Why does iron rust?

Galvanization rusts, but you can fight it!

Galvanizing iron using hot or cold galvanizing is considered a panacea for rust for at least 1015 years. In practice this does not always happen. A similar problem is faced, in particular, by motorists and operators of structures operating in wet, chemically aggressive environments. Why does galvanized steel rust?

Some reasons for the lack of durability of galvanized coatings

If we exclude from consideration the poor quality of the process itself (most often, corrosion of galvanizing is characteristic only when galvanizing is not carried out under specialized conditions), then the presence of rusty galvanizing is determined by several factors.

Contact medium

Hot-dip galvanized steel has an adverse effect, in particular, on the soil where the structure is placed. Since there are over 200 different types of soil actually found in nature, the effectiveness of hot-dip galvanizing in soil varies and is difficult to predict.

What corrodes galvanizing in the soil? The main factors determining the corrosiveness of soil are its moisture, pH level and the presence of chlorides. Additional characteristics must also be taken into account:

• Degree of soil aeration;
• Range of daily temperature fluctuations;
• Electrical resistivity;
• Texture on the size of soil particles.

It has been practically established that the protection of galvanized steel against corrosion is more effective on brown sandy soils and does not work so well on gray, clayey soils. This is due to the fact that soil with larger particles removes moisture from the surface faster. Therefore, the galvanized part is exposed to less moisture, which provokes the development of electrochemical corrosion.

The first step to assessing the durability performance of galvanized steel in soil is to classify the soil in the area where galvanized iron structures are used.

The rate of corrosion of steel in soil can range from less than 0.2 microns per year in favorable conditions, to 20 microns per year or more in very aggressive soils.

Thus, highly corrosive soils will dictate the need for a robust corrosion protection system, such as longer-lasting hot-dip galvanizing, to provide long-term protection.

Because soil changes even over a small area and soil corrosivity can vary greatly, incorrect soil classification often leads to unsatisfactory results.

Impact impacts

After rolling, the adhesion strength of the galvanized layer to the base metal is checked according to GOST R 52246-2004. Galvanic protection can be carried out in two ways - hot galvanizing or applying an iron-zinc coating. In this case, the thickness of galvanization is determined by the class of coating. There are four of them:

• Galvanizing of kitchenware products (coating thickness - at least 70 microns);
• Increased quality (coating thickness 4060 microns);
• Class 1 coating (coating thickness 1840 microns);
• Class 2 coating (coating thickness from 10 microns).

The strength of the zinc coating is directly assessed by the results of technological bending tests regulated by GOST 14019-2003. In this case, only the change in the shape of the tested samples is normalized, but not the rate of application of the deforming force.

Meanwhile, it is known that the impact nature of the interaction reduces the adhesion strength of surface coatings by 30-35%.

Thus, if a part is periodically exposed to mechanical impacts from solid particles (for a car, for example, small stones or gravel), then galvanized iron rusts due to the appearance of cracks and scratches at the points of contact.

Can galvanizing rust on its own?

Perhaps the main provocateur of the process is moisture. Any galvanized coating in water behaves completely differently than we would like.

As is known, a metal that only periodically comes into contact with water (which almost always has a fairly high acid potential) is actually a macrobattery, the electrodes of which have a certain potential difference.

If there is enough zinc in the layer, then the so-called sacrificial protection is triggered, as a result of which zinc, and not iron, is exposed to corrosion.

But, if the surface layer is damaged, then the galvanized iron rusts, especially if the galvanized surface is wet after the process begins.

When metal comes into contact with water containing dissolved salts, corrosion intensifies. The resulting iron oxide flakes off the metal surface and is exposed to fresh iron molecules, which continue the rusting process. Ultimately, large oxidized areas appear, which cause destruction of the entire metal structure of the part.

White corrosion and how to deal with it

The corrosion process of galvanized iron ends with the formation of white rust on the galvanized surface. It is a white chalky substance that forms zinc when exposed to hydrogen (from water) and oxygen (from air). As a result of this reaction, zinc hydroxide is obtained.

White rust on galvanized steel is typical for new material. This is due to the fact that such a coating has not yet had the opportunity to form stable oxides, so hydrogen and oxygen bind to pure zinc. White rust often appears on galvanized sheets during storage, as condensation can get into the gap between the individual sheets.

In most cases, white corrosion renders the protective coating useless. Unlike stable zinc oxides, zinc hydroxides do not adhere well to other materials. White rust is also unattractive from a visual point of view.

There are several ways to prevent white rust:

1. Eliminate exposure to water.
2. Eliminate condensation by allowing the zinc to form stable oxides.
3. Use passivating chemicals or oils.

In the first case, it is necessary to improve the circular airflow of the product. Reasonably increasing the coating temperature or decreasing the relative humidity level is also effective.

Galvanizing is also corroded by prolonged exposure of the structure to a warm environment, since this intensifies the formation of condensate and, accordingly, zinc hydroxide.

Another method of preventing white rust is to form stable oxides on the surface of the zinc. They will prevent the formation of white rust. To do this, allow the coating to remain in an environment with low humidity for some time.

Increasing carbon dioxide in contact with the coating will also accelerate the formation of stable zinc oxides.

Removing rust from galvanized steel

The procedure begins with cleaning the already noticed areas of corrosion. An effective method is to subsequently coat the galvanized surface with a passivating chemical or oil.

In the first case, oxidation is prevented (albeit for a short time), and in the second, a protective barrier is created between the zinc and water, which prevents the formation of a layer of zinc hydroxide.

Most of the oils used, however, evaporate after a short period of time, so periodic renewal of this protective coating is required.

