The introduction to this issue will be somewhat longer, although I will simplify and shorten it as much as possible. The reason for this is that anyone who needs to solve this problem needs to keep in mind everything that will be presented here, all at the same time, as one connected and related package. Above all, it is necessary to realize that steel corrosion is not only a physical and chemical process, but primarily an electro-chemical process. The word "electric" plays a vital role in corrosion. Iron (Fe) is "missing" 2 electrons in its atom, so it likes to take them from a number of "donors". The "donors" may not only be oxygen (O 2 ), but many other elements. Or, conversely, iron itself can donate its electrons to another element. But as soon as a "hole" in the electron shell of iron is filled or enlarged, this "hole" then jumps further along the Fe metal lattice. For steels, it is usual for it to travel about 30 cm away from the place of its origin. But steel is not just iron, but a mixture of dozens of elements. Some are alloying, changing the physical and chemical properties of steels in the desired direction, some are present thanks to the entire production process. And most of them have a great influence on the corrosion properties of steels. A number of them, in contact with iron, create a micro galvanic cell, i.e. the germ of corrosion. But micro galvanic cells can also be created in other ways. Contact with the machining fluid, impurities in the air, the presence of certain ions in washing fluids, etc. Then at the same time we have the problem of creating galvanic cells. (For example, roxors freshly embedded in concrete create a voltage of 1.2-1.4 V.) But these galvanic cells also arise if one machined surface comes into contact with another (contact, crevice corrosion), or if the product has an unevenly machined surface (different roughness on the same product), especially if different metals or different batches of the same grade of steel come into contact, but the overall shape of the product also plays an important role. And since it is an electro-chemical process, corrosion rates are several times higher if they take place in an electrically conductive environment. They can also take place in an electrically non-conductive environment, but the speeds in such cases tend to be low. The most common electrically conductive medium is water. And not only in its liquid form, but also in vapor. The dependence of the corrosion rate on the relative air humidity is shown in the following graph. It can be seen from this graph that if the relative humidity of the air is up to 65%, the corrosion rates are low. Above 65%, however, speeds rise steeply. At 100%, i.e. in liquid water, corrosion rates are highest. But corrosion rates are also dependent on temperature. They grow exponentially with higher temperature. But they also depend on which chemical compound they come into contact with during the manufacturing process. And the issue of pH is related to this. In acidic environments, corrosion rates are high and the curve begins to break at pH 7.5. Above pH 8.5 they are relatively small. A number of important general practical conclusions follow from the above. Steels (with the exception of stainless steel) will always corrode without treatment. How quickly, and on what corrosion products, it depends on many factors. There is no machining process that can remove or stop corrosion. None of them can work with electrons. So, even if the surface is beautifully shiny and without visible corrosion, the electrochemical corrosion processes in the material continue and it is only a matter of time before the corrosion reappears. It is also necessary to keep in mind that it can appear up to 30 cm from the place of its origin. Corrosion can be stopped or prevented only by chemical processes, to which some metallurgical ones belong as a smaller group. Some alloying additives, or elements present from the manufacturing process, accelerate corrosion processes. And their quantity also plays an important role. Even if it is the same grade of steel and from the same manufacturer, each batch of melting and processing will have slightly different corrosion behavior. Sometimes even significantly different. The formation of micro-galvanic cells can be reduced by not allowing dust from the air to settle on the steel product or raw material. But it is also necessary to pay attention to process fluids, such as machining emulsions, oils, quenching fluids, welding agents, etc. Over time, these become clogged with micro-dust from machining, which creates ions. And filters that could remove them are not used in the metal industry. Therefore, it is absolutely necessary to replace the given fluids with clean ones from time to time. Related to this is the issue that these liquids usually react with air. New compounds are often formed, which sometimes initiate the formation of corrosion. But they also get microbial contamination from the surrounding environment. And many microorganisms produce metabolites that are highly corrosive. This results in the need not only to continuously change process fluids, and to find the limit of their endurance in the process before problems start to arise, but also to clean and disinfect the equipment before a new process fluid is fed into it. And if steel and then copper or aluminum alloys are processed in the same production process, it can be called a nice mess. In such a case, reliance is placed on the fact that micro galvanic cells are created. And since chemicals that could stabilize such combinations are produced in small quantities and only for a few technological operations, it is better to machine and treat steels separately from copper alloys and aluminum alloys. Industrial washing and degreasing is "one big fun" that deserves a separate treatise. Another important factor affecting the corrosion of steel is the contact of the products with each other. The formation of crevice corrosion occurs just because, for example, sheets are stacked on top of each other during storage. Or when the products are packed in such a way that one touches the other. One company wanted to save on shipping costs, so they stacked the products as close as possible to each other in the shipping packaging, and they couldn't help but wonder how they started getting complaints. In such cases, it is necessary to use some kind of dielectric so that the products do not touch directly, or to make some suitable anti-corrosion protection before they come into contact. Another example of contact corrosion is fig.     