Corrosion resistance rating

To characterize the corrosion properties of materials, they are usually tested for resistance. against general corrosion, intergranular corrosion and stress corrosion cracking.

General corrosion tests. General corrosion tests are carried out on specimens with a large surface to volume ratio. The corrosive environment is selected taking into account the operating conditions of the material. Tests are carried out in liquid with constant or repeatedly repeated alternating immersion of samples, in boiling brine, in vapor or ambient atmosphere.

The corrosion rate of metals and alloys is characterized by a deep corrosion index h K , mm/year - tab. 2 or weight loss g K, g / (m 2 ∙ h) - table. 3.

The recalculation of both indicators is carried out according to the formula:

h K = 8,76 g K / ρ,(1)

where h K - corrosion rate, mm/year;

ρ – density, g/cm 3 ;

g K – sample weight loss, g/(m 2 h).

Characteristics h K and g K assume uniform corrosion and typically represent surface-averaged corrosion rates. However, it is known that local types of corrosion are the most dangerous. With a relatively small total loss of metal mass, a strong local destruction of the structure occurs, and this leads to premature failure of the equipment.

table 2

Ten-point scale of corrosion resistance of metals according to the depth of corrosion

Corrosion resistance score	Corrosion rate h K, mm/year	Resistance group
	≤ 0,001	Completely resistant
	(> 0,001) – 0,005	Very resistant
	(> 0,005) – 0,01	Very resistant
	(> 0,01) – 0,05	Persistent
	(> 0,05) – 0,1	Persistent
	(> 0,1) – 0,5	Persistent lowered
	(> 0,5) – 1,0	Persistent lowered
	(> 1,0) – 5,0	Low resistance
	(> 5,0) – 10,0	Low resistance
	> 10,0	unstable

Table 3

Ten-point scale of corrosion resistance according to the rate of sample corrosion

Core score resilience	Resistance group	Weight loss, g K, g / (m 2 ∙ h)
Black metals	Copper and alloys	Nickel and Alloys	Lead and alloys	Aluminum and alloys	Magnesium and alloys
	Completely resistant	<0,0009	<0,001	<0,001	<0,0012	<0,0003	<0,0002
	Very resistant	0,0009-0,0045	0,001-0,0051	0,001-0,005	0,0012-0,0065	0,0003-0,0015	0,0002-0,001
	Very resistant	(>0,0045)-0,009	(>0,0051)-0,01	(>0,005)-0,01	(>0,0065)-0,012	(>0,0015)-0,003	(>0,001)-0,002
	Persistent	0,009-0,045	0,01-0,051	0,01-0,05	0,012-0,065	0,003-0,015	0,002-0,01
	Persistent	(>0,045)-0,09	(>0,051)-0,1	(>0,05)-0,1	(>0,065)-0,12	(>0,015)-0,03	(>0,01)-0,02
	Persistent lowered	(>0,09)-0,45	(>0,1)-0,5	(>0,1)-0,5	(>0,12)-0,65	(>0,03)-0,15	(>0,02)-0,1
	Persistent lowered	(>0,45)-0,9	(>0,5)-1,02	(>0,5)-1,0	(>0,65)-1,2	(>0,15)-0,31	(>0,1)-0,2
	Low resistance	(>0,9)-4,5	(>1,02)-5,1	(>1,0)-5,0	(>1,2)-6,5	(>0,31)-1,54	(>0,2)-1,0
	Low resistance	(>4,5)-9,1	(>5,1)-10,2	(>5,0)-10,0	(>6,5)-12,0	(>1,54)-3,1	(>1,0)-2,0
	unstable	>9,1	>10,2	>10,0	>12,0	>3,1	>2,0

Therefore, it is necessary to check the corrosion resistance of materials under specific operating conditions, especially in cases where there is a risk of local corrosion.

Intergranular Corrosion Tests(GOST 6032-84). The main cause of intergranular corrosion of corrosion-resistant materials is heating during pressure treatment or welding, which leads to electrochemical heterogeneity between the boundary regions and the volume of grains.

