Materials Selection for Corrosion Prevention and Corrosion Control
CORROSION RESISTANT METALS
This page describes the process of Material Selection. You can find a list of the different types of corrosion process with explanation of the corrosion mechanism and corrosion prevention method in this page: corrosion forms and types of corrosion.
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, corrosion resistance 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 of corrosion 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.
Use of Corrosion Resistance Data.
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 & pitting corrosion or galvanic corrosion, can be identified as a specific requirement
with resistance to other forms of corrosion 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 Resistance Data.
Corrosion
resistance 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.In-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
corrosion interactions and the effects of crevices and crevice corrosion 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
resistance 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