Corrosion in heat exchangers: how does it develop and when does it become critical?

Why does corrosion develop in heat exchangers?

A heat exchanger continuously brings metal surfaces into contact with cooling water, seawater, process liquids, steam, oil, chemicals or combustion gases. At the same time, it is exposed to pressure, flow, temperature changes and fouling.

This combination makes heat exchangers particularly sensitive to corrosion.

Common contributing factors include:

• chlorides in seawater, brackish water or process water;
• an unsuitable or fluctuating pH;
• aggressive process chemicals;
• elevated operating temperatures;
• stagnant liquid during shutdown periods;
• deposits of limescale, sludge, salt, oil or biofilm;
• high flow velocities or local turbulence;
• solid particles in the medium;
• contact between different metals;
• incorrect cleaning chemicals or cleaning procedures;
• a plate or tube material that is unsuitable for the application.

Corrosion rarely has one single cause. For example, deposits may create a stagnant zone on a heat-transfer surface. Chlorides may then become concentrated underneath the deposit, while the local oxygen level and pH change. This can cause corrosion to develop even when the rest of the heat exchanger appears to be in good condition.

Stainless steel is also not corrosion-proof under all conditions. Chloride-containing media, higher temperatures, low pH values and deposits can damage the protective passive layer and cause localised corrosion.

The correct material choice must therefore be based on more than the name of the medium. Its concentration, temperature, velocity, contamination and operating cycle must also be considered.

Where does corrosion occur in a heat exchanger?

The location of the damage often provides important information about its cause. In plate heat exchangers, corrosion is commonly found:

• underneath or close to gaskets;
• at plate contact points;
• underneath limescale, sludge or biofilm;
• around the port holes;
• near the inlet and outlet areas;
• on plates exposed to chloride-containing media;
• at damaged or deformed sections of the plate pack.

In shell-and-tube heat exchangers, corrosion may occur:

• inside the tubes;
• at the tube inlets;
• around the tube-to-tube-sheet connections;
• underneath deposits on the shell side;
• near baffles and flow restrictions;
• in stagnant sections;
• at welded joints;
• at interfaces between different materials.

These areas are difficult to assess while the heat exchanger is still assembled and contaminated. Thorough cleaning and, where possible, dismantling are therefore important parts of the inspection process.

Common forms of corrosion in heat exchangers

Different corrosion mechanisms create different damage patterns. Recognising these patterns helps determine whether cleaning is sufficient or whether components must be repaired or replaced.

General or uniform corrosion

General corrosion affects a relatively large section of a plate, tube or other heat-transfer surface. When the material loss is distributed approximately evenly, it may also be described as uniform corrosion.

In a heat exchanger, this can lead to a gradual reduction in wall thickness. The damage may initially appear manageable because it is spread over a larger area. However, the plate or tube can eventually become too thin to withstand the operating and test pressure.

This type of corrosion is generally easier to identify and measure than localised corrosion. Its severity still depends on the remaining wall thickness and the load on the affected component.

Pitting corrosion

Pitting corrosion creates small but potentially deep holes in the material. It is particularly relevant in thin stainless-steel plates and tubes exposed to chlorides.

The affected surface may appear limited, while the pit has already penetrated deeply into the material. Corrosion products or deposits can also cover the opening, making the true depth difficult to see before cleaning.

For a plate heat exchanger, a single deep pit may eventually perforate a plate and create an internal leak between the hot and cold circuits. The media can then mix without any visible leakage on the outside of the unit.

In a tubular heat exchanger, pitting may perforate an individual tube. Depending on the application, this can contaminate cooling water, oil, process liquid or another medium.

This is why the depth of a pit is more important than the total size of the visibly affected area.

Crevice corrosion

Crevice corrosion develops in narrow spaces where liquid becomes trapped or is not refreshed sufficiently.

Typical heat-exchanger locations include:

• underneath plate gaskets;
• between deposits and the metal surface;
• around bolted or clamped connections;
• between tubes and tube sheets;
• underneath sealing surfaces;
• around overlapping or closely fitted components.

The liquid inside the crevice can develop a different chemical composition from the main flow. Oxygen may be depleted, while chlorides become concentrated. This can damage the protective layer of stainless steel and cause rapid localised corrosion.

In a gasketed plate heat exchanger, this damage may only become visible after the plate pack has been opened and the gaskets and deposits have been removed.

Galvanic corrosion

Galvanic corrosion may occur when different metals are electrically connected and exposed to a conductive medium.

Heat exchangers often contain several materials. Plates, tubes, tube sheets, frames, fasteners, soldered joints and connections may not all be made from the same metal. In seawater or conductive process water, an unsuitable material combination can cause the less noble metal to corrode more quickly.

