1. Brief introduction to the corrosion modes of stainless steel
In a wide range of industrial applications, stainless steel offers satisfactory corrosion resistance.
Based on experience, the corrosion of stainless steel, in addition to mechanical failures, mainly manifests itself as localized corrosion, which includes stress corrosion cracking (SCC), pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion.
1.1 Stress Corrosion Cracking (SCC)
Stress corrosion is a type of failure that occurs in stressed alloys in corrosive environments due to crack propagation. SAC has brittle fracture surface characteristics, but it can also occur in materials with high toughness.
The conditions necessary for SCC to occur include tensile stress (either residual stress, applied stress, or both) and the presence of a specific corrosive medium. Crack formation and propagation generally occur perpendicular to the direction of tensile stress.
The stress level that causes SCC is significantly lower than the stress level required to fracture the material in the absence of a corrosive medium.
On a microscopic scale, cracks that pass through grains are called transgranular cracks, while those that propagate along grain boundaries are called intergranular cracks.
When CAA progresses to a certain depth (where the stress in the loaded material's cross-section reaches its fracture stress in air), the material breaks normally (in tough materials, usually through the aggregation of microscopic defects).
Therefore, the fracture surface of a component that fails due to SCC will contain areas characteristic of SCC as well as areas of “ductile ripples” associated with the aggregation of microscopic defects.
The primary conditions for stress corrosion cracking typically involve a weak corrosive medium, a certain tensile stress, and a specific corrosive system composed of certain metallic materials. This topic will be discussed in detail below.
The. Stress corrosion cracking can occur only when weak corrosion forms an unstable protective film on the metal surface.
Experimental results indicate that a decrease in pH value reduces the susceptibility of austenitic stainless steel to stress corrosion cracking.
General structural steel, in neutral and high pH media, will experience stress corrosion cracking through different mechanisms.
B. Corrosion tends to occur under certain tensile stress conditions.
For stress corrosion cracking of Cr-Ni stainless steel, the relationship between stress (σ) and cracking time (ts) is generally considered to follow the equation 1gts=a+bσ, where a and b are constants.
This suggests that the higher the stress, the shorter the time before stainless steel suffers from stress corrosion cracking.
Studies on stress corrosion cracking in stainless steels show that there is a critical stress value for the occurrence of stress corrosion cracking, commonly represented by σSCC.
If the stress is below this value, stress corrosion cracking will not occur. The σSCC value varies with the type of medium, concentration, temperature and different compositions of the material. The environment that causes stress corrosion cracking failure is quite complex.
The stresses involved are not just operational stresses, but a combination of these stresses and the residual stresses generated in the metal due to manufacturing, welding or heat treatment.
w. Metallic media systems are prone to destruction by stress corrosion cracking.
The media that most commonly cause stress corrosion cracking are chlorides, alkaline solutions and hydrogen sulfide.
Research results on the effect of metal ions on various chlorides have led to stress corrosion cracking in Cr + Ni stainless steel. The effect of different chlorides decreases in the order of Mg2+, Fe3+, Ca2+, Na+, Li+ ions.
d. The influence of materials, structure and stress conditions.
Impurity elements greatly affect the sensitivity to stress corrosion cracking. In stainless steel, a nitrogen content greater than 30×10^-6 can significantly increase sensitivity to chloride embrittlement.
The sensitivity of steel to stress corrosion cracking varies with carbon content.
When the carbon content is low, the sensitivity of the steel increases as the carbon content increases. When ω(C) is greater than 0.2%, the stress corrosion resistance tends to stabilize. When ω(C) is 0.12%, the sensitivity to stress corrosion cracking is greater.
The structural state of the material significantly affects the sensitivity to stress corrosion cracking. The greater the heterogeneity of the material, the easier it is to generate active cathode channels and cause stress corrosion. As the grain size increases, the sensitivity of the steel to stress corrosion cracking increases.
The higher the concentration of the medium and the ambient temperature, the easier it will be for stress corrosion cracking to occur. Stress corrosion caused by chlorides generally occurs above 60℃, and sensitivity increases markedly with temperature.
