Forms of Metallic Corrosion
45
Impingement (i.e., direct contact of the corrodent on the metal surface)
occurs at bends, elbows, and tees in a piping system and causes intense
attack. Impingement is also encountered on the surface of impellers and turbines, in areas in front of inlet pipes in tanks, and in many other situations.
The attack appears as horseshoe-shaped pits with a deep undercut and the
end pointing in the direction of flow.
An additional subset of erosion corrosion is the case of cavitation that is
prevalent in pump impellers. This form of attack is caused by the formation
and collapse of tiny vapor bubbles near a metallic surface in the presence of
a corrodent. The protective film is again damaged, in this case by the high
pressure caused by the collapse of the bubbles.
When two metal surfaces are in contact and experience a very slight relative motion that results in damage to one or the other surfaces, fretting corrosion, a special form of erosion corrosion, takes place. The movement causes
mechanical damage to the protective film. This can lead to erosion corrosion if a corrodent is present. This corrosion usually takes the form of pitting attack. Attack is further aggravated at higher temperatures and with
solutions containing solids in suspension. Steam carrying water condensate
droplets provides an aggressive medium for corrosion of steel and cast iron
piping. The impingement of water droplets at the return bends destroys the
protective oxide film and accelerates attack on the substrate.
Soft and low-strength metals, such as copper, aluminum, and lead, are
especially susceptible to erosion corrosion, as are the metals and alloys that
are inherently less corrosion resistant, such as carbon steels.
Stainless steels, of all grades, are generally resistant to erosion corrosion.
The addition of nickel, chromium, or molybdenum further improves·their
performance. Stainless steels and chromium steels are resistant as the result
of their tenacious surface films.
As a rule, solid solution alloys provide better resistance than alloys hardened by heat treatment because the latter are heterogeneous in nature.
Cast iron usually performs better than steel. Alloy cast irons containing
nickel and chromium exhibit better performance. Duriron, containing 14.5%
silicon, gives excellent performance under severe corrosive conditions.
3.6.1 Preventive Measures
Changing the design can minimize or completely eliminate erosion corrosion. For example, the shape or geometry of the component has an effect.
Increasing wall thickness at the affected areas will extend the life of the
component. Other factors such as reducing velocity, reducing or eliminating
turbulence, and the proper design of the piping system or heat exchangers
will also reduce or eliminate erosion corrosion. Selecting a harder material
of construction is another alternative.
Impingement attack can be avoided by increasing pipe diameters, and
installing baffles will restrict the impact of the fluid.
45
Impingement (i.e., direct contact of the corrodent on the metal surface)
occurs at bends, elbows, and tees in a piping system and causes intense
attack. Impingement is also encountered on the surface of impellers and turbines, in areas in front of inlet pipes in tanks, and in many other situations.
The attack appears as horseshoe-shaped pits with a deep undercut and the
end pointing in the direction of flow.
An additional subset of erosion corrosion is the case of cavitation that is
prevalent in pump impellers. This form of attack is caused by the formation
and collapse of tiny vapor bubbles near a metallic surface in the presence of
a corrodent. The protective film is again damaged, in this case by the high
pressure caused by the collapse of the bubbles.
When two metal surfaces are in contact and experience a very slight relative motion that results in damage to one or the other surfaces, fretting corrosion, a special form of erosion corrosion, takes place. The movement causes
mechanical damage to the protective film. This can lead to erosion corrosion if a corrodent is present. This corrosion usually takes the form of pitting attack. Attack is further aggravated at higher temperatures and with
solutions containing solids in suspension. Steam carrying water condensate
droplets provides an aggressive medium for corrosion of steel and cast iron
piping. The impingement of water droplets at the return bends destroys the
protective oxide film and accelerates attack on the substrate.
Soft and low-strength metals, such as copper, aluminum, and lead, are
especially susceptible to erosion corrosion, as are the metals and alloys that
are inherently less corrosion resistant, such as carbon steels.
Stainless steels, of all grades, are generally resistant to erosion corrosion.
The addition of nickel, chromium, or molybdenum further improves·their
performance. Stainless steels and chromium steels are resistant as the result
of their tenacious surface films.
As a rule, solid solution alloys provide better resistance than alloys hardened by heat treatment because the latter are heterogeneous in nature.
Cast iron usually performs better than steel. Alloy cast irons containing
nickel and chromium exhibit better performance. Duriron, containing 14.5%
silicon, gives excellent performance under severe corrosive conditions.
3.6.1 Preventive Measures
Changing the design can minimize or completely eliminate erosion corrosion. For example, the shape or geometry of the component has an effect.
Increasing wall thickness at the affected areas will extend the life of the
component. Other factors such as reducing velocity, reducing or eliminating
turbulence, and the proper design of the piping system or heat exchangers
will also reduce or eliminate erosion corrosion. Selecting a harder material
of construction is another alternative.
Impingement attack can be avoided by increasing pipe diameters, and
installing baffles will restrict the impact of the fluid.