Previously, to prevent corrosion of galvanized iron, compositions based on hexavalent chromium compounds were used, but now such substances are recognized as toxic and are used extremely rarely.

How to treat galvanized steel against rust?

Special rust converters are used. In order to remove rust from galvanized steel, first clean the surface, then dry it thoroughly, and then, strictly following the manufacturer's instructions, apply a protective agent. Combining several compounds is undesirable, since they will require different application technologies, as a result of which it is possible to remove rust from galvanized steel.

Source: http://www.hugebuilding.ru/rzhaveet-li-ocinkovka.html

Rust on metal: harm, types of corrosion

We, sellers of rolled metal, are faced with this obsession - rust, like no one else. And we know for sure the harm caused by corrosion. In this article we will say a few words about this problem, its manifestations, its scale.

Damage, damage

Everyone has seen those orange-brown or yellowish rust spots on metal parts. The economic damage from metal corrosion is enormous. In the USA and Germany, the estimated damage from corrosion and the costs of combating it amount to approximately 3% of GDP. At the same time, metal losses, including due to the failure of structures, products, and equipment, amount to up to 20% of the total steel production per year. For Russia, exact data on losses from corrosion have not been calculated.

It is known for certain that it was rusted metal structures that caused the collapse of several bridges in the United States, including with numerous casualties. Environmental damage is also extremely unpleasant: leakage of gas and oil due to the destruction of pipelines leads to environmental pollution.

Types of corrosion and its causes

Before talking about rust on iron, let's briefly look at its other types.

Not only metals, but also non-metallic products are susceptible to corrosion. In this case, corrosion is also called “aging”. Plastics, rubbers and other substances are subject to aging. For concrete and reinforced concrete there is a term "fatigue". They are destroyed or their performance characteristics deteriorate due to chemical and physical exposure to the environment.

Metal alloys - copper, aluminum, zinc - also corrode: during their corrosion, an oxide film is formed on the surface of the product, tightly adhering to the surface, which significantly slows down the further destruction of the metal (and the patina on copper also gives it a special charm). Precious metals are such not only because of their beauty, valued by jewelers, but also because of their resistance to corrosion.

Gold and silver are still used to coat particularly sensitive electronic contacts, and platinum is used in the space industry.

The metal can corrode in some areas of the surface (local corrosion), cover the entire surface (uniform corrosion), or destroy the metal along the grain boundaries (intercrystalline corrosion). Corrosion accelerates noticeably with increasing temperature.

Types of Rust

Iron is more susceptible to corrosion. From a chemical point of view, rust is an oxidative process (like combustion). Elements resulting from oxidation in an oxygen environment are called Oxides. There are 4 main types.

1. Yellow rust - chemical formula FeO(OH)H2O (ferrous oxide). Occurs in a humid, oxygen-poor environment. Often found underwater. In nature, it exists in the form of the mineral wustite, while being a monoxide (it contains 1 oxygen atom).

2. Brown rust - Fe2O3 (double iron oxide): grows without water and is rare.

3. Black rust - Fe3O4 (tetravalent iron oxide). Formed with low oxygen content and without water, it is therefore stable and spreads very slowly. This oxide is ferromagnetic (under certain conditions it is magnetized in the absence of an external magnetic field), therefore it is potentially applicable for the creation of superconductors.

4. Red rust - chemical formula Fe2O3•H2O (ferric oxide). Occurs under the influence of oxygen and water, the most common type, the process proceeds evenly and affects the entire surface.

Unlike all of the above types of oxidation, which are not so dangerous for iron, this one forms iron hydroxide in its thickness, which, when it begins to peel off, opens up more and more layers of metal for destruction. The reaction can continue until the structure is completely destroyed.

 It is used in iron smelting and as a dye in the food industry. It occurs naturally in nature under the name hematid.

Several types of rusting can occur simultaneously without particularly interfering with each other.

Chemical and electrochemical corrosion

Iron rusts if it contains additives and impurities (such as carbon) and comes into contact with water and oxygen. If salt (sodium and potassium chloride) is dissolved in water, the reaction becomes electrochemical and the rusting process accelerates.

The massive use of these salts both in household chemicals and for combating ice and snow makes electrochemical corrosion a very common and dangerous phenomenon: losses in the United States from the use of salts in winter amount to $2.5 billion.

When exposed to water and oxygen simultaneously, iron hydroxide is formed, which, unlike oxide, peels off from the metal and does not protect it in any way. The reaction continues either until the iron is completely destroyed or until the system runs out of water or oxygen.

Electrochemical corrosion can be caused by stray currents that occur when part of the current leaks from an electrical circuit into aqueous solutions or into the soil and from there into a metal structure.

In those places where stray currents exit metal structures back into water or soil, metal destruction occurs. Stray currents occur especially often in places where ground electric transport moves (for example, trams and electric railway locomotives).

In just one year, stray currents with a force of 1A are capable of dissolving 9.1 kg of iron, 10.7 kg of zinc, and 33.4 kg of lead.

In the second part of the article we will tell you how you can protect your metal structures from this scourge or defeat it if it is already attacking.

Source: https://www.1metallobaza.ru/blog/kak-my-stradaem-ot-rzhavchiny

Has rust eaten the iron?

A dangerous enemy is rust! No matter how strong the metal, rust will still overcome it. Listen to a story about this. In ancient times, one unlucky king ordered a lot of various weapons to be hidden in reserve in the damp basements of the fortress: steel swords, guns, cannons, cannonballs. Only he didn’t order the gunpowder to be put there so it wouldn’t get damp. But with iron, they say, nothing will happen. Fortunately, there was no war for a long time, and the weapons lay in the basement for many years.