Stress corrosion caused by alkaline solutions generally occurs at 130°C or higher temperatures. Stress corrosion in hydrogen sulfide solution occurs mainly at low temperatures.
The effect of material strength and hardness on stress corrosion cracking sensitivity depends on the actual condition of the component. Under the same strain (deformation) control conditions, the greater the strength and hardness of the material, the greater the stress of its component and the greater the sensitivity to stress corrosion cracking.
Under the same stress control, as the strength and hardness of the material increases, the sensitivity of the component to stress corrosion cracking decreases.
Generally, when the external load (stress caused by deformation or external load) reaches more than 85% of the material's yield strength, the probability of the component suffering from stress corrosion cracking increases significantly.
The most effective method for preventing stress corrosion cracking is to select materials that are resistant to such cracking in a given environment.
1.2 Pitting corrosion
Pitting corrosion, also known as pit corrosion, is a form of electrochemical corrosion and is a common type of localized corrosion in stainless steel.
As mentioned previously, the excellent corrosion resistance of stainless steel is due to an invisible oxidized film that makes it passive. If this passive film is destroyed, the stainless steel will corrode. The characteristic appearance of pitting corrosion is corrosion spots located on the surface.
Removing the surface layer of the pit reveals severe corrosion craters, sometimes covered by a layer of corrosion products. Once removed, the serious corrosion pits are revealed. Furthermore, under specific environmental conditions, corrosion pits may exhibit a special pagoda-like morphology.
Factors that contribute to pitting corrosion include:
a) The environmental environment that causes pitting corrosion is the presence of central metallic ions, such as Fe3+, Cu2+, Hg2+ in solutions of Cl-, Br-, I- and ClO-4, or alkali and alkaline metal ions Na+, Ca2+ in solutions containing H2O2, O2.
The corrosion rate will increase with increasing temperature. The fluid state of the solution also affects the occurrence of pitting corrosion. When the flow rate reaches a certain level, pitting corrosion does not occur.
b) The addition of Mo to stainless steel can produce a dense and robust passive film on the surface of the stainless steel, leading to an increase in pitting corrosion potential and increasing the ability to resist pitting corrosion. As the Cr content increases, the pitting corrosion rate of stainless steel decreases.
c) The stainless steel heat treatment process greatly affects pitting corrosion. Heat treatment at temperatures comparable to carbide precipitation can increase the number of pitting corrosion events.
d) Processing and deformation will also increase sensitivity to pitting corrosion.
The following measures can prevent pitting corrosion:
- Avoid concentration of halide ions.
- Add appropriate amount of molybdenum and increase the chromium content in austenitic stainless steel. Adopt reasonable heat treatment process.
- Ensure uniformity of the oxidizing solution. Increase oxygen concentration or remove oxygen.
- Increase the pH value of the solution.
- Operate at the lowest possible temperature.
- Add passivators to the corrosive medium.
- Use cathodic protection to keep the material potential below the critical pitting corrosion potential.
1.3 Intergranular Corrosion
Intergranular corrosion of stainless steel is a type of corrosion that occurs along or immediately adjacent to grain boundaries.
This corrosion is caused by the precipitation of chromium carbides along grain boundaries under certain heat treatment conditions, which forms chromium-depleted zones near the grain boundaries and preferentially dissolves in the corrosive medium.
The corrosion that occurs between the grains is a serious form of degradation, as it results in the loss of bond strength between the grains, almost completely eliminating the material's strength.
After the metal has undergone intergranular corrosion, there are practically no changes in appearance – the geometric dimensions and gloss of the metal surface remain unchanged – but the length and elongation decrease significantly.
After exposure to cold bending, mechanical impact or intense shock with fluids, cracks appear on the metal surface, which may even become brittle. With a slight force, the grains fall on their own, losing their metallic sound.
Metallographic examination reveals uniform corrosion along grain boundaries and, in some cases, grain displacement can be observed. When examined with a scanning electron microscope, the fracture surface exhibits a granulated sugar-like morphology.
