Corrosion Control of High-Strength Fasteners

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 fasteners 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 uncoated (mill oil) or with different coatings, each with its own advantages and disadvantages. Bolt material and coatings are used for corrosion control of fastener installations.

Corrosion

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. Per AISC, “Where corrosion could impair the strength or serviceability of a structure, structural components shall be designed to tolerate corrosion or shall be protected against corrosion.” [1]

Environment

The environment that a fastener is exposed to can range from benign to extreme and might be different during stages of construction than its permanent exposure. A fastener assembly in a benign environment such as in a dry climate enclosed in a conditioned building might have negligible corrosion potential and may be suitable in a “black” condition, i.e. uncoated except for mill oil left over from fabrication. Fasteners that are exposed to a more aggressive environment, like what would be experience in a coastal marine area or structures exposed to corrosive chemical industrial processes, will require a coating to protect the fastener from corrosion. The marine environment combines atmospheric conditions that include high humidity, chloride content and high precipitation values. The chlorine, or specifically chloride ions in aqueous solution, acts as an oxidizer and adds oxygen to molecules that combine with the iron (Fe) to create FeO2 – creating corrosion that deteriorates the base metal as rust. The level of corrosion risk is associated with the environment that fastener is exposed to.

Hydrogen Embrittlement

Hydrogen embrittlement poses a significant risk to iron and steel fasteners, potentially leading to sudden and catastrophic metal failure. This concern arises when hydrogen atoms saturate the metal internally during the manufacturing process or externally when hydrogen atoms in the surrounding environment react with metals. The reaction can trap gas within the metal, creating a latent threat. 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.

• Internal Hydrogen Embrittlement (IHE): During the manufacturing or galvanizing process, the metal becomes saturated with hydrogen atoms. Processes like acid pickling and hot dipping are known to induce Internal Hydrogen Embrittlement (IHE).
• External Hydrogen Embrittlement (EHE): Embrittlement that occurs during the metal's service life. Free hydrogen atoms exposed to metallic surfaces trigger a cathode-anode reaction, leading to metal oxidation and the reduction of hydrogen ions to hydrogen gas. If hydrogen gas becomes trapped within the metal, External Hydrogen Embrittlement (EHE) occurs.

Hydrogen embrittlement several preventive measures are employed:

• Materials Selection. Choose materials that are less susceptible to hydrogen embrittlement. For instance, materials with higher alloy content or specific coatings may be less prone to embrittlement. Steels with tensile strength of 200 ksi and higher are potentially subject to embrittlement if hydrogen is permitted to remain in the steel and the steel is subjected to high tensile stress. ASTM A3125, Grade A325 with a minimum 120 ksi tensile strength is generally considered significantly below this strength threshold; however, Grade A490 with a minimum tensile strength of 173 ksi is not, because actual tensile strengths may exceed the 200 ksi threshold.
• Manufacturing Processes. Take measures to prevent fasteners from being exposed to hydrogen during manufacturing and service. This may involve controlling the atmosphere and process conditions to minimize hydrogen absorption. Heat treatment can help relieve internal stresses and reduce the susceptibility to hydrogen embrittlement.
• Protection. Employ proper handling and storage procedures to prevent contamination and exposure to conditions that could lead to hydrogen absorption. Avoid exposing uncoated high-strength fasteners to acidic environments, as acids can facilitate the penetration of hydrogen into the material.

Material selection and coating protections are strategies that can be employed to avoid hydrogen embrittlement of high-strength fasteners.

Extreme Environments

There are some installations that may be subject to extremely corrosive chemical environments in industrial applications. These will require special care in providing protection to fasteners, including multiple coating systems. ?

Corrosion Control

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.? Zinc works both as a barrier and as a sacrificial element in the electrochemical process of oxidation.? There are also several proprietary coatings available to provide barriers to prevent corrosion of the underlying carbon fastener material.

Protective Coatings

Protective coatings are the primary means of providing corrosion control to fastener systems. There are several types of protective coatings that are described below.

