Every 60 to 90 days, a corrosion coupon comes out of your cooling tower — weighed, analyzed, and sent back to you as a report. In most facilities, that report gets filed without anyone reading it closely. That’s a problem.
Left unchecked, the corrosion those numbers describe will eat through pipe walls and heat exchanger tubes quietly and expensively, until it becomes an emergency. A corrosion coupon report is one of the clearest signals your water treatment program will ever give you. Once you know what the numbers mean, it stops being a formality and starts being a management tool.
This guide breaks it down: what corrosion coupons measure, how to read every field on the report, what the thresholds mean, and what to do when the numbers are off.
A corrosion coupon report is a lab analysis that quantifies how much metal your cooling tower water is destroying over time. A small, precisely pre-weighed piece of metal — usually mild steel, copper, or both — is inserted into the system for 60 to 90 days, then removed, cleaned, and reweighed. The weight loss is converted into an annualized rate expressed in mils per year (MPY).
That gives your water treatment program a real-world measure of whether your metals are protected. Think of it as a controlled experiment running inside your actual system — not in a beaker. A water chemistry test measures what’s in the water. A corrosion coupon tells you what that water is doing to your equipment.
Water chemistry can look fine while corrosion is still happening — especially in areas with uneven flow, dead legs, or localized biological activity. Coupons catch what chemistry tests miss. Both belong in a well-run program.
The primary output of any corrosion coupon report is the corrosion rate in mils per year (MPY) — thousandths of an inch of metal lost per year. Industry benchmarks for cooling tower systems are:
| Metal | Rate (MPY) | Status | What it means |
|---|---|---|---|
| Mild steel | Under 3 | ✓ Good | Inhibitor program is working. Metal is protected. |
| Mild steel | 3 – 5 | ⚠ Marginal | Investigate inhibitor residuals and pH control. |
| Mild steel | Over 5 | ✕ Action required | System corroding at damaging rate — program adjustment needed immediately. |
| Copper / copper alloy | Under 0.5 | ✓ Good | Normal for a well-controlled system. |
| Copper / copper alloy | 0.5 – 1.0 | ⚠ Marginal | Review pH and oxidant levels. Check azole residuals. |
| Copper / copper alloy | Over 1.0 | ✕ Action required | Copper loss will damage heat exchangers. Immediate review required. |
MPY is an annualized rate, which means a coupon pulled at 60 days and one pulled at 90 days produce directly comparable numbers. That’s intentional — it lets your water treatment provider spot trends over time without results being skewed by exposure duration.
If your report doesn’t show a target range next to the result, ask for one. It should always be there.
Beyond the MPY number, a complete corrosion coupon report includes several additional fields. Each one adds context:
| Field | What it’s telling you |
|---|---|
| Coupon weight loss | Actual grams lost during the exposure period. Used to calculate MPY — not meaningful on its own without the rate conversion. |
| Exposure time | Days the coupon was installed. Under 30 days is too short for reliable data. 60–90 days is the standard for cooling towers. |
| Coupon appearance | Visual lab notes: pitting, staining, deposits, or biological film. A clean coupon with low MPY is the target. Pitting warrants attention even at low overall rates. |
| Deposit analysis | Identifies what’s on the coupon — scale, iron oxide, biological film. Helps pinpoint root cause when rates are elevated. |
| Pitting notation | Separate from overall corrosion rate. Pitting is localized and punches through metal much faster than the average rate suggests. |
Uniform corrosion removes metal evenly. Pitting concentrates corrosive attack in one spot — it can perforate a pipe wall or heat exchanger tube while the overall MPY still looks acceptable.
Always read the visual appearance notes alongside the rate.
ChemREADY’s cooling tower specialists can review your program chemistry, inhibitor residuals, and coupon history to find what’s driving the problem.
If your report comes back with a rate above the acceptable threshold, there are five common causes your water treatment provider should be investigating. Knowing these helps you ask the right questions:
None of these require a crisis response in isolation. But when your corrosion coupon report shows a rate trending upward over two or more pull cycles, these are the five levers your treatment provider should be adjusting — and documenting.
The coupon also checks the other direction. A low or negative Langelier Saturation Index signals water that won’t lay down a protective carbonate film, and coupon rates are how you confirm whether that tendency is showing up in the metal.
If you have a recent corrosion coupon report in front of you, work through this checklist:
A corrosion coupon report shouldn’t be a mystery document that gets filed and forgotten. It’s one of the most direct indicators of whether your cooling tower treatment program is actually protecting your equipment. Once you know how to read it, it becomes a management tool — not a formality.
A lab analysis that quantifies how much metal your cooling tower water is destroying over time. A pre-weighed piece of mild steel, copper, or both is installed for 60 to 90 days, then removed, cleaned, and reweighed. The weight loss is converted into an annualized corrosion rate in mils per year (MPY).
For mild steel, under 3 MPY is good, 3 to 5 MPY is marginal, and over 5 MPY requires action. For copper and copper alloys, under 0.5 MPY is good, 0.5 to 1.0 MPY is marginal, and over 1.0 MPY requires immediate review.
MPY means mils per year: thousandths of an inch of metal lost per year. Because it is an annualized rate, a coupon pulled at 60 days and one pulled at 90 days produce directly comparable numbers, which lets you track trends over time.
Sixty to 90 days is the standard for cooling towers. Under 30 days is too short for reliable data. Coupons are typically placed in a bypass rack at the tower return.
Pitting concentrates corrosive attack in one spot and can perforate a pipe wall or heat exchanger tube while the overall MPY still looks acceptable. A coupon showing 1.5 MPY with significant pitting is a more urgent concern than one showing 3.5 MPY with a clean, uniform surface.
The five common causes are low inhibitor residual, pH outside the 7.0–8.5 target, high chloride or sulfate levels, biological activity such as sulfate-reducing bacteria under biofilm, and cycles of concentration running out of control.
Water chemistry can look fine while corrosion is still happening, especially in areas with uneven flow, dead legs, or localized biological activity. A water chemistry test measures what’s in the water. A corrosion coupon tells you what that water is doing to your equipment.
Lay it up right so the metal stays protected while it sits, and it restarts clean in spring.
We’ll discuss your cooling tower, your coupon reports, and whether working together makes sense.
Adeline Johnston writes WaterFACTS for ChemREADY, covering cooling tower and boiler water treatment, corrosion control, and water treatment costs.
Technically reviewed by the ChemREADY cooling tower services team.