Application Technology

Here Freelancer Ian Parker looks into how the highly corrosive environments in which ships, boats and other ocean assemblies are used, leads to them needing special fastener combinations.

Although it doesn’t contain strong acids, the sea can be just as corrosive, causing pitting and crevice corrosion, while debris and sands contribute to erosion corrosion. Ship builders are also faced with the challenge of galvanic corrosion when salts convey electric currents between dissimilar metals. Fasteners used within these environments need to be resistant to such corrosions and may also need to be of superior strength.

US company Extreme Bolt & Fastener says that the Monel 400 and age hardened Monel K500 fasteners have long been the ‘gold standard’ of salt water corrosion resistance. Monel 400 bolts offer excellent resistance, while Monel K500 offer similar resistance at three times the yield strength and twice the tensile strength.

Though these are the most commonly used bolts for marine vessels and naval projects, other materials also serve to protect from salt water corrosion. AL6XN bolts were developed for marine conditions and offer excellent seawater resistance; Duplex 2507 offers high resistance to pitting, erosion and crevice corrosion, as well as stress corrosion cracking in salt water; and titanium Grade 2 and 5 (6AL-4V) offer excellent resistance to salt water and are essentially immune to corrosion.

High strength focus

Military vessels and marine structures require some of the strongest and most reliable materials to maintain their strength and safety. Often, speciality materials are needed for high strength in a saltwater environment. MP35N is the highest strength material Extreme Bolt & Fastener can supply with an ultimate tensile strength of over 225 KSI – 290 KSI. It offers excellent resistance to sea water.

Hastelloy

Although considered overkill for many marine applications, Hastelloy components are extremely corrosion resistant. They offer excellent corrosion resistance to reducing and oxidising environments and are well suited for hydrochloric and sulfuric acid environments. They are also strongly resistant to stress corrosion cracking; performing excellently at elevated temperatures; and are available in alloy grades C276, B2, B3, C22 and Hastelloy X.

Sea conditions

Conditions in the oceans vary and so does the resultant corrosion. The level of chloride ions in salt water, also referred to as salinity, is one of the most aggressive causes of seawater corrosion. Many things affect the salinity including evaporation (increasing saline levels), precipitation (decreasing), and dilution (decreasing).

The chlorides can also increase pitting corrosion in stagnant water. Titanium fasteners offer superior resistance to chloride and chlorine solutions as they are fully resistant to chlorides, hypochlorites, chlorates, perchlorates and chlorine dioxide. Duplex steels also offer good corrosion protection to chlorides, including high resistance to chloride stress corrosion cracking, as well as pitting and crevice corrosion.

Seawater temperature varies greatly from -2°C (28.4°F) at the poles, up to 35°C (95°F) in the tropics. The warmer it is, the higher the fastener corrosion because heat promotes the oxide reaction. Hot engines, processes or locations in tropical areas will see a higher rate of corrosion.

The speed and movement of sea water can also affect the corrosion resistance of a material. For high velocity marine environments, titanium is resistant to any erosion even up to a velocity of 120 ft/sec, as well as stagnant water. Like titanium, Monel is extraordinarily resistant to rapidly flowing seawater, but can experience pitting and cracking if the salt water is stagnant.

Smaller vessels

Last October, UK company Leyton Fasteners Limited announced an expansion of the number of nuts, bolts, washers, and machine screws for the maritime sector. The company explains: “Marine fasteners are different for a number of reasons, and there are factors that you need to take into account to ensure that you find the correct fastener for the job. At the top of that list is corrosion.”

Marine fasteners have to be ultra-resistant to corrosion simply because they receive so much more exposure to the elements, particularly salt water and sea spray. “You can’t afford for fasteners critical to the structure of your maritime vessel 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. That’s why you will tend to pick stainless steel as the material for marine fasteners, with its robust defences against corrosion agents such as acids, alkalines, salt and water. However, there’s another form of corrosion to consider – galvanic corrosion,” comments Leyton Fasteners.

What is galvanic corrosion?

Technically galvanic corrosion can take place anywhere, not just aboard marine vessels. It can happen when dissimilar metals make contact. For example, this might happen on your sailing boat if you use stainless steel screws and blind rivets to secure hardware onto the aluminium spars. You could revert to aluminium fasteners for non-critical fastening tasks whereby you don’t require the tensile strength of stainless steel.

The specific choice of fastener materials will depend largely on the purpose. However, Leyton Fasteners says that most marine fasteners boil down into two categories – A2-304 and A4-316. Their main characteristics and properties are as follows:

A2-304 grade – has good corrosion resistance and can be used for outside work and inside boats. However, Leyton Fasteners would not recommend it for use in severe or hostile marine conditions.

A4-316 grade – has a greater corrosion resistance than A2-304 and is often used in hostile and severe environments such as sea spray.

Protecting marine fasteners

While picking the correct stainless steel grade for the application is a good start, there are other steps that can be taken to protect fasteners from corrosion. Anti-corrosion grease can be very effective. These products offer protection against acidic and alkaline substances, such as sea spray.

