ASTM B874-96(2023)

Standard Specification for Chromium Diffusion Coating Applied by Pack Cementation Process

ABSTRACT
This specification covers the requirements for chromium diffusion of metals applied by pack cementation process. The four classes of chromium diffusion coating, defined by the type of base metal, are as follows: Class I (carbon steels); Class II (low-alloy steels); Class III (stainless steels); and Class IV (nickel-based alloys). Specimens shall adhere to processing requirements such as substrate preparation, materials (masteralloys, activators, and inert fillers), loading, furnace cycle, post cleaning, post straightening, visual inspection, and marking and packaging. Specimens shall also adhere to coating requirements such as diffusion thickness, decarburization, chromium content, appearance, and mechanical properties (tensile strength, and macro- and micro-hardness).
SCOPE
1.1 This specification covers the requirements for chromium diffusion of metals by the pack cementation method. Pack diffusion employs the chemical vapor deposition of a metal which is subsequently diffused into the surface of a substrate at high temperature. The material to be coated (substrate) is immersed or suspended in a powder containing chromium (source), a halide salt (activator), and an inert diluent such as alumina (filler). When the mixture is heated, the activator reacts to produce an atmosphere of chromium halides which transfers chromium to the substrate for subsequent diffusion. The chromium-rich surface enhances corrosion, thermal stability, and wear-resistant properties.  
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.3 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Oct-2023
• ICS: 25.220.40 - Metallic coatings
• Technical Committee: B08 - Metallic and Inorganic Coatings
• Drafting Committee: B08.03 - Engineering Coatings

Relations

• Replaces: ASTM B874-96(2018) - Standard Specification for Chromium Diffusion Coating Applied by Pack Cementation Process
• Effective Date: 01-Nov-2023
• Refers: ASTM D3951-18(2023) - Standard Practice for Commercial Packaging
• Effective Date: 01-Oct-2023
• Refers: ASTM D3951-18 - Standard Practice for Commercial Packaging
• Effective Date: 01-May-2018

Overview

ASTM B874-96(2023) is the international standard specification developed by ASTM International for the chromium diffusion coating of metals using the pack cementation process. This method employs chemical vapor deposition of chromium into a metal substrate at elevated temperatures, resulting in a chromium-enriched surface that significantly improves corrosion resistance, thermal stability, and wear properties.

The standard is applicable to four classes of base metals:

• Class I: Carbon steels
• Class II: Low-alloy steels
• Class III: Stainless steels
• Class IV: Nickel-based alloys

ASTM B874-96(2023) details requirements for substrate preparation, coating application, quality control, inspection, and documentation, ensuring consistency and performance across industries where reliable surface engineering is essential.

Key Topics

• Pack Cementation Process: Chromium diffusion takes place by embedding the substrate in a powder mixture containing chromium (source), a halide salt (activator), and an inert filler such as alumina. Controlled heating causes the formation of chromium halides, enabling chromium to diffuse into the substrate surface.
• Processing Requirements:
  • Substrate preparation, including cleaning and surface treatment.
  • Use of appropriate masteralloys, activators (e.g., ammonium chloride), and inert fillers.
  • Guidelines for loading, furnace cycling (heating between 1900 and 2400 °F), post-cleaning, and straightening.
• Coating Requirements:
  • Diffusion thickness: Minimum required thickness by material class.
  • Chromium content: Chromium concentration in the outer layer to ensure corrosion resistance.
  • Decarburization: Limits on surface decarburization for carbon and low-alloy steels.
  • Appearance: Uniform, adherent, nonporous coatings free from major imperfections.
  • Mechanical Properties: Optional requirements for tensile strength and hardness testing.
• Quality Assurance:
  • Sampling and inspection procedures.
  • Use of test specimens for destructive and non-destructive evaluation.
  • Certification and record retention guidelines.

