![]() ![]() In a report on bolt failures as part of a testing program by Det Norske Veritas (DNV), it was shown that washers are more likely to fail than any other component in a bolted system. In the real world, the bolts shown in Figure 1 have been exposed to constant salt-spray conditions for 18 months, within a structure design life of 20 years.Ĭrevice corrosion under washers and bolt heads, and disbonded coatings on the threads, also play a role in the litany of problems affecting bolted systems. For example, if it could last 1,000 hours in a salt-spray cabinet, the coating's lifetime should be 10 years under normal conditions. An accelerated weathering test such as this is designed to demonstrate the potential longevity of a coating system on the basis that test conditions are extreme. In a salt-spray test, a zinc/nickel coating would be expected to last at least 1,000 hours before "red rust" occurs. ![]() Figure 1 shows a nut after only 18 months in the splash zone, its polytetrafluoroethylene (PTFE) and zinc plating failing within six weeks of exposure. In nuts and bolts, assembly can cause damage and penetration of the coating, which can lead to corrosion. Galvanic, or bimetallic, corrosion can cause sustained corrosion of aluminium.) Bolt Damage and Crevice Corrosion However, under certain circumstances, this process cannot fully protect against corrosion. With that protective layer in place, aluminum's corrosion process normally does not advance. (Many people mistakenly believe aluminum is not corrosive because it contains a specific corrosion prevention mechanism involving the formation of a thin protective layer of aluminium oxides, which shields the material from further oxygen access. In instances where it's not possible to select a material for the bolts that will minimize galvanic corrosion, an insulating layer like a washer can be placed between the two mating areas to avoid contact and thus prevent galvanic corrosion. Selecting proper materials for these bolted connections may not be easy. Under these conditions, it is important that bolted joints are carefully designed to minimize the occurrence of galvanic corrosion between the aluminium structure and the metal bolts. In seaside, maritime or offshore applications containing an aluminum structure and metal bolts covered by sodium chloride- (sea salt-) containing water, a galvanic cell may be created. In these environments, metal structures are often made of aluminum - which, contrary to the common misconception, is extremely corrosive. This process is especially pertinent for bolted systems in seaside, maritime or offshore applications. (For more on galvanic corrosion, read: An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion. The electro-potential difference between the metals is a driving force for an accelerated corrosion attack to happen. Galvanic corrosion occurs when two dissimilar metals are in contact with each other while an electrolyte is covering the connecting area. The corrosion process in bolts can cause them to act as sacrificial anodes. In some environments, such bolts may show significant corrosion within weeks of installation, as they are affected in the same way as the sacrificial anodes in a cathodic protection (CP) system.įigure 2. In addition, the bolts and nuts are small relative to the structure to which they are joined, exacerbating the corrosion effect. The flange is high-alloy with a corrosion potential of around 0.5 - a difference of 0.35, well outside acceptable differences. The bolts are of low-alloy steel with a corrosion potential of around 0.85. ![]() Typical problems with mixed metals.Ĭorrosion in the low-alloy flange is apparent where coating damage has occurred, but most significant is the level of corrosion in the bolts. ![]()
0 Comments
Leave a Reply. |