Guide to Material Selection for Instrumentation Process Connections

Comprehensive Analysis of Material Types for Instrumentation Process Connections

The material of instrumentation process connections directly affects the system's corrosion resistance, temperature and pressure resistance, and service life. Reasonable material selection requires comprehensive consideration of media characteristics, operating parameters, industry standards, and cost. This article systematically reviews three major categories of materials—metals, non-metals, and special alloys—to provide reference for engineering selection.

I. Metal Materials (Most Commonly Used)

1. Carbon Steel

Carbon steel has high strength and low cost, but poor corrosion resistance, and requires galvanizing or painting. Its applicable temperature range is -30°C to 450°C, and it is widely used in ordinary industrial pipelines for water, air, steam, etc.

• Typical grades: ASTM A105, A350 LF2; EN 10222-2 P235GH; GB/T 699 20#; JIS S20C
• Pressure rating: Class 150 to Class 600 (PN10 to PN40)
• Standards and specifications: ASME SA-105, EN 10222-2, GB/T 12459

2. Stainless Steel

(1) Austenitic Stainless Steel

Austenitic stainless steel has excellent corrosion resistance, is non-magnetic, and is easy to machine. It is the most commonly used material series for instrument process connections. 304 is suitable for general corrosive environments, 316 is resistant to chloride ions due to its molybdenum content, 316L has low carbon content and resists intergranular corrosion, and 317L resists strong corrosion.

• 304 (ASTM A351 CF8): resistant to general corrosion, temperature resistance ≤800°C
• 316 (ASTM A351 CF8M): resistant to seawater and salt solutions, temperature resistance ≤1200°C
• 316L (ASTM A351 CF3M): strong resistance to intergranular corrosion after welding
• 317L: resistant to strong corrosion such as sulfuric acid and phosphoric acid
• Standards and specifications: ASME SA-351, EN 10088-3, GB/T 12771

(2) Duplex Stainless Steel

Duplex stainless steel has twice the strength of austenitic steel and resists stress corrosion cracking. It is suitable for marine engineering and petrochemical media containing Cl⁻ and H₂S. 2205 has better pitting corrosion resistance than 316, and 2507 is used in extremely corrosive environments.

• Typical grades: 2205 (UNS S32205), 2507 (UNS S32750)
• Standards and specifications: ASTM A890, EN 10213-6, GB/T 20878

3. Copper and Copper Alloys

Copper and copper alloys have good thermal conductivity and low-temperature resistance, and are suitable for low-temperature instruments, gas sampling lines, and instrument air supply lines. Pure copper withstands -200°C low temperatures, brass has higher strength, and bronze resists seawater corrosion.

• Pure copper (Cu≥99.9%): good thermal conductivity, low-temperature resistance
• Brass (H62, H68): resistant to atmospheric corrosion, used for low-pressure pipelines
• Bronze (QSn6.5-0.1): wear-resistant, seawater-resistant, used for marine instruments
• Standards and specifications: ASTM B88, GB/T 1527

4. Aluminum and Aluminum Alloys

Aluminum and aluminum alloys are lightweight, have good thermal conductivity, and resist atmospheric corrosion, but have relatively low strength. 6061 and 6063 aluminum alloys have improved strength after adding Mg and Si, and are used in aerospace instruments and low-temperature equipment.

• Pure aluminum: 1060, 1100
• Aluminum alloys: 6061, 6063
• Standards and specifications: ASTM B221, GB/T 4437.1

5. Nickel-Based Alloys

Nickel-based alloys resist strong corrosion and high temperatures, and are suitable for concentrated acids in chemical engineering, high-temperature and high-pressure water in nuclear power, and high-temperature aerospace components. Monel 400 resists hydrofluoric acid, Hastelloy C-276 resists aqua regia, and Inconel 625 resists high-temperature oxidation.

• Monel 400: resistant to hydrofluoric acid and seawater, ≤815°C
• Hastelloy C-276: resistant to concentrated hydrochloric acid and sulfuric acid, ≤1200°C
• Inconel 625: resistant to high-temperature oxidation and corrosion
• Standards and specifications: ASTM B564, GB/T 15007

6. Titanium and Titanium Alloys

Titanium and titanium alloys have high specific strength and resist seawater, chloride ions, and nitric acid corrosion, but their cost is about 3 to 5 times that of 316L. Pure titanium TA1/TA2 is used in marine engineering, and TC4 is used in aerospace.

