What is the orientation of a corrosion coupon?

What are corrosion coupons?

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A corrosion coupon is an in-line intrusive corrosion monitoring device used widely in oil and gas assets. Corrosion coupons are typically constructed from carbon steel. These can be generic carbon steel or may be selected to match the material used for a specific purpose; for example, X65 pipeline steel or weld consumables.

Any corrosion coupon should be certified by recording its serial number, weight in grams, dimensions, material, and surface finish.

Why are corrosion coupons used?

Corrosion coupons are used to monitor the corrosivity of a system. Corrosion coupons are installed to duplicate the corrosion rate experienced in piping, pipelines, or vessels. They can also be used to estimate the effectiveness of the mitigation program in place or evaluate the suitability of a specific material for a specific system.

What are the advantages and disadvantages of using corrosion coupons?

Advantages:

• Simplicity – No electronic monitoring equipment is required

• Versatility – The technique is extremely versatile because weight loss coupons can be fabricated from any commercially available alloy. Also, using appropriate geometric designs, a wide variety of corrosive environments may be studied

• Applicability – The technique is applicable to all environments and both corrosive and erosive species- gases, liquids, solids/particulates, stagnant or flowing conditions

• Reliability – Coupons are not subject to the various forms of interference that can lead to false corrosion readings or probe failures

• Visual Assessment – Visual and microscopic assessment can be undertaken

• Localised Phenomena – Localised corrosion phenomena, such as pitting, can be assessed

• Surface Analysis – Surface deposits can be observed and analysed

Disadvantages:

• Delayed Data – In a typical monitoring program, coupons are exposed for a three-month to one-year duration before being removed for laboratory analysis. As such, an indication of a corrosion event may only be obtained significantly after the event may have occurred

• Lack of Time Determination – If a specific corrosion event occurs during the period of exposure, the coupon will not be able to identify the time of occurrence

• Time Averaging – Depending upon the peak corrosion rate of a corrosion event and its duration, the coupon will not indicate the peak corrosion rate of the event, and short-term events may not even register a statistically significant increased weight loss 

• Galvanic Corrosion – Corrosion coupons are typically isolated from any metal-metal contact. Thus, they are unaffected by the galvanic interactions present in the system

What data does a corrosion coupon provide?

Corrosion coupons should be located where corrosion is expected. Corrosion coupons provide the general and pitting corrosion rates. The general corrosion rate is assessed through metal loss from the coupons over a certain time period. The worst pitting corrosion rate is assessed using the depth of the deepest pit over a period of time.

Several factors should be taken into consideration during the interpretation of corrosion coupon data; such as:

• Location
• Exposure time
• Corrosion product/scale composition
• Flow velocity and flow mode
• Corrosion mitigation

To ensure effective monitoring in a given system, coupons should be placed throughout the different areas in the system where the worst-case corrosion rates are predicted. This could include low points, stagnant areas, and high velocity/turbulent areas. Additionally, both the 6 o’clock and the 12 o’clock positions should be monitored for piping and pipelines.

Long exposure to the service fluid (3-6 months) is required to evaluate corrosion rates. It should be noted that some conditions such as bacteria fouling take time to develop on the coupon surface.

How should the coupons be oriented and positioned?

Corrosion coupons orientation should be consistent in order to assess and compare different data sets. Typically, a corrosion coupon should be oriented in a direction parallel to the process flow. Both of the 6 O’clock and the 12 O’clock positions should be monitored and this is especially important for systems with multiphase/stratified flow or systems where bottom of line deposits could occur.

The corrosion coupon should be positioned in the plane of the vessel or pipe wall to provide a good representation of the flow regime of the vessel/pipe. The best representation of flow in a given system will be achieved by using flush mounted coupons rather than intrusive coupons.

How many coupons should be installed?

Typically, at any coupon monitoring point, more than one coupon is installed for two reasons:

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• Many uncontrollable factors may exist and reduce the accuracy of a single coupon. The use of paired coupons, with an average value taken, substantially improves data reliability and accuracy.

• Corrosion coupons give an indication of the corrosion that has already occurred. If it is required to verify whether the corrosion rate was uniform or varying over a certain time, then multiple corrosion coupons could be installed at the same location and retrieved at different times.

Types of corrosion coupons

Figure 1: Corrosion coupon and holder types

There are many different coupon types. The most common coupon types are described below and presented in Figure 1:

• Strip coupons
  • The most common type
  • The most economical
• Disc coupons
  • Useful for multiple phase flow especially when used with ladder coupon holders to evaluate the corrosion rates across the diameter of the pipeline/piping
  • Flush Disc coupons can be mounted to align with the interior of the piping, pipeline or vessel. This has two advantages:
    • The coupon will more accurately present the flow in the vessel
    • For pipelines, this will enable the pipeline to be pigged
  • Orientation with product flow is not required
• Special purpose coupons
  • Scale coupons
    • Provide the deposition rate (coupon must be made of a non-corrodible material if weight gain is to be used)
  • Bio-film (mesh) coupons
    • Stainless steel mesh
    • Used to identify micro-organisms and resist corrosion
  • Elastomeric coupons
    • Assess materials selection for elastomeric testing and applications.

