Forms of Metallic Corrosion
31
report average or typical rates of corrosion for various metals in common
media (see, for example Reference 1).
Because corrosion is so uniform, corrosion rates for materials are often
expressed in terms of metal thickness loss per unit of time. The rate of uniform attack is often reported in various units. One common expression is
mils per year (mpy). Refer to Chapter 2 for a more complete discussion of the
various units used to measure loss.
Because of its predictability, low rates of corrosion are often tolerated
and catastrophic failures are rare if planned inspection and monitoring are
implemented. For most chemical process equipment and structures, general
corrosion rates of less than 3 mpy are considered acceptable. Rates between
3 and 20 mpy are routinely considered useful engineering materials for the
given environment. In severe environments, materials exhibiting high corrosion rates, between 20 and 50 mpy, might be considered economically
justifiable. Materials that exhibit rates of general corrosion beyond this are
usually unacceptable. It should be remembered that not only must the metal
loss be considered, but where the metal is going must also be considered.
Contamination of product, even at low concentrations, can be more costly
than the replacement of the corroded component.
Examples of uniform or general corrosion are illustrated by a steel tank or
steel roof rusting in the atmosphere, or a piece of zinc dissolving in hydrochloric acid.
Atmospheric corrosion is also an example of uniform corrosion, and takes
place under damp or wet conditions. A more detailed discussion of atmospheric corrosion is found in Chapter 4.
3.1.9 Protective Measures
The single most important protective measure is the selection of the proper
material of construction. Many corrosion-resistant alloys have been developed, of which the most widely used are the stainless steels. However, these
are not a cure-all. To provide protection where the stainless steels are unable
to resist the corrosion, other alloys and/or metals have been developed to
fill the voids. For example, to prevent the atmospheric corrosion of steel,
special weathering steels have been developed. These are steels containing
small amounts of copper or micro-alloyed steels containing copper, phosphorus, nickel, or chromium. The rust formed on these steels is adherent
and protective.
In some cases, a less expensive material of construction may be used when
a protective coating or lining is applied to provide protection against the
corrosive media.
Another option is to provide a corrosion allowance. If a vessel showing a
corrosion rate of 6 mpy is to have a 10-year life, an additional wall thickness
of 60 mils (or 0.06 inch) will be provided above what is required to meet the
operating conditions of pressure, temperature, and stress.
31
report average or typical rates of corrosion for various metals in common
media (see, for example Reference 1).
Because corrosion is so uniform, corrosion rates for materials are often
expressed in terms of metal thickness loss per unit of time. The rate of uniform attack is often reported in various units. One common expression is
mils per year (mpy). Refer to Chapter 2 for a more complete discussion of the
various units used to measure loss.
Because of its predictability, low rates of corrosion are often tolerated
and catastrophic failures are rare if planned inspection and monitoring are
implemented. For most chemical process equipment and structures, general
corrosion rates of less than 3 mpy are considered acceptable. Rates between
3 and 20 mpy are routinely considered useful engineering materials for the
given environment. In severe environments, materials exhibiting high corrosion rates, between 20 and 50 mpy, might be considered economically
justifiable. Materials that exhibit rates of general corrosion beyond this are
usually unacceptable. It should be remembered that not only must the metal
loss be considered, but where the metal is going must also be considered.
Contamination of product, even at low concentrations, can be more costly
than the replacement of the corroded component.
Examples of uniform or general corrosion are illustrated by a steel tank or
steel roof rusting in the atmosphere, or a piece of zinc dissolving in hydrochloric acid.
Atmospheric corrosion is also an example of uniform corrosion, and takes
place under damp or wet conditions. A more detailed discussion of atmospheric corrosion is found in Chapter 4.
3.1.9 Protective Measures
The single most important protective measure is the selection of the proper
material of construction. Many corrosion-resistant alloys have been developed, of which the most widely used are the stainless steels. However, these
are not a cure-all. To provide protection where the stainless steels are unable
to resist the corrosion, other alloys and/or metals have been developed to
fill the voids. For example, to prevent the atmospheric corrosion of steel,
special weathering steels have been developed. These are steels containing
small amounts of copper or micro-alloyed steels containing copper, phosphorus, nickel, or chromium. The rust formed on these steels is adherent
and protective.
In some cases, a less expensive material of construction may be used when
a protective coating or lining is applied to provide protection against the
corrosive media.
Another option is to provide a corrosion allowance. If a vessel showing a
corrosion rate of 6 mpy is to have a 10-year life, an additional wall thickness
of 60 mils (or 0.06 inch) will be provided above what is required to meet the
operating conditions of pressure, temperature, and stress.