The essence of the electrochemical process of intergranular corrosion is the emergence and action of the anode sections of galvanic pairs at the grain boundaries of steel. Anodic dissolution of metal at grain boundaries under electrolyte conditions also causes intergranular corrosion destruction.
Various austenitic and ferritic steels, non-ferrous alloys: duralumin, brass, etc., in which the supersaturated solid solution decomposes with the release of an excess phase along the grain boundaries, undergoes intergranular corrosion.
Stainless steel is generally corrosion resistant and resistant to most acids and alkalis. Due to its high resistance and versatility of processing, stainless steel can be used in all branches of industry and construction. It combines the highest standards of functionality, hygiene and aesthetics.
Some stainless steel alloys and their features
We supply you with billets in almost all tires and thicknesses of many alloys. In addition, we supply you with tapes from of stainless steel... The following overview gives short review features of various stainless steel alloys. This, however, reduces the corrosion resistance, thus avoiding the use of chlorine-containing media.
Generalizations on intergranular corrosion of stainless steels:
1. Chromium-nickel steel of the austenitic class without stabilizing additives has high corrosion resistance in the hardened state, as well as after cold plastic deformation to a certain degree. With delayed cooling from the hardening temperature or after welding, as well as after tempering in the range of 500–750 ° C, the corrosion properties of chromium-nickel austenitic steels change greatly. Steels are prone to intergranular corrosion. The tendency to intergranular corrosion is determined by the carbon content of the steel. Determined that intergranular corrosion in Cr-Ni steel does not occur when the carbon content in the steel does not exceed 0.02%. With a carbon content of about 0.06%, the steel becomes susceptible to intergranular corrosion. A further increase in the carbon content leads to a strong increase in the tendency of steel to intergranular corrosion. In Cr-Ni steels with 24% Ni, intergranular corrosion is observed even at a content of 0.003% C.
The most commonly used chromium-nickel steel. It features, among other things, high corrosion resistance and good workmanship. Its good welding properties also make it suitable for many applications! Material for machining! Its sulfur content of 0.15-0.25% makes cutting easier. However, it is not recommended to use higher corrosion resistance requirements.
Even after welding, it is resistant to intergranular corrosion. Due to its titanium content, machining increases tool wear. If for high temperatures strengths are necessary, this is the right choice. Standard for high temperature applications. Corrosion resistance is classified as low depending on the concentration of the contact media.
2. The addition of titanium, niobium and tantalum to stainless steel has a positive influence: steel becomes insensitive to intergranular corrosion. The complete lack of sensitivity to intergranular corrosion in chromium-nickel steels with additives of the indicated elements is observed only when the content of the shrinkable element is in a certain ratio to carbon. For titanium, this ratio corresponds to five to six times, and for niobium, ten to twelve times, of the amount of carbon in steel.
Even at high temperatures, it has good strength properties. All information is indicative and non-binding! The market has long been demanding the concentration of such materials. Over the years, more and more processors have discovered the benefits of stainless steel screws for their performance. If stainless steel screws are mainly used in food processing, pharmaceutical and cosmetic industries, chemical apparatus construction and medical technology, due to the higher costs in the past, stainless steel screws are increasingly taking place in general engineering, metalworking and metalworking. furniture production.
3. Steels that have coarse grains after heat treatment acquire a higher tendency to intergranular corrosion than steels with fine grains.
4. In chromium stainless steels of ferritic and semi-ferritic class, intergranular corrosion is observed only after rapid cooling from high temperatures, and upon slow cooling or after additional tempering, these steels are not subject to intergranular corrosion.
Due to its low carbon content and high chromium content, stainless steel has very good corrosion resistance. Its corrosion resistance is associated with chemically reacted stainless steel, in which, in combination with oxygen from air or water, a thin, thin passive layer is formed on the surface, which protects the steel from external influences. A special feature of stainless steel is that the formation of the layer occurs automatically even after damage to the surface, and the steel is completely protected.
