Living at, or having a boat, at the coast conjures up pictures of different things for different folks, but most of them will be positive, like sun-kissed beaches, fresh fish, surfing, and paddle-boarding. On the flip side (there’s always a flip side!), you have the daily challenges like coastal storms, sand, and – most challenging – salt air and saltwater.
The type of metal best for saltwater corrosion resistance is, in general, stainless steel. In particular, duplex stainless steel is the very best and is used on off-shore oil derricks. The top four contenders in order:
- Duplex Stainless Steel
- 316 S Steel
- 316L S Steel
- 304 S Steel
Several metals and metal alloys work well at the coast, but there is probably nothing in nature that corrodes metal as effectively as salt. Regardless of what material you use or in what thickness, the time will come when you will need a regimen of cleaning to maintain the integrity of any metal covered with saltwater.
What Type Of Metal Is Used On Oil Rigs?
While the rust and pitting on your yacht and the window frames of your beach house are certainly of importance, the biggest groups impacted by the threat of rust and salt corrosion are the oil derricks that are permanently standing in the ocean and shipping lines, whose existence is a constant struggle against the elements.
Saltwater and wind constantly attack the exposed equipment on oil rigs and their platform without respite. Protection against corrosion is essential, and painting steel parts or coating them in special oils is the most common way to protect the material. This maintenance regime is constant to one degree or another.
Some surfaces can be painted or coated, but stainless steel is becoming the material of choice. Generally, the austenitic stainless steel grade 316L (1.4404) is used for most corrosion protection. Most stainless steel structural sections that engineers plan to use on the topside of off-shore rigs are rectangular or square hollow sections.
Statoil, (the national oil company of Norway) is conscious about lifetime sustainability and costs, yet uses stainless steel extensively in many different forms; plates, pipes, grating, supporting structural bars, and more. For Statoil, the high cost of stainless steel and the maintenance required is worth the expense.
Why Does Stainless Steel Resist Corrosion So Efficiently?
Stainless steel has a content of at least 10.5% chromium, which reacts quickly with surrounding oxygen forming an oxide layer on the surface. The chromium oxide clings to the steel and acts as a protective barrier, unlike iron-oxide, which often takes the form of flaky rust.
Do Stainless Steel Structures Add Weight To A Rig?
Stainless steel is famously heavy and adds a lot of weight to any superstructure, and while keeping corrosion at bay is a major concern, weight control is also vital, particularly in heavy seas. The goal of reducing weight where possible has led to the creation of special stainless steel grades like ‘duplex steels’, which are very attractive for the rig’s topside.
Duplex and lean duplex are two steel grades with elevated corrosion-resistant properties. Their mechanical performance regarding the material’s yield allows design and engineering teams to plan lighter sections. These sections, though much lighter, will carry the same load.
What Is Duple Stainless Steel?
Duplex stainless steels are named such because they have a two-phase microstructure consisting of ferritic and austenitic stainless steel particles. Duplex stainless steels are approximately twice as strong as regular ferritic or austenitic stainless steels, which is why they have even better corrosion-resisting properties.
Does Duplex Stainless Steel Ever Rust?
All corrosion-resistant alloys, duplex stainless steels included, can and will suffer corrosion when exposed to suitably corrosive conditions. Duplex Stainless Steel contains a greater chromium content than standard grades, and their PREN (Pitting Resistance Equivalent Number) is higher, which indicates to engineers an increased resistance to pitting issues.
How Long Are Oil Rigs Expected To Last At Sea?
Incredibly, some have lasted over 50 years. Despite rupturing at the seafloor and turning the sea black in January 1969, Platform ‘A’, just off the Santa Barbara coast in California, is still there and still operating, along with dozens of others. The correct choice of metal for the platform and other parts exposed to seawater and salt air is vital.
Once the correct material is in place and correctly maintained, only then can the various petro-chemical giants justify their place off our coasts.
What Type Of Metal Is Best For Salt Water At A Human Level?
It’s all very well knowing what materials big industry uses, but what about normal folk living at the coast? What material do you use for any metal components on your boat dock, beach house, or similar? Stainless steel is the best way to go, but what is stainless steel?
Stainless steel is a corrosion-resistant alloy of chromium, iron, nickel, and sometimes other metals. Almost infinitely recyclable, stainless steel is a great material to use where any form of corrosion is an issue, but note it is not permanently impervious to salt.
There are different kinds of stainless steel, each with differing percentages of the metals it contains. 316 stainless steel is a very popular alloy that possesses molybdenum. Unlike the most common form of stainless steel (304), 316 has more nickel, increased resistance to corrosion from salts and many acids, and is nicknamed ‘Marine Grade’ for a good reason.
Copper, Brass & Bronze Offer An Alternative To Stainless Steel
These luxury metals are the so-called ‘Red Metals’, which will outlast all of us by a thousand years at least. If that’s long enough for you to avoid saltwater corrosion, then they might be a far more attractive option than stainless steel. All three will mature into a green tinge when they oxidize, a patina that is extremely attractive to many people.
If you want the best metal for saltwater with your boat or home in mind, stainless steel will be the obvious choice. However, if you want to know the absolute best metal for the job, it’s duplex stainless steel, though even that will not last forever against nature. If you like the patina of the oxidized Red Metals, they are definitely worthy of your consideration.