Coating Thickness and Salt-Spray Hours for CNC Finishes
Coating thickness vs salt-spray hours for 13 CNC finishes: anodize, zinc plate, HDG, zinc-flake, phosphate, black oxide, powder and Ni-Cr, endpoints stated.
Coating thickness vs corrosion resistance — 13 finishes (NSS per ASTM B117 / ISO 9227)
| Process | Coating thickness (µm) | Salt spray, NSS — endpoint stated | Substrate | Typical application | Cost tier (editorial, 1–5) | Standard | Source |
|---|---|---|---|---|---|---|---|
| Anodizing Type II (sulfuric, conventional) | 5–25 (spec window 2–25; decorative Class 2 commonly 8–15) | 336 h, sealed specimens, no corrosion beyond spec limits | Aluminum alloys | Enclosures, consumer electronics, trim | 3 | MIL-PRF-8625F (formerly MIL-A-8625F) Type II | S1 / S3 / S4 |
| Anodizing Type III (hardcoat) | 25–75 (nominal 50.8 µm / 0.002 in) | 336 h, sealed specimens, no corrosion beyond spec limits | Aluminum alloys | Hydraulic bores, wear plates | 4 | MIL-PRF-8625F Type III; ISO 10074 | S2 / S3 |
| Electroless nickel, mid-phosphorus (Type IV, 5–9% P) | SC1 5 / SC2 13 / SC3 25 / SC4 75 (minimums) | Typically 48–96 h to red rust (supplier data; B733 itself sets no NSS hours) | Steel, aluminum, stainless, copper alloys | Valve bodies, general industrial parts | 4 | ASTM B733 | S5 / S6 / S7 |
| Electroless nickel, high-phosphorus (Type V, above 10% P) | ≥25 (SC3 and up; marine commonly 50) | Typically ≥1,000 h to red rust at 25 µm; up to 2,000 h after diffusion treatment (supplier data) | Steel, aluminum, copper | Marine, oil and gas, non-magnetic parts | 5 | ASTM B733 (high-phosphorus salt-spray clause) | S5 / S7 / S8 / S9 |
| Zinc electroplate + clear trivalent passivate (Type V) | SC1 5 / SC2 8 / SC3 12 / SC4 25 (minimums) | 72 h to white rust (Type V) | Steel | Indoor and light-duty fasteners and parts | 2 | ASTM B633 | S10 / S11 / S12 |
| Zinc electroplate + colored passivate (Type II hexavalent / Type VI trivalent) | SC1 5 / SC2 8 / SC3 12 / SC4 25 (minimums) | 96 h (Type II) / 120 h (Type VI) to white rust | Steel | Outdoor-adjacent hardware | 2 | ASTM B633 | S10 / S11 / S12 |
| Hot-dip galvanizing (HDG) | 45–100 (A123 grade, by steel category and thickness); EN ISO 1461: 85 µm average on steel ≥6 mm | Not specified by standard — accepted by thickness; typically 500 h+ to red rust (supplier data) | Steel | Structural, long-life outdoor | 2 | ASTM A123 (parts) / A153 (fasteners); EN ISO 1461 | S14 / S15 / S16 / S26 |
| Zinc-flake coating (Dacromet / Geomet type) | 4–10 (sealer plus topcoat, reference) | 240 h at 4 µm / 480 h at 5 / 600 h at 6 / 720 h at 8 / 960 h at 10 µm, no red rust; 72 h no red rust at a scribe (sacrificial) | Steel (preferred on ≥1,000 MPa high-strength fasteners) | Automotive chassis bolts; service where hydrogen embrittlement must be ruled out | 3 | ISO 10683:2018 | S17 / S18 / S19 |
| Zinc phosphate (Type Z) + oil (Class 2) | 2–15 | ≥72 h to red rust with the specified oil supplement | Steel | Paint and powder pre-treatment, fasteners | 1 | MIL-DTL-16232G | S20 / S21 / S23 |
| Manganese phosphate (Type M) + oil (Class 2) | Heavy film (accepted by coating weight per area, not µm) | ≥48 h to red rust with the specified oil supplement | Steel | Gears and sliding parts for run-in and oil retention | 1 | MIL-DTL-16232G (DFL supplemental system: 100 h clause) | S20 / S22 / S23 |
