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Anodizing types I II III properties

Anodizing increases the natural aluminum oxide layer from approximately 4 nm / 0,16 µin to controlled thicknesses of 150 µm / 6 mils. It is an electrolytic process in which the aluminum part acts as an anode within an acid bath. When direct current is applied, oxygen released at the surface reacts with the metal forming a porous oxide film. Depending on the electrolyte and operating conditions, types I (chromic acid), II (sulfuric acid), and III (hard anodizing) are obtained. Types I and III require strict control of voltage and temperature, while type II is the most versatile for decorative and architectural applications. After layer formation, a hydrothermal or nickel acetate seal is applied to close the pores and maximize corrosion resistance.

Applicable materials

Section titled “Applicable materials”

Anodizing is mainly applied to wrought and cast aluminum alloys. The series that respond best are those with low copper and silicon content. The temper designation influences layer uniformity; artificially aged materials (T6) usually offer the best results.

Alloy (series) Examples Anodizing suitability Maximum recommended thickness (Type II) Remarks
1xxx (Pure Al) 1100, 1050 Excellent 25 µm / 1,0 mil Clear and bright layer. Good corrosion resistance.
2xxx (Al-Cu) 2024 Fair 10 µm / 0,4 mil Tendency to pitting and darkening; Type I is preferred to avoid acid entrapment.
3xxx (Al-Mn) 3003 Good 20 µm / 0,8 mil Slight grayish tone, good formability.
5xxx (Al-Mg) 5052, 5083 Excellent 25 µm / 1,0 mil High marine corrosion resistance, uniform anodizing.
6xxx (Al-Mg-Si) 6061, 6063 Very good 25 µm / 1,0 mil Widespread use in architectural profiles and mechanical components.
7xxx (Al-Zn) 7075 Fair‑good 15 µm / 0,6 mil Possible darkening; with temperature control, hard layers are achieved in Type III.
Cast Al-Si (>7% Si) A356, A380 Poor ≤ 10 µm / 0,4 mil The high silicon content produces stained and brittle layers; requires special processes.

Operating parameters

Section titled “Operating parameters”

The following table summarizes typical process values for each type of anodizing on aluminum. Current densities and voltages must be adjusted according to the part area and alloy.

Parameter Type I (CrO₃) Type II (H₂SO₄) Type III (Hard anodizing)
Electrolyte Chromic acid 3‑10% Sulfuric acid 15‑20% Sulfuric acid 10‑20% + organic additives
Temperature 35‑40 °C / 95‑104 °F 20‑22 °C / 68‑72 °F 0‑5 °C / 32‑41 °F
Voltage (DC) 40‑50 V 12‑18 V 30‑100 V
Current density 0,5‑1,0 A/dm² / 4,6‑9,3 A/ft² 1,0‑1,5 A/dm² / 9,3‑13,9 A/ft² 2,0‑5,0 A/dm² / 18,6‑46,5 A/ft²
Typical thickness 0,5‑2,5 µm / 0,02‑0,1 mils 5‑25 µm / 0,2‑1,0 mils 25‑150 µm / 1‑6 mils
Hardness (Vickers/HB) < 200 HV 200‑350 HV (≈190‑330 HB) 400‑600 HV (≈380‑570 HB)
Porosity Low High Medium‑low
Sealing Optional; improves corrosion resistance Required to fix dyes and seal pores Recommended for waterproofing; sometimes omitted to retain lubricant
Natural color Transparent to pale gray Colorless to silvery Dark gray to matte black
Typical applications Aerospace components, bonded joints Architectural profiles, consumer goods Pistons, cylinders, molds, wear parts

Properties of the anodic layer

Section titled “Properties of the anodic layer”

The anodic layer increases wear and corrosion resistance, and provides electrical insulation. A Type III coating of 50 µm / 2 mils thickness has a Taber abrasion loss (CS‑10 wheel, 1000 cycles) of less than 15 mg / 0,00053 oz. The dielectric strength ranges between 30‑40 V/µm / 760‑1016 V/mil for Type II, possibly exceeding 500 V breakdown voltage in 25 µm layers. Surface hardness, measured in Vickers, reaches 600 HV on 6xxx alloys treated with Type III, a value comparable to low-alloy hardened steels (approximately 55 HRC according to Engineering Toolbox conversions). Corrosion resistance in salt spray (ASTM B117) exceeds 1000 hours in sealed Type II and III layers, while Type I is used when the part cannot tolerate acid residues in pores (riveted structures or adhesive joints).