The king got ready for war and ordered the young recruits to be armed. They opened the heavy doors, took out the battle swords from the basement - they looked, and they were all rusty. We started cleaning - the swords became thinner than kitchen knives. Where are these good for? They took out guns - they were also rusty.

If you shoot one of these, it will explode in your hands. It's time for the guns. With kernels. They began to remove the rust from them. They cleaned it so much that the kernels the size of watermelons became smaller than potatoes. How to load such guns? The guns are too big for them now.

I had to cancel the trip! Dampness and moisture let us down.

And this story happened recently. The tractor was walking on the ice and landed in a snow-covered wormwood. The tractor driver was saved, but the tractor sank. Only a year later they managed to lift the heavy car. It took me a long time to clean the rust off, but I still couldn’t start the engine until many of its parts that had rusted in the water were replaced with new ones.

Where else does iron rust?

If only iron would rust in water! But metal rusts even in the hot desert. All around, no matter how hard you search, you won’t find a drop of water. But there are always tiny, completely unnoticeable particles of moisture in the air. And this little bit is enough for the metal to gradually begin to rust. And in a damp climate, it, of course, breaks down much faster.

How much iron does rust destroy? The answer is ready. In ten years, rust eats up as much metal as all the metallurgical plants in the world produce in a year. It turns out that rust eats millions of tons of metal! People have long declared war on it! How do you escape the rain? That's right, put on rubber boots and raincoats, or better yet, hide under the roof. They do the same with metal. Cars and machine tools are hidden under sheds and under the roofs of workshops.

Rust and protection of metal from corrosion

They lay a gas pipeline, an oil pipeline, a water supply system - they put a waterproof raincoat on the pipes - they wrap them in tarred cloth or paper.

What about cars and ships? They are painted with elegant, bright colors not only for beauty. Although the layer of paint is thin, it protects well from dampness, and therefore from rust. Bridges, carriages, ships, and roofs are also painted for this purpose.

But not only paint can protect metal; iron can be coated with a thin layer of another, more resistant metal - zinc. And the roof immediately becomes more durable. Tin cans are also iron - tin. Here a thin layer of molten tin is applied to the iron.

There are many other ways to protect metal from rust, and scientists are looking for new, more reliable ones.

Source: https://partnerkis.ru/pochemu-rzhavchina-sela-perochinnyiy-nozhik/

Why metal rusts - Metalist's Handbook

Let's remember where iron or, for example, aluminum comes from. That's right, they are smelted from ore - iron, manganese, magnesium, aluminum, etc. Metals in ores are contained mainly in the form of oxides, hydroxides, carbonates, sulfides, that is, in the form of chemical compounds with oxygen, water, sulfur, etc.

In nature, in a metallic, or free, state, you can mainly find only gold, platinum, and sometimes silver. These metals are stable, that is, they do not tend (or weakly tend) to form chemical compounds. Probably for this reason they received the name noble.

As for the vast majority of metals, in order for them to be in a free state, they must be restored from natural ore compounds, that is, smelted. It turns out that by smelting metal, we transfer it from a stable state to an unstable one.

So it tends to return to its original state - to oxidize. This is corrosion - a natural process of destruction for metals when interacting with the environment. A special case of corrosion is rusting - the formation of iron hydroxide Fe(OH)3 on iron.

This process can only occur in the presence of moisture (water or water vapor).

But why then don’t bridges collapse overnight, planes and cars don’t instantly fall apart? And pots and pans do not turn into red, black or gray powder before our eyes.

Fortunately, metal oxidation reactions do not occur so rapidly. Like any process, they proceed at a certain speed, sometimes very small. Moreover, there are many ways to slow down corrosion.

friend's shoulder

Have you noticed that there is no rust on stainless steel, although its basis is the same iron, which, when oxidized (in the presence of water or water vapor), turns into red hairy hydroxide. There is a trick here: stainless steel is an alloy of iron with other metals. The introduction of elements into metal alloys to give them certain properties is called alloying.

The main alloying element that is added to regular (carbon) steel to make stainless steel is chromium. This metal also tends to oxidize, which it successfully does much more readily and quickly than iron itself.

In this case, a film of chromium oxide quickly forms on the surface of stainless steel.

Unlike loose rust, compact dark chromium oxide does not allow aggressive environmental ions to penetrate the metal surface, that is, the oxide simply covers the metal and the corrosion process stops.

Such oxide films are called protective films. Stainless steels must contain a strictly defined amount of chromium, but not less than 13%. In addition to chromium, nickel, molybdenum, niobium and titanium are often added to stainless steels.

Thanks to protective films, many metals withstand exposure to various environments well. Let's take, for example, an aluminum saucepan in which milk is boiled or semolina is cooked.

Typically, such a saucepan does not shine like chrome or stainless steel, and has a slightly whitish color.

The fact is that on aluminum, like on other metals, a whitish oxide film (aluminum oxide) always forms in air, which perfectly protects the metal from corrosion.

Such films are called passive, and the metals on which they spontaneously form are called passivating. If you clean an aluminum pan with a wire brush, the coating will disappear and a metallic sheen will appear. But very quickly the surface will again be covered with a film of aluminum oxide and become whitish.

Taming the active

The metal can be forced into a passive state.

For example, iron, in addition to unprotective iron hydroxide or lower oxides (nitrous oxide and nitrous oxide), under certain conditions forms a higher oxide - iron oxide (Fe2O3).

This oxide protects the metal and its alloys well at high temperatures in air; it (one of its forms) is “responsible,” according to experts, for the passive state of iron alloys in many aquatic environments.

