44
Fundamentals of Corrosion
4. The use of a sacrificial anode, which has been found to prevent pitting in marine operations, or cathodic protection by means of an
impressed current, is another possible preventative measure. In
more aggressive solutions, this may not provide protection.
5. The most practical solution, if economically feasible, would be to use
a metal or alloy having a higher resistance to pitting. Table 3.3 can
be used as a guide. Titanium and its alloys provide excellent pitting
resistance in most aggressive media, such as those containing ferric
or cupric chloride. In seawater, type 316 stainless steel is far superior
to type 304 stainless steel. Going up the scale, nickel alloys such as
Hastelloy ® are even better.
3.6 Erosion Corrosion
The term “erosion” applies to deterioration due to mechanical force. When
the factors contributing to erosion accelerate the rate of corrosion of a metal,
the attack is called erosion corrosion. Erosion corrosion is usually caused by an
aqueous or gaseous corrodent flowing over the metal surface or impinging
upon it. The mechanical deterioration may be aggravated by the presence of
a corrodent, as in the case of fretting or corrosive wear.
The attack takes the form of grooves, that is, scooped out rounded areas,
horseshoe-shaped depressions, gullies or waves, all of which often show
directionality. At times, attack may be an assembly of pits. Ultimate perforation due to thinning or progressive pits, and rupture due to failure of the
thinned wall to resist the internal fluid pressure are common. All equipment
exposed to flowing fluid is subject to erosion corrosion, but piping systems
and heat exchangers are the most commonly affected.
Erosion corrosion is affected by velocity, turbulence, impingement, the
presence of suspended solids, temperature, and prevailing cavitation conditions. Acceleration of the attack is due to the destruction or removal of the
protective film by mechanical forces, thus exposing fresh metal surfaces that
are anodic to the uneroded neighboring film. A hard, dense, adhering film,
such as on stainless steel, is more resistant than a soft brittle film, such as
that on lead. The nature of the protective film depends largely on the corrosive itself.
In most metals and alloys, corrosion rates increase with increased velocity,
but a marked increase is experienced only when a critical velocity is reached.
Turbulence is caused when the liquid flows from a larger area to a small
diameter pipe, as in the inlet ends of tubing in heat exchangers. Internal
deposits in the pipes, or any obstruction to the flow inside a pipe by foreign
bodies, such as a carried-in pebble, can also cause turbulence.
Fundamentals of Corrosion
4. The use of a sacrificial anode, which has been found to prevent pitting in marine operations, or cathodic protection by means of an
impressed current, is another possible preventative measure. In
more aggressive solutions, this may not provide protection.
5. The most practical solution, if economically feasible, would be to use
a metal or alloy having a higher resistance to pitting. Table 3.3 can
be used as a guide. Titanium and its alloys provide excellent pitting
resistance in most aggressive media, such as those containing ferric
or cupric chloride. In seawater, type 316 stainless steel is far superior
to type 304 stainless steel. Going up the scale, nickel alloys such as
Hastelloy ® are even better.
3.6 Erosion Corrosion
The term “erosion” applies to deterioration due to mechanical force. When
the factors contributing to erosion accelerate the rate of corrosion of a metal,
the attack is called erosion corrosion. Erosion corrosion is usually caused by an
aqueous or gaseous corrodent flowing over the metal surface or impinging
upon it. The mechanical deterioration may be aggravated by the presence of
a corrodent, as in the case of fretting or corrosive wear.
The attack takes the form of grooves, that is, scooped out rounded areas,
horseshoe-shaped depressions, gullies or waves, all of which often show
directionality. At times, attack may be an assembly of pits. Ultimate perforation due to thinning or progressive pits, and rupture due to failure of the
thinned wall to resist the internal fluid pressure are common. All equipment
exposed to flowing fluid is subject to erosion corrosion, but piping systems
and heat exchangers are the most commonly affected.
Erosion corrosion is affected by velocity, turbulence, impingement, the
presence of suspended solids, temperature, and prevailing cavitation conditions. Acceleration of the attack is due to the destruction or removal of the
protective film by mechanical forces, thus exposing fresh metal surfaces that
are anodic to the uneroded neighboring film. A hard, dense, adhering film,
such as on stainless steel, is more resistant than a soft brittle film, such as
that on lead. The nature of the protective film depends largely on the corrosive itself.
In most metals and alloys, corrosion rates increase with increased velocity,
but a marked increase is experienced only when a critical velocity is reached.
Turbulence is caused when the liquid flows from a larger area to a small
diameter pipe, as in the inlet ends of tubing in heat exchangers. Internal
deposits in the pipes, or any obstruction to the flow inside a pipe by foreign
bodies, such as a carried-in pebble, can also cause turbulence.