8. Improving Corrosion-Resistant Materials and Coatings

The durability of metal connectors can be improved by using more corrosion-resistant materials such as Type 304 or Type 316 stainless steel or by treating metal connectors after fabrication with hot-dip or mechanical galvanizing. Improving corrosion resistance is discussed in this section.

Materials with improved corrosion resistance may cost more initially, but reduced maintenance over the life of a building can partially or completely offset the cost.

Another important consideration in selecting connectors and fasteners is accessibility of the connections. Fasteners and metal connectors located in areas that cannot be readily inspected or where the fasteners and metal connectors cannot be easily replaced should be made of materials with higher corrosion resistance. Since corrosion not only reduces the strength of a connection, but more importantly, can result in a load path failure, it is important to use durable connectors in building areas that are difficult to access.

Metals may be selected because of their strength, temperature resistance, durability, availability, cost, and other reasons. As mentioned in Section 5.1.2, dissimilar metals can undergo a process called galvanic corrosion if they come into contact with an electrolyte such as saltwater. When in contact with an electrolyte, charged particles flow between the dissimilar metals, which causes one of them to corrode and the other to be protected. Which metal corrodes and which one is protected depends on the relative affinity to attract or repel electrical changes. The reactivity, the rate of corrosion that occurs, which metal will corrode, and which one will be protected can be evaluated using a galvanic corrosion chart such as the one shown in Figure 7.

The chart shown in Figure 7 is simplified, but it shows individual metals’ relative affinity for attracting or repelling electrical charges. Metals closer to the top of the chart tend to function as anodes and undergo galvanic corrosion. Metals closer to the bottom tend to function as cathodes and are protected. Metals function as anodes or cathodes depending on the metal they are paired with. For example, nickel will function as a cathode and be protected when in contact with zinc or aluminum but will function as an anode and corrode when in contact with copper. Metals not affected by galvanic corrosion can still corrode through exposure to compounds like salt.

In addition to depicting which metals will function as anodes and cathodes, a galvanic chart can also depict the relative rate of corrosion that two metals may experience. Metals that are relatively close to each other on the chart will have low rates of corrosion, while those that are more widely separated will corrode more quickly. For example, when exposed to copper, bronze, which is relatively close to copper on the galvanic chart, will have a lower corrosion rate than steel, which is more widely separated from copper on the chart.

The strength of the electrolyte also comes into play. For any two dissimilar metals, the anodic metal will corrode more rapidly in an electrolyte that easily conducts electrical charges compared to a weaker electrolyte that does not easily conduct charges. Because of this, dissimilar metals in dryer environments, which tend to retard the flow of electrical charges, have lower corrosion rates than the same metals in areas with high moisture levels.

As shown by its location near the “Protected” end of the chart, stainless steel is more resistant to galvanic corrosion than most metals and is therefore the preferred metal in harsh environments and in areas where connector replacement will be difficult.

The galvanizing zinc protects steel in two ways: it acts as a physical barrier, and it slows the corrosion process because the zinc layer corrodes first, even protecting adjacent scratches in the coating. It should be noted that if a galvanized fastener is used in conjunction with steel plates, the galvanizing on the fastener will corrode much faster than when used in conjunction with galvanized plates.