Understanding Corrosion Mechanisms of Stainless Steel Flanges
Stainless steels are widely recognized for their high corrosion resistance, making them suitable for various environments. However, the degree of corrosion resistance varies among grades depending on their constituent elements. This variability necessitates careful selection of the appropriate stainless steel grade tailored to specific applications. In addition to selecting the right material grade, meticulous detailing and craftsmanship play crucial roles in minimizing the risk of staining and corrosion.
Pitting Corrosion: Pitting corrosion is a localized form of corrosion that primarily occurs in environments containing chlorides. It manifests as small pits on the metal surface, which, if not addressed promptly, can compromise the structural integrity. In applications such as pipelines, ducts, and containers, pitting corrosion is particularly critical. Choosing stainless steels with molybdenum content can significantly reduce the risk of pitting corrosion.
Crevice Corrosion: Crevice corrosion occurs in stagnant liquids where oxygen supply is severely restricted, such as in narrow gaps around nuts, bolts, and welds. The severity of corrosion depends on the depth and narrowness of the crevice. Accumulation of chlorides and surface deposits exacerbates crevice corrosion.
Bimetallic (Galvanic) Corrosion: Bimetallic corrosion occurs when two dissimilar metals are in contact in the presence of an electrolyte. In such cases, the less noble metal (anode) corrodes faster than it would if isolated, while the more noble metal (cathode) remains protected. The corrosion rate typically depends on the surface area ratio between the metals, which is a common issue in joints and fasteners. Selecting compatible metals or implementing isolation techniques can effectively mitigate this type of corrosion.
Electrochemical Corrosion: Dust containing metallic elements or foreign metal particles can accumulate on the surface of stainless steel flanges. In the presence of humid air, condensation water between these particles and the stainless steel surface can create micro-cells. This sets off electrochemical reactions that disrupt the protective oxide film, initiating localized corrosion.
Organic Acid Corrosion: Organic substances such as juices from fruits, vegetables, soups, or other organic fluids can adhere to stainless steel flanges. In environments with water and oxygen, these organic substances can metabolize into organic acids. Over time, these acids can attack the metal surface, compromising its corrosion resistance.
Chemical Corrosion: Exposure to acids, alkalis, or salts (e.g., from splashes of alkaline water, lime water used in construction) can lead to localized corrosion on stainless steel flanges. These chemical substances can react with the metal surface, breaking down the protective oxide layer and initiating corrosion processes.
When austenitic stainless steels are heated for extended periods between 450-850°C, carbon within the steel diffuses to the grain boundaries and forms chromium-rich carbides. This process depletes chromium from the solid solution, resulting in lower chromium content adjacent to the grain boundaries. Steels in this condition are referred to as 'sensitised'. The grain boundaries then become susceptible to preferential attack upon exposure to a corrosive environment. This phenomenon is known as weld decay when it occurs in the heat-affected zone of a welded joint.
Stainless steel grades with low carbon content (~0.03%) do not undergo sensitisation, even for plate thicknesses up to 20 mm welded using arc processes, which involve rapid heating and cooling. Moreover, modern steelmaking techniques typically achieve carbon contents of 0.05% or less in standard grades like 304 and 316, making these grades resistant to weld decay when welded using arc processes.
