ASTM B733
Standard Specification for Autocatalytic (Electroless) Nickel-Phosphorus Coatings on Metal
Covers autocatalytic nickel-phosphorus coatings on metals for engineering use, classified by substrate, service condition, phosphorus content, and heat treatment.
What ASTM B733 specifies
ASTM B733 controls electroless nickel-phosphorus deposits by substrate Type, service-condition thickness SC0 through SC4, phosphorus-content Type 1 through Type 5, and post-deposition heat-treatment Class 1 through Class 6. Minimum thickness rises from no specified minimum for SC0 to 25 µm for SC4. Phosphorus type affects hardness, corrosion behavior, solderability, and magnetic response; heat treatment may improve adhesion or hardness but can reduce corrosion resistance. A complete purchase order or drawing note should identify substrate type, service condition, deposit type, heat-treatment class, significant surfaces, and required tests.
ASTM B733 types and classes
Every designation the document defines, with the thickness it requires and the tests it imposes.
| Designation | Meaning | Thickness | Requirement / test |
|---|---|---|---|
SC0 | No minimum thickness specified | — | Thickness as agreed |
SC1 | Light service | ≥ 5 µm ≥ 0.2 mil | Thickness and adhesion |
SC2 | Mild service | ≥ 13 µm ≥ 0.51 mil | Thickness and adhesion |
SC3 | Severe service | ≥ 25 µm ≥ 0.98 mil | Thickness, adhesion, and agreed performance tests |
SC4 | Very severe service | ≥ 75 µm ≥ 3 mil | Thickness, adhesion, and agreed performance tests |
Type 1 | Low-phosphorus nickel deposit | — | Phosphorus content |
Type 2 | Medium-low-phosphorus nickel deposit | — | Phosphorus content |
Type 3 | Medium-phosphorus nickel deposit | — | Phosphorus content |
Type 4 | High-phosphorus nickel deposit | — | Phosphorus content |
Type 5 | Very-high-phosphorus nickel deposit | — | Phosphorus content |
Classes 1–6 | As-deposited or specified heat-treatment condition selected for substrate, adhesion, hardness, or dimensional stability | — | Heat-treatment records and applicable adhesion or hardness test |
Tests and the drawing call-out
Tests the document imposes
- Thickness
- Adhesion
- Phosphorus content
- Hardness when specified
- Porosity or corrosion testing when specified
How to cite it
Drawing call-out
ELECTROLESS NICKEL PER ASTM B733, TYPE <1–5>, SC<0–4>, CLASS <1–6>Angle brackets mark the choices the drawing must make; every finish page shows a completed example.
Finishes specified to ASTM B733
ASTM B733 — frequently asked
What is ASTM B733 SC3 thickness?
SC3 requires a minimum coating thickness of 25 µm. Functional dimensions should account for coating on every exposed surface.
What do ASTM B733 Type and Class mean?
Type identifies the phosphorus-content range. Class identifies the as-deposited or heat-treated condition.
Does heat treating electroless nickel always improve it?
No. Heat treatment can increase hardness or improve adhesion, but high-temperature treatment may reduce the corrosion resistance of an otherwise amorphous high-phosphorus deposit.
ASTM B733 is copyrighted by ASTM International. This page explains the document so a drawing can be read; it does not reproduce the standard. Purchase the current edition from the publisher. Official source ↗