This is sixth in a series on Corrosion Control. Find the previous articles here.
After talking about cathodic protection, and specifically some of the more complex impressed current cathodic protection systems, we now come to seemingly the most boring of the subsections of corrosion control: materials selection. However mundane choosing materials may seem (or may even actually be), it is, in fact, the most important factor in corrosion control. To be sure, there is an entire universe of materials selection for corrosion control. Several (possibly leather-bound) books have tried to illuminate areas of common concern, the most comprehensive being ASM International's Materials Selection for Corrosion Control by S.L. Chawla and R.K. Gupta. Although in the 24 years since that book was published, not only has the science of corrosion control developed, but the number of materials from which to select has also grown. An update may be needed, but this article shall not suffice for that job, it shall merely overview the subject.
Materials selection is fairly self-defining, in that, yes, it's about selecting materials. To clarify though, materials selection happens during the design phase of production, which is earlier in the process than many of the other corrosion control options. The ideal situation, and really the way anyone designing a product should do it, is that corrosion is thought about from the beginning, so that it can be "designed out" of the final product. If engineers think about all of the factors of corrosion, e.g. surface area, potential environment, and, of course, materials, during the design phase, they can produce much more robust, long-lasting products. So, as with other aspects of the design, materials selection must be informed by other potential corrosion factors, and only in combination will those design decisions create a corrosion resistant product.
The first choices needing to be made on the materials taxonomic tree may, at first glance, seem quite easy. If I'm making a house, I use wood or bricks; if I'm making a computer, I use plastic and metal. Seems pretty obvious to anyone who has never made a house or a computer. But once you try to make a house or a computer, you soon realize that there are so many decisions to be made among so many possibilities. Obviously different types of wood have been around since trees started growing millions of years ago, and man has been tinkering with different metal and stone concoctions for millennia. Plastic is the new guy of the ones mentioned, and even it is being overshadowed recently by carbon fiber and much more complex and interesting sponge-like materials. Even the older technologies, like metallurgy, are still being developed. The number of different specific alloys of aluminum alone could fill a lifetime of study. Needless to say, we don't have that kind of time. From a corrosion control perspective, the key is to find materials appropriate to their intended environment. The outside of the house will be exposed to rain and the elements, so it needs to resist that sort of thing. A good solution is vinyl; it does not corrode very much and is easily cleaned. The materials used to make the inside of the house can be much less robust towards corrosion, as they face a less harsh environment. Similar for the computer, the outside plastics protect the metals, semi-metals and other more easily corroded materials inside.
But the specifics really matter for controlling corrosion. Most aluminum alloys are particularly corrosion resistant (at least to general surface corrosion, they still get ravaged by pitting corrosion), while others are more easily attacked. Using the wrong alloy in an important piece of the product can be harmless; unfortunately it can also be disastrous. Similar specifics can be noted of any of the other major material groups. The point is: details matter. And if the details of corrosion are thought about during the design phase, products can be developed to avoid the failures of the past.
One aspect of material selection unmentioned here is the choice of packaging material, which is too often left until after the design is complete. This is a mistake that many companies have started to correct, although there are many more which have yet to learn that packaging should be included in design. As packaging is something near and dear to us here at Liberty Intercept, I will be discussing it more fully in the next article in this series, Corrosion Control - Packaging. Stay tuned.