The temperature-time region of precipitation along the grain boundaries of corrosion-resistant steels of chromium carbides is shown in fig. 4. Inside it is an area of ​​sensitization - increased sensitivity to intergranular corrosion. The tendency to intergranular corrosion manifests itself in the temperature range Т max –T min for the minimum time τ min during which sensitization occurs.

Rice. 4. Temperature-time propensity region

corrosion-resistant austenitic steel to intergranular corrosion (ICC) associated with the depletion of grain boundaries in chromium:

T p is the temperature of dissolution of carbides; γ – austenite;

K - carbides

When testing for MCC, chromium steels are subjected to provoking heating at a temperature of 1100 ° C for 30 hours, and chromium-nickel austenitic steels at a temperature of about 700 ° C for 60 hours. After heating, the samples are kept for a long time in a boiling aqueous solution of sulfuric or nitric acid . The choice of holding time and the type of corrosive medium depends on the specific steel grade and its purpose. To control the tendency to ICC, the samples are either bent on a mandrel at an angle of 90°, or subjected to etching with special reagents and metallographic examination. The absence of cracks on the surface of the sample indicates its resistance to ICC.

On fig. Figure 5 shows the microstructures of steel 08Kh18N10 after tests for intergranular corrosion in different media.

Fig.5. Microstructure of steel 08X18H10

after quenching from 1050°C in water and tempering at 700°C:

a - intergranular corrosion during testing

in a solution of 25% HNO 3 + 40 g/l Cr 6+, duration 200 hours;

b - the same in a solution of boiling 65% HNO 3 + Cr 6+, × 500

Stress corrosion cracking tests. This type of test is carried out when the sample is loaded in a corrosive environment corresponding to the operating conditions of the part. The environment should not cause general corrosion and affect unloaded metal samples. For austenitic chromium-nickel steels, an example of such a medium is a boiling solution of a mixture of salts MgCl 2 , NaCl and NaNO. The aggressiveness of the media should not be less than that in which the tested materials must serve.

Stress corrosion cracking tests can be carried out either under conditions that cause failure of materials (tensile, fracture toughness and fatigue tests) or by determining the time of the first crack. The last type of test consists in fixing loaded specimens in special fixtures or by applying stresses with a wedge in cut rings. The time to cracking characterizes the resistance of materials to stress corrosion cracking.

Test questions\

1. List the methods for protecting metals and alloys from corrosion.

2. What determines the choice of corrosion protection method?

3. What is steel alloying?

4. What are bimetals?

5. What method is used to make bimetals?

6. What are corrosion inhibitors?

7. What is the mechanism of protection of metals and alloys from corrosion using anodic inhibitors?

8. What is the mechanism of protection of metals and alloys from corrosion using cathodic inhibitors?

9. What are the benefits of using volatile inhibitors?

10. What form of products is preferable for slowing down corrosion processes?

11. How does the cleanliness of parts processing affect the corrosion rate?

12. What explains the high corrosion resistance of aluminum and its alloys?

13. Name the most corrosion-resistant ferrous alloys.

14. Name the most corrosion-resistant non-ferrous alloys.

15. What determines the choice of the type of corrosion protection?

16. What types of corrosion are investigated when testing for corrosion resistance?

17. In what corrosive environment are general corrosion tests carried out?

18. What indicators characterize the corrosion rate of metals and alloys?

19. What is the dimension of the deep corrosion index?

20. What is the dimension of sample mass loss during corrosion?

21. What is the corrosion rate of materials that are completely resistant?

22. What is the corrosion rate of materials that are highly resistant?

23. What is the rate of corrosion of resistant materials?

24. What is the corrosion rate of low-resistance materials?

25. What is the corrosion rate of unstable materials?

26. What is the mass loss of a ferrous alloy sample with a corrosion resistance score of 3?

27. What is the mass loss of a copper alloy sample with a corrosion resistance score of 7?

28. What is the mass loss of a nickel alloy sample with a corrosion resistance score of 4?

29. What is the mass loss of a lead alloy sample with a corrosion resistance score of 5?

30. What is the mass loss of an aluminum alloy sample with a corrosion resistance score of 9?

31. What is the mass loss of a magnesium alloy sample with a corrosion resistance score of 10?

32. What is the main cause of intergranular corrosion?

33. Decipher the brand of alloy 08X18H10.

34. In what corrosive environment are corrosion cracking tests carried out?

35. How are corrosion cracking tests carried out?

The rate of corrosion destruction of metal is characterized by a weight or depth indicator. The first expresses the change in weight of the sample due to corrosion, per unit of metal surface and unit of time. The second one shows the depth of corrosion destruction of a metal sample, expressed in linear units and related to a unit of time.

