ASTM B733-22

Standard Specification for Autocatalytic (Electroless) Nickel-Phosphorus Coatings on Metal

ABSTRACT
This specification establishes the requirements for autocatalytic (electroless) nickel-phosphorus coatings applied from acidic aqueous solutions to metallic products for use in engineering functions operating at elevated temperatures. The coatings covered here are alloys of nickel and phosphorus produced by self-sustaining autocatalytic chemical reduction with hypophosphite. The coatings are grouped into the following classification systems: types, which are based on the general composition with respect to phosphorus; service condition numbers, which are based on the severity of exposure to which the coating is intended to perform and the corresponding minimum thickness that will provide satisfactory performance; and post heat treatment class, which are based on post-plating heat treatment temperature and time to produce the desired adhesion and hardness improvements. Prior to plating, substrates should be pretreated by stress relief for reducing risks of hydrogen embrittlement, peening, and racking. The coatings shall be sampled and tested accordingly to evaluate both acceptance (appearance, thickness, adhesion, and porosity) and qualification requirements (composition, microhardness, and hydrogen embrittlement). Thickness shall be assessed either by microscopical method, a magnetic induction instrument, beta backscatter method, a micrometer, weigh-plate-weigh method, coulometric method, or X-ray spectrometry. Adhesion shall be examined either by bend, impact, or thermal shock tests. And porosity shall be inspected either by ferroxyl test, boiling water test, aerated water test, or alizarin test.
SCOPE
1.1 This specification covers requirements for autocatalytic (electroless) nickel-phosphorus coatings applied from aqueous solutions to metallic products for engineering (functional) uses.  
1.2 The coatings are alloys of nickel and phosphorus produced by autocatalytic chemical reduction with hypophosphite. Because the deposited nickel alloy is a catalyst for the reaction, the process is self-sustaining. The chemical and physical properties of the deposit vary primarily with its phosphorus content and subsequent heat treatment. The chemical makeup of the plating solution and the use of the solution can affect the porosity and corrosion resistance of the deposit. For more details, see ASTM STP 265  (1)2 and Refs  (2), (3), (4), and (5).  
1.3 The coatings are generally deposited from acidic solutions operating at elevated temperatures.  
1.4 The process produces coatings of uniform thickness on irregularly shaped parts, provided the plating solution circulates freely over their surfaces.  
1.5 The coatings have multifunctional properties, such as hardness, heat hardenability, abrasion, wear and corrosion resistance, magnetics, electrical conductivity provide diffusion barrier, and solderability. They are also used for the salvage of worn or mismachined parts.  
1.6 The low phosphorus (2 to 4 % P) coatings are microcrystalline and possess high as-plated hardness (620 to 750 HK 100). These coatings are used in applications requiring abrasion and wear resistance.  
1.7 Lower phosphorus deposits in the range between 1 % and 3 % phosphorus are also microcrystalline. These coatings are used in electronic applications providing solderability, bondability, increased electrical conductivity, and resistance to strong alkali solutions.  
1.8 The medium phosphorous coatings (5 to 9 % P) are most widely used to meet the general purpose requirements of wear and corrosion resistance.  
1.9 The high phosphorous (more than 10 % P) coatings have superior salt-spray and acid resistance in a wide range of applications. They are used on beryllium and titanium parts for low stress properties. Coatings with phosphorus contents greater than 11.2 % P are not considered to be ferromagnetic.  
1.10 Units—The values stated in SI units are to be regarded as standard.  
1.11 The following precautionary statement pertains onl...

General Information

• Status: Published
• Publication Date: 30-Apr-2022
• ICS: 25.220.40 - Metallic coatings
• Technical Committee: B08 - Metallic and Inorganic Coatings
• Drafting Committee: B08.03 - Engineering Coatings

Relations

• Refers: ASTM F519-23 - Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments
• Effective Date: 01-Dec-2023
• Refers: ASTM E352-23 - Standard Test Methods for Chemical Analysis of Tool Steels and Other Similar Medium- and High-Alloy Steels
• Effective Date: 15-Nov-2023
• Refers: ASTM B849-02(2023) - Standard Specification for Pre-Treatments of Iron or Steel for Reducing Risk of Hydrogen Embrittlement
• Effective Date: 01-Nov-2023
• Refers: ASTM B571-23 - Standard Practice for Qualitative Adhesion Testing of Metallic Coatings
• Effective Date: 01-Nov-2023
• Refers: ASTM B678-23 - Standard Test Method for Solderability of Metallic-Coated Products
• Effective Date: 01-Nov-2023
• Refers: ASTM D3951-18(2023) - Standard Practice for Commercial Packaging
• Effective Date: 01-Oct-2023
• Refers: ASTM G85-19 - Standard Practice for Modified Salt Spray (Fog) Testing
• Effective Date: 01-Nov-2019
• Refers: ASTM D2714-94(2019) - Standard Test Method for Calibration and Operation of the Falex Block-on-Ring Friction and Wear Testing Machine
• Effective Date: 01-May-2019
• Refers: ASTM B849-02(2019) - Standard Specification for Pre-Treatments of Iron or Steel for Reducing Risk of Hydrogen Embrittlement
• Effective Date: 01-Apr-2019
• Refers: ASTM F519-18 - Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments
• Effective Date: 01-Nov-2018
• Refers: ASTM B571-18 - Standard Practice for Qualitative Adhesion Testing of Metallic Coatings
• Effective Date: 01-Aug-2018
• Refers: ASTM D3951-18 - Standard Practice for Commercial Packaging
• Effective Date: 01-May-2018
• Refers: ASTM F519-17a - Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments
• Effective Date: 01-Dec-2017
• Refers: ASTM B678-86(2017) - Standard Test Method for Solderability of Metallic-Coated Products
• Effective Date: 01-Nov-2017
• Refers: ASTM F519-17 - Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments
• Effective Date: 01-Mar-2017

Overview

ASTM B733-22 is the internationally recognized standard specification for autocatalytic (electroless) nickel-phosphorus coatings on metallic products. Developed by ASTM International, this standard establishes requirements for nickel-phosphorus coatings applied from acidic aqueous solutions, mainly for engineering applications operating at elevated temperatures. The coatings, which are alloys of nickel and phosphorus, are produced through a self-sustaining autocatalytic chemical reduction process using hypophosphite. ASTM B733-22 outlines classification, performance, and testing requirements to ensure coatings meet industry expectations for durability, corrosion resistance, and overall functionality.

Key Topics

• Coating Composition: Nickel-phosphorus coatings are classified by phosphorus content: low (1-4%), medium (5-9%), and high (≥10%). The phosphorus percentage directly influences physical properties such as hardness, crystalline structure, and magnetic behavior.
• Service Conditions: The standard identifies five service condition levels based on severity of exposure and minimum required thickness, from SC0 (minimum) to SC4 (severe service).
• Post-Heat Treatment Classification: Coatings are further classified based on post-plating heat treatment processes performed to enhance adhesion and/or hardness, critical for high-performance applications.
• Testing and Quality Control: Acceptance and qualification tests help ensure coating uniformity, thickness, adhesion, porosity, composition, microhardness, and resistance to hydrogen embrittlement. Sampling and statistical process controls are encouraged.
• Substrate Preparation: Proper pretreatment, including stress relief and surface activation, is required, especially for high-strength steel, to minimize risks such as hydrogen embrittlement.

Applications

ASTM B733-22 electroless nickel-phosphorus coatings are widely used across multiple industries for their multifunctional properties, which include:

• Abrasion and Wear Resistance: Low-phosphorus coatings are microcrystalline and highly wear-resistant, making them ideal for components subjected to friction and harsh mechanical environments.
• Corrosion Protection: Medium- and high-phosphorus coatings provide enhanced corrosion and chemical resistance, especially in marine, chemical processing, and oil & gas sectors. High-phosphorus types are particularly suited to aggressive environments with exposure to acids or salts.
• Electronic Industry: Lower phosphorus deposits offer improved solderability, electrical conductivity, and resistance to strong alkali, making them suitable for connectors, printed circuit boards, and bonding applications.
• Dimensional Restoration: The process allows uniform coating thickness even on complex or irregularly shaped parts, making it valuable for salvaging or repairing worn or mismachined components.
• Diffusion Barrier/Solderability: The coatings are frequently used as barriers to prevent diffusion between dissimilar metals and to enhance solderability of electronic contacts.
• Magnetic/Electrical Properties: Depending on phosphorus content, coatings can offer tailored magnetic and electrical behaviors, beneficial for specialized engineering applications.

