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Sulfuric Acid Anodizing

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Parent
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Anodizing
Creating a protective oxide layer on finished aluminum
Current
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Sulfuric Acid Anodizing
Standard anodizing bath setup and procedure

Anodize aluminum by making the cleaned part the positive electrode in a 15-20% sulfuric acid bath at about 20 °C, passing 12-20 V at 1-2 A per square decimetre for 30-60 minutes, then rinsing and sealing. A 2 dm² part at 1.5 A/dm² draws about 3 A and 45 W, so unlike smelting this is within reach of a small generator or battery bank.

Part of Aluminum Smelting

Setting up and running a sulfuric acid anodizing bath that grows a protective oxide layer 5-25 μm thick on finished aluminum.

Anodizing Is Within Reach Where Smelting Is Not

The carrier gives the process: a tank of 15-20% sulfuric acid at 20 °C, a DC supply of 12-20 V, the aluminum workpiece as anode and a lead or aluminum plate as cathode; clean the part (degrease, then etch in sodium hydroxide), submerge, apply 15 V at 1-2 A/dm² for 30-60 minutes, rinse thoroughly, and seal (Sealing the Anodic Layer). The layer is 5-25 μm thick. The Phase 5 article Aluminum Anodizing has the thickness-estimation rule, an alloy compatibility table (copper-bearing and die-cast alloys anodize poorly, which matters for scrap-derived metal) and troubleshooting; they are not repeated here.

What You Need

  • Sulfuric acid (Sulfuric Acid). The corpus produces 60-70% chamber acid; see the worked example for dilution.
  • A tank of acid-resistant plastic, glazed ceramic or lead-lined.
  • A DC supply of 12-20 V able to give the current below.
  • A cathode plate of lead or aluminum.
  • Sodium hydroxide for etching (Caustic Soda), clean water, and a way to hold the bath at about 20 °C.

Method

Step 1: Clean. Degrease, then etch briefly in sodium hydroxide solution, and rinse.

Step 2: Make the bath. Add acid to water, never the reverse.

Step 3: Connect. The part to the positive terminal, the cathode plate to the negative. Check polarity twice.

Step 4: Run at about 15 V and 1-2 A/dm² for 30-60 minutes. Bath temperature climbs as current flows; keep it near 20 °C.

Step 5: Rinse in clean water, then seal.

Worked Example

Diluting village acid. From 65% acid (inside the corpus’s 60-70%), by mass: 1 kg of 15% bath needs 1 × 15 / 65 = 0.23 kg of acid and 0.77 kg of water; 1 kg of 20% bath needs 1 × 20 / 65 = 0.31 kg of acid and 0.69 kg of water. Chamber acid carries impurities; I recall that chloride in an anodizing bath causes pitting, but the corpus gives no purity specification.

Power. A part with 2 dm² of surface at 1.5 A/dm² draws 2 × 1.5 = 3 A. At 15 V that is 3 × 15 = 45 W. A sheet with 10 dm² draws 15 A and 225 W. Compare the 15 kW and 3,300 A of one smelting cell in Hall-Héroult Electrolysis.

Common Mistakes

  • Wrong polarity. The part must be the anode.
  • A warm bath. Higher temperatures give a softer, more porous layer.
  • Anodizing before the part is finished. The layer thickens dimensions.

Safety

Acid and hydrogen

Sulfuric acid burns skin and eyes; add acid to water, wear face shield, gloves and apron, and keep a bath of baking-soda solution ready for spills, as the neutralization article advises (Neutralization). Hydrogen forms at the cathode and is explosive when it collects: ventilate the tank area and keep sparks and flames away, as Rust Removal and Prevention warns for electrolysis. A lead cathode is toxic; wash after handling. The supply is 12-20 V; the current, not the voltage, is what makes burns at contacts, so make joints firm.

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