### Process Specification for Pickling, Etching, and Descaling of Metals

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| **Engineering Directorate**                                           |
|                                                                       |
| **Structural Engineering Division**                                   |
|                                                                       |
| February 2007                                                         |
|                                                                       |
| ![](media/image1.png)                                                 |
|                                                                       |
| National Aeronautics and                                              |
|                                                                       |
| Space Administration                                                  |
|                                                                       |
| **Lyndon B. Johnson Space Center**                                    |
|                                                                       |
| Houston, Texas                                                        |
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Process Specification for Pickling, Etching, and Descaling of Metals

+---------------+-------------------------------+---+--------------------+
| **Prepared    |                               |   |                    |
| by:**         |                               |   |                    |
+---------------+-------------------------------+---+--------------------+
|               | Jay E. Bennett                |   | date               |
|               |                               |   |                    |
|               | Materials and Processes       |   |                    |
|               | Branch ES4                    |   |                    |
+---------------+-------------------------------+---+--------------------+
|               |                               |   |                    |
+---------------+-------------------------------+---+--------------------+
|               |                               |   |                    |
+---------------+-------------------------------+---+--------------------+
|               | John D. Figert                |   | date               |
|               |                               |   |                    |
|               | Materials and Processes       |   |                    |
|               | Branch ES4                    |   |                    |
+---------------+-------------------------------+---+--------------------+

+---------------+-------------------------------+---+--------------------+
| **Approved    |                               |   |                    |
| by:**         |                               |   |                    |
+---------------+-------------------------------+---+--------------------+
|               | Bradley S. Files, Chief       |   | date               |
|               |                               |   |                    |
|               | Materials and Processes       |   |                    |
|               | Branch                        |   |                    |
|               |                               |   |                    |
|               | ES4                           |   |                    |
+---------------+-------------------------------+---+--------------------+

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

  VERSION     CHANGES                                        DATE

  \--         Original version                               2/03/00

  A           Changed Division name. Corrected errors in     10/2004
              metal removal thickness before penetrant       
              inspection in Sections 3.0 and 6.3.2.          
              Re-titled and revised Section 6.1.             

  B           Extensive Changes in sections 6, 7, and 8,     12/2004
              associated with the processing of titanium.    

  C           Retitled Section 4.0 (Applicable Documents) to 2/2007
              (References) and updated document titles and   
              numbers. Reformatted tables in Section 6.3     
              (Special Requirements),                        

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

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

This document provides the standard requirements for etching, pickling,
and descaling of metals by acids or alkali aqueous solutions.

It does not cover the light pickling of Monel to remove embedded foreign
metals, which is covered by the passivation PRC.

# **2.0 APPLICABILITY**

This specification shall be applicable whenever a pickling, etching, or
descaling is invoked per Section 3.0, "Usage". It is also in force
whenever shop procedures that meet this specification are invoked by
another process specification or shop instruction.

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

This specification covers a wide variety of processes and metals, so the
engineering drawing callout shall be specific enough to ensure that the
proper work instructions are used.

The term "Pickling" is usually used to describe a process that removes
surface oxides (scale) and removes contamination from surfaces that are
oxidized or corroded. The term "Etching" is usually used to describe a
process that removes parent metal in a controlled manner, sometimes to
achieve specific surface textures. "Descaling" is usually used to remove
scale or heat tint caused by heat treating or welding. However, the
terms are often used interchangeably.

The following drawing callouts are for standard JSC processes.

#### Stainless Steels 

#### (including CRES 300 series except for 303, 15-5PH, A286, and Custom 455)

**DESCALE TO REMOVE EDM RECAST LAYERS PER NASA/JSC PRC-5010.**

**ETCH FOR PENETRANT INSPECTION PER NASA/JSC PRC-5010.**

**DESCALE TO REMOVE HEAT TINT PER NASA/JSC PRC-5010.**

#### Nickel Alloys

(none)

[Titanium (6Al4V)]{.underline}

**DESCALE TO REMOVE EDM RECAST LAYERS PER NASA/JSC PRC-5010.**

**DESCALE TO REMOVE HEAT TINT PER NASA/JSC PRC-5010.**

[Aluminum (Most common alloys)]{.underline}

**ETCH FOR PENETRANT INSPECTION PER NASA/JSC PRC-5010.**

All new pickling, etching and descaling processes shall be qualified for
the alloy being processed. Once a process is qualified, the
qualification testing does not have to be repeated for that alloy if no
essential process variables have changed and no processing problems are
discovered. The details of the qualification will be recorded in a
Process Qualification Record. Limitations of the process shall be noted
in the PQR. Process Qualification Records will be made available on the
ES home page for all qualified processes.

