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Structural Nut Color: Why Color Doesn’t Equal Coating

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Picture it: you receive your order of hot-dip galvanized structural nuts, open the container, and see that the nuts are blue. There must be a mistake. Aren’t hot-dipped nuts supposed to be green? That’s not always the case. Let’s go over dyes and why the color of the structural nut may not be what you were expecting.

Why are Structural Nuts Dyed?

While there’s no requirement that the lubricant should be dyed, many manufacturers put colored dye into the lubricant to easily distinguish the structural nuts. Because there is no set standard, different manufacturers use different colors for each kind of structural nut. Though you may be used to hot-dip galvanized nuts being green and mechanical galvanized nuts being blue, this can be the opposite for different companies. Some companies will use an entirely different color at their discretion. The colors are not present for the purpose of the customer, but for the manufacturer’s organization. 

Do I Have the Correct Structural Nuts?

Nine times out of ten, yes, you received the correct structural nuts. The best way to tell the difference in a hot-dip galvanized and a mechanical galvanized nut is by feel. Hot-dip galvanized structural nuts will feel grittier, while mechanical galvanized nuts will feel smoother.  That said, if you are still unable to tell the difference, our sales representatives are more than happy to assist you in ensuring you received the correct order.

Bottom Line

The colored dye in the lubricant on the structural nut serves two purposes: it shows that the nut is covered in the lubricant, and it helps the manufacturer keep each type of nut separate and organized. Different manufacturers use different colors to signify types of coatings, so it can get confusing if you are used to one color, and you receive another. You can usually confirm you received the right nuts by feeling the coating.

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Structural Bolt Coatings

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Coating structural bolting assemblies can protect the steel and ensure durability of the bolt, nut, and/or washer. There are a multiple types of structural bolt coatings, and we will touch on a few here, focusing on their characteristics, advantages, and things to keep in mind when choosing a coating. The two most common types of structural bolt coatings for F3125 Grade A325 bolts are hot-dip galvanized and mechanical galvanized. F3125 Grade A490 bolts can be coated in a zinc/aluminum corrosion protective coating or a zinc-rich base coat and aluminum coating. Zinc is used to coat structural bolts, because this metal is more reactive than iron. This ensures that the zinc coating will corrode first, thus protecting the steel fastener underneath it.

Hot-Dip Galvanizing F3125 Grade A325 Fasteners

Hot-dip galvanizing involves submerging a fastener into molten zinc to coat it. This coating is very thick, and there is a strong bond between the zinc and steel. Sometimes, the finished product will look slightly uneven compared to mechanically galvanized parts, because of the thickness of the coating. That said, hot-dip galvanized coating has more zinc per unit volume than mechanical galvanized parts, making it marginally more corrosion resistant. Hot-dip galvanizing is also generally less expensive than other types of coating.

It’s important to note that because of the thicker coating, more friction is caused when installing a hot-dip galvanized bolt than other bolts. Therefore, a rotational capacity (ROCAP) test is required for hot-dip galvanized bolts.

Mechanical Galvanizing F3125 Grade A325 Fasteners

The alternative to hot-dip galvanizing is mechanical galvanizing. This method provides a much cleaner, neater finish. The threads are smoother, and the bolt is easier to install. Mechanical galvanizing is a cold process, where the fasteners are placed in a rotary barrel that allows for a uniform coating. Though mechanical galvanizing produces a cleaner fastener, the protective coating is slightly less corrosion resistant than hot-dip, due to a weaker bond between the steel and the zinc. That said, mechanical galvanizing is a great way to ensure your bolts have a clean finish and are easy to install.

Coatings for F3125 Grade A490 Fasteners

Due to the risk of hydrogen embrittlement, A490 bolts cannot be galvanized like A325 bolts. Instead, a zinc/aluminum corrosion protective coating or a zinc-rich base coat and aluminum coating must be used. Common brands of these coatings include Geomet® and Magni®. The coating is sprayed on or dip spun, and it has a strong corrosion resistance. This coating reduces the risk of hydrogen embrittlement on the bolt.

Bottom Line

No matter what type you use, coating fasteners with a zinc barrier can help protect them from corrosion. While mechanical galvanizing produces a cleaner bolt with smooth threads, hot-dip galvanizing yields a stronger coating. F3125 Grade A490 fasteners cannot be galvanized due to the possibility of hydrogen embrittlement, but there are other great options for zinc-based coating to protect them.

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Who Creates Structural Bolt Specifications and Standards?

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When we talk about structural bolt specification, we drop a lot of acronyms: RCSC, ASTM, ASME, and more. All of these are organizations which provide the standards over structural steel and bolts. It’s important to keep up with these organizations to ensure that all standards are being followed. Here’s an overview of each organization and their responsibilities.

RCSC

The Research Council on Structural Connections (RCSC) is a non-profit organization made up of leading experts across all fields related to structural steel. These fields include engineering, fabrication, bolting, and erection. Many fastener companies, Birmingham Fastener included, have representation in the RCSC. The purpose of the RCSC is to “promote the knowledge of economical and efficient practices relating to structural connections.” They create and publish structural bolt specifications and standards which are used for the installation of high-strength bolts. In our blogs, you will often see us refer to the RCSC’s Specification for Structural Joints Using High-Strength Bolts. They have provided an online version, available here.

