The main types of metal corrosion. Corrosion resistance

The main types of metal corrosion. Corrosion resistance
The main types of metal corrosion. Corrosion resistance
21.09.2019

What is Corrosion Resistance

The ability of a metal to resist corrosion is called corrosion resistance. This ability is determined by the rate at which corrosion occurs under certain conditions. To assess the rate of corrosion, quantitative and qualitative characteristics are used.

Qualitative characteristics are:

    changing the appearance of the metal surface;

    change in the microstructure of the metal.

Quantitative characteristics are:

    time before the appearance of the first focus of corrosion;

    the number of foci of corrosion formed over a certain period of time;

    thinning of the metal per unit of time;

    change in the mass of the metal per unit of surface area per unit of time;

    the volume of absorbed or released gas in the corrosion process per unit of surface per unit of time;

    electric current density for a given corrosion rate;

    a change in a particular property over a certain period of time (mechanical properties, reflectivity, electrical resistance).

Different metals have different corrosion resistance. To increase corrosion resistance, special methods are used: alloying for steel, chrome plating, aluminizing, nickel plating, painting, zinc plating, passivation, etc.

Iron and steel


In the presence of oxygen and pure water, iron quickly corrodes, the reaction proceeds according to the formula:

In the process of corrosion, a loose layer of rust covers the metal, and this layer does not at all protect it from further destruction, corrosion goes on until the metal is completely destroyed. More active corrosion of iron is caused by salt solutions: if even a little ammonium chloride (NH4Cl) is present in the air, the corrosive process will go much faster. In a weak solution of hydrochloric acid (HCl), the reaction will also go actively.

Nitric acid (HNO3) in a concentration of more than 50% will cause the metal to passivate - it will be covered with a protective layer, albeit a fragile one. Fuming nitric acid is safe for iron.

Sulfuric acid (H2SO4) in a concentration of more than 70% passivates iron, and if steel grade St3 is kept in 90% sulfuric acid at a temperature of 40 ° C, then under these conditions its corrosion rate will not exceed 140 microns per year. If the temperature is 90 ° C, then corrosion will proceed at a 10 times higher rate. Sulfuric acid with a concentration of 50% iron will dissolve.

Phosphoric acid (H3PO4) will not cause corrosion of iron, as well as anhydrous organic solvents such as alkali solutions, aqueous ammonia, dry Br2 and Cl2.

If you add a thousandth part of sodium chromate to water, then it will become an excellent inhibitor of iron corrosion, like sodium hexametaphosphate. But chlorine ions (Cl-) remove the protective film from iron and increase corrosion. Iron is technically pure, containing about 0.16% of impurities, and is highly resistant to corrosion.

Medium-alloy and low-alloy steels

Alloying additions of chromium, nickel or copper in low-alloy and medium-alloy steels increase their resistance to water and atmospheric corrosion. The more chromium, the higher the oxidation resistance of steel. But if chromium is less than 12%, then chemically active media will have a destructive effect on such steel.

High alloy steels

In high-alloy steels, alloying components are more than 10%. If the steel contains from 12 to 18% chromium, then such steel will withstand contact with almost any of the organic acids, with food, will be resistant to nitric acid (HNO3), to alkalis, to many salt solutions. In 25% formic acid (CH2O2), high-alloy steel will corrode at a rate of about 2 mm per year. However, strong reducing agents, hydrochloric acid, chlorides and halogens will destroy high alloy steel.

Stainless steels, which contain from 8 to 11% nickel and 17 to 19% chromium, are more resistant to corrosion than just high chromium steels. Such steels withstand acidic oxidizing environments, such as chromic acid or nitric acid, as well as strong alkaline ones.

Nickel as an additive will increase the resistance of steel to non-oxidizing environments, to atmospheric factors. But the medium is acidic, reducing and acidic with halogen ions, - they will destroy the passivating oxide layer, as a result, the steel will lose its resistance to acids.

Higher corrosion resistance than chromium-nickel steels are stainless steels with the addition of molybdenum in an amount of 1 to 4%. Molybdenum will give resistance to sulfuric and sulfuric acids, organic acids, seawater and halides.

Ferrosilicon (iron with the addition of 13 to 17% silicon), the so-called iron-silicon casting, has corrosion resistance due to the presence of an oxide film of SiO2, and which neither sulfuric, nor nitric or chromic acids can destroy, they only strengthen this protective film. But hydrochloric acid (HCl) will easily corrode ferrosilicon.

Nickel alloys and pure nickel


Nickel is resistant to many factors, both atmospheric and laboratory ones, to pure and salt water, to alkaline and neutral salts such as carbonates, acetates, chlorides, nitrates and sulfates. Not saturated with oxygen and not hot organic acids will not harm nickel, as well as boiling concentrated alkali potassium hydroxide (KOH) in a concentration of up to 60%.

Corrosion is caused by reducing and oxidizing environments, oxidizing alkaline or acidic salts, oxidizing acids such as nitric, wet gaseous halogens, nitrogen oxides and sulfur dioxide.

Monel metal (up to 67% nickel and up to 38% copper) is more resistant to the action of acids than pure nickel, but it will not withstand the action of strong oxidizing acids. Differs in a rather high resistance to organic acids, to a significant amount of salt solutions. Atmospheric and water corrosion does not threaten monel metal; fluorine is also safe for it. Monel metal will safely withstand 40% boiling hydrogen fluoride (HF) as platinum does.