The stability of stainless steel in strong sulfuric acid is associated precisely with the passivation of steel in this very aggressive environment.

If you place stainless steel in a weak solution of sulfuric acid, the steel will begin to corrode.

The paradox is explained simply: strong sulfuric acid has strong oxidizing properties, due to which a passivating film is formed on the surface of stainless steel, but not in a weak acid.

In cases where the aggressive environment is not “oxidizing” enough, special chemical additives are used to help form a passive film on the metal surface. Such additives are called corrosion inhibitors or retarders.

Not all metals are capable of forming passive films, even by force. In this case, the addition of an inhibitor to an aggressive environment, on the contrary, keeps the metal in “reducing” conditions, in which its oxidation is suppressed (it is energetically unfavorable).

Sacrifice

It is possible to artificially maintain the metal in “reducing” conditions in another way, because it is not always possible to add an inhibitor. Take, for example, an ordinary galvanized bucket.

It is made of carbon steel and coated with a layer of zinc on top. Zinc is a more active metal than iron, which means it is more willing to enter into chemical reactions.

Therefore, zinc not only mechanically isolates a steel bucket from the environment, but also “takes the fire upon itself,” that is, it corrodes instead of iron.

For underground communications, “reducing” conditions are created using electrochemical protection: a negative (cathode) potential is applied to the protected metal from an external current source, so that the oxidation process on the metal stops.

However, why do we need so many different complex methods of protecting metals? Can't you just paint the metal or enamel it?

First of all, it’s impossible to paint everything. And secondly, let’s take an enamel pan or a car as an example.

If the pan, breaking out of your hands, falls to the floor with a roar and knocks off its enamel barrel, then under the broken enamel there will be a “black eye”, the edges of which will gradually turn a telltale red color - the chip will be covered with rust.

A car will not have the best fate if suddenly a small hole in the varnish layer appears in its varnished side (or more often at the junction with the bottom).

This channel for the entry of aggressive agents into the body - water, air oxygen, sulfur compounds, salt - will immediately start working, and the body will begin to rust. So car owners have to do additional anti-corrosion treatment.

Invisible Villain

So, maybe the problem of metal corrosion has been solved? Alas, it's not that simple. Any corrosion-resistant alloys are stable only in certain environments and conditions for which they are designed.

For example, most stainless steels withstand acids and alkalis very well and do not like chlorides, in which they are often subject to local types of corrosion - pitting, pitting and intergranular. These are very insidious corrosion damages.

A structure made of beautiful, shiny metal without a hint of rust may one day collapse or crumble. It's all about the smallest pinpoint, but very deep lesions.

Or in microcracks that are not visible to the eye on the surface, but literally penetrate the entire thickness of the metal.

No less dangerous for many alloys that are not subject to general corrosion is the so-called corrosion cracking, when a huge crack suddenly penetrates the structure. This happens with metals that experience long-term mechanical loads - in airplanes and helicopters, in various mechanisms and building structures.

  Manual sheet metal bending

Train crashes, plane crashes, bridge collapses, gas emissions and oil spills from pipelines - corrosion is often the cause of such disasters. To tame it, there is still a lot to be learned about the most complex natural processes occurring around us.

Source: https://ssk2121.com/pochemu-rzhaveet-metall/

Iron corrosion

The process of iron corrosion most often comes down to its oxidation by atmospheric oxygen or acids contained in solutions and its transformation into oxides. Corrosion of metals (rusting) is caused by redox reactions occurring at the interface between the metal and the environment. Depending on the mechanism of occurrence, there are such types of iron corrosion as: chemical, electrochemical and electrical.

Chemical corrosion process of iron

Redox reactions in this case proceed through the transfer of electrons to the oxidizing agent. During this type of corrosion process, atmospheric oxygen interacts with the surface of the iron. This creates an oxide film called rust:

3Fe + 2O2 = Fe3O4 (FeO•Fe2O3)

Unlike tight oxide films that form during corrosion on alkali metals, aluminum, and zinc, a loose oxide film on iron freely allows air oxygen, as well as other gases and water vapor, to pass to the metal surface. This promotes further corrosion of the iron.

Electrochemical corrosion process

This type of corrosion occurs in a medium that conducts electrical current. Metal in the ground is subjected primarily to electrochemical corrosion. This type of corrosion process is the result of chemical reactions involving environmental components. Also, electrochemical corrosion occurs in the event of contact between metals located in a series of voltages at some distance from each other, resulting in the formation of a galvanic cathode-anode pair.

The atmospheric and ground corrosion process is expressed by the scheme:

Fe + O2 + H2O → Fe2O3 xH2O

As a result, rust of various colors is formed, which is due to the fact that various iron oxides are formed. What kind of substance is formed during the process of iron corrosion depends on the oxygen pressure, air humidity, temperature, duration of the process, composition of the iron alloy, condition of the surface of the product, etc. The rate of destruction of different metals is different.

The process of metal corrosion in electrolyte solutions is the result of the work of a large number of microscopic galvanic cells, in which impurities in the metal act as a cathode, and the metal itself acts as an anode. As a result, microscopic galvanic elements appear.

Also, iron atoms in different areas have different abilities to donate electrons (oxidize). The areas of the metal on which this process occurs act as the anode. The remaining sections are cathode sections, where water and oxygen reduction processes occur:

H2O + 2e– = 2OH– + H2

O2 + 2H2O + 4e– = 4OH–

The result is that iron (II) hydroxide is formed from iron (II) ions and hydroxide ions. Next comes its oxidation to iron (III) hydroxide, the main component of rust:

Fe2+ ​​+ 2OH– = Fe(OH)2
Fe(OH)2 + O2 + H2O → Fe2O3 xH2O

In order for a galvanic cell to work, it is necessary to have two metals of different chemical reactivity and a medium that conducts electric current - an electrolyte.

