Previous posts discussed the varieties and types of stainless steel, a metal formed by adding chromium to carbon steel to give corrosion resistant properties to the metal. Stainless steel derives its name from the fact that it doesn’t get rust stains like ordinary carbon steel— but does that mean that it’s always completely stainless?
Stainless steel can in fact corrode if the oxygen in the environment is low enough. When chromium is added to steel, it reacts with oxygen in the atmosphere to form chromium oxide, a thin, invisible passive film. If the metal is cut or scratched, more oxide will quickly form and recover the exposed surface to prevent corrosion. In environments where oxygen levels are low and chlorides (salts) are high, such as exposure to seawater, road salt, or chemicals, the passive film lacks the oxygen it needs to regenerate. The addition of certain alloying elements improves corrosion resistance for applications where the steel will be exposed to or near salt water. The key is to know which grade of stainless steel is appropriate for your environment.
When stainless steel does corrode, it can take several forms:
What it is: Pitting corrosion is the most common form of corrosion and usually appears as small, dark brown pits on the surface of the metal. Causes include high chloride (sea salt, road salt) or acidic environments, and high temperatures. While pitting doesn’t interfere with the mechanical properties of the stainless steel, the pits can spread and perforate the surface, marring its appearance.
How to avoid it: Choose a stainless steel grade high in chromium, molybdenum and nitrogen.
What it is: Crevice corrosion happens in sharp or unsealed areas, such as around bolts and screws, in stagnant, high chloride solutions. Crevice corrosion is similar to pitting in that it has no impact on the steel’s mechanical properties, but it can occur at lower temperatures.
How to avoid it: Stainless steel components can be designed to prevent crevice corrosion by eliminating areas where crevices form, or by making sure the crevices are large enough to allow oxygen to come in and reform the passive film. The grade of stainless steel should contain high percentages of chromium, molybdenum, and nitrogen.
What it is: Galvanic corrosion can occur when two different metals come in contact with each other in the presence of an electrically conductive liquid.
How to avoid it: Avoid mixed metal fabrications if possible, paint the metals with a protective coating or use some other means to prevent contact, or remove the electrolyte.
Stress corrosion cracking
What it is: Stress corrosion cracking occurs when mechanical stresses, the environment, and high temperatures combine to form small cracks in the metal that can quickly expand.
How to avoid it: Stress corrosion cracking is related to the nickel content of the steel. Steel with a nickel content between 8 -20% is particularly susceptible to stress corrosion cracking, while steel with a nickel content of greater than 30% is resistant to this type of corrosion.
What it is: Intergranular corrosion happens when stainless steels are exposed to prolonged, high heat. The chromium and carbon in the steel combine to form chromium carbide particles along the grain boundaries throughout the steel. As these carbides form, chromium is removed, making the metal susceptible to corrosion at the grain boundaries. Steel in this condition is said to be “sensitized.”
How to avoid it: Choose grades with a low enough level of carbon to prevent the formation of chromium carbides. Stabilized grades alloyed with titanium and niobium are good choices for applications such as welding.
From construction to transportation to food processing and surgical equipment, the appropriate grade of stainless steel must be used to avoid the kinds of issues discussed in this post. In fact, to prevent failure of the finished stainless steel products, all aspects of the manufacturing process must be carefully monitored, from the raw material mix through fabrication and finishing. A variety of tools are available to metallurgical labs, cast shops, and rolling mills to make sure their stainless steel meets specifications.
- X-Ray Fluorescence (XRF), Optical Emission Spectroscopy (OES), and X-ray Diffraction (XRD).
XRF, OES, and XRD are technologies that provide fast, accurate elemental and phase analysis of the composition of stainless steel during the manufacturing process.
- Thickness Gauges for hot and cold rolling mills. Cold rolling is a metal forming process that ensures the stainless steel meets the thickness requirements for the finished product. Cold rolling mills rely on x-ray based thickness sensors for accurate, real-time measurements to ensure the stainless steel is on spec. The variety of alloying elements in the steel makes x-ray thickness gauge measurement challenging because the alloying elements absorb the x-rays at different rates. Therefore it is very important to know the precise chemistry of the metal being rolled. This relationship makes it important to have both accurate elemental analysis and good alloy compensation.
- Metals Analysis Software. Software packages help labs perform sample analysis of metals and non-metals in iron and steel production.