The selection of a particular material for an industrial application is important under any circumstances, as even slight differences in an alloy’s chemical makeup leads to vastly different performances. However, when your application must withstand an extreme environment, then having the right material with the suitable durability and resistance becomes imperative.
This is especially true of marine environments. When you combine so many elements that are harmful to steel, including water, salt, and other minerals, it takes a special type of stainless steel to live up to its name. Let’s take a look at what causes marine corrosion and whether 304 stainless steel is a good choice for marine applications.
What Causes Marine Corrosion?
Most people are not aware of how serious the problem of marine corrosion is. A recent study entitled, “International Measures of Prevention, Application and Economics of Corrosion Technology (IMPACT)” found that corrosion causes US$2.5 trillion in damage annually. This amount is equal to three and half percent of the global GDP. Finding good solutions to the costly problem of wasted materials and energy would save a tremendous amount of money.
There are actually several types of corrosion that all can be considered marine corrosion. Most of the time, the damage is due to contact with salt water. But other causes include atmospheric corrosion of materials that are in close proximity to coastlines, and hot salt corrosion, which in particular affects engines and fast moving parts while underwater.
Saltwater corrosion is an electrochemical process caused by the electrical potential of the alloy or metal when it comes into contact with seawater, and is dependent on the seawater acidity or alkalinity. Because of its high chloride content, seawater also makes for an efficient electrolyte. The constant churning, combined with the presence of oxygen, means that metals are exposed to a variety of anodic and acidic solutions that can seep into hidden cracks and crevices, allowing for the start of corrosion.
The problem is compounded by a number of factors. The organic microorganisms found abundantly in the oceans also strongly influence the pH balance of the water. Structures and vessels must deal with deposits of sand and silt, which can exclude oxygen from coming in contact with the metal but allow for the creation of locally corrosive conditions. There is also the presence of sulphate reducing bacteria, which leads to hydrogen sulphide concentrations, a substance that is especially dangerous for steel and copper alloys. It’s always possible to avoid corrosion completely, with gold platinum or tantalum, but the cost of these corrosion resistant materials makes them probative. This means that manufacturers must generally factor in price into any selection.
How Do Metals Like Stainless Steel Resist Corrosion?
The majority of alloys with strong corrosion resistance properties have an oxide film on their surface. The most resistant grades of stainless steel are not only stable, but they have a self-healing feature that allows them to repair themselves and increases their durability.
Scientific American offered a good explanation of how stainless steel works in a recent post. It’s ability to avoid rust and other corrosion is due to its ability to interact with its environment. In addition to iron, steel is formed by adding proscribed amounts of chromium, manganese, silicon, carbon, nickel, and/or molybdenum. These alloying agents are able to react with the oxygen in the air, or in the water in marine environments. This reaction is what forms the oxide film that protects stainless steel.
Not only does the oxide film protect against corrosion in the way it blocks oxygen and water from coming into direct contact with the steel, it also continually reacts with its surroundings. This means that in the most effective alloys, the reaction will be able to repair any damage to the oxide layer, basically healing itself and effectively preventing corrosion.
The trick is that this thin layer, so thin it’s invisible to the human eye, is actually a layer of corrosion. Thus, stainless steel corrodes in such a way that it prevents further corrosion. In regular steel, the surface of the metal reacts with the oxygen in seawater and creates an unstable iron oxide/hydroxide film. This layer is not protected, and will continue to grow as long as it is exposed to seawater.
Is 304 Stainless Steel a Good Choice for Marine Environments?
Clinton offers SAE 304 stainless steel, which can also be found under the name A2 stainless steel or 18/8 stainless steel. 304 is the most popular stainless steel alloy available on the market. It is an austenite steel, meaning that it is non-magnetic in nature. This alloy was first developed by Firth Brown Steels nearly a century ago. It was originally sold under the trademark Staybrite 18/8, because of the presence of chromium (at around 18%) and nickel (around 8%) as alloying agents.
304 steel has a eutectoid point that occurs at 727 °C (1,341 °F). While it is not a good choice for applications that require electrical or thermal conductivity, it is well known for its excellent formability. With a carbon maximum of .03 percent, this alloy is also ideal for welding operations under 800 degrees F. While heat treatment is not an option, cold working can be performed to make the material both harder and stronger.
When it comes to corrosion resistance, 304 steel is generally an excellent choice. Its molecular composition protects it from all kinds of environmental damage, including intergranular corrosion (This is a kind of corrosion that results from stress relieving or welding). The chromium content of 304 makes it more corrosion resistance in oxidizing environments, while its nickel content helps protect it from organic acids.
However, 304 stainless steel does have one weakness, and that is it does not perform well in chloride environments, in particular, seawater. It is susceptible to corrosion in these cases. If marine corrosion is a major concern, a better choice would be 316 stainless steel, which has 2 percent molybdenum added to it. This difference is what makes it especially effective in marine applications.
Summary
At Clinton and Stainless Steel, our number one priority is to provide quality service to our customers in order to help them succeed. We invite you to contact a member of our knowledgeable and friendly staff to learn more.
Sources:
https://www.scientificamerican.com/article/why-doesnt-stainless-stee/
