Anodizing Reference Guide

IMPORTANT TECHNICAL DETAILS

Unlock the secrets of anodizing with our comprehensive Anodizing Reference Guide. Discover important technical details about various types of anodic coatings and their application methods. From sulfuric acid-based coatings to decorative and architectural finishes, our guide covers it all. Whether you’re a beginner or an experienced professional, this guide is a valuable resource for understanding the mechanical and chemical finishing processes involved in anodizing. Explore the possibilities and enhance your knowledge with the Aluminum Anodizers Council [AAC]. What is Anodizing? Find out today with our Anodizing Reference Guide.

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Mil Spec 8625 F

This specification is the model for much of the anodizing specified for military and aerospace products. It covers the requirements for six types and two classes of anodizing for nonarchitectural applications. MIL-A-8625 also forms the basis for many anodizing specifications that are proprietary to individual companies. Nearly all of these proprietary specifications refer to one or more requirements of this mil spec. The specification includes broad processing conditions for anodizing. It also gives important performance and acceptance criteria for such attributes as coating weight, corrosion resistance, light fastness, paint adhesion, abrasion resistance, and dying. Additionally, standards are set for general appearance, workmanship, contact marks, inspection, process control, and testing.

Type I A

Conventional coatings produced from chromic acid bath

Thickness:

0.5µ-7.6µ (microns)

Type I B

Low voltage chromic acid anodizing (20 volts) Used for 7xxx series alloys

Thickness:

0.5µ-7.6µ

Type II

Conventional coatings produced from sulfuric acid bath

Thickness:

1.8µ-25.4µ

Type III

Hard coat (Uniform anodic coatings)

Thickness:

12.7µ-115µ

Class I

Class II

TYPE I "Chromic Acid"

Color will vary from clear to dark gray depending on alloy. Copper bearing alloys only yield gray colors. Not as readily dyed as sulfuric anodize due to thinness of coating. New salt spray requirement is 336 hours (5% solution per method 811 or FED-STD-No. 151).

Type I

Chromic acid anodized coating. This process is used principally for the treatment of aircraft parts. An example is the Bengough-Stewart process where a 30 – 50 g/l chromic acid bath is maintained at 100° F and the voltage is gradually raised to 50V. Adjustments are made for high copper, zinc, and silicon alloys. Coating weights must be greater than 200 mg/ft2. Criteria for corrosion resistance, paint adhesion, and paint adhesion testing must be specified.

Type IB

Low voltage (22)2V) chromic acid anodized coating. Typically associated with higher temperature, more concentrated chromic acid electrolytes. Coating weights must be greater than 200 mg/ft2. Criteria for corrosion resistance, paint adhesion, and paint adhesion testing must be specified.

Type IC

Anodized coating produced in a non-chromic acid electrolyte. As with other Type I coating processes, the treatment is designed to impart corrosion resistance, paint adhesion, and/or fatigue resistance to an aluminum part. Coating weights must fall between 200 – 700 mg/ft2. Criteria for corrosion resistance, paint adhesion, and paint adhesion testing must be specified.

TYPE II "Sulfuric Acid"

Mechanical Finishing	A.A.	Description	Examples
As fabricated	M-10	Unspecified
	M-12	Nonspecular as fabricated	No particular reflectiveness
Buffed	M-21	Smooth specular	Polished first with coarser than 320 grit, followed by 320 grit, then buffed with Alum oxide


	M-22	Specular	Buffed with Alum oxide compound
Directional textured	M-31	Fine satin	Sanded with 320-400 grit Alum oxide
	M-32	Medium satin	Sanded with 180-220 grit Alum oxide
	M-33	Coarse satin	Sanded with 80-100 grit Alum oxide
	M-35	Brushed	Brushed with stainless steel wire brush

Chemical Finishing	A.A.	Common	Description	Examples
Nonetched Cleaning	C-11		Degreased	Organic solvent treated
	C-12		Inhibited chemical cleaned	Soap cleaner only
Etched	C-22	R-1	Medium matte	Sodium hydroxide (caustic soda) 30-45 gr/li @ 60-65˚C for 5 min
Brightened	C-31	R-5	Highly specular	Chemical bright dip solution of the proprietary phosphoric-nitric acid type, or electropolishing
	C-32		Diffuse bright	Etched finish C-22 followed by Brightened finish C-31

Anodic Coating	A.A.	Common	Description	Examples
General	A-11		Prep for other applied coatings	15% Sulfuric acid @ 20˚C, 12 amps/sq ft. for 10 min. sometimes not sealed
Decorative	A-21		Clear coating 2.5μ-7.5μ	15% Sulfuric acid @ 20˚C, 12 amps/sq ft.
	A-211	200	Clear coating min. 2.5μ	15% Sulfuric acid @ 20˚C, 12 amps/sq ft. for 10 min.
	A-212	201	Clear coating min. 5μ	15% Sulfuric acid @ 20˚C, 12 amps/sq ft. for 15 min.
	A-213	202	Clear coating min.7.5μ mil	15% Sulfuric acid @ 20˚C, 12 amps/sq ft. for 20 min.
	A-23		Coating with impregnated color	15% Sulfuric acid @ 20˚C, 12 amps/sq ft., followed by dyeing with organic or inorganic colors
	A-24		Coating electrolytically	15% Sulfuric acid @ 20˚C, 12 amps/sq ft. , deposited color followed by deposition of inorganic metallic salts
Architectural Class 2	A-31	204	Clear coating	15% Sulfuric acid @ 20˚C, 12 amps/sq ft.
10μ-18μ	A-33		Coating with impregnated color	15% Sulfuric acid @ 20˚C, 12 amps/sq ft. for 30 min., followed by dyeing with organic or inorganic colors.
	A-34		Coating electrolytically	15% Sulfuric acid @ 20˚C, 12 amps/sq ft. deposited color for 30 min., followed by deposition of inorganic metallic salts.
Architectural Class 1	A-41	215	Clear coating	15% Sulfuric acid @ 20˚C, 12 amps/sq ft.
18μ and more	A-43		Coating with impregnated color	15% Sulfuric acid @ 20˚C, 12 amps/sq ft. for 60 min., followed by dyeing with organic or inorganic colors.
	A-44		Coating electrolytically	15% Sulfuric acid @ 20˚C, 12 amps/sq ft. deposited color for 60 min., followed by deposition of inorganic metallic salts.