Related Standards

Organizations seeking compliance with ASTM B733-22 or integrating it into broader quality management systems should also consider these related standards and industry documents:

• ASTM B374: Terminology relating to electroplating.
• ASTM B487, B499, B504, B567, B568: Test methods for measuring coating thickness using various techniques.
• ASTM B849 & B850: Guidance on pre-treatment and post-treatment to reduce hydrogen embrittlement in steel.
• ASTM B571: Practice for evaluating adhesion of metallic coatings.
• ISO 4527: International specification for electroless nickel-phosphorus coatings and test methods.
• SAE AMS 2430, AMS 2590: Procedures for shot peening and rotary flap peening, often specified for surface treatment prior to plating.
• ANSI/ASQ Z1.4 & MIL-STD-1916: Sampling procedures for inspection by attribute - commonly used for military and government contracts.

Practical Value

ASTM B733-22 helps ensure the reliability and performance of autocatalytic nickel-phosphorus coatings across critical applications. By following this standard, manufacturers and users can achieve coatings optimized for specific functional, environmental, and durability requirements, supported by robust quality assurance and international acceptance. This standard is an essential reference for industries demanding high-performance metal finishing solutions, including aerospace, electronics, automotive, energy, and heavy machinery sectors.

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:B733 −22
Standard Specification for
Autocatalytic (Electroless) Nickel-Phosphorus Coatings on
Metal
This standard is issued under the fixed designation B733; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope* bondability, increased electrical conductivity, and resistance to
strong alkali solutions.
1.1 This specification covers requirements for autocatalytic
(electroless) nickel-phosphorus coatings applied from aqueous 1.8 Themediumphosphorouscoatings(5to9%P)aremost
widely used to meet the general purpose requirements of wear
solutionstometallicproductsforengineering(functional)uses.
and corrosion resistance.
1.2 The coatings are alloys of nickel and phosphorus pro-
1.9 The high phosphorous (more than 10% P) coatings
ducedbyautocatalyticchemicalreductionwithhypophosphite.
have superior salt-spray and acid resistance in a wide range of
Becausethedepositednickelalloyisacatalystforthereaction,
applications.Theyareusedonberylliumandtitaniumpartsfor
the process is self-sustaining. The chemical and physical
low stress properties. Coatings with phosphorus contents
properties of the deposit vary primarily with its phosphorus
greater than 11.2% P are not considered to be ferromagnetic.
content and subsequent heat treatment. The chemical makeup
oftheplatingsolutionandtheuseofthesolutioncanaffectthe
1.10 Units—The values stated in SI units are to be regarded
porosity and corrosion resistance of the deposit. For more
as standard.
details, seeASTM STP265 (1) and Refs (2), (3), (4), and (5).
1.11 Thefollowingprecautionarystatementpertainsonlyto
1.3 The coatings are generally deposited from acidic solu-
the test method portion, Section 9, of this specification. This
tions operating at elevated temperatures.
standard does not purport to address all of the safety concerns,
if any, associated with its use. It is the responsibility of the user
1.4 The process produces coatings of uniform thickness on
of this standard to establish appropriate safety, health, and
irregularly shaped parts, provided the plating solution circu-
environmental practices and determine the applicability of
lates freely over their surfaces.
regulatory limitations prior to use.
1.5 The coatings have multifunctional properties, such as
1.12 This international standard was developed in accor-
hardness, heat hardenability, abrasion, wear and corrosion
dance with internationally recognized principles on standard-
resistance, magnetics, electrical conductivity provide diffusion
ization established in the Decision on Principles for the
barrier, and solderability.They are also used for the salvage of
Development of International Standards, Guides and Recom-
worn or mismachined parts.
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
1.6 The low phosphorus (2 to 4% P) coatings are microc-
rystalline and possess high as-plated hardness (620 to 750 HK
2. Referenced Documents
100). These coatings are used in applications requiring abra-
2.1 ASTM Standards:
sion and wear resistance.
B368Test Method for Copper-AcceleratedAceticAcid-Salt
1.7 Lower phosphorus deposits in the range between 1%
Spray (Fog) Testing (CASS Test)
and 3% phosphorus are also microcrystalline. These coatings
B374Terminology Relating to Electroplating
are used in electronic applications providing solderability,
B380Test Method for Corrosion Testing of Decorative
Electrodeposited Coatings by the Corrodkote Procedure
B487Test Method for Measurement of Metal and Oxide
This specification is under the jurisdiction of ASTM Committee B08 on
Coating Thickness by Microscopical Examination of
Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee
Cross Section
B08.03 on Engineering Coatings.
Current edition approved May 1, 2022. Published May 2022. Originally
approved in 1984. Last previous edition approved in 2021 as B733–21. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/B0733-22. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
The boldface numbers given in parentheses refer to a list of references at the Standards volume information, refer to the standard’s Document Summary page on
end of the text. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B733−22
B499Test Method for Measurement of CoatingThicknesses Anodic Polarization Measurements
by the Magnetic Method: Nonmagnetic Coatings on G31Guide for Laboratory Immersion Corrosion Testing of
Magnetic Basis Metals Metals
B504Test Method for Measurement of Thickness of Metal- G59TestMethodforConductingPotentiodynamicPolariza-
lic Coatings by the Coulometric Method tion Resistance Measurements
B537Practice for Rating of Electroplated Panels Subjected G85Practice for Modified Salt Spray (Fog) Testing
to Atmospheric Exposure
2.2 SAE Documents:
B567Test Method for Measurement of Coating Thickness
AMS 2430Shot Peening
by the Beta Backscatter Method
AMS 2590Rotary Flap Peening of Metal Parts
B568Test Method for Measurement of Coating Thickness
2.3 ANSI Standard:
by X-Ray Spectrometry
ANSI/ASQ Z1.4Sampling Procedures and Tables for In-
B571Practice for Qualitative Adhesion Testing of Metallic
spection by Attributes
Coatings
2.4 Military Standard:
B578Test Method for Microindentation Hardness of Elec-
MIL-STD-1916DOD Preferred Methods of Acceptance of
troplated Coatings
Product
B602Guide for Attribute Sampling of Metallic and Inor- 11
2.5 ISO Standard:
ganic Coatings
ISO 4527 Metallic Coatings-Autocatalytic (Electroless)
B667Practice for Construction and Use of a Probe for
Nickel-Phosphorus Alloy Coatings—Specification and
Measuring Electrical Contact Resistance
Test Methods
B678Test Method for Solderability of Metallic-Coated
Products
3. Terminology
B697Guide for Selection of Sampling Plans for Inspection
3.1 Definitions:
of Electrodeposited Metallic and Inorganic Coatings
3.1.1 significant surfaces, n—thosesubstratesurfaceswhich
B762GuideofVariablesSamplingofMetallicandInorganic
the coating must protect from corrosion or wear, or both, and
Coatings
that are essential to the performance.
B849Specification for Pre-Treatments of Iron or Steel for
3.2 Other Definitions—Terminology B374 defines most of
Reducing Risk of Hydrogen Embrittlement
the technical terms used in this specification.
B850GuideforPost-CoatingTreatmentsofSteelforReduc-
ing the Risk of Hydrogen Embrittlement
4. Coating Classification
B851Specification for Automated Controlled Shot Peening
4.1 The coating classification system provides for a scheme
of MetallicArticles Prior to Nickel,Autocatalytic Nickel,
to select an electroless nickel coating to meet specific perfor-
or Chromium Plating, or as Final Finish
mancerequirementsbasedonalloycomposition,thicknessand
D1193Specification for Reagent Water
hardness.
D2670Test Method for Measuring Wear Properties of Fluid
4.1.1 TYPE describes the general composition of the de-
Lubricants (Falex Pin and Vee Block Method)
positwithrespecttothephosphoruscontentandisdividedinto
D2714Test Method for Calibration and Operation of the