The use of only one of these indicators often does not give a correct idea of ​​the danger of corrosion for the structure. So, for example, with the development of local corrosion, the weight indicator may be insignificant, and the structure may be in disrepair; on the contrary, with uniform corrosion, the total corrosion losses can be large, and at the same time, the risk of failure of the structure from corrosion with its slow development in depth and sufficient thickness of the product will be less. Therefore, for a more complete picture of the rate and nature of corrosion, both indicators should be used.

The risk of destruction of the structure in the soil is the greater, the less uniformly the corrosion is distributed over the surface of the structure. In the case of development of local corrosion, the most dangerous will be those corrosion lesions that have the smallest area, since they develop faster than others deep into the structure wall due to the concentration of anodic dissolution of the metal in a limited area.

The nature, rate of corrosion and features of its distribution over the surface of the structure are determined both by the properties of the metal itself and by external conditions. Depending on the combination of external conditions, the quantitative indicators of corrosion for the same metal can vary significantly.

Therefore, the actual corrosion resistance of a particular metal is relative. It cannot be expressed as an absolute measure without comprehensive consideration of the conditions under which the corrosion process develops. Therefore, in the ideal case, the determination of the scope and type of protective measures should be based on a thorough study and analysis of the totality of external and internal corrosion factors.

Corrosion resistance -- the ability of materials to resist corrosion, determined by the rate of corrosion 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.

For quantification, you can use:

• time elapsed before the appearance of the first corrosion focus;
• the number of corrosion centers 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 surface per unit time;
• the volume of gas released (or absorbed) during corrosion of a surface unit per unit of time;
• current density corresponding to the rate of the given corrosion process;
• · change of any property for a certain time of corrosion (for example, and other electrical resistance, reflectivity of the material, mechanical properties).

Different materials have different corrosion resistance, to improve which special methods are used. Thus, an increase in corrosion resistance is possible with the help of alloying (for example, stainless steels), applying protective coatings (chromium plating, nickel plating, painting products), passivation

Lab #8

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

1. Guidelines

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, chemical, electrochemical and biocorrosion are distinguished.

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 the metal, accompanied by the appearance of an electric current as a result of the work of many macro- and microgalvanic pairs. 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 electrons in the metal:

(-)A: Me + mH 2 O → 1+ + ne

2) the cathode process is the assimilation of excess electrons in the metal by some depolarizers (molecules or ions of the solution that can be reduced at the cathode). During corrosion in neutral media, the depolarizer is usually corrosion into oxygen dissolved in the electrolyte:

(+)K: O 2 + 4e +2H 2 O →4OH¯

During corrosion in acidic environments - hydrogen ion

(+)K: H H 2 O + e → 1/2H 2 +H 2 O

Macrogalvanic pairs are produced when different metals come into contact. In this case, the metal having a more negative electrode potential is the anode and undergoes oxidation (corrosion).

The metal with the more positive potential serves as the cathode. It acts as a conductor of electrons from the anode metal to the particles of the environment that are capable of receiving these electrons. According to the theory of micropairs, the cause of electrochemical corrosion of metals is the presence on their surface of microscopic short-circuited galvanic cells that arise due to the heterogeneity of the metal and its contact with the environment. Unlike galvanic cells specially made in the technique, they spontaneously appear 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 metal surface. This creates conditions for the contact of the metal 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 processed parts of the surface, different structural components of the alloy, impurities and the base metal.