| Black oxide + supplementary oil or wax | 0.5–2 (magnetite film ≈0.76 µm) | 24–48 h (S26) / 24–72 h (S24) to red rust — both intervals listed, see the register | Steel, stainless | Indoor machinery, appearance, zero dimensional change | 1 | MIL-DTL-13924 | S24 / S25 / S26 |
| Powder coating (epoxy / polyester / hybrid) | 60–120 | Typically epoxy 1,000+ h / polyester 500–1,000 h / hybrid 500–800 h; acceptance by scribe creepage ≤2–3 mm (ASTM D1654) | Steel, aluminum | Enclosures, furniture, architectural | 2 | Test methods ASTM B117 + D1654/D714 (no single governing spec; QUALICOAT for architectural) | S27 / S28 / S29 |
| Decorative nickel + chromium (Ni/Cr) | Ni 10–30 + Cr 0.13–0.8 (DB34 example: Ni ≥20 + Cr ≥0.25) | NSS 24–96 h depending on system (single-layer 12 µm Ni ≈24 h; copper under-layer + multilayer Ni passes 96 h); acceptance via CASS (ASTM B368), SC4 service condition 22–24 h | Steel, brass, copper alloys | Cosmetic hardware, hand tools | 4 | ASTM B456 / B368 | S30 / S31 |
How to read this table. All hours are neutral salt spray (NSS) — 5% NaCl, 35 °C — with the pass or fail endpoint (white rust, red rust or no basis-metal corrosion) stated per row. Cabinet hours are a screening number, not outdoor service life. Cost tier (1 = lowest … 5 = highest) is an editorial rating from published relative-cost indications, not a standard value. Selection in one line: aluminum parts default to anodizing — Type II for appearance, sealed Type III for wear and corrosion, high-phosphorus electroless nickel for blind holes or 1,000 h+ — and never zinc-family coatings; steel parts take zinc plate indoors (clear 72 h / colored 96–120 h class), hot-dip galvanizing or zinc-flake outdoors, electroless nickel for uniform wear-resistant thickness, powder coating for color at volume.
My Part Is 6061 Aluminum — Which Coating Survives 500 Hours of Salt Spray?
Only two finishes on this page are in that class: high-phosphorus electroless nickel at ≥25 µm typically exceeds 1,000 h, and powder coating reaches 500–1,000 h. Sealed Type II anodize is certified to 336 h (MIL-PRF-8625F) and sealed Type III to the same 336 h minimum — typically much higher in practice — so for a hard 500 h+ certificate on aluminum, electroless nickel is the spec-safe pick.
| Coating (aluminum-eligible) | Thickness (µm) | NSS hours | How to read it |
|---|---|---|---|
| Type II anodize, sealed | 5–25 | 336 h (spec) | The appearance default; dimensionally tight |
| Type III hardcoat, sealed | 25–75 | 336 h (spec); typically higher | Wear plus corrosion in one step |
| High-phosphorus electroless nickel | ≥25 | ≥1,000 h typical (supplier data) | Uniform in blind holes; the 1,000 h+ pick |
| Polyester powder | 60–120 | 500–1,000 h typical | Thickest barrier; adds the most dimension |
How Much Does Hard Anodizing Add to My Part Dimensions?
About half the coating builds outward and half penetrates, so a 50 µm Type III coating grows roughly 25 µm per face — about 50 µm on a fully coated diameter. Mask threads and bearing fits before hardcoat.
| Case | Thickness (µm) | Growth per face (µm) | Across a coated diameter (µm) |
|---|---|---|---|
| Type III at nominal 50 (spec 50.8 µm / 0.002 in) | 50 | ≈25 | ≈50 |
| Type II at the 8–15 mid-band (12 µm, calculated) | 12 | ≈6 | ≈12 |
What Zinc Plating Thickness Do I Need Outdoors?