fivecategorieswhichestablishdepositproperties(seeTable1).
Falex Block-on-Ring Friction and Wear Testing Machine
D3951Practice for Commercial Packaging
NOTE 1—Due to the precision of some phosphorus analysis methods, a
D4060Test Method for Abrasion Resistance of Organic
deviation of 0.5% has been designed into this classification scheme.
Coatings by the Taber Abraser Rounding of the test results due to the precision of the limits provides for
an effective limit of 4.5% and 9.5%, respectively. For example, coating
E60Practice for Analysis of Metals, Ores, and Related
with a test result for phosphorus of 9.7% would have a classification of
Materials by Spectrophotometry
TYPE V, see Appendix X5, Alloy TYPEs.
E140Hardness Conversion Tables for Metals Relationship
4.2 Service Condition Based on Thickness:
Among Brinell Hardness, Vickers Hardness, Rockwell
Hardness, Superficial Hardness, Knoop Hardness, Sclero-
scope Hardness, and Leeb Hardness
AvailablefromSAEInternational(SAE),400CommonwealthDr.,Warrendale,
E156Test Method for Determination of Phosphorus in
PA 15096, http://www.sae.org.
TheoriginalreferenceMIL-S-13165ShotPeeningofMetalPartswascancelled
High-Phosphorus Brazing Alloys (Photometric Method)
4 inFebruary1998andreferredtoSAEAMS-S-13165,whichwasalsocancelledand
(Withdrawn 1993)
superseded by AMS 2430.
E352TestMethodsforChemicalAnalysisofToolSteelsand 7
The original reference MIL-R-81841 Rotary Flap Peening of Metal Parts was
Other Similar Medium- and High-Alloy Steels cancelled in April 2012 and referred to SAE AMS 2590.
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
F519Test Method for Mechanical Hydrogen Embrittlement
4th Floor, New York, NY 10036, http://www.ansi.org.
Evaluation of Plating/Coating Processes and Service En-
The original reference MIL-STD-105 Sampling Procedures and Tables for
vironments
Inspection by Attributes was cancelled in February 2008 and referred to MIL-
STD-1916 or ANSI/ASQ Z1.4.
G5Reference Test Method for Making Potentiodynamic
Available from Standardization Documents Order Desk, Bldg. 4 Section D,
700 Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Available from International Organization for Standardization (ISO), ISO
The last approved version of this historical standard is referenced on Central Secretariat, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva,
www.astm.org. Switzerland, https://www.iso.org.
B733−22
TABLE 1 Deposit Alloy Types TABLE 3 Classification of Post Heat Treatment
Type Phosphorus % wt Temperature
Class Description Time (h)
(°C)
I No Requirement for Phosphorus
II 1to3 1 No Heat Treatment, As Plated
III 2to4 2 Heat Treatment for Maximum Hardness
IV 5to9 TYPE I 260 20
V 10 and above 285 16
320 8
400 1
TYPE II 350 to 380 1
TYPE III 360 to 390 1
TYPE IV 365 to 400 1
4.2.1 Serviceconditionnumbersarebasedontheseverityof
TYPE V 375 to 400 1
3 Adhesion on Steel 180 to 200 2 to 4
the exposure in which the coating is intended to perform and
4 Adhesion, Carburized Steel and 120 to 130 1 to 6
minimum coating thickness to provide satisfactory perfor-
Age Hardened Aluminum
mance (see Table 2).
5 Adhesion on Beryllium and 140 to 150 1 to 2
Aluminum
4.2.2 SC0 Minimum Service, 0.1 µm—This is defined by a
6 Adhesion on Titanium 300 to 320 1 to 4
minimum coating thickness to provide specific material prop-
erties and extend the life of a part or its function.Applications
include requirements for diffusion barrier, undercoat, electrical
conductivity, and wear and corrosion protection in specialized
environments.
4.3.3 Class 3—Heattreatmentat180°Cto200°Cfor2hto
4.2.3 SC1 Light Service,5µm—This is defined by a
4h to improve coating adhesion on steel and to provide for
minimum coating thickness of 5 µm for extending the life of
hydrogen embrittlement relief (see 6.6).
the part. Typical environments include light-load lubricated
4.3.4 Class 4—Heattreatmentat120°Cto130°Cfor1hto
wear, indoor corrosion protection to prevent rusting, and for
6 h to increase adhesion of carburized steel or heat-treatable
soldering and mild abrasive wear.
(age-hardened) aluminum alloys (see Note 3).
4.2.4 SC2 Mild Service,13µm—This is defined by mild
4.3.5 Class 5—Heattreatmentat140°Cto150°Cfor1hto
corrosion and wear environments. It is characterized by indus-
2h to improve coating adhesion for aluminum, non age-
trial atmosphere exposure on steel substrates in dry or oiled
hardenedaluminumalloys,copper,copperalloys,orberyllium.
environments.
4.3.6 Class 6—Heattreatmentat300°Cto320°Cfor1hto
4.2.5 SC3 Moderate Service,25µm—This is defined by
4h to improve coating adhesion for titanium alloys.
moderate environments such as non marine outdoor exposure,
alkali salts at elevated temperature, and moderate wear.
NOTE 3—Heat-treatable aluminum alloys such as Type 7075 can
4.2.6 SC4 Severe Service,75µm—This is defined by a very
undergo microstructural changes and lose strength when heated to over
130°C.
aggressive environment. Typical environments would include
acid solutions, elevated temperature and pressure, hydrogen
5. Ordering Information
sulfide and carbon dioxide oil service, high-temperature chlo-
ride systems, very severe wear, and marine immersion.
5.1 The following information shall be supplied by the
purchaser in either the purchase order or on the engineering
NOTE 2—The performance of the autocatalytic nickel coating depends
drawing of the part to be plated:
to a large extent on the surface finish of the article to be plated and how
it was pretreated. Rough, non uniform surfaces require thicker coatings
5.1.1 Title,ASTM designation number, and year of issue of
than smooth surfaces to achieve maximum corrosion resistance and
this specification.
minimum porosity.
5.1.2 Classification of the coating by type, service
4.3 Post Heat Treatment Class—The nickel-phosphorus
condition, class (see 4.1, 4.2, and 4.3).
coatings shall be classified by heat treatment after plating to
5.1.3 Specify maximum dimension and tolerance
increase coating adhesion or hardness, or both (see Table 3).
requirements, if any.
4.3.1 Class 1—As-deposited, no heat treatment.
5.1.4 Peening, if required (see 6.3).
4.3.2 Class 2—Heat treatment at 260°C to 400°C to
5.1.5 The tensile strength of the material in MPa (see 6.3.1
produce a minimum hardness of 850 HK100.
and 6.6).
5.1.6 Stress relief heat treatment before plating (see 6.3).
5.1.7 HydrogenEmbrittlementReliefafterplating(see6.6).
TABLE 2 Service Conditions
5.1.8 Significantsurfacesandsurfacesnottobeplatedmust
Coating Thickness Requirements
be indicated on drawings or sample.
Minimum Coating
5.1.9 Supplemental or Special Government Requirements
Service Condition Thickness µm in.
such as, specific phosphorus content, abrasion wear or corro-
Specification
sion resistance of the coating, solderability, contact resistance
SC0 Minimum Thickness 0.1 0.000004
and packaging selected from Supplemental Requirements.
SC1 Light Service 5 0.0002
SC2 Mild Service 13 0.0005
5.1.10 Requirement for a vacuum, inert, or reducing atmo-
SC3 Moderate Service 25 0.001
sphere for heat treatment above 260°C to prevent surface
SC4 Severe Service 75 0.003
oxidation of the coating (see S1.3).
B733−22
5.1.11 Test methods for coating adhesion, composition, 6.5 Plating Process:
thickness, porosity, wear and corrosion resistance, if required,
6.5.1 To obtain consistent coating properties, the bath must
selected from those found in Section 9 and Supplemental
be monitored periodically for pH, temperature, nickel, and
Requirements.
hypophosphite. Replenishments to the plating solution should
5.1.12 Requirements for sampling (see Section 8).
be as frequent as required to maintain the concentration of the
nickel and hypophosphite between 90% and 100% of set
NOTE 4—The purchaser should furnish separate test specimens or
point. The use of a statistical regimen to establish the control
couponsofthebasismetalfortestpurposestobeplatedconcurrentlywith
the articles to be plated (see 8.4). limits and frequency of analysis may be employed to ensure
quality deposits are produced.
6. Materials and Manufacture
6.5.2 Mechanical movement of parts, agitation of the bath,
and filtration is recommended to increase coating smoothness
6.1 Substrate—Defectsinthesurfaceofthebasismetalsuch
and uniformity and prevent pitting or streaking due to hydro-
as scratches, porosity, pits, inclusions, roll and die marks, laps,
gen bubbles.
cracks, burrs, cold shuts, and roughness may adversely affect