Aluminum alloys and pure aluminum


The protective oxide film of aluminum makes it resistant to common oxidants, acetic acid, fluorine, just the atmosphere, and a significant amount of organic liquids. Technically pure aluminum, in which impurities are less than 0.5%, is very resistant to the action of hydrogen peroxide (H2O2).

It is destroyed by the action of caustic alkalis in strong reducing environments. Diluted sulfuric acid and oleum are not terrible for aluminum, but sulfuric acid of medium concentration will destroy it, like hot nitric acid.

Hydrochloric acid can destroy the protective oxide film of aluminum. Contact of aluminum with mercury or mercury salts is destructive for the former.

Pure aluminum is more resistant to corrosion than, for example, an alloy of duralumin (in which up to 5.5% copper, 0.5% magnesium and up to 1% manganese), which is less resistant to corrosion. Silumin (an addition of 11 to 14% silicon) is more stable in this respect.

Copper alloys and pure copper


Pure copper and its alloys do not corrode in salt water or air. Copper is not afraid of corrosion: dilute alkalis, dry NH3, neutral salts, dry gases and most organic solvents.

Alloys such as bronze, which contain a lot of copper, withstand exposure to acids, even cold concentrated or hot dilute sulfuric acid, or concentrated or dilute hydrochloric acid at ambient temperatures (25 ° C).

In the absence of oxygen, copper does not corrode on contact with organic acids. Neither fluorine nor dry hydrogen fluoride have a destructive effect on copper.

But copper alloys and pure copper corrode from various acids if oxygen is present, as well as upon contact with wet NH3, some acidic salts, wet gases such as acetylene, CO2, Cl2, SO2. Copper interacts easily with mercury. Brass (zinc and copper) is not highly resistant to corrosion.

Pure zinc


Clean water, just like clean air, does not lead to zinc corrosion. But if there are salts, carbon dioxide or ammonia in water or air, then zinc corrosion will begin. Zinc dissolves in alkalis, especially quickly - in nitric acid (HNO3), more slowly - in hydrochloric and sulfuric acids.

Organic solvents and petroleum products, in principle, do not have a corrosive effect on zinc, but if the contact is prolonged, with cracked gasoline, for example, the acidity of gasoline will increase when it is oxidized in air, and zinc will begin to corrode.

Pure lead


The high resistance of lead to water and atmospheric corrosion is a well-known fact. It does not corrode even when it is in the soil. But if the water contains a lot of carbon dioxide, then the lead will dissolve in it, since lead bicarbonate is formed, which will already be soluble.

In general, lead is very resistant to neutral solutions, moderately resistant to alkaline, as well as to some acids: sulfuric, phosphoric, chromic and sulphurous. With concentrated sulfuric acid (from 98%) at a temperature of 25 ° C, lead can be slowly dissolved.

Hydrogen fluoride at a concentration of 48% will dissolve lead when heated. Lead strongly interacts with hydrochloric and nitric acids, with formic and acetic acids. Sulfuric acid will cover the lead with a sparingly soluble layer of lead chloride (PbCl2), and further dissolution will not proceed. In concentrated nitric acid, the lead will also be covered with a layer of salt, but dilute nitric acid will dissolve the lead. Chlorides, carbonates and sulfates are not aggressive to lead, while nitrate solutions are the opposite.

Pure titanium


Good corrosion resistance is a hallmark of titanium. It does not oxidize with strong oxidants, it withstands salt solutions, FeCl3, etc. Concentrated mineral acids will cause corrosion, but even boiling nitric acid in a concentration of less than 65%, sulfuric acid - up to 5%, hydrochloric acid - up to 5% - will not cause corrosion of titanium. Normal corrosion resistance to alkalis, alkaline salts and organic acids distinguishes titanium among other metals.

Pure zirconium


Zirconium is more resistant to sulfuric and hydrochloric acids than titanium, but less resistant to aqua regia and wet chlorine. Possesses high chemical resistance to most alkalis and acids, resistant to hydrogen peroxide (H2O2).

The action of some chlorides, boiling concentrated hydrochloric acid, aqua regia (a mixture of concentrated nitric HNO3 (65-68 wt.%) And hydrochloric HCl (32-35 wt.%), Hot concentrated sulfuric acid and fuming nitric acid - cause corrosion. In terms of corrosion, it is such a property of zirconium as hydrophobicity, that is, this metal is not wetted either with water or aqueous solutions.

Pure tantalum


Tantalum's excellent chemical resistance is similar to glass. Its dense oxide film protects the metal at temperatures up to 150 ° C from the action of chlorine, bromine, iodine. Most acids under normal conditions do not act on tantalum, even aqua regia and concentrated nitric acid do not cause corrosion. Alkaline solutions have practically no effect on tantalum, but hydrogen fluoride acts on it and concentrated hot alkali solutions are used to dissolve tantalum alkali melts.

