Formation And Prevention Of Intergranular Corrosion

Formation and prevention of intergranular corrosion of stainless steel

Problem raising

Unified technical regulations usually include "austenitic stainless steel vessels used in the environment that may cause intergranular corrosion, after welding should be a solid solution or stabilization treatment", put forward such a requirement, has its rationality. But even if the designer put forward this article in the technical requirements of the drawing, requiring the manufacturer to carry out the post-welding heat treatment of stainless steel containers (such as heat exchanger), because the actual heat treatment process parameters are difficult to control and some other unexpected difficulties, it is usually difficult to achieve the ideal requirements proposed by the designer most of the stainless steel equipment in service is used after welding.

This prompts us to think: intergranular corrosion is the most common corrosion form of austenitic stainless steel, so what is the mechanism of intergranular corrosion? In what medium does intergranular corrosion occur? What are the main methods to prevent and control intergranular corrosion? Austenitic stainless steel vessels are used in environments that may cause intergranular corrosion.Are they always heat-treated after welding?

Formation of intergranular corrosion of stainless steel

Problem raising

Production mechanism

Intergranular corrosion is a common local corrosion, corrosion along the metal or alloy grain boundary or its adjacent region development, and the grain corrosion is very slight, this corrosion is called intergranular corrosion, this corrosion makes the cohesion between grains greatly weakened. Severe intergranular corrosion can cause the metal to lose strength and ductility and fracture under normal load. Modern intergranular corrosion theory mainly includes chromium deficiency theory and grain boundary impurity selective dissolution theory.

Chrome-poor theory

Commonly used austenitic stainless steel, in oxidizing or weak oxidizing medium is caused by intergranular corrosion, mostly due to improper heat processing or use. The so-called improper heating refers to the heat or slow cooling of steel through the 450 ~ 850 ℃ temperature zone, steel will be sensitive to intergranular corrosion. So this temperature is a dangerous temperature for austenitic stainless steel. Stainless steel materials have solid solution treatment when leaving the factory, the so-called solid solution treatment is to heat the steel to 1050 ~ 1150 ℃ after quenching, the purpose is to obtain a homogeneous solid solution. Austenitic steel contains a small amount of carbon, and the solubility of carbon in austenitic steel decreases with the decrease in temperature. For example, at 1100 ℃, the solid solubility of carbon is about 0.02 %, and at 500 ~ 700 ℃, about 0.02 %. So the carbon in the solution-treated steel is supersaturated.

When the steel is heated or cooled through 450 ~ 850 ℃, carbon (Fe, Cr) 23C6 can be precipitated from austenite and distributed on the grain boundary. The chromium content of (Fe, Cr) 23C6 is much higher than that of the austenite matrix. Its precipitation naturally consumes a large amount of chromium near the grain boundary, and the chromium consumed cannot be timely replenished from the grain through diffusion, because the diffusion rate of chromium is very slow. Results The chromium content near the grain boundary was lower than the limit necessary for passivation (that is, 12% Cr), forming a chrome-poor zone, thus the passivity was destroyed, and the potential of the region near the grain boundary decreased, but the grain itself remained passivity, the potential was higher, the grain and the grain boundary formed an active state -- passivity micro galvanic battery, the battery had an area ratio of the large cathode to small anode. This results in corrosion at grain boundaries.

Selective dissolution theory of grain boundary impurities

In production practice, we also know that austenitic stainless steel in a strong oxidizing medium (such as concentrated nitric acid) can also produce intergranular corrosion, but the corrosion situation is different from that in the oxidizing or weak oxidizing medium. Usually occurs in steel after solution treatment, after allergic treatment of steel generally does not occur. When the solid solution contains impurities such as phosphorus up to 100ppm or silicon impurities of 1000-2000ppm, they will be segregated at the grain boundary. These impurities dissolve under the action of a strong oxidizing medium, leading to intergranular corrosion. However, in sensitized steel, carbon can form (MP) 23C6 with phosphorus, or the first segregation of carbon restricts the diffusion of phosphorus to the grain boundary, both of which will eliminate or reduce the segregation of impurities at the grain boundary, thus eliminating or reducing the sensitivity of steel to intergranular corrosion.