Pickling, etching and descaling processes for other metals and other
purposes can be qualified when necessary, but design should consult with
an ES4 materials engineer to ensure that a procedure can be developed
and successfully qualified before manufacturing needs the process.

A significant amount of material may be removed during pickling,
etching, or descaling processing. In most cases, this will be
compensated for during machining, but there may be some cases for which
dimensional control may be difficult to maintain. The typical maximum
metal removal after etching aluminum and titanium for penetrant
inspection is .0006 inches and for stainless and nickel-based alloys are
.0004 inches.

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

The following documents were used in developing this specification:

SOP-007.1 *Preparation and Revision of Process Specifications*

JPR 8500.4 *Engineering Drawing System Manual*

The following documents are called out as an extension of the
requirements given in this specification. All documents listed are
assumed to be the current revision unless a specific revision is listed.

ASTM A380 *Standard Practice for Cleaning, Descaling, and Passivation of
Stainless Steel Parts, Equipment, and Systems*

ASTM B600 *Descaling and Cleaning Titanium and Titanium Alloy Surfaces*.

NASA/JSC TI-5000-01 *Metal Finishing Technicians, Training Instruction
for*

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

N/A

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

**6.1 WORK INSTRUCTIONS**

All work shall be performed to written procedures. The work instructions
shall contain sufficient detail to ensure that the manufacturing process
produces consistent, repeatable products that comply with this
specification.

For work performed at JSC facilities, these work procedures consist of
Detailed Process Instructions (DPI's).

For contracted work, the contractor shall be responsible for preparing
and maintaining, and certifying written work procedures that meet the
requirements of this specification.

**6.2 GENERAL REQUIREMENTS**

Pickling, etching, and descaling processes shall be controlled to
effectively remove oxidation or damage on metal surfaces while
minimizing any deleterious effect on the surface. The surface finish
shall be smooth, without pitting or attack of grain boundaries. For
susceptible metals, the process should not have a negative effect on the
mechanical properties of the metal.

General requirements for bath compositions are listed in the sections
for each special process. The specific compositions and operating
parameters of each process shall be detailed in the work instruction and
summarized in the PQR. Work instructions for stainless steels and alloys
shall be in compliance with the standard practices of ASTM A 380. Work
instructions for titanium alloys shall be in compliance with the
standard practices of ASTM B 600.

Free machining stainless steels shall not be pickled, etched, or
descaled.

While descaling or removing the EDM layer on titanium parts, the surface
may become passive and resistant to further chemical cleaning. If this
occurs, the part shall be blasted with abrasives to reactivate the
surface and the part returned to the processing solution. Allowable
abrasives and maximum sizes are #13 glass bead, 220 grit alumina, and
220 grit silicon carbide.

High temperature caustic solutions may be used to precondition titanium
prior to acid treatments. This treatment may remove all visible scale or
oxidation, but a flash pickle afterwards is required at a minimum.

Oxygen and nitrogen diffuse into the titanium at high temperatures and
form a brittle phase called alpha case, which can readily fracture and
subsequently initiate several types of service or test failures. The
visual inspection of titanium parts after pickling or descaling shall
include verification that alpha case has been removed.

**6.3 SPECIAL REQUIREMENTS**

**6.3.1 ETCHING FOR SUBSEQUENT METAL FINISHING**

Etching processes used as preparation of metals surfaces for subsequent
metal finishing shall use process solutions controlled and qualified by
the subsequent metal finishing process and its work instructions.

**6.3.2 ETCHING FOR PENETRANT INSPECTION**

When metal parts are etched for penetrant inspection, a minimum layer of
metal must be removed to meet NDE reliability requirements. For aluminum
and titanium, the minimum metal to be removed is .0004 inches. For
stainless steel and nickel-based alloys, the minimum metal to be removed
is .0002 inches.

Process control coupons shall be used to verify the minimum metal
removed.