When iron and another metal (for example, zinc) come into contact, the corrosion of the iron slows down, and the corrosion of the more active metal (zinc) accelerates. This is due to the fact that the flow of electrons goes from a more active metal (anode) to a less active metal (cathode).

Thus, when iron comes into contact with a less active metal, corrosion of the iron accelerates.

Electrical corrosion process

This type of destruction of metal underground structures, cables and structures can be caused by stray currents emanating from trams, subways, electric railways and various electrical installations with direct current.

The current from metal structures exits into the ground in the form of positive metal ions - electrolysis of the metal occurs. The area where currents exit is the anode zone. It is in them that active processes of electrical corrosion of iron occur. Stray currents can reach 300 A and operate within a radius of several tens of kilometers.

Stray currents emanating from alternating current sources cause mild corrosion of underground steel structures, and strong corrosion of structures made of non-ferrous metals. Protecting metal structures from corrosion is a very important task, as it causes huge losses.  

Source: https://notehspb.ru/o_korrozii/korroziia_zheleza

Rust removal

According to statistics, losses from rust (corrosion) amount to up to 12% of all metal produced. Operating conditions for metal products and structures are constantly becoming more stringent, including due to air pollution. Thus, emissions of sulfur or nitrogen oxides lead to the formation of microdroplets of sulfuric or nitric acids, and the rate of rust formation increases several times.

Rust - the chemical basis of the process

Iron is a reactive metal that, in the presence of water and oxygen, easily oxidizes, forming several compounds - oxides, hydroxides and their hydrates. Oddly enough, there is no exact formula for rust: depending on environmental conditions, the iron oxidation product has a variable composition: nFe(OH)3*mFe(OH)2*pH2O.

Rust occurs over the entire surface of the metal, but the most vulnerable places are welds, internal corners of structures, and holes for threaded connections. The structure of rust is very loose; there is practically no adhesion to metal.

Due to its high porosity, the rust layer easily retains atmospheric moisture, creating favorable conditions for further destruction of the metal.

The danger of the process is that it is not possible to visually assess the degree of damage to a metal structure: under the red-brown layer of rust, the metal can be completely destroyed. If measures are not taken in a timely manner, the result may be disastrous, including complete destruction of the product. It’s one thing if it’s a rusty nail in the wall of a country house, and quite another thing if the rust has affected a power line support or the hull of a marine vessel.

Methods for removing rust

Popular wisdom says that it is easier to prevent any problem than to later make heroic efforts to eliminate its consequences. Rust is no exception. Over the past 20-30 years, chemists and physicists have proposed many ways to prevent corrosion - from protective coatings to complex engineering structures - electrochemical protection stations.

If rust does appear, this is not a reason to give up: there are many effective ways to remove it, and the sooner active measures are taken, the greater the effect of their use will be. So, first things first.

Mechanical rust removal

Corrosion products have low adhesion and are therefore easily removed from the metal surface by mechanical action - for example, when treated with a metal brush.

You can remove rusty deposits from large products using a grinding machine, while following a simple rule: you need to start with coarse grains, and use the finest grains for finishing. Areas of metal from which rust has been removed are completely defenseless against atmospheric influences.

If they are not treated with anti-corrosion compounds that prevent contact with water and oxygen, the rusting process will only accelerate.  

Chemical methods for removing rust

Knowing the nature and chemical composition of rust, it is logical to assume that it can be removed with the help of acids. It is known from a school chemistry course that metal oxides and hydroxides easily interact with acids, resulting in the formation of iron salts and the corresponding acid, and water.

For example, when exposed to hydrochloric acid, the following reactions occur:

• 2Fe(OH)3 + 6HCL → 2FeCl3 + 6H2O
• Fe (OH)2 + 2HCL → FeCl2 +2H2O

The resulting ferric chloride is a water-soluble salt that must be removed from the surface of the workpiece by simply rinsing in water and then wiping the surface dry. There is no need to wait until rust begins to form again; the cleaned areas should be treated with protective compounds.

When treating with acids, there is a danger of dissolving the metal, since iron in the electrochemical voltage series comes before hydrogen, it reacts actively with many dilute acids:

For this reason, before doing chemical experiments at home, you need to read the relevant literature. A corrosion inhibitor, methenamine, will help eliminate the side effect; when adding only 1-2 g per liter of hydrochloric acid solution, the reaction with iron does not occur.

Rust converters

Liquid compounds based on phosphoric acid are an excellent way to prevent further rust formation on the surface of steel and iron products. With this processing method, only those areas of rust that are weakly adhered to the base are first removed.

The iron orthophosphate formed during the reaction creates a durable protective film through which moisture and oxygen do not penetrate, thereby preventing further corrosion of the metal.

To speed up the drying process, you can add 30-40 ml of isobutyl alcohol or 15 grams of tartaric acid per liter of 25% phosphoric acid solution.

Modern equipment for rust removal

Mechanical methods of cleaning surfaces from rust using improvised means are not always applicable; if the product has a complex shape, then it is not possible to treat all areas. Chemical methods also have their drawbacks; if basic safety rules are not followed, you can get a chemical burn or poisoning.

Disposal of waste solutions also poses a certain problem.

The optimal way to remove rust, especially from the surface of products with complex geometric shapes, is soft blasting. The essence of the method is as follows: a stream of compressed air containing special abrasive granules is directed at the metal product.

