What to do if stainless steel rusts
Stainless steel is a high-quality metal that has been alloyed with the addition of a number of chemicals that impart anti-corrosion properties. Due to alloying, steel becomes impervious to moisture, air, and many aggressive environments. But sometimes even this material begins to deteriorate and unsightly rust spots appear on it. Why does stainless steel rust? There may be several reasons, and the main one is improper operation.
Can stainless steel rust?
There are three groups of stainless steels, each of which has its own characteristics and specific application:
- Corrosion resistant steel. It has high resistance to corrosion in uncomplicated conditions - at home, at work.
- Heat-resistant steel. It is heat resistant, does not rust at elevated temperatures, and can be used in chemical plants.
- Heat-resistant steel. Remains mechanically strong at high temperatures.
Thus, not all types of stainless steel are intended for use in a particular aggressive environment. For example, using ordinary stainless steel in food production and frequent washing with chlorine-containing products will cause rapid deterioration of the material. Similarly, the use of metal in seawater will increase the corrosion rate significantly.
Also, rust often appears on stainless steel after welding (heat treatment), which was carried out without following certain rules. After mechanical damage to the metal, the consequences will be similar: pitting corrosion will occur at the site of the defect. Smooth, polished material usually rusts less intensely than rough material: on the latter, corrosion elements can appear much faster.
Protection against rust is violated where hot scale gets in, since a strong increase in temperature in non-heat-resistant steel causes alloying substances (mainly chromium) to burn out. After holes burn through, their edges and adjacent areas become susceptible to corrosion, although deeper layers of metal most often remain intact. Treatment with etching pastes and special emulsions will help save stainless steel.
Other causes of stainless steel corrosion:
- contact of the material with ordinary carbon steel (including through tools that used to cut ordinary steel),
- regular cleaning with metal brushes,
- ignoring mechanical or chemical treatment of the weld.
The cause of metal corrosion can also be its initially low quality. The resistance of steel to rust is due to the presence of chromium in sufficient quantities. This element, after exposure to water, air, acids and alkalis, forms a thin impenetrable layer that prevents the material from rusting. If there is little chromium in the composition or it is unevenly distributed, the creation and maintenance of an oxide layer becomes impossible.
In order for the metal not to be subject to corrosion, it must undergo passivation - the transition of the surface to an inactive (passive) state, during which a thin protective layer is formed on it. Good stainless steel quickly and easily passivates under normal atmospheric conditions - contact with oxygen from the air. The more chromium in the steel, the higher its passivation ability and anti-corrosion properties.
In addition to chromium, steel is alloyed with nickel. It also contributes to passivation, but to a slightly lesser extent. Both metals provide the highest anti-corrosion resistance, although other elements can also be added to the steel: copper, niobium, molybdenum.
To enhance the protective properties, any additives must be in a standard state, and when their structure changes, the resistance to corrosion decreases (for example, when chromium transforms into the form of nitride, carbide).
This can happen during contact with strong acids: sulfuric, hydrochloric, hydrofluoric.
Passive layer
The passive layer is a thin oxide film that forms on steel after chromium reacts with oxygen. It only has a beneficial effect on the properties of stainless steel: on ordinary steel, oxygen, when interacting with iron atoms, provokes the formation of small pores and the appearance of rust. The corrosion layer will also be called passive, because it is reaction inert with respect to the environment.
Types of stainless steel corrosion
Based on the type of development, cause of appearance and symptoms, several types of stainless steel corrosion are distinguished.
Crevice corrosion of stainless steels
Crevice corrosion is a widespread type of rusting of stainless steel. It develops where there is a small gap in the structure, for example, when water penetrates under the fasteners into the product. The second surface is usually a rubber seal, a gasket, and sometimes a metal element.
The mechanism of formation of crevice corrosion is as follows:
- Accumulation of aggressive ions in the gap, displacing oxygen.
- The appearance of an anode in the gap (the material outside the gap plays the role of a cathode).
- The formation of corrosion due to changes in the acidity of the environment and electrochemical reactions.
To prevent crevice corrosion, structures must be designed correctly. It is important to provide cathodic protection, which will reduce acidity, as well as improve the fluidity of the medium.
General surface corrosion
General corrosion is the uniform destruction of the metal structure in part of the surface layer. It causes the destruction of the oxide film on most of the product or over its entire area. Usually the cause is contact with strong alkalis, acids, compounds of iodine, fluorine, bromine. The main “enemy” of stainless steel is considered to be chlorine, which is why chlorine-containing detergents should not be used to clean it.
Pitting
Stainless steels, as well as alloys based on aluminum and nickel, are most susceptible to pitting corrosion. Unlike ordinary steel, which more often suffers from general surface corrosion, such materials in most cases are covered with pitting - small defects. Local destruction of the passive layer occurs in the following situations:
- scratching, mechanical damage,
- local change in steel composition,
- point exposure to chlorine, sulfur, halide ions,
- temperature increase.
