1. Ask a question
Unified technical regulations generally require austenitic stainless steel vessels used in environments that may cause intergranular corrosion to undergo solid solution or stabilizing treatment after welding. This requirement is reasonable.
However, even if the designer includes this requirement in the technical specifications of the drawing, it is often difficult for the manufacturer to meet ideal standards due to challenges in controlling heat treatment process parameters and other unforeseen difficulties. In fact, most stainless steel equipment in use today is used without undergoing post-weld heat treatment.
This raises the question: what is the mechanism of intergranular corrosion, which is the most common form of corrosion in austenitic stainless steel? What are the environmental conditions that can lead to intergranular corrosion? What are the main methods to prevent and control intergranular corrosion? Are heat treatments necessary for austenitic stainless steel vessels used in environments that may cause intergranular corrosion after welding?
This article will explore these questions by referencing relevant standards, specifications, and monographs and presenting personal opinions based on production experience.
2. Intergranular corrosion mechanism
Intergranular corrosion is a type of localized corrosion that occurs along the grain boundaries or in close proximity to the grain boundaries of a metal or alloy. This corrosion is characterized by minimal corrosion within grains and significant corrosion along grain boundaries, which weakens the bond between grains.
If intergranular corrosion is severe, it can reduce the strength and ductility of the metal, causing it to fail under normal loads. The two main theories behind intergranular corrosion are the low chromium theory and the theory of selective dissolution of impurities at grain boundaries.
2.1 Lean Chromium Theory
Intergranular corrosion of austenitic stainless steel commonly used in oxidizing or weakly oxidizing environments is generally caused by inadequate heating during processing or use. Inadequate heating refers to the slow heating or cooling of steel in the temperature range of 450-850°C, which makes it vulnerable to intergranular corrosion. This temperature range is therefore considered dangerous for austenitic stainless steel.
Austenitic stainless steel undergoes solution treatment before leaving the factory. Solution treatment involves heating the steel to 1050-1150°C and then rapidly cooling it to create a homogeneous solid solution. Austenitic steel contains a small amount of carbon and its solid solubility decreases with decreasing temperature. For example, the solid solubility of carbon in 0Cr18Ni9Ti is about 0.2% at 1100°C and about 0.02% at 500-700°C.
The carbon in solution-treated steel is therefore supersaturated. When steel is heated or cooled to between 450-850°C, carbon can precipitate from austenite and distribute along grain boundaries in the form of (Fe, Cr) 23C6. The chromium content of (Fe, Cr) 23C6 is much higher than that of the austenitic matrix, and its precipitation consumes a large amount of chromium near the grain boundaries, which cannot be replenished in a timely manner by diffusion. The slow diffusion of chromium causes the chromium content near the grain boundaries to fall below the 12% Cr threshold required for passivation, creating a chromium-poor region and damaging the passive state.
The grain itself, however, still maintains a passive state with high potential. The grain and grain boundary form a microgalvanic battery, with a large cathode and a small anode, leading to corrosion in the grain boundary region.
2.2 Theory of selective dissolution of impurities at grain boundaries
In production practice, we have observed that austenitic stainless steel can also undergo intergranular corrosion in strong oxidizing media (such as concentrated nitric acid), but the nature of corrosion is different from that in oxidizing or weak oxidizing media. Intergranular corrosion in strong oxidizing media generally occurs in solid solution-treated steel, but does not occur in sensitized steel.
If impurities such as phosphorus or silicon reach 100 ppm or 1,000-2,000 ppm, respectively, in the solid solution, they will segregate along grain boundaries. These impurities will dissolve under the action of strong oxidizing media, causing intergranular corrosion.
When steel is sensitized, the formation of (MP)23C6 with phosphorus or the first segregation of carbon eliminates or reduces the segregation of impurities at grain boundaries, eliminating or weakening the steel's sensitivity to intergranular corrosion.
These two theories on the mechanism of intergranular corrosion apply to the structural state of a specific alloy and medium and are not mutually exclusive, but rather complementary. In production practice, most cases of intergranular corrosion occur in oxidizing or weak oxidizing media and can therefore be explained by the low chromium theory.
3. Medium environment causing intergranular corrosion
There are two main types of media that cause intergranular corrosion in austenitic stainless steel. The first type is an oxidizing or weakly oxidizing medium, and the second type is a strong oxidizing medium such as concentrated nitric acid. The first type of media is more common.
