Definition and classification of corrosion resistance of stainless steel
Stainless steel is resistant to corrosion, but not completely immune to it.
To date, there has been no discovery of a type of steel that is completely impervious to corrosion under any conditions.
As a result, different grades of stainless steel are suited for specific environments.
Corrosion resistance principle of stainless steel:
① The addition of alloying elements, such as Cr and Ni, to steel can effectively improve its corrosion resistance by increasing the electrode potential of the base metal and reducing the number of microcells.
② By adding alloy elements, the steel can achieve a single-phase solid solution structure at room temperature, which reduces the number of microcells and enhances its corrosion resistance.
③ The addition of chromium as an alloying element to steel forms a dense, Cr2O3 passivation film on the steel surface. This film is highly resistant and has no corrosion medium, providing a protective layer for the metal matrix. The thickness of the passivation film ranges from 1-6nm and increases in strength and thickness with the increase of chromium content in the steel.
Classification and prevention of stainless steel corrosion
Pitting corrosion, as depicted in Figure 1, is a prevalent form of local corrosion in stainless steel. The depth of the pits is typically greater than their diameter.
When active anions (C1 -) are present in the environment, they get adsorbed at specific points on the metal surface, thereby compromising the passive film on the stainless steel surface. The steel may contain defects, impurities, and solutes.
Once the protective passivation film is damaged, the base metal is readily exposed at surface defects, making it active, while the passivation film remains passive, resulting in the formation of an active-passive corrosion cell.
Due to the smaller size of the anode area compared to the cathode area, the anode current density is high, leading to rapid corrosion. Over time, the metal surface will become corroded into small holes.
Fig. 1 Pitting on SUS304 steel pipe used in brine
Ways to prevent pitting
① To enhance pitting resistance, use materials that have added aluminum to steel and increased chromium content, such as 316L steel.
② Implement an appropriate heat treatment system to ensure the stainless steel matrix is in a fully solid solution state.
③ Minimize the concentration of halogen ions in the solution and raise the pH value of the solution.
④ Stir the solution consistently to prevent local concentration and to avoid impurities from accumulating on the surface of the steel.
⑤ Enhance the surface finish of the stainless steel.
⑥ Lower the temperature of the medium.
⑦ Implement cathodic protection measures.
2. Crevice corrosion
A narrow gap is formed between metal and metal or between metal and a non-metal.
The flow of related substances in the gap is restricted, creating a concentration cell that leads to local corrosion.
This type of corrosion is known as crevice corrosion (as illustrated in Figure 2) and frequently occurs at the junction of stainless steel equipment.
Fig. 2 Crevice corrosion at the bottom of the gasket in a flat heat exchanger used in brine
Ways to prevent crevice corrosion
① To prevent crevice corrosion, materials such as aluminum-containing stainless steel and titanium-containing stainless steel can be selected.
② The design should be improved to minimize or eliminate gaps and avoid the use of metal and non-metallic connectors whenever possible.
③ To keep impurities and pollutants from accumulating in gaps, the flow of the medium liquid should be increased.
④ The pH value should be raised and the concentration of CI ions should be reduced to lower the sensitivity to crevice corrosion.
3. Stress corrosion
Stress Corrosion Cracking (SCC) refers to the cracking of metals and alloys that is caused by the simultaneous presence of corrosive agents and tensile stress.
This type of corrosion is characterized by the formation of cracks that result from the combined effects of corrosion and mechanical stress. These cracks can occur along grain boundaries and even penetrate through the grains, reducing the mechanical strength of the metal structure. In severe cases, this can lead to sudden and catastrophic failure of metal equipment.
Ways to prevent stress corrosion
① Materials should be chosen appropriately, avoiding materials that are prone to stress corrosion;
② The design should be reasonable to prevent excessive machining, large residual stress, or stress concentration points;
③ Consider the use conditions to avoid exposure to corrosive substances on the surface, particularly avoiding the concentration of chloride ions in specific areas.
4. Intergranular corrosion
Intergranular corrosion is a type of localized corrosion phenomenon where the corrosion occurs along the boundaries between grains, leading to a loss of adhesion between grains.
Although the surface of parts affected by this corrosion may still appear bright and undamaged, the loss of adhesion between grains results in a loss of material strength. In severe cases, the metal may even become brittle.
The cause of intergranular corrosion is generally thought to be the dilution of alloy elements at the grain boundaries, leading to the precipitation of chromium compounds at these locations and the formation of chromium-poor areas along the grain boundaries.
These chromium-poor areas are then more susceptible to corrosion under the influence of a corrosive medium.
Intergranular corrosion can occur in austenitic, ferritic, or duplex stainless steels. The sensitization temperature range that increases the risk of intergranular corrosion in austenitic and duplex stainless steels is between 450 ℃ and 850 ℃, while in ferritic stainless steels, it occurs above 850 ℃.
Fig. 3 The intergranular corrosion around the weld of an AISI316 hook used in sulfuric acid solution
Ways to prevent intergranular corrosion
① The sensitivity to intergranular corrosion can be reduced by increasing the content of trace elements.
② The addition of stabilizing elements such as titanium and niobium can form stable NbC and TiC, due to the strong bond between titanium, niobium and carbon, which reduces the formation of intergranular chromium-deficient zones by preventing the interaction between inscriptions and carbon.
③ The presence of elements such as carbon, nitrogen, phosphorus, silicon, and others, can negatively affect the material’s resistance to intergranular corrosion, so it’s best to minimize their content as much as possible.
④ During heat treatment, it is important to avoid prolonged exposure to sensitization temperature ranges and to prevent the precipitation of intergranular carbides and grain coarsening.