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Introduction of Corrosion Coupons in Pipelines

Corrosion coupons in pipelines aren’t just affordable; they’re also a powerful tool for tracking corrosion rates in any structure or system. Yet, getting meaningful results isn’t always as simple as it sounds. Things like the surface finish of your coupon, where you place it, and how long you leave it in place can seriously affect the quality of your data.

Corrosion coupon testing is an in-line monitoring method. You place the coupons right in the process stream and later take them out for measuring. It gives you a direct measure of metal loss, which lets you calculate the general corrosion rate. The cool thing about using corrosion coupons is that your results don’t depend on the phase of the environment where you do the measurement.

So, why should you use corrosion coupon testing? Here are a few reasons:

• It’s simple to understand and apply.
• You get to keep the specimens for post-test examinations.
• It lets you compare different alloys and inhibitors.
• It assesses all forms of corrosion.
• And it’s low-cost.

Corrosion coupons are usually used to study general corrosion and to figure out the corrosion rate based on the coupon’s weight loss. But they can also help you understand other forms of corrosion:

• Crevice corrosion: You can use special washers or spacers to simulate crevices where the coupon surface is partially blocked from the liquid.
• Pitting: You can evaluate this type of attack visually or with a microscope. It’s best to use coupons with a large surface area because the number of pits that form is proportional to the surface area of the specimen.
• Galvanic corrosion: You can place coupons of different alloys in electrical contact to study this.
• Stress corrosion cracking: Special coupons like C-rings and U-bends can help you investigate this form of corrosion.
• Scaling: There are special coupons with various hole diameters that you can use to visually assess the extent and severity of scaling problems.

Parameters of the Corrosion Coupons

Factors Affecting Corrosion in Subsea Pipeline

1. Salt Content: Seawater is salty, a lot saltier than most other waters. This saltiness affects the ability of the water to carry electricity and the amount of oxygen it holds. Both of these things influence how quickly metals corrode in the water.

2. Dissolved Oxygen: Oxygen plays a big part in how fast things rust. The more oxygen there is in the water, the faster metals can corrode. Seawater, especially at the surface, tends to have a lot of oxygen because it’s always mixing with the air.

3. Temperature: Another player in this game is temperature. Warmer water can speed up corrosion, but it also holds less oxygen. So, when the water gets hotter, there’s a bit of a tug-of-war between these two effects. And don’t forget, warmer water can also change the behavior of sea creatures and the buildup of certain deposits on metal.

4. pH Level: The pH level of seawater is pretty close to neutral—not too acidic or too alkaline. But as you go deeper, the pH drops a bit. While pH isn’t as big a factor as oxygen in causing corrosion, it does influence certain processes that can protect against corrosion.

5. Flow Rate: How fast the water moves can affect corrosion. If the water is moving quickly, it can bring more oxygen to the metal, speeding up corrosion. But for materials that form a protective layer, faster water can actually help keep them from corroding.

6. Marine Life: Lastly, we have to consider the tiny organisms living in the sea. These little creatures can produce chemicals that speed up corrosion. Interestingly, in areas where the seafloor is muddy and there’s not much oxygen, there’s usually less corrosion. But where there’s a lot of organic stuff and certain bacteria, corrosion can be even faster than in normal seawater.

Limitations of the Corrosion Coupons Testing

Corrosion coupon testing, while valuable, has some limitations:

• It measures only the average corrosion rate during the exposure period. This means it does not capture variations in corrosion rates that may occur during shorter time intervals.
• Corrosion rates can only be calculated after the coupon is removed. Hence, real-time monitoring of corrosion is not possible.
• The method can yield unrepresentative corrosion rates for short exposure periods, particularly for alloys that form passive films like stainless steel. As a result, normal exposure periods often extend to around 90 days.For example, ASTM G311 recommends a formula to calculate minimum exposure time in hours:“Exposure time (in hours) = 2000 / Expected corrosion rate (in mpy)`So, if a corrosion rate of one mil per year (0.001 inches) is expected, the minimum recommended exposure time would be 2000 hours, or approximately 83 1/3 days.