Stainless steel screws are therefore corrosion resistant, hygienic, low maintenance and very durable. The higher costs are more than offset by these benefits. Steel is one of the few materials suitable for use at low temperatures down to the absolute zero point. Weldability is good in accordance with all electrical processes, grease gas welding should not be used. The steel has very good polishing properties and especially good deformability due to thermoforming, bending, profiling, etc. Due to the slope for cold hardening, steel should be used with high alloy high speed steel or hard metal tools.
Reasons for intergranular corrosion:
1) depletion of grain boundaries in chromium due to the precipitation of chromium carbides;
2) precipitation of phases that are less resistant to corrosion (anode sections) along the grain boundaries;
3) the appearance of stresses along the grain boundaries due to the release of new phases with a different specific volume during the decomposition of a supersaturated solid solution.
Steel is not resistant to chloride ions. Application: The steel is resistant to water, steam, moisture, food acids, and weak organic and inorganic acids and has a wide range of applications in the food, mechanical engineering, beverage, pharmaceutical and cosmetic industries, architecture, vehicle construction, household items and appliances, for surgical instruments, in the construction of cabinets and kitchens, in plumbing, for jewelry and art objects.
The conditions leading to the occurrence of intergranular corrosion (ICC) in high-alloy steels, for example, structural classes such as ferritic, martensitic, austenite-ferritic and austenitic, are different. However, its appearance is practically the same and lies in the fact that with a sufficiently high general corrosion resistance, selective dissolution of metal grain boundaries occurs and the appearance of new phases or segregation on them, differing in composition from chemical composition become.
Corrosion resistance is greatly enhanced by electropolishing. This is especially necessary for pharmaceutical, food, medical and façade technology. Austenitic stainless steel with excellent corrosion resistance. It is a chromium-nickel steel with an addition of molybdenum. This steel is easy to cold form but not easy to work with. Due to its strong hardening, appropriate tools are needed. Strong deformation can magnetize the steel somewhat. If the steel is held at temperatures between 500 ° C and 900 ° C, chromium carbides can precipitate at the grain boundaries, which can catastrophically degrade intergranular corrosion resistance.
The reason for the occurrence of MCC is most often an incorrectly carried out heat treatment or carrying out technological operations (welding, bending, stamping) in a dangerous temperature range:
In addition, MCC can occur during long-term operation of equipment at elevated temperatures, as well as with the wrong choice of the structural class of steel or the alloying system for a certain corrosive environment.
Annealing of the solution is then required to dissolve the carbides, followed by rapid quenching to prevent any new precipitation. In any case, hardening after heat treatment is recommended. This steel is easily welded with all common methods, with the exception of the oxyacetylene flame. After welding, soft annealing is followed by hardening to eliminate the risk of intergranular corrosion.
Drive technology refers to technical systems in which generated energy is converted into motion through the transmission of force. The drive technology is derived from the word "drive", with the result that it does not matter if the drive is used, for example, mechanical, pneumatic or electrical. The control of the various drive elements is important in this respect. Together with the drive technology and the actual drive, they form the drive.
In the welded joints of the MCC, the following may occur:
in the heat-affected zone (HAZ), where the metal in the welding process heats up in the area of dangerous temperatures;
at the interface between the deposited and base metals (the so-called knife corrosion);
in the weld metal.
MCC in the metal of the heat-affected zone and the weld metal is mainly associated with heating in the temperature range during welding or operation of welded joints at these temperatures. MCC of the deposited metal can be caused by delayed cooling of the weld. The ICC mechanism in the HAZ metal and deposited metal is similar to the ICC mechanism in the base metal after provoking tempering.
Fittings are standardized accessories for pipeline construction. The term "fit" means "fit", "fit", "fit", "fit" or "fit". The term "ball bearing" is used colloquially and describes a subset of rolling bearings. In this case, the rolling body, as the name suggests, consists of a ball that lies between the so-called inner ring and the outer ring. Plastic cage or low-alloy, uncured steel. Thanks to the optimized lubrication between the balls, these rollers reduce the frictional resistance between the inner ring and the outer ring as a rolling element.
