Heat Exchanger Materials: Corrosion-Resistant Alloys for Shell and Tube and Plate Units

Material selection is a decision about the specific stream, not about a preferred alloy. The inputs that matter are the chemistry of each fluid, chloride content, pH, operating and design temperature, dissolved oxygen or oxidising species, solids and velocity, and the cleaning chemistry the unit will see in service. A corrosion resistant heat exchanger is one whose plates or tubes, tubesheets, nozzles and gaskets have all been matched to those conditions.

Carbon steel remains a sound choice for shells, frames, tubesheets and tubes in clean, non-corrosive utility services such as closed-loop hot water, steam condensate and many hydrocarbon duties. It is not suitable for aerated chloride-bearing water or acidic streams without protection. On shell-and-tube units a carbon steel shell is often combined with an alloy tube bundle when only one side is corrosive, which keeps cost proportionate to the risk.

Stainless 316L is the workhorse material for plate heat exchangers and clean tubular services, covering most clean water, glycol, oil and food-grade duties. 304 can be acceptable for low-chloride, low-temperature service. Both grades become vulnerable to pitting and crevice corrosion as chloride concentration and temperature rise together, so the chloride level and the maximum metal temperature should always be stated in the enquiry.

Where chlorides exceed what 316L can be relied on for but a fully corrosion-resistant alloy is not warranted, duplex stainless (2205) and super-austenitic grades (904L, 254 SMO) provide an intermediate step with higher pitting resistance. They are widely used in brackish water, moderately corrosive process streams and in duties where chloride concentration can rise during upsets or shutdown.

Titanium is the common choice for seawater, brine, hypochlorite and aggressive chloride-rich services in both plate and tubular constructions, because it resists pitting and crevice attack in conditions where austenitic stainless is at risk. It is standard in marine central cooling, aquaculture and many coastal chemical duties. Titanium is not universal — it is not the preferred answer in reducing acid environments — so the full chemistry should still be reviewed.

Hastelloy, Inconel, Alloy 20 and similar nickel-based alloys are reserved for the most aggressive services: concentrated or reducing acids, oxidising environments and high-temperature corrosive gases. They carry a significant cost premium, so selection should be confirmed against published corrosion data for the actual concentration and temperature and, where possible, against plant experience with the same stream.

In gasketed plate units the elastomer frequently limits the service before the metal does. NBR suits mineral oils and general water duties, EPDM is widely used for water, glycol, steam and caustic CIP but is not suitable for mineral oils, HNBR extends oil and temperature capability, and FKM is used for oils, fats and many aggressive chemicals but is unsuitable for hot caustic. Because CIP chemistry can be more demanding than the process fluid, the cleaning regime should be stated alongside the process data.

The mapping below shows typical starting points by service. It is a decision-support summary, not a corrosion guarantee: the final recommendation is confirmed against the actual chemistry, temperature and inspection requirements of the project.

Use the table as a first filter on which materials to evaluate for a given stream, then confirm the selection with the engineering team against the actual data. It does not replace a corrosion assessment for critical duties.

The comparison below summarises how the main wetted-alloy families differ on chloride tolerance, typical constructions and relative cost position. Cost position is qualitative only — actual pricing depends on market alloy surcharges, thickness and fabrication route.

Send the chemistry, chloride level, pH, operating and design temperature, and the cleaning regime, and let the engineering team recommend the material and the gasket compound together. Over-specifying drives cost; under-specifying drives failure. Where the stream is unusual or the consequence of failure is high, a documented material recommendation and, if required, third-party review is worth the time it takes.

The list below is what an engineering team needs before it can name an alloy and a gasket compound with any confidence. Partial data is still useful — the missing items simply become assumptions that have to be confirmed later.