Test Methods For Type II Anodized Aluminum

Oxide Coating Thickness
ASTM B 244-97 (2002)
ASTM B 487-85 (2007)

Min Thickness
Class I	18 Microns (µ)
Class II	10 Microns

Corrosion Resistance
ASTM B 117-07

	Min Hours	Max Spots
Class I	3,000	15
Class I	1,000	15

Oxide Coating Weight and Apparent Density
ASTM B 137-95 (2004)

	Min Weight	Min Density
Class I	4.18 mg/cm2	2.32 g/cm3
Class I	2.40 mg/cm2	2.32 g/cm3

Seal Quality
ASTM B 136-84 (2003)
ASTM B 680-80 (2004)
ISO 3210

	Max Weight Loss
Class I	40 mg/dm2
Class I	40 mg/dm2

TYPE III "Hard Coating"

Color will vary from light tan to black depending on alloy and thickness. Color overtones listed below may vary with the use of additives and/or the process. Can be dyed in darker colors depending on thickness. Coating PENETRATES base metal as much as builds up on the surface. The term THICKNESS includes both the buildup and penetration. Provides very hard ceramic type coating. Abrasion resistance will vary with alloy and thickness of coating. Good dielectric properties. Corrosion resistance is good, but recommend sealing in 5% dichromate solution where increased corrosion resistance is required. Where extreme abrasion resistance is required do not seal as some softening is encountered.

Type III Anodize Thickness Guide

Alloy	Major Constituent(in)	Maximum Thickness*	Color Overtones***
1100	99.5% pure Alum.	.003	Gray/Green**
2011	Copper	Not recommended	Not recommended
2014	Copper	.001	Bronze
2017	Copper	.001	Bronze
2024	Copper	.0015	Bronze
3003	Manganese	.002	Gray
4032	Silicon	.0012	Gray
5005	Magnesium	.0035	Gray/Brown
5052	Magnesium	.0035	Gray/Brown
5083	Magnesium	.0035	Gray/Brown
6061	Mag/Silicon	.003	Dark Gray
6063	Mag/Silicon	.004	Green
6105	Mag/Silicon	.0035	Gray/Green

Alloy	Major Constituent (in)	Maximum Thickness*	Color Overtones***
6262	Mag/Silicon	.0025	Gray
6463	Mag/Silicon	.003	Gray
7075	Zinc	.004	Bronze
355	Silicon	.0035	Gray
356	Silicon	.0035	Gray
357	Silicon	.0035	Gray
360	Silicon	.0005	Gray
380	Silicon	.0005	Gray
319	Silicon	.0025	Light Gray
MIC-6	Silicon	.0035	Dark Gray

50% Penetration and 50% Buildup per Surface
* Generally Accepted
** Over .0025″ Thick
*** May vary

Aluminum Alloy Reference for Anodizing

Series (AA)*	Alloying Constituants	Metal Properties	Coating Properties	Uses	A.Q. Types**	Non-A.Q. Types**
1xxx	None	soft conductive	clear bright	cans architectural	none	1100, 1175
Finishing advice: care should be taken when racking this soft material; good for bright coatings; susceptible to etch staining

2xxx	Copper	very strong hard low elongation	yellow poor protection	aircraft mechanical	none	2011, 2017 2219, 2224
Finishing advice: since copper content is >2%, these produce yellow, poor weather-resistant coatings; don’t mix with other alloys on load

3xxx	Manganese	strong small grains	grayish-brown	cans architectural lighting	none	3003, 3004
Finishing advice: difficult to match sheet-to-sheet (varying degrees of gray/brown); used extensively for lighting

4xxx	Silicon	strong fluid	dark gray	architectural welding wire	none	4043, 4343
Finishing advice: produce heavy black smut which is hard to remove; 4043 & 4543 used for architectural dark gray finishes in past years

5xxx	Magnesium	strong ductile fluid	clear good protection	architectural welding wire lighting	5005, 5657	5052, 5252
Finishing advice: for 5005-keep silicon<0.1% and magnesium between 0.7% and 0.9%; watch for oxide streaks; 5005 used extensively for architectural

6xxx	Magnesium & Silicon	strong ductile	clear good protection	architectural structural	6063, 6463	6061, 6101
Finishing advice: matte-iron>0.2%; bright-iron<0.1%; 6063 best match for 5005; 6463 best for chemical brightening

7xxx	Zinc	very strong	clear good protection	automotive	none	7029, 7046 7075
Finishing advice: zinc over 5% will produce brown tinted coating; watch zinc in effluent stream; good for bright coatings