CORROSION INHIBITION OF ALUMINIUM IN AN ACIDIC MEDIUM
1.0 Background of Study
Corrosion is an irreversible interfacial reaction of a material (metal, ceramic, and polymer) with its environment which results in consumption of the material or in dissolution into the material of a component of the environment. Corrosion could also be defined as thedegradation of materials’ properties due to interactions with their environments, and corrosion of most metals (and many materials for that matter) is inevitable of the 105 known chemical elements, approximately eighty are metals , and about half of these can be alloyed with other metals, giving rise to more than 40,000 different alloys. Each of the alloys will have different physical, chemical, and mechanical properties, but all of them can corrode to some extent, and in different ways .Corrosion eventually causes failure of components and systems both in the processing andmanufacturing industries and in the service life of many components. Corrosion control of metals and alloys is an expensive process and industries spend huge amounts to control this problem.We have all seen corrosion and know that the process produces a new and less desirable material from the original metal and can result in a loss of function of the component or system. The corrosion product we see most commonly is the rust which forms on the surface of steel.
Figure 1: STEEL RUST
Corrosion is a natural phenomenon. When newly made steel is first exposed to the air, its originally shiny surface will be covered with rust in a few hours. The tendency of metals to corrode is related to the low stability of the metallic state. Metals occur either in the pure metallic state, the zero oxidation state, or in the form of compounds with other elements (they acquire positive states of oxidation. The chemical reactions that take place in corrosion processes are reduction-oxidation (redox) reactions. Such reactions require a species of material that is oxidized (the metal), and another that is reduced (the oxidizing agent). Thus the complete reaction can be divided into two partial reactions: one, oxidation; the other, reduction. In oxidation, the metal loses electrons. The zone in which this happens is known as the anode. In the reduction reaction, the oxidizing agent gains the electrons that have been shed by the metal, and the zone in which this happens is the cathode.[3-5]
Corrosion processes not only influence the chemical properties of a metal but also generate changes in its physical properties and its mechanical behavior. This is why the effects of corrosion are manifested in a variety of forms. The most common form is uniform corrosion, whereby there is a generalized, overall “attack” of the entire exposed surface of the metal, leading to a more or less uniform reduction in the thickness of the affected metal.
Corrosion processes affect many areas of human activity in which metal products are used. In general, as levels of economic development increase, so do costs incurred as a result of corrosion. It is estimated that the costs attributable to the corrosion of metallic materials amount to 4 percent of the gross domestic product of the developed countries. And this cost, representing a loss of resources, would be even higher if methods of protection against corrosion were not so widely applied. It is estimated that because of this protection, populations are able to reduce these potential losses by a factor of about 30 percent.
1.4Scope of Study
Though the outcome of the project can be applied in nearly all sectors, in the oil and gas industry which have corrosion problem, the scope of this project as have been limited to Corrosion/Corrosion Inhibitors. Thus, corrosion detection and control or reduction techniques have been discussed in detail. Also, within the scope of the project is the overview of the different types of corrosion and corrosion inhibitionand the terrain they occur.
1.5Significance and Benefits of the Study
Corrosion failures can result to personal injuries, fatalities and cost billions of dollars, through spontaneous shutdown and environmental contamination. Hence, the critical need for better methods to monitor the actual deterioration of a component once it is placed in service in a corrosive environment, analyse that information, and, based on decision-making reasoning, provide a reasonable forecast of the time remaining before maintenance or replacement becomes necessary. Through corrosion inspection and monitoring, the condition of the metals works can always be ascertained and proper corrosion control and maintenance strategies put in place.
Many catastrophic incidences resulting from corrosion failure had been historically recorded.
Corrosion costs the oil and gas industry tens of billions of dollars in lost income and treatment costs every year. The total annual cost of corrosion in the oil and gas production industry is estimated to be $1.372 billion, broken down into $589 million in surface pipeline and facility costs, $320 million in capital expenditures related to corrosion. Corrosion costs US industries alone an estimated $170 billion a year in which the oil and gas industry takes more than half of these costs
1.6Factors That Influence Corrosion
The nature and extent of corrosion depend on the metal and the environment. The important factors which may influence the corrosion process are:
1.6.1 Primary factors related to metal:
1. Nature of the metal:
The tendency of a metal to undergo corrosion is dependent on the nature of the metal. Metals with lower reduction potential undergo corrosion easily whereas metals with higher reduction potential do not undergo corrosion easily. The reactive metals like Na, K, Mg, Zn are more susceptible for corrosion. The noble metals like Ag, Au, Pt, Pd are less susceptible for corrosion.
2. Surface state of the metal:
Corrosion is surface phenomenon, larger the surface area or finer the grain size of the metal, more will be the corrosion. Smooth surface resist corrosion than the rough surface. Due to ups and downs on the rough surface, there will be formation of large number of air concentration cells with anodic and cathodic regions. Hence the metal suffers corrosion.
3. Nature of the corrosion product:
It largely decides the rate of corrosion. If the corrosion product is insoluble, stable, uniform and nonporous, it acts as a protective film preventing the further corrosion. If the corrosion product formed is soluble, unstable, porous and non-uniform, the corrosion continues.
4. Hydrogen over voltage:
If the hydrogen over voltage of metal is low, it is more susceptible for corrosion. When the cathodic reaction is of hydrogen evolution type with lower hydrogen over voltage, hydrogen gas is evolved easily and thus cathodic reaction is faster and corrosion of metal becomes fast. In metals with higher hydrogen over voltage, cathodic reaction is slow and corrosion of metal becomes slow.
