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CORROSION RESISTANT METALS
SELECTING METALS FOR CORROSION RESISTANCE.
Selecting metals for their resistance to corrosion should be considered as a part
of the overall material selection process. Corrosion resistance as well as strength,
ductility, fabricability, availability, and cost are all factors that must be considered
in selecting a material. In this chapter, the corrosion resistance and applications
of many of the metals that are commonly used in shore facilities will be described.
However, in order to make specific materials selections, detailed information on
the corrosion properties as well as other materials properties must be obtained
and evaluated.
APPLICATION OF CORROSION PERFORMANCE DATA TO SELECTION OF MATERIALS.
Materials performance data, whether based on the analysis
of previous successful or unsuccessful application or on corrosion testing must
be carefully integrated into the design process. Only through an understanding of
how the type and amount of corrosion that is likely to occur in specific locations
in a system will affect the function of the system can corrosion failures be avoided.
Materials Selection as Part of the Design Process.
Material selection is often performed only toward the
end of the design process. In some cases, due to material availability, fabrication
problems, or even after initial use or test has shown that the originally specified
material is unsatisfactory, the first attempt to solve the problem is to change
the material without changing the design. This often results in a less than optimum
match between material properties and the requirements of the application. In some
cases, this process results in a materials “overkill” that has an adverse impact
on cost, fabrication difficulties, and use of non-standard components with attendant
problems in obtaining spares.
Designing to Fit Available Materials.
In many cases, it is appropriate to identify those materials
that have suitable corrosion properties for a specific application, then design
a system based on the properties of the available materials. This is often more
effective than selecting materials that have the properties required by a given
design.
Corrosion data. whether from experience or from testing,
must be used in a systematic manner. Corrosion data are used to quantitatively predict
the amount and distribution of corrosion that will occur on a given component in
a given application. Whether the design is performed in a “materials first” or “design
first” manner, some quantitative corrosion data must be used. It is important to
understand how the data were obtained in order to best apply it to design.
Definition of Required Material Properties.
In order to select materials, limits for material performance
must be established. While requirements for such properties as strength and ductility
are relatively easy to define, requirements for such properties as corrosion resistance
and fabricability are often less easy to define quantitatively.
For corrosion resistance, high levels of resistance to certain forms of attack such
as crevice corrosion, can be identified as a specific requirement with resistance
to other forms of attack being less important. A list of specific critical properties
for a specific application with “desirable” and “acceptable or threshold” requirements
for these critical properties is often useful in selecting materials for specific
applications.
Determination of Acceptable Performance.
In many cases, a material will be selected that will perform
adequately for a given service life but will deteriorate significantly during that
service period and will either have to be repaired or replaced if the application
is repeated. In this case, the performance could be considered adequate but not
desirable. In many cases this is appropriate. In cases where repetitive or extended
application is anticipated, select a more corrosion resistant material that will
not require repair or replacement for re-use.
Sources of Corrosion Data.
Corrosion data are available from a wide variety of sources.
However, it is not available from any single source. Data from similar service performance,
corrosion testing, or corrosion theory can all be used if properly interpreted and
applied.
Service Performance.
Data from previous service performance often address many
of the anticipated corrosion related factors but are often qualitative in nature.Iin-service
material performance must be
properly evaluated in order to be useful in design.
Corrosion Testing.
Data from corrosion tests are usually very quantitative
in nature but often does not address many of the factors anticipated in a given
application. Most marine corrosion data are obtained from the exposure of small
samples of isolated materials. Information on galvanic interactions and the effects
of crevices is more limited and is often semi-quantitative or qualitative. Care
must be taken to verify that the quantitative data presented are appropriate to
the form of corrosion that was experienced. Corrosion rates based on weight loss
should not be used to assess nonuniform corrosion attack. Corrosion data are usually
more useful in comparing the performance of materials with similar performance than
in comparing the performance of materials with widely different corrosion behavior.
In many cases, data from corrosion testing are difficult to use in the quantitative
prediction of long term system performance of complex systems.
Corrosion theory is useful in evaluating the causes of corrosion damage but is difficult to apply to the prediction of corrosion performance. If there are no data from either corrosion testing or from in-service performance of a given material it is likely that some aspect of its performance has made it unfit for marine service. The market for marine materials is highly competitive and if a material has not been used or tested in the marine environment it should be used only with great caution.
Source : "Corrosion Control" NAVFAC MO-307 September 1992
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