Posted February 26, 2016
By Ed Rubin
Those Are Grain Boundaries—Part of the Material
Have you ever experienced the appearance of “raindrops,” “snowflakes,” or galvanized metal patterns in your anodized finish? If so, don’t shoot your anodize supplier. Let me tell you why.Common metals, such as aluminum, are polycrystalline in nature. These crystals are generally referred to as grains. The grains vary in size and shape and are defined by the edges of the crystal. They are commonly described as grain boundaries.
- Does the defect resemble either of the photos included in this blog post?
- Does the pattern remain when the anodize coating is stripped?
- Does the condition worsen when the part is re-anodized?
If the answer is yes to any of the above questions, it is likely that what you are seeing are grain boundaries. The crystal patterns that resemble frost on a window pane or galvanized steel most often are not visible on fabricated products such as rolled sheet or machined parts. However, when exposed to cleaning etchants that are used during the pre-treatment process followed by the anodize operation, the grains become visible.
- Etchants preferentially attack the material at the grain boundaries. This action creates contrast between the boundaries and the grains themselves, and thus, gives definition and contrast to the surface.
- The anodizing process itself has the same effect by consuming more material at the boundaries than at the grain proper. Very often it is the combination of the two that causes the effect to be visible.
Can this Problem Be Fixed?
Because the grains are produced during the heating, cooling, and mechanical processing of the metal, they become an inherent characteristic of the material. Once the grains become apparent, they are very difficult to hide. The metal finisher has limited ability to mitigate this cosmetic appearance. Sometimes applying a new mechanical finish can hide the issue. Good heat-treating practice will measurably reduce this occurrence. What we can do is advise you on how to minimize this effect if it should appear.
It is important to note that both the size and shape of the grains is created by the thermal and mechanical history of the metal and becomes an integral part of its properties. Heating followed by slow cooling tends to produce grains with large cross sections. Examples of this can be found in castings, extrusions with large cross sections, and heat-treating with no quenching. Smaller grains are produced during mechanical processing, as well as tempering. Examples of these are forging, rolling, and heat treatment followed by quenching. Extruded shapes having both thick and thin cross sections may simultaneously contain both large and small grains.
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