Here is a list of common medium environments that cause intergranular corrosion in austenitic stainless steel:
3.1 Common medium causing intergranular corrosion of austenitic stainless steel
The “Corrosion Data Chart” prepared by GA. Nelson lists the common media that cause intergranular corrosion in austenitic stainless steel:
- Acetic Acid
- Acetic acid + salicylic acid
- Ammonium nitrate
- ammonium sulfate
- Chromic acid
- copper sulfate
- Fatty acid
- Formic acid
- iron sulfate
- Hydrofluoric acid + iron sulfate
- Lactic acid
- Nitric acid
- Nitric acid + hydrochloric acid
- oxalic acid
- Phosphoric acid
- Sea water
- Salt fog
- Sodium bisulfate
- Sodium hypochlorite
- Sulfur dioxide (wet)
- Sulfuric acid
- Sulfuric acid + copper sulfate
- Sulfuric acid + ferrous sulfate
- Sulfuric acid + methanol
- Sulfuric acid + nitric acid
- Sulfite
- Phthalic acid
- Sodium hydroxide + sodium sulfide.
3.2 Intergranular corrosion tendency test
When using austenitic stainless steel in an environment that may cause intergranular corrosion, the intergranular corrosion tendency test should be conducted in accordance with test methods GB4334.1 to GB4334 for intergranular corrosion of stainless steel. The selection and qualification requirements for test methods for intergranular corrosion tendency of austenitic stainless steel must meet the following criteria:
(1) Austenitic stainless steel and special stainless steel for concentrated nitric acid used in nitric acid with a temperature of 60°C or higher and a concentration of 5% or higher shall be tested according to test method GB4334.3 for 65% nitric acid corrosion of stainless steel. The average corrosion rate over five cycles or three cycles must not exceed 0.6g/m 2 h (or equivalent to 0.6 mm/a). The sample may be in use or sensitized.
(2) Chrome-nickel austenitic stainless steel (such as 0Cr18Ni10Ti, 0Cr18Ni9, 00Cr19Ni10 and similar steels): General requirements: according to GB4334.5 sulfuric acid copper sulfate corrosion test method for stainless steel, should not there are intergranular corrosion cracks on the surface of the sample after the bending test. Higher requirements: the average corrosion rate should not exceed 1.1g/m 2 h according to GB4334.2 sulfuric acid ferric sulfate corrosion test method for stainless steel.
(3) Molybdenum-containing austenitic stainless steel (such as 0Cr18Ni12Mo2Ti, 00Cr17Ni14Mo2 and similar steels): General requirements: according to GB4334.5 sulfuric acid copper sulfate corrosion test method for stainless steel, there should be no corrosion cracks intergranular particles on the sample surface after the bending test. Higher requirements: the corrosion rate should not exceed 1.5 according to GB4334.4 nitric acid hydrofluoric acid corrosion test method for stainless steel. The average corrosion rate should not exceed 1.1g/m 2h according to GB4334.2 sulfuric acid ferric sulfate corrosion test method for stainless steel.
(4) If the medium has special requirements, intergranular corrosion tests other than those specified above may be carried out, and the corresponding qualification requirements must be specified.
4. Measures to prevent and control intergranular corrosion
According to the corrosion mechanism, the following measures can be taken to prevent and control intergranular corrosion in austenitic stainless steel:
(1) Using ultra-low carbon stainless steel can help reduce carbon content to less than 0.03%.
For example, 00Cr17Ni14Mo2 can be chosen to avoid the formation of (Fe, Cr) 23C6 in the steel and the occurrence of a chromium-poor zone, thus avoiding intergranular corrosion.
Typically, for components that have low strength, low voltage and good plasticity, 0Cr18Ni9 can be selected for its cost-effectiveness.
(2) Stabilized stainless steel refers to stainless steel that contains titanium and niobium.
During steel production, a specific amount of titanium and niobium is added, and these elements have a strong affinity with carbon, forming tic or NBC within the steel.
Furthermore, the solid solubility of tic or NBC is much lower than that of (Fe, Cr) 23C6 and is almost insoluble in austenite at the solid solution temperature.
Therefore, even if (Fe, Cr) 23C6 is not precipitated at the grain boundary when the sensitization temperature is reached, the probability of intergranular corrosion in austenitic stainless steel is greatly reduced.
For example, steels such as 1Cr18Ni9Ti and 1Cr18Ni9Nb can function within a temperature range of 500-700°C without suffering intergranular corrosion.
(3) When welding austenitic stainless steel with electric arc, the temperature of the arc pool can reach up to 1300°C, and the temperature on both sides of the weld decreases with increasing distance, creating a sensitization temperature zone.
It is ideal to heat and cool austenitic stainless steel as slowly as possible within the sensitization temperature range.