Corrosion resistance

Corrosion resistance- the ability of materials to resist corrosion, which is determined by the corrosion rate under given conditions. Both qualitative and quantitative characteristics are used to assess the corrosion rate. A change in the appearance of a metal surface, a change in its microstructure are examples of a qualitative assessment of the corrosion rate. To quantify, you can use:

  • the time elapsed before the appearance of the first corrosion center;
  • the number of corrosion foci formed over a certain period of time;
  • reducing the thickness of the material per unit of time;
  • change in the mass of metal per unit of surface per unit of time;
  • the volume of gas released (or absorbed) during the corrosion of a unit of surface per unit of time;
  • current density corresponding to the rate of a given corrosion process;
  • a change in a property over a certain time of corrosion (for example, electrical resistance, reflectivity of a material, mechanical properties).

Different materials have different corrosion resistance, to increase which special methods are used. Thus, an increase in corrosion resistance is possible by alloying (for example, stainless steels), applying protective coatings (chrome plating, nickel plating, aluminizing, zinc plating, painting products), passivation, etc. salt spray.

Sources of


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See what "Corrosion resistance" is in other dictionaries:

    Corrosion resistance- the ability of the metal to resist the corrosive effects of the environment. Source: snip id 5429: Guidelines for the design and protection against corrosion of underground metal communications facilities Co. Dictionary-reference book of terms of normative and technical documentation

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    corrosion resistance- The ability of a material to withstand the effects of a corrosive environment without changing its properties. For metal, this can be a local damage to the surface - pitting or rusting; for organic materials, this is the formation of hair ... ... Technical translator's guide

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    corrosion resistance- atsparumas korozijai statusas T sritis Standartizacija ir metrologija apibrėžtis Metalo gebėjimas priešintis korozinės aplinkos poveikiui. atitikmenys: angl. corrosion resistance vok. Korrosionswiderstand, m; Rostbeständigkeit, f; Rostsicherheit ... Penkiakalbis aiškinamasis metrologijos terminų žodynas

    corrosion resistance- korozinis atsparumas statusas T sritis chemija apibrėžtis Metalo atsparumas aplinkos medžiagų poveikiui. atitikmenys: angl. corrosion resistance rus. corrosion resistance ... Chemijos terminų aiškinamasis žodynas

    corrosion resistance- the ability of a material, such as metals and alloys, to resist corrosion in a corrosive environment; assessed by the rate of corrosion; See also: Resistance chemical resistance relaxation resistance ... Encyclopedic Dictionary of Metallurgy

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Books

  • Corrosion resistance of materials in aggressive environments of chemical industries, G. Ya. Vorobieva. The book summarizes data on the properties and corrosion resistance of metallic and non-metallic materials. It contains tables and diagrams of corrosion resistance of metals and alloys, ...
  • Corrosion resistance and corrosion protection of metal, powder and composite materials, Vladimir Vasiliev. This manual is devoted to a description of the corrosion resistance of the most commonly used construction materials in modern technology and technology: iron, steel, cast iron, aluminum, ...
Corrosion resistance- the ability of materials to resist corrosion, which is determined by the corrosion rate under given conditions.

Both qualitative and quantitative characteristics are used to assess the corrosion rate. A change in the appearance of a metal surface, a change in its microstructure are examples of a qualitative assessment of the corrosion rate.

To quantify, you can use:

  • the number of corrosion foci formed over a certain period of time;
  • the time elapsed before the appearance of the first corrosion center;
  • change in the mass of metal per unit of surface per unit of time;
  • reducing the thickness of the material per unit of time;
  • current density corresponding to the rate of a given corrosion process;
  • the volume of gas released (or absorbed) during the corrosion of a unit of surface per unit of time;
  • a change in a property over a certain time of corrosion (for example, electrical resistance, reflectivity of a material, mechanical properties)

Different materials have different corrosion resistance, to increase which special methods are used. Increasing corrosion resistance is possible by alloying (for example, stainless steels), applying protective coatings (chrome plating, nickel plating, aluminizing, zinc plating, painting products), passivation, etc. The resistance of materials to corrosion typical for sea conditions is studied in salt fog chambers ...

The mildest form of corrosive attack is discoloration and loss of gloss, which, in principle, is hardly noticeable from afar. Surface renovation can usually restore steel to its previous attractive appearance.

Smallpox corrosion

Smallpox corrosion(pitting corrosion) is a type of corrosive attack caused by chlorides.

Usually, small dots of a dark red color appear first, and only in very difficult cases can they grow to such an extent that corrosion passes into a new stage, continuous surface corrosion. The risk of corrosion increases if foreign materials (varnish, etc.) remain on the surface after welding, if particles of another corroded metal get on the surface, if the tarnishing color has not been removed after heat treatment.

Corrosion cracking

Corrosion cracking- This is the destruction of the metal due to the occurrence and development of cracks with the simultaneous effect of tensile stresses and a corrosive environment. It is characterized by an almost complete absence of plastic deformation of the metal.

This type of corrosion occurs in environments with a high chloride content, such as swimming pools.

Crevice corrosion

Crevice corrosion- occurs at the junction points due to structural or operational requirements.

The degree of corrosive attack will be influenced by the geometry of the joint and the type of contacting materials. The most dangerous are narrow joints with small gaps and the connection of steel to plastics. If it is not possible to avoid joints, then we recommend using molybdenum-alloyed stainless steels.

