Fasteners is a general term for mechanical components used to connect or secure two or more parts, and bolts (which must be used in conjunction with nuts) are among the most critical and widely used types.
Fasteners in Engineering Systems
Serving as the “joints” of equipment and piping systems, bolts are most widely used in the automotive and aerospace industries.
In high-integrity systems within the oil and gas industry, bolted connections are particularly critical to the long-term performance of major structures such as structural components, pressure vessels, and pipelines..
Marine Environment and Failure Risk
The marine environment is one of the most corrosive natural environments for metal structures.
In this environment, bolt joint failure is a major cause of gas and oil leaks in offshore installations.
Bolt joint failure can be caused by various factors, including overloading, fatigue, and corrosion.
Inadequate performance and high maintenance costs stem from improper material selection, manufacturing defects, and insufficient installation procedures or processes.
Therefore, it is essential to ensure that the design and installation of bolted joints are suitable for various exposure and service conditions to meet the required design life requirements.
Systematic Engineering Approach to Bolting Technology
This requires a systematic engineering approach—“bolting technology”—that spans the entire lifecycle from design and material selection through manufacturing, installation, and inspection.
However, statistics on leakage incidents in the oil and gas industry indicate significant shortcomings in the application of this technology in offshore engineering.
Systematic research indicates that natural gas leaks caused by bolt failure are frequent in offshore facilities.
Among these, the installation phase—particularly improper control of preload (including under-tightening or over-tightening)—is the weakest link in the entire reliability chain.
Practice has confirmed that the reliability of mechanical bolted connections can be significantly improved through systematic measures such as the application of advanced torque control technology, optimized joint design, strict material selection standards, and standardized installation procedures.
Material Selection and Corrosion Considerations
Furthermore, the selection of bolt materials must prioritize key performance indicators such as strength, ductility, and corrosion resistance.
The effectiveness of corrosion protection depends on the material’s inherent corrosion resistance as well as supplementary protective measures such as coatings or cathodic protection.
Since the marine environment can be divided into different exposure zones—including the atmospheric zone, splash zone, immersion zone, and buried/sedimented zone— the mechanical and corrosion resistance requirements for materials vary significantly across these zones.
It is essential to specifically address the primary corrosion and degradation mechanisms that may lead to failure, including: general corrosion, localized corrosion (pitting and crevice corrosion), galvanic corrosion, fatigue/corrosion fatigue, stress corrosion cracking (SCC), and hydrogen embrittlement (HE).
Environmental Zone–Dependent Failure Mechanisms
In marine atmospheric environments, general corrosion, localized corrosion, and galvanic corrosion are the primary failure mechanisms affecting bolt performance.
Although coated low-alloy steel bolts are commonly used in this environment, their inherent lack of corrosion resistance often leads to higher maintenance costs;
Therefore, corrosion-resistant alloys are frequently used as bolt materials in critical module systems.
For submerged applications, cathodic protection is the primary corrosion prevention method; in such cases, special attention must be paid to the mechanical properties of the material and its susceptibility to hydrogen-induced cracking (HIC).
At the same time, proper installation procedures are critical; for example, insufficient preload may induce vibration, leading to fatigue or corrosion-fatigue failure and significantly shortening the service life of the bolts.
Scope and Focus of This Study
Based on the current situation and challenges outlined above, this study provides a systematic review of the application of fasteners in marine environments.
First, the characteristics and selection criteria of bolt materials suitable for different marine regions (such as low-alloy steel, stainless steel, nickel-based alloys, and titanium alloys) are analyzed;
Subsequently, the mechanisms and prevention strategies for common failure issues—including wear, hydrogen embrittlement, and other forms of corrosion—are explored in depth;
Finally, the study reviews the latest advancements and engineering practices regarding key protective measures, such as cathodic protection and coating technologies, with the aim of providing theoretical references and practical guidance for enhancing the reliability and safety of bolted connections in marine engineering.
Bolt Materials
Although the terms “bolt” and “fastener” are often used interchangeably, strictly speaking, a bolt refers specifically to a threaded fastener designed to be used in conjunction with a nut.
