Most companies that need stainless steel assemblies have two requirements for the finished product: it must have excellent dimensional stability and a clean, shiny appearance. The first requirement is accomplished by using the right grade of steel, establishing the correct distance between workpieces, using a sufficiently strong filler metal, and preventing oxidation from forming by maintaining a pure furnace atmosphere. The second requirement involves a bit more consideration, as the causes of stainless steel corrosion are easy to overlooked.
Types of Corrosion
While we often think of metal as corroding after years of exposure to harsh conditions, the fact remains: the basis for stainless steel corrosion often begins in the facility where the metal is treated and processed for delivery to the customer. To prevent stainless steel from corroding, providers of brazing services should have clear understanding of what types of corrosion can develop. With this knowledge, they can eliminate causes of corrosion in the treatment environment and heat treating process, and create improved products.
Pitting corrosion is caused when metal contacts corrosive chemicals, particularly the Chloride ion Cl- that is present in salt and bleach. In many cases, pitting corrosion is inadvertently set in motion when metal is cleaned with an inappropriate solvent after the heat treating process. If in doubt about whether a chemical would cause corrosion, the metal’s corrosion resistance can be deduced by calculating its Pitting Resistance Equivalent Number using the alloy content.
Stainless steel needs the presence of oxygen to ensure that a passive, anti-corrosive layer forms on its surface. In tight crevices where oxygen has difficulty accessing the metal’s surface, corrosion may develop unless a seal is placed over the crevice using flexible sealant. The alternative is using a grade of stainless steel whose corrosion resistance is high enough to prevent crevice corrosion.
Stainless steel tends to experience spots of corrosion instead of general corrosion. However, there are certain chemical agents that can easily cause general corrosion, particularly hydrochloric acid and sulphuric acid. In the factory setting, general corrosion is easily prevented by ensuring that different grades of stainless steel are cleaned using the right solvents with the right dilutions.
This type of corrosion typically occurs when a stainless steel alloy contains a high level of carbon and is heated to the point (450-850 degrees Celsius) where the carbon and chromium combine to form chromium carbide, which weakens the passive, anti-corrosive layer on the metal’s surface. Using low carbon alloys or alloys that contain titanium or niobium is the easiest way to prevent intergranular corrosion.
Galvanic corrosion occurs when dissimilar metal come in contact with each other and an electrolyte such as water is present at the point of contact. If dissimilar metals are stored together, separating them with a layer of rubber will prevent galvanic cells from forming.
Call Franklin Brazing
If you need stainless steel assemblies that are highly corrosion resistant, call Franklin Brazing. We have over four decades of experience performing brazing services on stainless steel and understand how to prevent sources of corrosion at the factory level. For more information about our brazing services, call us today at (800) 450-7822, or click here to use our contact form.