Intergranular corrosion

Intergranular corrosion- this type of corrosion currently occurs on steels after sensitization in combination with use in acidic environments.

During sensitization, chromium carbides are released and accumulate along the grain boundaries. Accordingly, areas with a low chromium content and more susceptible to corrosion arise. This happens, for example, during welding in the heat affected zone.

All austenitic steels are resistant to intergranular corrosion. They can be welded (sheet up to 6 mm, bar up to 40 mm) without the risk of MCC.

Bimetallic or galvanic corrosion

Bimetallic corrosion- occurs during the operation of a bimetallic corrosion element, i.e. a galvanic cell in which the electrodes are composed of different materials.

Very often it is necessary to use inhomogeneous materials, whose interface under certain conditions can lead to corrosion. When two metals are coupled, bimetallic corrosion is of galvanic origin. In this type of corrosion, a less alloyed metal suffers, which under normal conditions, not being in contact with a more alloyed metal, does not corrode. The consequence of bimetallic corrosion is at least a discoloration and, for example, loss of tightness of pipelines or failure of fasteners. Ultimately, these problems can lead to a sharp reduction in the service life of the structure and the need for premature overhaul. In the case of stainless steels, the less alloyed metal mating with them undergoes bimetallic corrosion.

Laboratory work No. 8

Purpose of the work: familiarization with the mechanisms and rates of corrosion destruction of metals.

1. Methodical instructions

Corrosion destruction of metals is a spontaneous transition of a metal to a more stable oxidized state under the influence of the environment. Depending on the nature of the environment, a distinction is made between chemical, electrochemical and biocorrosion.

Electrochemical corrosion is the most common type of corrosion. Corrosion of metal structures in natural conditions - in the sea, in the ground, in groundwater, under condensation or adsorption films of moisture (in atmospheric conditions) is of an electrochemical nature. Electrochemical corrosion is the destruction of a metal, accompanied by the appearance of an electric current as a result of the operation of a variety of macro- and microgalvanic couples. The mechanism of electrical corrosion is divided into two independent processes:

1) anodic process - the transition of a metal into a solution in the form of hydrated ions, leaving an equivalent amount of an electron in the metal:

(-) A: Me + mH 2 O → 1+ + ne

2) cathode process - assimilation of excess electrons in the metal by any depolarizers (molecules or ions of the solution, which can be reduced at the cathode). In case of corrosion in neutral media, the depolarizer is usually corrosion to oxygen dissolved in the electrolyte:

(+) K: O 2 + 4e + 2H 2 O → 4OH¯

For corrosion in acidic environments - hydrogen ion

(+) K: H H 2 O + e → 1 / 2H 2 + H 2 O

Macrogalvanic vapors arise from the contact of different metals. In this case, the metal with a more negative electrode potential is the anode and is subject to oxidation (corrosion).

The metal with a more positive potential serves as the cathode. It acts as a conductor of electrons from the metal-anode to environmental particles capable of receiving these electrons. According to the theory of microvapors, the cause of electrochemical corrosion of metals is the presence on their surface of microscopic short-circuited galvanic cells arising from the inhomogeneity of the metal and its contact with the environment. Unlike galvanic cells specially made in technology, they arise spontaneously on the metal surface. O 2, CO 2, SO 2 and other gases from the air are dissolved in a thin layer of moisture that always exists on the surface of the metal. This creates conditions for the metal to come into contact with the electrolyte.

On the other hand, different parts of the surface of a given metal have different potentials. The reasons for this are numerous, for example, the potential difference between differently treated parts of the surface, different structural constituents of the alloy, impurities and the base metal.

Areas of the shaped surface with a more negative potential become anodes and dissolve (corrode) (Figure 1.1).

Some of the freed electrons will transfer from the anode to the cathode. The polarization of the electrodes, however, prevents corrosion, since the electrons remaining on the anode form a double electric layer with the positive ions transferred into the solution, the dissolution of the metal stops. Consequently, electrical corrosion can occur if electrons from the anode sites are continuously removed at the cathode and then removed from the cathode sites. The process of removing electrons from the cathode sites is called depolarization, and the substances or ions that cause depolarization are called depolarizers. If there is contact of any metal with the alloy, the alloy acquires a potential corresponding to the potential of the most negative metal included in its composition. When brass (copper-zinc alloy) contacts iron, brass will corrode (due to the presence of zinc in it). With a change in the environment, the electrode potential of individual metals can change dramatically. Chromium, nickel, titanium, aluminum, and other metals whose normal electrode potential is sharply negative, under normal atmospheric conditions is strongly passivated, covered with an oxide film, as a result of which their potential becomes positive. In atmospheric conditions and fresh water, the following galvanic cell will work:

(-) Fe | H 2 O, O 2 | Al 2 O 3 (Al) +

(-) A: 2Fe - 4e = 2Fe 2+

(+) K: O 2 + 4e + 2H 2 O = 4OH¯

As a result: 2Fe 2 + 4OH¯ = 2Fe (OH) 2

4Fe (OH) 2 + O 2 + 2H 2 O = 2Fe (OH) 3

However, in an acidic, alkaline environment or in a neutral one containing chlorine ions (for example, in seawater), which destroy the oxide film, aluminum in contact with iron becomes an anode and undergoes a corrosive process. In NaCl solution and seawater, the following electrochemical cell will work:
(-) Al | H 2 O, O 2, NaCl | Fe (+)

(-) A: Al - 3e = Al 3+

(+) K: O 2 + 4e + 2H 2 O = 4OH¯

4Al 3 + 12OH¯ = 4Al (OH) 3

Very often, electrochemical corrosion occurs as a result of different aeration, that is, unequal access of air oxygen to individual areas of the metal surface. Figure 1.2. depicts a case of corrosion of iron and a drop of oxen. Near the edges of the drop, where it is easier for oxygen to penetrate, cathode areas appear, and in the center, where the thickness of the protective layer of water is greater and oxygen is more difficult to penetrate, the anode area.