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

Part of the released electrons will pass 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 that have passed into the solution, the dissolution of the metal stops. Therefore, electrical corrosion can occur if electrons from the anode sites are continuously withdrawn 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 in its composition. When brass (an alloy of copper and zinc) comes into contact with iron, brass will begin to corrode (due to the presence of zinc in it). When the medium changes, the electrode potential of individual metals can change dramatically. Chromium, nickel, titanium, aluminum, and other metals whose normal electrode potential is sharply negative, are strongly passivated under normal atmospheric conditions, 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 \u003d 4OH¯

As a result: 2Fe 2 + 4OH¯ \u003d 2Fe (OH) 2

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

However, in an acidic, alkaline or neutral environment containing chlorine ions (for example, in sea water), which destroy the oxide film, aluminum in contact with iron becomes an anode and undergoes a corrosion process. The following galvanic cell will work in NaCl solution and sea water:

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

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

(+)K: O 2 +4e + 2H 2 O \u003d 4OH¯

4Al 3 + 12OH¯ \u003d 4Al (OH) 3

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

The occurrence 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 reasons causing corrosion destruction of metals are numerous. There are various methods of protection against corrosion:

processing of the external environment;

protective coatings;

electrochemical protection;

production of specially corrosion-resistant alloys.

Treatment of the external environment is to remove or reduce the activity of some of the corrosive substances present in it. For example, the removal of oxygen dissolved in iodine (deaeration). Sometimes special corrosion-retarding substances are added to the solution, which are called retarders or INHIBITORS (urotropine, thiourea, aniline, and others).

Parts that are protected 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 the inhibition of electrode processes.

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

Metal coatings are divided into anodic and cathodic. In the case of ANODE coating, the electrode potential of the coating metal is more negative than the potential of the protected metal. In the case of a CATHODE coating, the electrode potential of the coating metal is more positive than that 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 anode and cathode coatings. When the integrity of the coating is violated, new conditions arise. A cathodic 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. To do this, the product to be protected is either connected to a metal that has a more negative electrode potential, capable of more easily giving up electrons (protective protection) or with the 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 coating layer is violated, will itself be destroyed, thereby protecting the base metal from corrosion (zinc is the anode in the resulting galvanic cell).

Production 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 emergence in it of such microgalvanic cells, in which the total EMF approaches zero due to mutual compensation. Such useful additives, especially for steel, are chromium, nickel and other metals.

1. Getting the job done

Exercise 1

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

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

Progress of work: 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.

Task 2

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

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

Pour 5-10 ml of an aqueous solution of NaCl into a U-shaped tube. Metal plates are lowered into it, interconnected with clamps.

The metal plates must be carefully cleaned with an emery cloth, and the place of contact between the plate and the clamp is out of the solution. When performing the experiment, it is necessary to note the 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 the metal ion was found in solution

3) a diagram of a galvanic cell.

1. 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.

Task 3

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

The experiment is carried out on the installation shown in Figure 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. What metal produces hydrogen bubbles? Write appropriate responses. Bring two metals into contact with each other. On which metal do hydrogen bubbles form when the metals come into contact? Draw a diagram of a galvanic cell and electrode processes on its electrodes. Write the overall reaction equation.

3. Examples of problem solving

Example 1

Consider the corrosion process in the 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, passing two electrons to lead, go into solution in the form of ions. Electrons on lead restore hydrogen ions that are in solution, tk.

HCl = H + + Cl¯

Anode process Fe 0 - 2e \u003d 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

Anode process Fe - 2e \u003d 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 the electrolyte solution.

Example 3

Why is chemically pure iron more resistant to corrosion than commercial iron? Compose the electronic equations of the anode and cathode processes occurring during the corrosion of technical iron.

Decision

The process of corrosion of technical iron is accelerated due to the formation of micro and submicrogalvanic elements in it. In microgalvanic pairs, the base metal, as a rule, serves as an anode; iron. The 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, the electrons that have passed here from the anode sites are bound either by oxygen in the air dissolved in water, or by hydrogen ions. In neutral environments, oxygen depolarization occurs:

K: O 2 + 4e + 2H 2 O \u003d 4OH¯

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

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

Example 4

Name, cathodic, or anodic is zinc and a 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?

Decision

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

Zn 2+ + 2OH \u003d Zn (OH) 2

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

K(Fe): O 2 + 4e + 2H 2 O = 4OH¯

Example 5

Cadmium and nickel plates, being immersed in dilute sulfuric acid, dissolve in it with the release of hydrogen. What will change if both of them are lowered simultaneously into a vessel with acid, connecting the ends with a wire?

