Best Practices for Preventing Galvanic Corrosion in Tooling

Galvanic corrosion is a common issue that arises when two dissimilar metals come into contact in the presence of an electrolyte, such as water or moisture. This electrochemical process causes the metal with the higher anodic index to corrode faster, while the more noble (lower anodic index) metal remains relatively unaffected. The severity of the corrosion is influenced by factors such as the environment (e.g., the presence of water, humidity, or salt), temperature, and the ratio of the surface areas of the metals in contact. Tooling, which relies on the longevity and accuracy of metallic components—whether in construction, tooling, or marine environments—understanding how to prevent galvanic corrosion is essential. With thoughtful material selection and design, the risks associated with galvanic corrosion can be significantly mitigated, ensuring the durability and performance of tools over time.

The process of galvanic corrosion is driven by the difference in anodic indices between metals. The anodic index measures how easily a metal will corrode when paired with another metal. It represents the tendency of metals to give up electrons when they are part of a galvanic couple. The larger the difference between two metals on the index, the greater the electrochemical potential between them, which drives the corrosion reaction. Metals like platinum and gold, with very low anodic index values, are highly resistant to corrosion, while metals such as zinc and magnesium, with higher indices, are much more susceptible. In application, the goal is to select metals with anodic indices that are as close as possible, reducing the likelihood of corrosion.

Metals with a large difference in anodic index, for example, zinc and stainless steel, are paired, the more anodic metal, zinc, in this case, will corrode quickly to protect the more cathodic metal, stainless steel. However, if two metals are closer together on the anodic index, such as aluminum and steel, the electrochemical reaction is much weaker, resulting in a slower rate of corrosion.

In controlled environments, such as climate-controlled indoor settings, it is generally safe to use metals with a difference of up to 0.50 volts in their anodic indices. However, in harsher environments—where factors like saltwater, humidity, or extreme heat come into play—differences of 0.15 volts or less are recommended to minimize the risk of corrosion. For example, stainless steel, with an anodic index of 0.60 volts, and aluminum, with an index of 0.90 volts, present a difference of 0.30 volts. While this may be manageable in a controlled indoor environment, additional precautions are necessary in more aggressive settings, such as outdoor or marine applications.

Material Selection & Coatings

When environmental conditions are uncontrolled, or likely to result in galvanic corrosion, the use of sacrificial coatings can play a crucial role. Zinc coatings, for instance, are often used to protect steel components. Zinc acts as a sacrificial layer, corroding before the underlying steel is affected. This method is employed in many Carr Lane Mfg products, where zinc-plated pins protect steel structures from corrosion, even after some wear and tear of the zinc layer. This approach ensures that the more vulnerable material (zinc) can be easily replaced, preserving the integrity of the steel beneath. Materials and coatings may also be chosen to direct inevitable corrosion to one material or another, based on the needs of the tool.

Material selection goes beyond just preventing corrosion; it also requires consideration of environmental and health impacts. Some materials commonly used for corrosion protection, such as Hexavalent Chromium or Cadmium, pose serious environmental and health hazards. Hexavalent Chromium, while effective at preventing corrosion, is banned in many regions, including the European Union, due to its carcinogenic properties. Similarly, Cadmium is highly toxic and should be avoided wherever possible. Safer alternatives, such as trivalent zinc plating, offer a more environmentally friendly solution with little compromising corrosion protection.

Considerations for Tooling

In tooling, coating tolerances also play a crucial role. Many electro-coating processes can increase the tolerance of components, which can complicate the fit of high-precision parts. For example, zinc plating typically results in a total tolerance of 0.005 inches, which is ten times greater than the typical tolerance of 0.0005 inches. This presents a challenge for designers who need to maintain tight tolerances. While finishing processes like grinding or honing can reduce these issues, they add time and cost to the manufacturing process. Therefore, it is important to consider these factors during the initial design phase, reducing the need for extensive post-coating finishing whenever possible.

It’s also important to be aware of hydrogen embrittlement, a phenomenon that can occur during certain electroplating processes and heat treatment. Hydrogen embrittlement weakens the material, making it more prone to fracture or failure under stress. In critical applications, careful consideration of both the materials and the processes used to protect them is essential to avoid this risk. Most ASTM plating processes include a post-plating Hydrogen embrittlement “bake,” to relieve Hydrogen from the material. Despite this, care should be taken when an item has been both heat treated and plated. Destructive testing may be required for critical applications.

Regular monitoring and maintenance of components are critical for preventing galvanic corrosion, particularly in environments where metals are exposed to moisture, saltwater, or temperature extremes. Regular inspections allow for early detection of plating wear or corrosion, and parts that have experienced significant wear can be replaced before they compromise the overall integrity of the structure. For example, in many applications, zinc-plated pins can be easily replaced once the zinc coating wears away, ensuring continued protection of tool.

One of the most practical examples of managing galvanic corrosion in tooling is the use of zinc-plated pins with stainless steel bushings. In this setup, the zinc coating on the pins serves as the sacrificial layer, corroding over time while the stainless-steel bushings remain largely unaffected. As the pins wear, they can be replaced easily, ensuring that the bushings and the overall system remain intact. This design not only reduces maintenance costs but also ensures long-term performance and durability, even in harsh environments.

Conclusion

Preventing galvanic corrosion requires thoughtful design and material selection, along with a clear understanding of the anodic properties of metals. By minimizing the contact between dissimilar metals, using sacrificial coatings where appropriate, and selecting materials with similar anodic indices, galvanic corrosion can be effectively managed. Regular maintenance and monitoring further ensures that components remain protected, extending the lifespan of tools, fixtures, and structures across a variety of environments.