ASTM B733 - Autocatalytic (Electroless) Nickel-Phosphorus Coatings

Published on 20/11/2022 by Matteo Bordiga

Standard ASTM B733 - Electroless Nickel

ASTM B733 - Standard Specification for Autocatalytic (Electroless) Nickel-Phosphorus Coatings on Metal

ATSM B733 is the international standard for electroless nickel plating, mainly used in the United States of America.

The most widely used international reference standard in Europe is ISO 4527 - Metallic coatings - Autocatalytic (electroless) nickel-phosphorus alloy coatings - Specification and test methods.

To date (October 2022), the latest revision is B733-22 (May 2022), following revisions B733-21 of 2021, B733-15 of 2015 and B733-04 of 2004.

Type of electroless nickel

Standard B733 defines a classification of electroless nickel coatings according to the percentage of phosphorus in the alloy:

TypePhosphorus % wt
INo Requirement for Phosphorus
II1 - 3
III2 - 4
IV5 - 9
V> 10

Coating thickness

The standard identifies and classifies electroless nickel thicknesses according to service:

SCServiceMinimum thicknessNotes
SC0Minimum Thickness0,1 µmMinimum coating thickness to provide specific material properties and extend the life of a part or its function
SC1Light Service5 µmIndoor corrosion protection to prevent rust, soldering, mild abrasive wear, light-load lubricated wear
SC2Mild Service13 µmExposure to industrial atmosphere, on steel in dry or lubricated environments
SC3Moderate Service25 µmOutdoor exposure (non-marine), moderate wear, alkaline salts at elevated temperatures
SC4Severe Service75 µmVery aggressive environments, acid solutions, elevated temperature and pressure, hydrogen sulfide and carbon dioxide oil service, high temperature chloride systems, very severe wear and marine immersion

Electroless nickel heat treatment

Six classes of heat treatment are identified:

ClassTemperature °C
1No heat treatment
2Max hardness 260 - 400
3180 - 200
4120 - 130
5140 - 150
6300 - 320


Electroless nickel ASTM B733 TYPE IV, SC1 CLASS 3

Medium phosphorus nickel plating (5-9% P), 5µm minimum thickness, heat treatment 180-200°C

Electroless nickel ASTM B733 TYPE V, SC2 CLASS 2

High-phosphorus electroless nickel plating (>10% P), minimum thickness 13µm, heat treatment 375-400°C