The appearance of corrosive galvanic cells is influenced by the difference in the concentration of the dissolved electrolyte, the difference in temperature and illumination, and other physical conditions.

Corrosion protection

The causes of corrosive destruction of metals are numerous. Methods of corrosion protection are also varied:

processing of the external environment;

protective coatings;

electrochemical protection;

production of specially corrosion-resistant alloys.

Treatment of the external environment consists in removing or reducing the activity of some of the substances in it that cause corrosion. For example, removal of oxygen dissolved in iodine (deaeration) Sometimes special corrosion inhibiting substances are added to the solution, which are called inhibitors or inhibitors (urotropine, thiourea, aniline and others).

Parts that are exposed to protection in atmospheric conditions are placed together with inhibitors in a container or wrapped in paper, the inner layer, which is impregnated with an inhibitor, and the outer layer, with paraffin. The inhibitor, evaporating, is adsorbed on the surface of the part, causing inhibition of the electrode processes.

The role of protective coatings is reduced to isolating the metal from the effects of protective external environment. This is achieved by applying varnishes, paints, metal coatings to the metal surface.

Metallic coatings are divided into anodic and cathodic ones. In the case of ANODE coating, the electrode potential of the covering metal is more negative than the potential of the protected metal. In the case of a CATHODE coating, the electrode potential of the covering metal is more positive than the potential of the base metal.

As long as the protective layer completely isolates the base metal from the environment, there is no fundamental difference between the anodic and cathodic coating. If the integrity of the coating is violated, new conditions arise. A cathode coating, for example, tin on iron, not only ceases to protect the base metal, but also enhances the corrosion of iron by its presence (in the resulting galvanic cell, iron is the anode).

With electrochemical protection, the reduction or complete cessation of corrosion is achieved by creating a high electronegative potential on the protected metal product. For this, the item to be protected is either connected to a metal that has a more negative electrode potential, which can more easily give up electrons (protective protection) or with a negative pole of an external current source (cathodic electrical protection).

An anode coating, for example zinc on iron, on the contrary, if the integrity of the covering layer is violated, it will itself undergo destruction, thereby protecting the base metal from corrosion (in the resulting galvanic cell, zinc is the anode).

Manufacturing of special corrosion-resistant alloys, stainless steels, etc. is reduced to the introduction of additives of various metals into them.

These additives affect the microstructure of the alloy and contribute to the appearance in it of such microgalvanic elements, in which the total EMF, due to mutual compensation, approaches zero. Such useful additives, especially for steel, are chromium, nickel and other metals.

1. Execution of work

Exercise 1

Carrying out high-quality chemical reactions that allow you to detect metal ions that have passed into the solution during the anodic corrosion process.

Devices and reagents: solutions of ZnSO 4, FeSO 4 and K 3, a set of test tubes.

Work progress: Pour 1-2 ml of salt solution into test tubes:

a) ZnSO 4 and a few drops of K 3;

b) FeSO and a few drops of K 3.

Note the precipitation. Write the corresponding reactions in molecular and ionic form.

Assignment 2

Study of the mechanism of metal corrosion by direct contact in a neutral medium.

The experiment is carried out on the installation shown in Fig. 1.7

Pour 5-10 ml of an aqueous solution of NaCl into the U-tube. Plates of metals are dropped into it, connected to each other with clamps.

The metal plates must be thoroughly cleaned with an emery cloth, and the place of contact between the plate and the clamp is outside the solution. When performing the experiment, it is necessary to note a change in the color of the solution at the cathode and anode.

Write:

1) anodic and cathodic corrosion processes

2) the corresponding reactions by which a metal ion was detected in a solution

3) circuit of a galvanic cell.

1.The Zn and Fe plates are lowered.

In the solution where the zinc electrode is located, add a few drops of K 3, where the iron electrode is located, a few drops of phenolphthalein.

2. Fe and Cu plates are lowered,

In the solution where the iron electrode is located, add a few drops of K 3, where the copper electrode is located, a few drops of phenolphthalein.

Compare the behavior of iron in both cases, draw the appropriate conclusions.

Assignment 3

Study of the mechanism of corrosion of metals during their direct contact in an acidic environment.

The experiment should be carried out on the installation shown in Fig. 1.8.

Pour 10% HCl solution into a porcelain cup. Dip two metals Al and Cu into the solution, and observe the behavior of the metals. Which metal produces hydrogen bubbles? Write the appropriate reactions. Bring honor metals into contact with each other. On which metal are hydrogen bubbles released when the metals come into contact? Draw up a diagram of a galvanic cell and electrode processes on its electrodes. Write the total reaction equation.

