What is Corrosion Coupon? (Corrosion Coupon)

What is Corrosion Coupon ?Corrosion coupons are a very simple and effective tool. This tool provides a quantitative estimate of the corrosion rate that is occurring within a particular operating system. Coupons also provide an objective signal of the type of corrosion that may occur in the monitored system.

This method involves placing a sample of material (coupon) in a process environment for a period of time, then sampling it for analysis . The basic measurement determined by corrosion coupons is weight loss. Weight loss, which occurs during the period of exposure to the environment, is expressed as the rate of corrosion.

Corrosion coupons are also known as corrosion monitoring coupons.

Therefore, most data for corrosion of alloys in high temperature gases are defined by factors such as weight / area change for relatively short contact times and conditions of exposure to an unknown corrosive environment. However, weight loss should be attributed to thickness reduction in design and functional decisions.

Corrosion in high temperature gases is affected by the parameters of corrosive environments such as temperature, alloy composition, time and gas composition.

Coupons are first weighed, then placed in a special environment, removed, and then cleaned and re-weighed. This process is done to determine the amount of corrosion in weight loss over a period of time.

NACE has a standard method for determining weight loss. The result of determining the rate of weight loss in miles per year (MPY) is the most used reference. This reference is also used in most other corrosion monitoring methods.

Millimeters per year and several different units of measurement are also used. But so far MPY is the most used.

Corrosion monitoring is usually done using the following:

• Coupons
• Electrical resistance probes
• Probes designed based on linear polarization resistance,

Many refineries use online analyzers for continuous monitoring of pH, Iron and Chloride. Continuous measurement provides a stepwise change in the rate of corrosion.

The simplicity of the measurement provided by the corrosion coupon is such that the coupon technique forms the basis of the measurement method in many corrosion monitoring programs. This technique is very various. Because weight loss coupons can be made from any available commercial alloy.

Also, using appropriate geometric designs, various types of corrosion phenomena may be studied, including:

• Tension corrosion
• Bimetallic invasion (galvanic)
• aeration cell corrosion
• HAZ regions

Corrosion coupon holders are used to suspend corrosion coupons in the monitored area.

Coupon holders work with a variety of corrosion coupons, including rods (cylinders), single-hole flats (strips), flat-holes (strips), double-holes and discs, to measure corrosion rates in pressurized processes such as pressurized tanks, pipelines, and oil wells. . Coupons can be installed in any direction to meet the project requirements.

Pipeline corrosion coupons:

Corrosion coupons placed in the pipeline can be weighed immediately. Therefore, it shows the material drop faster. It can also be considered a regular basis for corrosion.

The coupon can be a tube-like metal or a more active metal, such as Zinc or Aluminum, that responds more quickly to corrosive conditions.

However, the effects of injection, temperature rise and pressure are not measured at surface conditions. This is why more active metal is often used instead of sample Steel.

McNealy et al. (2009) showed how a special autoclave is used to test coupons for corrosion before application in the environment. Test conditions can be selected to simulate well conditions such as temperature, total pressure, gas mixture, brine, rotation speed, time and types of coupons.

Corrosion couponscan come in many forms and types. Such as rectangle, circle or spiral. They may also be ordinary or prestressed. When you find out that the size of the area is more exposed to the environment, the size does not matter. The readings will be more accurate and the reading will be faster.

Weight loss corrosion coupons are available in any material. However, the material is often made of environmentally friendly materials (eg, pipe or tank) or low carbon steel that is highly susceptible to corrosion. If you want to repeat the corrosion on the pipe, you must use coupons of the same material.

You can use low carbon steel coupons to determine if the inhibitor is forming a film. However, when using low Carbon coupons, keep in mind that if you can stop corrosion on these coupons, the corrosion rate of the tube itself will not necessarily be affected.

With two mounting holes, it can be used for various studies. This coupon is widely used for all corrosion monitors. These coupons are widely used in collections at line intervals with narrow ports or low pressure equipment.

