How corrosion rate probes estimate how fast metal is corroding in the water, and how to read the result alongside coupons.
Most online corrosion rate probes in water treatment use linear polarization resistance (LPR).
- Electrodes. The probe carries two or three small metal electrodes made from the same alloy as the system metal you want to protect — commonly mild (carbon) steel and copper or a copper alloy such as admiralty brass. Using the matching alloy is essential: the probe measures how fast its own electrodes corrode.
- Polarization. The instrument pushes the electrode potential a very small amount away from its natural corrosion potential — typically about 10–20 mV — and measures the current that flows.
- Resistance to corrosion. The ratio of that small voltage to the current is the polarization resistance. A metal that corrodes quickly passes more current for the same voltage, so it has a low polarization resistance. The corrosion rate is inversely proportional to it.
- Units. The instrument converts the result, using constants for the alloy, to a rate in mils per year (mpy), where 1 mil = 0.001 inch. The metric unit is mm/y: 1 mpy ≈ 0.0254 mm/y.
Some instruments also report an imbalance or pitting indication, from the current noise between electrodes, which hints at localized attack.
¶ What the reading does and does not represent
- It is an instantaneous, general corrosion rate on the probe electrodes, updated every few minutes. It responds within hours to changes in chemistry, inhibitor dose, or upsets — which is its strength.
- It does not measure pitting well. LPR assumes uniform corrosion across the electrode. Pitting, under-deposit, and microbiologically influenced corrosion can be severe while LPR reads low.
- It is the rate on the probe, not the pipe. Differences in temperature, flow, deposits, and metallurgy mean the system may corrode faster or slower.
Corrosion coupons are weighed metal strips left in a rack for a period (commonly 30–90 days) and weighed again. They give an average rate over the exposure and show the type of attack — pitting, deposits, and staining — but only after the fact.
|
LPR probe |
Coupon |
| Response |
Minutes to hours |
Once per exposure period |
| Result |
Instantaneous general rate |
Average rate over the exposure |
| Shows pitting and deposits |
Only indirectly |
Yes, by inspection |
| Use it for |
Catching upsets, tuning inhibitor dose |
Confirming program performance |
Use both: the probe to see when something changes, and the coupon to confirm what the metal actually experienced.
- Low-conductivity water. LPR needs conductive water between the electrodes. In very pure water (commonly below a few hundred µS/cm, depending on the instrument), the water’s own resistance adds to the measurement and the rate reads low. Some instruments correct for solution resistance.
- Deposits and fouling on the electrodes — scale, iron, biofilm — shield the metal and change the reading, often making it read low when corrosion under the deposit is high.
- New electrodes. Fresh electrodes read high for the first hours to days until a surface film forms. Allow them to stabilize before trusting the number.
- Wrong alloy constants set in the instrument give a consistent error.
- Oxidizing biocide and treatment slugs cause genuine short spikes; recognize them as real events, not faults.
- Air bubbles or low flow across the electrodes change the local conditions.
- Electrical noise and stray currents, including cathodic protection systems and nearby VFDs.
- Electrodes touching each other or the pipe short the measurement.
- Mount where the probe sees representative water — the same temperature and flow as the equipment you are protecting. Corrosion rate is sensitive to both.
- Keep the electrodes fully submerged in flowing water, and in a pipe or rack that does not drain or trap air.
- Orient the electrodes across the flow so each electrode sees the same conditions, as the manufacturer shows.
- Put probes and coupons in the same rack or loop, at similar velocity (coupon racks are commonly run at around 1–1.5 m/s, or 3–5 ft/s), so the results can be compared.
- Isolate the electrodes electrically from the pipe and from each other; use the insulating fittings supplied.
- Handle new electrodes carefully. Do not touch the metal with bare hands; skin oils affect the surface.
¶ Maintenance
- Inspect the electrodes when you change coupons. Note deposits and pitting; they tell you as much as the number.
- Replace electrodes when they are visibly pitted, heavily coated, or worn down — commonly every few months to a year, depending on how corrosive the water is. Electrodes are consumable by design.
- Clean lightly or not at all. Cleaning removes the natural surface film and resets the reading; replacing the electrodes is usually better than cleaning them.
- Check the instrument with the manufacturer’s test resistor or simulator if one is supplied. There is no field calibration of the rate itself; accuracy depends on the right alloy settings and healthy electrodes.
- Compare with coupons each time coupons come out. A large, persistent gap between probe and coupon points to fouled electrodes, the wrong alloy setting, or local conditions that differ between the two.
Isolate and depressurize the line before removing a probe from a pressurized rack — see Isolation and Pressurized Systems.
Models documented in this wiki:
- ASTM G96, Standard Guide for Online Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods)
- ASTM G59, Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements
- ASTM D2688, Standard Test Method for Corrosivity of Water in the Absence of Heat Transfer (Weight Loss Method) — coupons