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:
| Type | Phosphorus % wt |
|---|---|
| I | No Requirement for Phosphorus |
| II | 1 - 3 |
| III | 2 - 4 |
| IV | 5 - 9 |
| V | > 10 |
Coating thickness
The standard identifies and classifies electroless nickel thicknesses according to service:
| SC | Service | Minimum thickness | Notes |
|---|---|---|---|
| SC0 | Minimum Thickness | 0,1 µm | Minimum coating thickness to provide specific material properties and extend the life of a part or its function |
| SC1 | Light Service | 5 µm | Indoor corrosion protection to prevent rust, soldering, mild abrasive wear, light-load lubricated wear |
| SC2 | Mild Service | 13 µm | Exposure to industrial atmosphere, on steel in dry or lubricated environments |
| SC3 | Moderate Service | 25 µm | Outdoor exposure (non-marine), moderate wear, alkaline salts at elevated temperatures |
| SC4 | Severe Service | 75 µm | Very 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:
| Class | Temperature °C |
|---|---|
| 1 | No heat treatment |
| 2 | Max hardness 260 - 400 |
| 3 | 180 - 200 |
| 4 | 120 - 130 |
| 5 | 140 - 150 |
| 6 | 300 - 320 |
Examples
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