“Invincible” heat exchangers? A technical guide to anti-corrosion protection in extreme environments
In industrial refrigeration plants and high-capacity HVAC systems, corrosion is one of the main causes of premature degradation of heat-exchange coils. It is a progressive phenomenon: it acts on the metal surface, alters the fins’ morphology, can cause leaks in the tubes, changing the material’s effective conductivity and increasing air-side thermal resistance.
The reduction in the overall exchange coefficient k requires, for the same required thermal load, an increase in operating ΔT. In an evaporator, this translates into a lower evaporating temperature, a higher compression ratio and a reduced COP of the refrigeration system, with a consequent rise in electrical power input and a loss of overall energy efficiency in the medium term.
The economic impact is not limited to the cost of replacing the coil. Uncontrolled corrosion can generate:
- refrigerant loss through tube perforation
- unplanned plant downtime
- increased operating ΔT
- reduced evaporator service life
From a Life Cycle Cost (LCC) perspective, corrosion is a structural variable.
There is, however, no universal protection. There is only the solution consistent with the specific environment in which the heat exchanger must operate. A coastal zone, an industrial area, a food plant or a waste-treatment site are not merely geographical contexts: they are different chemical conditions, with distinct degradation mechanisms. Anti-corrosion design must start from their analysis.
The chemistry of degradation: why do metals give way?
Corrosion is an electrochemical phenomenon that activates when metals with different behaviour are in contact in the presence of moisture or contaminants. In a real heat exchanger this means condensation, marine aerosol, industrial vapours.
- Galvanic corrosion: in the typical case of a heat exchanger with copper tubes and aluminium fins, the aluminium tends to oxidise preferentially. The phenomenon becomes critical when the coil works in the presence of saline aerosol: moisture keeps the galvanic cell active. In coastal or industrial areas, without protection, service life can drop drastically.
- Pitting corrosion: in the presence of chlorides (Cl⁻), typical of saline atmospheres, the protective film of aluminium or stainless steel can break down locally. Deep pinhole perforations form, often invisible until the tube starts to leak.
- Chemical attack: in industrial, food or waste-treatment environments, organic acids, sulphur compounds or ammonia can alter the surface pH and degrade both the base metal and coatings not specific to that environment.
Understanding which mechanism prevails is the first step to correctly choosing materials and treatments.
Mapping aggressive environments
Anti-corrosion design does not start from the material, but from the environment. Before choosing alloys or coatings, it is necessary to identify which chemical agent prevails, with what intensity and with what exposure frequency.
- Saline atmospheres: within 5–10 km of the coast, marine aerosol carries sodium chloride (NaCl) that deposits on metal surfaces and, in the presence of moisture, promotes pitting corrosion.
- Industrial and urban areas: in industrial or high-density urban contexts, the presence of sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) promotes the formation of acidic compounds on contact with atmospheric moisture. The surface pH drops and accelerates the dissolution of metals, as well as progressively degrading coatings not specific to chemically aggressive environments.
- Food & Beverage sector: food environments present specific issues, from ammonia vapours in cheese-ageing rooms, for example, to organic acids such as acetic or lactic acid, smoking fumes and the alkaline detergents used for cleaning. The combination of condensation and organic compounds can generate more or less localised attacks on the exchange coils.
- Waste treatment and livestock farming: in waste-treatment plants, sewage works or intensive livestock farms, hydrogen sulphide (H₂S) and nitrogen compounds derived from urea create a highly aggressive environment. These gases, in contact with moisture, form acidic solutions that rapidly attack unprotected copper and aluminium alloys.
Every environment has a dominant degradation mechanism. Identifying it correctly means coherently defining the tube and fin material, the type of coating and the level of protection required.
Strategies for configuring the heat exchanger
Choosing anti-corrosion protection is not a single intervention. The heat exchanger’s configuration can be optimised on two levels: material selection and the application of surface treatments. The two strategies are not alternatives, but complementary and closely connected, and must be calibrated according to the environmental aggressiveness.
Strategy 1: material selection
The first barrier against corrosion is the consistent choice of the construction material. When the environment exceeds the protective capacity of standard copper and aluminium, it is necessary to act at the alloy level.
Stainless steel tubes
Stainless steel becomes the primary choice in environments with chlorides or chemical contaminants.
- AISI 304: offers good general resistance and is suitable for moderately aggressive indoor environments, where chloride exposure is limited.
- AISI 316L: thanks to the presence of molybdenum, it ensures greater resistance to chloride pitting. It is suited to marine environments, coastal areas or contexts with high salinity and a risk of localised corrosion.
The distinction between 304 and 316L is not merely formal: in the presence of chlorides, the difference in behaviour over the medium term can be substantial.
Aluminium-magnesium (AlMg) alloy fins
AlMg alloys offer superior resistance compared with pure aluminium. Adding magnesium improves the stability of the surface oxide film and increases resistance to salt spray. They represent an effective intermediate solution between standard aluminium and fully stainless-steel solutions.
Material selection must therefore balance environmental aggressiveness, expected life and the plant’s economic sustainability.
Strategy 2: coatings and surface treatments
When material choice alone is not enough, or when you want to optimise the cost/performance ratio, protective coatings come into play.
- Fin pre-painting (pre-coated fins): in this solution the aluminium strip used to produce the fins is painted before blanking. It is an economically advantageous configuration suited to moderately aggressive environments. The main limitation is the cut edges and mechanical-deformation zones, which can remain partly exposed.
- Cataphoresis (E-coating / KTL): this represents a high standard of protection for finned coils. The whole pack is immersed in an electrolytic bath containing epoxy resin, ensuring uniform coverage, protection of the tubes, fins and coil frame, with a controlled coating thickness.
- Post-assembly painting: applied with specific systems, it is suited to very large units or retrofit work on existing plants. It offers high surface protection, but requires careful control of thickness and application quality.
The initial investment in anti-corrosion materials and coatings can increase CAPEX by 15–20%. However, over a five-year horizon, the reduction in operating costs and unplanned downtime can amply offset the initial extra cost.
At Refteco we analyse the environmental conditions and the plant’s operating profile to define a tailor-made configuration of materials and surface treatments, consistent with the site’s real level of aggressiveness.
If you are designing a new plant or considering the refurbishment of a heat exchanger in an aggressive environment, share the site’s characteristics and operating conditions with us: a truly reliable heat exchanger is not the “standard” one.
It is the one designed for the environment in which it must work.
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