the appearance and performance of the deposit, despite the
6.6 Post Coating Treatment for Iron and Steel for Reducing
observance of the best plating practice. Any such defects on
the Risk of Hydrogen Embrittlement—Parts that are made of
significant surfaces shall be brought to the attention of the
steel with ultimate tensile strengths of 1000 MPa (correspond-
purchaserbeforeplating.Theproducershallnotberesponsible
inghardnessvalues300HV10,303HB,or31HRCorgreater),
for coatings defects resulting from surface conditions of the
as well as surface hardened parts, shall require post coating
metal, if these conditions have been brought to the attention of
hydrogen embrittlement relief baking when specified by the
the purchaser.
purchaser. The tensile strength shall be supplied by the
purchaser. Guide B850 contains a list of post treatments,
6.2 Pretreatment—Asuitable method shall activate the sur-
procedures, limitations, and guidelines that are permitted to be
faceandremoveoxideandforeignmaterials,whichmaycause
used to reduce the effects of hydrogen embrittlement.
poor adhesion and coating porosity.
6.6.1 Heat treatment shall be performed preferably within
NOTE 5—Heat treatment of the base material may affect its metallur-
1h, but not more than 3h, of plating unless the size or weight
gical properties. An example is leaded steel which may exhibit liquid or
of the part prevents the initiation of heart treatment within 3h.
solid embrittlement after heat treatment. Careful selection of the pre and
post heat treatments are recommended.
In this case, the part shall be heat treated as soon as possible.
In all cases, the duration of the heat treatment shall commence
6.3 Stress Relief:
from the time at which the whole of each part attains the
6.3.1 Pretreatment of Iron and Steel for Reducing the Risk of
specified temperature.
Hydrogen Embrittlement—Parts that are made of steel with
ultimatetensilestrengthofgreaterthan1000MPa(hardnessof
6.7 Heat Treatment After Plating to Improve Adhesion—To
31 HRC), that have been machined, ground, cold formed, or
improve the adhesion of the coating to various substrates, the
cold straightened subsequent to heat treatment require stress
heat treatments in Table 3 should be performed as soon as
relief heat treatment when specified by the purchaser. The
practical after plating (see 4.3).
tensile strength of the material shall be supplied by the
6.8 Heat Treatment After Plating to Increase Hardness:
purchaser. Specification B849 contains a list of pre-treatments,
6.8.1 To increase the hardness of the coating, a heat treat-
precautions, procedures, and caveats that shall be used.
ment of over 260°C is required. Table 3 describes the heat
6.3.2 Peening—Peening prior to plating may be required on
treatment for maximum hardness. See Appendix X3 and
high-strength steel parts to induce residual compressive
Appendix X4.
stresses in the surface, which can reduce loss of fatigue
6.8.2 Aheattreatmentat260°Cforgreaterthan20hshould
strength and improve stress corrosion resistance after plating.
be used to reduce the loss of surface hardness and strength of
(See Supplementary Requirements.)
some ferrous basis metals.Avoid rapid heating and cooling of
6.3.2.1 Steel parts which are designed for unlimited life
plated parts. Sufficient time must be allowed for large parts to
underdynamicloadsshallbeshotpeenedorrotaryflappeened.
reach oven temperature.
NOTE6—Controlledshotpeeningisthepreferredmethodbecausethere
6.8.3 Do not use gas containing hydrogen with high-
are geometry’s where rotary flap peening is not effective. See S1.11.2.
strength steel parts.
6.3.2.2 Unlessotherwisespecified,theshotpeeningshallbe
NOTE7—Thelengthoftimetoreachmaximumhardnessvarieswiththe
accomplished on all surfaces for which the coating is required
phosphorus content of the deposit. High phosphorus deposits may require
andallimmediateadjacentsurfaceswhentheycontainnotches,
longer time or a higher temperature, or both. Individual alloys should be
fillets, or other abrupt changes of section size where stresses
testedformaximumhardnessattainable,especiallyforconditionsoflower
will be concentrated.
temperatures and longer times.
NOTE 8—Inert or reducing atmosphere or vacuum sufficient to prevent
6.4 Racking—Parts should be positioned so as to minimize
oxidation is recommended for heat treatment above 260°C.
trapping of hydrogen gas in cavities and holes, allowing free
circulation of solution over all surfaces to obtain uniform
7. Requirements
coating thickness. The location of rack or wire marks in the
coating shall be agreed upon between the producer and 7.1 Process—The coating shall be produced from an aque-
purchaser. ous solution through chemical reduction reaction.
B733−22
7.2 Acceptance Requirements—These requirements are otherwise specified by purchaser in the purchase order or
placed on each lot or batch and can be evaluated by testing the contract(see5.1.12andS1.11.1).GuideB697andTestMethod
plated part. B762 also contain sampling plans that are designed for the
7.2.1 Appearance: sampling inspection of coatings. When Guide B697 or Test
Method B762 are specified, the purchaser and producer must
7.2.1.1 The coating surface shall have a uniform, metallic
appearance without visible defects such as blisters, pits, agree on the plan to be used.
pimples, and cracks.
NOTE10—Usually,whenacollectionofcoatedparts(theinspectionlot
7.2.1.2 Imperfections that arise from surface conditions of
8.2)isexaminedforcompliancewiththerequirementsplacedontheparts,
the substrate which the producer is unable to remove using
a relatively small number of parts, the sample, is selected at random and
conventional pretreatment techniques and that persist in the inspected. The inspection lot is then classified as complying or not
complying with the requirements based on the results of the inspection
coating shall not be cause for rejection (see 6.1). Also,
sample. The size of the sample and the criteria of compliance are
discoloration due to heat treatment shall not be cause for
determined by the application of statistics. The procedure is known as
rejection unless special heat treatment atmosphere is specified
sampling inspection.
(see 5.1.10).
TestMethodB602containsfoursamplingplans,threeforusewithtests
that are nondestructive and one for use with tests that are destructive.The
7.2.2 Thickness—The thickness of the coating shall exceed
purchaserandproducermayagreeontheplan(s)tobeused.Iftheydonot,
the minimum requirements in Table 2 as specified by the
Test Method B602 identifies the plan to be used.
service condition agreed to prior to plating. After coating and
Guide B697 provides a large number of plans and also gives guidance
if specified, the part shall not exceed maximum dimension on
on the selection of a plan.
significant surface (see 5.1.3). Test Method B762 can be used only for coating requirements that have
a numerical limit, such as coating thickness. Test Method B762 contains
NOTE 9—The thickness of the coating cannot be controlled in blind or
several plans and also gives instructions for calculating plans to meet
small diameter deep holes or where solution circulation is restricted.
special needs.The purchaser and producer may agree on the plan(s) to be
used. If they do not, Test Method B762 identifies the plan to be used.
7.2.3 Adhesion—The coating shall have sufficient adhesion
An inspection lot shall be defined as a collection of coated parts which
to the basis metal to pass the specified adhesion test (see 9.4
are of the same kind, that have been produced to the same specification,
and Practice B571).
that have been coated by a single producer at one time or approximately
7.2.4 Porosity—The coatings shall be essentially pore free
the same time under essentially identical conditions, and that are submit-
ted for acceptance or rejection as a group.
when tested according to one of the methods of 9.6. The test
method,thedurationofthetest,andnumberofallowablespots
8.3 All specimens used in the sampling plan for acceptance
per unit area shall be specified (see 5.1.11).
tests shall be made of the same basis material and in the same
metallurgical condition as articles being plated to this specifi-
7.3 Qualification Requirements—These requirements are
cation.
placed on the deposit and process and are performed on
specimens to qualify the deposit and plating process.The tests
