The corrosion rate is a multifactor parameter that depends both on the external environmental conditions and on the internal properties of the material. In the normative and technical documentation there are certain restrictions on the permissible values of metal destruction during the operation of equipment and building structures to ensure their trouble-free operation. In design, there is no universal method for determining the corrosion rate. This is due to the complexity of accounting for all factors. The most reliable method is to study the history of the operation of the facility.
Criteria
Currently, several corrosion indicators are used in the design of equipment:
- By a direct method of assessment: reduction in the mass of a metal part per surface unit — weight indicator (measured in grams per 1 m 2 in 1 hour); damage depth (or permeability of the corrosion process), mm / year; the amount of released gas phase of corrosion products; the length of time during which the first corrosion damage occurs; the number of corrosion centers per unit surface area appearing over a specified period.
- According to an indirect assessment: current strength of electrochemical corrosion; electrical resistance; change of physical and mechanical characteristics.
The first indicator by the direct method of assessment is the most common.
Calculation formulas
In the General case, the weight loss, determining the corrosion rate of the metal, is found by the following formula:
V kp = q / (St),
where q is the decrease in metal mass, g;
S is the surface area from which the material was transferred, m 2 ;
t is the time period, h
For sheet metal and shells made from it, the depth indicator (mm / year) is determined:
H = m / t
m is the depth of penetration of corrosion into the metal.
Between the first and second indicators described above, there is the following relationship:
H = 8.76V kp / ρ,
where ρ is the density of the material.
The main factors affecting the corrosion rate
The following groups of factors influence the rate of metal destruction:
- internal, associated with the physicochemical nature of the material (phase structure, chemical composition, surface roughness of the part, residual and working stresses in the material, and others);
- external (environmental conditions, speed of movement of a corrosive environment, temperature, composition of the atmosphere, the presence of inhibitors or stimulants, and others);
- mechanical (the development of corrosion cracks, the destruction of metal under the action of cyclic loads, cavitation and fretting corrosion);
- design features (choice of metal grade, the presence of gaps between parts, roughness requirements).
Physiochemical properties
Of the highest importance among internal corrosion factors are the following:
- Thermodynamic stability. For its determination in aqueous solutions, reference Purbe diagrams are used, on the abscissa of which the pH of the medium is deposited, and on the ordinate, the redox potential. A positive shift of the potential means greater stability of the material. Roughly it is defined as the normal equilibrium potential of the metal. In reality, materials corrode at different speeds.
- The position of the atom in the periodic table of chemical elements. Metals most susceptible to corrosion are alkaline and alkaline earth. The corrosion rate decreases with increasing atomic number.
- Crystal structure. It has a mixed effect on destruction. The coarse-grained structure per se does not lead to an increase in corrosion, but is favorable for the development of intergranular selective destruction of grain boundaries. Metals and alloys with a uniform distribution of phases corrode uniformly, and with an inhomogeneous one - by the focal mechanism. The mutual arrangement of the phases performs the function of the anode and cathode in an aggressive environment.
- Energy heterogeneity of atoms in the crystal lattice. The atoms with the highest energy are located in the corners of the microroughness faces and are active centers of dissolution during chemical corrosion. Therefore, thorough machining of metal parts (grinding, polishing, debugging) increases corrosion resistance. This effect is also explained by the formation of more dense and continuous oxide films on smooth surfaces.
The effect of acidity
In the process of chemical corrosion, the concentration of hydrogen ions affects the following points:
- solubility of corrosion products;
- the formation of protective oxide films;
- metal destruction rate.
At a pH in the range of 4–10 units (acidic solution), iron corrosion depends on the rate of penetration of oxygen to the surface of the object. In alkaline solutions, the corrosion rate first decreases due to surface passivation, and then, at pH> 13, increases as a result of dissolution of the protective oxide film.
Each type of metal has its own dependence of the intensity of destruction on the acidity of the solution. Noble metals (Pt, Ag, Au) are resistant to corrosion in an acidic environment. Zn, Al are rapidly destroyed both in acids and in alkalis. Ni and Cd are resistant to alkali, but easily corrode in acids.