ASTM B633 SC3 — 12 µm minimum — covers occasional rain and condensation; SC4 — 25 µm — is for saline or frequently wet service. Some catalogues print SC3 as 13 µm from rounding 0.0005 in; the standard tables this page pins say 12 µm.
| Service condition | Minimum zinc (µm) | Typical environment | Type V clear (h to white rust) | Type II colored (h to white rust) | Type VI colored (h to white rust) |
|---|---|---|---|---|---|
| SC1 | 5 | Indoor, dry | 72 | 96 | 120 |
| SC2 | 8 | Indoor humidity, light exposure | 72 | 96 | 120 |
| SC3 | 12 | Occasional rain and condensation | 72 | 96 | 120 |
| SC4 | 25 | Saline or frequently wet | 72 | 96 | 120 |
Blue-White Zinc or Yellow Zinc — Which Lasts Longer?
Colored trivalent passivate (Type VI) is rated 120 h to white rust, hexavalent colored (Type II) 96 h and clear trivalent (Type V) 72 h — the same zinc thickness with different conversion coatings. These are the three-source consistent ASTM B633 figures; a lone page claiming Type I 48 h or Type II 240 h stays quarantined in the conflict register below, not in this table.
| Passivate type | Chemistry | To white rust |
|---|---|---|
| Type V | Clear trivalent passivate (blue-white) | 72 h |
| Type II | Colored chromate, hexavalent (yellow) | 96 h |
| Type VI | Colored passivate, trivalent | 120 h |
Why Specify Zinc-Flake Coating on 12.9-Class Bolts?
It is non-electrolytic — no hydrogen-embrittlement risk — and ISO 10683 rates 480–960 h to red rust at only 5–10 µm, with 72 h of protection at a scribe. The hour pairings below follow the 2018-edition table; the 2014 edition pairs differently (8 µm = 480 h), so treat the purchased standard text as the final verification.
| Coating thickness (µm) | NSS to first red rust (h) |
|---|---|
| 4 | 240 |
| 5 | 480 |
| 6 | 600 |
| 8 | 720 |
| 10 | 960 |
Does Hot-Dip Galvanizing Need a Salt Spray Certificate?
No — ASTM A123 accepts hot-dip galvanizing by coating grade (45–100 µm average, set by steel category and thickness), not cabinet hours; EN ISO 1461 likewise specifies thickness, 85 µm average on steel ≥6 mm. Typical red-rust performance of 500 h+ is supplier data, not a standard requirement.
| Standard | Scope | Acceptance basis | Coating thickness |
|---|---|---|---|
| ASTM A123 | Articles and assemblies after fabrication | Coating grade by steel category and thickness | 45–100 µm average |
| ASTM A153 | Fasteners and hardware | Coating class | Per class; not tabulated on this page |
| EN ISO 1461 | After-fabrication galvanizing | Average minimum by steel thickness | 85 µm average on steel ≥6 mm |
Can Electroless Nickel Pass 1,000 Hours?
Yes — but only high-phosphorus: deposits above 10% P at ≥25 µm typically reach ≥1,000 h NSS, up to 2,000 h after diffusion treatment (supplier data), while mid-phosphorus of the same thickness usually manages 48–96 h. ASTM B733 itself sets no salt-spray hours; these are system values.
| Type | Thickness classes | Typical NSS | Typical use |
|---|---|---|---|
| Type IV, mid-P (5–9% P) | SC1 5 / SC2 13 / SC3 25 / SC4 75 µm minimums | 48–96 h to red rust | Valve bodies, general industrial |
| Type V, high-P (above 10% P) | ≥25 µm (SC3 and up); marine commonly 50 µm | ≥1,000 h to red rust; up to 2,000 h after diffusion | Marine, oil and gas, non-magnetic parts |
Is Black Oxide Enough for Outdoor Parts?