Spot rusting is considered the most common among different types of stainless steel. Because of it, holes appear in tanks and small cracks appear in pipes and tanks.
Usually their diameter is no more than 1 mm, while the depth can be significant - this is the insidiousness of this phenomenon. As in the case of crevice corrosion, the specific pitting will act as the anode, and the remaining (undamaged) surface will become the cathode.
Adding molybdenum to stainless steel during its production increases the product's resistance to pitting corrosion.
Intercrystalline corrosion
This process has another name - intergranular corrosion of stainless steels (ICC). It occurs when the temperature rises sharply, which happens, for example, during welding. Rusting begins if, with the participation of heating, chromium urea appears along the grain boundaries, that is, the structure of this alloying additive changes radically. For ferritic steel, the sufficient temperature for the formation of corrosion foci is +900 degrees, for austenitic steel – +450 degrees.
Contact corrosion
This type of corrosion develops when dissimilar metals come into direct contact with each other under the influence of electrolytes. For example, this happens when joining different metal products in an aggressive conductive environment - sea water. As a result, the steel locally deteriorates, and less noble metals may even dissolve.
Pitting Resistance Equivalent Numerical (PREN)
The RREN indicator is for reference; it shows the tendency of different types and brands of stainless steel to develop pitting. The numerical equivalent of pitting resistance is used as a guide, but not as an absolute guide, for determining corrosion resistance.
Typically, molybdenum, chromium and nitrogen are the most resistant to pitting rust as alloying additives. The higher the RREN number, the more resistant the steel will be to pitting. Here is some background information on RREN:
Steel grade RREN| 444 | 25 |
| 430 | 16 |
| 304 | 19 |
| 316 | 26 |
| 304LN | 21 |
| 904L | 36 |
| 316LN | 27,5 |
| SAF 2507 | 42 |
| Zeron 100 | 41 |
Ways to protect stainless steel from MCC
Cleaning rust from a surface can sometimes be difficult, especially when the defect penetrates deeply. A number of methods have been developed against intergranular corrosion, here are the main ones:
- Annealing (stabilization). Ferritic steels are treated at high temperatures (50+900 degrees), due to which the concentration of chromium on the surface increases, and the distribution of the element becomes more uniform.
- Carbon reduction. If the concentration of the substance is less than 0.03%, then the metal will become practically not susceptible to intergranular corrosion.
- Quenching in water. This method is applicable to austenitic steel, it helps the chromium carbides to change into a more suitable form and concentrate on the grain boundaries of the metal.
To remove the tendency of stainless steel to undergo MCC, new additives are introduced into it: titanium, tantalum, niobium, but this leads to a serious increase in the cost of the material. Their quantity should be 5-10 times greater than the carbon norm, and then the metal will not be subject to rust.
Corrosion and surface treatment of stainless steel
Corrosion can be removed chemically - using special rust converters. Also, the surface of stainless steel products can be processed by milling, stripping, grinding, and polishing. The choice of a specific technique depends on the preferences of the specialist and a number of other conditions.
The selection of a method for preventive treatment of metal will be determined by the initial corrosion resistance of a particular steel grade. Elements of pitting corrosion are more likely to form on rough surfaces, while rust spots rarely appear on smooth surfaces. Grades 304, 316 rust quickly when used in seawater conditions, they need to be protected more carefully.
Stainless steel care
In order for stainless steel items to remain attractive and functional for a long time, they need to be well cared for. Under normal conditions, products are washed regularly, at least once every 6 months, with mild surfactants without chlorine and ammonia. In harsh climates, washing should be more frequent. When stains are detected, they are immediately thoroughly scrubbed, and the holes are sealed with special means. Maintenance will help extend the life of stainless steel products and reduce the risk of corrosion.
Source: http://PechiStroy.ru/stroymaterialy/chto-delat-esli-rzhaveet-nerzhaveyka.html
What causes stainless steel to rust?
Such a thing as stainless steel often misleads people, and they mistakenly believe that if the stainless steel type is indicated on the marking, the instrument is not in danger of corrosion under any circumstances. In this article we will look at why this is not the case and what kind of prevention stainless steel requires in order not to rust.
All metals, without exception, are susceptible to corrosion in one form or another. However, some of them, when interacting with oxygen, form an oxide film on the surface of the product, which stops the corrosion process.
A well-known example in this case is aluminum - its oxide film serves as a guarantee against the development of corrosion. You can also mention copper, which, when oxidized, forms an emerald color on the surface.
It is precisely this property of some metals that allows stainless steel to not rust.
On most products, sudden changes in temperature cause microcracks to form; when liquid gets into these cracks, the base metal corrodes and breaks down. In the case of stainless steel, the surface of microcracks, reacting with oxygen, creates a protective film. Thus, a product made of such steel does not require special anti-corrosion coatings.