Applications

The chromium diffusion coating specified by ASTM B874 is widely used across industries where improved corrosion resistance, high temperature stability, and wear protection are essential. Typical applications include:

• Aerospace components: Protection of engine parts, fasteners, and structural components exposed to harsh environments.
• Power generation: Steam turbine blades, heat exchangers, and boiler tubes benefitting from oxidation and sulfidation resistance.
• Chemical processing equipment: Vessels, piping, and fittings subjected to aggressive chemicals and elevated temperatures.
• Automotive and rail: Components such as valves and shafts requiring increased surface durability.
• Industrial manufacturing: Machinery parts exposed to abrasive and corrosive media.

By enhancing the surface of carbon steels, low-alloy steels, stainless steels, and nickel-based alloys, ASTM B874 chromium diffusion coatings extend component life and reduce maintenance costs in demanding service conditions.

Related Standards

For effective implementation and quality control, ASTM B874 references several related international standards and practices, including:

• ASTM B374 - Terminology Relating to Electroplating
• ASTM B487 - Measurement of Metal and Oxide Coating Thickness
• ASTM B602, B697, B762 - Sampling Plans for Inspection of Coatings
• ASTM C664 - Thickness of Diffusion Coating
• ASTM D3951 - Commercial Packaging
• ASTM E766 - Calibrating Magnification of Scanning Electron Microscope
• ASTM E1077 - Estimating the Depth of Decarburization of Steel Specimens
• ASTM F1330 - Metallic Abrasive Blasting for Pipe