The generally accepted cause of intergranular corrosion is the existence of inclusions or the precipitation of certain compounds (such as carbides or sigma phase) at the grain boundaries, which reduce the electrode potential of the base metal at the grain boundary.
When an electrical dielectric is present at the surface, corrosion originates at the grain boundaries and gradually develops inward. Whether a given material will undergo intergranular corrosion depends on the characteristics of the material and the medium system.
In this system, the dissolution rate of the grain boundary region of the material is greater than that of the grain body, leading to intergranular corrosion.
Preventive measures for intergranular corrosion are as follows.
The. Reduction of carbon content: By reducing the carbon content in steel below the solubility limit, the precipitation of carbides is prevented. Alternatively, a slight elevation above the solubility limit allows only traces of carbides to precipitate at grain boundaries, which is insufficient to pose an intergranular corrosion risk.
B. Addition of strong carbide-forming elements: Alloy with stabilizing elements such as Titanium (Ti) and Niobium (Nb), or traces of grain boundary adsorbent elements such as Boron (B). These elements exhibit a strong affinity for carbon, forming insoluble carbides by combining carbon, nickel and niobium in the TiC and NbC forms. This effectively prevents chromium depletion caused by the precipitation of Cr23C6 compounds.
w. Employ appropriate heat treatment methods: This prevents or alters the type of precipitates formed at grain boundaries. Solution treatment allows the redissolution of precipitated carbides, eliminating the tendency to intergranular corrosion after sensitization. Prolonging the sensitization treatment allows the chromium plenty of time to diffuse into the grain boundary regions, mitigating localized chromium depletion.
1.4 Crevice corrosion
The. Causes of crevice corrosion:
In an electrolyte, a concentration cell is formed due to small gaps between stainless steel and another metal or non-metal. This results in localized corrosion in or near the crevice, known as crevice corrosion. Crevice corrosion can occur in various media, but is most severe in chloride solutions.
In seawater, the mechanism of crevice corrosion differs from pitting corrosion, but their diffusion mechanisms are similar, both involving autocatalytic processes. This reduces the pH value within the crack and accelerates the migration of chloride ions towards the corrosion area.
B. Preventive measures for crevice corrosion:
In corrosive media, cracks can be formed by deposits on the steel surface, corrosion products and other fixed substances. There are always cracks in flange joints and bolted connections, so to mitigate the damage caused by cracks, it is preferable to use welding instead of bolted connections or rivets.
Furthermore, deposits on the metal surface must be removed regularly. Waterproof sealing areas must be used at flange joints. Improving alloying elements resistant to pitting corrosion generally benefits crevice corrosion resistance. To improve resistance to corrosion in crevices, chromium-nickel stainless steel containing molybdenum can be used.
1.5 Galvanic Corrosion
Galvanic corrosion is corrosion caused by the connection of two or more dissimilar metals, also known as bimetallic corrosion.
The. Causes of galvanic corrosion:
Galvanic corrosion occurs when a metallic component immersed in an electrolyte solution comes into contact with other components with different electrode potentials, or when there is a potential difference in different parts of the same metallic component.
Metal or parts with lower electrode potential corrode more quickly, leading to galvanic corrosion. The degree of galvanic corrosion depends on the difference in corrosion potential between the two metals before the short circuit, which varies with different media.
B. Preventive measures for galvanic corrosion:
To prevent galvanic corrosion, the number of primary cells must be reduced as much as possible and the electrode potential difference must be reduced. Efforts must be made to form a stable, complete, dense and tightly combined passivation film on the surface of the steel.
2. Corrosion resistance of stainless steel in corrosive environments
2.1 Atmospheric Corrosion
The resistance of stainless steel to atmospheric corrosion varies basically with the chloride content in the atmosphere. In general atmospheric environments, the corrosion resistance of stainless steel is generally classified as follows: Cr13, Cr17 and 18-8.
In rural atmospheric environments, Cr13 and Cr17 steels can meet corrosion resistance requirements. In urban or industrial environments, Cr13 or Cr17 steel may be chosen for indoor use; Cr17 steel should be chosen at least for outdoor use.
