Stainless steel is known for its resistance to rust. The main alloying element in stainless steel is chromium (Cr). Only when the chromium content reaches a certain level can it exhibit corrosion resistance. Normally, the chromium content in stainless steel should be at least 10.5%.
Illustration of bell coating on stainless steel surface
The mechanism behind the corrosion resistance of stainless steel is known as passive film theory, which states that an ultra-thin, solid, thin and stable passive film rich in chromium forms on its surface, blocking the infiltration and oxidation of oxygen atoms. , thus providing protection against corrosion.
Although many people believe that “stainless steel does not rust,” this statement is incorrect. Under specific conditions, stainless steel can still corrode.
Related Reading: Why Does Stainless Steel Rust and How to Prevent It From Rusting?
It is important to highlight that by understanding the different types of corrosion that can affect stainless steel, measures can be taken to minimize losses due to stainless steel corrosion.
The majority of corrosion damage suffered by stainless steel is localized corrosion, with the most common types being intergranular corrosion (9%), pitting corrosion (23%), and stress corrosion cracking (49%).
Stainless steel can often provide adequate corrosion resistance in many industrial applications. Based on practical experience, in addition to mechanical failures, the corrosion of stainless steel is mainly characterized by local corrosion (such as stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion). These forms of local corrosion are responsible for more than half of failure cases. In reality, many of these failures can be avoided through proper material selection.
Stress Corrosion Cracking (SCC):
Stress corrosion cracking (SCC) is a general term that refers to the combined failure of stress alloys caused by the propagation of severe cracks in corrosive environments. Although it typically has a brittle fracture appearance, SCC can still occur in materials with high toughness. The necessary conditions for stress corrosion cracking are the presence of tensile stress (either residual or external, or both) and a specific type of corrosive medium. Crack formation and growth are generally perpendicular to the direction of tensile stress, and the stress level required for SCC is much lower than that required for fracture in the absence of a corrosive medium.
Microscopically, cracks that run across grains are called transgranular cracks, while cracks that run along grain boundaries are called intergranular cracks. When stress corrosion cracking reaches a certain depth (at which point the stress in the loaded material reaches its fracture stress in air), the material will break as would happen with normal cracks (in ductile materials, usually through aggregation). of microscopic defects). ). The section of a part that fails due to stress corrosion cracking will therefore contain both the area characteristic of stress corrosion and the “strength-rich” area associated with microdefect aggregation.
Point corrosion:
It is a type of localized corrosion that results in pitting.
Intergranular corrosion:
Intergranular boundaries are the limits between different crystallographic orientations of disordered and staggered intergranular elements. As a result, they are prone to the segregation of various solute elements or the precipitation of metallic compounds (such as carbides and δ phases) in the steel. In certain corrosive environments, it is not uncommon for grain boundaries to corrode first, leading to a type of corrosion known as intergranular corrosion. This type of corrosion can occur on most metals and alloys in specific corrosive media.
Interstitial corrosion:
Crevice corrosion is a type of localized corrosion that occurs in areas where the solution is stagnant or on shielded surfaces. This type of corrosion can occur at metal-to-metal or metal-to-nonmetal joints, such as at points of contact with bell pins, screws, gaskets, valve seats, loose surface deposits, and marine organisms.
General corrosion:
General corrosion refers to corrosion that occurs across the entire surface of an alloy in a relatively uniform manner. This type of corrosion can cause the material to gradually become thinner and, in severe cases, can result in the material becoming unusable due to corrosion. Stainless steel can suffer from general corrosion in strong acids and alkalis. Failure problems caused by general corrosion are not as concerning because this type of corrosion can usually be predicted through simple immersion tests or by consulting corrosion literature.
