A corrosion coupon measures average mass loss and the form of attack on a specific material during exposure. Results are comparable only when alloy, preparation, area, rack position, flow, temperature, duration, cleaning, and calculation stay consistent. Coupons do not reproduce heat transfer at condenser tubes, so interpret them with water chemistry, chemical residuals, deposits, leakage, inspection, and operating data.
Standards that control coupon testing
ASTM D2688-23 is the most direct reference for water systems. It determines corrosion rate by coupon weight loss and evaluates pitting in a side-stream rack; its stated applications include open recirculating cooling water, closed chilled water, and hydronic heating, but the method operates without heat transfer.
ASTM G1-25 covers specimen preparation, removal of corrosion products, and evaluation. Its cleaning procedures aim to remove corrosion products without significant base-metal loss because over-cleaning inflates the apparent corrosion rate. Use the standard edition named in the company procedure and document any alternative method.
| Document | Question answered | Interpretation boundary |
|---|---|---|
| ASTM D2688-23 | How to expose coupons in a side-stream water system and determine weight-loss corrosion rate/pitting | Does not reproduce heat flux, boiling, real equipment crevices, galvanic couples, or dead zones |
| ASTM G1-25 | How to prepare, clean, and evaluate a specimen without excessive base-metal loss | Cleaning must suit the alloy and be recorded; results from different methods are not automatically comparable |
| Site and OEM procedure | Alloy, rack point, flow, exposure period, control limit, response, and facility safety | Must not conflict with the contract standard, equipment metallurgy, or HSE approval |
Do not claim “ASTM compliant” when a laboratory only weighs the coupon without area, density, exposure time, cleaning blank/control, and water-condition records. The audit trail is part of the result.
Place the corrosion rack where it represents the system
The rack must receive continuously representative loop water, not stagnant water in a dead leg or flow only when an operator opens a valve. Identify the location on the P&ID and provide safe isolation/depressurization, a flow indicator or verification method, and a drain routed into the site’s treatment system.
| Placement check | Record required | Risk if wrong |
|---|---|---|
| Side-stream source | Supply/return point and relation to exchangers, basin, pump, filter, blowdown, and chemical injection | Sample does not represent the loop or sees only a chemical slug |
| Rack flow | Value/range, measurement method, valve position, operating hours, interruption | Low flow creates deposits/stagnation; changed flow invalidates trend comparison |
| Temperature | Minimum, normal, maximum, and seasonal/load changes | Reaction and deposits change; a cold rack does not represent a hot surface |
| Injection point | Distance and mixing after inhibitor, acid, and oxidizing/non-oxidizing biocide | Local concentration may attack the coupon or give an optimistic result |
| Coupon material | Alloy/grade representing carbon steel, copper alloy, stainless, galvanized, or another site material | A generic coupon does not answer the actual material risk |
| Coupon arrangement | Position, orientation, insulator, fastener, and order per the approved method | Galvanic contact, copper deposition, shielding, or an artificial crevice |
| Safe access | Isolation, pressure relief, labels, PPE, sampling, drain, and lockout | Hot/chemical exposure and loss of coupon/data |
Place the rack after complete chemical mixing when monitoring treatment performance, but avoid a point that only sees a transient high concentration near an injection quill. If supply/return or another risk area requires comparison, use separately identified racks or monitors; do not move one rack and merge two baselines.
Record sheet for every coupon
Capture data before insertion, through exposure, and at removal. Photograph the as-received coupon, the removed coupon before cleaning, and the cleaned coupon with a scale and visible coupon ID.
- Coupon ID, alloy/grade, heat/lot where available, density used, dimensions, exposed area, hole/edge treatment, initial mass, and balance.
- Installation/removal date and time, exposure hours, shutdown, flow interruption, draining, layup, and tower cleaning during the interval.
- Rack location on the P&ID, slot, orientation, fastener/insulator, flow, pressure, and temperature range.
- Makeup and circulating-water chemistry: pH, conductivity, cycles, hardness, alkalinity, chloride, sulfate, silica, iron, copper, turbidity/TSS, and the site’s microbiological parameter.
- Treatment: product, target and measured residual, dose, pump calibration, injection point, blowdown, acid feed, biocide event, and deviation.
- Final mass after cleaning, cleaning method and blank/control correction where required, visual morphology, pit data, deposit analysis, and reviewer.
Betaqua Sentinel CTS cooling-tower monitoring can help record operating parameters continuously. Coupons provide material evidence, while sensors and water analysis explain what happened between insertion and removal.
Corrosion-rate formula and worked example
A common metric weight-loss calculation is:
CR (mm/year) = 87.6 × W ÷ (D × A × T)
where W is mass loss in mg, D is alloy density in g/cm³, A is exposed area in cm², and T is exposure time in hours. The area must follow the approved method, and mass loss must be corrected for the applicable cleaning procedure.
Transparent worked example
A carbon-steel coupon has density 7.86 g/cm³, exposed area 28.0 cm², exposure 720 h, initial mass 32,415.0 mg, and cleaned final mass 32,375.0 mg. Its mass loss W is 40.0 mg.
