Crevice corrosion

The notion of corrosion of crevices has been around for centuries, but it has only become of greater interest in recent decades. The term has been applied to describe the destructive effects of corrosion that occur within generally inaccessible regions of products, components and structures. Corrosion of crevices is enhanced due to a localized concentration of corrosive agents and environments. This process is accelerated by lower oxygen concentrations, aggressive reagents and restricted areas.

A crevice is defined as an area with a width less than one or two times the thickness of the material, an open gap or an area covered by a thin film which can be the result of the construction materials, gaskets and other components. Crevice corrosion can cause catastrophic safety failures including pressure vessels, bridges, and aircraft components, and can also lead to an increased cost of maintenance and corrosion protection.

The corrosion of crevices is a very complicated phenomenon that can be affected by chemical, physical and electrochemical aspects. It is well known as a localized corrosion and is described in several different types, including pitting corrosion, caustic embrittlement, and stress corrosion cracking. Crevices play a key role in the corrosion process, as they can provide sites for the initiation of corrosion and then act as reservoirs for aggressive active species, creating a strong positive feedback and promoting further corrosion.

The coating of surfaces such as polymers and epoxy paints and coatings is one of the best ways to prevent crevice corrosion from occurring. The coating should be thick enough to eliminate any voids and prevent the intrusion of water and other corrosive substances. Coatings such as zinc, magnesium and aluminum are extremely effective in resisting crevice corrosion due to their strong galvanic protection.

However, although coatings provide long-term protection, they can be affected by certain environmental conditions such as humidity or water intrusion. Therefore, a proper inspection of the coating should be regularly performed in order to identify any potential defects. In cases where the coating has been damaged or is compromised, repair or repainting of the affected area is recommended.

When coating is not an option, or when will not be of sufficient protection, corrosion protection methods such as cathodic protection can be applied. Cathodic protection works by supplying a controlled current from an external source and elevating the potential of the protected surface to a value below the atmospheric corrosion potential, thus eliminating the corrosion potential. An alternative method for combating crevice corrosion is to apply a stainless steel, aluminum or zinc alloy covers.

In addition, the use of inhibitors can be an effective way of protecting against corrosion. Corrosion inhibitors form a protective barrier which block or reduce the access of corrosive agents to the exposed metal surface, thus reducing or prolonging the corrosion process. Inhibitors include chemicals such as phosphates, chromates and nitrites which are effective but can be toxic and dangerous to handle.

Crevice corrosion is a serious issue when it comes to safety and reliability of components, products and structures. A good understanding of the various aspects of the corrosion process and effective protection techniques are crucial for ensuring the integrity of these components. The application of proper coatings, cathodic protection, and inhibitors are thus invaluable and serve to avoid corrosion of crevices and the associated catastrophic consequences.

Gap erosion occurs when water flows through small gaps created by rocks and gravel left on the streambed. As the water flows, the gaps are widened and deepened. This process can cause changes in the streams structure and the types of aquatic species that inhabit it.

Gap erosion is primarily caused by high flow rates, or the amount of water flowing through the stream. The greater the flow rate, the more likely it is that gaps will be widened and deepened. In some cases, the gaps will become large enough for larger fish or other aquatic organisms to enter, which can cause changes in the fish population. Other factors that can contribute to gap erosion include changes in stream depth, rocks and gravel being moved downstream, and sediment building up along the banks of the stream.

To prevent gap erosion, there are some steps that can be taken. For example, adding objects to the streambed, such as large rocks or wooden frames, can help reduce the size of the gaps. This can help slow down the flow of the stream and reduce the impact of water on the gaps. Additionally, minimizing human activities in the area can help reduce the amount of sediment and debris that get into the water and contribute to the widening and deepening of the gaps.

Gap erosion is an important issue in stream ecology, as it can have a significant impact on aquatic species and their habitats. Understanding the causes and effects of gap erosion, as well as the steps that can be taken to prevent it, can help ensure that the stream is healthy and the aquatic species inhabiting it are protected.