In part 1 of this 2 part article, we discussed heat sink anodization, and how it affects radiation cooling in thermal management. In part 2 we’ll cover the heat sink anodization process and different types of anodization.
A typical anodizing process has five steps: cleaning, pretreatment, anodizing, coloring and sealing. First, alkaline and acidic cleaners are used to remove any residual grease or dirt on the surface. Next, the surface is etched using sodium hydroxide to remove minor surface imperfections. At this stage, a thin layer of metal is removed to create a matte or dull finish. This surface is then brightened to a near mirror finish with a concentrated mixture of phosphoric and nitric acids which chemically smooths the metal surface. The surface is now ready to be anodized by passing an electrical current through an acid electrolyte bath in which the metal is immersed. The coating thickness and surface characteristics are tightly controlled to meet end product specifications [3].
Coloring can be achieved in a number of different ways. The most technically advanced color quality is obtained from the two-step electrolytic coloring method. The anodized part is immersed in a bath containing an inorganic metal salt, and then current is applied which deposits the salt in the base of the pores. The resulting color is dependent on the metal used, the processing conditions and the dye intensity of the organic dyes. Commonly used metals in inorganic salt baths include tin, cobalt, nickel, and copper [3].
Even though the main concept of surface anodizing is the same, the process could differ in terms of the acidic electrolyte used. One of these is chromic acid, or type 1, anodizing. In this process, the color will vary from clear to dark gray depending on the alloy. Copper alloys only yield gray colors. The thickness of the surface anodizing for this process usually ranges from 0.5 to 7.6 microns [3]. Because the anodized thickness in type I is not thick, it is not as readily dyed as the other types of anodized surfaces uses sulfuric acid. This is known as type II anodizing and can produce an anodized layer with a thickness from 1.8 to 25.4 microns. Because the thickness of type II anodizing is sufficiently large, an organic dye or color could be easily applied with the two-step electrolytic coloring method.
Type III anodizing refers to hard coat anodizing (hard anodizing) with a thickness from 12.7 to 115 microns. This method is often used where the environment is harsh and extreme and the anodizing must protect the part against long term corrosion, weather and wear damages. The thickness for these three types of surface anodizing is summarized in Table 1.
References:
- Radiation Heat Transfer and Surface Area Treatment, Qpedia Thermal eMagazine, June 2008.
- Edwards, J., Coating and Surface Treatment Systems for Metals, Finishing Publications Ltd. and ASM International, 1997.
- Aluminum Anodizer Council Web Forum.
- Gustavsen, A. and Berdahl, P. Spectral Emissivity Of Anodized Aluminum And Thermal Transmittance Of Aluminum Windows Frames, Nordic Jounnal Of Building Physics, Vol.3, 2003.
- Ozisik, N.,Heat Transfer: A Basic Approach, McGraw-Hill, 1984.
- Highly Emissive Ion Beam Textured Surfaces for Improved Cooling of Electronic Devices, Electronics Cooling Magazine, September 1997.
- Chi, T. , Ballinger, R., Olds, R., and Zecchino, M., Surface Texture Analysis Using Dektak Stylus Profilers, Veeco Instruments, Inc.
