Products Finishing

OCT 2017

Products Finishing magazine is the No. 1 industrial finishing publication in the world. We keep our readers informed about the latest news and trends in plating, painting, powder coating, anodizing, electrocoating, parts cleaning, and pretreatment.

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PFonline.com/experts SJON WESTRE, Ph.D. Chemeon Surface Technology Sjon is vice president of technology at Chemeon. Visit chemeon.com. How to Apply the 720 Rule to Current Density Anodizing Q. What can you tell me about the 720 Rule as it applies to current density anodizing? A. We bring anodizers and manufacturers in from around the country to teach them the best practices for aluminum anod- izing. During this class, we typically discuss the 720 Rule, along with current density anodizing. This rule is a very powerful tool for managers, estimators and planners, as well as anodizers, but we often find that our students are not entirely familiar with what it is, or what they can do with it. The 720 Rule describes the relationship between the amount of current passed through an aluminum surface and the resultant anodic oxide thickness produced over time. It is known that 720 amp-minutes of current per square foot of load are required to produce one mil (0.001" or 25.4 microns) of anodic oxide (Westre 1997, 2017) (Westre 2000). Although the source of this relationship is not well documented, it has been suggested that the rela- tionship is derived from the Ilkovic equation using the half- cell potential for aluminum (Runge-Marchese 1999). At its most basic, the 720 Rule is a way to determine anod- izing build rate, anodizing time or the current density of the anodizing load when the other two parameters are known. Functionally, we express the relationship as Equation 1: where "CD" is the current density in amps per square foot, "t" is the exposure time in minutes and the resulting thick- ness is in mils. We can rewrite Equation 1 to emphasize the relationships of interest: Equation 2 allows us to calculate that, to achieve a 1-mil oxide thickness at 24 amps per square foot (ASF), 30 minutes is required: 720 A*min ft²mil CD*t Thickness = t(minutes)= Thickness CD *720 A*min ft²mil t(minutes)= 1mil *720 A*min ft²mil = A ft² 24 30 minutes Simple rearrangement of Equation 2 shows that the amount of oxide generated at a particular current density as a function of time is Equation 3: Table 1. Anodizing times as a function of current density CURRENT DENSITY/ASF TIME TO 1 MIL/MIN. 12 60 18 40 24 30 30 24 40 18 For anodizing in sulfuric acid electrolytes, this provides a very good estimate, regardless of alloy, electrolyte concen- tration and/or electrolyte temperature. In other words, it can be used to estimate the behavior of Type II anodizing on 6061 as well as Type III anodizing on 2024. It has been noted that perhaps the 720 Rule does not do as good of a job estimating 2024 thicknesses (Chesterfield 2008). For precision work, we must compensate for the amount of oxide added during the ramp period. Equation 2 provides the anodizing rate at a constant current. In real-world situations, the rectifier is ramped from zero current to the target final current density. What happens during the initial ramp of the rectifier? It turns out this is quite simple for linear ramps. If the current is increasing over time "t" to a final current density "CD," the amount of oxide generated during the ramp is: Table 1 shows the calculated time to 1 mil thickness for some common current densities: For example, if we ramp our rectifier to 30 ASF over a five-minute period, the ramp contributes 0.104 mil to the total thickness. Equation 4 can be derived graphically by realizing that, during a ramp, the area under the line in a current density vs. time plot is a triangle whose area is 0.5 t*CD. Or, that the average current density during ramp is one-half the target final current density. Another way to think about ramp compensation is that the amount of thickness formed during the ramp is half of what would be formed during the same amount of time under full current density. This allows us to simplify PLATING C L I N I C 74 OCTOBER 2017 — PFonline.com Thickness= CD*t A*min ft²mil 720 Thickness during ramp= * CD*t A*min ft²mil 720 1 2

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