Summary: An industrial recirculating cooling water system reuses water, in an open circuit (make-up from a cooling tower or pond) or a closed circuit, after it passes through heat-exchange equipment; its corrosion is a superposition of electrochemical corrosion, chloride pitting, microbially influenced corrosion (MIC) and under-deposit corrosion. Protection follows a layered logic of water stabilisation + a protective coating + cathodic protection or corrosion-resistant material where needed: under GB/T 50050-2017 the uniform corrosion rate of carbon-steel equipment is held below <0.075 mm/a and that of copper alloys and stainless steel below <0.005 mm/a; condenser water boxes and the bore of circulating water pipes are commonly lined with a 300–500 µm epoxy-phenolic (novolac) lining, while concrete cooling ponds use an epoxy sealer plus vinyl/glass-flake anti-permeation. The water box, the water-side tube and the pond each have a different governing corrosion form, so protection cannot be one-size-fits-all.
What counts as corrosion of a recirculating cooling water system
Recirculating cooling water flows repeatedly between the exchanger tube bundles, the water boxes, the circulating water pipework and the cooling-tower basin, concentrating by evaporation while it gains heat, so as the cycles of concentration rise the dissolved-solids content, chloride and alkalinity climb together. Carbon steel in this medium suffers uniform corrosion driven by oxygen depolarisation, and scale layers and biofoul create differential aeration cells that cause localised pitting; seawater or high-chloride make-up additionally triggers chloride pitting and stress-corrosion cracking of stainless steel. Microorganisms such as sulphate-reducing bacteria (SRB) form anaerobic micro-environments under the fouling and are the classic source of MIC.
How this relates to water-treatment dosing
Corrosion and scale inhibitors and biocides belong to water stabilisation: they hold corrosion rate, fouling thermal resistance and microbial counts within limits and are the first line of defence during operation; but chemistry cannot stop pit initiation, erosion-corrosion, or localised attack at crevices and welds, and cannot protect already-degraded base metal. Coatings and linings belong to the material barrier, separating metal from the medium; cathodic protection adds electrochemical protection for submerged steel such as water boxes and buried circulating water pipes. The three divide the work rather than replace each other — precisely the combination that corrosion engineers and the water-treatment discipline must settle together.
Key data
- Corrosion-rate limits (GB/T 50050-2017): carbon-steel heat-exchange and circulating-water equipment <0.075 mm/a (about 3 mpy); copper alloys and stainless steel <0.005 mm/a.
- Water-quality window: pH 6.8–9.5 (for acid-based programmes the lower bound should not fall below 6.8); for open indirect cooling the cycles of concentration should be no less than 5.0 and not below 3.0; turbidity generally ≤20 NTU and ≤10 NTU at the heat-exchange equipment.
- Microbes and fouling: total heterotrophic bacteria ≤1×10⁵ CFU/mL, biofoul ≤3 mL/m³, free residual chlorine 0.1–1.0 mg/L — fouling is the breeding ground for under-deposit pitting and MIC.
- Flow velocity and wall temperature: tube-side velocity >1.0 m/s, shell-side >0.3 m/s (preventing deposition while controlling erosion); metal wall temperature should not exceed 70 ℃ to suppress chloride pitting.
- Fouling criteria: fouling thermal resistance ≤3.44×10⁻⁴ m²·K/W and deposit rate ≤15 mg/(cm²·month) (relaxed to 20 for refinery service); the thicker the scale, the faster the under-deposit corrosion.
- Coating barrier: condenser water boxes and the bore of circulating water pipes typically carry a 300–500 µm epoxy-phenolic (novolac) lining, and concrete cooling ponds use an epoxy sealer primer plus a vinyl-ester/glass-flake anti-permeation top coat.
Main parts of the circulating water system × governing corrosion form × first-choice protection (a combination, not a single pick)
| Location | Governing corrosion form | First-choice protection | Key control points |
|---|---|---|---|
| Condenser/exchanger water box (carbon steel) | Oxygen corrosion + under-deposit/under-fouling pitting, MIC | Epoxy-phenolic lining 300–500 µm + sacrificial anodes | Stripe coat at corners and welds, spark pin-hole testing, dew point and drying |
| Carbon-steel circulating water pipe (bore) | Uniform thinning + localised pitting | Cement-mortar/epoxy internal lining + water stabilisation | Lower velocity limit against deposition, upper limit against erosion, combined with CP |
| Heat-exchanger tubes (copper alloy/stainless) | Chloride pitting, ammonia/stress corrosion | Material upgrade (duplex steel/titanium) + control of chlorine and temperature | Chloride–temperature interaction, avoid stagnant or dry walls |
| Cooling tower / concrete basin | Carbonation, rebar corrosion expansion, water leakage | Epoxy sealer + vinyl-ester/glass-flake anti-permeation | Substrate dryness, movement-joint displacement following, evaluation of tile substitution |
Where each part's governing corrosion comes from
Water boxes and circulating water pipes: localised attack under scale and fouling
Once the water-side flow path scales or accumulates biofoul, oxygen is depleted and chloride concentrates under the deposit, forming a differential aeration cell of large cathode to small anode, so pit depth can far exceed the average wall loss. The bare carbon-steel wall takes it directly; linings and cathodic protection are the main barrier.