Surprisingly, few people know that the differently treated surface of the same product has a great influence on the corrosion behavior. For example, one company used high machining speeds to create "notches" in one spot on the product, while the rest of the workpiece was smooth. The place corroded very quickly, even though the entire product was preserved. A suitable solution is for the entire product to have the same surface roughness as possible, or an effective conservation procedure needs to be found. It is also neglected that the very shape of the product has an effect on the formation of corrosion and its speed. Sharp corners and L-shapes support this cause of corrosion. Another company prevented the development of corrosion by starting to machine a rounded inner corner instead of an almost sharp right angle. Relative humidity alone provides big corrosion surprises. It is quite often forgotten that even if in production there is 50% relative humidity at 20°C, at a temperature of 11°C this air reaches 100% relative humidity. Then the companies are surprised that their products corroded during transport, when they are fine in the warehouse directly at the factory. It's just that they forgot about vapor condensation due to the reduced temperature during transport. And there is one very important matter related to that. Many companies pack their products in PE films. Some of them even fuse smaller products into them. However, the less permeable the packaging is to water vapor, the harder it breathes, the more moisture condensation occurs inside. Basically, in this way, a person creates a condensation chamber, which accelerates corrosion. And even though there are foil materials that contain vapor corrosion inhibitors, their use is very risky. No film contains enough inhibitor to be effective more than a few mm from its surface. Most products cannot be wrapped in such a way that their entire surface is in contact with such an anti-corrosion film. The idea is to pack in paper laminates impregnated with a vapor corrosion inhibitor, but in such a way that the packaging has enough slits through which it can breathe and that there is as little condensation of water vapor as possible. It is also a harmful habit to put a bag with silica gel in the foil package and then seal the package hermetically. But no one dries the silica gel just before putting it in the package, so by the time the factory receives the shipment of silica gel bags, they are already unusable because they are saturated. And with that, the user only adds additional moisture to the moisture contained in the air in the package, which is retained by the silica gel. Please do not underestimate the effect of temperature on corrosion. Every 10°C, the rates of chemical reactions double. This means that if someone processes at 20°C and processes the exact same material at 80°C, the reactions will be 64 times more intense and faster at that 80°C. When machined under emulsion, it is an electrically conductive environment with high corrosion rates. Therefore, it is necessary that emulsions contain anti-corrosion additives. But there is a catch in that too. The entire production process and the means used in it must be compatible with each other. Or it can easily happen that one anti-corrosion additive reacts with another to create spectacular corrosion. If the manufacturer is not sure of the composition of the process fluids and what they will do to each other, it is a good idea to consult. For example, combining contact corrosion inhibitors in oil systems with vapor corrosion inhibitors is a nonsense that will come pretty expensive. But many manufacturers also believe that if they machine under oil, nothing can happen. The opposite is true. Mineral oils contain 4-8% water, which is more than enough to cause corrosion. With synthetic oils, the situation is even more complicated. Some of them do not accept water at all and it then accumulates somewhere, or some of them are infinitely soluble in water. In the next installment, I will focus on the most common production corrosion problems and how to deal with them.     Ing. Peter Stuchlík, CSc., CTex ATI

Anti-corrosion protection using vapor corrosion inhibitors Anti-corrosion protection using vapor corrosion inhibitors Ing. Peter Stuchlík, CSc., CTex ATI   Motto: "It is cheaper and faster to prevent problems than to solve them when they arise."   Although corrosion degradation processes concern a whole range of materials, further attention will be paid only to corrosion processes of metals, especially iron alloys. Once the corrosion reaction is started, it is very difficult and expensive to stop it because electrons or ions are transferred through the crystal lattice of metals. Therefore, it is important to always keep in mind that potential accumulation and corrosion processes can occur relatively far from the place where the corrosion started (for iron alloys, the usual distance is 30 cm). And it is delusional to think that there is a mechanical process that can stop corrosion. Grinding does not remove one particular electron. The following figure roughly shows the dependence of the costs of stopping and removing corrosion.   Giant. no. 1 Dependence of the costs of stopping the corrosion of steels according to their type.   According to the method of attack, corrosion is divided into uniform, where corrosion processes take place on the entire surface at the same speed and to the same depth. Further to uneven, which can be dimple, point, intercrystalline, transcrystalline, lamellar and selective. According to the physical process, we distinguish corrosion under voltage, thermal corrosion, cavitation, corrosion by stray currents, corrosion by induced voltage, form corrosion, etc. According to the chemical process, it is divided into corrosion in an electrically non-conductive environment, which can be in reducing gases, in oxidizing gases and in non-conductive liquids. Or for corrosion in a conductive environment (electrochemical corrosion), when electrochemical, electrolytic or concentration cells are formed (concentration cells include joint or crevice corrosion). Special types of corrosion include microbial or high-energy radiation. According to the phase interface, it is divided into solid phase (metal) with gas, solid phase/liquid, solid phase/solid phase (contact corrosion). In practice, we most often encounter two types of corrosion in metals: Chemical corrosion in an electrically non-conductive environment. That is, most often a reaction between a metal and a reducing or oxidizing gas at the phase interface. These are mostly reactions with oxidizing gases. But the case of hydrogen depolarization using (H + ), i.e. corrosion in reducing gases, is also quite common. In the case of iron alloys, this type of corrosion has a significant rate only when the temperature exceeds 580°C. However, the most common and also the most dangerous are cases of corrosion in an electrically conductive environment, i.e. electrochemical corrosion. Corrosion velocities here are ten to a thousand times higher than in a non-conductive environment, and fractions of seconds are enough to initiate corrosion processes. The critical value for the development of these processes is 60% relative humidity. Once this value is exceeded, corrosion rates increase exponentially. A number of basic methods of corrosion protection have been discovered and others are being worked on. In practice, it is not possible to use one corrosion protection system for all types of corrosion processes, materials and metal products. In the same way, it is necessary to find an optimal compromise between economic, useful, ecological, health and technological factors. The basic methods of protecting metals against corrosion include: Increasing the chemical inertness of the surface using protective coatings (passive protection). It is a surface blocking that prevents contact of aggressive chemicals with the metal surface and prevents the diffusion of gases. This is done using metals - either alloying, or barrier plating, or cathodic plating. Using non-metals - oxidized, nitrided, chromated and phosphated conversion coatings. Using silicates - enamels and glass. Using polymers – paints, varnishes, polymer melt coating. Or hydrophobic treatment using "waxes", oils, etc. is used. The latest is blocking using nanoparticles, which can be metallic or non-metallic. Modification of the corrosion environment. By removing the aggressive component (degassing, drying, inertization). Change of environment (inhibition). Physical inhibition is used - blocking of active nuclei on the surface, or chemical inhibition of the surface. It can be anodic or cathodic. Increasing the thermodynamic stability of the surface (cathodic protection). Increasing the electrochemical stability of the surface by shifting the reaction balance (anodic protection or passivation). Active protection using a sacrificial electrode (anodic protection) or active cathodic protection.   