8.4 Allspecimensshallbeprovidedbythepurchaserunless
for these qualification requirements shall be performed
otherwise agreed to by the producer.
monthly or more frequently.
NOTE 11—The autocatalytic nickel process is dynamic and a daily
7.3.1 Composition—Type II, III, IV, V deposits shall be
sampling is recommended. For coatings requiring alloy analysis and
analyzed for alloy composition by testing for phosphorus (see
corrosion testing, weekly sampling should be considered as an option.
9.1). The weight percent of phosphorus shall be in the range
designated by type classification (see 4.1).
9. Test Methods
7.3.2 Microhardness—The microhardness of Class 2 depos-
9.1 Deposit Analysis for Phosphorus:
its shall be determined by Test Method B578 (Knoop). For
9.1.1 Phosphorus Determination—Determinemass%phos-
Class 2 coatings, the microhardness shall equal or exceed a
minimum of 850 HK100 (or equivalent Vickers) (see 4.3 and phorus content according to Practice E60,Test Methods E352,
orTest Method E156 on known weight of deposit dissolved in
9.5).TheconversionofVickerstoKnoopusingTablesE140is
warm concentrated nitric acid.
not recommended.
7.3.3 Hydrogen Embrittlement—Theprocessusedtodeposit 9.1.2 Compositioncanbedeterminedbyatomicabsorption,
a coating onto high strength steels shall be evaluated for
emission, or X-ray fluorescence spectrometry.
hydrogen embrittlement by Test Method F519.
NOTE 12—Inductively coupled plasma techniques can determine the
alloy to within 0.1%.The following analysis wavelength lines have been
8. Sampling
used with minimum interference to determine the alloy.
Ni 216.10 nm Cd 214.44 nm Fe 238.20 nm
8.1 The purchaser and producer are urged to employ statis-
P 215.40 nm Co 238.34 nm Pb 283.30 nm
ticalprocesscontrolinthecoatingprocess.Properlyperformed
P 213.62 nm Cr 284.32 nm Sn 198.94 nm
thiswillensurecoatedproductsofsatisfactoryqualityandwill
Al 202.55 nm Cu 324.75 nm Zn 206.20 nm
reduce the amount of acceptance inspection.
9.2 Appearance—Examine the coating visually for compli-
8.1.1 Sampling plans can only screen out unsatisfactory
ance with the requirements of 7.2.1.
products without assurance that none of them will be accepted
9.3 Thickness—Measure the coating thickness for compli-
(6).
ance with the requirements of 7.2.2.
8.2 The sampling plan used for the inspection of a quantity
of coated parts (lot) shall be Test Method B602 unless NOTE 13—Eddy-current type instruments give erratic measurements
B733−22
due to variations in conductivity of the coatings with changes in
thecoatingonthetensionsideofthebendarenotanindication
phosphorus content.
of poor adhesion. Insertion of a sharp probe at the interface of
the coating and basis metal to determine the adhesion is
9.3.1 Microscopical Method—Measure the coating thick-
ness of a cross section according to Test Method B487. suggested.
NOTE 17—Appropriate test specimens are strips approximately 25mm
NOTE 14—To protect the edges, electroplate the specimens with a
to 50mm wide, 200mm to 300mm long, and 3mm to 6mm thick.
minimum of 5 µm of nickel or copper prior to cross sectioning.
9.4.2 Impact Test—A spring-loaded center punch with a
9.3.2 Magnetic Induction Instrument Method—Test Method
point having 2mm to 3mm radius is used to test adhesion of
B499isapplicabletomagneticsubstratesplatedwithautocata-
the coating on nonsignificant surfaces of the plated part. Make
lytic nickel deposits that contain more than 11 mass %
three closely spaced indentations and examine under 10×
phosphorus (not ferromagnetic) and that have not been heat-
magnification for flaking or blistering of the coating, which is
treated.The instrument shall be calibrated with deposits plated
cause for rejection.
in the same solution under the same conditions on magnetic
9.4.3 Thermal Shock—The coated part is heated to 200°C
steel.
in an oven and then quenched in room temperature water. The
9.3.3 Beta Backscatter Method—Test Method B567 is only
coating is examined for blistering or other evidence of poor
applicable to coatings on aluminum, beryllium, magnesium,
adhesion at 4× magnification.
andtitanium.Theinstrumentmustbecalibratedwithstandards
having the same composition as the coating.
9.5 Microhardness—The microhardness of the coating can
be measured by Test Method B578 using Knoop indenter and
NOTE15—Thedensityofthecoatingvarieswithitsmass%phosphorus
is reported in Knoop Hardness Number (HK). It will vary
content (See Appendix X2).
depending on loads, type of indenter, and operator. A 100 g
9.3.4 Micrometer Method—Measure the part, test coupon,
load is recommended. The rhombic Knoop indenter gives
orpininaspecificspotbeforeandafterplatingusingasuitable
higherhardnessreadingsthanthesquare-basepyramidalVick-
micrometer. Make sure that the surfaces measured are smooth,
ersdiamondindenterfor100gto300gloads;seeRef (7).For
clean, and dry.
maximum accuracy, a minimum coating thickness of 75 µm is
9.3.5 Weigh, Plate, Weigh Method—Using a similar sub-
recommended. Conversions of Vickers or Knoop hardness
strate material of
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: B733 − 21 B733 − 22
Standard Specification for
Autocatalytic (Electroless) Nickel-Phosphorus Coatings on
Metal
This standard is issued under the fixed designation B733; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope*
1.1 This specification covers requirements for autocatalytic (electroless) nickel-phosphorus coatings applied from aqueous
solutions to metallic products for engineering (functional) uses.
1.2 The coatings are alloys of nickel and phosphorus produced by autocatalytic chemical reduction with hypophosphite. Because
the deposited nickel alloy is a catalyst for the reaction, the process is self-sustaining. The chemical and physical properties of the
deposit vary primarily with its phosphorus content and subsequent heat treatment. The chemical makeup of the plating solution
and the use of the solution can affect the porosity and corrosion resistance of the deposit. For more details, see ASTM STP 265
(1) and Refs (2), (3), (4), and (5).
1.3 The coatings are generally deposited from acidic solutions operating at elevated temperatures.
1.4 The process produces coatings of uniform thickness on irregularly shaped parts, provided the plating solution circulates freely
over their surfaces.
1.5 The coatings have multifunctional properties, such as hardness, heat hardenability, abrasion, wear and corrosion resistance,
magnetics, electrical conductivity provide diffusion barrier, and solderability. They are also used for the salvage of worn or
mismachined parts.
1.6 The low phosphorus (2 to 4 % P) coatings are microcrystalline and possess high as-plated hardness (620 to 750 HK 100).
These coatings are used in applications requiring abrasion and wear resistance.
1.7 Lower phosphorus deposits in the range between 1 % and 3 % phosphorus are also microcrystalline. These coatings are used
in electronic applications providing solderability, bondability, increased electrical conductivity, and resistance to strong alkali
solutions.
1.8 The medium phosphorous coatings (5 to 9 % P) are most widely used to meet the general purpose requirements of wear and
corrosion resistance.
This specification is under the jurisdiction of ASTM Committee B08 on Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee B08.03 on
Engineering Coatings.
Current edition approved Oct. 15, 2021May 1, 2022. Published November 2021May 2022. Originally approved in 1984. Last previous edition approved in 20152021 as
B733 – 15.B733 – 21. DOI: 10.1520/B0733-21. 10.1520/B0733-22.
The boldface numbers given in parentheses refer to a list of references at the end of the text.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B733 − 22
1.9 The high phosphorous (more than 10 % P) coatings have superior salt-spray and acid resistance in a wide range of applications.
They are used on beryllium and titanium parts for low stress properties. Coatings with phosphorus contents greater than 11.2 %
P are not considered to be ferromagnetic.
1.10 Units—The values stated in SI units are to be regarded as standard.
1.11 The following precautionary statement pertains only to the test method portion, Section 9, of this specification. 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.
1.12 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.
2. Referenced Documents
2.1 ASTM Standards:
B368 Test Method for Copper-Accelerated Acetic Acid-Salt Spray (Fog) Testing (CASS Test)
