Don’t let its name deceive you, stainless steel can indeed corrode.
Why is stainless steel considered highly resistant to corrosion?
Stainless steel contains a passive film that generally combats against corrosion. This is due to the chromium (all stainless steel contains at least 10.5%) reacting with oxygen to form a passive layer (hydrated chromium oxide) which can rebuild if damaged mechanically.
Now, let’s address those instances when corrosion does occur with stainless steel. In fact, there are seven specific ways stainless steel can corrode. It’s all dependent on two factors: environment and alloy chemical composition.
Each corrosive scenario can be combatted by choosing different grades.
Here is a look at the seven types of corrosion to stainless steel and ways to combat each, with information courtesy of Outokumpu Stainless USA, LLC :
|Type of corrosion
||How to combat
||This is not a very common condition for most of the grades. This could occur uniformly over the entire
exposed surface when the passive layer is destroyed
||Cleaning practices avoiding chlorines and exposing the grades to a clean oxidizing atmosphere (air). Keep
surface free of foreign impurities.
||This occurs when chloride ions cause a local breakdown of the passive film of the stainless steel. Pitting
may occur in acidic or almost neutral solutions containing chlorides (halogenides).
||Depending on the atmospheric conditions and the specific alloy, Increasing the chromium, nitrogen, and
molybdenum content in the grade will improve pitting corrosion resistance.
||This type of corrosion occurs in crevices or other shielded areas in acidic or nearly neutral solutions
mainly containing chlorides.
||Improve the design to avoid crevices depleted in oxygen. Increasing the chromium, nitrogen, and molybdenum
content of the grade will improve crevice corrosion resistance.
|Stress corrosion cracking
||This can be caused by a combination of corrosive environments, mostly chlorides, tensile stresses in the
steel, and mostly austenitic grades.
||Selecting the right alloy composition and reducing the tensile stresses and presence of chlorides in the
equipment. Keeping lower temperatures is favorable.
||This can occur if the stainless steel has been heat treated at, or cooled through the temperature
interval, 500 - 900°C) and if the
environment is corrosive. Chromium carbides precipitate and produce chromium depleted grain boundaries. This
is rare with modern low carbon stainless grades.
||Select low carbon alloys such as 304L and 316L. The addition of titanium or niobium in stabilized grade
will help to diminish the intergranular corrosion.
||This can occur if two dissimilar metallic materials are in electric contact with each other in a
conductive liquid or wet atmosphere.
||Select alloys with similar galvanic tendency according to a galvanic table. To separate the two dissimilar
metals with a non-conductive material such as polymer.
||This can occur when the steel is exposed to an aggressive atmosphere at ambient temperatures. This is
further impacted by
- Atmospheric contaminants/salt
- Surface roughness
Most stainless steel is considered resistant to this form of corrosion. To further combat, it is very
important to practice surface cleaning maintenance. Be sure to specify a lower surface roughness along with
the correct grade for the application.
Stainless Steel Sheet
Thin and corrosion resistant, stainless steel sheets are used in a variety of applications, ranging from lightweight residential and architectural or industrial and marine applications.
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Stainless Steel Plate
Stainless steel plate come in a variety of thicknesses and tolerances. Stainless 304 & 304L stainless steel plate is often used in stamped and machined parts for processing equipment while 316 & 316L stainless steel plate are employed by the chemical, marine and power transmission industries.
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Stainless Steel Bar
Stainless steel bar comes in a wide variety of grades and many finishes such as hot rolled, cold drawn, annealed, rough turned and bearing quality. The various alloys offer a range of corrosion resistance, hardness, strength, weldability, machinability and heat treatability.
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