When the atmosphere contains C12, H2S and CO2, 18-8 steel and 18-14-2 austenitic stainless steel can meet the corrosion resistance requirements.
In marine atmospheric environments, corrosion by chloride ions is particularly prominent. Cr13 and Cr17 steels cannot meet corrosion resistance requirements. Rust and pitting corrosion will occur in a very short time.
The corrosion resistance of 18-8 steel in this environment is also not ideal, as evidenced by the appearance of fine, easily removable rust. The corrosion resistance of 18-12-2 steel is comparatively ideal.
This steel generally has a very low corrosion rate (0.0254 μm/a) and surface pitting corrosion (0.024 cm). Under marine atmospheric conditions, molybdenum-containing stainless steels oCr17Ni12Mo2 and 30Cr-2Mo basically meet the requirements of corrosion resistance.
2.2 Water Medium
Based on salt content, water is classified into high purity water, fresh water (salt content below 0.05%), sea water (salt content between 3.0% and 3.5%), brackish water (salt content between fresh water and sea water) and acidic water.
The corrosion rate of stainless steel in high purity water is the lowest (below 0.01 mm/a). The high purity water environment is often the nuclear industry. Generally, 0Cr19Ni9, 00CrNi11, 0Cr17Ni12Mo2, 0Cr17Ni14Mo2 steels meet the corrosion resistance requirements.
Under industrial water (fresh water) conditions, Cr13, Cr17 and 18-8 steels generally meet corrosion resistance requirements. Parts that work in aquatic environments are subject to cavitation. Cr13Ni4, M50NiL, 16CrNi4Mo are high strength and cavitation resistant stainless steels.
Stainless steels 0Cr13, Cr13, Cr17, 0Cr18Ni9 or 0Cr18Ni11Ti are commonly used for products exposed to the atmosphere and often subject to freshwater corrosion. Medical equipment generally uses 3Cr13, 4Cr13, 9Cr18 martensitic stainless steels.
The main forms of damage to stainless steel in seawater are pitting corrosion, crevice corrosion and stress corrosion cracking. It is also influenced by many factors such as seawater oxygen content, chloride ion concentration, temperature, flow rate and pollution.
Generally, in seawater below 30℃, ω(Mo)2%-4% stainless steel can meet the corrosion resistance requirements.
Acidic water refers to contaminated natural water that is leached from ores and various substances. Acidic water generally contains a large amount of free sulfuric acid and a large amount of iron sulfate. Under such conditions, austenitic stainless steel exhibits greater corrosion resistance.
2.3 Soil
Metals buried in the ground are subject to constant change due to climate and several other factors. Austenitic stainless steels typically exhibit corrosion resistance in most soils.
Steel types 1Cr13 and 1Cr17 tend to suffer from pitting corrosion in many soils. 0Cr17Ni12Mo2 stainless steel demonstrates resistance to pitting corrosion in all soil types.
2.4 Nitric Acid
Almost all stainless steels readily passivate in dilute nitric acid, showing fairly good corrosion resistance. Ferritic stainless steels and austenitic stainless steels with a chromium content of not less than 14% have excellent resistance to nitric acid corrosion.
Under working conditions with less than 65% (by weight) dilute nitric acid, type 18-8 stainless steel is generally used. Under conditions with 65% to 85% (by weight) dilute nitric acid, Cr25Ni20 stainless steel can meet corrosion resistance requirements.
When the concentration of nitric acid is very high, Si stainless steels (such as 0Cr13Si4NbRE, 1Cr17Ni11Si4, 00Cr17Ni17Si6, etc.) can meet the corrosion resistance requirements.
Mo-containing stainless steels are generally not resistant to nitric acid corrosion, but are sometimes used to prevent pitting corrosion in conditions involving nitric acid with chloride ions.
2.5 sulfuric acid
Standard stainless steel grades are rarely used in sulfuric acid solutions. At room temperature, 0Cr17Ni12Mo2 stainless steel is resistant to corrosion when the sulfuric acid concentration exceeds 85% or is less than 15%.
