forms at the surface when they are exposed to oxygen (aluminum oxide and
chromium oxide, respectively). Pitting
corrosion develops when the protective
film is penetrated or removed by an electrical, chemical, or mechanical process.
Crevice Corrosion. Crevice corrosion
occurs in protected or shielded areas in
pipelines that can capture or create a
Stress Crack Corrosion. Stress cracking is actually a form of corrosion. It occurs in nearly all metals and most often
in high-strength materials. The fracturing
is normally along grain boundaries and
can be intergranular in nature.
Steels that are stressed beyond grain
boundary limits are very susceptible
to corrosion attack in severe environ-
ments. The corrosion can lower the grain
boundary resistance to cracking and
even change the chemical makeup of the
grain boundary zones. Stress cracking is
the result of a triad of requirements: a
susceptible alloy, high tensile stress, and
a corrosive-specific environment.
Impingement Corrosion. This is
caused by materials in a flow system, the
flow system itself, and the flow system
design. The direction of impingement
appears to come from the opposite direction of the flow lines. The corrosion
can be caused by particles in the flow
mixture making contact with, and becoming embedded in, the piping system.
It also can be caused by the flow of the
mixture through turbulence, aeration,
and wave erosion.
The design of the piping system can
contribute to the impingement and erosion of the inside walls of the pipes.
Steep angles create impact zones and
should be avoided in the design stage.
Gentle angles and curves decrease impact likelihood and severity.
Micro-organism Corrosion. Anyone
familiar with hydraulic system maintenance knows that bacteria can thrive in
extreme environments, including high-temperature, high-pressure hydraulic
circuits. Corrosion by such micro-organisms is one of the hardest forms of corrosion to stop. Micro-organisms can attack metal directly or indirectly. That is to
say, some can actually ingest the metals,
whereas others deposit corrosion-caus-ing chemicals onto metals.
Stopping this type of attack usually
involves chemicals that are benign to the
metal but hostile to micro-organisms.
Because corrosion develops in an electrical circuit, stopping corrosion is a matter of disrupting the electrical circuit.
Cathodic Protection. Cathodic protection (CP) is by far the best way to
stop corrosion on pipelines. It uses impressed currents from a fixed anode to
interfere with the electrical circuit in the
corrosion cell. It is 100 percent effective
against most forms of external pipe corrosion.
Galvanic CP connects a high-energy
metal, such as zinc or magnesium, to the
pipe (anode) (see Figure 4). The zinc or
magnesium acts as a sacrificial anode,
protecting the pipe. The sacrificial anode
operates as a galvanic anode with impressed voltages as high as -1.4 to - 2.1 V
(and in some cases, even more negative).
If a counter-impressed current from a
sacrificial anode is more negative than
-0.85 V, corrosion in steel stops. Aluminum corrosion stops if the counter-impressed current is between -0.8 V and
-1.00 V. If the current magnitude exceeds
-1.00 V, an alkaline solution forms on
the aluminum and attacks the metal. In
some cases, a shift as small as -0.003 V
Proper CP levels also can be achieved
Typical Cathodic Protection
Metallic Wire Connecting
Zinc Anode to Iron Cathode
Occurs at Anode
Occurs at the Cathode
2H 2e¯ H
A cathodic protection system uses a high-energy metal, such as magnesium, as a sacrificial anode.
Stress cracking is actually a form of corrosion.
It occurs in nearly all metals and most often in
high-strength materials. The fracturing is normally
along grain boundaries and can be intergranular