In the field of marine engineering, bolt materials are primarily classified into several major categories, including low-alloy steel, copper-based alloys, nickel-based alloys, stainless steel, and titanium alloys.
When selecting materials, it is necessary to comprehensively evaluate key performance indicators such as strength, ductility, cost, supply stability, weight, fatigue resistance, and corrosion resistance.
Additionally, structural criticality levels, expected service life, and inspection and maintenance requirements must be considered in light of the specific operating conditions at sea.
Low-alloy Steel
As a commonly used material in the marine industry, low-alloy steel requires tailored protective measures depending on the specific environment:
In atmospheric environments, protection is typically provided by galvanizing (hot-dip galvanizing or electrogalvanizing) or organic coatings (Note: The use of cadmium plating is prohibited in Norway).
For underwater applications, ASTM A193 Grade B7 or ASTM A320 Grade L7 low-alloy bolt materials are traditionally the primary choices.
Both of which are based on the AISI 4140/4142/4145 alloy system; the primary distinction between Grade B7 and Grade L7 is that the latter includes additional impact test requirements for low-temperature service conditions.
It is worth noting that in cathodic protection environments, high-strength quenched and tempered steels are susceptible to hydrogen embrittlement; therefore, materials of the L7M or B7M grades, which specify maximum hardness limits, may be considered to reduce the probability of failure.
Copper-based and Nickel-based Alloys
In the field of marine engineering, copper alloys are important materials for bolted connections; commonly used grades include UNS C61400 and UNS C65100.
To meet higher strength requirements, special copper-based materials such as beryllium copper alloys, nickel-copper-silicon bronzes, or high-strength copper-nickel alloys can be selected.
In addition, nickel alloy bolts primarily fall into two categories: nickel-copper-based (e.g., Alloys 400 and 500) and nickel-chromium-molybdenum-based (e.g., Alloys 625 and 725).
These materials demonstrate excellent suitability in harsh marine environments due to their superior comprehensive performance.
When nickel-based alloys 400 and 500 form galvanic couples with stainless steel or NiCrMo alloys, they are prone to uniform and localized corrosion, and may trigger environmental-assisted cracking issues such as hydrogen-induced stress cracking.
Among these, the Ni-30Cu-3Al (K-500) alloy, due to its high-strength properties, was once widely used by the U.S. Navy and in marine engineering for underwater fasteners;
However, research has found that it poses a significant risk of hydrogen-induced stress cracking under cathodic protection (CP) conditions.
The failure mechanisms of this alloy are primarily attributed to: carbide precipitation at grain boundaries; and intergranular brittle failure under specific heat treatment conditions (even in the absence of cathodic protection).
Given these limitations, the marine engineering sector is actively developing alternative solutions, with a focus on new material systems such as high-strength stainless steels, nickel-based alloys, and titanium alloys.
Stainless Steel
In the marine engineering sector, stainless steel bolt materials are primarily divided into two categories: traditional and new types.
Traditional types are represented by B8 (AISI 304) and B8M (AISI 316) under the ASTM 193 standard.
Among these, B8M is widely used in the marine industry due to its superior corrosion resistance; however, its lower strength limits its application in high-strength connections.
It is important to note that both austenitic and martensitic stainless steels are susceptible to stress corrosion cracking (SCC) in chloride-containing environments.
To address harsher marine environments, new stainless steel materials are continually being developed and applied:
A-286 (UNS S66286), a precipitation-hardening austenitic stainless steel, is considered for underwater applications due to its properties similar to those of AISI 316;
High-molybdenum alloys (UNS S31254) are suitable for special operating conditions; and duplex stainless steels (such as UNS 31803, S32750, and S32760) are attracting significant attention due to their excellent balance of strength and corrosion resistance.
However, these materials still have their respective limitations: austenitic stainless steel is prone to wear, while martensitic stainless steel lacks sufficient corrosion resistance in marine environments and is susceptible to crevice corrosion and pitting corrosion.
These are critical factors that must be carefully considered during material selection and engineering applications.
Titanium Alloys
Titanium alloys are ideal materials for bolted joints due to their excellent corrosion resistance;
However, only specific high-strength alloys such as UNS 56400 (ASTM B 265 Grade 5) and UNS R58640 (β-C grade) are suitable for use as bolting materials.
