**Process Specification for Electropolishing of Corrosion-**

**Resistant Steel**

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| **Engineering Directorate**                                           |
|                                                                       |
| **Structural Engineering Division**                                   |
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| **March 2002**                                                        |
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| ![](media/image1.emf){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**                                    |
|                                                                       |
| Houston, Texas                                                        |
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**Process Specification for Electropolishing of Corrosion-Resistant
Steel**

+-----------------+----------------------------------+------+---------+
| **Prepared      | ##### Signature on File          |      | 3/28/02 |
| by:**           |                                  |      |         |
+-----------------+----------------------------------+------+---------+
|                 | Julie A. Henkener                |      | Date    |
|                 |                                  |      |         |
|                 | Materials and Processes          |      |         |
|                 | Branch/ES4                       |      |         |
+-----------------+----------------------------------+------+---------+
|                 |                                  |      |         |
+-----------------+----------------------------------+------+---------+
|                 | ## Signature on File             |      | 3/28/02 |
+-----------------+----------------------------------+------+---------+
| **Approved      | Gail K. Horiuchi, Chief          |      | Date    |
| by:**           |                                  |      |         |
|                 | Materials and Processes          |      |         |
|                 | Branch/ES4                       |      |         |
+-----------------+----------------------------------+------+---------+
|                 |                                  |      |         |
+-----------------+----------------------------------+------+---------+

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  REVISIONS                                                   

  VERSION     CHANGES                                         DATE

  Baseline    Original version                                1/14/99

  A           Reviewed for accuracy, changed EM2 references   3/29/02
              to ES4, removed MMPTD references                

                                                              

                                                              

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

**1.0 [SCOPE]{.underline}**

This process specification establishes the technical engineering
requirements for electropolishing of corrosion-resistant stainless steel
alloys. Electropolishing of aluminum, copper, or titanium alloys is not
permitted. This specification is not intended to be used for
electropolishing of parts with internal surfaces, such as tubing. In
addition, this process specification is not intended to be used to
electropolish parts with faying surfaces or deeply recessed areas where
internal cathodes cannot be effectively applied.

**2.0 [APPLICABILITY]{.underline}**

This specification applies to electropolishing treatments for use on
austenitic, ferritic, martensitic, and precipitation-hardening
corrosion-resistant steel alloys*.* In addition, it applies to heat and
corrosion resistant materials, including cobalt and nickel alloys. When
this electropolish process is used, passivation is not required. Also,
electropolishing may be used in lieu of etching prior to dye penetrant
inspection.

**3.0 [USAGE]{.underline}**

This specification shall be called out on the engineering drawing by
using a drawing note that identifies this process specification. A
sample drawing note is:

  -----------------------------------------------------------------------
  **ELECTROPOLISH PER NASA/JSC PRC-5009.**

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

Metal removal by this electropolishing process shall be between 0.0002
in. and 0.0005 in. per surface. If a different range or tighter
tolerances are required, the engineering drawing must specify the range
of material removal that is acceptable. For example:

**REMOVE 0.0002 TO 0.0004 INCHES OF MATERIAL PER SURFACE**

Note however, that depending on the location of the electrodes, the
surface that results from less material removal may have more of a matte
finish than a uniformly shiny surface.

Designers should be aware that the operating parameters of the
electropolish process are somewhat dependent on the geometry of the
component. A separate test coupon from the same material lot as the
production part(s) must be provided for the process qualification (see
section 7.0). For extremely complex parts, a test article that is more
detailed than a flat coupon will be required and should be negotiated
with the manufacturing facility.

1.  **[REFERENCES]{.underline}**

The following documents were used to develop this PRC. Documents listed
are assumed to be the current revision unless a specific revision is
listed.

AMS 2759/9 *Hydrogen Embrittlement Relief (Baking) of Steel Parts*

10107-70972D *General Process Specification for Electropolishing of
Corrosion Resistant Alloys,* Oceaneering Space Systems

SOP-007.1 *Preparation and Revision of Process Specifications (PRCs)*

JPG 8500.4 *Engineering Drawing System Manual*

MAO103-308E *Electropolishing,* Rockwell International

MS 150.4 *Manufacturing Specification for Electropolishing of Corrosion
Resistant Steel,* NASA Marshall Space Flight Center

**5.0 [MATERIAL REQUIREMENTS]{.underline}**

**5.1 PRE-ELECTROPOLISH CLEANING**

The pre-electropolish cleaning step shall be accomplished using any
organic solvent or non-etching detergent cleaner.

**5.2 ELECTROPOLISH SOLUTION REQUIREMENTS**

The electropolish solution shall be composed of a relatively
concentrated acid mixture of either a glycolic-sulfuric acid or
phosphoric-sulfuric acid composition. However, it is noted that a rather
large number of different electropolish formulations are commercially
available. Exceptions to the above acid solution compositions must be
submitted to ES4 materials engineering for approval.

