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Guidelines for Corrosion Surface Treatment of Aluminum Alloy Parts
Date: 2023-03-24
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Guidelines for Corrosion Surface Treatment of Aluminum Alloy Parts

Aluminum is the second most abundant metal we get from the earth. Plus, it has several great features. This makes it ideal for metalworking. We practice a variety of manufacturing processes on aluminum. However, there are certain conditions that can shorten the life of the metal. Corrosion of aluminum can lead to structural damage, reducing its functional strength. Cracks and partial fractures can occur in metals, which can eventually lead to complete material failure. The following guidelines will help you understand the types of corrosion that affect aluminum.

What is the corrosion process of aluminum alloy parts?

It refers to the slow degradation of aluminum molecules into oxides. This changes its physical and chemical properties. Aluminum is both naturally reactive and passive. It first reacts with oxygen in the environment to form oxides. The resulting oxide layer is a protective layer.

Also, it doesn't peel off easily. The etch process is slow, unlike a deliberate etch like laser etching. It can happen for months or years. Understanding its different variations is key. It can help you reduce or prevent them from happening.

What are the processes for surface treatment of aluminum parts?

Natural aluminum alloy corrosion process

It is the most common type of corrosion process for aluminum alloy parts. It occurs due to exposure to natural elements. There are three subcategories based on moisture content. They are wet, dry and wet. Moisture content varies by environment. Also, changes in temperature, wind direction and precipitation can increase it.

The concentration and type of pollutants in the air play a significant role. Atmospheric corrosion increases if the design does not allow drainage. Creating pockets for condensing rain is a detrimental design flaw.

Plating

We also call it dissimilar metal corrosion. This happens when you join aluminum alloys to precious metals. The connection can be physical or through an electrolytic connection. A noble metal is anything below aluminum in the reactivity series.

You can easily form a primary battery by placing aluminum and brass next to each other in salt water. Aluminum alloy parts are easily corroded when anodized. A classic case is where the brass fittings approach the aluminum fittings on board. Galvanic corrosion is faster than atmospheric corrosion.

Pitting

It is a corrosion that has small holes in the surface of the metal. Fortunately, these holes do not affect the strength of the material. However, it raises aesthetic issues. Ultimately, it can lead to material failure. It usually occurs in areas with salt in the atmosphere. The presence of chloride ions is a key factor. This can be worse in the presence of acidic and basic salts. If the potential of the salt is lower than that of the alloy, pitting corrosion will occur.

Crack

As one of the corrosion processes. Inadvertent design flaws and overlapping materials lead to crack formation. These pockets can collect salt water and cause crevice corrosion. Even a tiny gap between the bolt and the structure is enough to initiate this corrosion. Over time, the aluminum dissolves and precipitates in the brine. Ionic aluminum absorbs oxygen and hydroxide ions to form aluminum hydroxide. The reduction in oxygen makes the crevices acidic. In addition, the presence of chloride ions accelerates corrosion.

Intergranular

For aluminum, the grain boundaries are electrochemically distinct from its alloy microstructure. This results in the formation of an electrochemical potential between the two. Thus, an electronic exchange occurs. There are different types of intergranular corrosion that occur at the base of the metal structure. It can also be caused by thermochemical treatments. Also, it varies according to its position in the aluminum family. For example, the 6xxx series are less prone to intergranular corrosion.

Exfoliation

It is a specific form of intergranular corrosion that occurs in aluminum alloys and is common in alloys with a pronounced directional structure. This is especially evident in our products formed using cold or hot rolling processes, where it occurs with a microstructure of elongated grain boundaries. The term spalling stems from the fact that corrosion products appear large. This gives the illusion of being elevated from the surface of the material. Corrosion scales up and out. This contributes to the build-up of pressure in the product.

Eventually, a wedging action occurs from the surface. Then it moves on to the rest of the product. Eventually, the material weakens and becomes severely delaminated. The surface may be pitted, peeled and blistered.

General corrosion

We can also call it a uniform aluminum alloy corrosion process. It appears evenly on the surface of the aluminum. It is caused by continuous exposure to strong alkaline or acidic media. Furthermore, it may arise from exposure to high electrochemical potentials. A classic example is aluminum rusting in acidic solutions. A high or low pH can distort the stable oxide layer on the aluminum surface. This exposes them to the active metal underneath. The thickness of the material gradually decreases. It eventually dissolves completely.

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