The selection of these titanium alloys requires a comprehensive evaluation of their mechanical properties, corrosion resistance, and resistance to hydrogen embrittlement.
It is worth noting that the susceptibility of titanium alloys to hydrogen embrittlement under cathodic protection conditions may limit their application in marine engineering.
Currently, the industry has conducted extensive research to verify the compatibility of titanium alloys with cathodic protection systems, with the aim of promoting their safe use in the marine industry.
Frequently Asked Questions
In marine environments, bolt materials face multiple corrosion threats, primarily including wear corrosion, hydrogen embrittlement, bio-corrosion, crevice corrosion, and galvanic corrosion.
Wear
The primary mechanism of wear and corrosion lies in the micrometer-scale relative slippage that occurs between contact surfaces in fastened joints subjected to vibration.
This micro-motion wears away the protective oxide layer on the metal surface, exposing the metal substrate to corrosive media.
The exposed metal rapidly undergoes oxidation or electrochemical corrosion, producing corrosion products.
The next micro-motion wears away these corrosion products, which are more brittle than the substrate, and may act as abrasives to exacerbate wear while exposing a fresh surface.
This process repeats cyclically, resulting in the superposition of wear and corrosion and exhibiting a synergistic effect.
Hydrogen Embrittlement
Among the various failure phenomena in fasteners, hydrogen damage warrants particular attention.
This term refers to the general phenomenon where hydrogen atoms penetrate the interior of a metal and cause a degradation of material properties through physicochemical interactions.
Specific manifestations include hydrogen embrittlement, hydrogen blistering, hydrogen-induced cracking, and other failure modes.
Although hydrogen embrittlement is often associated with SCC, it is itself an independent failure mode, and the mechanisms of hydrogen damage vary significantly among different metals:
For metals prone to forming hydrides, such as titanium, dissolved hydrogen directly generates brittle hydride phases;
Whereas the mechanism of hydrogen-microstructure interaction in materials like iron-based alloys is more complex.
Material Sensitivity and Structural Implications
It is worth noting that material strength is positively correlated with hydrogen embrittlement sensitivity, making high-strength steel particularly susceptible to fracture failure induced by hydrogen embrittlement.
This characteristic directly influences the selection of high-strength steel fasteners for marine engineering applications and the formulation of protective strategies.
Hydrogen Sources During Manufacturing and Service
Hydrogen can be generated through various pathways during both the manufacturing and service life of bolt materials:
During the manufacturing stage, it primarily originates from pickling and electroplating processes, while during the service stage, it mainly stems from hydrogen produced by cathodic protection systems.
Given that nearly all offshore subsea facilities are equipped with cathodic protection systems, subsea mechanical fasteners must possess hydrogen embrittlement resistance or low sensitivity to hydrogen embrittlement.
Research indicates that the hydrogen embrittlement susceptibility of steel is directly related to its hardness;
The risk increases significantly when hardness exceeds HRC 35, whereas electroplating processes typically do not induce hydrogen embrittlement issues when hardness is below HRC 35.
It is worth noting that hydrogen embrittlement sensitivity increases with rising material strength; laboratory data indicate that low-alloy steels with a yield strength of approximately 800 MPa exhibit good resistance to hydrogen embrittlement.
Furthermore, a frequently overlooked phenomenon is that high-strength steels and ferroalloys may also crack in atmospheric environments due to hydrogen generated by corrosion processes.
Microbial Corrosion
Microbial corrosion does not involve microorganisms directly corroding the metal surface; rather, their metabolic activities directly or indirectly contribute to accelerating the corrosion process.
Aerobic iron-oxidizing bacteria form iron hydroxide deposits on the metal surface, consuming oxygen and creating an ideal environment for the underlying anaerobic sulfate-reducing bacteria.
Under anaerobic conditions, sulfate-reducing bacteria can reduce sulfate to hydrogen sulfide, which directly corrodes the metal to form corrosion products such as ferrous sulfide.
Furthermore, ferrous sulfide acts as an effective cathode, accelerating the anodic dissolution of the metal.
Bacterial metabolism produces organic or inorganic acids, locally lowering the pH and further exacerbating corrosion.
