Material selection goes wrong most often when it starts with a shortlist of alloys rather than with the conditions the metal has to survive. Here is the order the decisions should actually be made in.
Material selection goes wrong most often when it starts at the wrong end — with a shortlist of alloys rather than with the conditions the metal has to survive. Before any grade is considered, five pieces of process data must be on the table:
To that add the mechanical duty: pressure, cyclic loading, erosion from entrained solids, and the design life. A material that resists the chemistry but fatigues in three years has not solved the problem.
Corrosion is not one phenomenon. The mechanism determines the countermeasure.
| Mechanism | Trigger | Typical countermeasure |
|---|---|---|
| General / uniform corrosion | Acid or alkali attack across the whole surface | Corrosion allowance, or a more resistant alloy |
| Pitting | Chlorides breaking the passive film locally | Higher PREN — 316, duplex, super-austenitic, titanium |
| Crevice corrosion | Stagnant chemistry under gaskets, deposits, lap joints | Design out the crevice; raise PREN |
| Stress corrosion cracking | Tensile stress + chlorides + temperature above ~60 °C | Duplex, ferritic, nickel alloy or titanium; stress relief |
| Intergranular attack | Sensitisation from welding or slow cooling | Low-carbon (L) grades, stabilised grades, solution anneal |
| Galvanic corrosion | Dissimilar metals in electrical contact in an electrolyte | Isolate, or select metals close on the galvanic series |
| Erosion-corrosion | High velocity flow, entrained solids or droplets | Harder alloy, thicker wall, velocity limits, flow redesign |
| Sulphide stress cracking | Wet H2S with susceptible microstructure | NACE MR0175 compliant material, hardness control |
A material choice that solves general corrosion but ignores chloride stress corrosion cracking will pass every acceptance test and fail in service. Identify the mechanism before you compare alloys.
Start with the cheapest material that could plausibly work and move up only when the data says you must. Over-specification is expensive; under-specification is worse.
Still the correct answer for a great many duties: hydrocarbons, dry gas, non-aggressive water, steam. A 3 mm corrosion allowance and periodic thickness surveys are often far cheaper over plant life than an alloy upgrade. Carbon steel fails only where the corrosion rate is unpredictable or where product contamination matters.
Chromium and molybdenum additions improve high-temperature strength and resistance to hydrogen attack and sulphidation. This is the standard family for refinery hydroprocessing and elevated-temperature service. They are not corrosion-resistant alloys in the aqueous sense.
The first genuinely corrosion-resistant step. 304 for clean water, steam and mild atmospheres; 316 where chlorides are present up to roughly 1,000 ppm at ambient temperature. Use the L grades where welded sections will not be solution annealed. Both remain vulnerable to chloride SCC above approximately 60 °C.
A mixed austenite-ferrite microstructure delivers roughly double the yield strength of 316 with markedly better chloride SCC resistance and higher PREN. The trade-off is strict welding control and a maximum service temperature around 300 °C to avoid embrittlement.
Alloy 625, 825, C-276 and similar handle severe reducing acids, high-temperature chlorides and mixed acid service where stainless steels cannot survive. They are expensive and are usually justified only for the worst service or as a clad layer over a carbon steel backing.
In oxidising and chloride environments, titanium Grade 2 is effectively immune. In hot seawater, in chlorine dioxide bleaching, in desalination brine heaters and in hypochlorite service it frequently outperforms alloys that cost more. Its weakness is strongly reducing acids — hot hydrochloric or sulphuric — and dry chlorine gas, where it can ignite.
Record why the material was chosen, against which process data, and what the assumed corrosion rate and design life are. Five years later, when someone proposes a cheaper substitution, that record is what prevents an expensive mistake being repeated.
Bring the process data to the enquiry rather than only a grade. If you tell us the fluid, the temperature, the chloride level and the pressure, we can tell you honestly whether 316 will do, whether you need duplex, or whether titanium Grade 2 is actually the cheaper answer over plant life. That conversation costs nothing and routinely changes the specification for the better.
Custom fabrication, special sizes and complete export documentation are handled in house. Send us your drawing or specification and our technical team will come back with a detailed offer, usually within one working day.
