##### Process Specification for the 

**Anodizing of Aluminum Alloys**

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| **Engineering Directorate**                                           |
|                                                                       |
| **Structural Engineering Division**                                   |
|                                                                       |
| **May 2003**                                                          |
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| ![](media/image1.png){width="1.3944444444444444in"                    |
| height="1.1145833333333333in"}                                        |
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| National Aeronautics and                                              |
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| Space Administration                                                  |
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| **Lyndon B. Johnson Space Center**                                    |
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| Houston, Texas                                                        |
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**Process Specification for the**

**Anodizing of Aluminum Alloys**

+-----------------+----------------------------------+------+---------+
| **Prepared      | ***Signature on file***          |      | 5/19/03 |
| by:**           |                                  |      |         |
+-----------------+----------------------------------+------+---------+
|                 | Julie A. Henkener                |      | Date    |
|                 |                                  |      |         |
|                 | Materials and Processes          |      |         |
|                 | Branch/ES4                       |      |         |
+-----------------+----------------------------------+------+---------+
|                 |                                  |      |         |
+-----------------+----------------------------------+------+---------+
|                 | ***Signature on file***          |      | 5/19/03 |
+-----------------+----------------------------------+------+---------+
| **Approved      | Jose Hernandez, Acting Chief     |      | Date    |
| by:**           |                                  |      |         |
|                 | Materials and Processes          |      |         |
|                 | Branch/ES4                       |      |         |
+-----------------+----------------------------------+------+---------+
|                 |                                  |      |         |
+-----------------+----------------------------------+------+---------+

  ----------- --------------------------------------------- ---------------
  REVISIONS                                                 

  VERSION     CHANGES                                       DATE

  \--         Original version                              5/96

  A           Reformatted, Type III tight tolerance issue   3/2/99
              addressed, provisions for addition of glass   
              bead or grit blasting prior to anodizing      

  B           Added color callout in section 3.0, re-write  7/7/00
              of definition of "sealing"                    

  C           Changed EM references to ES                   6/4/03

                                                            
  ----------- --------------------------------------------- ---------------

**1.0 SCOPE**

This process specification establishes the technical requirements for
anodizing aluminum alloys for use in JSC flight hardware.

**2.0 APPLICABILITY**

This process specification covers the requirements for six types and two
classes of electrolytically formed anodic coatings on aluminum and
aluminum alloys. It does not cover the requirements for anodic coatings
with controlled optical properties for long-duration exposure to
spacecraft external environments.

**3.0 USAGE**

This process specification shall be called out on the engineering
drawing by using a drawing note that identifies the process
specification and the anodic coating type and class to be used. For
example:

  -----------------------------------------------------------------------
  **ANODIZE PER NASA/JSC PRC-5006, TYPE II, CLASS 2, COLOR BLUE.**

  -----------------------------------------------------------------------

The following types of anodic coating are covered by this specification:

Type I Chromic acid anodizing, conventional coatings produced from
chromic acid bath

Type IB Chromic acid anodizing, low voltage process, 22 [+]{.underline}
2 volts

Type IC Non-chromic acid anodizing, for use as a non-chromate
alternative for Type I and IB coatings

Type II Sulfuric acid anodizing, conventional coatings produced from
sulfuric acid bath

Type IIB Thin sulfuric acid anodizing for use as a non-chromate
alternative for Type I and Type IB coatings

Type III Hard anodic coatings

The two classes of anodic coatings are:

Class 1 \-- Non-dyed

Class 2 -- Dyed

For Class 2 anodize, color must be included in the drawing callout. If
the color is required to be a matte finish, this shall also be specified
on the drawing.

Surface preparation is critical to the quality of the anodize that is
produced. For example, if the anodized surface will be subsequently
coated with a dry film lubricant, it is important to glass bead blast
the surface prior to anodizing to produce a surface with less roughness
(more of a matte finish). If glass bead or grit blasting is required by
the design prior to the anodize process, it shall be called out on the
drawing in the following manner:

  -----------------------------------------------------------------------
  **GLASS BEAD BLAST WITH SIZE xx BEADS PRIOR TO ANODIZING**

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or

  -----------------------------------------------------------------------
  **GRIT BLAST PRIOR TO ANODIZING**

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1.  **SPECIFIC DESIGN INFORMATION ON TYPE III COATINGS**

The Type III hard anodize coatings are especially difficult to anodize
to very tight tolerances (\< +/- 0.0012 in.). When the Type III anodic
coating is applied, typically half of the coating thickness is comprised
of the penetration into the "before coating thickness" and the other
half consists of the coating growth, as shown below:

![](media/image2.wmf)

$$T2 = T1 + \frac{C}{2}$$

This process is complicated even more for intricate geometries, such as
the tube shown below. It should be noted that the Inner Coating
Thickness (IC) does not necessarily equal the Outer Coating Thickness
(OC). The resulting dimensions and tolerances are determined as a
function of part geometry and anodize bath conditions (i.e. uniformity
of the supplied current density). The more complex the geometry, the
more difficult it becomes to hold tight-tolerances. Normally for tubing,
the current density is such that IC \< OC.

![](media/image3.wmf)

$$\begin{array}{r}
\text{OD}2 = \text{OD}1 + \text{OC} \\
\text{ID}2 = \text{ID}1 - \text{IC}
\end{array}$$

For these reasons, if component tolerances will be held to less than +/-
0.0012 in. and the part geometry is intricate, the designer should
consult with EM2 materials engineering personnel. It is also recommended
that for such Type III applications, both the final dimensions and the
"machine to" dimensions should be specified on the engineering drawing.

## 3.2 WORK INSTRUCTIONS

Work instructions shall be generated for implementing this process
specification. The work instructions shall contain sufficient detail to
ensure that the manufacturing process produces consistent, repeatable
products that comply with this specification.

**4.0 REFERENCES**

All documents listed are assumed to be the current revision unless a
specific revision is listed.

MIL-A-8625 Military Specification*, Anodic Coatings for Aluminum and
Aluminum Alloys*

**5.0 MATERIAL REQUIREMENTS**

Material requirements shall be as-specified in MIL-A-8625.

**6.0 PROCESS REQUIREMENTS**

All anodizing of aluminum alloys shall be conducted in accordance with
the technical requirements of MIL-A-8625.

**7.0 PROCESS VERIFICATION**

Process verification shall be in compliance with MIL-A-8625.

**8.0 TRAINING AND CERTIFICATION OF PERSONNEL**

All anodizing of aluminum alloys shall be performed by personnel
qualified to conduct the process through training or experience. If
these processes are to be performed by an outside vendor, the
development of an appropriate training program shall be the
responsibility of the vendor.

**9.0 DEFINITIONS**

Anodizing Process Electrochemical procedure by which aluminum and
aluminum alloys are treated electrolytically in a bath containing
chromic or sulfuric acid to produce a uniform oxide coating on the
metallic surface.

Sealing Process by which the chemically treated aluminum is immersed in
boiling, deionized water for 15-30 minutes; partially converts the
alumina of the anodic coating to aluminum monohydroxide. Nickel acetate
and sodium dichromate solutions may also be used as anodic coating
seals.