Sulfate-reducing bacteria can also destroy the passivation film on stainless steel, inducing pitting corrosion.
Crevice Corrosion
Crevice corrosion is often an easily overlooked form of corrosion; not all crevices lead to severe corrosion, but those with widths between 0.025 and 0.1 mm are typically the most dangerous.
This size is sufficient to allow the electrolyte to be drawn in by capillary action, yet also sufficient to trap it in place, hindering convection and diffusion.
In the early stages of corrosion, metal dissolution and oxygen reduction occur both inside and outside the crevice.
As oxygen inside the crevice is depleted, the oxygen reduction reaction shifts to the outer surface of the crevice.
To maintain electrical neutrality inside the crevice, anions such as chloride ions migrate in.
The increase in chloride ion concentration, combined with a drop in pH within the crevice caused by the hydrolysis of metal ions, creates an environment in which chloride ions rapidly destroy the passivation film, leading to an exponential increase in the corrosion rate.
This phenomenon is commonly observed at flange joint surfaces, nut clamping surfaces, and threaded contact areas.
Galvanic Corrosion
The potential difference between the metals in contact serves as the driving force.
Here, the potential difference refers to the galvanic series in the service medium;
The metal with the more negative potential acts as the anode and undergoes accelerated corrosion, while the metal with the more positive potential acts as the cathode and is protected.
A combination of a large cathode and a small anode is the most dangerous, as the high anode current density leads to an extremely high corrosion rate.
For example, in ship piping systems, large-area titanium alloy flanges (cathodes) connected to small-area stainless steel bolts (anodes) can cause significant corrosion of the bolts.
In seawater, there is a significant potential difference between aluminum alloy and stainless steel, resulting in an extremely high risk of galvanic coupling.
The polarization rate of the cathode material determines the intensity of the galvanic effect.
If the cathodic reaction proceeds easily on the cathode material, the anodic corrosion current will be high.
Conversely, if the cathodic reaction is hindered, the galvanic effect will be weakened.
Protection Technology
In the harsh marine engineering environment, bolts made of austenitic stainless steel, nickel-chromium alloys, and certain titanium alloys used in critical connections face severe long-term challenges.
The combined effects of wave impact, salt spray corrosion, and continuous vibration loads can easily lead to loosening and wear of bolted connections, posing a serious threat to the structural integrity and operational safety of the entire system.
Therefore, establishing a systematic, multi-level comprehensive protection system—encompassing design, material selection, surface engineering, and auxiliary protection—is crucial for ensuring the reliability of these connections.
Material Selection
Corrosion protection for fasteners is a full-lifecycle process that spans design, manufacturing, and operation and maintenance.
Carbon steel and alloy steel are the most widely used fastener materials.
Due to their cost-effectiveness and good mechanical properties, they cover a broad range of applications, from general structural connections to the fastening of critical components.
However, in chloride-corrosive environments, corrosion-resistant alloys, such as duplex steel, are preferred.
With a structure combining austenite and ferrite, they offer higher strength and excellent resistance to pitting and crevice corrosion.
In high-temperature environments, high-temperature-resistant steels or nickel-based alloys are selected.
Materials such as martensitic stainless steel, austenitic precipitation-hardening stainless steel, and nickel alloys utilize techniques like solution strengthening, precipitation hardening, and grain boundary control to maintain sufficient strength and creep resistance at elevated temperatures.
In highly acidic environments, Hastelloy or titanium alloys can be selected.
Alternatively, carbon steel fasteners combined with high-performance coatings may be chosen;
The selection of coatings must be appropriate for the specific environment, while also taking into account a comprehensive assessment of the full life-cycle cost.
Cathodic Protection
In marine engineering applications, a zoned protection strategy must be adopted for bolt materials with insufficient corrosion resistance: coating protection can be used in the atmospheric and splash zones.
The conventional approach for corrosion protection of underwater fasteners is galvanizing followed by coating; however, this method cannot ensure electrical continuity, and underwater structures are costly to construct.
Construction is difficult to implement, and both manufacturing and maintenance costs are substantial.
Given these issues, cathodic protection is typically employed in the design of coatings for underwater fasteners.