Compliance with ASTM B874-96(2023) ensures consistent, high-quality chromium diffusion coatings and compatibility with global requirements for coated metallic products.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: B874 − 96 (Reapproved 2023)
Standard Specification for
Chromium Diffusion Coating Applied by Pack Cementation
Process
This standard is issued under the fixed designation B874; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope B697 Guide for Selection of Sampling Plans for Inspection
of Electrodeposited Metallic and Inorganic Coatings
1.1 This specification covers the requirements for chromium
B762 Guide of Variables Sampling of Metallic and Inorganic
diffusion of metals by the pack cementation method. Pack
Coatings
diffusion employs the chemical vapor deposition of a metal
C664 Test Methods for Thickness of Diffusion Coating
which is subsequently diffused into the surface of a substrate at
D3951 Practice for Commercial Packaging
high temperature. The material to be coated (substrate) is
E766 Practice for Calibrating the Magnification of a Scan-
immersed or suspended in a powder containing chromium
ning Electron Microscope
(source), a halide salt (activator), and an inert diluent such as
E1077 Test Methods for Estimating the Depth of Decarbur-
alumina (filler). When the mixture is heated, the activator
ization of Steel Specimens
reacts to produce an atmosphere of chromium halides which
F1330 Guide for Metallic Abrasive Blasting to Descale the
transfers chromium to the substrate for subsequent diffusion.
Interior of Pipe
The chromium-rich surface enhances corrosion, thermal
stability, and wear-resistant properties.
3. Terminology
1.2 This standard does not purport to address all of the
3.1 Definitions used in this specification are in accordance
safety concerns, if any, associated with its use. It is the
with Terminology B374.
responsibility of the user of this standard to establish appro-
3.2 Definitions of Terms Specific to This Standard:
priate safety, health, and environmental practices and deter-
3.2.1 diffusion coating—a diffusion coating is one produced
mine the applicability of regulatory limitations prior to use.
by causing an element to react with or diffuse into, or both, the
1.3 This international standard was developed in accor-
surface of a metallic substrate, thus, chemically altering the
dance with internationally recognized principles on standard-
surface of the substrate.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
3.2.2 retorts—containers in which powder and parts are
mendations issued by the World Trade Organization Technical
packed for processing. They can be constructed of carbon,
Barriers to Trade (TBT) Committee.
stainless, or high alloys and fabricated in all shapes and sizes
to accommodate parts being processed.
2. Referenced Documents
3.2.3 significant surface—areas that are essential to the
2.1 ASTM Standards:
serviceability or function of the article. These surfaces must be
B374 Terminology Relating to Electroplating
identified on a drawing or marked-up sample of product. Areas
B487 Test Method for Measurement of Metal and Oxide
can fall into one of three categories as follows:
Coating Thickness by Microscopical Examination of
3.2.4 coating required—these surfaces must be in accor-
Cross Section
dance with all quality requirements of this specification.
B602 Guide for Attribute Sampling of Metallic and Inor-
3.2.5 no coating required—these surfaces are areas where
ganic Coatings
no coating is allowed due to a number of reasons including
dimensional, fabrication, and welding, as well as others.
This specification is under the jurisdiction of ASTM Committee B08 on
Materials used for masking are commercially available.
Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee
B08.03 on Engineering Coatings.
3.2.6 optional—these surfaces do not require coating, but at
Current edition approved Nov. 1, 2023. Published November 2023. Originally
the same time do not require masking.
approved in 1996. Last previous edition approved in 2018 as B874 – 96 (2018).
DOI: 10.1520/B0874-96R23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or 4. Classification
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
4.1 There are four classes of chromium diffusion defined by
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. base (basis) metal category.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B874 − 96 (2023)
4.1.1 Class I—Carbon steel. 6.5 Post-Cleaning—Retorts should be allowed to cool suf-
ficiently before opening. When parts are removed from retorts,
4.1.2 Class II—Low-alloy steels.
residual pack mix is removed from surfaces by a supplier-
4.1.3 Class III—Stainless steels.
approved method.
4.1.4 Class IV—Nickel-based alloys.
6.6 Post-Straightening—Long tubes may have distortion
5. Ordering Information
due to the high temperature of process. Use rotary straightener
or hydraulic press to restore straightness and ovality.
5.1 In order to make the application of this specification
complete, the purchaser shall supply the following information
6.7 Visual Inspection—Inspect in accordance with 7.3.
to the vendor through a purchase order and drawings:
6.8 Marking and Packaging—To be defined by the pur-
5.1.1 Title, ASTM designation number, and date of issue of
chaser in the purchase order. Parts processed for the U.S.
this specification.
government and military, including subcontract, shall be pack-
5.1.2 Deposit by classification (see Section 4).
aged in accordance with Practice D3951.
5.1.3 Composition and metallurgical condition of substrate
to be coated.
7. Coating Requirements
5.1.4 Location of significant surfaces (see 3.2.3).
7.1 Diffusion Thickness—The following are minimums for
5.1.5 Samples for destructive metallurgical test (see 8.1).
the different classes of materials based on standard process
5.1.6 Any post heat treatment required.
parameters. Thicker coatings require special processing and
5.1.7 Acceptance inspection procedure to be used (see
must be called out for in the purchase order if required.
Section 9).
Class Minimum, in.
5.1.8 Any requirement for certification (see Section 11).
Class I (carbon) 0.003
Class II (low alloys) 0.003
Class III (stainless steels) 0.002
6. Processing Requirements
Class IV (nickel base alloys) 0.001
6.1 Substrate Preparation—The metal to be chromized shall
7.2 Decarburization—For certain Class I and Class II
be free of flaws and defects that will be detrimental to the
materials, the maximum depth of decarburization to be defined
coating. Thorough cleaning is essential to ensure satisfactory
by the purchaser.
diffusion. Materials used for cleaning should not damage the
7.3 Chromium Content—The outer 15 % of coating shall
base metal. Oils, dirt, grease, and stains must be removed.
contain a minimum of 20 % (by weight) chromium.
When blasting is also required, use aluminum oxide (90 to 220
mesh) at 60 to 80 psi from 4 to 6 in. standoff. (Refer to Guide
7.4 Appearance—The diffusion zone shall be nonporous
F1330.)
and adherent to the base metal and shall have a uniform surface
free from objectionable imperfections. Minor variations in
6.2 Materials:
color and surface appearance shall be considered acceptable,
6.2.1 Masteralloy with 50 to 100 % Pure Chromium —Sold
providing the requirements of 7.1 and 7.2 are met (see 8.7).
under a number of trade names, in various mesh sizes.
Percentage in mix depends on material being processed.
8. Test Methods
...