To make ordinary carbon steel stainless, a certain amount of metal is added to it, which forms a similar protective film. Most often, chromium (Cr) is added to steel, but in addition to it it can also be tungsten (W), nickel (Ni), vanadium (V), molybdenum (Mo), silicon (Si), manganese (Mn), etc. Added the element must be distributed evenly throughout the metal mass, otherwise a protective film may not form on the entire surface of the product, and holes will lead to corrosion.
Steel to which additional elements have been added to improve some of its properties is called alloyed. Carbon steel is often harder than alloy steel, but this is not always an advantage, since it makes the blade prone to chipping.
Depending on the type and quantity of added elements, the properties of steel change according to certain parameters. For example, chromium increases resistance to corrosion and high temperatures, while nickel increases ductility, strength and heat resistance. Chromium oxidizes slowly.
Therefore, new products are especially “sensitive” to adverse factors and require more careful care.
There is a conditional division of steel grades into low-, medium- and high-alloy steel. Stainless steels, as a rule, are high-alloy steels. Low alloy alloys, in which Among the various types of stainless steels, three groups are in greatest demand:
Austenitic is a group of steels that makes up 90% of all stainless steel consumption. This type includes stainless steels with the addition of 15-20% chromium and 5-15% nickel. Such steel not only has corrosion resistance, but is also easy to heat treat and weld. Such steels are not magnetic.
Martensitic - have a high percentage of carbon content, due to which they have a higher hardness index compared to other groups of steel, but at the same time they are more susceptible to corrosion. Often used for the production of cutting tools.
Ferritic - a group with a low carbon content and a high content of alloying elements, which is why they have low hardness. However, in terms of corrosion resistance in many aggressive areas, they are superior to the austenitic group of steels.
All these groups of steels have different properties that are suitable for certain conditions of use of the tool.
But such a protective film on the metal surface still has its weaknesses, which are worth knowing about so that one day you do not find unwanted brown stains on the instrument.
First of all, let's talk about the reasons that can cause corrosion - this will help resolve an existing problem or even play it safe in advance and prevent any signs of rust.
The causes of rust may be the following:
— Contact of the surface of stainless steel with carbon (or low-alloy)
The presence of even a very small amount of free iron on the surface of a stainless steel can lead to the formation of rust, since, once formed on ordinary steel, rust begins to spread deep into the stainless material. Therefore, the use of alloyed and non-alloyed steel grades in the same design is not recommended. And speaking of tools, you shouldn’t store carbon and stainless steel tools in the same place.
— The tool is exposed to aggressive environments
Stainless steel is much more resistant to corrosion than conventional steels, but rust is likely to form when in contact with certain types of chemicals. Stainless steel is most vulnerable to sulfuric acid, although other types of acids are also not harmless. Sea water also removes the protective film due to the ions it contains.
In addition, cleaning agents containing chlorine can pose a threat, so to clean stainless steel it is recommended to use other types of cleaners (or thoroughly rinse the parts to remove any cleaning agent residues). It is also worth saying here that, depending on the elements added to the metal, the alloy may be resistant to certain types of substances.
— The surface is poorly cleaned
Rust can also form under stubborn dirt, so it is highly advisable to clean the tool after use.
— The steel grade differs from that declared by the seller or manufacturer
This point is not directly related to the formation of rust on stainless steel, it is about the human factor. Unfortunately, there are cases when, for some reason, a tool made of ordinary steel, or low-alloy steel, is mistaken for stainless steel. It is extremely difficult, if not impossible, to determine the brand of steel by appearance, which is why such cases happen.
The formation of corrosion in a given situation is influenced by several factors; temperature is one of the key ones, since when it increases, corrosion processes significantly accelerate. The combination of high temperature and aggressive chemicals is especially undesirable. Therefore, before using the drying oven, it is imperative to thoroughly clean the instrument from any residues of sterilizers and disinfectants, and dry it.
— First of all, keep the instrument clean
— Do not store with tools made of ordinary steel
— Use cleaning, disinfecting and sterilizing agents suitable for the steel grade
— Follow the instructions for using disinfectants and sterilizers, and in no case overexpose the instrument in them
We hope this article was useful to you, share your opinion in the comments
and may your instrument last as long as possible!
Source: http://ns.zatochka-etalon.ru/statji/26
Why does stainless steel rust - reasons, types of corrosion
Stainless steel is a high-quality metal that has been alloyed with the addition of a number of chemicals that impart anti-corrosion properties. Due to alloying, steel becomes impervious to moisture, air, and many aggressive environments. But sometimes even this material begins to deteriorate and unsightly rust spots appear on it. Why does stainless steel rust? There may be several reasons, and the main one is improper operation.
Can stainless steel rust or not, reasons for rusting
Catalog Choose a tool Where to buy
Can stainless steel rust or not? If we talk about stainless steel with a chromium content of more than 10.5%, then the occurrence of rust cannot be completely excluded.
Even austenitic steel with a chromium content greater than 20% and a nickel content greater than 8% can rust if improperly handled and processed, or due to design defects. This is why it is so important to use abrasive tools with special properties when processing stainless steel.
