Question 1

How much sulphuric acid do I need for an anodizing bath?

Accepted Answer

Standard Type II sulphuric anodizing runs at 180–200 g/L of H₂SO₄ in solution. For a 1,000-litre bath at 190 g/L, you need approximately 190 kg of H₂SO₄. Industrial sulphuric acid is typically supplied at 98% concentration (density 1.84 kg/L), so you need 190 ÷ 0.98 ≈ 194 kg of commercial acid, or 194 ÷ 1.84 ≈ 105 litres by volume. Always add acid to water — never water to acid.

Question 2

What concentration of caustic soda is used for anodizing etch?

Accepted Answer

Decorative etch: 40–60 g/L NaOH at 55–65 °C for 1–3 minutes gives a matte, even etch. Aggressive etch: 80–120 g/L NaOH at 60–70 °C for 2–5 minutes for heavily machined or as-cast surfaces. Add aluminium gluconate or sodium gluconate (1–2 g/L) to inhibit over-etching and reduce drag-out loading.

Question 3

How do I make up a nickel acetate sealing bath?

Accepted Answer

Dissolve nickel acetate powder in de-ionised water at 5–8 g/L. pH should be 5.5–6.5; adjust with acetic acid (lower) or sodium acetate (raise). Seal temperature 95–98 °C, time 1 minute per micron of coating. A 500-litre bath at 6 g/L requires 3 kg of nickel acetate sealing salts.

Question 4

What chemicals do I need for desmut after caustic etch?

Accepted Answer

The standard desmut for most aluminium alloys is 20–30% v/v nitric acid (HNO₃ 65%) at room temperature for 1–3 minutes. For high-copper alloys (2xxx series), add 1–2% v/v hydrofluoric acid or use ferric sulphate-based proprietary desmuts. A 500-litre bath at 25% v/v requires 125 litres of nitric acid (65% commercial grade).

Question 5

How do I calculate chemical replenishment for an anodizing line?

Accepted Answer

Sulphuric acid is consumed at approximately 1–2 g/L per 1,000 A·min of anodizing current. Caustic soda is consumed at approximately 0.5–1 g of NaOH per gram of aluminium etched. Nickel acetate sealing salt is consumed at approximately 0.1–0.2 g per dm² of sealed area. Monitor bath concentrations by titration weekly and replenish to target; this calculator handles both make-up and replenishment calculations.

Bath	Chemical	Target (g/L or % v/v)	Operating temp	Notes
Sulphuric acid anodizing (Type II)	H₂SO₄ 98%	180 – 200 g/L	18 – 22 °C	Monitor by titration weekly; aluminium build-up > 20 g/L slows growth
Hard anodizing (Type III)	H₂SO₄ 98%	150 – 180 g/L	−2 to +2 °C	Lower acid than Type II; chiller critical; add glycolic acid additive optional
Caustic soda etch (decorative)	NaOH flakes/beads 99%	40 – 60 g/L	55 – 65 °C	Add gluconate 1–2 g/L to control etch rate; Al build-up 10–15 g/L = change bath
Caustic soda etch (aggressive)	NaOH flakes/beads 99%	80 – 120 g/L	60 – 70 °C	For heavily machined surfaces; watch weight loss on 6063
Nickel acetate sealing	Ni(CH₃COO)₂ sealing salts	5 – 8 g/L	95 – 98 °C	pH 5.5 – 6.5; use DI water only; 1 min per µm of coating
Nitric acid desmut	HNO₃ 65%	20 – 30 % v/v	Ambient (20 – 25 °C)	For 6061/6063; add HF 1–2% v/v for 2xxx alloys (copper smut)
Alkaline degreaser	Proprietary powder (pH 11–12)	30 – 50 g/L	50 – 60 °C	Spray or immersion; rinse conductivity < 50 µS/cm before etch

Anodizing Chemical Dosing Calculator

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Bath chemistry reference — standard concentrations

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