CR = 87.6 × 40.0 ÷ (7.86 × 28.0 × 720) = 0.022 mm/year.
That is about 0.87 mil per year (mpy) because 1 mpy equals 0.0254 mm/year. Retain the unrounded calculation. Recalculate whenever area, time, density, or cleaning correction changes; never copy a constant without checking its units.
Interpret the number without losing context
ASTM D2688-23 describes results as relative: compare one material with the same material in another water condition, or compare inhibitor performance with a controlled baseline. A universal limit for every cooling tower is therefore not defensible.
| Finding | Supported conclusion | Not yet proven |
|---|---|---|
| Mass-loss rate falls for the same alloy, rack, and interval | Average general corrosion improved under that exposure | Every exchanger/tube is safe or localized corrosion is absent |
| Low weight loss with a deep/local pit | General corrosion is low, but localized attack may be significant | Average rate is adequate as the only KPI |
| Thick coupon deposit | Deposit loading/fouling occurred at the rack | Deposit composition or cause without analysis |
| One high interval during loss of rack flow | Stagnation affected the result, so it is not directly comparable to the normal-flow baseline | The whole loop experienced the same rate |
| Copper/alloy result changes with dissolved iron/copper | A change needs correlation with water chemistry and metallurgy | The corrosion source location without further inspection/trending |
At minimum report average corrosion rate, before/after-cleaning photographs, attack morphology, pits/crevices, deposits, and exposure deviations. Heat exchangers with heat flux, crevices, stress, deposits, high velocity, or mixed metallurgy still need inspection/NDT under the mechanical-integrity program.
Inspection and coupon-replacement cadence
Use the fixed exposure period in the site or contractual procedure based on ASTM D2688-23; compare intervals only when material, rack, flow, and duration remain consistent. Pulling the only coupon early “for a quick look” produces a non-comparable result dominated by initial surface conditioning.
- At commissioning or a program change: install new coupons after rack flow, water chemistry, and dosing are stable; freeze the baseline before another change.
- Each operator round: inspect for leakage, pressure, flow indication, valve position, air lock, and whether the rack remains full. Do not remove coupons for a visual check.
- At the site/laboratory interval: record chemistry and residuals with timestamps that map to the exposure.
- At the fixed endpoint: remove, photograph, identify, seal/transport, clean, weigh, evaluate pits, and install a new coupon without changing its position.
- After a major upset: retain and event-mark the running coupon; if short-term evidence is required, add a separately identified parallel coupon rather than break the primary series.
Review the frequency after a makeup-source change, higher cycles, inhibitor/biocide change, tower cleaning, exchanger leak, process contamination, side-stream filtration failure, or system layup/startup.
Response checklist when results deteriorate
Do not immediately raise inhibitor dose. First prove the result is valid and identify what changed during exposure.
| Signal | First 24-hour checks | Next action |
|---|---|---|
| Corrosion rate rises while chemistry looks normal | Coupon ID/alloy/area/time, cleaning, balance, rack flow, shutdown history | Repeat with a controlled coupon if data are defective; inspect equipment if valid |
| pH falls or acid is overfed | Analyzer calibration, grab sample, dosing pump, valve/interlock, injection mixing | Stop/isolate overfeed per SOP, restore the envelope, inspect for localized damage |
| Conductivity/chloride/cycles rise | Blowdown valve, conductivity controller, makeup, evaporation/load, leak | Restore blowdown and cycle target; evaluate chloride/material limit |
| Inhibitor residual is low | Tank level, solution strength, pump calibration, quill/check valve, flow-paced signal | Restore measured dose and find the cause; do not raise stroke without calibration |
| Biofilm/deposit appears | Biocide residual/contact, microbial data, nutrients, side-stream filter, dead legs | Coordinate cooling-tower biocides, cleaning, and deposit removal with corrosion control |
| Iron/copper or turbidity rises after an upset | Process leak, makeup quality, transported corrosion products, tower cleaning | Isolate the source, sample deposits, and inspect the relevant exchanger/piping |
Coordinate cooling-tower corrosion inhibitors, scale inhibitors, biocide, blowdown, and filtration. Where chemical delivery is unstable, Watermart’s water-treatment dosing pumps are a relevant equipment handoff.
Data package for cooling-water program review
Provide the P&ID and water balance; tower volume and circulation/makeup/blowdown flow; metallurgy for every exchanger and pipe circuit; dated makeup/circulating chemistry; temperature/load; cycle target; all chemicals, residuals, doses, and pump calibration; rack drawing/flow; coupon certificates, raw weights, area, density, exposure log, cleaning method, photographs, and pit data; corrosion/scale/biofilm inspection; process leaks; and maintenance history.
Laboratory cleaning solution, deposits, and rack drain water belong in the site’s waste procedure. Indonesia’s Government Regulation No. 22 of 2021 covers water-quality protection and waste management; classification and disposal must follow actual composition and the facility’s environmental approval.