An example of such tools are fiber grinding wheels or abrasive cutting wheels with the symbol Fe, S, Cl < 0.1%.
Corrosion
There are two reasons why rust occurs on stainless steel:
- the passive layer has not formed;
- the passive layer was destroyed.
The absence of a passive layer can only be caused by a high degree of purity. The treated surfaces are thoroughly cleaned of all contaminants.
The types of corrosion described below are caused by subsequent destruction of the passive layer:
Why does stainless steel rust?
As materials for containers, equipment parts, components, cutting tools - in the food industry, as well as for the manufacture of dishes, cutlery, knives - in everyday life, aluminum-based alloys and stainless steels . But lately, quite often we have to deal with the fact that stainless steel rusts. Why?
Advantages of stainless steels
Stainless steels have become more widely used , the advantages of which are relatively low cost compared to aluminum-based alloys, high corrosion resistance , and high mechanical properties. A characteristic feature of stainless steels is their high resistance to atmospheric corrosion and oxidation at high temperatures.
These remarkable properties are due, first of all, to the influence of chromium, which is part of stainless steels .
Due to its special tendency to form a very stable protective film of metal oxide on the surface, chromium has exceptional self-protection properties against atmospheric corrosion and the action of a number of chemicals.
This is explained by the fact that the free surface of chromium or an iron-chromium alloy in air becomes very quickly oxidized, and this invisible and at the same time very stable film protects the metal from further oxidation.
The tendency of pure chromium to passivate also extends to its solid solutions in iron (at a chromium content above 12%, iron-chromium alloys become stainless).
With increasing chromium content in an iron-chromium alloy, the resistance to atmospheric corrosion and the action of many chemicals increases even more.
However, it should be remembered that iron-chromium alloys show the greatest resistance to corrosion in an environment that provides the formation of a stable protective film on the metal surface.
Also, the corrosion resistance of stainless steels is highly dependent on the carbon content. In steels with 13-15% chromium, used for the manufacture of knives and cutting tools in the food industry, corrosion resistance decreases at a carbon content of 0.3-0.4%.
Materials used
Recently, quite often questions have arisen about reducing the corrosion resistance of stainless steels during the operation of cutting tools and components made of stainless steels used in the food industry, as well as dishes, cutlery, knives used for household purposes.
The most widely used steel for household products (except knives) and for equipment in the food industry is steel type 18-8 (08Х18Н10Т, 12Х18Н10Т), foreign analogues - AISI 304 and AISI 321. Combining sufficient strength with very high ductility, at the same time, These materials have very high chemical resistance and excellent weldability.
The main suppliers of relatively inexpensive dishes, cutlery and other household products to Ukraine are China, India, as well as domestic Ukrainian manufacturers. In Ukraine, the main producers of stainless steel were the Dneprspetsstal plant, the Volnyansky cutlery plant and the Donetsk iron and steel plant.
Currently, due to the high cost of products from these manufacturers, they are not competitive, and their production volume has fallen sharply or stopped altogether.
There are no complaints about the quality of products manufactured by well-known companies Zeptor, Vinzer, but their cost is 5-10 times higher than that of products made in China or India, so the demand for these products is insignificant.
Reasons for the decline in quality of stainless steels
Let us consider the main reasons for the decrease in corrosion resistance , and, as a consequence, the occurrence of either continuous or pitting corrosion on the surface of products.
Steels of type 18-8 are prone to intergranular corrosion (destruction of steel along grain boundaries), which they acquire as a result of slow cooling or heating in the temperature range 500-850 ° C, as well as during welding.
It is difficult to determine the susceptibility to intergranular corrosion of a material on finished products, since it does not always manifest itself during visual inspection, but can be detected during the operation of the products.
Type 18-8 steels with a carbon content of less than 0.06% are not prone to intergranular corrosion. Heat treatment and surface condition have a great influence on corrosion resistance Steels acquire the highest corrosion resistance stainless steel used for the manufacture of household products is not subject to quenching.
In addition, the steel used for the manufacture of the products in question must have a polished surface.
The presence of deep scratches, scratches, pitting, as well as the presence of traces of scale, due to damage to the passive film, usually leads to the formation of local corrosion.
The main way to obtain a polished surface on sheet material is electropolishing, after which surface defects can have a shiny appearance, but during the use of the products, local corrosion is possible on them.
One of the reasons for steel corrosion is the mismatch of steel grade for the manufacture of products, namely the replacement of relatively expensive steel type 18-8 (due to the presence of nickel in it) with nickel-free stainless steel type X13, which has lower corrosion resistance .
The easiest way to determine type 18-8 steel is to test the product with a permanent magnet. Products made from this steel are non-magnetic, since its structure is austenite. It can be replaced with steel type 03Х17Н14М2 (manganese), which is also non-magnetic and has high corrosion properties.
Products made in China are made mainly from cheaper nickel-free steels, so they are susceptible to corrosion, especially when heated to high temperatures. Unlike 18-8 steels, they are magnetic.
