Heat exchanger corrosion: causes and prevention, component by component

How do you prevent corrosion in a heat exchanger?

You prevent corrosion in a heat exchanger by matching each component to its own mechanism: tube materials and water velocity against pitting and erosion, water treatment against deposits and microbial attack, coatings and cathodic protection on tube sheets and water boxes against galvanic and crevice corrosion, and gasket and plate selection on plate exchangers. One fix never covers the whole unit.

A shell-and-tube exchanger puts several metals, a heated wall, crevices and often partly treated water in one box; a plate exchanger adds hundreds of gasketed contact points. The damage follows the design, so it is predictable. Below, each component with its symptoms, causes, prevention and repair, linked to our detailed guides. For the mechanisms themselves, see types of industrial corrosion.

Tubes: pitting, inlet erosion, deposits, MIC and cracking

The tubes carry most of the heat transfer surface and most of the risk: a leak mixes the two fluids.

Pitting. Symptom: isolated, deep pits, often found by eddy current testing before they leak. Cause: chlorides breaking down the passive film on stainless steel, or local cells under deposits on any alloy. Pitting resistance of stainless grades is ranked by their pitting resistance equivalent number, which is why higher-PREN grades, duplex or titanium are specified for chloride-bearing water. See our guide to pitting corrosion.

Erosion-corrosion at tube inlets. Symptom: smooth, horseshoe-shaped metal loss on the first part of each tube, worst facing the water box inlet. Cause: turbulence where water enters the tube strips the protective oxide film faster than it reforms, a classic failure on copper-alloy condenser tubes. Erosion-corrosion gets worse with entrained sand, debris or air, and with poor flow distribution in the water box. Prevention: keep tube-side velocity within the range the thermal designer and the tube alloy allow, screen debris upstream, and fit tube inlet inserts or sleeves that take the turbulence away from the tube-to-tube-sheet joint.

Deposits and under-deposit corrosion. Symptom: scale, silt or biofilm, with pits underneath when it is removed. Cause: low velocity, hardness salts precipitating on the hot wall, or poor filtration. The oxygen-starved area under a deposit becomes an anode against the clean surface around it. Prevention: water treatment (scale and corrosion inhibitors, filtration, blowdown control), regular cleaning, and a design velocity that is high enough to limit fouling without eroding the tube. TEMA, and API 660 for petroleum, petrochemical and natural gas service, frame these design choices; the values belong to the designer, not to a rule of thumb.

Microbiologically influenced corrosion (MIC). Symptom: tubercles with aggressive, often cup-shaped pits underneath. Cause: sulfate-reducing and iron-related bacteria in cooling water, favored by stagnant periods. A frequent case is an exchanger filled with untreated hydrotest water and left standing. Prevention: biocide programs, draining and drying after hydrotest, and a lay-up procedure for idle units. See our guide to MIC microbial corrosion.

Stress corrosion cracking. Symptom: fine, branched cracks, often with little visible metal loss. Cause: tensile stress plus a specific environment — chlorides on austenitic stainless steel at elevated temperature, ammonia on brass, caustic on carbon steel. Stress corrosion cracking is a material-and-environment problem: the prevention is alloy selection (duplex, ferritic, high-nickel or titanium in chloride service), avoiding zones where chlorides concentrate by evaporation, and stress relief where the code allows. Coatings do not cure it.

Tube coatings. Two different things go by that name. Tube-end coating carries the water box coating a short distance into each tube, over the joint, to protect the inlet and avoid leaving bare noble metal next to coated steel. Full-length internal tube coating is a specialist process that adds a thin film to the whole bore; it adds thermal resistance, so the thermal designer has to confirm the duty before it is specified. Repair options for damaged tubes are plugging, sleeving, or retubing when too many tubes are lost.

Tube sheets and tube-to-tube-sheet joints: galvanic and crevice corrosion

Galvanic corrosion. Symptom: craters around tube holes, thinned ligaments, rolled joints that start to weep. Cause: a carbon-steel tube sheet connected to copper-alloy, stainless or titanium tubes. The tubes are rolled into the sheet, so the metallic contact is permanent, and the cooling water closes the circuit. The very large tube surface acts as a cathode facing the small exposed steel. Galvanic corrosion at the tube sheet is covered in detail in galvanic corrosion in heat exchangers, including why coating only the steel is dangerous.

Crevice corrosion. Symptom: attack in gasket seats, pass partition grooves and at the edge of rolled joints. Cause: stagnant, oxygen-depleted water trapped in a narrow gap. On stainless tube sheets in chloride water, the gap between tube and hole beyond the rolled zone is a typical site. Crevice corrosion is explained in our guide to crevice corrosion.

Prevention combines material selection at design (clad tube sheets, matched alloys), coating of the tube sheet face extended into the tube ends, cathodic protection in the water box, and joint design that limits crevices. Repair: metal-filled epoxy rebuild of craters, ligaments and gasket seats, then an immersion-grade coating, as described in tube sheet and water box repair. The tube sheet is a pressure part: remaining thickness and ligament acceptance are the owner's inspector's call, and registered exchangers follow the jurisdiction's requirements, in Canada CSA B51 and the provincial authority.

