What is a corrosion coupon?

Introduction to Corrosion Coupons - AIE

A corrosion coupon serves as a crucial tool in assessing metal degradation within various industrial systems. According to recent insights, corrosion coupons are standard measurement devices that provide data on metals' reaction to specific environmental conditions, such as moisture, chemicals, and heat. In essence, corrosion coupons are metal strips used to measure the rate of corrosion over time.

What are corrosion coupons?

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A corrosion coupon is an in-line intrusive corrosion monitoring device commonly employed in oil and gas operations. Typically made from carbon steel, these coupons can be generic or specifically chosen to match the characteristics of particular materials, like X65 pipeline steel or weld consumables.

Each corrosion coupon must be certified by documenting its serial number, weight, dimensions, material, and surface finish.

Why are corrosion coupons used?

Corrosion coupons are crucial for assessing a system's corrosivity. They are integrated to replicate the corrosion rates present in piping, pipelines, or vessels. Furthermore, these coupons can help evaluate the effectiveness of existing corrosion mitigation strategies or determine the suitability of materials for specific operational environments.

Advantages and Disadvantages of Using Corrosion Coupons

Advantages:

• Simplicity: No electronic monitoring equipment is necessary.
• Versatility: The technique's flexibility allows for the fabrication of weight loss coupons from various commercially available alloys; appropriate geometric designs enable the examination of diverse corrosive environments.
• Applicability: This method applies to all environments, accommodating both corrosive and erosive agents—whether gases, liquids, or solids, and regardless of flow conditions.
• Reliability: Unlike electronic systems, coupons avoid interference leading to inaccurate corrosion readings or probe failures.
• Visual Assessment: Coupons allow for both visual and microscopic evaluations.
• Localised Phenomena: Identification and analysis of localized corrosion phenomena, such as pitting, can be conducted.
• Surface Analysis: Observations and examinations of surface deposits are facilitated.

Disadvantages:

• Delayed Data: The typical exposure duration ranges from three months to a year, meaning indications of corrosion events may emerge significantly after occurrences.
• Lack of Time Determination: Coupons do not indicate when specific corrosion events happen if they occur during the exposure period.
• Time Averaging: The coupons may not reveal peak corrosion rates during events due to prolonged exposure periods.
• Galvanic Corrosion: Corrosion coupons are isolated from any metal-metal interactions, rendering them ineffectual against galvanic corrosion.

What data does a corrosion coupon provide?

Strategically placed where corrosion is anticipated, corrosion coupons generate data on general and pitting corrosion rates. General corrosion rates derive from the metal loss over time, while the worst pitting corrosion rates correspond to the depth of the deepest pit recorded over a set period.

Factors for Data Interpretation:

• Location
• Duration of exposure
• Composition of corrosion products/scale
• Flow velocity and mode
• Corrosion mitigation strategies implemented

For effective monitoring, coupons should be strategically distributed across the system's high-risk areas, such as low points, stagnant zones, and turbulent regions. It's critical to monitor both the 6 o’clock and 12 o’clock positions in piping and pipelines.

Long exposure to the service fluid—ranging from 3 to 6 months—is generally required to accurately evaluate corrosion rates, as certain conditions, like bacterial fouling, need exposure time to manifest on coupon surfaces.

Coupon Orientation and Positioning

The orientation of corrosion coupons should remain consistent for reliable data comparisons. Typically, coupons are installed parallel to flow direction. Monitoring the 6 o’clock and 12 o’clock positions proves vital in multiphase flow systems or when bottom deposits are present.

Positioning should enable the coupon to align with the pipeline or vessel wall for optimal flow representation. Using flush-mounted coupons is preferable for a more accurate representation.

How many coupons should be installed?

Typically, it is advisable to install multiple coupons at any monitoring point for improved data accuracy and reliability. Using paired coupons to obtain averaged measurements enhances dependability. Additionally, retrieval of multiple coupons at varying times can provide insight into the uniformity of corrosion rates.

Types of Corrosion Coupons

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Figure 1: Corrosion coupon and holder types

• Strip Coupons: Most common and economically viable.
• Disc Coupons: Valuable for multiphase flow applications, especially with ladder coupon holders.
• Special Purpose Coupons: Includes scale coupons for deposition rates and bio-film cups for microbial assessments.

Corrosion Coupon Holders

A coupon holder secures two or more coupons in place and is selected based on parameters such as operating conditions and system entry methods.

• Fixed Coupon Holder: Requires system depressurization; ideal for systems with frequent shutdowns.
• Adjustable Coupon Holders: Suitable for water treatment with a pressure capacity of up to 125 psi.
• Retractable Coupon Holders: Common in refineries, prohibiting leakage without requiring process shutdowns.
• Retrievable Coupon Holders: Frequently used in oil and gas production, allowing for maintenance without system shutdown.

Additionally, ladder coupon holders can facilitate monitoring across pipeline diameters for a comprehensive overview of corrosion rates.

For further understanding of how our corrosion management team can assist with your corrosion assessment and management needs, please visit our contact us page.

Through an integrated approach to corrosion management, chemical, and flow assurance, our leading software, Veracity CCM, empowers operators to manage potential threats effectively, sustaining operational uptime while minimizing unforeseen maintenance and repairs.

To learn more about Veracity CCM, click here.

To request a free demo, contact us.

What Are Corrosion Coupons and Do I Need Them?

Corrosion occurs when metal interacts with moisture, leading to rust formation; this surface issue hints at deeper problems within the systems. Protecting industrial water from corrosion requires the use of inhibitors and effective monitoring.

Conducting the Corrosion Test

Coupons are secured in a coupon rack installed within the recirculation loop post-system flow. Precise placement ensures that active metals yield accurate readings, and the vertical orientation of strips mitigates debris accumulation.

Value of Corrosion Coupons

Utilizing corrosion coupons can be a cost-effective strategy to maintain your HVAC or process systems. While there are initial setup fees, the long-term savings outweigh the costs associated with replacing corroded components.

Interpreting Results

Corrosion rates are derived from measuring metal loss over time and can be analyzed based on pitting surface areas as well. Numerous variables can affect corrosion rates including placement, exposure, and treatment protocols.

Each metal possesses distinct acceptable corrosion rates, thus thorough assessments are critical for operational integrity.

Next Steps After Corrosion Rate Assessment

If the corrosion rate is acceptable, continue the ongoing monitoring regimen. If rates surpass acceptable limits, collaborate with your water treatment team to reassess strategies and conditions.

For additional details regarding corrosion coupons or corrosion protection, don’t hesitate to reach out. An experienced HOH Water Quality Engineer is ready to assist you!

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