What Is Corrosion & The Rate of Corrosion
Corrosion is a?chemical reaction?between a metal and its surrounding environment under the formation of corrosion products.
For initiation and propagation of a corrosion attack, a “corrosion cell” must be established.
?A corrosion cell is a condition on a metal surface where electrochemical reaction occurs between two metal surfaces.
?It consists of four fundamental components:
The electrodes (metals) in a corrosion cell
The driving force behind a corrosion cell is a potential or voltage difference between the anode and cathode.?
?In most cases connecting two differently charged electrodes (different metallic materials), as in a battery, creates the driving voltage. As we will see later the electrodes can be two separate metallic phases of the same alloy, or impurities in an alloy (i.e. uniform corrosion).
?In addition, one piece of metal exposed to different environments may serve as two different electrodes and give the same effect (i.e. crevice corrosion).
Conductor 1: Electrical contact in a corrosion cell.
?The first conductor in a corrosion cell is established when the anode is in metallic contact with the cathode. This can be either by a direct metal-to-metal contact or via an external, electrical conductor (wire, bolt, deck support, suspension systems or similar). Metallic contact enables transport of electrons (electrical current), between the anode and the cathode.?
?Conductor 2: The Electrolyte in a corrosion cell.
?In most cases corrosion takes place in an aqueous environment, or at least in the presence of humidity. Dry high-temperature corrosion only takes place under special conditions, for example in engines and exhaust systems. Thus, the primary mechanism of most forms of corrosion is of electrochemical nature. A prerequisite is the presence of an electrolyte containing ions that can transport electrical current. The higher the concentration of ions the better is the conductivity and the faster is the corrosion rate. When metals are immersed in a liquid (e.g. seawater), the liquid itself will be the electrolyte.?
?An electrochemical reaction may also take place above the waterline as an electrolyte can be formed by moisture in the atmosphere which under certain atmospheric conditions such as high humidity and low air temperature can lead to the dew point and the formation of condensation or dew (water) on the surface.?Water can easily collect impurities or salt from the steel surface or from the atmosphere, and together they form a conductive electrolyte.
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?Oxygen to promote corrosion in a corrosion cell
The above mentioned factors would be sufficient to give corrosion, but at a very low corrosion rate. In many cases air is present, dissolved in the electrolyte.?
?The presence of air will speed up the corrosion rate as the oxygen helps (speeds up) the reaction at the cathode.?
?The corrosion process on steel
The electrochemical process that occurs during the corrosion of steel can be represented by the following equation in the figure below.The simultaneous presence of water and oxygen is a prerequisite for the corrosion process. In the absence of either, corrosion does not occur.
Some Materials Corrode Faster than Other
It is a fact that some materials are more intrinsically resistant to corrosion than others when exposed to a particular environment. For example carbon steel will corrode rapidly in seawater, while gold is inert. Why do they behave so differently? The explanation is found by looking at the energy levels of the various materials.
?The primary reason iron or steel corrodes is that elemental iron is thermodynamically unstable.To produce iron or steel large amounts of energy is introduced. Thus, the energy levels of the materials we actually take into use are much higher than the natural energy level of the metals found in nature.
?In contrast, metals like Gold & Platinum can be found in their natural form and are thermodynamically unfavourable. These metals are called “noble metals” and do not need to go through energy consuming purification or a refining process before taken into use.
NOBLE METAL VS BASE METAL
In a simplified manner, you may say that the more energy required to produce a metal or alloy the higher is the driving force to start a corrosion process. As seen later, alloys can be ranked to find out which one of them are stable and which ones are likely to corrode.
THE VARYING CORROSION RESISTANCE & ELECTROCHEMICAL PROPERTIES OF DIFFERENT METALS FURTHER EXPLAINED.