Water boxes, channels and covers: raw water, seawater and galvanic attack

Symptom. General wastage, pits and tubercles on the water box wall and cover, worst near the tube sheet flange and the inlet nozzle. On cast iron, surfaces that look sound but can be scraped into soft graphite.

Cause. Raw river, lake or seawater is conductive, oxygenated, often loaded with silt and organisms. Steel and cast iron water boxes are the least noble metal in the system, so galvanic attack adds to the general corrosion. Graphitic corrosion of cast iron is a form of selective leaching (dealloying).

Prevention. An immersion-grade epoxy lining on all wetted surfaces, inspected by holiday testing, plus cathodic protection: sacrificial anodes bolted in the water box or an impressed-current system. The coating reduces the current demand; the anodes protect its defects. Cathodic protection is sized and designed by a cathodic protection specialist; with titanium tubes, potentials must stay within the tube supplier's limits.

Repair. Rebuild of pitted and thinned zones with metal-filled epoxy, coating, renewal of anodes. Thinned walls below the owner's minimum, or graphitized cast iron with no sound metal left, call for replacement.

Shell and shell side: deposits, impingement and external corrosion

When cooling water flows on the shell side, low-velocity zones behind baffles, dead spots near the tube sheet and areas under supports collect deposits, and suffer under-deposit corrosion and MIC. Baffle holes in steel enlarge by corrosion and let tubes vibrate and fret. Under the inlet nozzle, impingement erodes the first row of tubes; TEMA sets when an impingement plate or equivalent protection is required. A gas pocket at the top of the shell or water left at the bottom during shutdown creates wet-dry or stagnant zones.

On the outside, insulated shells are exposed to corrosion under insulation. Prevention is mostly design and operation: water on the tube side where possible, adequate velocity, impingement protection, full drainage at shutdown. Shell repairs follow the pressure code; internal coating applies where the shell side is water and access allows.

Plate heat exchangers: corrosion under gaskets and plate material

Corrosion in a plate exchanger concentrates where the design makes crevices: under the gaskets, at the contact points where corrugated plates touch, and at the ports.

Crevice corrosion under the gasket. Symptom: pits or a dark line along the gasket groove, leaks between plates or to the outside. Cause: chloride water trapped in the gasket groove, made worse by deposits. On stainless plates in chloride water, it often appears before pitting on the open surface.

Stress corrosion cracking of plates. Pressed stainless plates carry residual forming stress. In hot chloride service they can crack at the corrugations or near the ports.

Plate material. The choice is qualitative and service-driven: standard austenitic stainless for clean, low-chloride water; higher-alloy austenitic or nickel alloys as chloride, temperature and oxidizing conditions rise; titanium for seawater and brackish water. Gasket elastomers are chosen for chemical and temperature compatibility.

Repair. Plates are thin pressed sheet: they are cleaned, crack-tested and replaced, not rebuilt. The carbon-steel frame and pressure plates are a coating job.

Graphite heat exchangers

Impregnated graphite resists acids that few metals tolerate, so graphite units rarely fail by corrosion of the graphite. They fail by thermal shock, mechanical shock, ageing of the impregnation resin and gasket degradation. The metallic parts around them — steel shells, heads, tie rods, flanges — corrode in acid vapours and next to graphite, which is noble in the galvanic series. Graphite cannot be welded; restoration relies on plugging, sealing, re-impregnation and composite resurfacing, covered in graphite heat exchanger repair.

Component × mechanism × prevention × repair

FAQ

What is the most common cause of heat exchanger corrosion? In cooling-water service: deposits and under-deposit corrosion, galvanic attack on steel tube sheets and water boxes, and erosion-corrosion at tube inlets. Which one dominates depends on water, materials and velocity, so the first step is to identify the mechanism at inspection.

Does coating a tube sheet stop galvanic corrosion? Only if it also covers the tube ends, is free of defects and is ideally backed by cathodic protection. Coating the steel alone turns each holiday into a small anode facing the whole tube surface.

Can heat exchanger tubes be coated on the inside? Yes: tube-end coating protects the inlet and the joint; full-length coating is a specialist process that must be checked against the thermal design.

How do you stop crevice corrosion in a plate heat exchanger? Choose a plate alloy suited to the chloride level and temperature, use compatible gaskets and keep the plates clean. Attacked plates are replaced.

Which standards apply to shell-and-tube exchanger design? TEMA covers mechanical design, including impingement protection; API 660 supplements it for petroleum, petrochemical and natural gas service. Pressure-part repairs follow the owner's procedure and, in Canada, CSA B51 and the provincial authority.

In practice

The water side of tube sheets, water boxes, channels and covers is where a cold repair makes the most difference. Induscoat rebuilds and coats these surfaces with metal-filled epoxy and immersion-grade linings, working with the owner's inspector and, where anodes are involved, with the cathodic protection specialist. This work is part of our maintenance and rehabilitation service and our heat exchanger repair work for plants in Quebec and Ontario. Photos of the opened channel, the materials of tubes and tube sheet, and the water type are enough to start: request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.