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Materials Selection for Corrosion Prevention and Corrosion Control
Material Selection for Corrosion Control
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.
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.
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.
Corrosion resistance 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.
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.
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.
Source : "Corrosion Control" NAVFAC MO-307 September 1992
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