For the manufacture of knives and other cutting products, steels must be used that, after heat treatment, acquire high hardness and, consequently, high cutting ability. The most suitable material for knives is steel type 20Х13-40Х13 (AISI 420). These steels acquire their best properties after hot plastic deformation (reforging), but nowadays knives are often made from sheet material.
Knives made in China are made from this type of steel, but they are often not subjected to heat treatment, have lower hardness, and therefore cannot be sharpened. A simple way to check the hardness of knives is to check with a needle file (file) or a gentle blow on soft steel, in which there should be no nicks on the cutting edge (harder steels that have undergone hardening heat treatment leave marks on soft, unhardened steel).
The reason for the formation of corrosion on knives made of material that has not undergone hardening heat treatment is surface defects, as well as significant overheating of the metal during sharpening. But this does not always appear immediately, and can “let you down” during operation.
Currently, knives are not produced in large quantities in Ukraine, and besides China, suppliers of fairly high quality knives at a relatively low price are Brazil and Russia. The cutting ability of knives depends on the carbon content: the higher the carbon content, the higher the performance properties, but with a significant increase in its content, corrosion resistance . In steels of the X13 type, this is observed at a carbon content of more than 0.5%.
The main reasons for the decrease in the corrosion resistance of containers, products and parts in the food industry are the inconsistency of the stainless steels with the environments with which they are in contact.
It should also be remembered that in no case should we allow contact between stainless steels and ordinary carbon steels because stainless steel itself begins to rust as a result of contact with “rusting” steels. There are many known cases where stainless containers installed on a platform made of ordinary steel began to leak over time.
The guarantee that stainless steels will not rust during operation is strict adherence to steel manufacturing technology and the use of each material in accordance with its properties and purpose.
Menshikov A. G.,
Ph.D., Associate Professor, Rebrova E.M., Senior Lecturer, Department of Materials Science
NTU "Kharkiv Polytechnic Institute"
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Source: https://www.metalika.ua/articles/pochemu-rzhaveet-nerzhaveika.html
Why does stainless steel rust? Unforeseen situation
This situation may still happen. Let's say you bought stainless steel products, no matter what - a stainless sheet or a profile stainless pipe, for example, steel grades AISI 430 or AISI 201, and decided to use it in your production for its intended purpose, let's say, without fanaticism. And they bought it, one might say, straight from the ship from which the container with stainless steel had just arrived at the seller’s warehouse. Bought and forgotten.
It lies in your warehouse and is waiting in the wings. One fine day, production workers get their hands on the stainless steel you bought, and it’s rusty. They look at her and their eyes become so 0_o. They call you and you have the same expression on your face. How so? - you think. Here you have a fresh certificate for metal products in your hands. You've been waiting so long for this stainless steel to arrive! Aah and aah, arguing with the supplier.
Failure to meet production deadlines. Everything is decay.
natural disasters can damage stainless steel during transportation
What actually happened? Yes, a common emergency during the sea transportation of stainless steel containers on a container ship. The ship was caught in a storm. Filled with water. Sea water.
And although transportation containers are made not of stainless steel, but of Corten steel, resistant to atmospheric corrosion, sea salt water still seeps into the container, and water with thirty-five ppm (‰), which is an indicator of the average salinity of the World Ocean, does enter in contact with stainless steel, and you have already seen the result of the interaction of salty sea water with steel in your warehouse.
So another option for answering the question of why stainless steel rusts is the situation described above. And, as you already understand, stainless steel AISI 201, and even more so AISI 430, are not intended for use in sea water.
Why does stainless steel rust? Simple carelessness
a short queue for loading in our warehouse with stainless steel
Let's look at another example. You sent a soldier in a car to a metal dealer to get metal for your production needs. Yes, not for simple metal, but for different grades. For black and for stainless steel. Although stainless steel is already classified as a ferrous metal, that’s not what we’re talking about now. So, the fighter is at the base. It's being loaded.
It is loaded with sheet metal. And they stack everything on top of each other. Black sheet on stainless steel sheet. Without any spacers between sheets. And during the loading process, the black sheet scratched the stainless steel a little. It’s also drizzling a little outside.
In general, all conditions have been created for the stainless steel to begin to rust.
And why? Because the protective layer of the oxide film is damaged and iron is drawn onto the surface of the stainless steel sheet, which will corrode. For remembering the table from GOST 9.
005 of the 72nd year of production, which indicates the permissibility of contacts of various metals with each other, you can see that stainless chromium-nickel and chromium steels should in no way come into contact with low-alloy and carbon, that is, black, steel. From the word absolutely.
Except that some chromium steels have limited permissible contacts in atmospheric conditions, and then only under the condition of nitrided, oxidized or phosphated coating of low-alloy and carbon steel. Here is another answer to the question why stainless steel rusts.
Why does stainless steel rust? On a note
In this example, we will not be able to answer the question of why stainless steel rusts, since we will simply consider the option of incorrect use of a specific grade of steel under certain conditions.
