Coating Structure

There are several reasons why the Tagnite coating provides superior corrosion resistance when compared to HAE and Dow 17 anodize coatings. One reason is its superior structure or coating morphology. The smaller the pores and the more uniform the coating layer, the more difficult it is for corrosive environments to penetrate the coating and damage the base magnesium metal.

During the anodization process, some degree of porosity is going to be created. Since the creation of porosity or pores cannot be prevented, emphasis needs to be placed on keeping the pores as small as possible and building the coating layer uniformly. The Tagnite coating system does this by building predominately magnesium oxide and not depositing large quantities of electrolyte or coating bath constituents.

Shown below are a number of scanning electron microscope (SEM) photographs which show the surface of the coating layer formed by the Tagnite, HAE and Dow 17 coatings. These photographs provide an excellent comparison of pore sizes formed on each of the coatings and, to a fair degree, the uniformity of the coating layer formed. The first set of photos is of Type 1 (0.20 mil - 0.40) mil coating thickness.

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The samples photographed below had Type II coating thicknesses as follows: Tagnite, 80 mil; HAE, 3.5; Dow 17, 1.9 mil. As mentioned previously, the HAE and Dow 17 coatings can be applied much thicker than the Tagnite coating. As you can see, the layer formed on the Tagnite Type I samples look like the Type II Tagnite samples. However, the HAE and Dow 17 Type II layers look very different from their Type I layers.

The HAE coating in a Type II thickness produces far fewer pores than in the Type I thickness range. However, the pores created are positively enormous making an easy route for corrosive environments to penetrate to the base magnesium metal.

The Dow 17 layer in a Type II thickness range also looks very different when compared to a layer formed in a Type I thickness. It is clearly a deposition coating in this thickness range and large pieces of electrolyte constituents are loosely built on to the layer. When compared to Tagnite, this Dow 17 layer is built very rapidly.

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