The Short Answer
Match the valve body material to the media corrosivity, design temperature and pressure, and cost target. For water, steam, and hydrocarbons: cast carbon steel (WCB) covers the vast majority of process duty. For corrosive chemicals and chloride-containing media: 316 stainless (CF8M) or higher alloys. For severe corrosive, high-temperature, or sour service: alloy steels (WC6/WC9), duplex, Alloy 20, or Hastelloy. When in doubt, use a corrosion data source (e.g., NACE, manufacturer corrosion guides) and cost the material against the valve’s expected life — a cheap body that fails early is the most expensive choice.
The Material Groups at a Glance
| Material (Cast Designation) | Typical Service | Key Strengths | Limits |
|---|---|---|---|
| Cast carbon steel (WCB) | Water, steam, hydrocarbons, general process | Economical, strong, weldable, rated to ~425°C | Poor corrosion resistance; needs coating/liner for corrosive media |
| Low-temperature carbon steel (LCB/LCC) | Cold climates, cryogenic gas, low-temp service | Tough at low temperature (to ~−46°C) | Same corrosion limits as WCB |
| Chrome-moly alloy (WC6/WC9) | High-temperature steam, hydrogen service | Strength at high temperature (to ~590°C) | Higher cost; needs PWHT; not corrosion-resistant to acids |
| 316 stainless (CF8M) | Corrosive chemicals, chlorides (moderate), food, pharmaceuticals | Excellent general corrosion resistance | Chloride stress corrosion cracking above ~60°C; pitting in hot chlorides |
| 304 stainless (CF8) | Less corrosive service, clean media | Cheaper than CF8M | Lower corrosion margin than CF8M |
| Duplex / super-duplex (CD3MN, CE3MN) | Seawater, chlorides, offshore, high strength | High strength + chloride resistance | Limited temperature range; sensitive to heat treatment |
| Alloy 20 (CN7M) | Sulfuric acid, aggressive chemicals | Broad acid resistance | Expensive; limited availability in all sizes |
| Hastelloy C (CW2M/CW12MW) | HCl, wet chlorine, extreme corrosives | Extreme corrosion resistance | Very expensive, difficult to cast |
| Titanium (CZ100) | Seawater, chlorides, oxidizing media | Excellent seawater resistance | Expensive; pyrophoric risk in some media |
| Cast iron / ductile iron | Water, air, utilities, non-critical | Lowest cost | Not for process hydrocarbons, shock, or low temperature |
How to Choose — Step by Step
Step 1: Define the Duty
Write down: media (with concentrations and impurities), operating and design temperature, design pressure, flow velocity, and any special conditions (sour gas, chlorides, erosion, thermal cycling). The body material choice starts from this sheet.
Step 2: Screen by Corrosion
For common media:
- Water and steam: carbon steel (or cast/ductile iron for low-pressure water utilities).
- Hydrocarbons (sweet): carbon steel WCB is standard; use LCB below about −29°C.
- Sour gas (H2S): carbon steel complying with NACE MR0175/ISO 15156; trim and hardness controlled.
- Chlorides and seawater: 316 SS for moderate duty; duplex or super-duplex for hot chlorides and offshore; titanium or high-nickel alloys for severe seawater.
- Acids: match the specific acid and concentration — sulfuric acid commonly uses Alloy 20 or ductile iron (concentrated); HCl requires high-nickel alloys or lined valves.
Step 3: Check the Temperature Limits
Use the ASME B16.34 rating tables for each material group:
- WCB: to ~425°C (800°F) in B16.34 ratings.
- WC9: to ~590°C (1100°F).
- 316 SS: to ~815°C in B16.34, but practical limits are set by corrosion and oxidation, not just strength.
- Duplex: typically −50°C to +300°C (check the standard); above ~300°C the material embrittles in service.
Step 4: Match the Trim
The body is only half the story. Trim (seat, disc, stem) must resist corrosion and erosion at least as well as the body:
- WCB body + 13Cr trim = standard for water/steam/hydrocarbons.
- CF8M body + CF8M or hardened trim = corrosive service.
- For erosive service, hard-face the seat (Stellite) regardless of body material.
- For bubble-tight shutoff, soft seats (PTFE/PEEK) suit the temperature limit of the seat material, not the body.
Step 5: Cost the Lifecycle
Body material cost rises roughly: cast iron < WCB < LCB < CF8 < CF8M < WC6/WC9 < duplex < Alloy 20 < Hastelloy < titanium. For a 10–20 year plant life, the cheapest material that meets corrosion and temperature requirements is almost always the right choice. Only step up to premium alloys when the media demands it — not "for safety."
Common Mistakes
- 316 SS for hot chlorides. Stress corrosion cracking can occur above ~60°C in chlorides; use duplex or higher.
- WCB for sour service without NACE. Sulfide stress cracking can fail the valve catastrophically; hardness and material must comply with NACE MR0175.
- Cast iron for hydrocarbons or shock service. Cast iron fails by brittle fracture; process piping uses steel.
- Ignoring trim. A stainless body with a carbon-steel trim is not corrosion-resistant — trim must match or exceed the body’s resistance.
- Choosing premium alloys without a corrosion basis. Specify alloys from corrosion data or a corrosion specialist’s recommendation, not habit.
Lined Valves — An Alternative
For severe corrosives, lined valves (PTFE, PFA, or rubber-lined bodies) offer corrosion resistance at lower cost than solid high alloys, especially in large sizes. Limits: lined valves have temperature limits (~180–200°C for PTFE), are not for slurries with abrasives, and can have permeation issues with some media. Consider them when the media is corrosive but temperature and abrasion are moderate.
Conclusion
Body material selection is a corrosion problem, a temperature problem, and a cost problem. Start from the duty, screen by corrosivity, verify the temperature rating in ASME B16.34, match the trim, and cost the lifecycle. For standard process duty, WCB covers most cases; step up to CF8M, duplex, or high alloys only where the media justifies it — and always document the corrosion basis for the choice.