The above two theories to explain the intergranular corrosion mechanism apply to the microstructure of certain alloys and certain media respectively. They are not mutually exclusive but mutually complementary. Most of the most common intergranular corrosion of stainless steel in production practice occurs in weak oxidizing or oxidizing media, so most of the corrosion cases can be explained by the chromium deficiency theory.

The resulting medium environment

Two main types of media cause common intergranular corrosion of austenitic stainless steel. One is oxidizing or weak oxidizing medium, and one is a strong oxidizing medium, such as concentrated nitric acid. Common is the first type, listed below is the common cause of austenitic stainless steel intergranular corrosion media environment.

Common medium causing intergranular corrosion of austenitic stainless steel

Common mediums that cause intergranular corrosion in austenitic stainless steel are listed in the Corrosion Data Chart compiled by G. A. Nelson: Acetic acid, acetic acid + salicylic acid, ammonium nitrate, ammonium sulfate, chromic acid, copper sulfate, fatty acid, formic acid, ferric sulfate, hydrofluoric acid + ferric sulfate, lactic acid, nitric acid, nitric acid + hydrochloric acid, oxalic acid, phosphoric acid, seawater, salt spray, 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.

Intergranular corrosion tendency test

When austenitic stainless steel is used in an environment that may cause intergranular corrosion, the intergranular corrosion tendency test shall be carried out according to GB4334.1-GB4334 "Test Method for intergranular Corrosion of Stainless Steel". The selection of the test method for the intergranular corrosion tendency of austenitic stainless steel and its qualification requirements shall comply with the following provisions:

(1) in the temperature is greater than or equal to 60 ℃, and the concentration is greater than or equal to 5% nitric acid used in austenitic stainless steel and concentrated nitrate special stainless steel, should be tested according to GB4334.3 "stainless steel 65 % nitric acid corrosion test method", The average corrosion rate of five cycles or three cycles shall not be greater than 0.6 g/ m2 h (or equivalent to 0.6 mm/a). The sample can be used or sensitized.

(2) chromium-nickel austenitic stainless steel (such as 0Cr18Ni10Ti, 0Cr18Ni9, 00Cr19Ni10, and similar steel): general requirements: according to GB4334. 5 "stainless steel sulfuric acid - copper sulfate corrosion test method", after bending test, the sample surface shall not have intergranular corrosion cracks. Higher requirements: according to GB4334.2 "stainless steel sulfuric acid - iron sulfate corrosion test method", the average corrosion rate should not be greater than 1.1g/m2 h.

(3) molybdenum austenitic stainless steel (such as 0Cr18Ni12Mo2Ti, 00Cr17Ni14Mo2, and similar steel): general requirements: according to GB4334. 5 "stainless steel sulfuric acid - copper sulfate corrosion test method", after bending test, the sample surface shall not have intergranular corrosion cracks. Higher requirements: according to GB4334.4 "stainless steel nitric acid - hydrofluoric acid corrosion test method", the corrosion ratio is not greater than 1.5. Also according to GB4334.2 "sulfuric acid - iron sulfate test method", the average corrosion rate should not be greater than 1.1g /m2 h.

(4) If the medium has special requirements, the intergranular corrosion test other than the above provisions can be carried out, and the corresponding qualification requirements shall be specified.

Measures to prevent and control intergranular corrosion

According to the corrosion mechanism, the measures to prevent and control intergranular corrosion of austenitic stainless steel are as follows:

(1) The use of ultra-low carbon stainless steel to reduce the carbon content to 0.03% below, such as 00Cr17Ni14Mo2, so that the steel does not form (Fe, Cr) 23C6, no chrome-poor zone, prevents the generation of intergranular corrosion. Generally, the strength is not high, the force is not large, and the plastic parts are required. From the economic point of view, 0Cr18Ni9 can be selected.