The most common form is usually called a bar coupon and is often available in the following sizes:

What is corrosion coupons; Disc coupons:

It is commonly used in systems where the coupon cannot be extended into the pipe to interfere with flow. It is also used in multiple or stacked monitoring programs.

A disc coupon has an mounting hole that is removed and a head screw, insulating washer and insulating disc are used for mounting.

What is the advantage of coupons?

They are relatively inexpensive, the result is positive weight loss, corrosion product samples or bacteria can be obtained from the surface for further observation and testing. Also, coupons are not as prone to tool failure as electronic corrosion monitoring methods.

• This technique is applicable to all environments: Gases, liquids and solids / particles.
• A visual inspection can be performed.
• Corrosion deposits can be observed and analyzed.
• Weight loss can be easily determined and, the corrosion rate can be easily calculated.
• Local corrosion can be identified and measured.
• The performance of inhibitors can be easily assessed.

Online Corrosion Coupons:

Online corrosion coupons are widely used in critical process circuits to evaluate fluid corrosion. Typically, alloy Steel and Carbon Steel coupons are used to monitor corrosion. Coupons can also be fixed or retractable.

Proper sealing should be used on retractable coupons in high temperature locations.

Assessing the nature of fluid corrosion:

Based on the corrosion rate of the coupons (for both uniform corrosion and cavity corrosion),the corrosive nature of the fluid can be assessed and the amount of additives can be optimized. In a typical monitoring program, coupons are displayed for laboratory analysis for 90 days prior to removal.

These initial measurements estimate the amount of corrosion repeated four times a year. Weight loss will be followed by fatigue and constant tiredness.

Corrosion coupons can be made by observing the amount of corrosion per year (MPY) in the system. Valuable data on material life can also be extracted. Valuable data on material life can also be extracted.

Coupons must be made of the same chemical composition as the pipeline or ship or the equipment being monitored.

Coupons exposed to corrosive solution similar to regular process solutions can provide information on the extent and type of corrosion.

Proper identification of stencils (pattern):

Corrosion coupons are usually stamped with alloy numbers and order for proper identification. They also usually include Mill test reports that specify the composition of the materials used.

Coupon monitoring:

Coupon monitoring is useful in environments where corrosion rates do not change much over long periods of time. However, it can provide useful communication with other techniques, such as electrical resistance and linear polarization resistance methods.

This tool can greatly help in accurately monitoring the corrosion rate in any critical environment. This tool is useful for:

• Repair costs
• Repair and maintenance
• Measurement of material degradation
• Determine the useful life of equipment

We used corrosion detection technique to prevent scale formation and scale in pipelines, wells and injection molding lines, etc.

Coupons and material selection:

Another use of coupons is to choose materials. If a pipe, ship, or tank is to be replaced, it is often recommended that you install multiple coupons of different alloys to check the performance of each. This allows the engineer to consider the choice of new pipe material or location for the appropriate materials for the particular environment we may be dealing with.

Coupons should always be used to confirm probe readings and to confirm long-term corrosion rates.

The disadvantage of corrosion coupons is that it takes a long time to get results and the coupons can only be mediocre. Coupons tell you the average corrosion from the time the coupon is installed until it is removed. However, it should be noted that corrosion is usually not uniform and constant.

Another disadvantage of the coupon technique is that if a corrosion disturbance occurs during the period of exposure to the corrosive environment, the coupon alone will not be able to determine when the disturbance occurred. Depending on the amount of turbulence peak and its duration, it may not even statistically increase weight loss significantly.

The uniform corrosion rate can be calculated by reducing the sample weight using a simple ASTM formula:

where in:

V: Corrosion rate (μm/year)

M∆: Sample weight change (mg)

ρ: Alloy density (g/cm³)

S: Sample area exposed to the environment (cm²)

t: Duration of exposure (h)

K: Fixed coefficient (10⁶*8.76، With units used for other terms)

Corrosion probes are electrically resistant monitoring devices (ER probe) that are used in many industries to monitor the corrosion rate without removing the corrosion probe.