1.6.2 Secondary factors related to environment:
1. pH of the medium:
In general, lower the pH higher is the rate of corrosion. If the pH is greater than 10, corrosion of iron is very less due to the formation of protective coating of hydrous oxides of iron. If pH is between 10 and 3, then presence of oxygen is essential for corrosion of iron. If the pH is 3 or lower than 3 severe corrosion occurs in the absence of air due to the continuous evolution of H2 at cathode. However metals like Al, Zn etc. undergo fast corrosion in highly alkaline medium.
As temperature increases, rate of corrosion also increases. This is because increase in temperature increases the conductance of the aqueous medium .As a result rate of diffusion also increases.
3. Presence of oxidizing agents:
The presence of oxidizing agents increases the corrosion rate of the metal. Even noble metals undergo corrosion in the presence of oxidizing agents.
Most of the metals corrode faster in a humid atmosphere than in dry air. There is a particular value of humidity called critical humidity above which corrosion rate steeply increases. Humidity (moisture) provides conducting medium which helps in
(i) Formation of electrochemical cell on the surface.
(ii) Dissolution of gases like O2, CO2, SO2etc.That help in corrosion.
5. Presence of impurities in the atmosphere:
Presence of impurities like SO2, HCl in the environment increases the rate of corrosion due to acidic conditions created by their dissolution. For example, when SO2 is present as impurity in the atmosphere, it combines with moisture or rain water forming sulphuric acid. In the presence of an acid metals like iron are more susceptible for corrosion.
6. Conductance of the medium:
Presence of conducting species in the atmosphere increases the rate of corrosion. This is because, higher the conductivity of the medium, faster the ions can migrate between anodic and cathodic regions of the corrosion cell, in turn faster will be the exchange of electrons at the electrode surfaces. Therefore, corrosion problem is more in the sea water than in fresh water.
7. Area effect:
Smaller the anodic area, larger the cathodic area, faster will be the rate of corrosion and conversely, larger the anodic area, smaller the cathodic area, slower will be the rate of corrosion. This is because electrons liberated at anode (smaller the anodic area) are consumed quickly by the large cathodic area and hence, the rate of corrosion will be more.
8.Polarization at anodic and cathodic area:
Polarization of cathode or anode decreases the rate of corrosion. If anodic polarization takes place due to some reaction, then tendency of metal to undergo oxidation decreases hence dissolution of metals as metal ion decreases. This is usually due to increase in concentration of ions of the dissolved metals in the vicinity of electrode or also due to the anodic passivity.
Cathode polarization decreases the cathodic reaction hence hindering the combination of cathode reactant and electron. For the corrosion to continue both anodic and cathodic reaction should take place simultaneously if any one reaction is slower than the rate of corrosion is slower. Use of depolarizes reduces the polarization effect hence the rate of corrosion reaction increases.
1.4 Types of Corrosion
There are different types of corrosion which depend on the environment surrounding the material, type of material, chemical reaction etc. Some general types of corrosion are described below.
This is also called General corrosion. It is a very common method of corrosion. It deteriorates the whole surface of the metal and makes the surface thin. The damage is done at a constant rate on the entire surface. It can be easily detected by its appearance. It can be controlled but if it is not, it then destroys the whole metal.
This type of corrosion occurs with an electrolyte like seawater. Metals have different values of electrical potentials. When they become electrically connected and put in an electrolyte, the more active metal which has a high negative potential becomes the anode. Due to its high negative potential, it corrodes fast. But the less active metal becomes the cathode.
The flow of electric current continues till the potentials are equal between both electrodes. So at thejoint where the two non-similar metals meet, the galvanic corrosion appears. The Galvanic Series shows the list of metals from the most active to the least active (most noble). Thus galvanic corrosion can be controlled by selecting the two metals which are close in series. As platinum is the least active, it is also less active for corrosion.
This occurs because of random attacks on particular parts of the metal’s surface. This makes holes which are large in depth. These holes are called “pits”. The pit acts as the anode while the undamaged part of the metal is the cathode. It begins with a chemical breakdown in the form of a scratch or spot. The pitting process makes the metal thinner and increases fatigue. For example, it can be very harmful in gas lines.
This occurs in the presence of a corrosive environment like saltwater. It is a combination of cyclic stress and corrosion. Corrosion fatigue is produced when a metal breaks at a stress level which is lower than its tensile strength. It is strongly affected by the environment in which the metal resides which affects the initiation and growth rate of the cracks. These cracks are too fine to detect easily. So the stress coupons (metal sample) are used to detect the corrosion.
It can be produced by the influence of various types of stress like stresses applied, thermal expansion, thermal contraction, welding, soldering, cleaning, heating treatment, construction process, casting etc. To prevent corrosion fatigue, the designing and construction process of the materials should be done properly, by eliminating any stress and environmental factors and by eliminating crevices.
In the granular composition of metals and alloys, grains (small crystals) are present and their surfaces join with each other. This forms the grain boundaries. Thus the grains are separated by grain boundaries. Intergranular corrosion is also known as inter crystalline corrosion. The Intergranular corrosion is developed on or near the grain boundaries of a metal. This can be due to welding, stress, heat treating or improper service etc. The metal can lose its strength due to the Intergranular corrosion.