The risk depends on factors such as:

• the combination of metals;
• the relative surface areas;
• the conductivity of the medium;
• the location of the connection;
• the operating temperature;
• the presence of coatings or insulation.

Galvanic corrosion should therefore be considered not only during maintenance, but also when replacement components or fasteners are selected.

Erosion-corrosion

Erosion-corrosion occurs when flow conditions repeatedly damage the protective surface layer of a plate or tube.

In heat exchangers, this type of damage is often concentrated around:

• tube inlets;
• bends;
• nozzles;
• port areas;
• restrictions;
• baffles;
• areas with high turbulence;
• locations where air bubbles or solid particles strike the surface.

Once the protective layer has been removed, the exposed material corrodes more quickly. The resulting surface damage may create additional turbulence, causing the process to accelerate.

In a shell-and-tube heat exchanger, erosion-corrosion around the tube inlet can gradually thin the tube wall. In a plate heat exchanger, high local flow velocity may contribute to damage around the port area or other highly loaded sections.

Stress corrosion cracking

Stress corrosion cracking occurs when a susceptible material is exposed to both tensile stress and a corrosive environment.

In a heat exchanger, these stresses may be caused by:

• operating pressure;
• temperature cycles;
• vibration;
• deformation during assembly;
• incorrect tightening;
• welding;
• residual manufacturing stresses.

Certain stainless-steel grades are susceptible to chloride-induced stress corrosion cracking, particularly at elevated temperatures.

The damage appears as fine cracks rather than general material loss. These cracks may be difficult to identify during a basic visual inspection and can continue to grow under changing pressure and temperature conditions.

Microbiologically influenced corrosion

Microbiologically influenced corrosion, or MIC, is associated with microorganisms and biofilms that alter the conditions at the metal surface.

This is particularly relevant in heat exchangers that operate with untreated cooling water or that remain filled with stagnant water during shutdown periods.

Biofilm can create local differences in oxygen level, pH and chemical concentration. Corrosion may then develop underneath the biological layer, especially in low-flow areas and underneath deposits.

MIC may occur together with pitting or crevice corrosion and is often difficult to confirm without proper cleaning and further investigation.

What are the warning signs of corrosion?

Corrosion inside a heat exchanger does not always produce a clear external warning. However, several operational and visual signs may indicate that further inspection is required.

These include:

• external leakage;
• contamination or mixing of the two media;
• unexplained changes in process-liquid quality;
• rust marks or local discolouration;
• pits in plates or tubes;
• corrosion around connections or welds;
• damaged areas underneath gaskets;
• recurring leakage after repairs;
• locally thinned tubes;
• recurring damage in the same position;
• unusually rapid deterioration between maintenance intervals.

A reduced thermal capacity, higher pressure drop or abnormal outlet temperature can also justify inspection. However, these symptoms are often caused by fouling rather than corrosion.

Fouling mainly restricts flow and heat transfer. Corrosion mainly affects the material integrity, pressure boundary and separation between the two media. Both problems can occur at the same time.

Why is cleaning necessary before corrosion can be assessed?

Deposits can hide corrosion almost completely.

A plate may appear to be covered only with limescale or sludge. Once it has been cleaned, deep pits, damaged sealing surfaces or thin areas may become visible. In a tubular heat exchanger, deposits inside the tubes may conceal wall thinning or localised attack.

Cleaning is therefore not only performed to restore heat transfer. It is also required to expose the surface for inspection.

Cleaning removes fouling, but it does not restore material that has already been lost through corrosion.

At Blue Orange, the heat exchanger is first assessed to determine the type of fouling and the appropriate cleaning method. Depending on the design and contamination, this may involve chemical cleaning, high-pressure cleaning, ultrasonic cleaning or a combination of methods.

After cleaning, the relevant components can be inspected, including:

• individual plates;
• plate ports and contact points;
• gasket grooves and sealing surfaces;
• tubes and tube inlets;
• tube sheets;
• welds;
• connections;
• coated surfaces.

Only after this inspection can the actual extent of the corrosion be determined.

When does corrosion become critical?

There is no universal corrosion percentage at which every heat exchanger must be rejected.

A small pit in a thin plate may already create a leakage risk, while limited general corrosion in a much thicker component may still leave sufficient material. The consequences also differ by application. A minor internal leak may be unacceptable when it allows oil, chemicals, food products or seawater to mix with another process stream.