By establishing an electrical connection between the fasteners and the flange structure, this ensures that sacrificial anodes on the underwater structure can provide cathodic protection to the fasteners.
This approach prevents premature corrosion of underwater structures after prolonged service, ensures quality control, guarantees structural safety, extends design life, and significantly reduces the likelihood of safety incidents.
Consequently, manufacturing and maintenance costs resulting from accidents can be controlled, and repair frequency is minimized.
Electrical Continuity Requirements for Cathodic Protection
To ensure the effectiveness of cathodic protection, electrical continuity between the bolts and the main structure must be maintained.
For multi-component assemblies without welding, electrical connection can be achieved through metal seals and uncoated threaded connections;
In other cases, wire connections must be used to ensure electrical continuity.
It is particularly worth noting that this electrical connection design is a key prerequisite for the cathodic protection system to function effectively.
Conductive Coatings and Engineering Measures
There are various methods to ensure electrical continuity between fasteners and flange structures, such as using conductive coatings.
Conductive coatings allow current to flow between the fasteners and the underwater structure, thereby protecting the fasteners.
Targeted grinding of the coating on the contact surface between bolts and flanges actively incorporates the bolts into the cathodic protection system of the entire flange circuit, transforming “passive insulation” into “active conductivity.”
This eliminates the risk of galvanic corrosion caused by insulating coatings and serves as an effective and necessary engineering measure to ensure the long-term safe operation of critical equipment bolts.
In addition, the use of serrated washers is a necessary measure to establish electrical continuity and ensure that the sacrificial anodes on underwater structures can provide cathodic protection to the fasteners.
Coating Protection and Physical Protection
For specific application environments, alloy steel fasteners may also require special surface treatments or material modifications.
Surface engineering techniques are the primary means of enhancing wear resistance.
Hard coatings such as titanium nitride (TiN) and chromium nitride (CrN), produced via physical vapor deposition (PVD) technology, as well as tungsten carbide coatings formed using the high-velocity oxygen-fuel (HVOF) process, can impart extremely high hardness (up to HRC 60 or higher) and excellent wear resistance to bolt surfaces.
In addition, chemical heat treatment technologies such as plasma nitriding and carburizing can form a thick, dense hardened layer on the component surface while maintaining the toughness of the core, significantly improving resistance to fatigue and fretting wear.
Balance Between Protection and Installation Performance
The application of bolt coatings involves a delicate balance between protection and installation.
On the one hand, coatings can significantly extend bolt service life (reducing maintenance requirements by more than 50%), reduce the coefficient of friction by 30%–40% during installation, and facilitate subsequent disassembly; on the other hand, due to thread fit tolerances (typically requiring coating thickness to be controlled between 20 and 50 μm), mechanical stresses during installation can easily cause coating damage.
Compounding the issue, electroplated coatings may introduce a risk of hydrogen embrittlement (particularly significant when hardness exceeds HRC 35), while organic coatings can block cathodic protection currents (becoming completely ineffective when insulation resistance exceeds 1 MΩ).
As the service life increases, coating aging and peeling may also lead to secondary installation damage.
Therefore, in practical engineering applications, it is essential to precisely match the coating type (zinc-aluminum coating/PTFE, etc.), application process (electroplating/thermal spraying, etc.), and thickness parameters (15–200 μm) based on the bolt grade, service environment, and required protection duration.
Application Limitations and Selection Considerations
For small-diameter bolts (approximately 10 mm), hot-dip galvanizing may not be suitable because the coating thickness could result in insufficient bolt tolerances.
Hot-dip galvanized bolts require larger nuts to ensure sufficient thread tolerance.
For process piping, using coated bolts facilitates disassembly when necessary; hot-dip galvanized bolts should only be used when disassembly is not required during the service life.
Experience shows that when disassembly is required, galvanized bolts typically need to be cut.
Bolt Protection Systems: Coating and Physical Protection
Bolt protection systems are primarily divided into two categories: coating protection and physical protection.
In coating systems, both organic coatings (such as Teflon) and metallic coatings (such as zinc and cadmium plating, among which cadmium coatings are banned on the Norwegian continental shelf) can be used;
Eee Table 1 for typical coating types. Teflon coatings are typically applied over phosphate primers or electroplated substrates.