No — the 0.5–2 µm magnetite film gives only 24–72 h even with the supplementary oil or wax (two sources split the interval; both stay listed). Use it indoors, for appearance, or as a paint base. The phosphate rows only hold their hours with the specified oil in place — strip the supplement and the protection largely goes, which is why phosphate Class 3 (no supplement) has no salt-spray requirement at all.
| Process | Film | NSS with supplement | Supplement |
|---|---|---|---|
| Black oxide (MIL-DTL-13924) | 0.5–2 µm (≈0.76 µm magnetite) | 24–48 h (S26) / 24–72 h (S24) to red rust | Oil or wax required |
| Zinc phosphate, Type Z (MIL-DTL-16232G) | 2–15 µm | ≥72 h to red rust | Specified oil, Class 2 |
| Manganese phosphate, Type M (MIL-DTL-16232G) | Heavy film, accepted by coating weight | ≥48 h to red rust; DFL supplemental system 100 h | Specified oil, Class 2 |
Powder Coat or Anodize for an Outdoor Aluminium Enclosure?
Polyester powder at 60–120 µm typically reaches 500–1,000 h; Type II anodize is certified to 336 h. Powder is the thicker barrier, anodize the harder and dimensionally tighter finish. Powder acceptance runs on scribe creepage (ASTM D1654, typically ≤2–3 mm) rather than a pass or fail hour, and there is no single governing specification — architectural work commonly cites QUALICOAT.
| Resin family | Thickness (µm) | Typical NSS | Note |
|---|---|---|---|
| Epoxy | 60–120 | 1,000+ h | Highest hours of the three families |
| Polyester | 60–120 | 500–1,000 h | The common outdoor choice |
| Hybrid (epoxy-polyester) | 60–120 | 500–800 h | Balance of the two |
How Much Nickel Is Required Under Decorative Chrome?
Expect at least 20 µm nickel plus 0.25 µm chromium for SC4-grade decorative service, accepted through a 22–24 h CASS test (ASTM B368) rather than plain salt spray; single-layer 12 µm nickel manages about 24 h and a copper under-layer with multilayer nickel reaches 96 h. CASS hours for SC1–SC3 were not verifiable, so this page states SC4 only. Every disagreement in the sources stays registered below, unmerged.
| Conflict point | Source A | Source B | Handling on this page |
|---|---|---|---|
| Black oxide + oil salt-spray interval | 24–48 h (S26) | 24–72 h (S24) | Both intervals listed in the main table; size from the conservative 24 h |
| ASTM B633 salt-spray hours | Type II 96 h / Type V 72 h / Type VI 120 h to white rust (S10, S11, S12 agree) | Type I 48 h / Type II 240 h to red rust (S13, single source) | Table uses the three-source set; S13 is quarantined here, not in the tables |
| Zinc SC3 thickness | 12 µm (S10, S11, S12) | 13 µm (S15, rounding of 0.0005 in) | Table pins 12 µm; the rounding is noted in the footnotes |
| Type II anodize salt spray | Sealed 336 h per MIL-PRF-8625F (S1, S3, S4) | Up to 168 h (low-authority SEO page) | 336 h per the specification; the SEO figure is dropped |
| HDG salt-spray hours | Not specified by the standard — accepted by thickness grade (S14, S15, S16) | 500 h+ to red rust (S26, supplier data) | Table states standard-silent; the typical value is listed separately |
How to Use These Numbers
- Salt-spray hours are a screening number, not outdoor service life: ISO 10683 states the accelerated result cannot be directly related to service, and powder-coating practice treats cabinet hours as a comparative screening tool only.
- Read the endpoint before comparing rows: zinc-family hours default to white rust (the coating itself corroding — cosmetic); red rust means the substrate is corroding, the functional failure. Always ask which endpoint a certificate reports.
- The zinc-flake hour pairings follow the ISO 10683:2018 table as republished by S17; the 2014 edition pairs them differently (8 µm = 480 h) — treat the purchased standard text as the final verification.
- Zinc electroplating on steel at 1,000 MPa or higher (~31 HRC) requires a bake per ASTM B850, and above 1,700 MPa plating is prohibited; zinc-flake is non-electrolytic and carries no such risk — the default for high-strength fasteners.