A good anti-corrosion treatment technology usually combines several principles at the same time. Most often, it is a combination of protection using a passive or hydrophobic layer, using inhibition and increasing the electrochemical stability of the surface.   The basic methods of anti-corrosion protection and their properties are listed in the following table: When designing the most suitable technology and method of anti-corrosion protection, it is necessary to take into account: What kind of metal is it, and which risky additives does it contain. What technology was used to make the product? What pollution occurs on the surface of the product. What method of anti-corrosion protection is used by the manufacturer in individual production steps, or what is required by the customer. Here, the compatibility of the protective means used, both with each other and with the contamination on the product, is very important. How the product will be stored and transported. And under what conditions. What protection period is required. How the product will be handled in the future. (For example, it is not possible to use a difficult-to-remove anti-corrosion protection that completely blocks the surface of the product, if further surface treatment is to be carried out on it.)   Since corrosion prevention is the cheapest anti-corrosion measure, it is important to pay extra attention to it. Several principles apply to prevention: The product must not be exposed to relative humidity above 60% . This cannot be avoided in a number of processes, because they are machined under aqueous emulsions, water quenching and cooling or pickling baths are used, products are washed in aqueous media, etc. In these cases, it is imperative to ensure that the products are dried as quickly as possible. The same is true of condensed moisture or rain. If corrosion is detected during any manufacturing step, the necessary corrective measures must be taken immediately . Once the corrosion reaction is started, it will not stop on its own even if the corrosive conditions are removed. It is absolutely necessary to shift the reaction balance by means of some anti-corrosion system (passivation, inhibition). Pay attention to the compatibility of the means used in the entire production process from start to finish. This is especially true if different machining and process fluids are used. They represent a rather complex chemical system that contains a number of additives. Therefore, it is imperative to carry out appropriate corrosion tests for compatibility. If any corrosion protection methods and means are used in the manufacturing process, they should be maintained throughout the process. In the event that for some reason the combination of different resources cannot be avoided, it is usually necessary to include an operation that completely removes the previous resources. Here too, it is absolutely necessary to carry out suitable corrosion tests. For iron alloys, all operations with increased humidity or in the presence of an electrolyte (water) should take place in conditions above pH 7.5 (optimally in the range of pH 8-10). If any operation takes place in an acidic region (i.e. below pH 7), neutralization should be included immediately and a check made for both corrosion and the remains of the neutralization reaction. For most metals, it is desirable that the product does not come into contact with chlorides, chlorine derivatives, sulfur dioxide (SO 2 ) or compounds that release it, and with most monoacids. The corrosion reactions started by them are difficult to stop. Make sure that the contact of two surfaces in the presence of water vapor or water does not create an electrochemical (galvanic) cell . This occurs when two metals with different compositions are in contact (even different grades of steel are enough) or when two differently treated surfaces come into contact. In such cases, it is necessary to separate the surfaces from each other dielectrically, ensure a reduction of humidity below 60% relative humidity, prevent water condensation and take anti-corrosion measures. Limit the formation of electrolytic or concentration cells by means of shape and design solutions. Choose rounded corners over sharp ones and prevent the formation of capillaries. If this cannot be prevented for some reason, then it is necessary to ensure that the humidity is reduced below 60% relative humidity, prevent water condensation and take anti-corrosion measures.   One of the ways to prevent corrosion is the use of vapor corrosion inhibitors (VCI). These inhibitors are evaporated from a carrier under normal pressure and temperature, the molecules of the inhibitor are attached to the surface of the metal by physical or chemical bonds, where they form a molecular layer. This layer usually has a pressure greater than the partial pressure of water vapor, so they have a hydrophobic effect. They therefore have the function of physical inhibition. At the same time, they also function as cathodic or anodic chemical inhibition. Some of them are also capable of passivation reactions during the formation of conversion layers. Other of them can also neutralize corrosive ions, or function as buffers shifting the pH to the alkaline region, or as scavengers of free radicals. The diagram of the action of vapor corrosion inhibitors is shown in the following figure.   Giant. No. 2 A method of attaching VCI vapors to a metal surface     There are a number of vapor corrosion inhibitors and they are divided into several groups. Currently, amines are the most widely used vapor corrosion inhibitors in the world, followed by chelates, nitrites (although the last two types are on the decline due to legislative restrictions) and urea (for short-term protection). Advantages and disadvantages of vapor corrosion inhibitors: Advantages: wide range of applications and applicability to all metals, high ratio of effectiveness to the price and the amount of inhibitor used, the surface of the products is not greasy and it is usually not necessary to remove the inhibitor before further surface treatment, simple application. Disadvantages: must be used in closed or semi-closed containers, they are mostly water soluble, so rain or condensed moisture will break them down, are sensitive to the pH of the environment, have limited thermal stability, it takes some time for the vapors to reach an effective concentration.   All inhibitors of this group are very similar and have some common features. Their thermal decomposition temperature is about 70° C, the "start" temperature is about 15° C, they are soluble in water, they provide a high efficiency-to-mass ratio, the protective effect is about 1 year for packaged goods stored in a tempered warehouse (if they are used correctly ), a certain concentration of vapors must be reached in the package to be effective and they reach 80-95% efficiency in protecting roughly 4 dm 3 of the interior space of the package per 1 g of inhibitor. See Chart.   Giant. No. 3 Anti-corrosion efficiency of the amount of VCI used depending on the volume of the package to be protected [g/dm 3 ].   