3. Examples of problem solving

Example 1

Consider the corrosion process upon contact of iron with lead in an HCl solution

In an electrolyte solution (HCl), this system is a galvanic cell, in the internal circuit of which Fe is the anode (E ° = 0.1260). iron atoms, transferring two electrons to lead, pass into the solution in the form of ions. Electrons on lead, reduce the hydrogen ions in the solution, because

HCl = H + + Cl¯

Anodic process Fe 0 - 2e = Fe 2+

Cathodic process 2H + + 2e = 2H 0

Example 2

Corrosion process upon contact of Fe with Ph in NaCl solution. Since the NaCl solution has a neutral reaction (salt formed by a strong base and a strong acid), then

Anodic process Fe - 2e = Fe 2+,

Cathodic process O 2 + 4e + 2H 2 O = 4OH¯

Sodium chloride (NaCl) does not participate in corrosion processes; it is shown in the diagram only as a substance capable of increasing the electrical conductivity of an electrolyte solution.

Example 3

Why is chemically pure iron more corrosion resistant than technical iron? Make the electronic equations of the anodic and cathodic processes occurring during the corrosion of technical iron.

Solution

The process of corrosion of technical iron is accelerated due to the formation of micro and submicrogalvanic elements in it. In microgalvanic vapors, as a rule, the base metal serves as the anode, i.e. iron. Cathodes are inclusions in the metal, for example, grains of graphite, cement. At the anode sites, metal ions go into solution (oxidation).

A: Fe - 2e = Fe 2+

At the cathode sites, electrons that have passed here from the anode sites are bound either by atmospheric oxygen dissolved in water or by hydrogen ions. In neutral media, oxygen depolarization occurs:

K: O 2 + 4e + 2H 2 O = 4OH¯

In acidic environments (high concentration of H - ions) common depolarization

K: 2H + + 2e = 2H 0

Example 4

Call, cathodic, or anodic is zinc and coating on an iron product? What processes will take place if the integrity of the coating is violated and the product is in humid air?

Solution

The algebraic value of the zinc electrode potential is lower than the iron electrode potential; therefore, the coating is anodic. In case of violation of the integrity of the zinc layer, a corrosive galvanic couple is formed, in which zinc will be the anode, and iron will be the cathode. The anodic process involves the oxidation of zinc:

Zn 2+ + 2OH = Zn (OH) 2

The cathodic process takes place on iron. In humid air, oxygen depolarization occurs predominantly.

K (Fe): O 2 + 4e + 2H 2 O = 4OH¯

Example 5

Cadmium and nickel plates, immersed in dilute sulfuric acid, dissolve in it with the evolution of hydrogen. What will change if you put both of them at the same time in a vessel with acid, connecting the ends with a wire?

Solution

If you connect the ends of the cadmium and nickel plates with wire, cadmium is formed, a nickel electrochemical cell in which cadmium, as the more active metal, is the anode. Cadmium will oxidize:

A: Cd - 2e = Cd 2+,

Excess electrons will transfer to the nickel plate, where the process of hydrogen ion reduction will take place:

K (Ni): 2H + 2e = 2H 0.

Thus, only cadmium is subjected to dissolution, nickel will become only a conductor of electrons and will not dissolve itself. The hydrogen will be released only on the nickel plate.

Example 6

How does the PH of the medium affect the corrosion rate of aluminum?

Solution

Reducing the PH of the medium, i.e. an increase in the concentration of H-ions sharply increases the corrosion rate of nickel, since the acidic environment prevents the formation of protective films of nickel hydroxide, active oxidation of nickel occurs in an acidic environment

A: Ni - 2e = Ni 2+

Decrease in the concentration of H-ions, i.e. an increase in OH concentration, promotes the formation of a layer of nickel hydroxide:

Ni 2+ - 2OH¯ = NI (OH) 2

Aluminum hydroxide has amphoteric properties, i.e. dissolves in acids and alkalis:

Al (OH) 3 + 3HCl = AlCl 3 + 3H 2 O

Al (OH) 3 + NaOH = Na AlO 2 + 2H 2 O

More precisely, this reaction proceeds as follows:

Al (OH) 3 + NaOH = Na

Thus, the lowest corrosion rate of nickel is in an alkaline medium, and aluminum is in a neutral one.

4. Tasks

1. An iron plate immersed in hydrochloric acid releases hydrogen very slowly, but if you touch it with a zinc wire, it immediately becomes covered with hydrogen bubbles. Explain this phenomenon. What metal goes into solution in this case?

2. The iron product contains parts made of nickel. How will this affect iron corrosion? Write down the appropriate anodic and cathodic processes if the item is in a humid atmosphere.

3. In what environment is the rate of destruction of iron higher? What environment promotes the anodic oxidation of zinc? Write the appropriate reactions.

4. How does atmospheric corrosion of tinned iron and tinned copper occur when the integrity of the coating is broken? Make the electronic equations of the anodic and cathodic processes.

5. Copper does not displace hydrogen from dilute acids. Why? However, if a zinc plate is touched to a copper plate, then a violent evolution of hydrogen begins on the copper. Give an explanation for this by making the electronic equations of the cathodic and anodic processes.