- The electroless-nickel, phosphate and black-oxide hours are system values with the specified oil, wax or passivate supplement in place — strip the supplement and the bare film loses most of its resistance (phosphate Class 3 has no salt-spray requirement at all).
- Zinc SC3 is pinned at 12 µm here (some catalogues round 0.0005 in up to 13 µm); decorative Ni/Cr CASS acceptance is tabulated for service condition SC4 only (22–24 h) because SC1–SC3 hours could not be verified.
- Cost tier 1 (lowest) to 5 (highest) is an editorial rating compiled from published relative-cost indications — it is not a standard value and not a quote.
Frequently Asked Questions
My part is 6061 aluminum — which coating survives 500 hours of salt spray?
Sealed Type II anodize is certified to 336 h (MIL-PRF-8625F); for 500 h+ move to high-phosphorus electroless nickel ≥25 µm (typically 1,000 h+) or polyester powder (500–1,000 h).
Sealed Type II anodize is certified to 336 h (MIL-PRF-8625F); for 500 h+ move to high-phosphorus electroless nickel ≥25 µm (typically 1,000 h+) or polyester powder (500–1,000 h).
How much does hard anodizing add to my part dimensions?
A 50 µm Type III coating grows about 25 µm per face, so a fully coated diameter gains about 50 µm — mask threads and bearing fits before hardcoat.
A 50 µm Type III coating grows about 25 µm per face, so a fully coated diameter gains about 50 µm — mask threads and bearing fits before hardcoat.
What zinc plating thickness do I need outdoors?
ASTM B633 SC3 is 12 µm minimum for occasional rain and condensation; SC4 is 25 µm for saline or frequently wet service.
ASTM B633 SC3 is 12 µm minimum for occasional rain and condensation; SC4 is 25 µm for saline or frequently wet service.
Blue-white zinc or yellow zinc — which lasts longer?
Colored trivalent passivate (Type VI) is rated 120 h to white rust, hexavalent colored (Type II) 96 h and clear trivalent (Type V) 72 h — same zinc, different conversion coating.
Colored trivalent passivate (Type VI) is rated 120 h to white rust, hexavalent colored (Type II) 96 h and clear trivalent (Type V) 72 h — same zinc, different conversion coating.
Why specify zinc-flake coating on 12.9-class bolts?
It is non-electrolytic, so there is no hydrogen-embrittlement risk, and ISO 10683 rates 480–960 h to red rust at only 5–10 µm.
It is non-electrolytic, so there is no hydrogen-embrittlement risk, and ISO 10683 rates 480–960 h to red rust at only 5–10 µm.
Does hot-dip galvanizing need a salt spray certificate?
No — ASTM A123 accepts by coating grade (45–100 µm average depending on steel thickness), not cabinet hours; typical red-rust performance is 500 h+ anyway.
No — ASTM A123 accepts by coating grade (45–100 µm average depending on steel thickness), not cabinet hours; typical red-rust performance is 500 h+ anyway.
Can electroless nickel pass 1,000 hours?
Yes — high-phosphorus (above 10% P) at ≥25 µm typically reaches ≥1,000 h NSS to red rust; mid-phosphorus of the same thickness is usually only 48–96 h.
Yes — high-phosphorus (above 10% P) at ≥25 µm typically reaches ≥1,000 h NSS to red rust; mid-phosphorus of the same thickness is usually only 48–96 h.
Is black oxide enough for outdoor parts?
No — the 0.5–2 µm film gives only 24–72 h even with supplementary oil; use it indoors, for appearance, or as a paint base.
No — the 0.5–2 µm film gives only 24–72 h even with supplementary oil; use it indoors, for appearance, or as a paint base.
Powder coat or anodize for an outdoor aluminum enclosure?
Polyester powder at 60–120 µm typically reaches 500–1,000 h; Type II anodize is certified to 336 h — powder is the thicker barrier, anodize the harder, dimensionally tighter finish.