Specific types of inhibitors from individual manufacturers differ from each other within a certain range in terms of solubility, surface tension, vapor pressure, protected metals, percentage of anti-corrosion efficiency, health safety, biological degradability, or other details. Benzamine corrosion inhibitors are solid substances that usually have the character of micro crystals. It is possible to sprinkle them directly on the protected goods, but this method is used only exceptionally. It is more common to find that inhibitors are filled into bags, or most often a carrier is filled with them. The most common carrier is carbonized VCI papers. These are produced both smooth and creped, without lamination, or laminated with PE film, or with textile reinforcement. PE films filled with VCI are currently a popular carrier. Such foils are used by those who know little about the issue. By adding VCI to the polymer, the mechanical properties of the film deteriorate. An acceptable value of fulfillment is between 2-4%. While most packaging films have an area weight of up to 100 g/m 2 . Therefore, the packaging film contains a very small amount of the active substance relative to its surface area. Moreover, the release of the inhibitor is slowed down by the necessity of diffusion of VCI through PE, and the specific surface area of the foil is on the order of thousands of times smaller than that of paper. Therefore, the use of foils for anti-corrosion protection is very illusory. VCI are also filled into PE granulate and sprinkled with packaged goods. This method of protection has all the disadvantages of a film, as well as the disadvantage of being removed as a powder, but without its effectiveness. Furthermore, various sponges, felts, non-woven fabrics or bulky papers are used as carriers. These carriers are cut into blanks and then inserted into the protected area. Since there is a wide range of VCIs, attention will be paid only to general knowledge and a little more detailed information will be focused on benzamine VCIs. With regard to chemical substances, the most important legal measure is the REACH-CLP regulation. But there are other rules and regulations. Lists of dangerous substances are attached to these legislative measures. It is therefore necessary to check that the VCI in question does not appear on the listed lists and, if so, which dangerous category is assigned to it. (A number of historical VCIs are on this list in terms of carcinogenicity, or suspected carcinogenicity.) Moreover, the risks of a specific agent should be expressed in its Safety Data Sheet. Current commercial quality benzamine inhibitors are only irritants, and this is proven by LD50 measurements, and they are biodegradable. Furthermore, they do not contain secondary amines, N-nitroamines or their precursors. Some corrosion inhibitors that were used in the past were more effective, but their properties are now unacceptable from a health or environmental point of view. Regardless of how vaporous corrosion inhibitors (VCI) are applied, product packaging plays a very important role. Several principles apply. The products must not be packed wet , because they would be exposed to conditions of increased humidity, such as in a condensation chamber, at the very moment when they are most vulnerable (before the protective atmosphere is created). Such an environment would very quickly nullify the inhibitor's protective effect. At the same time, the products must not be packed corroded , because VCI do not have the ability to remove corrosion and are able to stop corrosion as much as possible, but at the cost of their consumption. One of the most common mistakes when preserving metal products with VCI is to first preserve the product with oil and then place it in a VCI vapor atmosphere for "better" protection. Most of the time it turns out the opposite, i.e. badly. Even though VCI vapors are able to penetrate remote spaces and very small capillaries, it should be taken into account that the VCI source should not be further than 30 cm from the protected surface. In order to achieve the desired concentration of VCI vapors, it is necessary to close the product. Either completely closed or semi-closed packages are used. In the case of closed packages, care must be taken to prevent moisture condensation inside the package during storage or transport. Semi-closed packaging allows air ventilation between the external environment and the internal contents. At the same time, they are a sufficient barrier to create the necessary concentration of VCI vapors. When packaging products, it is also necessary to take into account corrosion, which is caused by the contact of two metals or two unequally treated surfaces in an electrically conductive environment. It is absolutely necessary to separate these surfaces from each other dielectrically . For example, the product must not touch the walls of the metal box. Dielectric separation can be achieved using VCI carbonated paper or laminated paper. The foils themselves may not be so suitable, because in the case of moisture condensation, a concentration cell can form between them and the metal (pitting corrosion). Last but not least, it is necessary to pay attention to how the products are stored in group packaging. In the case of packaging, it cannot be completely ruled out that condensation of atmospheric moisture will occur. This moisture then flows down due to gravity to the lowest places, or is held in the crevices due to capillary forces. Removing capillary water is an energy-intensive and lengthy process. It is therefore necessary to solve the storage of the material in the package in such a way that possible condensed water does not penetrate into the cracks and capillaries, and that this water is removed as quickly as possible from the places where it flows.

Anti-corrosion protection of cast iron Cast irons are alloys of iron, carbon and accompanying elements (desirable and undesirable), where the carbon content is higher than 2%, and the sum of all accompanying elements does not exceed 2%. Cast iron is a popular construction material, especially for its simplicity of production. Basic types of cast iron are distinguished according to the shape of the graphite in the alloy, and each group has its own characteristic physical-mechanical and chemical properties. Cast iron with flake graphite (grey), with nodular graphite (ductile), with worm (vermicular) graphite, white cast iron, tempered, ADI cast iron. Each type of cast iron can be further modified with alloying additives, inoculation and heat treatment. Therefore, it is not possible to establish clear general rules for the behavior of cast irons against corrosion and their resistance. However, due to the fact that the resistance to corrosion cannot be increased much by low alloying, it is true that cast irons tend to be more sensitive to corrosion than alloy steels. Both chemical and electrochemical corrosion are used for cast iron. (Non-chemical corrosion, for example cavitation, comes into consideration only for some products.) Chemical corrosion is the action of oxidizing or reducing substances without the presence of an electrolyte and leads to the formation of a layer of waste products at the point of phase contact. Its speed increases at temperatures above 580°C. More dangerous is electrochemical corrosion, which causes the transfer of charge along the metal grid based on the action of the galvanic cell. It is created in the electrolyte not only in the presence of two different metals, but also an unevenly treated surface of one metal is enough to create it. Electrochemical corrosion is accelerated if chlorine ions (Cl - ), carbonates (CO 3 2- ) or sulfates (SO 4 2- ) are present in the water. Corrosion resistance does not depend only on the composition of cast iron, but also on the concentration of corrosive substances. Preventing the occurrence of corrosion during the production and processing of cast iron is very problematic. Here are some general guidelines to reduce this risk: use such binders and form separators that do not contain oxidizing or reducing substances and these are not created even by their thermal decomposition, unmould the castings at temperatures below 500°C - the lower the temperature, the better, work with products above the dew point temperature, prevent contact with water, if the technological process requires contact of the product with water, it should not contain chlorine (free or bound), carbonates or carbonic acid, sulfates, saturated oxygen. These conditions are almost impossible to meet (a number of aqueous degreasing liquids contain NaHCO 3 , there is always some oxygen in the water, etc.). That is why procedures that prevent the formation of corrosion or its spread are used more in practice. They are both physical and chemical. Physical ones work by creating an impermeable layer on the surface that resists the diffusion of electrolytes, oxidizing or reducing substances and has a hydrophobic character. plating Cr, Ni, Co, Au, Zn, etc., plastic coating, most often PVC, PP, PE, measures with protective coating, varnish, paint, by applying a hydrophobizing agent, oil, wax, silicone, fluorinated hydrocarbons, amines, etc., Chemical methods work on the basis of a chemically or physically bound impermeable layer on the surface of the metal, which either transforms the metal into another corrosion-resistant compound, or prevents the transfer of the corrosion ion to the metal through a chemical redox reaction, or acts as a free radical scavenger, or acts as a cathode/ anode. passivation by oxidation to Fe 3 O 4 , blackening, passivation with organic salts, oxalate, citrate, tannate, chelate, etc., passivation with inorganic salts, chromating, phosphating, inhibition, e.g. amines, inhibition using free radical scavengers, cathodization/anodization, A combination of both principles. The following table provides a basic comparison of the mentioned principles: principle benefits disadvantages plating High corrosion resistance, aesthetic appearance, more expensive They are laboriously removed, they cannot be repaired, only for final products plastic coating High corrosion resistance, aesthetic perception They are very difficult to remove, they cannot be repaired, only for final products protective coating Easy application, wide range of uses, repairable As they age, they will not prevent the progression of corrosion already in progress * hydrophobization Easy application, mostly easily removable Temporary protection, limited only by some corrosive processes, must be removed before surface treatment blackening Aesthetic perception, mechanical resistance Medium level of protection, only for final products passivation of org. salts Easy application, stopping previous corrosion Lower level of protection passivation of inorg. salts Easy application, stopping previous corrosion Medium degree of protection, ecologically problematic inhibition Easy application, easy to remove Temporary protection sensitive to solvents scavengers of radicals Easy application, easy to remove Temporary protection, may block further surface treatments cathodic protection High efficiency Only for final products and some electrochemical processes * If it is not a coating that also combines chemical protection In practice, a combination of both of the above-mentioned principles is very often used. E.g. the base paint contains zinc (cathodic protection) and the top paint has hydrophobic and barrier properties. The blackened surface is treated with a preservative. The ongoing corrosion is stopped by the reaction to the organic salt and simultaneously converted to a metal polymer with barrier properties (corrosion converters). The preservative oil with hydrophobic properties contains corrosion inhibitors and free radical scavengers. From the point of view of interoperational protection of cast iron products against corrosion (ie short-term up to 1 year), the following means and principles are most often used today: Painting. It works with its hydrophobic and barrier principle, but it is very problematic because the varnish must be removed before using the product or its final treatment. Oil preservation. Again, it is necessary to degrease the product before use (in most cases). However, this is easier than removing the paint. On the other hand, an oiled surface sticks the products together, catches dust and deteriorates the packaging. Inhibition using contact water-soluble corrosion inhibitors. It is a very easy application, where usually it is not necessary to remove the inhibitor micro layer before the next process or final treatment, but this micro layer is sensitive to moisture. Running water, rain or condensed moisture will wash away the inhibitor. Inhibition using vapor/Volatile Corrosion Inhibitors (VCI). They are similar to contact inhibitors, but they also have the property of slowly evaporating. The vapors then remain attached to the metal surface where the inhibitor did not reach. This method is used where the products are packed in a closed package. Inhibition of organically soluble corrosion inhibitors. These agents are easy to apply, the size of the deposit can be controlled by viscosity. After application, the solvent evaporates, leaving only a microfilm on the surface of the product, which has hydrophobic properties. A disadvantage may be the flammability of the vapors. An example of such a product is the washing, cleaning and preservative liquid KORING 141 (for ferrous metals) or KORING 145 (for ferrous and non-ferrous metals). It is a solution of corrosion inhibitors in organic solvents. The liquid combines several processes. It degreases the product and cleans it of chips and mechanical dirt stuck to the surface. Its viscosity can be adjusted within a certain range to the customer's requirements. Thanks to the composition of corrosion inhibitors, pitting corrosion is simultaneously removed in many cases. After the solvent has dried, the surface remains dry and at the same time protected against corrosion. For most final surface treatments, it is no longer necessary to remove the inhibitor from the product. It has excellent compatibility with paint systems and other methods of corrosion protection. When using some preservative oils, it is only necessary to perform a preliminary test to see if the additives contained in them somehow react with the inhibitor. But since this liquid replaces the use of preservative oils, their use is only relevant in tropical climate conditions or sea transport, assuming long-term contact of the product with water or salt water. When using the product, make sure the workplace is well ventilated both for work safety and fire protection reasons, as this liquid is class 3 flammable. Peter Stuchlik MSc, PhD., CTex ATI

PROTECTION OF GAUGES AGAINST CORROSION Ing. Peter Stuchlík, CSc., CTex ATI KORCHEM s.r.o Introduction All materials are subject to chemical, biological and energetic processes. If the physical, chemical and mechanical properties of the product change during these processes, we speak of corrosion in the case of metals, degradation in the case of plastics, corrosion and degradation in the case of glass and ceramics, etc. The common denominator of these processes in most cases is irreversible changes, which lead to such a change in properties that the object loses its functionality. Since there are a large number of processes mentioned, this lecture will be significantly limited to length gauges and their construction materials. At the same time, it will also provide practical advice on how to prevent destructive processes. It is always better and cheaper to prevent problems than to eliminate their consequences. Materials For length gauges, we can most often encounter the following construction materials: metals (Fe alloys, Al alloys, Cu alloys, Zn, Au, Pt alloys, semiconductors), polymers (varnishes, paints, PE, PTF, PAD, ABS, PS, PC, PVC, rubber), ceramics (porcelain, glass, enamel) For metals, in terms of their corrosion damage, the most important properties are their conductivity, crystal lattice and electrode potential. In terms of resistance to atmospheric or electrolytic corrosion, the listed metals can be ranked as follows: Fe, Cu, Zn, Al, Au, Pt. Depending on the corrosion environment present, especially the type of electrolyte, some elements may be interchanged. The corrosion of metals is influenced not only by the environment, but also by the alloying elements of the alloys, the contact of two metals of different potential, the contact of two differently treated surfaces, and the shape solution. The environment from which they were applied (made), their resistance to UV or other high-energy radiation, and their resistance to water and solvents is decisive for polymers. Varnishes and paints are usually applied in the form of solutions, suspensions or emulsions. Some polymers are produced from solution or are produced by emulsion polymerization. In these cases, the environment from which the polymer was applied or produced can damage it. Every polymer is sensitive to high-energy radiation, when the polymer chain breaks, i.e. degradation leading to a change in physical-mechanical properties. In practice, this is most often the effect of UV radiation. This problem is solved with polymers using UV stabilizers, so it is not possible to present an unambiguous range of their resistance. Some polymers hydrolyze in water (e.g. PAD) or swell (e.g. lacquers and paints containing cellulose derivatives). A number of organic solvents attack polymers. There are also many ceramic materials, but in general they can be said to have a high resistance to chemical degradation processes, while their resistance decreases with the amount of admixture of alkali metal compounds that serve as fluxes. These reduce the resistance, especially in the case of the action of the electrolyte. Types of degradation and corrosion events For metals, where we talk about corrosion, from a chemical point of view, it is chemical corrosion in an electrically non-conductive environment. That is, the reaction between the metal and the reducing or oxidizing gas at the phase interface. They mostly involve reactions with oxidizing gases such as sulfur dioxide (SO2), sulfur dioxide (SO3), ozone (O3), atomic oxygen (O-), nitrogen oxides (NOx), carbon dioxide (CO3), hydrogen chloride (HCl) , halogens. But the case of hydrogen depolarization using (H+), i.e. corrosion in reducing gases, is quite common. Furthermore, chemical corrosion occurs in an electrically conductive environment, in the electrolyte, i.e. electrochemical corrosion. I.e. where a conductive cell is formed and the metal(s) form the anode and cathode. Corrosion caused by processes other than chemical or electrochemical also belongs to this division. This is usually cavitation and corrosion caused by microorganisms. Due to the fact that for iron alloys, corrosion in an electrically non-conductive environment is minimal up to 580°C, electrochemical corrosion has a decisive influence. Its speeds are up to two orders of magnitude higher than in a non-conductive environment. According to the method of attack on the material, we distinguish uniform, uneven, point, pitting, lamellar, intercrystalline, transcrystalline and selective corrosion. According to the phase interface, it is divided into solid phase (metal) with gas, solid phase/liquid, solid phase/solid phase (contact corrosion). Polymers degrade due to temperature. In thermoplastics, at the glass transition point and simultaneous mechanical stress, deformations and changes take place in amorphous regions without chemical changes. In the region around the melting point, the supramolecular structure of the polymers changes and the mechanical properties are lost. By further increasing the temperature, thermal decomposition is achieved. Thermosets go directly into this decomposition process. Most polymers were formed by a radical polymerization reaction and are therefore sensitive to radical depolymerization reactions. The initiators of these reactions can be radicals of sulfur oxides, nitrogen, ozone, atomic oxygen, or high-energy radiation, which creates a radical directly in the polymer by breaking the chain. An unpleasant feature of radical reactions is that the free radical has the ability to travel along the chain of the macromolecule to an energetically weaker place, cleave the chain there, releasing another radical that travels on. Therefore, despite the fact that radical reactions at the polymer/gas phase interface take place only in the molecular layer, or by the effect of UV usually only to a depth of 350 nm, by traveling along the chain they are able to disrupt the polymer in the entire volume. Some microorganisms also cause degradation of polymers. A number of microorganisms produce enzymes that have the ability to split macromolecules into simple substances (usually carbohydrates or acids), which then serve as food for the microorganism. Stopping the enzymatic reaction is very problematic. Polymers that have an amine, alcohol, or ethoxy group are susceptible to hydrolytic degradation by water. These reactions can be quite fast if an electrolyte is present. For example, PAD degrades easily in an acidic environment. At the same time, in a number of polymers, the residual monomer is washed out, which is then replaced by the degradation of part of the polymer, until equilibrium is established. If this process continues repeatedly, the length of the polymer chain can be reduced to such an extent that the product loses its mechanical properties. Solvents do not have a direct effect on changing the chemical composition of the polymer, but they can either dissolve it (for example, PS and a number of colors in ketones or acetates) or they can wash out auxiliary substances in the polymer, such as plasticizers, UV stabilizers, etc. In this case, then polymer degradation by mechanical stress or high energy radiation. Ceramic materials, including glass, are very resistant to mechanical, microbiological and high-energy effects. Their attack occurs due to the content of K, Na, Mg and Ca salts. These salts are either present directly in the raw material itself, for example in porcelain, or are added during production to lower the melting point. They are used as fluxes, especially in the production of glass and ceramics. These salts are decomposed by aqueous solutions of strong acids and bases. As the reaction takes place in an aqueous environment, decomposition products are washed away and corrosion can then continue to the depth. Protection basics Protection against degradation and corrosion processes begins directly with the manufacturer. If the manufacturer of the device or device neglects something, then the user will hardly be able to correct such a mistake, and if corrosion develops, very often he does not have the means to remove the corrosion without impairing the function or accuracy of the meter. Therefore, it is very important to choose a meter already when purchasing it. Furthermore, the criteria for the correct selection according to the production procedures and materials used will be presented. The following applies to metals: Basic methods of corrosion protection. They are both physical and chemical. (When we talk about passivation, we are talking about chemical protection, where a chemical reaction occurs between the metal and the anti-corrosion agent. If the chemical reaction does not occur and the agent shifting the reaction balance is attached to the surface of the metal only by physical forces, we are talking about inhibition.) Physical ones work by creating an impermeable layer on the surface that resists the