Knife corrosion (NK) is one of the types of MCC. It occurs in a very narrow zone and is most often observed in multilayer seams as a result of heating to high temperatures close to solidus during the first pass and heating to critical temperatures in subsequent passes and manifests itself in oxidizing environments.
To a first approximation, the NC mechanism is analogous to the MCC mechanism of the base metal in oxidizing environments, subjected to tempering in the critical temperature range after high-temperature quenching. The influence of the stresses arising during welding on the occurrence of NC is not excluded.
A ball bearing serves as a locating point for axles or shafts and can absorb radial or axial forces. Both directions of force can also be obtained, depending on the design. Long before the turn of the century, it was discovered that with the addition of nickel and chromium, the corrosion behavior of steel could be improved. But the individual steels enriched with these elements left many desires open. The combination of nickel and chromium, combined with precisely metered heat treatment, has for the first time ensured optimum corrosion resistance and, at the same time, good mechanical properties.
The most correct is to identify the tendency to MCC by testing in real environments, however, this is practically impossible due to the variety of corrosive environments and the need for very long tests.
For an accelerated assessment of the tendency of steel to MCC, a number of control solutions with different potentials have been developed. The main methods for controlling the tendency of steel to MCC in accordance with GOST 6032-84 are tests in boiling solutions of copper sulfate with addition of copper (AM and AMU methods), sodium fluoride (AMUF method), sulfuric acid with addition of iron sulfate (VU method) and nitric acid (remote control method). These methods cover a wide range of potentials. For corrosion-resistant steels (GOST 5632-72 and GOST 6032-84), a specific test method is specified.
Material properties and advantages
It is approved for temperatures up to 300 degrees Celsius. The steel is resistant to water, steam, moisture, food acids, and weak organic and inorganic acids and has a wide range of applications, for example, in the food industry, the beverage industry, the pharmaceutical and cosmetic industries, chemical equipment and medical technology.
The advantages of stainless steel at a glance. Corrosion resistant Heat resistant Conductive Weldable Hygienic Low maintenance Durable. Its corrosion resistance is due to a simple chemical reaction due to the chromium content of steel combined with oxygen in air or water, forming a thin, thin passive layer on the surface. It repels all aggressive substances. And as soon as it is damaged by external influences, it is again formed from the matrix of steel in a fraction of a second. Corrosion resistance is mainly due to the chromium content.
In foreign practice, similar test methods are used. In the US standard ASTM A262-81, method E corresponds to methods AM and AMU GOST 6632-84, method VU - method B and DU method - method C (respectively, tests by Strauss, Streicher and Ggoy). Standard A262-81 also provides for testing molybdenum-containing steels in a nitrogen-fluoride solution (method D) and method A for preliminary assessment of the quality of steels by short-term (no more than 1.5 minutes) electrolytic etching in 10% oxalic acid (current density 1 A / cm 2, temperature is not higher than 50 ° С). This method is only suitable for molybdenum-free chromium-nickel steels tested after provoking heating.
The increase can be achieved with nickel and molybdenum, as well as other alloying agents. Today there are many grades without stainless steel that are adapted to specific applications in rather specific alloy variations. In addition, the corrosion resistance depends on the surface, the hour the smoother and more uniform, the better the corrosion resistance. Particularly on inclusions or deposits, for example, pressed rust or dust particles from processing, localized corrosion can occur, which quickly engages.
To determine the tendency to MCC, either welded samples or, more often, samples after special heat treatment provoking the emergence of MCC (provocation or sensitization) are tested. The "provocation" mode depends on the structural class of steel, and for standard test methods and standard steels is specified in GOST 6032-84. For new steels, the MCC control method is selected taking into account the oxidizing potential of the environment for which the steel is intended.
The following types of corrosion occur in stainless steel
As a result, in environment depletion of chromium occurs, which means that the passivation effect is lost. As a result, dimples or holes are formed on the surface. Pitting is mainly due to halogen ions, mainly chlorine ions. Pitting corrosion can occur mainly in the water and wastewater sector as it is often associated with chloride and chloride ions.
