Erosion Corrosion Flashcards
What is erosion corrosion? What kinds of surfaces does it occur on?
> combined effect of mechanical and environmental forces
> it occurs on surfaces that don’t form good protective oxide films
What is fretting corrosion?
caused by slight reciprocating movement on two metal surfaces
–> What are some recognizing features of erosion corrosion?
> accelerated thinning in regions of fluid flow
rounded and smooth pits from impingement attack
often horse-shoe shaped
fine powder may ooze from the fretting surface
cavitation damage appears as pits
grooves, waves, gullies, teardrop shaped pits, horseshoe shaped depressions
undercutting
What influence does flow have on erosion corrosion?
> transports reactants or products; higher flow = higher rxn rate
disrupts the passive film on surface; film breakdown occurs above the critical velocity Vc
–> Turbulent flow simulations
> Reynolds #: ratio between intertial and viscous forces
for the rotating cylinder electrode w/ outer diameter d, reynold’s # is R = Ud(p/u)
–> u: absolute viscosity
–> p: soln. density
–> U: linear velocity at outer surface of cylinder: U = pid(F/60)
–> F = rotations/minute
Shear stresses at rotating electrode surface
> shear stresses τ = 0.0791pR^-0.3*U^2
____ temperature = ____ rate of erosion corrosion
higher; higher
Normalized erosion resistance =
rate of volume loss of a standard material / rate of loss of the test material from the same test
How do you prevent erosion corrosion?
> redesign them to reduce surface velocity and turbulence of the stream, larger radius elbows, larger pipe diameters
design for greater section thickness or easy part replacement
change soln. corrosivity
inhibitor or passivator additions
–> What is cavitation corrosion?
> caused by the implosion of gas bubbles on a metal surface due to sudden variations in pressure
a smooth surface decreases the # of potential sites for formation of vapor bubbles
–> increase in fluid pressure is often sufficient to maintain a single phase fluid, avoiding formation of vapor bubbles
–> occurs when the local static pressure in a fluid goes below the vapor pressure of the liquid
Cavitation #:
σ = (P - Pv)/0.5*p*u^2 > P: reference pressure > Pv: liquid vapor pressure > p: density > u: free stream velocity
–> What happens when σ=0?
the pressure is reduced to the vapor pressure and cavitation will occur
What happens below the critical cavitation #?
cavitation will occur
How do you prevent cavitation?
> avoid low pressure
use cathodic protection-hydrogen evolved on the surface protects by cushioning
remove dissolved gases to eliminate bubbles
remove pressure drops across surface
What is the mechanism for cavitation corrosion?
> collapse of a spherical void in liquid when low pressure is applied
vapor gases evaporate into the cavity (cavity has lower gas pressure, almost like a vacuum)
low pressure cavitation bubble in the liquid collapses due to higher pressure of surrounding medium
–> pressure and temp of vapor within increases
gas dissipates into surrounding liquid via violent mechanism
point of total collapse where temp and pressure of the vapor in the bubble are very high