Decision

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

A: Cd - 2e \u003d Cd 2+,

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

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

Thus, only cadmium undergoes dissolution, nickel will only become an electron conductor and will not dissolve itself. Hydrogen will be released only on the nickel plate.

Example 6

How does the pH of the environment affect the rate of aluminum corrosion?

Decision

Reducing the pH of the environment, i.e. an increase in the concentration of H-ions sharply increases the rate of nickel corrosion, - since an 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+

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

Ni 2+ - 2OH¯ \u003d NI (OH) 2

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

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

Al (OH) 3 + NaOH \u003d 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 environment, aluminum - 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 is immediately covered with hydrogen bubbles. Explain this phenomenon. What metal goes into solution in this case?

2. There are nickel parts in the iron product. How will this affect the corrosion of iron? Write the corresponding anodic and cathodic processes if the product is in a humid atmosphere.

3. In what medium is the rate of iron destruction greater? What environment favors the anodic oxidation of zinc? Write appropriate responses.

4. How does atmospheric corrosion of tinned iron and tinned copper occur when the integrity of the coating is violated? Compose the electronic equations of the anode and cathode processes.

5. Copper does not displace hydrogen from dilute acids. Why? However, if a zinc plate is touched to a copper plate, then a rapid evolution of hydrogen begins on the copper. Explain this by writing the electronic equations for the cathode and anode processes.

6. A zinc plate and a zinc plate partially covered with copper were lowered into an electrolyte solution containing dissolved oxygen. In which case does the zinc corrosion process occur more intensively? Compose the electronic equations of the cathode and anode 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 the equations of the corresponding processes.

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

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

10. Iron plates omitted:

a) distilled water

b) sea water

In which case 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 does industrial zinc interact with acid more intensively than chemically pure zinc?

13. A plate is lowered into the electrolyte solution:

b) copper, partially covered with tin

In which case is the corrosion process more intense?

Motivate the answer

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

Compose the electronic equations for the reactions that occur in the corrosion processes when the nickel coating is damaged.

15. An iron product was coated with cadmium. What kind of coating is it - anode or cathode?

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

16. Which of the metals:

b) cobalt

c) magnesium

can be a protector to an iron-based alloy. Compose the electronic equations of the corresponding processes (acid medium).

17. What processes will occur on zinc and iron plates if each is immersed separately in a solution of copper sulphate? What processes will take place if the outer ends in the solution of the plates are connected with a conductor? Write electronic equations

18. Aluminum plate lowered

a) distilled water

b) in a solution of sodium chloride

In which case is the corrosion process more intense? Make equations for the anodic and cathodic corrosion processes of technical aluminum in a neutral environment.

19. If a nail is driven into a damp tree, then the part that is inside the tree is covered with rust. How can this be explained? Is this part of the nail anode or cathode?

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

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Basic concepts, terms, definitions

Corrosion resistance - the ability of a material to withstand the action of aggressive environments (corrosion).

Corrosion (from lat. sorrosio - corrosion) - the destruction of materials due to chemical or electrochemical interaction with the environment.

Building materials, and primarily their surfaces, during long-term operation are destroyed mainly as a result of two types of impact: corrosive, associated with the influence of an external, aggressive environment on the material, and erosive, caused by mechanical action.

Erosive destruction proceeds intensively with a relatively fast movement of the medium or material. Erosion reaches a particularly large value when the material comes into contact with melted metals and slags, as well as with gaseous oxidizers, etc.

The phenomena of corrosion and erosion often accompany each other, and therefore it is not always possible to separate them. In building materials science, these phenomena are considered separately. Erosion processes are considered when studying the performance properties of floor coverings, road surfaces, etc.

Types of corrosion of building materials

Corrosion of building materials differs in the type of corrosive environment, the nature of destruction and the processes occurring in them:

corrosive environment:

gas: (inert gas; reactive gas);

liquid: (acid; saline; alkaline, marine; river; in the melt of metals, silicates)

nature of destruction: (uniform, saline, uneven, selective, surface, cracking, local, intergranular);

types of influences (processes):(chemical; electrochemical; biological).

