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. Common 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

In This Section

Resources Anodizing 101 Course AAC Buyers’ Guide ANO-TALK Podcast Technical Bulletins AAC Webinar Program Anodizing Publications Reference Guide Useful Links