Anodize Time and Current Calculator for Type II and Hard Coat
Anodize time is set by how much charge you pass per square foot, not by clock time alone. This calculator turns your current density and target thickness into minutes in the tank, rectifier amps, amp-hours, kWh and the dimensional growth you must allow for in machining.
Your numbers
Include rack area that is not masked if it will anodize. 1 ft2 is 9.29 dm2.
Type II sulfuric commonly 12 to 18 ASF. Type III hard coat often 24 to 36 ASF with a chilled bath.
0.7 mil is about 18 um. Check the drawing and whether it means total or build-out thickness.
The common rule of thumb is 720 ASF-minutes per 0.001 inch for Type II. Calibrate on your own bath with test coupons.
Typical Type II 12 to 20 V. Hard coat can reach 40 to 80 V.
Type II roughly 50 percent grows outward. Hard coat is often 40 to 50 percent. Verify with the process supplier.
Your results
The ASF-minutes rule is an empirical approximation for sulfuric acid anodizing. Alloy, bath temperature, acid concentration and aluminum content change the real coating rate. Validate against MIL-A-8625 or your customer specification with coupons before production.
Anodize time (minutes) vs current density (ASF)
Sweeps Current density from 6 to 36, holding your other inputs fixed.
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How to use it
Enter the total area in the load in square feet, including any rack or exposed fixture surface, and the current density you plan to run. Enter the thickness from the drawing, the time constant for your bath, the operating voltage and rectifier efficiency. The result gives anodize time, the amps your rectifier must deliver, and energy. Use the chart to compare running at lower ASF for gentler coating against higher ASF for throughput.
- Check the drawing for whether thickness is total or build-in only
- Use coupons to calibrate the ASF-minute constant on each alloy family
- Keep amps within the rectifier rating with headroom
- Note masked areas and subtract them from the area
The 720 rule and what it assumes
The widely used shop rule says that 720 ampere-minutes per square foot builds roughly 0.001 inch of Type II sulfuric acid anodic coating. At 15 ASF that is 48 minutes per mil. The rule assumes a bath around 70 degrees F, 15 to 20 percent sulfuric acid, and a 6061-type alloy. Copper-rich alloys like 2024 and high-silicon castings coat more slowly and burn more easily, and hard coat baths are cold and use different constants. Treat the time as a starting point and calibrate.
Dimensional growth and tolerances
Anodic coating converts aluminum into oxide, so about half of the thickness grows outward and half penetrates the metal. A 0.7 mil coating therefore adds about 0.35 mil per surface, or roughly 0.7 mil on a shaft diameter, while a bore shrinks by the same amount. Machine tight-fit features undersized or oversized accordingly, or mask them. Hard coat to 2 mil total adds about 1 mil per surface at 50 percent growth.
Common mistakes
Estimators often forget rack contact area, which draws current and raises amperage, and they quote time using the nominal 720 constant for every alloy. Another mistake is applying one voltage to every job; voltage rises with coating thickness and bath temperature, so energy cost estimates should use an average rather than an initial value. Finally, confirm that the thickness requirement is not the post-seal figure.
Frequently Asked Questions
How long does it take to anodize to 0.001 inch?
Using the 720 ASF-minute rule, 0.001 inch takes about 48 minutes at 15 ASF, or 60 minutes at 12 ASF. Your actual time depends on bath temperature, acid strength and alloy, so verify with a coupon.
How much does anodizing add to dimensions?
Roughly half of the total coating thickness grows outward from the original surface. A 0.0007 inch coating adds about 0.00035 inch per surface. A shaft diameter increases by about twice that and a hole diameter decreases by about the same.
What current density is used for hard coat?
Type III hard anodize commonly runs about 24 to 36 ASF in a bath held near freezing, often with a voltage ramp. Time constants differ from Type II, so enter your own measured ASF-minutes per mil.
Can I use this for chromic acid anodize?
Chromic baths use lower current density and different voltage programs, so the 720 constant does not apply. Enter your own constant from process data if you want a rough estimate, but follow the specification's voltage and time schedule.
If your anodizing routings and customer specs sit in an ERP or quality system, we can help build an assistant that checks thickness, growth and process notes against the drawing on your own infrastructure.
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