Offshore

By Ian Parker, freelancer, ianparkerwriter@aol.com

The UK is the greatest sailing nation on Earth and its boating and shipping history goes back thousands of years. For instance, yachting goes back hundreds of years and the Americas Cup actually started on the Solent, the strait between the Isle of Wight and the mainland of England, in 1861. Sailing is huge around the shores, and inland lakes, of the UK – with the sailors and boats needing to be robust, but what about the fasteners that hold them together? 

The sea, salty air and sun are the enemies of many materials and can lead to corrosion and fastener failure when it comes to boat and ship maintenance. Boats and ships can also experience huge loads in storms, which means the fasteners must be up to the job. Pontoons and docks suffer similar conditions and again the requirements on fasteners are many. 

Leyton Fasteners, a UK supplier of industrial nuts, bolts, washers, and fixings, explains: “You can’t afford for fasteners critical to the structure of maritime vessels to rust and fail. The risks involved are simply too significant. You need to have peace of mind that your fasteners will stand the test of time.”

In the challenging world of maritime applications, ensuring the structural integrity and safety of projects is paramount. The cornerstone of this reliability lies in selecting the right marine fasteners. Among the many factors to consider,
load-bearing capacity stands as a critical one, but others include installation procedures, coatings, maintenance and, of course, corrosion resistance. 

Fasteners are among the maritime components for which load ratings are specified by organisations such as the American Boat and Yacht Council (ABYC). The right load-bearing capability can be found by consulting these specifications.

Including safety margins in the load-bearing capacity calculation is a good idea. Catastrophic failures can result from overloading fasteners, so including a margin of safety adds a degree of security and guarantees that fasteners can withstand unforeseen strains. Making the proper material selection and installation of maritime fasteners is also equally important. Fasteners operate at peak efficiency, they retain their structural integrity and are also corrosion resistant when installed correctly.

Pre-drilling holes for fasteners is crucial, particularly in composite or hardwood materials. Pre-drilling guarantees that fasteners sit firmly and keep wood from breaking. Use of a torque wrench will prevent over-tightening or under-tightening. While under-tightening might lead to loose items, over-tightening can damage fasteners or the material they’re fastening. It is important that users tighten the fasteners evenly and gradually, they need to refrain from pushing them into position – as this may cause an unequal tension distribution.

Fasteners should also be the appropriate length for the job. They shouldn’t go in too far, as this surprisingly could reduce the fastener’s grip, but just far enough into the material to provide a strong connection. When sealing fasteners, especially in below waterline installations, use a marine grade sealer, as this lowers the chance of corrosion by preventing water intrusion.

It is key that fasteners and materials of the same metal type are used whenever feasible. This helps prevent galvanic corrosion, which happens when different metals come into contact, particularly in salt water. 

Applying coatings and platings is also essential for improving corrosion resistance and extending fastener life. Galvanising is a zinc coating that offers a barrier that prevents fasteners from corroding. This is an affordable choice that works well for a variety of nautical applications. However, it’s not really suitable for below waterline applications. Ceramic coatings have a remarkable ability to resist corrosion. They perform well in severe marine situations because they offer a long-lasting barrier against moisture and exposure to seawater.

The role of stainless steel

Selecting the appropriate alloy of stainless steel can improve corrosion resistance. Due to its increased nickel and molybdenum content, Grade 316 stainless steel provides better resistance to corrosion from seawater. This makes stainless 316 fasteners a great option if they are subjected to continuous water contact and dynamic pressures. They are also generally more resistant to corrosion than their 18-8 counterparts because of the molybdenum content. For above water conditions, austenitic stainless steel Grades 1.4401, 1.4404 and 1.4542 will be sufficient but for anything that is to be submerged – it should be at least 6% molybdenum.

Docks and piers require solid and long-lasting fasteners. It is advised to use stainless steel fasteners, such as expansion anchors, lag screws and throughbolts. It is also crucial to ensure proper installation methods are used, such as pre-drilling and applying marine sealants.

Underwater structural fasteners need to be able to endure extended submersion. Titanium or Grade 316 stainless steel fasteners are frequently the best options for these uses. To guarantee their functionality, routine maintenance and inspections are essential. The US Marine Boat Supply reports: “Compared to the common type 304 or type 18-8 stainless steels, type 316 stainless provides the highest resistance to rust and corrosion when it comes to fasteners and/or pipe fittings. Type 316 stainless steel can withstand harsh conditions and is an absolute must in any type of salt water (marine) environment.”

Where possible, fasteners should be regularly cleaned and they should be lubricated with a corrosion resistant substance. This preserves functionality and aids in protecting against exposure to salt water. Fasteners that exhibit noticeable wear or corrosion should be changed right away. Leaving broken or corroding fasteners may be a false economy, causing structural integrity to be compromised. Users should also consider using sacrificial anodes or other galvanic protection techniques to protect fasteners against galvanic corrosion from dissimilar metals.

