Did you know that the reason stainless steel resists rust and corrosion is that it has a very thin film coating its surface? This stainless steel coating is a byproduct of the metal elements within the stainless steel interacting with oxygen in the air, and it only forms under specific circumstances.
If these circumstances aren’t met, either because the stainless steel’s elemental composition is off or because it’s not in the right environment, the stainless steel coating won’t form and the steel will rust like any other.
Making sure the conditions are right for stainless steel coating to form is an essential part of the manufacturing and fabrication processes of any stainless steel product. Here is how stainless steel coating works and why testing it is so important.
What is stainless steel coating and how does it help stainless steel resist rust?
Though it’s colloquially referred to as steel, stainless steel is actually a metallic alloy that can contain metals including iron, manganese, silicon, carbon, and, most importantly, chromium.
When chromium reacts to oxygen in the air, it forms a very thin layer of chromium oxide across the surface of the stainless steel. This stainless steel layer is called the “passive layer” because it renders the surface of the stainless steel electrochemically passive in the presence of corrosive environments.
This passive layer protects against rust and corrosion by limiting oxygen and water access between the outside environment and the underlying surface. Though the chromium oxide layer itself may corrode, the layers of steel beneath it will not – and the passive layer is usually no more than a few atomic layers thick, so its corrosion is invisible to the naked eye.
Why does some stainless steel still rust and corrode over time?
Stainless steel products will rust and corrode over time if something either destroys the passive layer or prevents it from forming.
What stops stainless steel coating from forming?
First, any stainless steel product must contain at least 10% chromium by definition. Without at least that much chromium, the chromium oxide passive layer won’t even form in the first place. The passive layer also won’t form if the stainless steel isn’t exposed to sufficient oxygen.
Even if the stainless steel coating does form successfully, however, it can break down and lose effectiveness under several circumstances. For example:
- Composition: If the stainless steel’s surface contains too many deposits of minerals and other elements, the passive layer could lose effectiveness.
- Covering: Covering up portions of the stainless steel’s surface (by painting them, for example) can keep oxygen from reaching that area of the surface and prevent the passive layer from forming there. After corrosion reaches the unprotected area, it can spread across the rest of the surface from beneath the passive layer.
- Damage: Physical damage to the surface of the stainless steel (such as scratches) can destroy the passive layer on the damaged area and allow corrosion to spread.
- Harsh environments: Some environments can wear away at the passive layer of stainless steel over time. For example, chlorine is so corrosive that it can wear away the passive layer. Salt water can also accelerate stainless steel corrosion.
- Metallic cross-contamination: If another type of metal comes into contact with the stainless steel for an extended period of time (if they’re welded together, for example), it can cause galvanic corrosion at the point where they meet.
- Fabrication mistakes: If stainless steel is exposed to temperatures above 800 degrees Fahrenheit for extended periods during the fabrication process, areas near the metal’s grain boundaries can lose their passive layer.
One of the most important steps in manufacturing and performing quality assurance on stainless steel products is ensuring the passive layer forms successfully and properly protects the steel from corrosion and rust.
Why is testing stainless steel coating important?
Obviously, the primary purpose of testing stainless steel coating is to determine whether the stainless steel meets the scientific requirements to prevent rust and resist corrosion. When conducted by experts, however, stainless steel coating tests can provide a great deal more useful information about the stainless steel product.
Performing expert stainless steel coating tests via ESCA can help determine the full chemical composition of the stainless steel. This allows experts to understand the chromium-to-iron and chromium oxide-to-iron-oxide ratios present in the product. In some cases, detailed ESCA testing of stainless steel coating can even specifically determine what is preventing a stainless steel product from resisting rust properly.
How does stainless steel coating testing work?
Innovatech uses Electron Spectroscopy for Chemical Analysis (ESCA), also known as X-ray Photoelectron Spectroscopy (XPS) to test the passive layer of stainless steel.
ESCA works by shining an X-ray beam over the surface of a sample of the stainless steel. This beam excites the atoms on the steel’s surface until they emit photoelectrons. The amount of energy an atom can absorb from the X-ray beam until it emits its photoelectrons differs for each element, so by monitoring when each photoelectron appears, experts can determine the elemental structure of the surface of the steel.
The amount of energy required for photoelectron emission also differs depending on the chemical environments the elements are in during emission (this is called “chemical shift”). By determining first which elements are present on the surface of the steel and then figuring out their elemental shift, experts can also determine the chemical environments present on the steel.
Using the information ESCA/XPS provides, Innovatech’s experts can help you determine whether your stainless steel coating can capably prevent rust and corrosion. If it isn’t, our testing can even help you figure out what’s going wrong in your manufacturing or fabrication process and what you need to do to fix it.
If you need stainless steel coating testing, Innovatech is ready to help. Reach out to us online and we can get started.
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