B374 Terminology Relating to Electroplating
B380 Test Method for Corrosion Testing of Decorative Electrodeposited Coatings by the Corrodkote Procedure
B487 Test Method for Measurement of Metal and Oxide Coating Thickness by Microscopical Examination of Cross Section
B499 Test Method for Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis
Metals
B504 Test Method for Measurement of Thickness of Metallic Coatings by the Coulometric Method
B537 Practice for Rating of Electroplated Panels Subjected to Atmospheric Exposure
B567 Test Method for Measurement of Coating Thickness by the Beta Backscatter Method
B568 Test Method for Measurement of Coating Thickness by X-Ray Spectrometry
B571 Practice for Qualitative Adhesion Testing of Metallic Coatings
B578 Test Method for Microindentation Hardness of Electroplated Coatings
B602 Guide for Attribute Sampling of Metallic and Inorganic Coatings
B667 Practice for Construction and Use of a Probe for Measuring Electrical Contact Resistance
B678 Test Method for Solderability of Metallic-Coated Products
B697 Guide for Selection of Sampling Plans for Inspection of Electrodeposited Metallic and Inorganic Coatings
B762 Guide of Variables Sampling of Metallic and Inorganic Coatings
B849 Specification for Pre-Treatments of Iron or Steel for Reducing Risk of Hydrogen Embrittlement
B850 Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement
B851 Specification for Automated Controlled Shot Peening of Metallic Articles Prior to Nickel, Autocatalytic Nickel, or
Chromium Plating, or as Final Finish
D1193 Specification for Reagent Water
D2670 Test Method for Measuring Wear Properties of Fluid Lubricants (Falex Pin and Vee Block Method)
D2714 Test Method for Calibration and Operation of the Falex Block-on-Ring Friction and Wear Testing Machine
D3951 Practice for Commercial Packaging
D4060 Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser
E60 Practice for Analysis of Metals, Ores, and Related Materials by Spectrophotometry
E140 Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness,
Superficial Hardness, Knoop Hardness, Scleroscope Hardness, and Leeb Hardness
E156 Test Method for Determination of Phosphorus in High-Phosphorus Brazing Alloys (Photometric Method) (Withdrawn
1993)
E352 Test Methods for Chemical Analysis of Tool Steels and Other Similar Medium- and High-Alloy Steels
F519 Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments
G5 Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements
G31 Guide for Laboratory Immersion Corrosion Testing of Metals
G59 Test Method for Conducting Potentiodynamic Polarization Resistance Measurements
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
B733 − 22
G85 Practice for Modified Salt Spray (Fog) Testing
2.2 SAE Documents:
AMS 2430 Shot Peening
AMS 2590 Rotary Flap Peening of Metal Parts
2.3 ANSI Standard:
ANSI/ASQ Z1.4 Sampling Procedures and Tables for Inspection by Attributes
2.4 Military Standard:
MIL-STD-1916 DOD Preferred Methods of Acceptance of Product
2.5 ISO Standard:
ISO 4527 Metallic Coatings-Autocatalytic (Electroless) Nickel-Phosphorus Alloy Coatings—Specification and Test Methods
3. Terminology
3.1 Definitions:
3.1.1 significant surfaces, n—those substrate surfaces which the coating must protect from corrosion or wear, or both, and that are
essential to the performance.
3.2 Other Definitions—Terminology B374 defines most of the technical terms used in this specification.
4. Coating Classification
4.1 The coating classification system provides for a scheme to select an electroless nickel coating to meet specific performance
requirements based on alloy composition, thickness and hardness.
4.1.1 TYPE describes the general composition of the deposit with respect to the phosphorus content and is divided into five
categories which establish deposit properties (see Table 1).
NOTE 1—Due to the precision of some phosphorus analysis methods, a deviation of 0.5 % has been designed into this classification scheme. Rounding
of the test results due to the precision of the limits provides for an effective limit of 4.5 % and 9.5 %, respectively. For example, coating with a test result
for phosphorus of 9.7 % would have a classification of TYPE V, see Appendix X5, Alloy TYPEs.
4.2 Service Condition Based on Thickness:
4.2.1 Service condition numbers are based on the severity of the exposure in which the coating is intended to perform and
minimum coating thickness to provide satisfactory performance (see Table 2).
4.2.2 SC0 Minimum Service, 0.1 μm—This is defined by a minimum coating thickness to provide specific material properties and
extend the life of a part or its function. Applications include requirements for diffusion barrier, undercoat, electrical conductivity,
and wear and corrosion protection in specialized environments.
TABLE 1 Deposit Alloy Types
Type Phosphorus % wt
I No Requirement for Phosphorus
II 1 to 3
III 2 to 4
IV 5 to 9
V 10 and above
Available from SAE International (SAE), 400 Commonwealth Dr., Warrendale, PA 15096, http://www.sae.org.
The original reference MIL-S-13165 Shot Peening of Metal Parts was cancelled in February 1998 and referred to SAE AMS-S-13165, which was also cancelled and
superseded by AMS 2430.
The original reference MIL-R-81841 Rotary Flap Peening of Metal Parts was cancelled in April 2012 and referred to SAE AMS 2590.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
The original reference MIL-STD-105 Sampling Procedures and Tables for Inspection by Attributes was cancelled in February 2008 and referred to MIL- STD-1916 or
ANSI/ASQ Z1.4.
Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700 Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Available from International Organization for Standardization (ISO), ISO Central Secretariat, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva, Switzerland,
https://www.iso.org.
B733 − 22
TABLE 2 Service Conditions
Coating Thickness Requirements
Minimum Coating
Service Condition Thickness μm in.
Specification
SC0 Minimum Thickness 0.1 0.000004
SC1 Light Service 5 0.0002
SC2 Mild Service 13 0.0005
SC3 Moderate Service 25 0.001
SC4 Severe Service 75 0.003
4.2.3 SC1 Light Service, 5 μm—This is defined by a minimum coating thickness of 5 μm for extending the life of the part. Typical
environments include light-load lubricated wear, indoor corrosion protection to prevent rusting, and for soldering and mild abrasive
wear.
4.2.4 SC2 Mild Service, 13 μm—This is defined by mild corrosion and wear environments. It is characterized by industrial
atmosphere exposure on steel substrates in dry or oiled environments.
4.2.5 SC3 Moderate Service, 25 μm—This is defined by moderate environments such as non marine outdoor exposure, alkali salts
at elevated temperature, and moderate wear.
4.2.6 SC4 Severe Service, 75 μm—This is defined by a very aggressive environment. Typical environments would include acid
solutions, elevated temperature and pressure, hydrogen sulfide and carbon dioxide oil service, high-temperature chloride systems,
very severe wear, and marine immersion.
NOTE 2—The performance of the autocatalytic nickel coating depends to a large extent on the surface finish of the article to be plated and how it was
pretreated. Rough, non uniform surfaces require thicker coatings than smooth surfaces to achieve maximum corrosion resistance and minimum porosity.
4.3 Post Heat Treatment Class—The nickel-phosphorus coatings shall be classified by heat treatment after plating to increase
coating adhesion or hardness, or both (see Table 3).
4.3.1 Class 1—As-deposited, no heat treatment.
4.3.2 Class 2—Heat treatment at 260 °C to 400 °C to produce a minimum hardness of 850 HK100.
4.3.3 Class 3—Heat treatment at 180 °C to 200 °C for 2 h to 4 h to improve coating adhesion on steel and to provide for hydrogen
embrittlement relief (see 6.6).
4.3.4 Class 4—Heat treatment at 120 °C to 130 °C for 1 h to 6 h to increase adhesion of carburized steel or heat-treatable
(age-hardened) aluminum alloys (see Note 3).
TABLE 3 Classification of Post Heat Treatment
Temperature
Class Description Time (h)
(°C)
1 No Heat Treatment, As Plated
2 Heat Treatment for Maximum Hardness
TYPE I 260 20
285 16
320 8
400 1
TYPE II 350 to 380 1
TYPE III 360 to 390 1
TYPE IV 365 to 400 1
TYPE V 375 to 400 1
3 Adhesion on Steel 180 to 200 2 to 4
4 Adhesion, Carburized Steel and 120 to 130 1 to 6
Age Hardened Aluminum