By changing the pressure, you can adjust the depth of the layer removed from the surface - this way only the layer of rust or scale is removed, and the metal remains untouched. ARMEX granules used in the Nordblast NB 28-2 soft blasting machine consist of tiny particles of soda and chalk.

By applying high pressure to the surface, they easily remove not only rust, but also paint and varnish materials.

A distinctive feature of the method is its absolute environmental safety: the components used are chemically inert. Numerous studies have proven that there are practically no scratches or other microscopic defects on the metal surface, which can subsequently become centers for the re-formation of rust. The alkaline nature of the granules promotes the formation of a passive film on iron or steel products, protecting the bulk of the metal from corrosive destruction.

The best results from using the Nordblast NB 28-2 soft blasting machine were obtained when processing machine or yacht parts. The rust removal process depends on the degree of corrosion, usually it takes 1 day to completely treat a car, 2 days for a yacht.

The earlier the problem is detected, the easier it is to fight rust. Which method is most preferable - everyone decides for themselves, but you should not use old-fashioned methods if there is equipment that removes rust with 100% efficiency!

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What is rust and how to deal with it: iron oxides

As practice shows, rust (corrosion) affects almost 12% of the iron produced in the world. Due to the fact that the environment is becoming more and more aggressive, including due to its pollution, metals are used in increasingly difficult conditions. Sulfur or nitrogen oxides released from the atmosphere form microdrops of nitric or sulfuric acid, which leads to a significant increase in the rate of rust formation.

Chemical basis of the iron corrosion process

Iron is a reactive metal. In the presence of oxygen and water, it undergoes oxidation, forming various compounds: oxides, hydroxides, oxide hydrates. Khimki states that there is no specific formula for rust. What is rust? This is corrosion that is formed due to the oxidation of iron. It has a variable composition that depends on the environment.

Rust affects iron over its entire surface. However, the most vulnerable are the internal and external components of products, welding seams, and threaded connections. The structures of rusty iron are characterized by a significant degree of looseness. Rust does not have any adhesion to metal. Due to the fact that the highly porous corrosion surface freely retains atmospheric moisture, optimal conditions are created for further destruction of iron.

Usually the color of rust is red-brown, brown, which does not allow one to assess the condition of the iron under the layer of corrosion. Under rust, the metal can be completely destroyed.

If measures are not taken to prevent its spread, the effects of corrosion on iron can be catastrophic and lead to complete destruction of structures. This is especially dangerous if rust has corroded power line supports or the bottom of a marine vessel.

What rust is for a car, and what harm it causes, is known to every motorist.

Causes of rust

Rust begins to appear when metal comes into contact with oxygen, water, oxidizing agents or acids. One of the conditions for metal to rust is the presence of impurities or additives in it. If there is contact of iron with external irritants in the presence of salt (salt water), then corrosion destroys it much faster due to the onset of electrochemical reactions.

If iron is pure, without impurities, then it is much more resistant to the effects of oxygen and water. Just like metals such as aluminum, a dense oxide coating (passivation layer) is formed on its surface, which protects the bulk of the iron from deeper oxidation. However, this layer can also be destroyed if iron begins to interact with oxygen and water together.

Other factors that actively destroy iron are carbon dioxide in water and sulfur dioxide. When exposed to them, various types of iron hydroxide are very actively formed. They, unlike iron oxides, cannot protect the metal.

The hydroxide, when formed, begins to peel off from the surface of the iron, after which the lower layer is negatively affected, which also peels off.

And this process lasts until all the metal is destroyed, or there is no oxygen, carbon dioxide, sulfur and water left in the environment.

If iron, undergoing combustion in air, comes into contact with oxygen, then the formation of iron oxide ii occurs.

When burned in pure oxygen - oxide IV.

Iron III oxide is formed when air or oxygen passes through a metal in a molten state.

Rust composition

Rust, which forms under normal conditions, is usually a mixture of 3 iron oxides. They are not formed at one moment and have different physical and mechanical properties. Iron oxides from the lowest layer towards the surface are a combination of the following components:

1. Wustite (iron oxide) is a soft structure, depending on the conditions in which the metal is located. If the storage temperature is high, then this layer is the largest.
2. magnetite (magnetic iron ore) is an iron oxide with higher porosity than the vustid layer and lower hardness. This structure has pronounced magnetic properties.
3. Hematite (red iron ore) is usually a red-gray structure, a hard abrasive substance. Hematite has a higher density, corrodes metal and increases the coefficient of friction when in contact with surfaces.

Before starting work to eliminate rust, it is necessary to know the composition of the metal, especially on its surface, and also to establish the conditions that contributed to its appearance. With this information, it’s easy to find the best option for removing iron oxide and choose the most effective rust control products.

Classification of methods to combat corrosion

Taking into account the main components of corrosion, methods for removing rust are divided into the following:

• Mechanical - the elimination of the oxide layer is carried out using hard metal brushes, sandpaper, etc.
• Thermal - carried out by exposing corrosion to high temperatures, usually in combination with water and (or) air flows.
• chemical - removal of iron oxides is carried out as a result of exposure to special agents that dissolve rust when applied to the metal surface.

It must be taken into account that the effectiveness of the above methods varies. So, if the process of corrosion formation is established in a timely manner, and this is a small spot, then the surface of the iron can be effectively treated with a steel brush, coarse-grained sandpaper, or an angle grinder with an appropriate attachment.

However, if it is determined that rust has affected large surfaces, then chemical methods will be the best.

If the areas of rusty metal are very large and cannot be transported, then heat treatment is considered optimal, but it is associated with high labor intensity.

