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Hannes Blaas Hannes BlaasCEO at Rotho Blaas USA IncPublished Sep 8, 2021 A frequently asked question in the context of metal connections is how dissimilar metals behave and if galvanic corrosion can take place for example between carbon steel and stainless steel, or aluminum and carbon steel. The short answer to this question is: usually galvanic corrosion is not a problem. The long answer is: for galvanic corrosion to take place, the following 3 conditions need to be met simultaneously:
The potential for galvanic corrosion between metals is dictated by how far they are apart on the galvanic series of metals (source). The higher the potential difference, the higher the risk for corrosion to take place. The most common structural materials in timber buildings are stainless steel, carbon steel, aluminum, and zinc (as a coating for carbon steel). The fact that zinc is commonly used in close contact with steel as a coating provides us with a couple of interesting insights:
One of the 3 conditions for galvanic corrosion to take place is the presence of a electrolyte. The electrolyte enables the movement of ions and as a consequence starts to corrode the less noble material. Water is one of the most common electrolytes and if you keep the water out of the connection, you prevent galvanic corrosion to take place. But, doesn't wood contain moisture that could potentially act as an electrolyte and trigger galvanic corrosion? Even after years in a closed environment timber only reaches an equilibrium moisture content, but it will never reach 0% moisture content. Here it's important to distinguish between free water and bound water. Free water could potentially act as a electrolyte but even then the risk associated with it would be very very low because the electrolyte needs to touch both dissimilar materials, and free water is not exactly pouring in streams out of the wood cells. Bound water in turn can't act as a because it's bound within the cells of the timber. It's safe to say that there's no free water left in timber with a moisture content below 20% (source) and given that the moisture equilibrium of timber is closer to 10%, the timber that surrounds the connection can actually protect the connection from galvanic corrosion by absorbing excess moisture and prevent a buildup of water. Sometimes we cannot avoid using dissimilar metals. Or by using a mix of different metals we yield efficiencies for the project and we don't want to give up on those. And we don't need to give up on them if we adhere to a few principles:
The conclusion is, that the most commonly used materials in timber construction do not show a significant risk for galvanic corrosion if we follow the basic principles, first of all to keep water out of the structure. The theoretical considerations outlined in this article are backed up with rothoblaas' decades-long experience in testing and in production of structural solutions for timber construction.
Others also viewedExplore topicsDoes aluminum corrode with brass?The aluminum plate will corrode more heavily due to it being more anodic to stainless steel than brass is. The aluminum will also corrode where its exposed surface contacts the brass plate because brass is more cathodic.
How is galvanic corrosion prevented?Galvanic corrosion can be prevented by: Selecting materials with similar corrosion potentials. Breaking the electrical connection by insulating the two metals from each other. Applying coatings to both materials.
How do you separate brass from aluminum?Dump all the pieces into a vibrating separator. This machine has a tub filled with sand whose particles are between the specific gravity of aluminum (2.6) and brass (8.5). Vibrating the tub causes all the aluminum pieces to go to the top and brass to sink to the bottom. Your aluminum is now separated from your brass.
How do you protect aluminum from galvanic corrosion?For aluminum, a single coat of chrome phosphate pretreatment followed by primer and high-performance paint may effectively prevent galvanic corrosion. Alternatively, a single field-applied coat of heavy-bodied bituminous paint can be employed. Anodic coatings alone are usually insufficient for galvanic separation.
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