diffusion of electrolytes, oxidizing or reducing substances and has a hydrophobic character. plating Cr, Ni, Co, Au, Zn, etc., plastic coating, most often PVC, PP, PE, measures with protective coating, varnish, paint, by applying a hydrophobizing agent, oil, wax, silicone, fluorinated hydrocarbons, amines, etc. Chemical methods work on the basis of a chemically bound impermeable layer on the surface of the metal, which either transforms the metal into another corrosion-resistant compound, or using a chemical redox reaction prevents the transfer of the corrosion ion to the metal, or acts as a free radical scavenger, or acts as a cathode or anode . passivation by oxidation to Fe 3 O 4 , blackening, passivation with organic salts, oxalate, citrate, tannate, chelate, etc., passivation with inorganic salts, chromating, phosphating, inhibition, e.g. amines, inhibition using free radical scavengers. Electrochemical methods of protection are based on the creation of a sacrificial anode or the connection of a passive or active cathode. A combination of several principles. If Fe alloy is used for the bearing part of the measuring device, it is most suitable to be made of stainless steel. However, rust-free stainless steel does not exist, so even this material requires maintenance. If carbon steel is used, it is best to protect it by chrome plating, or if the part is not mechanically stressed, then by PE coating. Conversion layers (such as blackening and phosphating) do not have sufficient resistance in wet environments. Paints and varnishes have limited mechanical resistance and a shorter lifespan against UV. Furthermore, parts of measuring devices are usually made of Al alloys. The best corrosion resistance in this case is achieved by anodizing. If a Cu or Zn alloy is used in the meter, and it is not possible to protect it in the factory by gold plating, chrome plating, application of plastic or at least paint, corrosion cannot be prevented. It can only be kept at an acceptable level with certain rules and maintenance procedures. However, metals are also found in the electrical components of the electronic components of the meters. There are two rules here. Parts exposed to the atmosphere should be gold plated. The electronic part of the given device should have its own dustproof and waterproof case. Dust forms condensation and corrosion nuclei, and electrochemical corrosion is the most progressive. A high-quality device can also be recognized by the fact that it contains a minimal combination of different metals, especially that there are no combinations of Fe with Cu or Cu with Al. And if it is necessary to use several types of metals, they are non-conductively separated. An overlooked but nevertheless important element of corrosion protection is the shape solution. Sharp edges increase corrosion attack, while rounded corners reduce it. While notched surfaces hold up well, corrosive chemicals, including sweat, cling to them and moisture condenses well. With polymers, the situation is somewhat more complicated. Each of them has its own specific characteristics. Therefore, attention will be paid to individual most common polymers separately. Varnishes and paints. So-called powder paints have the greatest resistance to chemical influences. This is because they do not use any solvent. Other solution paints are sensitive to organic solvents, especially esters and ketones. From the point of view of water resistance, the most suitable two-component systems are PES or epoxies. The least durable are single-component systems that contain cellulose derivatives as a film-forming component. Disperse paints and varnishes have little resistance to water if they are not cross-linked. PUR and PA have little UV resistance. Inks applied by pad printing are generally soluble in a wide range of organic solvents, including alcohols. Since, with the exception of powder paints, it is impossible to tell what color was used and the manufacturer of the device does not indicate this, it is best if there is no color on the device (if its construction and the materials used allow it). This eliminates the problems of how to protect and renovate the paint or varnish surface. PE (Polyethylene) It belongs to the group of polyolefins, but unlike PP, it has excellent UV resistance. In terms of resistance to the effects of water and other chemicals, it is one of the most resistant polymers. It is worse with its resistance to abrasion. A whole range of polyethylenes is produced, which differs not only in the degree of polymerization, but also in structure. Therefore, the basic types are designated: LDPE (low density, branched), HDPE (high density, linear), UHDPE (very high density, linear). PTF (polytetrafluoroethylene, Teflon) It has the lowest friction of all known substances, high resistance to water and good resistance to most chemicals. Not resistant to halogenated solvents and UV. It is less resistant to abrasion. PAD (polyamide) Due to its good mechanical properties, it is used for gears and sliding bearings. There are several basic types of PAD 6 (Silon), PAD 6.6 (Nylon), aramid (Kevlar). For all types, they have a worse resistance to water, which dissolves residual monomer from them, and to acidic solutions, which can decompose them when heated. ABS (Acrylobutadiene Styrene) It is a terpolymer with excellent mechanical properties. It is less resistant to halogenated solvents, esters and ketones. Its UV resistance is also weaker. PS (polystyrene) It is cheaper than ABS, so it is used as its cheap replacement. It also has worse mechanical properties, above all it is more brittle. Its resistance to solvents, aging and UV is minimal. If possible, avoid equipment where this polymer is used. PC (polycarbonate) Due to its good mechanical properties, it is used for transparent covers. However, it is sensitive to a wide range of chemicals, water detergents and UV. PVC (polyvinyl chloride) A polymer with excellent resistance to chemicals, but softens at 40°C, becomes brittle at low temperatures and loses its shape properties around 80°C. Rubber Since rubber includes dozens of different polymers, with different fillers and possibly cross-linking, it is impossible to find common characteristics. In any case, care should be taken with organic solvents. Ceramic materials There is also a wide range of them, but one can generalize about their excellent resistance to water, chemicals and weathering. Porcelain is attacked by strong acids, glass and glazes by strong hydroxides, few ceramics are resistant to hydrofluoric or fluorosilicic acid. Procedures and means There is no device that will work forever and that does not require a certain amount of care and maintenance. Next, the most basic principles and procedures will be presented. To assess the suitability of chemical agents for the treatment and maintenance of devices, a so-called safety data sheet can be used, which by law every agent must have, and which the manufacturer or supplier is obliged to supply upon request. It states which main hazardous substances the product consists of, what the risks are to personnel, the environment, etc. However, not all manufacturers (suppliers) provide complete and true information. Safety data sheets from German manufacturers tend to have the most shortcomings. The most common enemy of structural materials is water. In addition, with regard to the surrounding conditions, it is in the form of an electrolyte. Water gets on the devices either as rain, or by condensation of atmospheric moisture, such as dew, from detergents, or from fingers. In the event of rain, the measuring devices should be designed so that water does not get into the interior, where a condensation chamber would form. The surface of