Corrosion Protection of Screw Joints

A screw flange, butt or lap joint is a special part of an installation, structure or foundation. Despite the identical environment of the whole installation/structure, the corrosion exposure is different, more intense, more dangerous in the zone of the joint itself than in the rest of the area.

The reason for this is the geometry and variety of materials of the joint components. An additional factor accelerating corrosion is stresses that sometimes reach high values, which can, for example, initiate a crack in the steel and thus create new areas of potential corrosion. While the coating of the structure is sufficient to protect against general corrosion, in the case of a flange joint, for example, there is usually a defective coating or even no coating at all.

Corrosion

Corrosion, as a natural process resulting in the deterioration of materials, usually metals/alloys, occurs as a result of direct contact and interaction of the corroding surface with the environment. The simplest protective measure is therefore to limit or even block access of the environment to the surface (create a barrier). Direct ”metallic” flange-to-flange contact, with the wrong choice of materials (different potential values, i.e. greater than 0.05 mV), is the reason for increased electrochemical corrosion. Depending on the potential, flanges (photo below) or, worse, bolts or nuts (photo below) may corrode.

Furthermore, crevice corrosion can occur in almost all types of screw joints, resulting from uneven oxygenation in the gaps between hole and bolt or gasket and rabbet, or between the components of a lap joint. This type of corrosion also requires a continuous ”supply” of corrosive agents such as oxygen, chlorides and moisture from the surroundings, i.e. the environment. Pitting and other corrosion damage on joint components can result in the loss of full cathodic protection, even though the protective potential is maintained throughout the pipeline/structure. The value of the local corrosion potential at the point of damage will depend on the surface area of the damage, and therefore another source of local corrosion will develop over time. As always when dealing with point corrosion, i.e. usually of a spontaneous nature, the effects of the corrosion are difficult to predict and are usually charged with sudden failure e.g. leakage, loss of fasteners etc. The common denominator of the corrosion mechanisms mentioned in the article is that, in order for them to emerge and operate in full development, it is necessary – in addition to the initiation of the process – contact with the environment. In other words, if a section of the structure were separated from contact with the environment, corrosion would not occur. This is what was used in the idea of protecting flexible screw joints with a membrane coating.

Belzona 3412

The coating surrounding the flange joint should fulfil at least two basic functions, i.e. protect against corrosion (corrosive environment) and allow easy and reproducible access to all parts of the joint in the event of repair. Such functions are fulfilled by the special flexible coating (membrane) Belzona® 3412, which acts as a barrier preventing moisture, chlorides and other contaminants from reaching the protected surfaces. The flexibility of the coating material is maintained down to a temperature of –22°C, and the total elongation (determined in turn for 20°C) according to ASTM D412 guidelines is 260%. The tensile strength of this material, which is 3,180 kg/m, is one of the most important mechanical parameters and represents the value of the force required to rupture and maintain a crack until the specimen fails, and is therefore calculated based on the thickness of the specimen as a function of force. However, the key parameter for the Belzona® 3412 coating, demonstrating its barrier properties, is primarily its tightness (impermeability), as determined by a test conducted in accordance with the requirements of ASTM B117, during exposure of the coating material in a salt chamber to a 5% NaCl solution at 35°C. After three thousand hours of exposure, it was found that there was no corrosion damage under the Belzona® 3412 membrane, while the unprotected part of the pipe of the tested joint corroded significantly. In addition, the coating is resistant to UV radiation.

Advantages of Belzona 3412 Coating

The images above show various applications of the Belzona® 3412 coating. The coating is easily applied by brush or spray, and is reusable (see images below). Together with the required Belzona® 8411 primer, which acts as a separating agent (the membrane can be peeled off the metal surface) and contains corrosion inhibitors (to form a protective layer), it provides a very effective barrier system to inhibit corrosion processes. Thanks to the method of application using simple hand tools, the product can protect both typical and regular joints, as well as parts with more complex shapes.

Written by Roman Masek, Technical Director, Belse Sp. z o. o.