| 1. Match the flange material to the pipe material | Carbon steel | Carbon-steel water, air, oil, and general industrial pipelines when the internal and external corrosion controls are defined. | Moderate corrosion resistance. | Suitable for many ambient and elevated-temperature services when the design code, gasket, coating, and pressure rating are verified. | Use compatible carbon-steel pipe, bolts, and welding consumables. Specify an internal lining or external coating when moisture, soil, or corrosive chemicals are present. |
| 2. Check chloride exposure before selecting stainless steel | 304 stainless steel | Fresh water, indoor process lines, food-related services, and mildly corrosive atmospheric environments. | Can suffer pitting and crevice corrosion in chloride-rich water, marine atmospheres, and stagnant wet areas. | Often used for moderate-temperature service, subject to pressure, gasket, and code limitations. | Do not select 304 solely because it is stainless. Consider a more chloride-resistant alloy when salt, brackish water, or chemical cleaners are present. |
| 3. Prefer molybdenum-bearing stainless steel for wet and saline service | 316 or 316L stainless steel | Brackish water, many chemical-processing environments, outdoor wet service, and systems exposed to moderate chlorides. | Better resistance to pitting and crevice corrosion than 304 because of molybdenum content, but not immune to chloride attack. | Suitable for a broad range of industrial temperatures when the complete piping system is rated accordingly. | Use 316L where welding is extensive or where reduced risk of weld-related sensitization is important. Confirm chloride concentration and temperature. |
| 4. Evaluate high-strength alloys for demanding chloride environments | Duplex stainless steel, such as 2205 | Seawater-related systems, high-chloride process lines, desalination equipment, and applications requiring high strength. | Generally higher resistance to chloride stress-corrosion cracking and pitting than common austenitic stainless steels, when correctly fabricated. | Temperature limits depend strongly on design code, phase balance, and service conditions; avoid assuming that higher strength eliminates temperature restrictions. | Choose duplex material when both corrosion resistance and mechanical strength are required. Require controlled welding procedures and proper heat treatment practices. |
| 5. Verify non-metallic compatibility with the fluid and loads | PVC or other thermoplastic flange systems | Many ambient-temperature water, dilute chemical, and wastewater applications, subject to the specific resin and chemical concentration. | Excellent resistance to many aqueous chemicals, but vulnerable to solvents, strong oxidizers, ultraviolet exposure, impact, and creep. | Lower temperature capability than metallic systems; the pressure rating normally decreases as temperature rises. | Use only when the pipe, flange, gasket, bolts, support system, and operating temperature are all rated for the service. Do not use as a direct substitute for metal in high-temperature or high-load anchoring. |
| 6. Prevent galvanic corrosion at dissimilar-metal joints | Mixed-metal assembly | Systems combining carbon steel, stainless steel, copper alloys, or other metals in the same wet electrical environment. | A less noble metal can corrode faster when electrically connected to a more noble metal in the presence of an electrolyte. | Risk increases with conductivity, moisture, temperature, exposed cathode-to-anode area ratio, and coating damage. | Use compatible bolts and washers, insulating kits where appropriate, protective coatings, dielectric separation, and drainage. Confirm that the isolation method does not compromise grounding or safety requirements. |
| 7. Confirm gasket, bolt, coating, and soil compatibility | Complete flange assembly | Buried pipelines, submerged lines, above-ground process piping, and systems subject to external moisture or chemical exposure. | Flange material alone does not determine service life. Gaskets, fasteners, coatings, welds, crevices, and damaged surfaces can become the primary corrosion points. | Verify the temperature range of the gasket and coating, as well as bolt relaxation and thermal expansion effects. | Check fluid chemistry, pH, chlorides, dissolved oxygen, soil resistivity, stray current, pressure, temperature, and maintenance access before final selection. |
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