Hot Dip Galvanizing ASTM F2329-11 (replaces A153-09)

ASTM 153 covers hop dip zinc coatings on steel hardware while F2329 is specifically used for the subject of steel bolts, screws, washers and nuts and special threaded fasteners. The process is outlined as follows:

• Cleaning and Surface Preparation. The first step consists of a caustic solution to remove oil, grease, dirt and paint. Then the surface is rinsed and then sent to a solution of hydrochloric acid (HCl) (pickling) to remove mill scale. Fluxing is the last step and removes oxides and prevents any further from forming. Cleaning is finished with a final rinse.?
• Galvanizing. The steel is submerged in molten zinc resulting in the formation of iron/zinc alloy coatings. Excess zinc is removed by spinning the parts in a centrifuge.

Data collected by the American Galvanizers Association shows a coating thickness of 43μm exceeding a 20-year life expectancy.

Advantages

• Long history of use
• Provides both anodic (sacrificial) protection and barrier.
• Hot Dip galvanizing results in a strong bond strength usually 3500 psi. The coating is uniform even on edges and threads. Average coating thickness for fasteners and nuts over 3/8" according to ASTM F2329 - 11 Table 2 is 0.0017in (43μm).

?Disadvantages

• Pickling is a process that may introduce hydrogen embrittlement in the steel, especially in steel above 130 ksi tensile or a specified hardness of 33 HRC. Baking immediately after pickling and before hot dipping can decrease embrittlement. The actual hot dipping stage also contains risk of embrittlement.
• Hot dip galvanized bolts can have a construction fit problem due to excess coating thickness. Requires use oversize nuts.
• No clear evidence has been seen to hot dip galvanization protecting against acids (pH<2) like HCl, topcoat must be used.

Mechanical Galvanizing ASTM B695

Mechanical galvanizing, also known as mechanical plating or peen plating, is a method of applying a zinc coating to a substrate by using mechanical means rather than hot-dip galvanizing.

• Cleaning and Surface Preparation. Can be accomplished in a similar manner as in the hot dip process, involving alkaline bath and acid pickling. Cleaning can also be accomplished by mechanical means.
• Galvanizing. Mechanical galvanizing is accomplished by placing the fasteners in a tumbler with zinc powders, accelerators and promoters. Zinc is cold welded to the works pieces through impacts induced by tumbling.

Advantages

• Provides both anodic (sacrificial) protection and barrier.
• Avoids hydrogen embrittlement. Coatings can vary based on tumbling times however ASTM B695 lists a class 55 coat (53μm) as the most common which is thicker than typically accomplished by hot dip galvanizing (Appendix 1 Figure 5)
• Easier fit-up than hot-dip galvanized nuts and doesn’t require the use of oversize nuts

Disadvantages

• Bonding between the steel and zinc is much weaker, typically 500 psi, than accomplished through hot dip galvanizing. Coating can be somewhat non-uniform or thinner on intricate pieces, edges, corners and threading.
• No clear evidence has been seen to mechanical galvanization protecting against acids (pH<2) like HCl, top coat must be used.

Zinc Aluminum Inorganic Coatings ASTM F1136 Grade 3

GEOMET?, - Metal Coatings International (MCII) Chardon, OH.

Geomet (chrome free) is a water based Zinc/Aluminum (Zn/Al) corrosion protective coating in accordance with ASTM F1136 Grade 3. Cleaning is accomplished by an alkaline bath or vapor degreasing along with shot blasting to remove rust and scale. Coating is applied via conventional dip-spin, dip-drain or spray method. Can include the addition of Plus? Sealer top coat.

The product has been tested through the Industrial Fasteners Institute, IFI 144 evaluation method. IFI 144 conducted testing of the following qualities according to ASTM standards:

• Coating thickness - ASTM D1186
• Paintability - ASTM D3359
• Coating Adhesion - ASTM B571
• Rotational Capacity - ASTM A325
• Salt Spray Exposure - ASTM B117
• Tensile Strength - ASTM F606
• Hardness - ASTM F606
• Cyclic Exposure - GM9540P
• Hydrogen Embrittlement - ASTM F1940

Geomet coated bolts meet all the requirements for the above listed ASTM standards and is approved for use on ASTM A490 and ASTM A325 Bolts. Per AISC [2]

Investigations in accordance with IFI-144 were completed and presented to the ASTM F16 Committee on Fasteners (Brahimi, 2006, 2011, 2014, 2017). These investigations demonstrated that a Zn/Al Inorganic Coating applied in accordance with the relevant standards to ASTM F3125 Grade A490 bolts does not cause delayed cracking by internal hydrogen embrittlement, nor does it accelerate environmental hydrogen embrittlement by hydrogen absorption. Thus, this is an acceptable finish to be used on such bolts.