Here, the less noble metal is exposed to the electrolyte and dissolves. Corrosion resistance depends on the amount of current flowing in this galvanic cell. Contact corrosion is very common. As a well-known example, it is necessary to emphasize the connection of steel and stainless steel flanges. Contact corrosion is also known for threaded flanges with stainless steel screws.
The reliable operation of equipment made of corrosion-resistant steels is ensured by the correct choice of the material of welded joints for the working conditions and the absence of violations of technological conditions in the manufacture and operation of welded structures. To prevent the emergence of addiction to IWC in advance:
reduce the carbon content (no more than 0.03%) in steel and seams for operation at temperatures up to 350 ° C;
carry out stabilization with carbide formers (alloying of the weld metal with Ti, Nb, V) when operating the equipment at 550 ° C;
create a two-phase austenite-ferrite structure (alloying the weld metal with ferritisers - up to 22-25% Cr, Si, etc.).
For example, the presence of aggressive media with a simultaneous lack of oxygen. For this reason, crack corrosion is often found in narrow crevices and small cavities such as under seals or under screw heads. An example to be mentioned here is a gas hood in towers, where inside there is an aggressive environment in the form of sewer gas. On the other hand, oxygen is completely absent. Particular attention should be paid to corrosive corrosion.
Cleaning and maintenance. The flushing solution is usually sufficient to remove fingerprints. Several manufacturers of detergents offer special products in which the cleaning effect is complemented by a care component. For stubborn dirt, you can use a household cleaning powder that also removes limescale marks and slight discoloration. After cleaning, the surface is rinsed with clean water. Severe greasy and greasy stains can be treated with alcohol cleaning and solvents, remove alcohol or acetone.
Stabilized steels should be used in cases where, along with corrosion resistance, requirements are imposed on strength, since low-carbon steels have a lower strength. One method to reduce the tendency to MCC in some environments is to use materials of increased purity. Due to their high cost, their use must be reasonably justified.
Resistance to MCC of welded joints can be increased by applying high speed cooling in the area of critical temperatures (500-600 ° C), forced cooling, limiting the strength of the welding current, using thread rollers.
MCC can be prevented by heat treatment (quenching and stabilizing annealing), which ensures complete dissolution of chromium carbides or reduces the segregation of impurities along grain boundaries. For most austenitic steels, the quenching regime is usually adopted, which consists in rapid cooling (in oil, in water or in air) after heating at 1020-1060 ° C. For low carbon high alloy steels, especially with boron additives, and for molybdenum steels intended for use in oxidizing environments, the hardening temperature must be increased. Stabilizing annealing is usually carried out in the range of 850-950 ° C with a duration of 2-4 hours. Stabilizing annealing is most effective for steels with titanium or niobium, since there is a more complete binding of carbon by stabilizing additives, as well as the formation of large uncoupled chromium carbides. Subsequent provoking heating does not result in the dangerous formation of boundary carbides, and there is no MCC. Stabilizing annealing is applicable to increase the resistance against MCC and unstabilized steels; however, complete elimination of the tendency to MCC in this case is impossible due to the retention of a significant supersaturation of the solid solution with carbon. Stabilizing annealing may increase the strength properties and decrease the ductility of the steel, and excess phases (for example, the σ-phase) may also form, which reduce the resistance, especially in oxidizing environments.
The methods for eliminating MCC, indicated above, are applicable to those environments where MCC is associated with the formation of chromium-depleted border zones. In ferritic steels, annealing at 700-800 ° C promotes uniform precipitation of chromium carbides throughout the grain, eliminates the predominant precipitation of chromium carbides at grain boundaries. When carrying out annealing of chromium ferritic steels, it should be borne in mind that as a result of prolonged heating at the annealing temperature or in the case of delayed cooling at temperatures below 500 ° C, embrittlement processes can develop as a result of the formation of the σ-phase or 475-degree brittleness. Elimination of MCC in austenitic and austenite-ferritic steels by quenching at 950-1080 ° C is based on the complete dissolution of Cr 23 C 6 carbide. The upper temperature limit of heating should be lower than the dissolution temperature of special titanium or niobium carbides and the temperature of intensive grain growth.
Source - "Welder in Russia" 1 (17) / 09