Advantages and limitations

Section titled “Advantages and limitations”

Advantages

  • Increases the surface hardness of aluminum from ~30 HV to 600 HV (Type III).
  • Improves resistance to scratching and frictional wear.
  • Excellent anchor for paints and adhesives (the controlled porosity of Type II absorbs primers).
  • Allows stable decorative colors through dyes, especially in Type II.
  • Electrically isolates (25 µm layers withstand >500 V).
  • Does not modify the mechanical properties of the part’s core.

Limitations

  • Reduces fatigue strength by 10 % to 30 %, with Type I being the least aggressive.
  • Sharp corners cause current concentration and brittleness, so minimum radii of 0,5 mm / 0,02 in are recommended.
  • Thick coatings (>125 µm / 5 mils) can chip under impact.
  • Limited applicability in alloys with high copper or silicon content.
  • Layer growth alters critical dimensions (approximately 50 % of the total thickness remains above the original surface).
  • Type III layers are difficult to dye in bright colors due to their natural dark shade and pore size (10‑30 nm / 0,39‑1,18 µin).

Selection guide by type

Section titled “Selection guide by type”
  • Type I (chromic acid) → choose when the part is subject to fatigue or cannot retain electrolyte in cracks (riveted joints, aeronautical structures). Provides a thin but very corrosion-resistant layer without embrittling the aluminum.
  • Type II (sulfuric acid) → ideal for decorative and architectural purposes. Accepts a wide range of dyes and provides good weather protection. Serves as a base for paint on bodies and housings.
  • Type III (hard anodizing) → suitable for components subject to friction, wear or high surface pressures, such as pistons, bearings, molds and military equipment. Reaches thicknesses of 150 µm / 6 mils and hardnesses of 600 HV. Can retain lubricants if not sealed, reducing the coefficient of friction.

Frequently Asked Questions (FAQ)

Section titled “Frequently Asked Questions (FAQ)”

What is the typical hardness of Type III anodizing?

Section titled “What is the typical hardness of Type III anodizing?”

The hardness is between 400 and 600 HV (approximately 50‑55 HRC or 380‑570 HB), depending on the alloy and electrolyte temperature.

What maximum thickness can be obtained with sulfuric anodizing (Type II)?

Section titled “What maximum thickness can be obtained with sulfuric anodizing (Type II)?”

The practical maximum thickness is 25 µm / 1,0 mil; greater thicknesses require the hard anodizing process (Type III).

How much does the part dimensionally increase after anodizing?

Section titled “How much does the part dimensionally increase after anodizing?”

About 50 % of the total coating thickness builds outward and 50 % penetrates into the base metal. Thus, a 50 µm / 2 mils coating increases the external dimension by approximately 25 µm / 1 mil.

Can hard anodizing be colored?

Section titled “Can hard anodizing be colored?”

The pore diameter of Type III ranges between 10‑30 nm / 0,39‑1,18 µin, which only allows absorbing dark tone dyes (black, bronze, olive green). Bright colors are not viable without additional decorative sealing.

How much does anodizing affect fatigue strength?

Section titled “How much does anodizing affect fatigue strength?”

The anodic layer produces a reduction between 10 % and 30 % in the fatigue limit of aluminum, with Type I being the least penalizing. For critical parts, previous or subsequent shot peening is recommended.

What is the maximum service temperature of an anodic layer?

Section titled “What is the maximum service temperature of an anodic layer?”

The layers retain their properties up to 200‑250 °C / 392‑482 °F. Above 350 °C / 662 °F oxide dehydration begins and microcracks appear that degrade the protection.

References

Section titled “References”