Copper and copper alloys have high electrical and thermal conductivity, can be machined, have good resistance to corrosion, and therefore are actively used in many industries. But when it enters a certain environment, corrosion of copper and its alloys still appears. What is it and how to protect products from spoilage, we will consider in this article.
What is corrosion
This is the destruction of metals as a result of exposure to the environment. In countries with well-developed industry, corrosion damage accounts for 4–5% of national income. Not only metals are spoiled, but also the mechanisms and parts made from them, which leads to very high costs. Pipeline rusting often results in leakage of harmful chemicals, resulting in pollution of soil, water, and air. All this adversely affects people's health. Corrosion of copper is its spontaneous destruction under the influence of individual elements of the human environment. The cause of metal damage lies in its instability to individual substances in the air. The greater the corrosion rate , the higher the temperature.
Copper properties
Copper is the very first metal that man began to use. It is golden in color, and in the air it is covered with an oxide film and acquires a red-yellow color, which distinguishes it from other metals that have a gray tint. It is very plastic, has high thermal conductivity, is considered an excellent conductor, second only to silver. In weak hydrochloric acid, fresh and sea water, copper corrosion is negligible.
In the open air, metal oxidizes to form an oxide film that protects the metal. Over time, it darkens and turns brown. The copper coating layer is called patina. It changes color from a brownish tint to green and even black.
Electrochemical corrosion
This is the most common type of destruction of metal products. Electrochemical corrosion destroys machine parts, various structures located in the ground, water, atmosphere, cutting fluids. This is damage to the surface of metals under the influence of electric current, when during a chemical reaction there is a recoil and transfer of electrons from the cathodes to the anodes. The heterogeneous chemical structure of metals contributes to this. When copper and iron come into contact with the electrolyte, a galvanic cell appears, where iron becomes the anode and copper becomes the cathode, because iron in the series of voltages according to the periodic table is to the left of copper and has more activity.
In the pair of iron with copper, corrosion of iron occurs faster than copper. This is because when iron is destroyed, the electrons from it pass to copper, which remains protected until the entire layer of iron is completely destroyed. This property is often used to protect parts and mechanisms.
The effect of impurities on metal spoilage
It is known that metals in their pure form practically do not corrode. But in practice, all materials contain a certain amount of impurities. How do they affect the safety during the operation of products? Suppose that there is a part made of two metals. Consider how corrosion of copper with aluminum occurs. When in the air, its surface is covered with a thin film of water. It should be noted that water decomposes into hydrogen ions and hydroxide ions, and carbon dioxide dissolved in water forms carbonic acid. It turns out that copper and aluminum, immersed in a solution, create a galvanic cell. Moreover, aluminum is the anode, copper is the cathode (aluminum in the voltage range is to the left of copper).
Aluminum ions enter the solution, and excess electrons pass to copper, discharging hydrogen ions at its surface. Aluminum ions and hydroxide tones are combined and deposited on the surface of aluminum in the form of a white substance, causing corrosion.
Corrosion of copper in acidic environments
Copper exhibits good resistance to corrosion in any conditions, since it infrequently displaces hydrogen, because it is near noble metals in the electrochemical series of voltages. The widespread use of copper in the chemical industry is caused by its resistance to many aggressive organic environments:
- nitrates and sulfides;
- phenolic resins;
- acetic, lactic, citric and oxalic acid;
- potassium and sodium hydroxides;
- weak solutions of sulfuric and hydrochloric acid.
On the other hand, there is a strong destruction of copper in:
- acidic solutions of chromium salts;
- mineral acids - perchloric and nitric, and corrosion increases with increasing concentration.
- concentrated sulfuric acid, intensifying with increasing temperature;
- ammonium hydroxides;
- oxidizing salts.
Metal Protection Methods
Almost all metals in a gaseous or liquid medium undergo surface destruction. The main way to protect copper from corrosion is to apply a protective layer to the surface of the product, consisting of:
- Metal - a metal layer is applied to the copper surface of the product, which is more resistant to corrosion. For example, brass, zinc, chromium and nickel are used as it. In this case, contact with the environment and oxidation will occur with the metal used for the coating. If the protective layer partially deteriorates, then the destruction of the base metal - copper.
- Non-metallic substances are inorganic coatings consisting of a vitreous mass, cement mortar, or organic - paints, varnishes, bitumen.
- Chemical films - protection is formed chemically, creating compounds on the metal surface that reliably protect copper from corrosion. To do this, use oxide, phosphate films or saturate the surface of the alloys with nitrogen, organic substances or treat with carbon, the compounds of which reliably preserve it.
In addition, an alloying component is introduced into the composition of copper alloys, which enhances the anticorrosive properties, or changes the composition of the environment, removing impurities from it and introducing inhibitors that slow down the reaction.
Conclusion
Copper is not a chemically active element, because of this, its destruction occurs very slowly in almost any environment. Therefore, it is widely used in many sectors of the economy. For example, metal behaves very stably in clean fresh and sea water. But with increasing oxygen content or accelerating the flow of water, corrosion resistance decreases.