Stainless steel is known for its corrosion resistance in many environments in which carbon and low alloy steels would corrode. The corrosion resistance is a result of a very thin (about 5 nanometers) oxide layer on the steel’s surface. This oxide layer renders the surface electrochemically passive in the presence of corrosive environments.

The passive layer forms because of the chromium added to stainless steel. It must have at least 10.5% chromium for the passive layer to form. The more chromium that is added, the more stable the passive layer becomes, and the better the corrosion resistance. Other elements such as nickel, manganese and molybdenum can be added to enhance stainless steel corrosion resistance.

Another requirement for the formation and maintenance of the passive layer is that the steel surface must be exposed to oxygen. Corrosion resistance is greatest when the steel is boldly exposed and the surface is maintained free of deposits. If passivity is destroyed under conditions that do not permit restoration of the passive film, then stainless steel will corrode much like a carbon or low alloy steel. 

Covering a portion of the surface, for example, by biofouling, painting or installing a gasket, produces an oxygen-depleted region. It is anodic relative to the well aerated boldly exposed surface, possibly resulting in the corrosion of the covered region.

Under certain circumstances, the passive layer can break down at localised spots on a well exposed stainless steel surface. When this happens, the metal can corrode in these spots. This is called pitting corrosion. One common cause of pitting corrosion is exposure to aqueous environments that contain chloride. Examples are coastal atmospheres, road salt combined with rain water, and even tap water containing high levels of chloride.

During the fabrication of stainless steel components or structures, it is possible to degrade the corrosion resistance. This occurs when austenitic stainless steels (such as 304 grade) are exposed to temperatures between about 425°C and 870°C. If the exposure time is too long, then the areas near the metal’s grain boundaries lose their corrosion resistance and can be preferentially attacked when exposed to a corrosive environment. The grains fall out and the metal loses strength. The increased susceptibility to corrosion by this change in microstructure is called sensitisation.  

The only metals that are completely corrosion resistant are gold and platinum. While stainless steel isn’t completely corrosion resistant, it is much more economical and practical. It also far surpasses other common (and cheaper) metals when it comes to corrosion. 

Other material options

Brass is an alloy of copper and zinc, known for its strength and corrosion resistance, and is often used in marine applications, particularly in cabin fittings. Brass also often boasts a higher tensile strength than stainless steel, ranging from 40ksi – 85ksi (thousand pounds per square inch), depending on how much zinc is present in the alloy.

However, only certain classifications of brass fasteners should be used below the waterline – due to the process of dezincification. Zinc is corroded by the chloride in seawater, meaning high zinc brass alloys would have very little structural integrity after prolonged exposure to salt water. They will also readily tarnish when exposed to seawater, usually leaving a green patina.

Silicon bronze contains copper, tin, silicon and sometimes small amounts of other metals, such as zinc and manganese. This alloy is highly resistant to corrosion and is particularly effective in salt water environments, where other metals may corrode quicker. It forms a protective oxide layer on its surface, preventing further corrosion and ensuring long-lasting performance. When building a wooden boat, users should consider using silicon bronze fasteners because they complement wood perfectly and provide superior corrosion resistance.

There is also titanium fasteners, which have the unique ability to rapidly regrow their protective passive layer in the presence of oxygen – giving them unparalleled resistance to salt water environments. They’re also naturally lighter than other metal alloys and composites, making the material an excellent choice for weight sensitive vessels. Like silicon bronze, titanium boasts the ability of resistance to extreme temperatures and fluctuations, making it an ideal material for marine applications.

Making the right thread choice

When choosing the appropriate fasteners, it’s essential to understand the distinctions between coarse and fine threads. Every thread has particular benefits and drawbacks, with fine threaded fasteners generally stronger in tension and shear. They will also be less likely to loosen over time and permit finer tightening adjustments, as well as being more easily tapped into harder materials. Coarse threads on the other hand have more profound and broader ridges, making them less prone to cross-thread problems and stripping. They are also resilient enough to endure little scratches and work well.

Coarse threaded fasteners are appropriate when cleanliness may be an issue – since they can function well even in ‘less than ideal’ settings. Because coarse threads are less likely to seize when tightening, installation can be completed more quickly and effectively. They are also more appropriate for various applications because they gall less frequently than fine threads.

What is galling?

Galling is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together. Galling is caused by a combination of friction and adhesion between the surfaces, followed by slipping and tearing of the crystal structure beneath the surface.

This will generally leave some material stuck or even friction welded to the adjacent surface, whereas the galled material may appear gouged with balled-up or torn lumps of material stuck to its surface. Galling is most commonly found in metal surfaces that are in sliding contact with each other. 

It is especially common where there is inadequate lubrication between the surfaces. However, certain metals will generally be more prone to galling, due to the atomic structure of their crystals. For example, aluminium is a metal that will gall very easily, whereas annealed (softened) steel is slightly more resistant to galling. Steel that is fully hardened is very resistant to galling.

Galling can cause fasteners to be impossible to undo (cold welding) and may even lead to catastrophic failure, which is why galling prevention is highly recommended.