In addition to coatings, physical protection measures include: tapes, covers, and thread protectors made of plastic or elastic materials; thermoplastic thread protectors combined with grease injection technology;
Spraying thermoplastic materials to form integral protective coatings (e.g., full flange wrapping), which can further enhance protection by adding corrosion inhibitors;
And maintenance-oriented protection solutions such as penetrating wax systems.
These methods can be combined based on the bolt’s service environment and protection requirements, with thermoplastic spraying and penetrating wax systems being particularly suitable for post-installation corrosion protection maintenance.
Zinc Coating Performance and Protection Mechanisms
The performance of zinc coatings—the most common form of bolt protection—depends directly on their thickness and application process.
Electrogalvanizing produces a thin layer of 2–25 μm, but its protective lifespan is limited in marine environments (5–15 μm/year in the atmospheric zone and 15–85 μm/year in the submerged zone).
Although hot-dip galvanizing provides a thicker protective layer (10–50 μm), traditional processes often lead to thread tolerance issues, particularly for bolts with diameters <10 mm.
To address this challenge, the spin-coating method (centrifugal galvanizing) achieves a uniform coating of approximately 40 μm through a 370 °C rotational process, perfectly meeting the protection requirements for small-diameter bolts.
It is worth noting that the actual service life of a zinc coating can be accurately predicted by the ratio of corrosion rate to coating thickness, providing a quantitative basis for engineering selection.
PTFE Coating Systems and Composite Protection
Teflon (PTFE) coating, as the preferred solution for bolt surface treatment, typically exists as a fluoropolymer layer 20–30 μm thick and offers multiple significant advantages: superior corrosion resistance, an extremely low coefficient of friction (reducing installation torque by 20%–30%), and exceptional wear resistance.
Its outstanding chemical inertness enables it to withstand most corrosive media;
When combined with a galvanized substrate (requiring solvent cleaning and phosphate pretreatment), it forms a composite protective system.
However, in practical engineering applications, it is important to note that the limited mechanical strength of the PTFE coating makes it susceptible to scratches and damage during installation (with a damage rate of up to 15%–25%);
Once the coating is damaged, it will lead to accelerated localized corrosion.
Therefore, for bolted connections that require frequent disassembly or are subjected to dynamic loads, it is recommended to use a PTFE composite coating (e.g., with added ceramic particles) or specialized installation tools to maintain the integrity of the coating.
In highly corrosive environments such as marine settings, the condition of the coating must also be inspected regularly;
When the area of surface damage exceeds 5%, recoating should be considered.
Under regular maintenance, fastener corrosion protection can meet the design service life;
However, in maintenance-free conditions, the design must account for the risk of subsequent coating damage.
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Conclusion
The harsh challenges posed by the marine environment to metal fasteners require the adoption of systematic engineering strategies.
Extensive experience and lessons learned from long-term engineering practice are crucial for ensuring the safety and durability of offshore structures.
Regarding material selection, different marine regions impose varying requirements on materials.
Due to its alternating wet and dry conditions, the splash zone is the area most severely affected by corrosion.
In this zone, corrosion-resistant alloys demonstrate superior performance.
For fully submerged applications, although titanium alloys have a higher initial cost, their life-cycle cost advantages are significant, making them particularly suitable for critical connection points.
The design of cathodic protection systems requires special attention to the uniformity of current distribution, and preload control is a critical factor in ensuring long-term reliability.
At the same time, establishing a “preventive maintenance” system is essential to monitor current and potential at vulnerable points of metal fasteners, thereby enabling the timely and effective detection of potential failures in concealed areas.
The application of fasteners in marine environments requires the establishment of a comprehensive technical system encompassing material selection, protective design, assembly, and subsequent monitoring.
Future research should focus more on the development of new corrosion-resistant materials and the refinement and innovation of intelligent monitoring technologies.
These experiences are not only crucial for enhancing the reliability of marine engineering but also provide a practical basis for the formulation of relevant standards.
Only through continuous technological innovation and rigorous engineering management can the long-term reliable service of fasteners in harsh marine environments be ensured.