The corrosion process is a tendency of metals to return to their natural oxidized state. Chemical corrosion is the result of chemical reactions with substances such as acids and other chemical compounds present in the environment. Metals like chromium, aluminum, and titanium show increased corrosion resistance in certain environments. Iron and its iron-carbon alloys have very low corrosion resistance.
The different behavior of metals is due to the formation of a thin surface layer that protects the metal against corrosion. This phenomenon is called surface passivation. Chromium and nickel show a particular tendency for passivation, which justifies their use as major alloying elements in stainless steels. Even at 12% chromium content, steel exhibits passivating properties. In more aggressive environments, higher-chromium steels with additional alloying elements must be used.
For final stainless steel products, a service life of over 50 years is expected.
However, cases of stainless steel corrosion may occur due to:
Symptoms of stainless steel corrosion can vary. Corrosion mostly appears on all kinds of internal inhomogeneities (inclusions, precipitates, deformations) and external ones (edges, scratches, dents, oxides, deposits, etc.). Smooth and uniform surfaces, on the other hand, show much higher resistance to corrosion. Hence, proper pickling and passivation of the surface is so important..
Metal corrosion is a natural process of degradation of metallic materials as a result of their interaction with the environment. It is a chemical reaction that gradually breaks down metals due to environmental factors like moisture, oxygen, salts, or pollution. This process leads to loss of weight, weakened structure, reduced strength under stress, and the formation of characteristic corrosion products (such as rust) on its surface. Metal corrosion is a common and inevitable phenomenon, but its effects can be minimized through proper preventive measures.
Metal corrosion can be accelerated by various environmental factors and conditions. The most common include: moisture, oxygen, salts, acids, atmospheric pollution, temperature, pressure, and mechanical stresses. Moisture and oxygen are the primary factors that initiate the corrosion process, while salts and acids can significantly speed it up. Atmospheric pollutants, like sulfur dioxide, also negatively impact the rate of corrosion. High temperature and pressure can increase the aggressiveness of the environment, and mechanical stresses can cause micro-cracks that become corrosion initiation sites.
Uniform (general) corrosion — occurring across the entire surface. This is the least dangerous as it evenly attacks the whole surface of stainless steel. This causes uniform thickness loss and a decrease in the strength of the corroded part. Prevention is typically achieved by periodic passivation.
Intergranular corrosion — occurs when the active environment attacks grain boundaries without penetrating into the grains. This type spreads along grain boundaries and weakens the bond between grains. Aggressive media may “pluck” grains from the surface, decreasing wall thickness, or cause damage inside the material without obvious visual signs. Intergranular corrosion is one of the most dangerous types.
Stress corrosion cracking — happens when internal or externally applied stresses exist in the metal. Different areas under stress hold different internal energy and in the presence of aggressive solutions can produce local cells that lead to corrosion. It can result from residual stresses due to processes like bending or welding. Manifested as cracks in the metal.
Pitting corrosion — occurs when metal is attacked at specific spots, creating small cavities or pits — a local loss of material. It is driven by localized electrochemical cells where a large passive surface acts as a cathode and small depassivated spots act as an anode. Metal dissolution at the anode is very rapid, leading to perforation of walls with minimal overall weight loss. Pitting is especially common in water containing halides like chlorides, bromides, or iodides.
Corrosion fatigue — occurs due to the combined effect of cyclic or variable stresses and a corrosive environment, resulting in cracks. Stresses break the protective passive layer and expose the underlying metal, allowing corrosion to proceed.
Crevice corrosion — takes place in crevices and tight gaps (e.g. under gaskets, rivet heads, deposits, oxides). It occurs due to the gradual depletion of the protective passive film in areas of poor ventilation and restricted oxygen supply. Preventing crevice corrosion mostly requires eliminating crevices during the design of equipment and products.
Galvanic (contact) corrosion— occurs at the junction of two metals or alloys with different potentials, creating a galvanic cell. The larger the difference in electrochemical potential, especially in a chloride-containing environment, the faster the less noble metal corrodes, especially if its surface is much smaller than the noble one. This is a classic case of galvanic corrosion.
Electrochemical corrosion — happens when two different metals are in contact in the presence of an electrolyte, generating a galvanic couple and leading to accelerated corrosion of the less noble metal.
Metal corrosion has numerous adverse consequences — both economically and in terms of safety. Damaged metal structures like bridges, buildings, and pipelines can require costly repairs. Corrosion also reduces the efficiency of machines and industrial equipment, leading to downtime and production losses. Furthermore, corrosion can cause catastrophic failures of machinery and structures, endangering human safety. This is why it’s so important to monitor the condition of metallic structures and take preventive action.
There are many methods for preventing or slowing corrosion. One is to coat the metal with a corrosion-resistant material — like anti-corrosion paints, zinc, or ceramics. Another is to isolate the metal from water and oxygen using protective covers or gaskets. Corrosion inhibitors — chemicals that slow corrosion processes — can also be effective. Finally, careful design can minimize corrosion risk, for example by eliminating crevices and pockets that might accumulate moisture.
The international standard describing corrosion protection for steel structures is ISO 12944. It contains 9 sections that detail corrosion factors and prevention methods. This standard is an essential tool for professionals involved in surface treatment and protective coatings, providing systematic guidance on best practices in anti-corrosion. Adhering to ISO 12944 ensures greater durability of steel structures and minimizes corrosion risks, which translates into increased safety and economic savings.
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