Metal Corrosion - Causes And Methods Of Protection

2023 Author: Douglas Hoggarth | [email protected]. Last modified: 2023-11-26 20:16

• What is metal corrosion
• Chemical corrosion
• Electrochemical corrosion
• Other causes of metal corrosion
• Corrosion protection measures for metals
• Anti-corrosion protection with non-metallic coatings
• Iron protection against corrosion by coatings of other metals
• Improving corrosion resistance by adding alloying additives to steel alloys
• Anti-corrosion measures
• Protection against stray currents

The expression "costs a pretty penny" in relation to metal corrosion is more than true - the annual damage caused by corrosion is at least 4% of the annual income of any developed country, and in Russia the amount of damage is calculated in ten digits. So what causes corrosive processes in metals and how to deal with them?

What is metal corrosion

Destruction of metals as a result of electrochemical (dissolution in a moisture-containing air or water environment - electrolyte) or chemical (formation of metal compounds with chemical agents of high aggression) interaction with the environment. The corrosion process in metals can develop only in some areas of the surface (local corrosion), cover the entire surface (uniform corrosion), or destroy the metal along the grain boundaries (intergranular corrosion).

Metal under the influence of oxygen and water becomes a loose light brown powder, better known as rust (Fe ₂ O ₃ · H ₂ O).

Chemical corrosion

This process takes place in environments that are not conductors of electric current (dry gases, organic liquids - oil products, alcohols, etc.), and the corrosion intensity increases with increasing temperature - as a result, an oxide film forms on the metal surface.

All metals, both ferrous and non-ferrous, are subject to chemical corrosion. Active non-ferrous metals (for example, aluminum) become covered with an oxide film under the influence of corrosion, which prevents deep oxidation and protects the metal. And such a low-active metal like copper, under the influence of moisture in the air, acquires a greenish bloom - patina. Moreover, the oxide film does not protect the metal from corrosion in all cases - only if the crystal-chemical structure of the formed film is consistent with the structure of the metal, otherwise the film will not help.

Alloys are susceptible to another type of corrosion: some elements of the alloys are not oxidized, but are reduced (for example, in the combination of high temperature and pressure in steels, the reduction of carbides with hydrogen occurs), while the alloys completely lose the necessary characteristics.

Electrochemical corrosion

The process of electrochemical corrosion does not require the mandatory immersion of the metal in the electrolyte - a sufficiently thin electrolytic film on its surface (often electrolytic solutions impregnate the environment surrounding the metal (concrete, soil, etc.)). The most common cause of electrochemical corrosion is the widespread use of household and industrial salts (sodium and potassium chlorides) to remove ice and snow on roads in winter - cars and underground utilities are particularly affected (according to statistics, annual losses in the United States from the use of salts in winter are $ 2.5 billion).

The following happens: metals (alloys) lose some of their atoms (they pass into the electrolytic solution in the form of ions), electrons replacing the lost atoms charge the metal with a negative charge, while the electrolyte has a positive charge. A galvanic pair is formed: the metal is destroyed, gradually all its particles become part of the solution. Electrochemical corrosion can be caused by stray currents arising from the leakage from the electric circuit of a part of the current into aqueous solutions or into the soil and from there into a metal structure. In those places where stray currents leave metal structures back into water or soil, metal destruction occurs. It is especially common for stray currents to occur in places where ground electric vehicles (for example, trams and railway locomotives powered by electric traction) move. In just a year, wandering currents of 1A are able to dissolve iron - 9.1 kg, zinc - 10.7 kg, lead - 33.4 kg.

Other causes of metal corrosion

The development of corrosive processes is facilitated by radiation, waste products of microorganisms and bacteria. Corrosion caused by marine microorganisms damages the bottoms of ships, and corrosive processes caused by bacteria even have their own name - biocorrosion.

The combination of the effects of mechanical stresses and the external environment accelerates the corrosion of metals many times over - their thermal stability decreases, surface oxide films are damaged, and in those places where inhomogeneities and cracks appear, electrochemical corrosion is activated.

Corrosion protection measures for metals

An inevitable consequence of technological progress is the pollution of our environment - a process that accelerates the corrosion of metals, since the external environment is increasingly aggressive towards them. There is no way to completely eliminate the corrosive destruction of metals; all that can be done is to slow down this process as much as possible.

To minimize the destruction of metals, you can do the following: reduce the aggression of the environment surrounding the metal product; increase the resistance of metal to corrosion; exclude the interaction between the metal and substances from the external environment that show aggression.

For thousands of years, mankind has tried many ways to protect metal products from chemical corrosion, some of them are used to this day: coating with fat or oil, other metals that corrode to a lesser extent (the most ancient method, which is more than 2 thousand years old, is tinning (coating tin)).