Typically, metal processing to remove rust is carried out using combined methods, in which various methods are used in a certain sequence.

Mechanical methods

The choice of a specific machining method depends on the type of iron surface. So, for coils of steel wire, it is used to rewind it from one carrier to another. In this case, when kinked, the rust is separated from the metal surface.

When removing corrosion mechanically, stiff steel bristle brushes or sandpaper (coarse-grained) are usually used.

The disadvantages of mechanical methods of getting rid of rust include the fact that marks left by the cleaning tool remain on the surface of the iron. Therefore, it is recommended that the surface of purified iron be polished to give it its previous appearance.

Thermal cleaning

To remove rust using thermal methods, special installations are required (industrial steam generators or construction hair dryers). The method of cleaning from iron oxides is based on the fact that the contact of rust with the base metal is not strong. The impact of elevated temperature and hot moisture with high-speed air flow is such that rust is removed almost completely.

This method is most effective when hot steam is also supplied to the surface being treated. The steam-air mixture in the jet, which is supplied to the metal surface under pressure, leads to softening of the rust, crushing it into separate fragments, which are removed from the surface of the iron by an air flow.

These methods are especially effective when it is necessary to remove rust from steel doors, ventilation structures, and metal structures that are impossible or difficult to dismantle.

Chemical cleaning

Currently, methods for chemically cleaning metal surfaces from rust are very diverse. However, they are all based on the same process - removing corrosion through chemical action on it with acid solutions.

The most effective ways to get rid of iron oxides include treating rust with hydrochloric acid, especially when its concentration in the solution is at least 15%. If the concentration is lower, then the dissolution of rust is significantly slower.

Acidic compounds based on sulfuric acid should not be used, since as a result of exposure, a layer of hydrides is formed on the surface of the iron, which increases the fragility of the metal.

If it is necessary to chemically clean metal at home, then it is possible to use non-aggressive substances such as lemon, vinegar, etc. The principle of influencing corrosion is the same. These substances dissolve rust quite well, which can then be easily removed with a rag. Most housewives probably know what rust is and how to remove it.

Use of other equipment to remove metal corrosion

Mechanical methods of combating rust are not always possible, especially if metal products have complex shapes.

Chemical methods also have certain disadvantages. If you do not follow safety precautions, you can get a chemical burn or poisoning. There are difficulties with the disposal of waste solutions.

As a result, the most optimal method is to use the so-called soft blasting method. Its principle is that a stream of compressed air, which contains abrasive components, is directed onto the surface of a metal damaged by rust.

By changing the pressure in the jet, you can adjust the depth of the layer that is removed. This results in only the rust being removed, while the metal itself remains intact. Granules that act on corrosion consist of finely dispersed soda and chalk; very fine sand can also be used.

Source: https://FB.ru/article/452298/chto-takoe-rjavchina-i-kak-s-ney-borotsya-oksidyi-jeleza

How to stop rust

Ferrous metal is susceptible to corrosion - a process of destruction due to the influence of external factors. If you don’t know how to stop rust, the corrosion process becomes irreversible and, as a result, the destruction of the structure.

Rusting occurs not only with the metal that is above the ground in the open air, but also with that that is underground, inside concrete products (concrete products).

“Red plaque” or what is metal rust

Very often one can come across an equation between corrosion and rust, which is not true. Corrosion is a process and rust is a product. Thus, metal rust is a product of the corrosion process, which is iron oxide (scale) formed on the surface of a metal product with a penetrating nature of development. It has a rich brown-red color and a loose consistency that is amenable to mechanical action.

Causes of rust formation:

• Lack of protective coating (in order to reduce the cost of the finished product, manufacturers rarely treat products such as reinforcement and chain-link mesh with protective coatings, so you can often see rusty reinforcement on a construction site).
• Improper storage (metal is stored outdoors or in a damp room).
• Negative effects of the environment (smog, frequent precipitation, fog, ultraviolet radiation) on metal.

Since it is not always possible to prevent the impact of one of the corrosion factors, you need to know how to stop rust and thereby prevent the destruction of the structure.

Ways to stop and prevent metal rusting

In construction practice, several methods are used to stop the corrosion process and remove already formed rust on the metal surface.

You can stop rust in the following ways:

• Mechanical.
• Thermal.
• Chemical.

The mechanical method of removing rust is to clean the rusty surface with sandpaper with an appropriate abrasive coating. Cleaning can also be done with a metal brush, scraper and similar tools. If rust has affected large areas, the jet-abrasive method is used. This is the easiest and most affordable way to get rid of rust. However, if the metal is significantly damaged, mechanical cleaning alone will not be enough.

The thermal method is carried out using the gas-flame method. In addition to removing rust, it increases the strength of steel reinforcement. Unlike “mechanics”, it requires skills in working with specialized equipment and strict adherence to safety regulations.

The chemical method of stopping rust is the most progressive and highly effective. It consists in applying a solution of a special preparation to rusty surfaces, under the influence of which the adhesion of scale to metal is disrupted, as a result of which the surface is cleaned and the destruction of the metal is stopped.

Chemical preparations differ in their base, which can be acidic, alkaline or neutral. Unlike the first two, the neutral rust converter is absolutely safe for humans and the environment. At the same time, it effectively removes rust, converting it into a protective coating.

Source: https://syntilor.ru/preobrazovatel-rzhavchiny/kak-ostanovit-rzhavchinu/

Characteristics of rust

Chemically speaking, rust is an iron oxide. It is formed by the action of oxygen on iron under high humidity conditions. From a physical point of view, this formation on a metal surface is a rich orange coating that has a rather fragile consistency. The color of rust can be green under some conditions.