The specific gravity of the solution shall be in compliance with that
which is specified by the solution vendor at the 170-180°F operating
temperature when the recommended current density is used. The specific
gravity of this solution shall be checked with a hydrometer every two
weeks. Immediately after checking the solution's specific gravity, a
sample of the chemical bath shall be titrated to determine the correct
percentages of dissolved solids. The electropolish solution shall be
maintained between 2.5% and 3.5% iron. If these limits are not met, the
solution shall be removed and replaced.

**5.3 POST-ELECTROPOLISH RINSING**

Post-electropolish rinsing shall use a cold water bath of either
distilled or deionized water. If subsequent part finishing includes
plating operations, the final rinse shall be in an acidic, cold water
solution (2.5\<pH\<3.5) that is maintained using additions of HCl.

**6.0 [PROCESS REQUIREMENTS]{.underline}**

All forming, tube flaring, machining, and heat treatment must be
performed prior to the electropolish process. If stainless steels 420,
440A, 440B, or 440C are to be electropolished, they must first be
annealed at 300-700°F in order to avoid pickling cracks. If components
have been heat treated, they may require a wet or dry blasting procedure
to remove the scale.

Parts shall be cleaned (degreased and descaled) so that a clean, uniform
surface is obtained before electropolishing begins. Chemical cleaning
methods other than those referenced in section 5.1 require prior
approval by an ES4 materials engineer.

After cleaning, parts shall be racked with attention to the following
details:

• Ample spacing to permit proper current density and gas evolution

• Rack mark placement

• Proximity to cathodes

Racked components are placed in the electropolish tank and the positive
connection is made.

Specific cycles and parameters of the electropolish process are
determined by the metal finishing personnel, depending on the material
and nature of finish required. However, they shall meet the following
general operating parameters for corrosion-resistant steel alloys:

Temperature- 120°F to 180°F (controlled to ± 5°F)

Time/Cycle- 30 sec. to 8 min. (controlled to ±5 sec.)

Current Density- 1.0-2.0 amp/sq. in.

Metal removal by this electropolishing process shall not be less than
0.0002 in. nor greater than 0.0005 in. per surface. Note that if tighter
tolerances are specified on the engineering drawing, this process shall
meet the specified range of material removal.

Automated air or mechanical agitation shall be used whenever possible to
reduce or prevent gas streaks. If automated mechanical agitation is
used, it shall possess a stroke of 4-6 in., a maximum velocity of 50
ft/min horizontal, and a frequency of 15-25 cycles/min. If the
electropolish bath is not equipped for automated air or mechanical
agitation, the solution shall be manually stirred prior to each use.

After electropolish, the components shall be thoroughly rinsed with
distilled or deionized water (meeting the requirements of section 5.3)
until clean. Use of chemical brighteners as part of the final cleaning
is specifically prohibited due to the potential for causing hydrogen
embrittlement.

If parts are to be subsequently plated, the cleaning step must also
include a spray rinse with a 150 ± 5°F deionized water source. In this
case, the final rinse shall be a cold water solution (2.5\<pH\<3.5) that
is maintained using additions of HCl.

After cleaning, the parts shall be dried using clean, dry gas or low
heat. In order to prevent hydrogen embrittlement, the stainless steel
parts shall then be baked out at 375°F for 8 hours, according to the
requirements of AMS 2759/9.

The clean, dry parts shall be packaged in clean plastic bags or other
containers, which shall be labeled with the part number or other
suitable identification, unless further processing is needed. Appearance
of finished parts shall conform to the requirements of section 8.0.

**7.0 [PROCESS QUALIFICATION]{.underline}**

Process qualification is required using coupons for each material to be
electropolished. After electropolishing, the coupons shall be examined
to ensure that all of the requirements of section 8.0 are met.

**8.0 [PROCESS VERIFICATION]{.underline}**

The verification of the adequate completion of the electropolishing
process shall consist of a visual inspection for evidence of pitting,
scale, localized material removal, preferential etching, burns, cloudy
films, stains, oxidation, and gas streaks. Negative relief (grooving or
undercutting) which occasionally results from electropolishing is
specifically prohibited. Minor positive relief and/or minor surface
discoloration is acceptable if it is at the positive connection point.

In addition, verification that metal removal is in the 0.0002-0.0005 in.
per surface range shall be performed using traditional dimensional
measurement techniques or go/no-go gages that are appropriate to the
design. A current probe that is designed and calibrated to the metal
removal that is obtained by the specific material-solution combination
for a given electropolish cycle may only be used if its accuracy is
demonstrated.

**9.0 [TRAINING AND CERTIFICATION OF PERSONNEL]{.underline}**

All electropolishing treatments shall be performed by trained personnel.
Proper training for electropolishing shall, at a minimum, be structured
in such a way as to ensure that each trainee is capable of applying an
electropolish process that meets this specification. Training and
certification records shall be kept.

**10.0 [DEFINITIONS]{.underline}**

Electropolishing Procedure by which a very small amount of metal
(typically 0.0002-0.0005 in.) is electrochemically removed from the
workpiece, thereby smoothing the surface and diminishing scratches,
burrs, etc. It is a controllable process that is the reverse of plating.