Gas corrosion is corrosion in a gaseous environment in the complete absence of moisture condensation on the surface of the material. Materials operating at high temperatures in a dry gas environment (ceramics) are susceptible to this type of corrosion. Gas corrosion refers to chemical destruction processes. Its speed depends on the nature of the material, its structure and the properties of neoplasms on its surface.

Liquid corrosion natural and artificial stone materials, occurring under the action of electrolyte and non-electrolyte solutions, as well as various melts, is mainly chemical in nature, although, depending on the type and properties of the liquid, it differs in a number of features. The most important feature of liquids is the presence of forces of intermolecular interaction in them. This is due to two properties of the liquid state: molecular pressure and the surface tension associated with it. The surface tension of a liquid has a great influence on the intensity of material destruction, which is also determined by the wetting properties of the liquid.

uniform corrosion occurs as a result of the action of an aggressive environment with a sufficient thickness of the product and a uniform distribution of compressive, bending or tensile stresses. Corrosion of this type, unlike others, affects the strength properties of the material to a much lesser extent.

Uneven or localized corrosion(spots, ulcers, streaks) occurs at different concentrations of the aggressive medium in individual areas or the heterogeneity of the material itself (its composition and structure). Thus, as a result of the uneven distribution of the crystalline and glassy phases in a ceramic material, corrosion destruction in its individual sections proceeds at different rates. In this case, the process develops much faster in the glassy phase than in the crystalline phase. The presence of heterogeneous porosity in the material also contributes to the formation of uneven corrosion in it.

Selective corrosion characteristic of materials in which one of the components during the formation of the structure forms easily soluble compounds. During operation, these compounds can go into solution, forming the so-called "efflorescence" on the surface of the material.

Intergranular corrosion It arises as a result of the destruction of the material along the grain boundaries and quickly spreads into the depth of the material, sharply reducing its properties. This type of corrosion is inherent in some firing materials, during the sintering of which new phases, solid solutions, etc. are formed, and, consequently, interfaces.

Corrosion action in the general case can have two fundamentally different mechanisms: chemical interaction and dissolution.

Chemical interaction is reduced to a reaction between the medium and the material with the formation of new compounds. In the presence of impurities in aggressive media, and in the material - additives, chemical reactions can occur between all elements of interaction. Since stone materials are dielectrics and their interaction with an aggressive environment is not accompanied by the appearance of electric currents, the process of destruction of materials is called chemical corrosion.

Under the influence of corrosive media on metals, an electrochemical process of electron transfer from a metal layer with a lower electric potential to a layer with a higher potential occurs, and electropositive ions are restored, followed by destruction of the surface layer. This destruction process is called electrochemical corrosion.

biological corrosion- the destruction of the material under the direct influence of plant and animal organisms, as well as microorganisms. Higher plant organisms (root system, stems, leaves, seeds, etc.) in the process of life produce various types of substances, most of which are aggressive in relation to building materials. Animal organisms cause biodamage of materials both directly by their mechanical action (rodents, birds, etc.) and by the products of their vital activity. Lower plant organisms and microorganisms (algae, lichens, mosses, fungi, bacteria, etc.) destroy the surface layers of concrete and create conditions for decay of wood structures.

Corrosion resulting from the impact on building materials of products of technological processing of organic substances, both biogenic (fruits, vegetables, vegetable oils, blood, juices, fats, etc.) and non-biogenic origin (oil, coal, shale, shell limestone, exhaust gases, soot, etc.), commonly called organogenic corrosion.

Factors affecting the corrosion resistance of building materials

Corrosion resistance of building materials depends on many factors, which are divided into external and internal.

External factors determine the aggressiveness of the medium and its effect on the material. These include the pH of the medium, the temperature and its difference, as well as the intensity of the impact of the medium on the material.

The hydrogen index of the electrolyte solution, which characterizes the activity of hydrogen ions in it, is a very important factor influencing the process of chemical corrosion. The corrosion rate of silicates in electrolyte solutions largely depends on the nature of the solutions and proceeds differently in acidic, alkaline, or neutral media.

Water as a participant in the technological process is considered in two aspects: as a neutral component that serves to give the mixture the necessary properties, and as a solvent and ion carrier.

