In a chemical plant, a instrumentation tube fitting is a small component with a large consequence. A single leaking connection can mean lost product, an unplanned shutdown, environmental release or a safety incident. Most fitting failures trace back to one root cause: the wrong material for the service.

We will cover the most widely used tube fitting materials, shows how they behave in common process chemicals, and gives you a repeatable method for choosing between them.

Where to use Tube Fittings

If Your Process Involves Start With
Mild chemicals, general service 316/316L stainless steel
Chlorides, brine, seawater Super duplex, 6Mo or Titanium
Hot hydrochloric acid Hastelloy C276, PTFE or PVDF
Sulfuric acid Alloy 20, Hastelloy C276
Strong caustic Alloy 400 or nickel alloys
Hydrofluoric acid Alloy 400 or PTFE
Oxidizing acids (nitric) 304L or 316L, Titanium for hot concentrated
High temperature and pressure with aggressive media Alloy 625 or C276

Material selection decides the life of every connection

Why Material Selection Matters

Chemical processing environments combine several stressors at once: aggressive media, elevated temperature, pressure cycling, vibration and sometimes chloride rich enviroments. The right fitting material must resist all of them for the intended service life.

Failure Mode What Happens Typical Cause
General corrosion Uniform thinning of the wall Acid or caustic attack on an under rated alloy
Pitting corrosion Localized deep pits that penetrate the wall Chlorides, especially on 304 or 316 stainless
Crevice corrosion Attack inside tight gaps such as under ferrules Stagnant chloride bearing fluid
Stress corrosion cracking (SCC) Sudden cracking under tensile stress Chlorides plus heat on austenitic stainless
Galvanic corrosion Accelerated attack on the less noble metal Dissimilar metals in a conductive fluid
Intergranular corrosion Attack along grain boundaries Sensitization from welding or heat on non low carbon grades
Permeation / swelling Polymer absorbs fluid and loses strength Incompatible plastic or elastomer

Key Factors to Consider Before Selecting a Material

Define the service conditions first. Use this checklist before requesting quotes.

Factor Questions to Answer Why It Matters
Media Which chemicals, at what concentration and purity? Corrosion resistance is chemical specific
Temperature Normal and upset operating range? Corrosion rates rise sharply with heat; polymers soften
Pressure Working pressure and pressure spikes? Determines wall thickness and fitting rating
Chlorides Any chloride in the process or the environment? Drives pitting, crevice corrosion and SCC
Flow conditions Velocity, stagnant zones, particles? Erosion corrosion and crevice risk
Cleanliness Any contamination or purity limits? Affects material and surface finish choice
Service life Expected years of operation? Balances upfront cost against replacement cost
Standards NACE, ASTM, ASME, PED requirements? Compliance and traceability

Metallic Tube Fitting Materials

Material Typical Grades Key Strengths Key Limitations Relative Cost
Stainless steel 316 tube fittings UNS S31600 / S31603 Broad chemical compatibility, widely available, good strength Vulnerable to chlorides and hot acids Medium
Stainless steel 304 instrumentation fittings UNS S30400 / S30403 Lower cost, good for mild media Poorer pitting resistance than 316 Low – Medium
Duplex 2205 Tube Fittings UNS S32205 High strength, better chloride SCC resistance than 316 Limited above about 300 °C (embrittlement) Medium – High
Super duplex 2507 Tube Fittings UNS S32750 Excellent chloride resistance, very high strength Harder to machine, higher cost High
SMO 254 Tube Fittings UNS S31254 Outstanding pitting and crevice resistance Cost, limited supplier range High
Alloy 20 Tube Fittings UNS N08020 Designed for sulfuric acid service Less versatile outside acid duty High
Monel Tube Fittings UNS N04400 Hydrofluoric acid, seawater, alkalis Poor in oxidizing acids, ammonia compounds can crack it High
Inconel 625 Instrumentation Fittings UNS N06625 Excellent all-round resistance, high strength, high temperature High cost Very High
Hastelloy C276 Instrumentation Tube Fittings UNS N10276 Superior in mixed acids, wet chlorine, oxidizing and reducing media Highest cost of common alloys Very High
Titanium Grade 2 Tube Fittings UNS R50400 Superb in chlorides, oxidizing acids, wet chlorine Poor in reducing acids and dry chlorine; galling Very High
Brass C36000 / C37700 Low cost, easy to machine Attacked by ammonia, acids and many process fluids Low
Carbon steel Various Cheapest, strong Corrodes quickly in wet or acidic service Low

Pitting Resistance at a Glance

The Pitting Resistance Equivalent Number is a quick way to rank alloys for chloride environments. Higher is better. Values are approximate and depend on actual composition.

Alloy Approximate PREN Suitability for Chloride Service
304 18 – 20 Poor
316 23 – 26 Limited
Duplex 2205 34 – 36 Good
Super duplex 2507 41 – 43 Very good
6-Moly (6Mo) 43 – 47 Very good
Alloy 625 50+ Excellent
Alloy C276 65+ Excellent

Corrosion Resistance Guide by Chemical

These ratings are general guidance at moderate temperature. Concentration, temperature and impurities change results
dramatically, so always check against a current corrosion database or manufacturer data.

