Corrosion Control High-Strength FastenersApr 01 2014 · 0 comments · NISHKIAN DEAN, Technical notes ·0
Bolts, washers and other types of fasteners might be small, but they are a fundamental part of a structure. That is why having the right corrosion protection for the bolts that hold together a structure and knowing the environment it is exposed to is crucial to the safety and strength of the structure.
There are many types of metal high-strength carbon steel fastener assemblies available offered with different coatings, each with its own advantages and disadvantages. Some coatings are highly resistant to chipping, high heat, or certain chemicals. The specifications that cover the performance of coatings are covered by various ASTM (American Society for Testing and Materials) committees who investigate and review what fastener requirements currently are and specify how there are to manufactured, applied and used. These committees continue to develop coating standards specifically for metal fasteners.
Corrosion of high-strength carbon steel fasteners, along with carbon steel in general, is an electrochemical process that requires the simultaneous presence of moisture and oxygen. Essentially, the iron in the steel is oxidized to produce rust (FeO2), which occupies approximately six times the volume of the original material. The rate at which the corrosion process progresses depends on a number of factors, but principally the ‘micro-climate’ immediately surrounding the structure. The corrosion of high-strength fasteners leads to their deterioration and eventual failure resulting in a structure falling apart at critical bolted joints.
One particular concern encountered with iron and steel fasteners is Hydrogen Embrittlement that can result in catastrophic failure of metal without warning. This can occur when metal is saturated by Hydrogen atoms during the manufacturing process (internally) or when Hydrogen atoms in the surrounding area are exposed to metals, causing a reaction that can trap gas within the metal (environmental). For instance, fasteners that will be exposed to high heat, humidity and salty sea air would require different corrosion-resistant coatings than bolts in milder environments.
Corrosion protection is provided through the careful selection of materials that are corrosion resistant or by coatings. The most common coating is the application of Zinc either by mechanical tumbling or dipping in molten Zinc. The Zinc works both as a barrier and as a sacrificial element in the electrochemical process of oxidation. There are also a number of proprietary coatings available to provide barriers to prevent corrosion of the underlying carbon fastener material.
Accurately evaluating the environment can help prevent issues with fasteners during the life of the structure. Experienced engineers should be able to assess all the factors so the best combination of fasteners and coatings can be used in the project. If you have an upcoming project and are looking for skilled engineers, please contact any of our offices today.