The reason for the corrosion of many building materials in water or in other electrolytes is the thermodynamic instability of the compounds contained in these materials, which is associated with the development of hydration processes accompanied by exothermic or endothermic effects.

The exothermic effect indicates a creative process in the material, for example, during the hydration of cement, and the endothermic effect indicates a destructive process, for example, during the hydration of a ceramic shard.

The behavior of chemical elements in solutions largely depends on the magnitude of the radii of the ions and their valence, or rather, on the ratio of the valence of the ion to its radius, called the ionic potential:

where PI - ionic potential, Å-1;

V - valency, units;

R - ionic radius, Å..

The lower the ionic potential, the stronger the basic properties of the elements are manifested, the larger it is - acidic. For example, K and Na, characterized by low ionic potentials, respectively 0.75 and 1.02, have pronounced alkaline properties. Elements with an ionic potential in the range of 4.7 ... 8.6 have amphoteric properties, and at pH> 8.6 acidic properties

Comparing the activity of elements by ionic potential, we obtain the following distribution of cations in descending order:

SiO2 → TiO2 → MgO → Fe → Cu

The high ionic potential of the silicon cation causes the formation of strong anionic groups with oxygen ions.

Temperature- one of the most important variables affecting corrosion and erosion resistance. An increase in temperature, as a rule, contributes to an increase in the corrosion effect due to an increase in the limiting solubility, diffusion rate, and intensity of chemical reactions.

temperature fluctuations thermal mass transfer is caused in the system, which can make it unsuitable to use a material that under normal conditions has little solubility.

The intensity of the impact of the environment affects the rate of corrosion processes. Increasing the volume of media in contact with the material can increase the corrosive effect by increasing the average dissolution rate of the material.

Internal factors - this is the composition, structure of the material and its properties.

Due to the structural features of various materials, the influence of external factors on them is not the same, and therefore the corrosion resistance of firing, fused, hydration materials, as well as metals and wood, is considered separately. And we will start studying the properties of specific materials with the next lecture.

General principles for improving corrosion resistance

Corrosion resistance is determined by the mass of material converted into corrosion products per unit time per unit area, which is in interaction with an aggressive environment, as well as the size of the destroyed layer in mm per year.

The main principles of increasing the corrosion resistance of building products and structures are:

Selection of the composition of the compositions, characterized by low activity in aggressive environments;

The use of special coatings for chemical, thermal and mechanical protection of products and structures from the effects of aggressive environments.

It should be noted that the main criterion that determines the performance properties of building materials is time. Therefore, such material characteristics as water resistance, frost resistance, and corrosion resistance are not true physical properties, but only conditional indicators of a change in the state of its structure during prolonged constant or cyclic exposure to an aggressive environment.

Maintaining performance over time is commonly referred to as durability of building materials .

Corrosion resistance

Corrosion resistance- the ability of materials to resist corrosion, determined by the rate of corrosion 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. For quantification, you can use:

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

Different materials have different corrosion resistance, to improve 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, product painting), passivation, etc. The resistance of materials to corrosion, typical for marine conditions, is studied in chambers salt fog.

Sources

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See what "Corrosion resistance" is in other dictionaries:

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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 may be a local surface damage - pitting or rusting; for organic materials, this is the formation of hair ... ... Technical Translator's Handbook

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CORROSION RESISTANCE- property of materials to resist corrosion. Corrosion resistance is determined by the mass of the material converted into corrosion products per unit time per unit area of ​​the product that interacts with an aggressive environment, as well as the size ... ... Metallurgical Dictionary

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

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

Metals, the ability of a metal or alloy to resist the corrosive effects of an environment. K. s. determined by the corrosion rate under given conditions. The corrosion rate is characterized by qualitative and quantitative indicators. To the first ... ... Great Soviet Encyclopedia

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 provides tables and diagrams of the corrosion resistance of metals and alloys, ...
• Corrosion resistance and corrosion protection of metal, powder and composite materials, Vladimir Vasiliev. This manual is devoted to the description of the corrosion resistance of the most commonly used structural materials in modern engineering and technology: iron, steels, cast irons, aluminum, ...