The general view of stainless steel is that it is a high grade alloy that never rusts, when in reality it does. While stainless steel in theory should not corrode, there are many occasions where rust forms on a stainless steel product. This is especially true in marine environments.
In lower quality stainless steel, or poorly made stainless steel products, it could be that the chromium levels are not high enough or evenly distributed. Stainless steel generally contains between 12-30 per cent of chromium, which protects it as long as the localised concentration is in excess of 12 per cent.
The simplest condition for this rusting to occur is when ordinary steel is rubbed off onto or comes in contact with a stainless steel surface. This causes the localised concentration of chromium to fall below 12 per cent. Hence the chromium oxide barrier fails and causes a pitting corrosion to occur, which will be explained below.
Depending on its crystalline structure, stainless steel falls into one of three main categories (as mentioned in our previous article found here):
After differentiating the types of stainless steel, it is important to recognise different types of corrosion that affect stainless steel, which include:
Where the entire stainless steel surface can be attacked uniformly when the passive film disappears from the surface without being able to reform. This corrosion occurs when the surface is exposed to reducing mediums.
Two forms of corrosion (pitting and crevice corrosion) are typical of stainless steels.
Before determining the appropriate grade of stainless steel for your project, you must acquire information about the temperature, pH level and chemical composition of the contents. Of those prerequisites, it is extremely important to ask how much chloride (salt) is present in its chemical composition.
Data reports often provide the measurement as milligrams per litre (mg/L) or sometimes as parts per million (ppm) of Cl. But it is imperative to not confuse chloride with chlorine, as Cl is also the symbol used for the element chlorine.
In marine environments where stainless steel is exposed to salt water and sun for long periods, special types of stainless steel are usually required. The most appropriate stainless steel applications are designed using the following grades of stainless steel:
One of the most common grades for making components that will hold well when exposed to salt water but not submerged in it. It is an austenitic alloy that includes chromium and nickel and is lower in carbon than the standard 304 grade of stainless. Often used in water theme parks with salt water tanks and pools.
Also commonly used for marine environments, similarly to 304L with its lower carbon, austenitic alloy containing chromium and nickel. The addition of molybdenum provides an increased level of corrosion resistance than 304L, and proves particularly useful in environments where salt water meets fresh water.
Duplex stainless steel is a combination of ferritic and austenitic steels, resistant to pitting, stress corrosion cracking, and crevice attack. 2507 duplex stainless steel is one of a group of super duplex grades often used in the construction of off shore oil rigs and found in marine cranes for hauling boats in and out of salt water.
Although stainless steel has its drawbacks in a marine environment, it is still the most preferred material in the industry and one that offers the greatest corrosion resistance underwater.