Anti-corrosion protection with non-metallic coatings

Non-metallic coatings - paints (alkyd, oil and enamels), varnishes (synthetic, bituminous and tar) and polymers form a protective film on the surface of metals, which excludes (with its integrity) contact with the external environment and moisture.

The use of paints and varnishes is advantageous because these protective coatings can be applied directly at the assembly and construction site. Methods of applying paints and varnishes are simple and amenable to mechanization, damaged coatings can be restored "on the spot" - during operation, these materials have a relatively low cost and their consumption per unit area is small. However, their effectiveness depends on compliance with several conditions: compliance with the climatic conditions in which the metal structure will be used; the need to use exclusively high-quality paints and varnishes; strict adherence to the technology of application on metal surfaces. It is best to apply paints and varnishes in several layers - their amount will provide the best protection against weathering on the metal surface.

Polymers such as epoxy resins and polystyrene, polyvinyl chloride and polyethylene can act as protective coatings against corrosion. In construction work, reinforced concrete embedded parts are covered with coatings from a mixture of cement and perchlorovinyl, cement and polystyrene.

Iron protection against corrosion by coatings of other metals

There are two types of metal inhibitor coatings - tread (zinc, aluminum and cadmium coatings) and corrosion resistant (silver, copper, nickel, chromium and lead coatings). Inhibitors are applied chemically: the first group of metals has a high electronegativity in relation to iron, the second - a high electropositiveness. The most widespread in our everyday life are metal coatings of iron with tin (tinplate, cans are made from it) and zinc (galvanized iron - roofing), obtained by pulling sheet iron through the melt of one of these metals.

Cast iron and steel fittings, as well as water pipes are often galvanized - this operation significantly increases their resistance to corrosion, but only in cold water (when hot water is supplied, galvanized pipes wear out faster than non-galvanized ones). Despite the effectiveness of galvanizing, it does not provide ideal protection - zinc coating often contains cracks, which require preliminary nickel plating of metal surfaces (nickel plating) to eliminate them. Zinc coatings do not allow the application of paints and varnishes to them - there is no stable coating.

The best solution for corrosion protection is an aluminum coating. This metal has a lower specific gravity, which means it is less consumed, aluminized surfaces can be painted and the paint layer will be stable. In addition, the aluminum coating, in comparison with the galvanized coating, is more resistant to aggressive environments. Aluminum is not widely used due to the complexity of applying this coating to a metal sheet - aluminum in a molten state shows high aggression to other metals (for this reason, the aluminum melt cannot be contained in a steel bath). Perhaps this problem will be completely solved in the very near future - the original method of performing aluminizing was found by Russian scientists. The essence of the development is not to immerse the steel sheet in the aluminum melt,and raise the liquid aluminum to the steel sheet.

Improving corrosion resistance by adding alloying additives to steel alloys

The introduction of chromium, titanium, manganese, nickel and copper into a steel alloy makes it possible to obtain an alloy steel with high anti-corrosion properties. The steel alloy is especially resistant to a large proportion of chromium, due to which a high-density oxide film forms on the surface of structures. The introduction of copper into the composition of low-alloy and carbon steels (from 0.2% to 0.5%) makes it possible to increase their corrosion resistance by 1.5-2 times. Alloying additives are introduced into the steel composition in compliance with the Tamman rule: high corrosion resistance is achieved when there is one alloying metal atom for every eight iron atoms.

Anti-corrosion measures

To reduce it, it is necessary to reduce the corrosive activity of the medium by introducing non-metallic inhibitors and to reduce the number of components capable of initiating an electrochemical reaction. This method will reduce the acidity of soils and aqueous solutions in contact with metals. To reduce the corrosion of iron (its alloys), as well as brass, copper, lead and zinc, carbon dioxide and oxygen must be removed from aqueous solutions. In the power industry, chlorides are removed from water, which can affect localized corrosion. By liming the soil, you can reduce its acidity.

Protection against stray currents

It is possible to reduce electrocorrosion of underground utilities and buried metal structures if several rules are observed:

• the section of the structure serving as a source of stray current must be connected with a metal conductor to the rail of the tramway;
• heating network routes should be located as far as possible from the railways along which electric vehicles travel, to minimize the number of their intersections;
• the use of insulating pipe supports to increase the transition resistance between the soil and pipelines;
• at the inputs to objects (potential sources of stray currents), it is necessary to install insulating flanges;
• on flange fittings and stuffing box expansion joints, install conductive longitudinal jumpers - to increase longitudinal electrical conductivity on the protected section of pipelines;
• in order to equalize the potentials of parallel pipelines, it is necessary to install transverse electrical jumpers in adjacent sections.

The protection of insulated metal objects and small steel structures is accomplished with a protector that acts as an anode. The material for the protector is one of the active metals (zinc, magnesium, aluminum and their alloys) - it takes on most of the electrochemical corrosion, collapsing and preserving the main structure. One magnesium anode, for example, protects 8 km of pipeline.