5 Adhesion on Beryllium and 140 to 150 1 to 2
Aluminum
6 Adhesion on Titanium 300 to 320 1 to 4
B733 − 22
4.3.5 Class 5—Heat treatment at 140 °C to 150 °C for 1 h to 2 h to improve coating adhesion for aluminum, non age-hardened
aluminum alloys, copper, copper alloys, or beryllium.
4.3.6 Class 6—Heat treatment at 300 °C to 320 °C for 1 h to 4 h to improve coating adhesion for titanium alloys.
NOTE 3—Heat-treatable aluminum alloys such as Type 7075 can undergo microstructural changes and lose strength when heated to over 130 °C.
5. Ordering Information
5.1 The following information shall be supplied by the purchaser in either the purchase order or on the engineering drawing of
the part to be plated:
5.1.1 Title, ASTM designation number, and year of issue of this specification.
5.1.2 Classification of the coating by type, service condition, class (see 4.1, 4.2, and 4.3).
5.1.3 Specify maximum dimension and tolerance requirements, if any.
5.1.4 Peening, if required (see 6.3).
5.1.5 The tensile strength of the material in MPa (see 6.3.1 and 6.6).
5.1.6 Stress relief heat treatment before plating (see 6.3).
5.1.7 Hydrogen Embrittlement Relief after plating (see 6.6).
5.1.8 Significant surfaces and surfaces not to be plated must be indicated on drawings or sample.
5.1.9 Supplemental or Special Government Requirements such as, specific phosphorus content, abrasion wear or corrosion
resistance of the coating, solderability, contact resistance and packaging selected from Supplemental Requirements.
5.1.10 Requirement for a vacuum, inert, or reducing atmosphere for heat treatment above 260 °C to prevent surface oxidation of
the coating (see S1.3).
5.1.11 Test methods for coating adhesion, composition, thickness, porosity, wear and corrosion resistance, if required, selected
from those found in Section 9 and Supplemental Requirements.
5.1.12 Requirements for sampling (see Section 8).
NOTE 4—The purchaser should furnish separate test specimens or coupons of the basis metal for test purposes to be plated concurrently with the articles
to be plated (see 8.4).
6. Materials and Manufacture
6.1 Substrate—Defects in the surface of the basis metal such as scratches, porosity, pits, inclusions, roll and die marks, laps,
cracks, burrs, cold shuts, and roughness may adversely affect the appearance and performance of the deposit, despite the
observance of the best plating practice. Any such defects on significant surfaces shall be brought to the attention of the purchaser
before plating. The producer shall not be responsible for coatings defects resulting from surface conditions of the metal, if these
conditions have been brought to the attention of the purchaser.
6.2 Pretreatment—A suitable method shall activate the surface and remove oxide and foreign materials, which may cause poor
adhesion and coating porosity.
NOTE 5—Heat treatment of the base material may affect its metallurgical properties. An example is leaded steel which may exhibit liquid or solid
embrittlement after heat treatment. Careful selection of the pre and post heat treatments are recommended.
6.3 Stress Relief:
B733 − 22
6.3.1 Pretreatment of Iron and Steel for Reducing the Risk of Hydrogen Embrittlement—Parts that are made of steel with ultimate
tensile strength of greater than 1000 MPa (hardness of 31 HRC), that have been machined, ground, cold formed, or cold
straightened subsequent to heat treatment require stress relief heat treatment when specified by the purchaser. The tensile strength
of the material shall be supplied by the purchaser. Specification B849 contains a list of pre-treatments, precautions, procedures,
and caveats that shall be used.
6.3.2 Peening—Peening prior to plating may be required on high-strength steel parts to induce residual compressive stresses in
the surface, which can reduce loss of fatigue strength and improve stress corrosion resistance after plating. (See Supplementary
Requirements.)
6.3.2.1 Steel parts which are designed for unlimited life under dynamic loads shall be shot peened or rotary flap peened.
NOTE 6—Controlled shot peening is the preferred method because there are geometry’s where rotary flap peening is not effective. See S1.11.2.
6.3.2.2 Unless otherwise specified, the shot peening shall be accomplished on all surfaces for which the coating is required and
all immediate adjacent surfaces when they contain notches, fillets, or other abrupt changes of section size where stresses will be
concentrated.
6.4 Racking—Parts should be positioned so as to minimize trapping of hydrogen gas in cavities and holes, allowing free circulation
of solution over all surfaces to obtain uniform coating thickness. The location of rack or wire marks in the coating shall be agreed
upon between the producer and purchaser.
6.5 Plating Process:
6.5.1 To obtain consistent coating properties, the bath must be monitored periodically for pH, temperature, nickel, and
hypophosphite. Replenishments to the plating solution should be as frequent as required to maintain the concentration of the nickel
and hypophosphite between 90 % and 100 % of set point. The use of a statistical regimen to establish the control limits and
frequency of analysis may be employed to ensure quality deposits are produced.
6.5.2 Mechanical movement of parts, agitation of the bath, and filtration is recommended to increase coating smoothness and
uniformity and prevent pitting or streaking due to hydrogen bubbles.
6.6 Post Coating Treatment for Iron and Steel for Reducing the Risk of Hydrogen Embrittlement—Parts that are made of steel with
ultimate tensile strengths of 1000 MPa (corresponding hardness values 300 HV10, 303 HB, or 31 HRC or greater), as well as
surface hardened parts, shall require post coating hydrogen embrittlement relief baking when specified by the purchaser. The tensile
strength shall be supplied by the purchaser. Guide B850 contains a list of post treatments, procedures, limitations, and guidelines
that are permitted to be used to reduce the effects of hydrogen embrittlement.
6.6.1 Heat treatment shall be performed preferably within 1 h, but not more than 3 h, of plating unless the size or weight of the
part prevents the initiation of heart treatment within 3 h. In this case, the part shall be heat treated as soon as possible. In all cases,
the duration of the heat treatment shall commence from the time at which the whole of each part attains the specified temperature.
6.7 Heat Treatment After Plating to Improve Adhesion—To improve the adhesion of the coating to various substrates, the heat
treatments in Table 3 should be performed as soon as practical after plating (see 4.3).
6.8 Heat Treatment After Plating to Increase Hardness:
6.8.1 To increase the hardness of the coating, a heat treatment of over 260 °C is required. Table 3 describes the heat treatment for
maximum hardness. See Appendix X3 and Appendix X4.
6.8.2 A heat treatment at 260 °C for greater than 20 h should be used to reduce the loss of surface hardness and strength of some
ferrous basis metals. Avoid rapid heating and cooling of plated parts. Sufficient time must be allowed for large parts to reach oven
temperature.
6.8.3 Do not use gas containing hydrogen with high-strength steel parts.
B733 − 22
NOTE 7—The length of time to reach maximum hardness varies with the phosphorus content of the deposit. High phosphorus deposits may require longer
time or a higher temperature, or both. Individual alloys should be tested for maximum hardness attainable, especially for conditions of lower temperatures
and longer times.
NOTE 8—Inert or reducing atmosphere or vacuum sufficient to prevent oxidation is recommended for heat treatment above 260 °C.
7. Requirements
7.1 Process—The coating shall be produced from an aqueous solution through chemical reduction reaction.
7.2 Acceptance Requirements—These requirements are placed on each lot or batch and can be evaluated by testing the plated part.
7.2.1 Appearance:
7.2.1.1 The coating surface shall have a uniform, metallic appearance without visible defects such as blisters, pits, pimples, and
cracks.
7.2.1.2 Imperfections that arise from surface conditions of the substrate which the producer is unable to remove using conventional
pretreatment techniques and that persist in the coating shall not be cause for rejection (see 6.1). Also, discoloration due to heat
treatment shall not be cause for rejection unless special heat treatment atmosphere is specified (see 5.1.10).