Suppose your grandson, who is a big fan of Yuri Gagarin and the head of Tesla and Space X, comes up to you and says: “Grandfather, let’s make a rocket?” Why are we worse than Americans? – and really, with what? And you, being passionate about rocket science in your youth, decided with your grandson to launch a small rocket in the backyard of your dacha during the summer holidays. Not an R-7, of course, but a smaller one.
After watching old recordings, as well as videos of the same enthusiasts on YouTube, you begin to work in your garage. Fortunately, you still have a small amount of fuel based on a pair of liquid oxygen and kerosene, and there is a metal depot nearby.
And now, after several weeks of construction, your miracle is ready to launch. The aluminum body of a one and a half meter rocket and the engine, whose tanks are welded from AISI 304 stainless steel sheets, are displayed in the backyard, and you have already called all the neighbors, your grandson managed to take a few selfies with the guests, and the countdown to the launch will soon begin. Clear skies and high spirits promote a speedy launch.
Phone cameras are pointed at your rocket, your grandson is counting down the cherished “three, two, one!” Go!" The fuel is ignited and the launch is complete! Noise is heard from the nozzle; the chemical oxidation reaction followed by the release of heat is in full swing. Metal clamps welded to profile pipes, which are like support trusses, are removed from the rocket body and the streamlined structure is carried upward.
In a matter of seconds, the rocket takes off with a whistle to the enthusiastic exclamations of those watching, leaving behind a small amount of smoke. Phone camera optics are trying to track a rapidly receding object in the sky. Ten seconds pass, when suddenly a small flash in the sky makes you understand that your rocket cannot reach the stratosphere.
Surprised cries of guests and a drawn-out “N-e-e-t!” of your grandson, turning his gaze to you, a silent question arises in the subsequent silent pause - How do you like this, Elon Musk? What could have happened?
There are suspicions that an explosion occurred in the liquid fuel compartment. And it happened because AISI 304 stainless steel does not withstand the temperatures at which kerosene fuel with oxygen burned. GOST 5632-72, where the domestic analogue of imported steel is stainless steel 08Х18Н10, states that the recommended maximum application temperature is 800 °C.
The combustion of fuel occurred at temperatures twice higher than this indicator. By the way, the engine itself should have been better made of copper, because thanks to its much higher thermal conductivity than stainless steel, the rocket would have flown much higher due to the fact that the walls of the tanks in the engine would have burned out later.
So for the future, keep in mind that it is better to use stainless steel according to its specific application, rather than darken your grandson’s memories of the summer spent at his grandfather’s dacha.
But seriously speaking, you can simply contact us at StanlisPro, and we will save you from the hassle of choosing a particular brand of stainless steel for your needs. Call - (812) 320-14-01
Source: https://generalsteel.ru/pochemu-rzhaveet-nerzhavejka/
How does stainless steel rust?
It would seem that from the very name “stainless steel” it follows that this material never rusts. However, under certain unfavorable environmental conditions, stainless alloys can be subject to corrosion. To avoid unpleasant consequences, you should understand the causes of corrosion and ways to prevent it.
Pitting or pitting corrosion
Such damage to the metal is of a point nature. It is caused by galvanic processes that begin when there is a lack of oxygen at some points of the product. Such zones acquire negative potential.
Zones with excess oxygen acquire a positive potential. This creates an anode-cathode galvanic couple, and the flowing current causes oxidation of the metal. The process extends into the depth of the product and after some time leads to the formation of through holes.
Various containers and pipelines are susceptible to such damage.
Sensitization of stainless steels and corrosion of welds
This type of corrosion damage is caused by the separation of individual alloy crystals from the surface of the part. It is caused by the influence of working environments saturated with metal salts. In such environments, calcium salts are wedged between separated crystals, and the growth of carbide crystals leads to further destruction of the crystal structure.
Welded seams are susceptible to this influence, during which the welding technology was violated. Sensitization of areas of container shells is also possible. It's called knife corrosion and spreads in narrow stripes.
The crystal structure, weakened by sensitization, is especially susceptible to galvanic influence. Under the influence of induced potentials, the corrosion rate increases many times.
To prevent such negative processes, the method of passivation of stainless alloys and welds is used.
Corrosion in aquatic environments
In a liquid environment, predominantly pitting and intergranular types of corrosion develop. Welds are especially susceptible to corrosion and must be protected with special care.
The rate of development of corrosion phenomena, their properties and the depth of damage to the metal are determined by the chemical composition of the liquid and additional conditions for using the product. Among the most influential factors are:
- hardness of water;
- the presence of iron and other metal ions;
- oxygen saturation of the liquid;
- concentration of heavy metal salts;
- liquid temperature;
- galvanic effect.
An increase in temperature and the formation of stagnant zones of liquid near parts made of stainless alloys significantly accelerate the development of corrosion phenomena.