ASTM International

ASTM International is an all-encompassing organization that produces over 12,000 standards for many different types of products globally. In relation to structural bolt specification, the ASTM provides the standards for how the bolts are made and what raw materials can be used to create them. Each ASTM standard is reviewed at a minimum of every three years, meaning standards can change. For example, prior to 2015, ASTM A325 and A490 were separate specifications. After a review of the standards, these were replaced by F3125, making A325 and A490 grades under the F3125 specification. ASTM International also provides the standards on testing methods and what tests have to be done on all structural bolts prior to installation. To learn more about ASTM International, visit their website.

AISC

The American Institute of Steel Construction (AISC) is a technical institute and trade association that “serves the structural steel design community and construction industry in the United States.” The AISC provides continuing education in the industry, steel design guides, and the Steel Construction Manual. This manual provides design considerations which can relate to the RCSC. To learn more about the AISC’s resources, visit their website.

ASME

The American Society of Mechanical Engineers (ASME) is a not-for-profit organization that works towards collaboration and skill development for engineering disciplines. They produce codes and standards for many different technical fields, including the fastener field. In regards to fasteners, the ASME provides manufacturers with dimensional standards for all types of bolts, including structural bolts. For more information on the ASME, visit their website.

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What is the Difference in Type 1 vs. Type 3 Structural Steel?

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When we’re talking about structural bolts, particularly ASTM F3125 Grades A325 and A490, there are two types of structural steel: Type 1 and Type 3. On the outside, they look the same, but there’s one key difference: Type 3 structural steel is designed to weather, or rust, as a means of protection. Type 1 bolts can be galvanized to protect the bolts from corrosion. There are advantages to each type of steel in regards to structural bolts. Here, we’ll discuss their make-up, what types of coatings they can have, and their differences.

Type 1 Medium Carbon and Alloy Steel

According to the RCSC, Type 1 covers medium carbon steel for F3125 Grade A325 bolts and alloy steel for F3125 Grade A490 bolts. Type 1 Grade A325 bolts can be mechanically coated or hot-dip galvanized to protect them from corrosion.  Type 1 Grade A490 bolts on the other hand are only approved for F1136 Grade 3 and F2833 Grade 1 coatings per ASTM F3125 and the IFI 144 standard. Unlike Type 3 weathering steel, any rust on Type 1 bolts will not act as a protective coating to the bolt. Though Type 1 A325 bolts can have a protective coating (mechanical or hot-dipped), it is extremely important to ensure proper jobsite storage throughout your project to prevent any corrosion.

Type 3 Weathering Steel

The alternative to Type 1 steel is Type 3 weathering steel. These high strength fasteners have atmospheric corrosion resistance and weathering properties. This means that Type 3 structural bolts are designed to weather and rust over time. Unlike Type 1, their rust acts as a protective barrier or “coating” that seals the bolt.

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Structural Bolt Head Markings

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The ASTM requires structural bolts to be distinctively marked. These bolt head markings include the manufacturer’s mark, the grade of bolt (ex. A325, A490), and markings to classify the type of bolt (Type 1 or Type 3).

In addition to the mandatory markings, certain manufacturers may include additional markings to distinguish them from others. Below are the mandatory bolt head markings using our manufacturer’s mark. To learn more about our structural bolting products, visit our main structural bolting page.

Bolt Head Markings Table

bolt-head-markings

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How to Reapply Lubricant to Structural Bolting Components

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Structural bolting components, whether uncoated or galvanized, are lubricated for installation purposes. Though the types of structural bolt lubricant vary, both have a chance of coming off prior to installation. This is typically caused by exposure to the elements from improper jobsite storage. Reapplication of the lubricant is typically required prior to installation. Here, we’ll go over how and when to reapply lubricant on structural bolting assemblies.

Structural Bolt Lubricant for Uncoated Assemblies

While there are no specific ASTM requirements that uncoated bolting assemblies be lubricated, their lubricant comes from the left over water-soluble oil from production. This oil ensures ease of installation. The lubricant can wash away when exposed to condensation, or it can evaporate when their containers are left open.  If this happens, the assemblies can be difficult to install, and the bolt’s ductility is lessened due to the stress of tightening the non-lubricated bolt.

If the bolting assemblies have lost their lubrication, you may reapply an oil-based structural bolt lubricant, like stick wax or liquid wax, prior to installation. To do this, put the lubricant on the nut’s threads and inside face. It’s important that the lubricant is on the inside face of the nut, because much of the torque used to tighten the bolt is to get past the friction between the nut and washer or steel. Once the lubricant is reapplied, you may be required to retest the bolting assemblies or recalibrate your wrenches prior to installation. Refer to the RCSC for more details.

Structural Bolt Lubricant for Galvanized Assemblies

In contrast to uncoated bolting assemblies, galvanized nuts are lubricated with a dry, wax-based lubricant. Because it isn’t oily, you may not be able to feel this lubricant. The ASTM requires that the lubricant be dyed to ensure that the nuts have been lubricated (Note: the color is not specified and varies from manufacturer to manufacturer).

Similar to the uncoated bolting assemblies, if the nuts have lost their lubrication, reapply a wax-based lubricant on the nut’s threads and inner face. There is no need to lubricate the bolt or washer. Once the lubricant has been reapplied, you must retest the assembly. Refer to the RCSC for more details on testing.

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