7.2.2 Thickness—The thickness of the coating shall exceed the minimum requirements in Table 2 as specified by the service
condition agreed to prior to plating. After coating and if specified, the part shall not exceed maximum dimension on significant
surface (see 5.1.3).
NOTE 9—The thickness of the coating cannot be controlled in blind or small diameter deep holes or where solution circulation is restricted.
7.2.3 Adhesion—The coating shall have sufficient adhesion to the basis metal to pass the specified adhesion test (see 9.4 and
Practice B571).
7.2.4 Porosity—The coatings shall be essentially pore free when tested according to one of the methods of 9.6. The test method,
the duration of the test, and number of allowable spots per unit area shall be specified (see 5.1.11).
7.3 Qualification Requirements—These requirements are placed on the deposit and process and are performed on specimens to
qualify the deposit and plating process. The tests for these qualification requirements shall be performed monthly or more
frequently.
7.3.1 Composition—Type II, III, IV, V deposits shall be analyzed for alloy composition by testing for phosphorus (see 9.1). The
weight percent of phosphorus shall be in the range designated by type classification (see 4.1).
7.3.2 Microhardness—The microhardness of Class 2 deposits shall be determined by Test Method B578 (Knoop). For Class 2
coatings, the microhardness shall equal or exceed a minimum of 850 HK100 (or equivalent Vickers) (see 4.3 and 9.5). The
conversion of Vickers to Knoop using Tables E140 is not recommended.
7.3.3 Hydrogen Embrittlement—The process used to deposit a coating onto high strength steels shall be evaluated for hydrogen
embrittlement by Test Method F519.
8. Sampling
8.1 The purchaser and producer are urged to employ statistical process control in the coating process. Properly performed this will
ensure coated products of satisfactory quality and will reduce the amount of acceptance inspection.
8.1.1 Sampling plans can only screen out unsatisfactory products without assurance that none of them will be accepted (6).
8.2 The sampling plan used for the inspection of a quantity of coated parts (lot) shall be Test Method B602 unless otherwise
specified by purchaser in the purchase order or contract (see 5.1.12 and S1.11.1). Guide B697 and Test Method B762 also contain
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sampling plans that are designed for the sampling inspection of coatings. When Guide B697 or Test Method B762 are specified,
the purchaser and producer must agree on the plan to be used.
NOTE 10—Usually, when a collection of coated parts (the inspection lot 8.2) is examined for compliance with the requirements placed on the parts, a
relatively small number of parts, the sample, is selected at random and inspected. The inspection lot is then classified as complying or not complying
with the requirements based on the results of the inspection sample. The size of the sample and the criteria of compliance are determined by the application
of statistics. The procedure is known as sampling inspection.
Test Method B602 contains four sampling plans, three for use with tests that are nondestructive and one for use with tests that are destructive. The
purchaser and producer may agree on the plan(s) to be used. If they do not, Test Method B602 identifies the plan to be used.
Guide B697 provides a large number of plans and also gives guidance on the selection of a plan.
Test Method B762 can be used only for coating requirements that have a numerical limit, such as coating thickness. Test Method B762 contains several
plans and also gives instructions for calculating plans to meet special needs. The purchaser and producer may agree on the plan(s) to be used. If they
do not, Test Method B762 identifies the plan to be used.
An inspection lot shall be defined as a collection of coated parts which are of the same kind, that have been produced to the same specification, that
have been coated by a single producer at one time or approximately the same time under essentially identical conditions, and that are submitted for
acceptance or rejection as a group.
8.3 All specimens used in the sampling plan for acceptance tests shall be made of the same basis material and in the same
metallurgical condition as articles being plated to this specification.
8.4 All specimens shall be provided by the purchaser unless otherwise agreed to by the producer.
NOTE 11—The autocatalytic nickel process is dynamic and a daily sampling is recommended. For coatings requiring alloy analysis and corrosion testing,
weekly sampling should be considered as an option.
9. Test Methods
9.1 Deposit Analysis for Phosphorus:
9.1.1 Phosphorus Determination—Determine mass % phosphorus content according to Practice E60, Test Methods E352, or Test
Method E156 on known weight of deposit dissolved in warm concentrated nitric acid.
9.1.2 Composition can be determined by atomic absorption, emission, or X-ray fluorescence spectrometry.
NOTE 12—Inductively coupled plasma techniques can determine the alloy to within 0.1 %. The following analysis wavelength lines have been used with
minimum interference to determine the alloy.
Ni 216.10 nm Cd 214.44 nm Fe 238.20 nm
P 215.40 nm Co 238.34 nm Pb 283.30 nm
P 213.62 nm Cr 284.32 nm Sn 198.94 nm
Al 202.55 nm Cu 324.75 nm Zn 206.20 nm
9.2 Appearance—Examine the coating visually for compliance with the requirements of 7.2.1.
9.3 Thickness—Measure the coating thickness for compliance with the requirements of 7.2.2.
NOTE 13—Eddy-current type instruments give erratic measurements due to variations in conductivity of the coatings with changes in phosphorus content.
9.3.1 Microscopical Method—Measure the coating thickness of a cross section according to Test Method B487.
NOTE 14—To protect the edges, electroplate the specimens with a minimum of 5 μm of nickel or copper prior to cross sectioning.
9.3.2 Magnetic Induction Instrument Method—Test Method B499 is applicable to magnetic substrates plated with autocatalytic
nickel deposits that contain more than 11 mass % phosphorus (not ferromagnetic) and that have not been heat-treated. The
instrument shall be calibrated with deposits plated in the same solution under the same conditions on magnetic steel.
9.3.3 Beta Backscatter Method—Test Method B567 is only applicable to coatings on aluminum, beryllium, magnesium, and
titanium. The instrument must be calibrated with standards having the same composition as the coating.
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NOTE 15—The density of the coating varies with its mass % phosphorus content (See Appendix X2).
9.3.4 Micrometer Method—Measure the part, test coupon, or pin in a specific spot before and after plating using a suitable
micrometer. Make sure that the surfaces measured are smooth, clean, and dry.
9.3.5 Weigh, Plate, Weigh Method—Using a similar substrate material of known surface area, weigh to the nearest milligram
before and after plating, making sure that the part or coupon is dry and at room temperature for each measurement. Calculate the
thickness from the increase in mass, specific gravity, and area as follows:
coating thickness, µm5 10 W/ A 3D (1)
~ !
where:
W = mass gain, mg,
A = total surface area, cm , and
D = specific gravity, g/cm (see Appendix X2).
9.3.6 Coulometric Method—Measure the coating thickness in accordance with Test Method B504. The solution to be used shall
be in accordance with manufacturer’s recommendations. The surface of the coating shall be cleaned prior to testing (see Note 14).
9.3.6.1 Calibrate standard thickness specimens with deposits plated in the same solution under the same conditions.
9.3.7 X-Ray Spectrometry—Measure the coating thickness in accordance with Test Method B568. The instrument must be
calibrated with standards having the same composition as the coating.
NOTE 16—This method is only recommended for deposits in the as-plated condition. The phosphorus content of the coating must be known to calculate
the thickness of the deposit. Matrix effect due to the distribution of phosphorus in layers of the coating also effect the measurement accuracy and require
that calibration standards be made under the same conditions as the production process.
9.4 Adhesion:
9.4.1 Bend Test (Practice B571)—A sample sp
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