Deviations of the pH value of the liquid from the neutral corridor of 6-7 also significantly accelerate the corrosion process. The more acidic or, conversely, the more alkaline the environment, the higher the rate of metal damage.
Salts of chemically active elements, primarily chlorides, accelerate corrosion. Carbonates and copper ions activate the effects of chlorides.
Not all substances dissolved in water negatively affect the rate of corrosion processes. For example, the presence of bicarbonates and iron sulfates slows down
The most detrimental effect on the safety of parts made of stainless alloys in a liquid environment is the iron ions dissolved in it. They react with oxygen contained in the air and dissolved in the liquid. The oxidation products that arise during these reactions precipitate on the surface of the part and greatly accelerate the corrosion processes, adding a galvanic component to them.
A clear example of such phenomena is the formation of rusty sediment on the surface of sinks and toilets due to leaking fittings.
Such processes occur most actively at the boundary of the liquid and air environments, when the influence of oxygen in the air and oxygen dissolved in the liquid alternates.
In practice, if it is necessary to use water contaminated with salts and having a pH different from neutral, the method of settling the liquid in intermediate tanks is used. In addition, the water is purified in special filters filled with ion exchange resin granules. Vessels used for storing, processing and heating such waters are subject to periodic inspection and maintenance
Passivation of stainless steel
The passivation process consists of the deliberate formation of a layer of poorly soluble oxides on the metal surface, protecting the thickness of the part from exposure to an aggressive environment.
Oxidizing compounds are applied to the surface, such as an 8% solution of citric acid. In industrial settings, stronger reagents are used. To speed up passivation, the temperature of the part and the solution is increased.
It should be taken into account that under mechanical stress the passivating layer is destroyed and the product again becomes susceptible to corrosion. Therefore, the passivation process must be repeated as necessary.
Source: https://pechiexpert.ru/kak-rzhaveet-nerzhavejka-10/
Why does stainless steel rust - Metalworker's Guide
In this article we will partially answer the question of why stainless steel rusts, but we will not answer this question from a technical point of view, describing such banal and boring reasons for rusting as the appearance of general, intergranular, pitting, or crevice corrosion. No. Today we will look at the reasons why stainless steel rusts purely due to the presence of the human factor. And not only him.
One of the reasons for rusting of stainless steel due to the human factor can be the following situation. A swimming pool manufacturing company receives an order to equip a small fitness center with an infinity pool. And this order appears thanks to the tender won by the enterprise. As a result of fierce competition, the cost of manufacturing the pool had to be significantly reduced.
The company declined due to an invoice for AISI 316 stainless steel, from which the pools are made, from one of the suppliers who offered the lowest price for stainless steel sheets. All documents and specifications are signed. The metal has already been received. However, upon receipt at the warehouse, we noticed that there were no markings on the sheets.
But the supplier attached the certificate to the documents, and even gave a short deferment of payment. After some time, the company produced an order in its production facility, installed the pool and water treatment equipment, and even provided the customer with software for monitoring the stainless steel pool pumps and filters. Acceptance certificates were signed.
We marked the completion of the order and safely forgot about it. Because other orders appeared.
stainless steel swimming pool
And six months later, a representative of the customer contacted the manufacturer with a complaint about the appearance of dark red dots in various places in the pool. After inspecting the pool bowl, it was revealed that rust spots had formed as a result of exposure to reagents used to disinfect water.
But acid-resistant stainless steel AISI 316 was used in production! How could this happen? After much investigation and searching for possible reasons for what happened, small pieces of sheets that had once been purchased were found in the warehouse and the piece was sent to a chemical specialist. analysis.
It turned out that the steel from which the pool was made was not comparable in chemical composition to AISI 316 steel.
What actually happened? Of course, you can say: you shouldn’t be chasing cheapness, priest. But a low price may not always mean that they want to deceive you. Here, for example, the fact that the supplier has metal that he purchased at a very good price from the manufacturer may play a role. But in this case something different happened.
At production, when accepting stainless steel sheets, they did not attach much importance to the absence of markings on the surface of the sheets, but as you know, it is by markings on a stainless steel sheet that the data in the certificate can be correlated when checking for authenticity.
And the metal dealer, who provided the low price, himself bought these sheets from someone and simply provided a certificate from another batch. Here is the result.
For the future: if you have high demands on stainless steel products, check for markings on stainless steel sheets and purchase goods from trusted suppliers.
How are consumers deceived and why does stainless steel rust?
Let's answer these questions regarding our main activity, the manufacture and installation of stainless steel railings and fences.
First you need to understand a basic thing, what makes steel stainless? This is a useful and important property that steel acquires when chromium, nickel and other impurities such as molybdenum are added to it.
It is the presence of chromium in steel that allows it to form a protective anti-corrosion layer. This process occurs when chlorine atoms and oxygen atoms react, which leads to the formation of a protective oxide film.
If oxygen reacts with steel atoms without the presence of chromium, this leads to porosity and eventually corrosion.
