended solid particles
• Impact of suspended liquid droplets in high-speed gas flow
• Impact of suspended gas bubbles in aqueous flow
• The violent collapse of vapor bubbles following cavitation
For single-phase turbulent flow in pipes distinct “breakaway”
velocities above which damage occurs in copper tubes have been
observed, giving rise to the concept of a critical shear stress [31].
The largest values are obtained during quasicyclic bursting events
close to the wall, which are said to be responsible for most of the
turbulent energy production in the entire wall-bounded shear flow
[32]. This is true of both nondisturbed and disturbed pipe flows. In
practice, film removal in single-phase aqueous flow is invariably
associated with the vortices created under disturbed flow conditions
produced by sudden macro or microscale changes in the flow
geometry. Table 6.3 summarizes the critical flow parameters that
should be considered when designing a copper alloy tubing system
for carrying seawater [28].

6.4.1 Erosion–Corrosion
The properties of surface films that naturally form on metals and
alloys are important elements to understand the resistance of metallic
materials to erosion–corrosion. Most metals and alloys used in