Advantages

• Testing of coating thickness as per ASTM D1186 showed Geomet having a thickness of 6 to 12 μm, approximately one order of magnitude less than hot dip or mechanical galvanizing. Hex nuts will not be required to be tapped oversized.
• IFI - 144 found Geomat coated bolts showed significantly less red rust in ASTM B117 salt spray testing after 1000 and 5000 hours than both mechanical and hot dip galvanized bolts.
• No risk of internal hydrogen embrittlement during the coating process additionally no risk of environmental hydrogen embrittlement during service life according to testing conducted by IFI -144.
• Approved for use on A325 and A490 bolts.
• Some protection against solvents has been documented but unclear as to specifics (length of exposure, pH level, type of solvent).

?Disadvantages

• Less common than galvanizing
• Top coat likely required

Top Coating

PLUS? Sealer – Metal Coatings International (MCII) Chardon, OH.

Inorganic silicate topcoat

• Extended bi-metallic protection with Aluminum
• Consistent torque/tension values
• Increased mar resistance and barrier protection
• Excellent resistance to solvents, gasoline, and brake fluids
• Good temperature resistance
• Available in a variety of colors

Application Details

The application of coatings to bolts requires some specific detailing considerations. The thickness of the coatings affects the bolt nut fit up. The thickness of the coatings will require overtapping the nuts in order for the fasteners to fit up.? Additionally, the coatings generate additional friction that can lock-up the nuts from turning and result in a reduction in the pre-tension that can be applied. To counteract this a lubricant (typically a wax) is applied to the threads in the nut.

Coated high-strength bolts and nuts must be considered as a matched bolting assembly, and three principal factors must be considered are:

1. The effect of the coating process on the mechanical properties of bolting materials;
2. The effect of overtapping coated nuts on the nut’s stripping strength; and
3. The effect of coating and lubrication on the torque required for pretensioning.

Corrosion Resisting Materials

ASTM F3125 Type 3 bolts, with their enhanced atmospheric corrosion resistance and weathering characteristics, offer a robust solution for environments prone to corrosion challenges. Meanwhile, stainless steel bolts, renowned for their alloy composition featuring a minimum of 10.5% chromium, exhibit inherent resistance to corrosion, rust, and staining. Understanding the material performance characteristics in mitigating corrosion risks, addressing the unique properties that make them suitable for diverse applications, and underscoring the importance of selecting materials that align with environmental demands and structural integrity requirements is an important consideration.

Type 3 Weathering Steel Structural Fasteners

The ASTM F3125 structural bolt standards currently distinguish between two types of high-strength bolts based on metallurgical classification. Type 1 bolts can be crafted from medium carbon steel, carbon boron steel, alloy steel, or alloy steel with added boron. On the other hand, Type 3 bolts exhibit enhanced atmospheric corrosion resistance and weathering characteristics, offering a higher level of corrosion resistance.

Type 3 structural bolts are particularly effective in environments with alternating wet and dry conditions. In contrast, hot dip galvanized structural bolts are commonly utilized where fasteners are consistently exposed to continuous moist conditions or in a more aggressive environment. This means that Type 3 structural bolts are designed to weather and rust over time, but not corrode. Unlike Type 1, the Type 3 rust acts as a protective barrier or “coating” that seals the bolt.

It is important to note that weathering steel is not suitable for high-chlorine environments or in marine or aggressive environments, a coating is essential to provide effective corrosion resistance. Understanding the specific environmental conditions and selecting the appropriate type of structural bolts and coatings is crucial to ensuring long-term durability and performance.

