Corrosion Resistance Gray Ductile Iron
Who this helps: Design Engineers / Buyers specifying gray/grey & ductile (SG) iron parts for outdoor use, chemicals, salt, humidity or wash-down.
What you’ll get: a practical coating-selection matrix (with DFT/µm & mil), when material upgrades beat paint, design rules that stop under-film rust, plus copy-paste drawing notes.
Prepared by YB Metal Solution. Share your drawing via /rfq—YB Metal will return a part-specific coating stack, material option and test plan.
Author: YB Metal Solution Engineering Team (hereafter YB Metal)
Table of contents
- Corrosion basics for cast irons
- When to solve it with coatings vs material
- Real-world coating stacks (with DFT & use cases)
- Material options & where they win
- Design for durability: geometry, edges & galvanic pairs
- Surface prep, Ra & adhesion checks
- What to put on the drawing (copy–paste)
- What YB Metal delivers
- FAQs
Corrosion basics for cast irons
- Graphite effect: Graphite acts cathodic to iron; in wet electrolytes it can form micro-galvanic cells, accelerating local iron loss. Gray iron (flakes) exposes more area than ductile (nodules), so gray tends to rust faster in the same environment.
- Matrix matters: More pearlite/carbides can be slightly more corrosion-resistant than purely ferritic matrices, but surface condition dominates.
- Defects drive failure: Porosity, burn-on and sand inclusions trap moisture → under-film rust. See: Surface Finish (Ra) guide and Sand Inclusion & Burn-On for prevention.
When to solve it with coatings vs material
- Pick coatings when the environment is atmospheric (C2–C4), intermittent splash, or you can maintain paint.
- Pick material upgrades when media is continuous (seawater, hot brine, acids/alkalis), or the part cannot be coated or maintained.
- Often the most robust is both: modest material improvement (Cu/Ni-bearing DI or Ni-resist for chloride service) + duplex coating.
Real-world coating stacks (with DFT & use cases)
Values are typical; set exact test & DFT during PPAP. Units show µm / mil (1 mil = 25.4 µm).
| Environment & need | Recommended stack | Total DFT |
|---|---|---|
| Indoor dry / C1–C2 (machinery frames) | Phosphate + powder coat (polyester/epoxy-polyester). | 60–90 µm / 2.4–3.5 mil |
| Outdoor general / C3 (transport housings) | Zinc-rich epoxy primer (60–80 µm / 2.4–3.1) + epoxy mid (80–120 µm / 3.1–4.7) + PU top (40–60 µm / 1.6–2.4). | 180–260 µm / 7–10 mil |
| High salt / C4–C5 (coastal, road salt) | Zinc-rich epoxy + high-build epoxy + aliphatic PU (or fluoropolymer) top; consider duplex e-coat + powder. | 220–320 µm / 9–13 mil |
| Wash-down / chemicals | Epoxy primer + novolac epoxy or epoxy + vinyl ester system; seal crevices. | 200–300 µm / 8–12 mil |
| Complex geometry / inside cavities | E-coat (cathodic) for coverage + powder top for UV. | 25–35 µm e-coat + 60–90 µm powder |
| Touch-up / field repair | Zn-rich touch-up + 2-pack epoxy/PU; clean to Sa2½ equivalent. | As specified |
Prep & blast profile: target Ra 6–12 µm (240–475 µin) after shot-blast (S280–S330) for paint/powder. Avoid over-polish that kills adhesion.
Material options & where they win
| Option | Where it helps | Notes |
|---|---|---|
| Standard gray iron (EN-GJL / ASTM A48) | Indoor, painted exterior | Easiest to machine; relies on coating for life. |
| Standard ductile iron (EN-GJS / ASTM A536) | Slightly better base resistance than GI; higher strength | Good default for painted outdoor parts. |
| Cu/Ni-bearing ductile iron | Splash seawater, de-icing salts | Minor alloy adds base resistance; still paint for long life. |
| Austempered DI (ADI) | Strength/fatigue ↑; corrosion similar to DI | ADI is not a corrosion upgrade by itself—still coat. |
| High-Si irons (SiMo) | High-temperature oxidation (600–900 °C) | Not a general wet-corrosion solution. |
| Ni-Resist (austenitic cast iron) | Seawater/hot brine; some chemicals | Cost ↑↑; lower modulus; use for media-wetted internals. |
| Cast steel + coating | Shock/impact + heavy corrosion | Often heavier machining cost; coat as for iron. |
Design tip: if media is aggressive only inside the cavity, consider unpainted exterior + corrosion-resistant liner or Ni-resist insert internally to control cost.
Design for durability: geometry, edges & galvanic pairs
- Edges & radii: break/shamfer R 0.5–1.0 mm (0.02–0.04 in); sharp edges thin the coating.
- Drain/vent: add weep holes to avoid liquid traps; slope flats ≥ 2–3° for drainage.
- Crevices: avoid tight lap joints; specify sealant where unavoidable.
- Galvanic couples: isolate iron from stainless/aluminum with non-conductive gaskets/washers; use compatible fasteners and seal threads.
- Hangers & masking: provide hanging points and mask areas (fits/threads) on the drawing.
Surface prep, Ra & adhesion checks
- Blast to clean metal; remove burn-on/silica (these undercut paint).
- Target Ra: 6–12 µm profile for paint; 1.6–3.2 µm after machining on sealing faces (do not coat).
- Tests to call out (as needed): dry film thickness (DFT), cross-cut adhesion, impact, salt spray (neutral, ISO 9227/ASTM B117) or better cyclic tests, holiday (pinhole) test for thick films, gloss & color Δ.
Related articles on your site:
What to put on the drawing (copy–paste)
Coating stack (example for C3/C4):
- Prep: shot-blast to clean metal; blast profile Ra 6–12 µm (240–475 µin).
- Primer: zinc-rich epoxy, DFT 60–80 µm (2.4–3.1 mil).
- Mid-coat: high-build epoxy, DFT 80–120 µm (3.1–4.7 mil).
- Topcoat: aliphatic polyurethane, DFT 40–60 µm (1.6–2.4 mil).
- Color/Gloss: per spec; edges broken R0.5–1.0 mm (0.02–0.04 in).
- Mask: threads, fits, sealing faces (see view).
Inspection & tests:
- DFT at 5+ locations/part; cross-cut adhesion (per method); salt-spray ISO 9227 (hours per spec) or customer cyclic test; visual—no holidays/rust.
- Provide batch certificates for coating materials and blast media cleanliness.
Material call-out (if upgrading):
- Ductile iron EN-GJS-500-7 (ASTM A536 80-55-06) with Cu/Ni window for corrosion resistance.
- Alternative (media-wetted only): Ni-Resist austenitic cast iron, grade per service.
What YB Metal delivers
YB Metal Solution quotes with a corrosion plan attached:
- A coating selection matched to your environment (DFT numbers, cure, adhesion tests).
- Surface prep & blast profile setup, plus masking & hanging instructions.
- If required, material upgrade options (Cu/Ni DI or Ni-resist) with cost/lead-time impact.
- Pilot evidence pack: DFT logs, adhesion photos, salt-spray/cyclic results, and coated-part dimensional check.
Need a plan for your environment? Upload your drawing—we’ll return recommendations and a quote.
FAQs
CTA — specify with proof, not guesses
Pick the right coating stack (and material only when it pays). Upload your drawing to /rfq and YB Metal will send a corrosion plan, test matrix and quotation.