Effect of Gap Size on Flange Face Corrosion in Offshore Wind Turbines

Abstract

Bolted flanged joints are key components in offshore wind turbine towers, connecting different sections of the structure. This study investigates how the size of gaps between flanges—specifically, the thickness and depth of the crevices—affects corrosion on stainless steel (SS) flanges. Experiments in a saltwater solution at 50 °C with gaskets to create gaps between flange surfaces were conducted. A custom-designed fixture was used to mimic real-world conditions, such as gasket stress and fluid flow. Our results showed that as the gap thickness increased from 1.58 mm to 6.35 mm, the corrosion rate also increased significantly, from 0.09 mm per year to 1.03 mm per year. However, the onset of crevice corrosion increased from 0.23 hours to over 3 hours. When the crevice depth was reduced from 7.49 mm to 0 mm, the general corrosion rate dropped drastically, with smallest depths showing almost no sign of crevice corrosion. These findings help improve our understanding of how gap size affects corrosion in bolted flanged joints, offering insights for better design and maintenance of offshore wind turbine structures.

Keywords: offshore wind turbine; bolted flanged joints; crevice corrosion; gap size.

Corrosion of Flanged Joints in Offshore Wind Turbines

Offshore wind turbines (OWTs) are powerful machines that harness energy from the wind to generate electricity. These turbines consist of three main parts: 1) tower, 2) transition piece, and 3) foundation [1]. To connect the tower sections, a special connection of the type bolted flanged joint is used. This joint involves two metal plates (flanges) tightened together with bolts, often separated by a gasket (seal) for added protection [2].

While this design typically works well, the harsh marine environment poses a serious challenge, making the OWT structure susceptible to corrosion. The combination of strong wind and seawater creates conditions that wear down the metal, leading to fatigue and cracks over time [3]. This is especially true for the bolts used in these joints, which have been studied extensively [4]. However, corrosion that occurs at the flange interface is an area that has been less researched.

In most cases, gaps and crevices exist between the flanges and the gasket. These small spaces trap water and other corrosive elements, leading to what is known as crevice corrosion. Detecting this type of corrosion before it causes damage is highly challenging but crucial, because neglecting it can result in costly repairs or even structural failure [5].

This research study focuses on understanding how particular designs influence crevice corrosion in these bolted joints. A special setup to study how different gasket dimensions affect the corrosion process, aiming to find better solutions for OWT longevity is developed.

Mechanism of Crevice Corrosion

In environments with saltwater, SS components like flanged joints can suffer from crevice corrosion. This type of corrosion occurs when oxygen is unevenly distributed between tight gaps (or crevices) and the surrounding water (as shown in Figure 1a). In these small enclosed spaces, chloride ions (Cl^-) from the water react with metal ions, leading to a chemical process that makes the environment more acidic. This acidity weakens the protective layer on the metal, promoting corrosion (Figure 1b).

Effect of Gap Thickness and Depth on Flange Face Corrosion

It was found that thinner gaps reduce fluid circulation, creating perfect conditions for the onset and propagation of crevice corrosion, as shown in Figure 2. On the other hand, wider gaps let more fluid flow through, which can prevent crevice corrosion but may cause other damage to the metal protective layer, leading to deeper pitting on the metal surface.

Overall, our findings suggest that while larger gaps can slow down crevice corrosion, they can increase general corrosion, leading to the weakening of the structure. Since crevice corrosion poses an immediate risk to the integrity of structures like flanged joints, it is recommended to eliminate the gap by using full face gaskets or using thicker gaskets to minimize the occurrence of this type of hidden and localized corrosion.

Crevice corrosion is influenced by the depth of the gap (crevice), between the flange and gasket in flanged joints. This depth plays an important role in the circulation of fluids and oxygen in and out of the crevice.

When the crevice is deep, oxygen gets used up faster in the small space, speeding up the corrosion process. This leads to a quicker breakdown of the protective layer in the crevice. In contrast, with smaller crevice depths or no gap at all, fluids circulate better, preventing the buildup of acids that can damage the protective layer on the metal surface.

Our study showed that the higher gradient of oxygen content—lower in the crevice and higher in the bulk solution—led to the development of distinct anodic and cathodic regions, which promoted galvanic corrosion.

Corrosion Morphology

When SS flanges are exposed to corrosive environments, the way corrosion develops and its morphology, can vary based on factors like stress and the type of gasket used. In our study, a SS flange pair are examined after 12 hours of testing, focusing on how corrosion propagates in the crevice between the flanges and the gasket.

The most severe corrosion occurred near the gasket, as shown in the color map (Figure 3a). This happens because the oxygen level drops inside the crevice, allowing chloride ions (Cl^-) to react and make the environment acidic. This acidic condition breaks down the protective layer on the metal surface, leading to crevice corrosion.

Due to the high stress applied to the gasket (15 MPa), the corrosive solution had difficulty seeping into the gap at first. As a result, the corrosion initially spreads downward, then gradually under the gasket. In these acidic conditions, the grain boundaries of the metal became more prone to corrosion (Figure 3c).

The corrosion patterns differed based on whether the area was exposed to the solution or hidden under the gasket. On the exposed area (Figure 3b), there was a sharp line separating the corroded and non-corroded areas, with no visible cracks. However, under the gasket (Figure 3d), numerous cracks were found, likely due to high contact stress in that region. These cracks worsened the corrosion over time, as shown in greater detail in Figure 3e.

The profile of the corroded area (Figure 3f) confirms that the deepest corrosion occurs near the gasket and gradually decrease further away. Compared with other studies on crevice corrosion, the higher stress levels applied to the gasket study likely contributed to the cracking, as other research usually reported pitting without cracking.

Conclusion

In this study, the effects of gap size (both thickness and depth) on the corrosion of SS flanges, commonly used in bolted flanged joints of wind turbines and pipelines were explored. Using electrochemical tests and surface analyses under simulated real-world conditions, including stress on the gasket, fluid flow, shape of the crevice between the gasket and flange was scrutinised.

Key findings are:

• Gap size influences the contribution of crevice corrosion and pitting corrosion on the flange surface.
• Gap thickness increases the corrosion rate on the flange surface. Additionally, the reduction of crevice depth decreases, in the general, the corrosion rate on the flange surfaces.
• Crevice corrosion starts at the flange-gasket interface and spreads under the gasket. Interestingly, the time it takes for this corrosion to begin increases larger gap. No crevice corrosion was found on samples where no gap is present or with the case of full face gasket.

These findings show the importance of controlling gap size in bolted flanged joints to prevent corrosion, and extend component life.

References

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