Accordingly, if this oxide layer does not form or is destroyed, you and I will experience corrosion.
Now let's figure out what can destroy the protective layer of stainless steel?
(There are three possible options here)
FIRST. Poor quality work when installing metal barriers. That is, the option is damage to the protective layer due to poor-quality welding, or in the process of poor-quality grinding.
Often installation work is carried out carelessly. Quality is usually monitored by large and medium-sized companies. Small “basement” organizations grab any orders, send non-specialists, and you get the corresponding result.
SECOND. In the process of saving and in the desire to sell you cheaper, they may slip you steel with a low chromium and nickel content.
We need to clarify here.
WHAT ARE THERE TYPES OF STEEL?
Depending on the presence of impurities in carbon steel, it is divided into grades. The simplest and lowest-containing protective anti-corrosion steel that we use is AISI 201 steel. Fencing made from this steel can be used indoors, provided that the environment in these rooms is not aggressive. That is, there should be no framing of fountains and pools with such railings and bumpers. Otherwise, after 2-3 months you will get corrosion.
If you are going to use your stainless steel fences on the street, then you can use steel no lower than AISI 304. This grade of steel contains five times more nickel and chromium than AISI 201. Then you are guaranteed to get a product for many years without risk replacements soon.
But even the AISI 304 category will not save you if you are going to use railings and fences in a particularly aggressive environment, for example, fencing for a pool with chlorinated water or in an area with a maritime climate. Here it is better to use steel with the addition of AISI 316 molybdenum.
Also, unscrupulous companies often use stamped fittings and pass them off as cast, more expensive ones. Be careful and meticulous.
AND THE THIRD OPTION , when you can get unplanned rust on your brand new railings, is improper care of the product. Even though your fence appears to be metal, it still requires special care. And improper cleaning can lead to damage to the protective oxide layer and the appearance of rust.
How to avoid corrosion of your fences?
(Always make a contract)
FIRST , when you choose a contractor to install your product, make sure that they sign a contract with you and give you a guarantee of at least one year.
Once again, clarify the terms of the guarantee, and what can you count on and in what case? Carefully accept the result of the installation team’s work. Inspect how well the welds are processed and without damage.
For example, the Modern Group company uses steel at least 1.5 mm thick, which guarantees that the welds will be completely invisible.
SECONDLY , always be wary of offers that are too “lucrative”. Perhaps they are trying to deceive you by passing off AISI 201 category steel as higher grade 300+ steel. This is the most common buyer deception in the metal fencing market today.
Even a specialist cannot determine the quality of steel by eye, so what can we say about the common man? Here, oddly enough, bureaucracy and literalism will also save you. Demand that the steel grade be specified in the contract.
This seemingly trifle will save you from subsequent expenses and other headaches.
Remember, saving on the quality of a product whose service life is estimated at decades is the last thing to do. Don't chase bargain prices. At the trading stage, demand an explanation for any reduction in price for you. Don’t hesitate to clarify why and how the company is giving you a discount, to see if this “saving” will come back to bite you later. After all, money is a gainful business, and nerve cells are not restored.
Proper care of railings
(Here is a little more detail since you will encounter this every day)
REMEMBER! When servicing stainless steel products, do not use detergents containing chlorine, alkali or abrasive elements. Otherwise, over time, you will simply “erase” the protective oxide layer of your product. Or in the case of abrasive additives, damage the polished surface.
FOR CLEANING it is best to use plain water. With daily care, serious dirt simply will not have time to accumulate and regular wet wiping will be enough. If regular wet cleaning does not work, add 1% ammonia to the water. Acetone can also be used for cleaning.
WHEN, DURING REPAIRS or construction, your stainless steel fences become dirty with cement or lime, they need to be washed as soon as possible. You can use acetic acid, but avoid cleaners containing hydrochloric acid.
IF THE RAILINGS ARE SPOILED WITH PAINT, simply remove it with regular paint removers using a sponge or brush with a nylon cleaning surface. Never use a wire brush.
IF YOU NEED TO REMOVE A STICKER from a polished steel surface, use a hair dryer. The heated glue will easily release the glued object.
WHEN REMOVING GLUE RESIDUE from a sticker, chewing gum or patch, add eucalyptus oils to your cleaning fluid. You can buy it at almost any pharmacy. Carefully remove all adhesive residues with this cleaning composition, otherwise dirt and dust will begin to stick in this place.
If, nevertheless, you damaged your product and caused a deep scratch that began to rust, do not rush to sound the alarm and replace the entire product. Treat the rust with nitric acid at room temperature, then rinse with clean water and dry. Over time, the protective oxide layer will be restored.
If you follow all the recommendations listed here, your railings and fences will always be clean and beautiful. And frequent contact with your palms will only benefit them, because it is a natural polish for your product. So use it correctly and these beautiful and comfortable products will delight you for years.
Source: http://zakazperil.ru/kak-obmanyvayut-potrebitelej-i-pochemu-rzhaveet-nerzhavejka.html
