Vol.13 Explaining the Rust Source "Galvanic Corrosion"

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Have you ever been surprised by rust suddenly appearing on rust-resistant aluminum?
If this happens, the reduced strength resulting from the corrosion brings fears of possible damage.
This phenomenon is known as "galvanic corrosion". Today, I will explain this principle and how it can be handled.

So what is "galvanic corrosion"?

Galvanic corrosion is corrosion that appears when dissimilar metals are in a state where electricity can be easily conducted (under water or in wet areas). It is also sometimes called bimetallic corrosion.

What causes this?

If metals with different electric potentials come in contact with water, a battery structure is formed and electrons are exchanged between the dissimilar metals, causing corrosion. This occurs depending on the natural electric potential of the metals, and the metal with the lower natural electric potential corrodes.

Although the term "electrolytic corrosion" (or electric corrosion) often refers to this galvanic corrosion, according to JIS Z 0103 electrolytic corrosion is defined as "corrosion caused by current flowing in an irregular circuit", and is a different phenomenon than stray current corrosion.

Effects of Galvanic Corrosion

As a specific example, if an aluminum stand is fixed with steel screws, corrosion will occur when water comes in contact with the different metals through rain or condensation. In this case, the aluminum forms white rust, as the natural electric potential of aluminum is less than that of steel.

In this way, even if aluminum or stainless steel are otherwise rust-resistant, corrosion may occur at an accelerated rate due to galvanic corrosion, so care must be taken when handling dissimilar metals.

How can we prevent this rusting?

There are two main solutions.
(1) Use the same materials, such as fixing aluminum frames with aluminum screws
(2) Do not allow dissimilar metals to contact each other directly
However, (1) may be difficult due to strength or cost of materials.
If so, it is effective to electrically insulate dissimilar metals using a resin washer or other method as part of method (2).

Screws with a special surface treatment that suppresses electrical conductivity are also available. This is effective not only for washers for screws, but also for the contact parts of male and female threads, so it is certainly worth trying out.

Well, at that point, let's finish for today.

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