6. In an electrolyte solution containing dissolved oxygen, a zinc plate and a zinc plate partially covered with copper were dipped. When does the process of zinc corrosion occur more intensively? Make up the electronic equations of the cathodic and anodic processes.

7. What can happen if a product, in which technical iron is in contact with copper, is left in the air at high humidity? Write down the equations of the corresponding processes.

8. Aluminum is riveted with iron. Which metal will corrode? What processes will take place if the product gets into the sea water?

9. Why, when iron products come into contact with aluminum ones, do iron products undergo more intense corrosion, although aluminum has a more negative standard electrode potential?

10. The iron plates are omitted:

a) into distilled water

b) into sea water

When is the corrosion process more intense? Motivate your answer.

11. Make the equations of the processes occurring during the corrosion of aluminum immersed in a solution:

a) acids

b) alkalis

12. Why technical zinc interacts with acid more intensively than chemically pure zinc?

13. A plate is lowered into the electrolyte solution:

b) copper, partially covered with tin

when is the corrosion process more intense?

Motivate the answer

14. Why, when nickel-plating iron products, are they coated first with copper and then with nickel?

Make the electronic equations of the reactions occurring in corrosion processes when nickel plating is damaged.

15. The ironwork was coated with cadmium. Which coating is this - anodic or cathodic?

Motivate your answer. What metal will corrode if the protective layer is damaged? Write the electronic equations of the corresponding processes (neutral environment).

16. Which metal:

b) cobalt

c) magnesium

can be a protector to iron-based alloy. Make the electronic equations of the corresponding processes (acidic environment).

17. What processes will take place on the zinc and iron plates if you immerse each one separately in a solution of copper sulfate? What processes will occur if the outer ends, which are in the solution of the plates, are connected with a conductor? Make electronic equations

18. Aluminum plate lowered

a) into distilled water

b) in a solution of sodium chloride

when is the corrosion process more intense? Make the equations of the anodic and cathodic corrosion processes of commercial aluminum in a neutral environment.

19. If a nail is driven into a damp wood, the part inside the wood becomes rusty. How can this be explained? Is this part of the nail anode or cathode?

20. Recently, other metals have been coated with cobalt for corrosion protection. Is the cobalt coating of steel anodic or cathodic? What processes take place in humid air when the integrity of the coating is violated?

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Table. Corrosion resistance of metals and alloys under normal conditions

Table. Corrosion resistance of metals and alloys under normal conditions

This corrosion resistance table is intended to provide an overview of how different metals and alloys react with certain media. Recommendations are not absolute, as the concentration of the medium, its temperature, pressure and other parameters can affect the applicability of a particular metal and alloy. The choice of metal or alloy can also be influenced by economic considerations.

CODES: A - usually does not corrode, B - minimal to negligible corrosion, C - not suitable