Stainless Steel Fasteners

Stainless steel fasteners are not typically used in structural applications but are found in mechanical installations. Stainless steel is a steel alloy that includes a minimum of 10.5% chromium (Cr) and nickel in austenitic stainless steel used for fasteners. In combination with iron (Fe) and other alloys stainless steels have inherently good corrosion resistance. Stainless steel fasteners are commonly made from austenitic stainless steel grades due to their excellent corrosion resistance and overall durability. The most widely used stainless steel grades for fasteners include:

• 304 Stainless Steel: Known as A2 stainless steel, it contains 18-20% chromium and 8-10.5% nickel. It is corrosion-resistant and suitable for a wide range of applications, including fasteners used in everyday environments.
• 316 Stainless Steel: Known as A4 stainless steel, it contains 16-18% chromium, 10-14% nickel, and 2-3% molybdenum. 316 stainless steel offers superior corrosion resistance, particularly in marine and chloride-rich environments. It is often used in marine applications and chemical processing.
• 18-8 Stainless Steel: This term refers to fasteners made from a group of stainless steel grades, including 302, 303, 304, 305, and 384. The "18-8" signifies the approximate composition of 18% chromium and 8% nickel. These fasteners are corrosion-resistant and suitable for various applications.

These stainless steel grades are chosen for their resistance to corrosion, staining, and rust, making them ideal for use in diverse environments. The selection of a specific grade depends on factors such as the environmental conditions the fasteners will be exposed to, the required strength, and other application-specific considerations. It's important to match the properties of the stainless steel with the demands of the intended use to ensure optimal performance and longevity.

Use of stainless steel does not eliminate all corrosion issues and introduces some other important considerations that need to be addressed.

Dissimilar Metals

When stainless steel fasteners are used in conjunction with carbon steel plates or structures, there can be potential issues related to dissimilar metal corrosion. The two primary types of dissimilar metal corrosion are galvanic corrosion and stress corrosion cracking.

Galvanic Corrosion: Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte (such as moisture). In the case of stainless steel fasteners and carbon steel plates, the difference in electrode potential between the two metals can lead to galvanic corrosion.

To minimize galvanic corrosion, it's crucial to choose fasteners and plates with similar corrosion resistance properties. For example, using stainless steel fasteners with a higher alloy content (e.g., 316 stainless steel) may reduce the galvanic corrosion potential when paired with carbon steel. Introducing insulating materials or coatings between the stainless steel fasteners and carbon steel plates to electrically isolate them and reduce the potential for galvanic corrosion.

Stress Corrosion Cracking: Stress corrosion cracking (SCC) can occur in environments where a combination of tensile stress and a corrosive environment is present. Stainless steel is generally more resistant to stress corrosion cracking than carbon steel. However, when the two metals are in contact, the risk of stress corrosion cracking can increase.

To mitigate stress corrosion cracking, it's essential to control the environment (e.g., minimizing exposure to corrosive chemicals) and ensure that fasteners and plates have compatible mechanical properties. Minimize exposure to corrosive environments, especially those that can contribute to stress corrosion cracking, which can be done with coatings on the fasteners or plates to provide an additional barrier against corrosion.

Crevice Corrosion

Crevice corrosion is a common concern in stainless steel fasteners, especially when they are used in environments with conditions that can promote this form of localized corrosion. Crevice corrosion occurs in confined spaces or crevices, and it can be particularly challenging for stainless steel fasteners because they rely on a passive oxide layer for corrosion resistance. Crevices exist in stainless steel fasteners in areas where two surfaces come into close contact, such as the threads of a bolt and the surface it is threaded into, or under the head of a bolt where it contacts the material. Crevice corrosion is often initiated by factors such as a lack of oxygen within the crevice, the accumulation of aggressive ions (e.g., chlorides), and differential aeration within the crevice. These conditions can disrupt the passive oxide layer on the stainless steel surface. Chlorides are particularly problematic as they can penetrate crevices and contribute to the breakdown of the passive layer. Environments such as coastal areas, marine environments, or areas where de-icing salts are used can increase the risk of crevice corrosion. To see this one only needs to walk through a marina and see the rust running out from under the stainless steel bolts commonly used to fasten hardware on boats.

Preventive Measures: Preventive measures that can be use to mitigate the potential for crevice corrosion include:

• Material Selection: Choosing stainless steel alloys with high corrosion resistance, such as austenitic grades like 316, can be beneficial.
• Regular Maintenance: Regular inspection and maintenance practices can help identify and address crevice corrosion issues before significant damage occurs.

