Corrosion is the one of the biggest problem of the metal sector. Corrosion is the alteration or abrasion incident which proceeds uniformly from the top to the depth of the material by a chemical or electrochemical reaction. In brief, it is a formation process of natural chemical reaction on the metal surface. The chemical structure formed at the end of this process is called rust. Rust is one of the most common problem in metal industry which is encountered by manufacturers. It is a realy important process for the producers to protect raw materials and even treated products during the transportation and storage. Corrosion process is increased especially the high humidity cases such as overseas transportation. Thus, the parts should keep best during the transportation. Even in best package some air can be existed and for that matter rusting process can start. It is always truer to coat the parts with protective layer if cannot bear the consequences of the potential risk. Rusting which occurs on the metal in addition to creating a bad appearance at the surface, besides make difficulties for the processing of the material. Also in time it goes down to deep of the material and decrease the material lifetime. In order to use rusted materials, some mechanical or chemical processes must be applied. This situation causes extra costs such as labor cost, material loss, time loss. For this reason, it will be more advantageous to protect the material from corrosion at the very beginning. Corrosion can occur in two different ways which are chemical and electrochemical.
1.Electrochemical Oxygen Corrosion
It is a corrosion type which happens through a series of reduction-oxidation reactions. In electrical respect neutral atom gain or lose electron according to the results of some effects. In aqueous media reactions are occurred in the form of electron losing (oxidation) and electron gaining (reduction) are called electrochemical reaction. All corrosion reactions that occur in the water, in the atmosphere and under the soil are electrochemical reactions. It can also be called as wet corrosion.
Corrosion Cell Structure and Actual Reactions
Electrochemical corrosion is occurred either micro scale or macro scale can be modeled with corrosion cell. Corrosion cell is composed of anode, cathode, conductive media (electrolyte) and conductive connection between anode and cathode.
If any of four components are not to be existed, the corrosion is not be happened. Corrosion occurs on the material which pretend to be like an anode. As an electrolyte, film layer, water residual or a slight dampness in a crack and even finger sweat is sufficient. Corrosion based occurrences are arised from the fact that oxygen in the air acts with the water above the metal material.
Metal ions begin to dissolve under a water droplet.
At the edge region of the droplets OH- ions which are formed from the dissolved air react with dissolved metal and is firstly formed metal hydroxide Me(OH)3 and from here rust MeO(OH) is occured.
Anode : Corrode (oxidized) metal (oxidation)
Me → Me+n + ne–
Cathode : The reaction that consumes the electrons that are released in the anode (reduction) where is occurred on the metal surface
O2 + 2H2O + 4e– → 4OH–
Electronic Conductor : It is a metallic conductive that carries electrons released at the anode to cathode. The interaction of anode and cathode also provides this communication.
Electrolyte : Electrolytic conductive, aqueous solution. An aqueous environment which provides an ionic bond between anode and cathode.
Anodic Reaction : Metal atoms were transforming to positive loaded metal ions while were losing negative ions. As a result of this reaction, electrons are produced.
Me → Me+n + ne–
Cathodic Reactions : The function of the cathodic fact is to consume electrons produced in the anodic reaction.
Me+n + ne– → Me
2.Chemical Corrosion (Dry Corrosion)
It is an oxidation of metal and their alloys at the gases media. At the atmospheric conditions the most important corrosive substances are O2, H2S and halogens in general it forms as oxides and sulphides which are corrosion products on the metal surface. However, the corrosion caused by the moist air surrounding us is out of this description. The most common sample is oxide layers formed on the iron and steel surfaces at the high temperatures.
Corrosion reactions according to the metals;
Corrosion reactions of iron;
Fe+2 + 2OH– → Fe(OH)2 (k)
4Fe(OH)2+ O2 + 2H2O → 4Fe(OH)3(k)
2Fe(OH)3 → Fe2O3.H2O(k) + 2H2O
It is the main component of the red-brown rust and it can be written as Fe(OH)3. It can be composed a mineral which is called hematite.
Fe2O3.FeO is usually green as it shown here but it can be dark blue in the presence of organic complexes.