Today, there are several types of rust. They depend on how the plaque is formed.

Types of this type of corrosion include:

• Red oxides. They are formed by the action of oxygen on iron under the influence of water.
• Green rust. It is formed under the influence of chlorine on iron without the participation of oxygen in the process. In the modern world, this type of rust is not uncommon. It is known to many thanks to the plaque that forms on the reinforcement that is used to construct concrete sea pillars.

There are several other types and forms of rust. They are all visually different. In some cases, spectroscopy is used to determine the type of corrosion. The formation of corrosion on iron is almost inevitable.

Gradually, any amount of this metal, under the influence of oxygen and water, turns into a pile that is completely covered with a rich orange coating. Subsequently, this can lead to the destruction of iron.

Under the influence of rust, this metal begins to acquire a loose structure, which leads to rust corroding and destroying it.

Rust belongs to one of the products of the process known as corrosion. As a result, various types of metals are damaged. Metals that are formed from iron alloys are susceptible to corrosion. In some cases, steel also undergoes this process, if it does not belong to the category of stainless steel. However, rust is precisely the process of formation of iron oxide.

The cause of iron rusting is most often the presence of water, access to oxygen and other strong oxidizing agents. Under their influence, the iron begins to become covered with a rusty coating. In order to speed up this process, just add salt. As a result of the electrochemical reaction, the iron will begin to rust more strongly and the destruction of an object made from this metal will occur faster.

In some cases, iron begins to rust if it is in an aggressive environment. Such a medium can be a solution consisting of water, sulfur dioxide and carbon dioxide.

Table. Basic quantitative indicators of corrosion and corrosion resistance

Type of corrosionMain quantitative indicators of corrosion and corrosion resistanceCorrosion effect (integral corrosion indicator)Rate (differential) corrosion indicatorCorrosion resistance indicator

Complete corrosion	Corrosion penetration depth	Linear corrosion rate	Time of penetration of corrosion to the permissible (specified) depth*
Mass loss per unit area	Mass loss rate	Time until the mass decreases by the permissible (specified) value*
Corrosion spots	Degree of surface damage	Time to reach the permissible (specified) degree of damage*
Pitting corrosion	Maximum pitting depth	Maximum pitting penetration rate	Minimum time for pitting to penetrate to the permissible (specified) depth*
Maximum diameter of pitting at the mouth	Minimum time to achieve the permissible (specified) size of the pitting diameter at the mouth*
Degree of surface damage by pitting	Time to reach the permissible (specified) degree of damage*
Intergranular corrosion	Corrosion penetration depth	Corrosion penetration rate	Penetration time to permissible (specified) depth*
Decrease in mechanical properties (elongation, contraction, impact strength, tensile strength)	Time required for mechanical properties to decrease to an acceptable (specified) level*
Corrosion cracking	Depth (length) of cracks	Crack growth rate	Time until first crack appears**
Decrease in mechanical properties (relative elongation, narrowing)	Time until sample failure** Safe stress level** (conditional limit of long-term corrosion strength**) Threshold stress intensity factor for corrosion cracking**
Corrosion fatigue	Depth (length) of cracks	Crack growth rate	Number of cycles before sample failure** Conditional limit of corrosion fatigue** Threshold stress intensity factor for corrosion fatigue**
Exfoliation corrosion	Degree of damage to the surface by delamination Total length of ends with cracks	—
Corrosion penetration depth	Corrosion penetration rate

Fighting rust

In the modern world, a large number of iron products are produced. They are represented by both industrial goods and products for household use. You always want them to last a long time. Rust formation is not beneficial for items made of iron. It leads to their breakdown and failure. It is for this reason that you should know how to remove rust and how to prevent its appearance.

In order to prevent rust from harming the products, it is necessary to use special means to form a film on the surface of iron objects that protects against the penetration of air and water into the metal structure.

Today, the following methods are used to protect against rust:

• Galvanization. This method is used in the production of stainless steel. A layer of copper or zinc is applied to the metal. Cadmium is also used in some cases. These substances form an invisible film on the surface, which gives the iron material density and high resistance to moisture and oxygen.
• Cathodic protection. This method is used primarily for pipes that are laid deep underground. An electric charge is applied to them, which causes an electrochemical reaction, preventing the appearance of rusty deposits on the surface of the pipes.
• Application of paints and varnishes to the surface of iron objects. This method is to, in addition to decorating the product, protect it from rust. The paint covers the metal with a thin layer and prevents moisture and air from reaching the iron structure.

Important: To prevent rust from forming on a painted product, you must ensure that the paint lies in an even layer and does not have any chips. Otherwise, the surface of the metal will be affected by humidity and air.

Rust removers are now available. They can be used when plaque has already formed. They are aimed at making the plaque structure more fragile so that it can be removed from the metal surface.

The most popular means of removing rust deposits is a rust converter. It is a solution that turns plaque into a substance that can be easily removed. Many of these products make the rust structure more uniform, which makes it possible to leave it on the metal surface for paint and varnish work, if it does not disturb its evenness.

Rust removers

Today it is not uncommon to find special paint for rust. It is represented on the domestic market by a large number of brands. Its advantage is that it gives a fairly dense coating. It has a triple effect.

It combines the functions:

• rust converter,
• primers,
• coloring matter with a high level of density.

It not only removes traces of rust, but also makes the coating smoother and more attractive. Paints for working with rusty objects have a high level of color saturation, so that even in one layer all traces of rusty deposits are hidden. In this case, a small layer of film is formed on the metal, which prevents rust from spreading further and developing a new one.

rust removal

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