ASTM B875-96

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: B 875 – 96
Standard Specification for
Aluminum Diffusion Coating Applied by Pack Cementation
Process
This standard is issued under the fixed designation B 875; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope F 1330 Guide for Metallic Abrasive Blasting to Descale
Surfaces of Pipe
1.1 This specification covers the requirements for aluminum
diffusion of metals by the pack cementation method. Pack
3. Terminology
diffusion employs the chemical vapor deposition of a metal
3.1 Definitions used in this specification are in accordance
which is subsequently diffused into the surface of a substrate at
with Terminology B 374.
high temperature. The material to be coated (substrate) is
3.2 Definitions of Terms Specific to This Standard:
immersed or suspended in a powder containing aluminum
3.2.1 diffusion coating—a diffusion coating is one produced
(source), a halide salt (activator), and an inert diluent such as
by causing an element to react with or diffuse into, or both, the
alumina (filler). When the mixture is heated, the activator
surface of a metallic substrate, thus, chemically altering the
reacts to produce an atmosphere of aluminum halides which
surface of the substrate.
transfers aluminum to the substrate for subsequent diffusion.
3.2.2 retorts—containers in which powder and parts are
The aluminum-rich surface enhances corrosion, thermal stabil-
packed for processing. They can be constructed of carbon,
ity, and wear-resistant properties.
stainless, or high alloys and fabricated in all shapes and sizes
1.2 This standard does not purport to address all of the
to accommodate parts being processed.
safety concerns, if any, associated with its use. It is the
3.2.3 significant surface—areas that are essential to the
responsibility of the user of this standard to establish appro-
serviceability or function of the article. These surfaces must be
priate safety and health practices and determine the applica-
identified on a drawing or marked-up sample of product. Areas
bility of regulatory limitations prior to use.
can fall into one of three categories as follows:
2. Referenced Documents 3.2.4 coating required—these surfaces must be in accor-
dance with all quality requirements of this specification.
2.1 ASTM Standards:
3.2.5 no coating required—these surfaces are areas where
B 374 Terminology Relating to Electroplating
no coating is allowed due to a number of reasons including
B 487 Test Method for Measurement of Metal and Oxide
dimensional, fabrication, and welding, as well as others.
Coating Thickness by Microscopical Examination of Cross
Materials used for masking are commercially available.
Section
3.2.6 optional—these surfaces do not require coating, but at
B 567 Test Method for Measurement of Coating Thickness
the same time do not require masking.
by Beta Backscatter Method
B 602 Test Method for Attribute Sampling of Metallic and
4. Classification
Inorganic Coatings
4.1 There are three classes of aluminum diffusion defined by
B 697 Guide for Selection of Sampling Plans for Inspection
2 base (basis) metal category.
of Metallic and Inorganic Coatings
4.1.1 Class I—Carbon steel and low alloy.
B 762 Method of Variables Sampling for Metallic and
2 4.1.2 Class II—Stainless steels.
Inorganic Coatings
2 4.1.3 Class III—Nickel-based alloys.
C 664 Test Methods for Thickness of Diffusion Coatings
D 3951 Practice for Commercial Packaging
5. Ordering Information
E 3 Methods of Preparation of Metallographic Specimens
5.1 In order to make the application of this specification
complete, the purchaser shall supply the following information
to the vendor through a purchase order and drawings:
This specification is under the jurisdiction of ASTM Committee B-8 on
Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee
5.1.1 Title, ASTM designation number, and date of issue of
B08.15 on Diffusion Coatings.
this specification.
Current edition approved June 10, 1996. Published August 1996.
Annual Book of ASTM Standards, Vol 02.05.
Annual Book of ASTM Standards, Vol 15.09.
4 5
Annual Book of ASTM Standards, Vol 03.01. Annual Book of ASTM Standards, Vol 01.07.
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NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
B875–96
5.1.2 Deposit by classification (see Section 4). 6.5 Post-Cleaning—Retorts should be allowed to cool suf-
ficiently before opening. When parts are removed from retorts,
5.1.3 Composition and metallurgical condition of substrate
residual pack mix is removed from surfaces by a supplier-
to be coated.
approved method.
5.1.4 Location of significant surfaces (see 3.2.3).
6.6 Diffusion Heat Treatment—Parts processed in highac-
5.1.5 Samples for destructive metallurgical test (see 8.1).
tivity packs are placed in the retort and under inert atmosphere
5.1.6 Any post heat treatment required.
of argon or hydrogen are heated to 1900 to 2000°F and held for
5.1.7 Acceptance inspection procedure to be used (see
the amount of time needed to produce the desired level of
Section 9).
diffusion.
5.1.8 Any requirement for certification (see Section 11).
6.7 Post-Straightening—Long tubes may have distortion
due to the high temperature of process. Use rotary straightener
6. Processing Requirements
or hydraulic press to restore straightness and ovality.
6.1 Substrate Preparation—The metal to be aluminized
6.8 Visual Inspection—Inspect in accordance with 7.3.
shall be free of flaws and defects that will be detrimental to the 6.9 Marking and Packaging—To be defined by the pur-
coating. Thorough cleaning is essential to ensure satisfactory
chaser in the purchase order. Parts processed for the U.S.
diffusion. Materials used for cleaning should not damage the government and military, including subcontracts, shall be
base metal. Oils, dirt, grease, and stains must be removed.
packaged in accordance with Practice D 3951.
When blasting is also required, use aluminum oxide (90 to 220
7. Coating Requirements
mesh) at 60 to 80 psi from 4 to 6 in. standoff.
7.1 Diffusion Thickness—The following are minimums for
6.2 Pack Mix—Can be categorized by the following:
the different classes of materials based on standard process
6.2.1 Types of Activity:
parameters. Thicker coatings require special processing and
6.2.1.1 High Activity—Deposit aluminum rapidly and per-
must be called out for in the purchase order if required.
form at temperatures (1000 to 1500°F). This is usually fol-
Class Minimum, in.
lowed by a diffusion heat treatment at (1900 to 2000°F).
Class I (carbon and low alloy) 0.005
6.2.1.2 Low Activity—Deposit aluminum slowly and per-
Class II (stainless steels) 0.003
form at temperatures in excess of 1650°F. Class III (nickel-base alloys) 0.001
6.2.2 Materials:
7.2 Aluminum Content—The outer 15 % of coating shall
6.2.2.1 Masteralloy with 50 to 100 % Pure Aluminum—
contain a minimum of 28 % (by weight) aluminum.
Sold under a number of trade names in various mesh sizes.
7.3 Appearance—The diffusion zone shall be nonporous
Percentage in mix depends on material being processed.
and adherent to the base metal and shall have a uniform surface
6.2.2.2 Activator—Most commonly used is ammonium
free from objectionable imperfections. Minor variations in
chloride, but many others are available. Percentage in mix
color and surface appearance shall be considered acceptable,
depends on material and type of activity.
providing the requirements of 7.1 and 7.2 are met (see 8.6).
6.2.2.3 Inert
...