the device is dried as soon as possible by wiping with an absorbent material. Condensed moisture is a bigger problem. The devices should be handled in such a way that there are no sudden changes in temperature, especially if the two environments have different relative humidity. If this cannot be avoided, it is advisable to have a storage cabinet or case for the devices, in which you can put a desiccant (preferably a bag with Silikagel, which, however, needs to be replaced, because it absorbs moisture from the air) and if the device contains materials such as metals , as well as plastics that are sensitive to water, as well as a bag with a vapor corrosion inhibitor (VCI, which lasts up to 10 years). However, it is necessary to consider which preservation system will be used for the given device. In the event that "oil" protective agents are used, the VCI becomes irrelevant because the vapors of the inhibitor through the "oil film" do not reach the surface of the material and sometimes these chemicals can fight with each other. In general, it is recommended to minimize the use of aqueous detergents. Not only are they electrolytes, but few of them contain anti-corrosion additives, but they usually contain chemicals that are dangerous to a variety of construction materials. However, every finger touch poses a corrosion or degradation risk. Fingerprints should be removed from the meters as soon as possible. In addition, the composition of each person's sweat is different and changes. Another common enemy is dust and possibly other pollution. It can act as condensation nuclei, it can have a direct chemical effect, or it can act as a breeding ground for microorganisms. It follows that it is necessary to clean the gauges after use. Dry absorbent materials (cloths) are not able to remove most dirt and present a risk of abrasion. Therefore, it is advisable to use auxiliary washing liquids. If possible without water, and which have the ability to displace water. Combinations of polar and non-polar solvents are best because they will remove a wide variety of contaminants. Solvents containing halogenated hydrocarbons, ketones and esters are not suitable, as they corrode metals and degrade a range of polymers. The most suitable are agents containing desulfurized alkanes and pure alcohols. As an example, I can mention KORING 792-10, which is a mixture of heptanes and isopropanol. If the device contains polymers sensitive to alcohols (this is usually some rubber or PC), it is necessary to use only alkane agents, even if they have a smaller spectrum of effectiveness. For example KORING 702 (slower drying) or KORING 792-4 (fast drying). These products degrease at the same time. Considering that simultaneously with the removal of surface impurities, preservatives are also washed away, if they were applied to the surface before that, it is necessary to renew the protective coating or lubricant. To protect metals, it is necessary to decide in advance which of the paths will be used. Metals can be protected with vapor corrosion inhibitors (VCI) or with "oil" preservatives. Combining both methods is not recommended and is sometimes risky. The advantage of most preservatives is that they work on both metals and polymers. Vapor inhibitors are not resistant to liquid water. However, they hardly affect the accuracy (up to nm) and electrical conductivity. The so-called "Oil" preservatives are well resistant to water, but affect accuracy. The protective film of the inhibitor is usually 2-4 m for evaporating, drying "oils", 8-20 m for classic, non-drying "oils", and over 40 m for petroleum jelly. It is also important to note that these corrosion inhibitors are effective dielectrics, so they electrically insulate. Preservative "oils" can also attack some types of rubber, PS and PC. All non-drying "oils" have the unpleasant property of picking up dust and dirt. From both groups, you can use, for example: vapor corrosion inhibitor for non-ferrous and ferrous metals KORING 505, vapor-water soluble inhibitor KORING 555, washing and preservative drying agent KORING 145-K for non-ferrous and ferrous metals, or preservation oil for non-ferrous and ferrous metals KORING 205 . If it is a matter of lubrication of non-corroding parts or equipment with a small risk of corrosion, the most appropriate solution is to use silicone oil. It displaces water and creates a strongly repellent surface for a wide range of dirt. The disadvantage is that where it has been used, it will almost no longer stick to anything (it will not have adhesion). The advantage is that it is produced in a wide range of different viscosities, so with low viscosities it can literally be poured into the intercrystalline spaces of a metal or polymer material, so that the moving part is lubricated, but almost nothing remains on the surface. Or you can use high viscosities where it works like petroleum jelly. The viscosity is simply chosen according to need. In the case of gauges where it has been tested that the agent does not attack the rubber, and where the reduction of electrical conductivity does not play a role, agents of the CLP type (Claening, Lubricating, Protection/Preservation) can be used. These agents clean and wash excellently in one step, at the same time neutralize corrosion reactions, preserve for a long time and provide thixotropic lubrication. One of the best preparations in the world is the CLP Professional from CX Dynamics, originally developed for weapons. Finally, a few notes on generally used means and procedures. Very often pure petrochemical products are used for gauges. This has its justification in healthcare products, but it is a mistake with meters. From the very nature of the occurrence of corrosion events, if two differently treated surfaces come into contact in an electrically conductive environment, a galvanic cell is formed. In addition, so-called mineral oils and petroleum jelly are able to bind 4-8% of air moisture, so they are partially electrically conductive. Therefore, it is necessary to use lubricants that contain corrosion inhibitors. Technical gasoline is a relatively health-hazardous mixture of hydrocarbons, which has very little degreasing and washing power. It is always better to use alkane desulfurized and dearomatized agents. Kerosene lubricates poorly rather than degreasing. In addition, it is also a rather "dirty" mixture. The fact that some device manufacturers recommend it is an alibi to avoid the risk of polymer damage. There are suitable remedies for everything, so there is no reason to use kerosene. The vast majority of ethanols (commonly known as alcohol) are not pure, they are already contaminated during production and denatured. Only ethanol marked pa and partially even pharmaceutical grade can be considered pure. However, the washing ability of ethanol is much lower than that of isopropanol. Therefore, it is better to use isopropanol or special products that contain it. Preservatives with the longest possible shelf life are always chosen. If, for some reason, it is necessary to use a product with a short protection period, or the protective effect is canceled by an external influence (washing, etc.), it is necessary to modify the instructions for using the device so that it corresponds to the given case. The transmission mechanisms of the gauges do not need to be lubricated if they are made of some sliding polymer, e.g. PAD. In other cases it is necessary. Either a suitable silicone oil, or an evaporative drying preservative, or a water-soluble VCI. If washing or de-preserving is carried out, it is desirable to carry out preservative treatment as soon as possible. In particular, corrosion processes in a conductive environment are very fast, taking seconds.