Polyester powder at 60–120 µm typically reaches 500–1,000 h; Type II anodize is certified to 336 h — powder is the thicker barrier, anodize the harder, dimensionally tighter finish.
How much nickel is required under decorative chrome?
At least 20 µm nickel plus 0.25 µm chromium for SC4-grade decorative service, accepted through a 22–24 h CASS test rather than plain salt spray.
At least 20 µm nickel plus 0.25 µm chromium for SC4-grade decorative service, accepted through a 22–24 h CASS test rather than plain salt spray.
Standards & Sources
- S1 — Fabdigit, anodizing Type II (MIL-PRF-8625F requirements digest)
- S2 — Fabdigit, hard anodizing Type III (MIL-A-8625F thickness, weight and salt-spray clauses)
- S3 — Micron Coatings anodizing guide (MIL-PRF-8625F / ISO 10074 336 h clauses)
- S4 — T&T Anodizing, MIL-A-8625F specification text page (336-hour clause)
- S5 — Micron Coatings, ASTM B733 class table (Type / SC / Class)
- S6 — First Mold, nickel plating (B733 Type I–V, SC classes, hydrogen-embrittlement clauses)
- S7 — MechDatum, high-phosphorus ENP 25 µm ≈ 1,000 h NSS
- S8 — Micro Plating, high-phosphorus ENP above 25 µm at 1,000 h+ field data
- S9 — Verbrugge, high-P 10–13%: 1,000 h as-plated, up to 2,000 h diffusion-treated
- S10 — Trojan Special Fasteners, ASTM B633 SC/Type table
- S11 — Triangle Manufacturing, ASTM B633 salt-spray hour data sheet (white rust)
- S12 — JMD Industries, B633 classes and salt-spray table
- S13 — Triplex Plating B633 interpretation (single-source conflict item, quarantined in the register)
- S14 — Indo East, ASTM A123 Table 1 coating grades
- S15 — Baowu Steel, A123 / EN ISO 1461 / GB/T 13912 thickness comparison
- S16 — HDG Community, A123 vs ISO 1461 grade finder
- S17 — TorqBolt, ISO 10683 body text (240 h at 4 µm through 960 h at 10 µm)
- S18 — ISO 10683:2018 official OBP catalogue (scope and NSS clause structure)
- S19 — Chaoshuo Trading, red-rust hour ranges by coating family
- S20 — Innovation Fasteners, MIL-DTL-16232: M+oil 48 h, Z+oil 72 h
- S21 — Fabdigit, zinc phosphate 2–15 µm, 24–72 h
- S22 — MnPhos, MIL-DTL-16232G PDF digest (DFL system, 100 h B117)
- S23 — Fabdigit, MIL-DTL-16232 Type/Class salt-spray requirement table
- S24 — Fabdigit, black oxide 24–72 h with oil or wax
- S25 — Acton Metal, black oxide film 0.000030 in (≈0.76 µm)
- S26 — RR Hydraulics coating comparison (black oxide 24–48 h, HDG 500+ h, Dacromet 720+ h)
- S27 — Huaxia Steel, powder coating 60–120 µm and per-resin salt-spray bands
- S28 — Sundial Powder Coating, salt-spray test interpretation (scribe-creepage criterion)
- S29 — Anrele, anodizing vs powder coating comparison
- S30 — Finishing.com nickel/chrome engineering thread (single-layer 12 µm Ni ≈ 24 h; copper under-layer + multilayer past 96 h)
- S31 — Electroplating & Environmental Protection journal reprint (ASTM B456-79 SC4: CASS 24 h, Cr 0.25 µm)
- Paid standards (ASTM B633 / B733 / A123, ISO 10683, MIL specs) were not purchased; every load-bearing value is cross-checked across three or more independent sources — treat purchased originals as the final verification
Related Pages
- /wiki/surface/coating/ — Long-form coating guide: process selection and specification practice
- /wiki/surface/anodizing/ — Anodizing in detail: Type II vs Type III, sealing and masking practice
- /wiki/reference/surface-finish-ra/ — Sibling quick table: surface roughness Ra for machined and finished parts