Leyton Fasteners offers LanoShield, a well regarded, 100% natural anti-corrosion grease that comes in 5 litre containers complete with a hand sprayer. It is salt water resistant (which is obviously crucial for marine applications) and boasts a power wash resistance of 300 psi.

Looking more broadly at fasteners materials, there are various pros and cons that depend on the strength required and the exposure to corrosive environments. From stainless steel to brass, choosing the right fastener material is crucial to the success of any project. Each respective material has different levels of corrosion resistance, tensile strength, and brittleness, and the choice will depend on the specific application.

Brass

Brass harnesses the inherent strengths of copper and zinc, making a very effective fastener material. It’s resistant to corrosion and delivers the added benefit of being conductive to electricity. However, it’s one of the softer fastener materials, meaning its load-bearing capabilities are limited. Although its distinctive look makes brass fasteners popular for more decorative or stylistic items such as furniture.

Aluminium

Aluminium has the benefit of being an extremely light fastener material, and, like brass, it’s corrosion resistant. However, it suffers from the same drawback of not being particularly strong, since it’s also a light and soft metal. It is useful for light load fastening in environments where other more robust materials would struggle with corrosion, such as within the maritime industry.

Silicon bronze

Another fastener material in the marine industry is silicon bronze. This alloy combines copper, tin, and silicon, to make this material ultra corrosion resistant. These bolts and screws are preferred for wooden boat constructions. However, it’s not cheap, often coming in at significantly more than other comparable fastener materials.

Alloy steel

Alloy steel is carbon steel with the elements such as silicon, manganese, and chromium, added to increase strength and enhance ductility (the capacity of a material to deform permanently when exposed to heat, such as stretching, bending or spreading). However, it is susceptible to hydrogen embrittlement.

Stainless steel

Perhaps the most common fastener material, stainless steel is an alloy created by combining low carbon steel and chromium. Always take care to avoid mixing fastener materials when conjoining materials since that will likely lead to galvanic corrosion.

Standard stainless steel fasteners are also more prone to galling as they are considerably weaker than hardened steel fasteners. They also tend to be the least magnetic of the available steel alloys.

The role of plastics

E&T Fasteners in the US says that the only metals that don’t corrode are gold, platinum, and palladium, but because they’re so expensive, they don’t make good marine building materials. That leaves builders with other materials that can corrode, especially under water, which ultimately weakens the structure. Other metal corrosion is incorrectly called rust, but only iron does this, since rust is iron oxide.

Metal corrosion is essentially the material’s attempt to return to its original form found in the Earth’s crust. Most metals readily combine with other elements due to their reactivity.

The solution to this problem is often stainless steel. Chromium is added to steel, creating a passive layer of chromium oxide when exposed to oxygen. This passive layer is impervious to water and air, even when scratched. The trouble is, salt water does something much worse to stainless steel than a simple scratch.

Salt water is highly corrosive because it contains chloride. It actually attacks the passive film that gives stainless steel its protective properties, which means even it will erode when exposed to salt water.

Plastics don’t have this problem. For example, polyvinyl chloride (PVC) is often used in underwater plumbing pipes and boat accessories because it is chemically inert and corrosion resistant. Polyethylene Terephthalate (PET) is often used in aquaculture applications. Isoplast is even stronger than both PVC and PET, providing users with even more options. It doesn’t absorb water and isn’t affected by salt water, gasoline, Clorox, and many other chemicals. It is also strong enough to be used with metal materials, making it an especially good choice in marine environments.

UV resistance

In a marine environment, considering the impact of water on the finished product is vital, but it isn’t the only consideration. Being out on the water means being under the harsh rays of the sun too. Not only will metal corrode under water, you can expect it to degrade when exposed to UV radiation. It isn’t uncommon for metal to become more brittle as it spends time out in the sun.

Plastic doesn’t have the same problem. Many plastics are actually resistant to UV rays. Some are so resistant that they can handle outer space. Fluoropolymers and polyimides have even been used in the Hubble Space Telescope and the International Space Station.

It’s important to choose plastics carefully because not all plastics have UV resistant properties. At the very least, some plastics will fade in the sun. Some will actually corrode in the sun and become brittle, ultimately compromising the strength of the plastic fastener.

Combined materials

Plastics can retain their strength over time when exposed to salt water or UV radiation, but they’re usually weaker than metal. That’s why builders still turn to metal materials when strength is of the utmost importance.

Just because metal is stronger than most plastics doesn’t negate the benefits that plastics have to offer in the marine environment. Some plastic fasteners have a metal core. The metal core makes the fastener exceptionally strong, while the plastic coating protects the fastener from the corrosion of salt water and UV radiation.

Plastic marine fasteners also require very little maintenance. To keep metal fasteners from corroding in salt water, they will have to be rinsed regularly with fresh water. If metal fasteners are corroding in the sun, they will have to be replaced. Plastic fasteners with metal cores can offer the best of both worlds.