Austenitic stainless steels and ferritic-austenitic duplex stainless steels containing Mo, Cu, Si (with a weight percentage of 3% to 4%) have the best corrosion resistance to sulfuric acid.
2.6 Phosphoric Acid Medium
Austenitic stainless steels show good corrosion resistance in phosphoric acid solutions. However, in practical applications, phosphoric acid often contains various impurities, such as fluorine, chloride ions, and metal ions such as aluminum, magnesium, and sulfate ions, which tend to accelerate the corrosion of stainless steel.
Austenitic stainless steels 00Cr27Ni31Mo3Cu and 00CtNi35Mo3Cu are the best stainless steels in terms of comprehensive performance and corrosion resistance to phosphoric acid impurities such as fluorine and chloride ions.
Under these working conditions, 0Cr17Ni14Mo2, 00Cr19Ni13Mo3 and others with Mo content of 2% to 4% by weight, high Cr duplex steel 00Cr26Ni6Mo2Cu3 and high Mo stainless steel 00Cr20Ni25Mo4.5Cu and high Cr super ferritic stainless steels 00Cr26Mo1 , 00Cr30Mo2, etc. ., all have good resistance to corrosion by phosphoric acid.
Martensitic and ferritic stainless steels exhibit notably lower resistance to phosphoric acid corrosion compared to austenitic stainless steels.
2.7 Hydrochloric Acid
At room temperature, hydrochloric acid in various concentrations can quickly corrode stainless steel, so stainless steel cannot be used under conditions involving hydrochloric acid.
2.8 Acetic Acid
Austenitic stainless steels generally exhibit excellent resistance to acetic acid corrosion. As the molybdenum (Mo) content in steel increases, its corrosion resistance improves. However, in acetic acid containing chloride ions, the corrosion rate accelerates significantly.
Stainless steels such as 0Cr17Ni12Mo2 and 00Cr18Ni16Mo5 with molybdenum content of 2% to 4%, duplex 00Cr18Ni16Mo3N and some nickel-based alloys show excellent corrosion resistance.
2.9 Formic Acid
At room temperature, austenitic stainless steels exhibit excellent resistance to formic acid corrosion. But under conditions involving hot formic acid, it can quickly corrode molybdenum-free stainless steel.
0Cr17Ni12Mo2 and 0Cr19Ni13Mo3 have heat-resistant formic acid corrosion properties. Formic acid is corrosive to martensitic and ferritic stainless steels at all temperatures.
2.10 Oxalic Acid
Stainless steel has excellent corrosion resistance at room temperature with a concentration of 50%.
At higher temperatures or 100% concentration, all stainless steels show poor resistance to oxalic acid corrosion.
2.11 Lactic Acid
At a maximum temperature of around 38°C, 0Cr18Ni9 stainless steel exhibits excellent corrosion resistance.
Types resistant to higher temperatures include 0Cr17Ni12Mo2 and 0Cr19Ni13Mo3. Generally, martensitic and ferritic stainless steels have poor resistance to lactic acid corrosion.
2.12 Hydrofluoric Acid
Most stainless steels are not resistant to hydrofluoric acid corrosion. When oxygen and oxidants are present in hydrofluoric acid, the corrosion resistance of austenitic stainless steels with high nickel, molybdenum and copper content improves significantly.
2.13 Alkalies
Stainless steels generally have good resistance to weak alkalis. Both chromium and nickel in steel contribute positively to resistance to alkaline corrosion. Ferritic stainless steel with 26% to 30% chromium and austenitic stainless steel with more than 20% nickel have strong resistance to alkaline corrosion.
2.14 Urea
Austenitic stainless steels and ferritic stainless steels such as Cr-Ni and Cr-Mn-N with 2% to 4% nickel content (e.g. 0-1Cr18Ni12Mo2Ti, urea grade 001Cr17Ni14Mo2, 00Cr25Ni22Mo2N) are used in the production of urea. They have excellent corrosion resistance in urea solutions.