Heat-exchanger tubes: pitting/stress corrosion driven by chlorine and temperature together
304 stainless steel readily pits and cracks from chloride stress corrosion at high temperature and high chloride, and copper alloys are threatened by ammonia and erosion; keeping the wall below 70 ℃ and the chloride within the range the material can tolerate is fundamental, using duplex steel or titanium tubes where necessary.
Microbially influenced corrosion MIC
SRB reduce sulphate to sulphide in the anaerobic micro-zones of the fouling, forming corrosive FeS and locally acidifying the environment; biocide dosing, keeping fouling ≤3 mL/m³, and eliminating stagnant dead zones are the practical levers that suppress MIC.
Coating system and application practice — key points
- System selection: for water boxes and the water side use solvent-free epoxy or epoxy-phenolic (novolac) water- and heat-resistant linings; for the concrete pond body use a sealer primer plus a vinyl-ester/glass-flake top coat to block permeation.
- Surface preparation: blast steelwork to Sa2½ with Rz roughness on target, grind welds flush, round sharp edges and stripe coat them — corners are the weakest point of any lining.
- Application window: control ambient temperature and relative humidity, with the steel at least 3 ℃ above the dew point, to avoid condensation that ruins adhesion.
- Quality inspection: wet-film thickness plus dry-film magnetic gauging (SSPC PA 2 logic), spark pin-hole/holiday detection, cross-cut or pull-off adhesion, with the test voltage set from the film thickness.
- Interface with CP and water treatment: when lining and sacrificial anodes are combined, verify the protection potential (seawater commonly Ag/AgCl reference −0.80 to −1.05 V) so as not to damage the coating by over-protection.
- Operating traceability: log corrosion-coupon/probe rates, weekly water reports and holiday-test records in the equipment file; when limits are exceeded, check chemistry and fouling first, then coating damage.
FAQ
If the circulating water is already dosed with corrosion and scale inhibitors, do we still need coatings or cathodic protection?
Yes — the two are not mutually exclusive. Chemistry holds the uniform corrosion rate within limits but protects poorly against pit initiation, under-deposit/under-fouling attack, erosion and crevice corrosion; the linings and CP of the water box and pipes provide a second and third barrier at the material and electrochemical levels, and are especially critical at stagnant zones, corners and welds.
Why are the corrosion-rate limits for carbon steel and stainless steel an order of magnitude apart?
GB/T 50050-2017 sets carbon steel at <0.075 mm/a and copper alloys/stainless at <0.005 mm/a because, once stainless steel suffers pitting or stress-corrosion cracking in chloride, it is a penetration failure that cannot be judged by 'average thinning'; the tighter limit forces prevention of localised corrosion through material choice, chlorine and temperature control.
Why do condenser water boxes favour epoxy-phenolic rather than plain epoxy?
The water box stays under water, sees temperature differences and repeated cooling during maintenance; phenolic-cured epoxy (novolac) resists hot water, temperature and chemicals better than amine-cured plain epoxy, and a 300–500 µm thick lining plus holiday testing markedly reduces under-deposit pitting and leakage; corners and welds still need a stripe coat build-up.
How exactly is biofoul quantified and controlled?
Per GB/T 50050-2017 the fouling level ≤3 mL/m³ (measured by the biocide-shooting method) with heterotrophs ≤1×10⁵ CFU/mL. Control combines oxidising biocide (residual chlorine 0.1–1.0 mg/L, intermittent shock dosing) with rotational non-oxidising biocides to prevent resistance, together with lower velocity limits to reduce deposition and periodic blowdown to shed deposits.
The concrete cooling pond leaks and its rebar is rust-expanding — how do coatings help?
First treat the substrate: hack off hollows, de-rust the rebar, flexibly seal cracks and movement joints; then build an anti-permeation layer of epoxy sealer primer plus vinyl-ester/glass-flake, ensuring the substrate dryness and moisture content are within limits and the top coat can follow displacement at the movement joints, evaluating corrosion-resistant tiles or flexible waterproof coatings as alternatives where needed.
Is a cement-mortar internal lining on circulating water pipes still worthwhile?
For water-conveying steel pipes where protection doubles as scale mitigation, the cement-mortar lining shields carbon steel through high-alkalinity passivation + physical separation and suppresses roughening of scale on the wall — low cost, long life; its weaknesses are erosion resistance and cold-weather application, so it is usually combined with water stabilisation and local epoxy repair, with solvent-free epoxy internal coating for critical pipe sections.
Last updated: 2026-09-22
References: GB/T 50050-2017 Design Code for Industrial Recirculating Cooling Water Treatment (corrosion rates, water-quality and microbial limits, fouling thermal resistance); GB 50049-2011 Design Code for Small Steam Power Plants, clause 13.12 Condenser and auxiliary facilities; Microbially Influenced Corrosion of Steel in Marine Environments: A Review (PMC10535020, 2023, SRB and MIC mechanisms); SSPC PA 2 (magnetic measurement of dry film thickness).
Kexin New Materials (Guangdong) Co., Ltd.