Key: E = Excellent, G = Good, L = Limited (check conditions), P = Poor / not recommended

Chemical 304 SS 316 SS Duplex 2205 Alloy 400 Alloy 625 C276 Titanium Brass
Sulfuric acid (dilute, cold) L G G G E E P P
Sulfuric acid (concentrated) P L L L G E P P
Hydrochloric acid P P P L G E L P
Nitric acid E E E P E G E P
Phosphoric acid G G G G E E G P
Acetic acid G E E G E E E L
Sodium hydroxide (caustic) G G G E E E L P
Ammonia / ammonium hydroxide G G G P E E G P
Seawater / brine P L G E E E E L
Wet chlorine P P L L G E E P
Hydrofluoric acid P P P E G G P P
Organic solvents (alcohols, ketones) E E E E E E E G
Hydrocarbons E E E E E E E G
Hydrogen sulfide (sour service) L L G L E E G P

Seal and Ferrule Materials

The fitting body is only half the story. Elastomer seals and ferrule coatings must also survive the process fluid.

Seal Material Temp Range (approx.) Good With Avoid With
FKM (Viton) -20 to 200 °C Oils, fuels, many acids, hydrocarbons Ketones, amines, hot steam
FFKM (Kalrez, Chemraz) -20 to 260 °C Nearly all aggressive chemicals Few; very high cost
EPDM -50 to 150 °C Hot water, steam, alkalis, many polar solvents Oils, hydrocarbons
PTFE (encapsulated or solid) -200 to 260 °C Virtually all chemicals Cold flow under load; needs good design
Nitrile (NBR) -30 to 100 °C Oils, fuels, water Strong oxidizers, ozone, ketones
Silicone -60 to 200 °C Clean, dry heat Steam, strong solvents, acids

Fitting Types and Why They Matter

Fitting Type Description Best Applications Material Considerations
Double ferrule compression Front and back ferrule grip and seal the tube General process and instrumentation lines Ferrule hardness should match or exceed tube hardness
Single ferrule compression One ferrule does both jobs Lower-pressure lines Simpler but less vibration resistant
Face seal (VCR-type) Metal gasket compressed between beads High-purity, vacuum, toxic gases Gasket material must suit the fluid
Welded (butt or socket weld) Permanent joint Leak-critical, hazardous service Use low-carbon or stabilized grades to avoid sensitization
Flared (37° / 45°) Tube end flared against a cone Hydraulic and high-pressure systems Needs ductile tube material
Threaded NPT Taper threads with sealant Secondary connections Prone to crevice and galling; avoid in severe service

Cost vs. Lifecycle Value

Material Upfront Cost Index (316 SS = 1.0) Typical Service Life in Suitable Service Lifecycle Comment
Carbon steel 0.3 – 0.5 Short in corrosive media Frequent replacement, higher leak risk
Brass 0.5 – 0.8 Short – Medium Only for benign media
304 SS 0.8 Medium Economical for mild service
316/316L SS 1.0 Long Default choice for most plants
Duplex 2205 1.5 – 2 Long Savings where chlorides are present
Super duplex / 6Mo 2.5 – 4 Very long Justified in seawater and brine
Alloy 400 4 – 6 Long Justified in HF and caustic
Alloy 625 6 – 10 Very long For severe or high-temperature duty
C276 8 – 12 Very long Reserve for the harshest media
Titanium 6 – 10 Very long Excellent for chlorides and oxidizers
PTFE / PFA 2 – 6 Long (within temperature and pressure limits) Cost effective for hot acids

Installation and Maintenance Best Practices

Practice Benefit
Use tube with the same or lower hardness than the fitting ferrules Proper ferrule bite and reliable seal
Cut tubes square and deburr the ends Prevents leaks and ferrule damage
Follow manufacturer turns-from-finger-tight instructions Avoids over- or under-tightening
Use a gap inspection gauge Confirms the fitting is properly made up
Avoid reusing ferrules after multiple remakes Prevents leaks from fatigued ferrules
Keep fittings clean and protected before installation Avoids contamination and thread damage
Support tubing to limit vibration Reduces fatigue at the connection
Inspect periodically for weeping, discoloration and corrosion Catches failures before they escalate

Common Mistakes to Avoid

Mistake Consequence Fix
Assuming 316 SS suits every chemical Pitting, cracking or rapid corrosion Check chemical specific data
Ignoring trace chlorides Unexpected SCC or crevice attack Evaluate chloride level and temperature
Mixing brands of ferrules and nuts Leaks and unsafe joints Use components from a single manufacturer
Overlooking seal compatibility Swelling, cracking and leakage Choose elastomers by media, not habit
Choosing on price alone High replacement and downtime costs Use lifecycle cost analysis
Skipping documentation Compliance and traceability gaps Require MTRs and certificates

Conclusion

There is no single best tube fitting material for chemical processing. The best choice is the one that matches your specific media, temperature, pressure and lifecycle expectations. For most general purpose applications, 316/316L stainless steel remains the default. When chlorides, strong acids or extreme purity are concerns, step up to duplex, nickel alloys, titanium or fluoropolymers, and make sure that seals, ferrules and fitting design are matched to the same standard.

If you are looking to buy tube fittings in any material or size, please feel free to contact us on sales@fit-lok.com