In the case of intergranular corrosion tendencies, unstable stainless steel should be heated to 1000-1120°C for 1-2 minutes per millimeter and then quenched.
For stabilized stainless steel, heating to 950-1050°C is recommended.
After undergoing solution treatment, it is important to avoid heating the steel to the sensitization temperature, as this may cause chromium carbide to precipitate along the grain boundary again.
(4) Choosing the correct welding method is important to reduce the sensitivity of welded joints to intergranular corrosion. If the operation remains unchanged or the welding material is very thick, a longer welding time increases the chances of remaining within the sensitized temperature zone.
To minimize the sensitivity of welded joints, it is necessary to minimize line energy input during welding.
Generally speaking, argon arc welding has a lower input line power compared to electric arc welding, making it a better choice for welding and repair.
For welding parts, it is recommended to use ultra-low carbon stainless steel or stainless steel with stabilizing elements such as titanium and niobium. Furthermore, it is recommended to use very low carbon welding rods or welding rods containing niobium.
When using argon arc welding, to prevent overheating of the welding joint, the operation must be quick and the base metal on both sides of the weld must be cooled quickly after welding to minimize the time spent within the temperature range of awareness.
5. Post-welding treatment
Post-weld heat treatment is not always a priority in the welding area.
Typically, a solid solution treatment is carried out at a temperature range of 1100-1150°C for a certain period and then quenched. Cooling in the range of 925-540°C should be completed within three minutes, followed by rapid cooling below 425°C.
For stabilized treatment, the workpiece must be air-cooled after being kept in a temperature range of 850-880°C for several hours.
The effectiveness of post-weld heat treatment is highly dependent on key process parameters such as furnace temperature, rate of temperature rise, temperature difference between various parts of the workpiece during temperature rise, furnace atmosphere, holding time, temperature difference between various parts during heat preservation, cooling rate and oven temperature.
For austenitic stainless steel vessels that can cause intergranular corrosion, solution treatment or stabilized treatment of parts in general can be carried out. However, post-welding heat treatment of the entire vessel (usually a heat exchanger) presents many difficulties.
This type of treatment is not a local post-welding heat treatment, but rather a post-welding heat treatment of all welded parts or containers.
Due to the complex structure and shape of most chemical vessels, such as the commonly used shell and tube heat exchanger, it is almost impossible to control the key process parameters for post-welding solid solution or stabilized treatment of the entire shell heat exchanger. and tube. alone ensuring the quality of post-welding heat treatment.
In many cases, this treatment may even prove to be counterproductive, not only failing to improve the weld structure, but also unnecessarily deteriorating the structure of the base metal.
Therefore, more than 90% of austenitic stainless steel chemical containers used in intergranular corrosion environments are still used in their post-welding state rather than undergoing post-welding heat treatment.
6. Summary
Chromium-nickel austenitic stainless steel is the most widely used corrosion-resistant material, and intergranular corrosion is the most common form of failure in chromium-nickel austenitic stainless steel vessels.
Intergranular corrosion significantly weakens the bond between grains and, in severe cases, can completely eliminate mechanical strength. The surface of stainless steel that has undergone this type of corrosion remains shiny, but can be easily broken into fine particles with gentle tapping.
Intergranular corrosion is difficult to detect, which can cause sudden damage to equipment and should be taken seriously.
Chromium-nickel austenitic stainless steel vessels are typically formed by welding, and the two sides of the welded joint are corrosion-sensitive intergranular areas, which are more susceptible to corrosion damage compared with the base metal.
Post-weld heat treatment can improve the intergranular corrosion resistance in the weld zone to the same level as the base metal. This is the ultimate goal of post-weld heat treatment.
However, in practice, there are many factors to be considered, such as the complex overall structure and welding shape, which makes it difficult to guarantee the parameters of the post-welding heat treatment process.
As a result, most chromium-nickel austenitic stainless steels in service are used after welding.
If the weld zone of a chromium-nickel austenitic stainless steel container used for intergranular corrosion resistance undergoes solid solution treatment or stabilized treatment, it cannot be generalized. The structural form of the container must be analyzed to determine whether heat treatment can be carried out effectively. Otherwise, even if post-welding heat treatment is necessary, it could have adverse effects, not only failing to achieve the desired result, but also affecting the structure of the base metal.
To increase the intergranular corrosion resistance of chromium-nickel austenitic stainless steel vessels, it is necessary to select ultra-low carbon stainless steel and stabilized stainless steel based on the specific corrosion environment and mechanism, choose the correct welding method during welding and appropriately combine the previously mentioned prevention and control measures to achieve good results.
Relying on solid solution or stabilization treatment after welding is not enough.