+------------------+------------------+-----------+------------------+
| Alloy            | Process Solution | Te        | Notes            |
|                  |                  | mperature |                  |
+------------------+------------------+-----------+------------------+
| Stainless Steels | Nitric Acid      | Ambient   | Concentrations   |
|                  |                  |           | per ASTM A380    |
|                  | HF Acid          |           |                  |
+------------------+------------------+-----------+------------------+
| Nickel           | TBD              |           |                  |
| Based-Alloys     |                  |           |                  |
|                  |                  |           |                  |
| (other than      |                  |           |                  |
| Monel)           |                  |           |                  |
+------------------+------------------+-----------+------------------+
| MP35N            | TBD              |           |                  |
+------------------+------------------+-----------+------------------+
| Monel 400 and    | TBD              |           |                  |
| K500             |                  |           |                  |
+------------------+------------------+-----------+------------------+
| Titanium         | NaOH 40-50%      | 240-280F  | Descaling        |
|                  |                  |           | pretreatment     |
+------------------+------------------+-----------+------------------+
| Titanium         | Nitric Acid 7%   | Ambient   | Total process    |
|                  |                  |           | time shall be no |
|                  | HF Acid 3%       |           | longer than 30   |
|                  |                  |           | minutes.         |
+------------------+------------------+-----------+------------------+
| Titanium         | Nitric Acid      | Ambient   | 10:1 ratio or    |
|                  | 20-29%           |           | greater          |
|                  |                  |           |                  |
|                  | HF Acid 1.5-2.5% |           |                  |
+------------------+------------------+-----------+------------------+
| Aluminum         | NaOH 5%          | Ambient   |                  |
+------------------+------------------+-----------+------------------+

6.3.3 DESCALING TO REMOVE EDM RECAST LAYER

Qualification coupons shall be processed that verify that the recast
layer will be completely removed. The EDM cutting schedule used in
qualification shall have a cutting current (usually denoted as IP) that
is the same or higher than the production parts.

+------------------+------------------+-----------+------------------+
| Alloy            | Process Solution | Te        | Notes            |
|                  |                  | mperature |                  |
+------------------+------------------+-----------+------------------+
| Stainless Steels | Nitric Acid      | Ambient   | Concentrations   |
|                  |                  |           | per ASTM A380    |
|                  | HF Acid          |           |                  |
+------------------+------------------+-----------+------------------+
| Nickel           | TBD              |           |                  |
| Based-Alloys     |                  |           |                  |
|                  |                  |           |                  |
| (other than      |                  |           |                  |
| Monel)           |                  |           |                  |
+------------------+------------------+-----------+------------------+
| MP35N            | TBD              |           |                  |
+------------------+------------------+-----------+------------------+
| Monel            | FeCl3            |           |                  |
|                  |                  |           |                  |
|                  | HCl              |           |                  |
+------------------+------------------+-----------+------------------+
| Titanium         | NaOH 40-50%      | 240-280F  | Descaling        |
|                  |                  |           | pretreatment     |
+------------------+------------------+-----------+------------------+
| Titanium         | Nitric Acid 7%   | Ambient   | Total process    |
|                  |                  |           | time shall be no |
|                  | HF Acid 3%       |           | longer than 30   |
|                  |                  |           | minutes.         |
+------------------+------------------+-----------+------------------+
| Titanium         | Nitric Acid      | Ambient   | 10:1 ratio or    |
|                  | 20-29%           |           | greater          |
|                  |                  |           |                  |
|                  | HF Acid 1.5-2.5% |           |                  |
+------------------+------------------+-----------+------------------+
| Aluminum         | Phosphoric Acid  | 212°F     |                  |
|                  | 80%              |           |                  |
|                  |                  |           |                  |
|                  | Acetic Acid 15%  |           |                  |
|                  |                  |           |                  |
|                  | Nitric Acid 5%   |           |                  |
+------------------+------------------+-----------+------------------+

**6.3.4 DESCALING TO REMOVE SCALE AND HEAT TINT**

When parts are to be descaled to remove scale or heat tint, the metal
parts shall be immersed in the process bath for as long as necessary to
remove the oxides, up to the maximum time allowed by the work
instructions. Usually the dimensional changes are uncontrolled.
Qualification coupons shall be processed at the maximum time allowed by
the work instructions.

The compositions of the descaling process solutions shall be as follows:

+------------------+------------------+-----------+------------------+
| Alloy            | Process Solution | Te        | Notes            |
|                  |                  | mperature |                  |
+------------------+------------------+-----------+------------------+
| Stainless Steels | Nitric Acid      | Ambient   | Concentrations   |
|                  |                  |           | per              |
|                  | HF Acid          |           |                  |
|                  |                  |           | ASTM A380        |
+------------------+------------------+-----------+------------------+
| Nickel           | TBD              |           |                  |
| Based-Alloys     |                  |           |                  |
|                  |                  |           |                  |
| (other than      |                  |           |                  |
| Monel)           |                  |           |                  |
+------------------+------------------+-----------+------------------+
| Monel            | FeCl3            | Ambient   |                  |
|                  |                  |           |                  |
|                  | HCl              |           |                  |
+------------------+------------------+-----------+------------------+
| Titanium         | NaOH 40-50%      | 240-280F  | Descaling        |
|                  |                  |           | pretreatment     |
+------------------+------------------+-----------+------------------+
| Titanium         | Nitric Acid 7%   | Ambient   | Total process    |
|                  |                  |           | time shall be no |
|                  | HF Acid 3%       |           | longer than 30   |
|                  |                  |           | minutes.         |
+------------------+------------------+-----------+------------------+
| Titanium         | Nitric Acid      | Ambient   | 10:1 ratio or    |
|                  | 20-29%           |           | greater          |
|                  |                  |           |                  |
|                  | HF Acid 1.5-2.5% |           |                  |
+------------------+------------------+-----------+------------------+
| Aluminum         | NaOH 5%          | Ambient   |                  |
+------------------+------------------+-----------+------------------+

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

The work instructions for each process shall be qualified on non-flight
hardware or coupons before the flight hardware is subjected to the
process. The processes shall be individually qualified for each alloy
and are valid only for a specific alloy or processing facility. Each
process shall be evaluated with qualification coupons to determine
adherence to the general requirements for pickling, etching, or
descaling and for any special requirements. Once a process is qualified,
the testing does not have to be repeated if no essential process
variables have changed and no processing problems are discovered.

Process qualification results shall be recorded on a Process
Qualification Record (PQR) and signed by the metal finishing supervisor
and an engineer from ES4, Materials and Processes Branch. The PQR shall
include, as a minimum, the alloy processed, the process solution
compositions, the essential processing variables, and the results of the
examinations.

Approved PQR's shall be stored on the ES Home page.

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

Each part shall be inspected after processing to verify that the surface
finish is smooth, free from oxide, scale, pitting or grain boundary
etching.

Process Control coupons shall be used to determine the etch rate when
the surface will be etched before NDE inspection. These coupons shall be
made of the same alloy as the part being processed. The verification
coupons shall be thin and flat to maximize the differential thickness
measurement. Verification coupons shall be etched and the thickness
changed determined on the day of processing before the production work
is processed. The bath's etch rate shall be determined and the etch time
adjusted to guarantee the minimum metal removal is achieved on that day.

Records of self-inspection and MIPS shall be kept as quality assurance
records.

**8.1** **ADDITIONAL REQUIREMENTS FOR TITANIUM**

Titanium parts shall be visually inspected to verify that alpha case has
been removed.

Process verification coupons to verify freedom from hydrogen
embrittlement are required when titanium parts are processed with nitric
acid/hydrofluoric acid solutions.

Tensile coupons are required when the Nitric Acid 7% \-- HF Acid 3%
solution is used. The coupons shall be made of the same lot of material
as the production parts. Coupons for EDM layer descaling process
hydrogen verification shall have their gage section cut by the EDM
process. Coupons for NDE etch process hydrogen verification shall be
conventionally machined (unless the part is EDM machined). Tensile
coupons made by the EDM process shall be rectangular sheet type (Figure
1 of ASTM E8) and shall have the same thickness as the minimum section
of the part or 0.300 inches, whichever is less. The preferred coupon for
conventionally machined parts is the ASTM E8 cylindrical-type and shall
have a diameter less than the minimum section of the part or 0.500
inches diameter, whichever is less. Tensile coupons shall be tested to
ASTM E8, except that coupon shall be chilled to 32°F immediately before
testing. Hydrogen verification tensile test coupons made of 6-4 titanium
shall have an elongation greater or equal to 10.0% and a reduction of
area greater or equal to 25%.

Tensile coupons are not required when a solution with 10:1 or greater
ratio of nitric acid to hydrofluoric acid is used. Instead a total
hydrogen pickup test may be performed on a daily or lot basis. The
coupon for this test shall be processed, at the minimum, the maximum
time the coupon is immersed in the acid solution. The coupon is not
required to be from the same lot of titanium, but it does have to be the
same alloy and heat treatment. The details of this test and the
calibration of the equipment shall be documented in the facility work
instructions for the process.

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

Training requirements for metal finishing technicians shall be written
to cover the requirements of this specification. Trainee shall be
certified following the successful working under the supervision of the
metal finishing facility manager. Training and certification records
shall be kept.

For work performed at JSC facilities, these requirements shall be
satisfied by the training and certification of personnel per TI-5000-01
Training Instruction.

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

Mandatory A second-party inspection process designated during a

Inspection Point manufacturing operation.

(MIP)

PQR Process Qualification Record.