The severity must therefore be assessed based on:

• the original and remaining wall thickness;
• the depth and shape of the damage;
• the location of the defect;
• the heat-exchanger design;
• the plate, tube or shell material;
• the operating and test pressure;
• the operating temperature;
• the medium on both sides;
• the corrosion rate;
• the consequences of external leakage;
• the consequences of internal mixing.

Immediate technical assessment is required when there is evidence of:

Internal or external leakage

External leakage shows that a seal, connection or pressure-retaining component is no longer reliable.

Internal leakage may be more difficult to recognise. A perforated plate or tube can allow the two media to mix without liquid appearing on the outside of the heat exchanger.

Deep pitting

Deep pits in plates or tubes may leave very little sound material. In thin heat-transfer surfaces, a small additional amount of corrosion can cause full perforation.

Cracks

Cracks can continue to grow under pressure fluctuations, vibration and thermal cycling. Their visible length does not always indicate their full depth.

Insufficient remaining wall thickness

A plate, tube, shell or tube sheet must retain sufficient material to withstand its operating conditions. If the remaining wall thickness is too low, repair or replacement may be necessary.

Severe erosion damage

Rapid local wall thinning near tube inlets, ports or turbulent flow zones may continue unless the underlying flow conditions are corrected.

Recurring corrosion

When similar damage returns after each maintenance interval, replacing the affected part alone may not solve the problem. The underlying cause may involve water quality, material selection, flow velocity, shutdown conditions or the cleaning procedure.

How is corrosion in a heat exchanger inspected?

The correct inspection method depends on the heat-exchanger type, material and suspected defect.

Possible methods include:

• visual inspection after cleaning;
• pressure testing;
• leak testing;
• wall-thickness measurements;
• dye penetrant testing for surface-breaking defects;
• eddy current testing of suitable tubes;
• ultrasonic or other specialised tube inspection;
• testing for internal leakage or cross-contamination.

A test result should always be considered together with the visible condition, material specifications and operating data.

A pressure test can demonstrate whether the heat exchanger is leak-tight under the applied test conditions. It does not automatically explain why the damage occurred or guarantee that corrosion will not continue.

Cleaning, overhauling or replacing?

After cleaning, inspection and testing, an informed decision can be made.

When is cleaning sufficient?

Cleaning may be sufficient when fouling is the main problem and no unacceptable corrosion damage is found.

The heat exchanger must still be capable of being rebuilt and tested reliably. For a plate heat exchanger, this also means checking the condition of the plates, gasket grooves and sealing surfaces. For a tubular unit, the tubes, tube sheet and connections must remain suitable for service.

When can a heat exchanger be overhauled?

Overhauling is possible when damaged or worn components can be repaired or replaced safely.

Depending on the design, this may involve:

• replacing damaged plates;
• renewing gaskets;
• replacing or plugging damaged tubes where technically permitted;
• repairing connections;
• restoring protective coatings;
• replacing other affected components;
• rebuilding and testing the complete unit.

At Blue Orange, an overhaul may include dismantling, cleaning, inspecting, repairing, replacing components, rebuilding and pressure or leak testing.

The objective is not simply to make the heat exchanger look clean again. The unit must be returned in a condition in which its function and leak tightness can be assessed reliably.

When is replacement the better option?

Replacement may be more appropriate when:

• corrosion affects too many plates or tubes;
• critical parts cannot be repaired reliably;
• the remaining material strength is insufficient;
• cracking is extensive;
• the original material is unsuitable for the medium;
• the underlying operating conditions cannot be corrected;
• overhaul is no longer technically or economically responsible;
• the risk of renewed leakage remains too high.

The age of the heat exchanger alone should not determine the decision. Its condition, design, duty and operating environment are more important.

How can corrosion in heat exchangers be reduced?

Corrosion cannot always be prevented completely, but the risk can be reduced through correct material selection, operating control and maintenance.

Important measures include:

• monitoring cooling-water and process-water quality;
• controlling pH, temperature and chloride levels;
• preventing prolonged stagnation during shutdowns;
• draining or preserving the heat exchanger correctly when required;
• selecting plate and tube materials for the actual operating conditions;
• avoiding unsuitable combinations of different metals;
• controlling flow velocity and turbulence;
• removing deposits before they create local corrosion cells;
• using cleaning chemicals that are compatible with the heat-exchanger material;
• applying the correct concentration, temperature and contact time;
• rinsing cleaning chemicals out thoroughly;
• documenting inspection and test results;
• comparing damage patterns between maintenance intervals;
• investigating recurring corrosion instead of only replacing the damaged component.

The correct maintenance interval depends on the application. A seawater cooler, oil cooler or process heat exchanger handling aggressive chemicals may require more frequent inspection than a unit operating with clean, controlled water.