(2) stabilized stainless steel selection of steel containing titanium and niobium stainless steel, (that is, we often say stabilized stainless steel), smelting steel with a certain amount of titanium and niobium two components, they and carbon affinity, TiC or NbC is formed in the steel, and the solid solubility of TiC or NbC is much smaller than (Fe, Cr) 23C6, almost insoluble in austenite at solution temperature. In this way, the tendency of intergranular corrosion of austenitic stainless steel is largely eliminated, although (Fe, Cr) 23C6 does not precipitate at the grain boundary in large quantities at the allergenic temperature. Such as 1Cr18Ni9Ti, and 1Cr18Ni9Nb steel, can work in the range of 500 ~ 700 ℃, and will not tend to intergranular corrosion.

(3) resolution treatment when the austenitic stainless steel is welded, the temperature of the arc molten pool is up to more than 1300 ℃, and the temperature on both sides of the weld decreases with the increase of the distance, among which there is a sensitization temperature zone. Should try to avoid austenitic stainless steel in the sensitized temperature range of heat and slow cooling, if found to tend intergranular corrosion, generally on the unstable stainless steel heating to 1000 ~ 1120 ℃, insulation according to 1 ~ 2 minutes per millimeter, and then cold; It is appropriate to heat the stabilized stainless steel to 950 ~ 1050 ℃. The steel after solution treatment should still be prevented from heating at the sensitization temperature, otherwise, the chromium carbide will precipitate along the grain boundary again.

(4) When selecting the correct welding method for welding, if the operation is not skilled or the welding material is too thick, the longer the welding time, the more chance to stay in the sensitization temperature zone, resulting in the sensitivity of the base metal on both sides of the weld to intercrystalline corrosion. To reduce the sensitivity of welded joints, the input of line energy should be minimized. Generally, the input line energy of argon arc welding is lower than that of arc welding, so welding and welding should adopt argon arc welding. For welding parts, ultra-low carbon stainless steel or stainless steel containing Ti and Nb stabilizing elements should be selected, and for welding rods, ultra-low carbon electrodes or electrodes containing Nb should be selected. When using argon arc welding, to avoid overheating of welded joints, the operation should be fast, and the welding should be cooled quickly, to minimize the time that the base metal on both sides of the weld stays in the sensitized temperature range.

Post-welding treatment

The Weld zone does not necessarily emphasize post-welding heat treatment, general solution treatment is to be in the 1100 ~ 1150 ℃ range of heat preservation after a certain period of cooling, three minutes to complete the temperature range of 925 ~ 540 ℃ cooling, continue to cool to 425 ℃ below; Stabilization treatment should be kept in the temperature range of 850 ~ 880 ℃ for a few hours after air cooling. The expected post-welding heat treatment effect is closely related to the key process parameters in the whole process of heat treatment (such as the temperature entering the furnace, the temperature rise rate, the temperature difference of each part of the workpiece during the temperature rise process, the atmosphere in the furnace, the holding time, the temperature difference of each part during the holding process, the cooling rate, the temperature coming out, etc.).

For austenitic stainless steel vessels used in environments that may cause intergranular corrosion, solution treatment or stabilization of common parts can be achieved. The post-weld heat treatment of the whole container (mostly the heat exchanger) will face many difficulties. This kind of treatment is not partial post-welding heat treatment, but the whole welding parts or the whole container post-welding heat treatment. Due to the complex structure and shape of most chemical containers (such as our commonly used shell and tube heat exchanger).

If it is required to solve or stabilize the weld zone of the whole tube and shell heat exchanger after welding, the above key process parameters can not be controlled at all, let alone guarantee the quality of the heat treatment after welding. Even if the treatment is often self-defeating, not only the weld structure has not been improved, but also the base metal structure has unduly deteriorated. Therefore, more than 90 % of chemical vessels made of austenitic stainless steel used in intergranular corrosion environments are still used in the post-welding state rather than the post-welding heat treatment state.