It is known as magnetite and its color is black.
1.Red-brown rust 2.Black rust 3.Blue rust
Corrosion reaction of Aluminium;
4Al(k) + 3O2(g) → 2Al2O3(k) (White-grey color)
Corrosion reaction of zinc;
2Zn(k) + O2(g) → 2ZnO(k)
Corrosion reaction of copper;
2Cu + H2O ↔ Cu2O + 2H+
Cu2+ + H2O ↔ CuO + 2H+
Color of copper rust is green.
Atmospheric corrosion is the most common type of corrosion. The research shows that about 80% of metallic structures are affected under the atmospheric conditions. Poles, bridges, railings, railways, warehouses, roof materials, all sorts of vehicles, etc. many steel structures are constantly under the influence of the atmosphere. Iron and steel are exposed to corrosion by oxygen and humidity which are present in the atmosphere and these are create a rust. The rust, which is physically soft and permeable, does not exhibit a protective shell characteristic of corrosion, as it is in other metals. There against it constitutes a suitable environment for condensation of water vapor and absorption of sulfur oxides on the metal surface. In the natural clean atmosphere composition, there is no other component to form corrosion other than water vapor. Water vapor in the air can cause corrosion even when less than saturated. The most important factor in terms of atmospheric corrosion is industrial pollution. Many chemical gases, vapors and solid particles are mixed to the atmosphere particularly in combustion events at the industrial processes. The most prevalent and effective one of those is sulfur oxides. These oxides can be combined with water vapor which present in the air and mixed with acids, ammonia and chloride into the atmosphere. Particularly near the coastlines there are microscopic salt particles carried by the wind. These salts precipitate on the metal surfaces and form deposits.
TYPES OF CORROSION
Some of the corrosion types are stated as follows according to their construction.
As it is seen in the figure, it is a corrosion type which evenly throughout in all areas the exposed surface of the metal. The most material loss occurs in this type of corrosion and is the most common type of corrosion. It is seen at the metals produced from the same type of material that are not affected atmospheric environment and any external influence.
Pitting corrosion is the localized corrosion type. Loss of material is less but it is hard to recover the material from the corrosion because it can be caused piercing of the material. Pitting corrosion is especially prevailed in environments containing NaCl, CaCl2, MgCl2, AlCl3 and NaBr.
It is a type of corrosion that occurs in a crack or a narrow hollow which is present on the metal surface. The main reason of this corrosion is that the oxygen concentration or the metal ion concentration is different between crack inner and electrolyte environment.
Galvanic corrosion occurs when two electrode potentials different metals or alloy are located together in an electrolyte environment. It is the most common in old and new pipe connections, between metal and welding, particularly in areas where the properties of the ground which the metal passes are changed.
The most prominent feature of intergranular corrosion is despite very low weight loss, corrosion rate reach very high values near the grain boundary phase. In this condition parts are subjected to corrosion through the entire cross-sectional area in a short time.
Intergranular corrosion which proceeds along parallel to the surface of die or extrusion is called exfoliation corrosion. The damage of this type of corrosion, which is seen in aluminum and its alloys, occurs in the elongated grain boundaries of the rolling direction. Corrosion exposed metal layers are separated from each other and the comprised corrosion products are caused delamination of the layers.
Materials operating under dynamic loads can crack under the lower effect of stress which they can withstand due to fatigue. This type of corrosion is common especially on train wheels.
This kind of corrosion, which is common in pipe systems and ports accelerate the rate of corrosion attack in metal due to the relative motion between the metal and the corrosive environment. It appears in the form of holes, grooves and ditches on the metal surface. It manifests itself in many structures under moving conditions in the water. The presence of solid particles are enhanced the corrosion rate.
It is a corrosion event which occurs to the divergence of one element from an alloy by the cause of corrosion. It occurs a consequence of primarily dissolution of a specific metal or a specific phase from an alloy. This corrosion doesn’t make any difference except for the loss of strength in the part and the color change in the outer appearance of the part.
Reference : Net Kimya – R&D Departmant / Reyhan ÇELİK