Corrosion coupon holders

A coupon holder is used to fix two or more coupons at a certain location. Depending on the operating conditions, number and type of coupons, configuration and system entry method, the coupon holder is selected.

The common type of holders used are:

• Fixed coupon holder
  • System depressurization is required during insertion and removal
  • It is used in systems that have frequent shutdowns or systems that can be isolated easily
  • Simple to install
  • Can be made from a variety of materials
  • Most economic
• Adjustable coupon holders
  • Pressure up to 125 psi
  • Commonly used in the water treatment industry
• Retractable Coupon Holders
  • Commonly used in refineries
  • Pressure up to 1500 psi
  • Leakage is prohibited
  • A safety cable is provided to prevent accidental discharge
  • No process shutdown is required
• Retrievable Coupon Holders
  • Commonly used in oil and gas production
  • No process shutdown is required

Additionally, ladder coupon holders may be used instead of single or paired coupon holders to facilitate monitoring across the diameter of piping or pipelines allowing simultaneous monitoring of the 6 o’clock position, 12 o’clock position and the center line.

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If you're in charge of a cooling tower, you know that constant vigilance against corrosion is part of the game. While some corrosion is inevitable, minimizing it is an ongoing effort. Also, changes in corrosion rates can indicate problems that need your attention.

That's where corrosion coupons come in. They are simply pre-weighed, pre-measured metal strips which are mounted in a coupon rack--a dedicated piping system for monitoring corrosion. Available in a wide variety of materials, corrosion coupons help assess corrosion in a variety of systems. After exposure to the recirculating water in the system for a specified interval of time, they're removed, cleaned of corrosion, and weighed. The weight loss is converted to a corrosion rate (CR) or metal loss (ML).

This method of calculating corrosion rates is the most widely used and longest-established method of estimating corrosion losses in plants and equipment. Because it's simple, direct and versatile, it continues to prevail despite advances in technology. 

The following are best practices to get the most of out of corrosion monitoring with coupons, as we've developed in our many years of experience at Chemtex.

Coupon Test Rack Duration

The recommended time frame for coupons in a test rack is a minimum of 90 days. 

Short-term testing durations (30 days or less) usually have higher corrosion rates compared to coupons remaining in the rack for longer terms (90 days). With short terms, coupon surfaces are active and have not acclimated to the environment, resulting in greater metal loss from the coupon—longer terms significantly reduce this effect. This is because as the metal coupon corrodes, the corrosion products can act as a barrier, reducing the rate of attack. 

Coupon Placement & Orientation 

• Placement should follow the galvanic series, with the most active metal or alloy placed in the number one position downstream of the water flow. This procedure prevents the more noble metal or alloy from cathodically depositing on the active metal or alloy. 
• Coupons should never be placed in the rack such that water flows from the coupon holder. 
• Coupons should be orientated with the broad face in a vertical position, reducing the accumulation of debris onto the face of the coupon, which can accelerate corrosion.

Temperature & Flow Characteristics 

• Water flow through a coupon test rack should be 3-5 feet per second (or 3 to 5 gpm). High velocities can cause erosion on soft alloys. Low velocities can cause particulate matter to settle onto the coupon, resulting in underdeposit (corrosion) attack. 
• Discharged rack water should be open to the atmosphere to eliminate back pressure. 
• Corrosion racks should be connected to the cooling tower riser, where the water is the warmest. Lower temperature supply line connections will cause bias results with lower rates. 

Water Quality & Process Contamination 

• pH, temperature, conductivity, TDS and SS will influence corrosion (attack) on coupons. 
• Contaminations can accelerate attack. For example, ammonia generation from nitrifying bacteria in closed systems will raise the pH and attack copper. 

Level of Treatment & Coupon Racks 

Corrosion racks should not be coupled to brass or galvanized steel valves. Brass will cause copper ions to deposit onto steel causing accelerated corrosion. Galvanized will release zinc ions to deposit onto steel, also accelerating corrosion. Galvanized will release zinc ions which act synergistically with the chemical treatment to provide improved corrosion protection to low carbon steel. 

To sum it all up…Corrosion Coupons mounted correctly in a flow controlled coupon rack are an excellent way to monitor corrosion. If you are not using coupons and getting quarterly reports from your Water Treatment Professional, you should discuss this offering with them.

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