Description

What is a Corrosion Coupon?

A corrosion coupon is a small, pre-weighed, pre-measured specimen of metal (or sometimes a non-metal) that is inserted into a pipeline, vessel, or process stream for a controlled period. After retrieval, it is analyzed to directly measure the rate and type of corrosion occurring within the system.

How It Works (The Process)

Fabrication & Preparation: Operators machine the coupon from a material identical or similar to the system’s construction material (e.g., carbon steel, stainless steel). They then meticulously clean it, document its surface finish, and precisely weigh and measure it.

Installation: Operators place the coupon in a coupon holder or retriever and then install the assembly into the process stream via an access fitting. The coupon must experience the same conditions—flow, temperature, and chemistry—as the equipment wall.

Exposure: The coupon remains in the system for a predefined period (typically 30, 60, 90, or 180 days). During this time, it corrodes at the same rate as the internal walls of the pipe or vessel.

Retrieval & Analysis: After retrieval, the coupon is cleaned using specific chemical methods to remove corrosion products without attacking the base metal. It is then:

Re-weighed to determine weight loss.

Calculation & Reporting: Data is used to calculate the corrosion rate, usually in mils per year (MPY) or millimeters per year (mm/y).

Applications of Corrosion Coupon

Measure Corrosion Rate: Provides a tangible, time-averaged corrosion rate for the system.

Identify Corrosion Mechanisms: Reveals if corrosion is general (uniform) or localized (pitting, crevice), which is critical for assessing risk.

Evaluate Corrosion Control Programs: Tests the effectiveness of inhibitors, coatings, or material upgrades.

Provide Early Warning: Serves as a sacrificial indicator, warning of increased corrosion before it damages critical equipment.

Benchmarking & Compliance: Creates a historical record for regulatory compliance and system integrity management.

Types & Common Designs

Strip Coupons: Rectangular strips (common for rack holders).

Disc Coupons: Round discs (often used in high-pressure retrievable systems).

Typical Placement & Use in Industry

• Oil & Gas Production: Downstream of separators, in water injection lines, at wellheads.

• Refineries & Chemical Plants: In critical product streams, cooling water systems, overhead lines of distillation columns.

• Pipeline Systems: In traps or side-stream loops.

• Water Treatment & HVAC: Cooling towers, boilers, and domestic water lines.