Understanding the specific conditions in which the stainless steel fasteners will be used and implementing appropriate preventive measures are crucial for mitigating the risk of crevice corrosion. Regular inspection and maintenance are essential to identify and address potential crevice corrosion issues in a timely manner.

Galling

Galling is a form of wear that can occur in metal-to-metal sliding contact, particularly in stainless steel fasteners. It is a type of adhesive wear where material from one surface adheres to and transfers molecules onto the other surface, leading to friction, heat, and surface damage. Galling is also sometimes referred to as "cold welding" because the metal surfaces can adhere to each other without the application of significant heat.

In stainless steel fasteners, galling is more likely to occur because stainless steel has a tendency to gall compared to other materials like carbon steel. This is due to the specific characteristics of stainless steel, which include its high hardness, low thermal conductivity, and a strong tendency for the oxide layer on the surface to adhere to itself. Galling often starts with microscopic adhesive interactions between the contacting surfaces. As sliding continues, these adhesions can grow, leading to increased friction and the possibility of seizing, where the surfaces essentially become stuck together. To prevent galling a lubricant (such as Krytox?) or anti-galling compound is applied between the contacting surfaces.

Stainless steel fasteners, though not typically employed in structural applications, play a vital role in mechanical installations due to their corrosion resistance and durability. Commonly made from austenitic stainless steel grades, such as 304, 316, and 18-8, these fasteners are selected based on the specific environmental conditions, required strength, and application needs. Understanding corrosion issues and implementing preventive measures are crucial for ensuring the optimal performance and longevity of stainless steel fasteners in diverse applications.

While bolts are a small part of any structure, they play an outsized role in the integrity and stability of that structure. Accurately evaluating the environment can help prevent corrosion and potential deterioration 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.

The information and content provided in this article is intended only for general informational and educational purposes only. It is not to be construed as professional engineering advice. The content provided should not be considered a substitute for seeking advice from a qualified engineering professional.

Edwin T. Dean, PE, SE is an engineering consultant and former Client Executive with IMEG.

NOTES:

[1]?? Specification for Structural Steel Buildings, ANSI/AISC 360-16, Section B3.13, dated July 7, 2016

[2]? Specification for Structural Joints Using High Strength Bolts, AISC, dated June 11, 2020, page 16.2-7

REFERENCES:

1.?????? AISC 360-16, Specification for Structural Steel Buildings, July 7, 2016

2.?????? AISC Specification for Structural Joints Using ASTM A325 or A490 Bolts, June 11, 2020.

3.?????? ASTM F3125-15, Standard Specification for High Strength Structural Bolts, Steel and Alloy Steel, Heat Treated, 120 ksi (830 MPa) and 150 ksi (1040 MPa) Minimum Tensile Strength, Inch and Metric Dimensions

4.?????? ASTM A563-21 Standard Specification for Carbon and Alloy Steel Nuts (Inch and Metric)

5.?????? ASTM F436-19 Standard Specification for Hardened Steel Washers Inch and Metric Dimensions Washers

6.?????? ASTM A153- 16a Standard Specification for Zinc Coating (Hot Dip) on Iron And Steel Hardware

7.?????? ASTM B695 - 21 Standard Specification for Coatings of Zinc Mechanically Deposited on Iron and Steel

8.?????? ASTM F2329 -15 Standard Specification for Zinc Coating, Hot-Dip, Requirements for Application to Carbon and Alloy Steel Bolts, Screws, Washers, Nuts, and Special Threaded Fasteners

9.?????? ASTM F1136 - 19 Standard Specification for Zinc/Aluminum Corrosion Protective Coatings for Fasteners

10.?? ASTM F2833 - 17 Standard Specification for Corrosion Protective Fastener Coatings with Zinc Rich Base Coat and Aluminum Organic/Inorganic Type

11.?? IFI 144 "Qualifications of ASTM F1136 Non Chrome Coating (GEOMET? 321) for use on ASTM A490 High-Strength Structural Bolts"? Salim Brahimi, April 8 2014, IBECA Technologies Corp. Canada.