• Pure titanium: TA1, TA2 (ASTM B338 Gr.1/Gr.2)
• Titanium alloy: TC4 (Ti-6Al-4V)
• Standards and specifications: ASTM B338, GB/T 3624

II. Non-Metallic Materials

1. Plastics

Plastics are corrosion-resistant, insulating, and lightweight, and are suitable for low-pressure and corrosive media. PE resists weak acids, PP resists acids and bases up to 90°C, PVC has low cost but poor temperature resistance, PVDF resists aqua regia, and PPS resists high temperatures up to 200°C.

• PE: instrument sampling tubes, FDA compliant
• PP: chemical pipelines, flange PP-H/PP-B
• PVC: water supply and drainage, ≤60°C
• PVDF: semiconductors, lithium batteries, -40°C to 150°C
• PPS: instrument fittings for high-temperature corrosive environments
• Standards and specifications: ASTM D1785, GB/T 18742, ISO 161-1

2. Rubber

Rubber is mainly used for seals and hoses. NBR resists oil, FKM resists high temperatures and chemicals, and VMQ resists low temperatures and weathering and is FDA compliant.

• NBR: O-rings, sealing gaskets
• FKM: -20°C to 200°C, resistant to acids and solvents
• VMQ: -60°C, food and pharmaceutical sealing
• Standards and specifications: ASTM D2000, GB/T 5720

3. Ceramics

Ceramics resist high temperatures and corrosion and have good insulation. Alumina ceramics withstand 1600°C, and silicon carbide ceramics have high strength and high thermal conductivity, and are used for high-temperature sensor connections and slurry pipeline instruments.

• Al₂O₃: thermocouple protective sleeves
• SiC: high-pressure wear-resistant environments
• Standards and specifications: ISO 6872, GB/T 5593

4. Graphite

Graphite resists high temperatures above 3000°C and is chemically resistant (except to strongly oxidizing acids), and is used for high-temperature sealing gaskets. Impregnated graphite has improved strength and is used for instrument connections in corrosive liquid pipelines.

• Pure graphite: flexible graphite gaskets
• Impregnated graphite: corrosive liquid pipelines
• Standards and specifications: ASTM D5656, GB/T 30747

III. Special Alloys and Composite Materials

1. Zirconium Alloys

Zirconium alloys resist strong corrosion (hydrochloric acid, sulfuric acid, nitric acid) and are superior to Hastelloy, but their cost is extremely high (about 10 times that of 316L). They are used for instrument flanges in the nuclear industry and concentrated hydrochloric acid environments.

• Standard: ASTM B551

2. Composite Materials

FRP/GRP is lightweight and corrosion-resistant, and is used for instrument connections in low-pressure large-diameter pipelines; CFRP has high strength and high-temperature resistance, and is used for lightweight aerospace instruments.

• Standards: ISO 10414, GB/T 1447

IV. Materials for Special Industry Requirements

1. Food and Pharmaceutical Industry (Sanitary Grade)

Must comply with 3-A standards (Ra≤0.8μm), FDA 21 CFR 177.2600, EHEDG, and GB 14880. 316L electropolished, food-grade silicone, and PTFE are commonly used.

2. Explosion-Proof Environments

Aluminum alloy (EN 1706 AlSi9Cu3) is used for explosion-proof instrument housings, and cast steel (ASTM A216 WCB) is used for high-pressure explosion-proof flanges.

3. Low-Temperature Environments (below -200°C)

Austenitic stainless steel 304L/316L has no low-temperature brittleness and complies with ASME B31.3; copper-nickel alloys Cu-Ni 90/10 and 70/30 are used for instrument connections in LNG receiving stations.

V. Core Basis for Material Selection

Summary

In global industrial automation instrumentation process connections, stainless steel (304/316L) is the most widely used material, covering most conventional operating conditions; extreme environments rely on nickel-based alloys, duplex steel, and titanium alloys; non-metallic materials (such as PVDF and rubber) are used in corrosion-resistant or lightweight scenarios; special industries (food, nuclear power) must additionally meet hygiene or safety standards. During selection, media, temperature, pressure, and industry specifications must be combined to ensure the safety, reliability, and economy of the connection.