Anodizing Calculator

These are the three classes defined by military specification MIL-A-8625: Type I uses a chromic acid bath and produces a thin coating with good corrosion resistance but less dimensional buildup; Type II uses a sulfuric acid bath and is the common general-purpose anodizing most parts receive; Type III (hardcoat) also uses sulfuric acid but runs at higher current density and colder bath temperature to produce a much thicker, denser, more wear- and abrasion-resistant coating, at the cost of higher voltage and process complexity.

A colder bath (typically in the high 20s to low 30s Fahrenheit) slows the chemical dissolution of the oxide layer as it forms, which lets a thicker, denser coating build up before the acid bath can re-dissolve it. Standard Type I and Type II anodizing run at closer to room temperature since they target a thinner coating where this trade-off matters less -- running them too warm still measurably reduces their own plating efficiency, just not to the same degree hardcoat depends on cold.

Current density is the primary driver of how fast oxide builds up (following Faraday's-law logic: higher current moves more material per minute), so raising it directly cuts the time needed to hit your target thickness. But total current draw is current density multiplied by part surface area, so a higher current density also increases total amps and therefore power (volts times amps) -- you reach the target thickness faster, but you're pulling more power to do it.

Anodic oxide doesn't just sit on top of the aluminum surface -- roughly half of the oxide layer's thickness grows outward above the original surface, while the other half consumes (converts) the substrate metal beneath the original surface. This calculator reports only the outward-growth half, since that's the dimension that matters for parts with tight tolerances (threaded features, mating surfaces, or press-fit dimensions) where oxide buildup can affect fit.

They're planning-level estimates based on typical rate constants and cathode efficiencies for each anodizing type, not a guarantee for your specific line. Real outcomes also depend on alloy composition, surface preparation and etch quality, bath chemistry details beyond temperature (like acid concentration and dissolved aluminum content), rack and fixture design, and rectifier ripple -- always validate against qualification coupons and your own process specification before committing production parts.