ASTM B633 — electrodeposited zinc on iron and steel: SC1–SC4 and the Types

Scope — and what it does not cover

ASTM B633 covers electrodeposited zinc coatings on iron and steel parts, classified by service condition — SC1 mild, SC2 moderate, SC3 severe, SC4 very severe, at 5 / 8 / 13 / 25 µm minimum thickness — and by Type, which describes the supplementary treatment over the zinc: as-plated, coloured chromate, colourless chromate, phosphate conversion, and the hexavalent-free passivate types added in later revisions. It does NOT cover threaded-fastener specifics (ASTM F1941 / F1941M does), mechanically deposited zinc (B695), zinc alloys, or any correlation between cabinet hours and years of service.

Standards are copyrighted documents. We explain scope, intent and practical application in our own words; obtain the current edition from ISO, ASTM, DIN or your national standards body.

What actually bites in production

• 1Service condition is a thickness requirement with a severity label attached. SC3 means 13 µm minimum on significant surfaces; the words "severe service" carry no additional obligation.
• 2Type and service condition are independent decisions. Both must appear, because the Type determines colour, passivation chemistry and white-corrosion behaviour.
• 3The salt-spray requirement attached to a chromate or passivate Type is a check on the conversion film applied to freshly processed parts. It is process control, not a prediction of field life.
• 4Baking for embrittlement relief is invoked for hardened parts, and the same promptness discipline as ISO 4042 applies.
• 5B633 is written for parts in general. When the part is a screw or a nut, thread fit decides whether SC3 or SC4 is even physically possible.

What to write on the drawing or PO

• Type and service condition together — for example "ASTM B633, Type III, SC3" — plus which surfaces are significant.
• Whether hexavalent chromium is prohibited, since that decides which Types are allowed.
• The pre-plate thread tolerance class for threaded parts, or a note that the GO gauge must assemble after plating.
• The de-embrittlement baking requirement and record-keeping for hardened parts.
• Any salt-spray requirement as a separate line with hours and the corrosion type being judged.

What goes wrong when it is cited loosely

• "B633 SC4" on an M5 screw. 25 µm does not fit inside a standard thread allowance; the thread class, the thickness or the drawing has to change.
• A service condition with no Type. The passivation is what an inspector sees first.
• Quoting the standard's salt-spray hours as a warranty of outdoor life.
• Mixing ISO and ASTM designations in one callout so that nobody can say which thickness table governs.