Table. Corrosion resistance of metals and alloys under normal conditions
№ Wednesday Aluminum Brass Cast iron and
carbonaceous
steel 		Stainless steel Alloy Titanium Zirconium
416 and 440C 17-4 304 acc. 08X18H10 316 acc. 03Х17Н142 Duplex 254 SMO 20 400 C276 B2 6
1 Acetate aldehyde A A C A A A A A A A A A A A A A
2 Acetic acid, air free C C C C C C A A A A A A A A A A
3 Air-saturated acetic acid C C C C B B A A A A C A A A A A
4 Acetone B A A A A A A A A A A A A A A A
5 Acetylene A A A A A A A A A A A A A A A A
6 Alcohols A A A A A A A A A A A A A A A A
7 Aluminum sulphate C C C C B A A A A A B A A A A A
8 Ammonia A C A A A A A A A A A A A A A A
9 Ammonia C C C C C C B A A A B A A B A A
10 Ammonia caustic A C A A A A A A A A C A A A A B
11 Ammonium nitrate B C B B A A A A A A C A A A C A
12 Ammonium phosphate B B C B B A A A A A B A A A A A
13 Ammonium sulfate C C C C B B A A A A A A A A A A
14 Ammonium sulphite C C C C A A A A A A C A A A A A
15 Aniline C C C C A A A A A A B A A A A A
16 Asphalt, bitumen A A A A A A A A A A A A A A A A
17 Beer A A B B A A A A A A A A A A A A
18 Benzene A A A A A A A A A A A A A A A A
19 Benzoic acid A A C C A A A A A A A A A A A A
20 Boric acid C B C C A A A A A A B A A A A A
21 Bromine dry C C C C B B B A A A A A A A C C
22 Bromine wet C C C C C C C C C C A A A C C C
23 Butane A A A A A A A A A A A A A A A A
24 Calcium chloride C C B C C B B A A A A A A A A A
25 Calcium hypochlorite C C C C C C C A A A C A B B A A
26 Dry carbon dioxide A A A A A A A A A A A A A A A A
27 Carbon dioxide wet A B C C A A A A A A B A A A A A
28 Carbon disulfide C C A B B A A A A A A A A A A A
29 Carbonic acid A B C C A A A A A A A A A A A A
30 Carbon tetrachloride A A B B A A A A A A A A A A A A
31 Chlorine dry C C A C B B B A A A A A A A C A
32 Chlorine wet C C C C C C C C C C B B B C A A
33 Chromic acid C C C C C C C B A C C A B C A A
34 Lemon acid B C C C B B A A A A A A A A A A
35 Coke acid C B A A A A A A A A B A A A A A
36 Copper sulphate C C C C C C B A A A C A A C A A
37 Cottonseed oil A A A A A A A A A A A A A A A A
38 Creosote C C A A A A A A A A A A A A A A
39 Dowtherm A A A A A A A A A A A A A A A A
40 Ethane A A A A A A A A A A A A A A A A
41 Ether A A B A A A A A A A A A A A A A
42 Ethyl chloride C B C C B B B A A A A A A A A A
43 Ethylene A A A A A A A A A A A A A A A A
44 Ethylene glycol A A A A A A A A A A A A A A A A
45 Iron chloride C C C C C C C C B C C A C C A A
46 Fluorine dry B B A C B B B A A A A A A A C C
47 Fluorine wet C C C C C C C C C C B B B C C C
48 Formaldehyde A A B A A A A A A A A A A A A A
49 Formic acid B C C C C C B A A A C A B B C A
50 Freon wet C C B C B B A A A A A A A A A A
51 Freon dry A A B A A A A A A A A A A A A A
52 Furfural A A A B A A A A A A A A A A A A
53 Gasoline stable A A A A A A A A A A A A A A A A
54 Glucose A A A A A A A C A A A A A A A A
55 Hydrochloric acid saturated with air C C C C C C C C C C C B A C WITH A
56 Hydrochloric acid, no air C C C C C C C C C C C B A C WITH A
57 Hydrofluoric acid, air-saturated C C C C C C C C C C B B B C WITH C
58 Hydrofluoric acid, no air C C C C C C C C C C A B B C WITH C
59 Hydrogen A A A C B A A A A A A A A A WITH A
60 Hydrogen peroxide A C C C B A A A A A C A C A A A
61 Hydrogen sulfide C C C C C A A A A A A A A A A A
62 Iodine C C C C C A A A A A C A A A WITH B
63 Magnesium hydroxide B B A A A A A A A A A A A A A A
64 Mercury C C A A A A A A A A B A A A WITH A
65 Methanol A A A A A A A A A A A A A A A A
66 Methylethylglycol A A A A A A A A A A A A A A A A
67 Milk A A C A A A A A A A A A A A A A
68 Natural gas A A A A A A A A A A A A A A A A
69 Nitric acid C C C C A A A A A A C B C WITH A A
70 Oleic acid C C C B B B A A A A A A A A A A
71 Oxalic acid C C C C B B B A A A B A A B WITH A
72 Oxygen C A C C B B B B B B A B B B WITH C
73 Mineral oil A A A A A A A A A A A A A A A
74 Air-saturated phosphoric acid C C C C B A A A A A C A A A WITH A
75 Phosphoric acid, no air C C C C B B B A A A B A A B WITH A
76 Picric acid C C C C B B A A A A C A A A A A
77 Potassium carbonate / potassium carbonate C C B B A A A A A A A A A A A A
78 Potassium chloride C C B C C B B A A A A A A A A A
79 Potassium hydroxide C C B B A A A A A A A A A A A A
80 Propane A A A A A A A A A A A A A A A A
81 Rosin, resin A A B A A A A A A A A A A A A A
82 Silver nitrate C C C C B A A A A A C A A A A A
83 Sodium acetate A A A A A A A A A A A A A A A A
84 Sodium carbonate C C A B A A A A A A A A A A A A
85 Sodium chloride WITH A C C B B B A A A A A A A A A
86 Sodium Chromate Decahydrate A A A A A A A A A A A A A A A A
87 Sodium hydroxide WITH WITH A B B B A A A A A A A A A A
88 Sodium hypochlorite C C C C C C C C C C C A B C A A
89 Sodium thiosulfate C C C C B B A A A A A A A A A A
90 Tin chloride C C C C C C B A A A C A A B A A
91 Water vapor A A A A A A A A A A A A A A A A
92 Stearic (octadecanoic) acid C B B B B A A A A A A A A B A A
93 Sulfur A B A A A A A A A A A A A A A A
94 Sulfur dioxide dry C C C C C C B A A A C A A B A A
95 Sulfur trioxide dry C C C C C C B A A A B A A B A A
96 Sulfuric acid saturated with air C C C C C C C A A A C A C B WITH A
97 Sulfuric acid, no air C C C C C C C A A A B A A B WITH A
98 Sulfurous acid C C C C C B B A A A C A A B A A
99 Tar A A A A A A A A A A A A A A A A
100 Trichlorethylene B B B B B B A A A A A A A A A A
101 Turpentine A A B A A A A A A A A A A A A A
102 Vinegar B B C C A A A A A A A A A A A A
103 Chemical purified water A A A A A A A A A A A A A C A A
104 Distilled water A A C C A A A A A A A A A A A A
105 Sea water - on land
RF is little known, but
extremely unpleasant environment,
applicability - "relative"		 WITH A C C C C B A A A A A A A A A
106 Whiskey, vodka, wine A A C C A A A A A A A A A A A A
107 Zinc chloride C C C C C C C B B B A A A B A A
108 Zinc sulphate WITH WITH WITH WITH A A A A A A A A A A A A

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