The composition and concentration of neutral solutions
The corrosion rate in neutral solutions depends to a greater extent on the properties of the salt and its concentration:
- When salts are hydrolyzed in a corrosive environment, ions are formed that act as activators or moderators (inhibitors) of metal destruction.
- Those compounds that increase pH also increase the rate of the destructive process (for example, soda ash), and those that reduce acidity reduce it (ammonium chloride).
- In the presence of chlorides and sulfates in the solution, the destruction is activated until a certain salt concentration is reached (which is explained by the strengthening of the anode process under the influence of chlorine and sulfur ions), and then gradually decreases due to a decrease in oxygen solubility.
Some types of salts can form a sparingly soluble film (for example, iron phosphate). This helps protect the metal from further destruction. This property is used when using rust neutralizers.
Corrosion inhibitors
Retarders (or inhibitors) of corrosion differ in the mechanism of action on the redox process:
- Anode Thanks to them, a passive film is formed. This group includes compounds based on chromates and dichromates, nitrates and nitrites. The latter type of inhibitor is used for interoperational protection of parts. When using anode corrosion inhibitors, it is necessary to first determine their minimum protective concentration, since the addition of small amounts can lead to an increase in the rate of destruction.
- Cathodic. Their mechanism of action is based on a decrease in oxygen concentration and, accordingly, a slowdown of the cathode process.
- Shielding. These inhibitors isolate the surface of the metal by the formation of insoluble compounds deposited in the form of a protective layer.
The last group includes rust neutralizers, which are also used for cleaning oxides. Their composition, as a rule, includes phosphoric acid. Under its influence, metal phosphating occurs - the formation of a strong protective layer of insoluble phosphates. The neutralizers are applied with a spray or roller. After 25-30 minutes, the surface becomes white-gray. After drying, the composition is applied paints and varnishes.
Mechanical impact
Increased corrosion in an aggressive environment contribute to such types of mechanical stress, such as:
- Internal (during molding or heat treatment) and external (under the influence of externally applied load) stresses. As a result, electrochemical heterogeneity occurs, the thermodynamic stability of the material decreases, and corrosion cracking is formed. Particularly quickly, failure occurs under tensile loads (cracks form in perpendicular planes) in the presence of anions of oxidizing agents, for example, NaCl. A typical example of devices prone to this type of destruction is the details of steam boilers.
- Alternating dynamic action, vibration (corrosion fatigue). An intensive decrease in the fatigue limit occurs, multiple microcracks are formed, which then merge into one large one. The number of cycles to failure is more dependent on the chemical and phase composition of metals and alloys. Axes of pumps, springs, turbine blades and other items of equipment are subject to such corrosion.
- Friction parts. Rapid corrosion is caused by mechanical wear of protective films on the surface of the part and chemical interaction with the environment. In a liquid, the rate of destruction is lower than in air.
- Cavitation shock. Cavitation occurs when the continuity of the fluid flow is violated as a result of the formation of vacuum bubbles, which collapse and create a pulsating effect. As a result, deep local damage occurs. This type of corrosion is often observed in chemical apparatuses.
Design factors
When designing elements operating in aggressive conditions, it must be borne in mind that the corrosion rate increases in the following cases:
- upon contact of dissimilar metals (the greater the difference in electrode potential between them, the higher the current strength of the electrochemical destruction process);
- in the presence of stress concentrators (grooves, grooves, holes, and others);
- at low cleanliness of the treated surface, as this results in local short-circuited galvanic pairs;
- with a significant difference in temperature of individual parts of the device (thermovoltaic cells are formed);
- in the presence of stagnant zones (cracks, gaps);
- during the formation of residual stresses, especially in welded joints (to eliminate them, it is necessary to provide for heat treatment - annealing).
Assessment Methods
There are several ways to assess the rate of destruction of metals in aggressive environments:
- Laboratory - tests of samples in artificially simulated conditions close to real ones. Their advantage is that they reduce the time of the study.
- Field - conducted in vivo. Take a long time. The advantage of this method is to obtain information about the properties of the metal under conditions of further operation.
- Full-scale - tests of finished metal objects in the natural environment.