This introduction is about corrosion and we have covered this topic in the corrosion introduction and cost of corrosion in chemical and Process industries. Here we have highlighted the facts of Corrosion and its effects. corrosion is defined as a chemical reaction between the metal and its environment due to the presence of oxygen. Due to increase in temperature of all around the globe, it has started to diminish then natural resources of the world, so that’s why a lot of importance has given to the production of hydrogen fuel.
As of its importance it is vital in oil reforming and in Haber’s Process for NH3 Production. Hydrogen is gaining importance as a future energy source for all over the world in the upcoming years. There would be enough storage to meet the required demands.
Aluminum will form Hydrogen when mixed with water due to its reactive nature. Aluminum forms a defensive shield of aluminum oxide layer to protect the whole material due to its reaction in the atmosphere with the O2.
Such type of Layer is insoluble and avoids the Al-H2O reaction. Due to its Al2O3 it forms a layer to stop the reaction this is the reason that aluminum is not a source for hydrogen production. To control corrosion, material selection should be adequate to avoid loss of material and avoid degradation of the process plant. The corrosion prevention is necessary for the plant life span. The material should be a metal or an alloy.
There should be adequate knowledge of metal corrosion rate and other terminologies for the material selected. We have studied about the electrochemistry and thermodynamics of corrosion in this material about aluminium corrosion here the chemistry of aluminium corrosion is completely elaborated.
The hydrogen Production form aluminium can be increased by milling it. Due to milling it will produce more Hydrogen which is more beneficial instead of getting it form steam reforming. The by product in this case can be safely handled. Therefore, with the milling step, it is time consuming and energy. We can see the effect of temperature and pH on corrosion rate between milled and unmilled aluminium.The Hydrogen production at more than 500C by the milled sample is rapid and can be determined by the displacement of water by hydrogen. By the Evolution of gas, aluminium corrosion rate can be determined.2Al + 6H20 === 2Al (OH)3 + 3H2 2Al + 4H20 === 2AlOOH + 3H2 Anode: Al === Al+3 + 3e-Cathode: 6H+ + 6e- === 3H2The Eq. 1 consumes more water than 2 so it would be difficult to carry out such massive reaction for Portable conditions. By such reactions we can measure the extent of reaction of aluminium corrosion from the amount of hydrogen produced. The presence or absence of salts in washed did very small impact on corrosion rate.
When the reaction proceeds it suddenly increase the pH and temperature due to evolution of hydrogen from the reaction tank. Such type of increase of temperature and pH can be reduced by using excess of water. This will form the corrosion pits in the same way as the corrosion did.Effect of pH This section tells about the environment of corrosion at which temperature, pH and what will be the required surface area for the material. And we have completely known how about it. At 1st we will do the comparison of reactivity of aluminium disks and ball milled aluminium pellets in water and borax solution.
We will check the potential of the reactions by comparing their strength. BM solution will be more reactive by order of two magnitude. Test run have also shown that pH of the solution increases up to 9 which will use borax as a buffer. The overall corrosion rate for aluminium can be seen form the fig 4 which will show the reactivity of BM sample of aluminium and pH shift. It will be seen that pH for loose aluminium will lie in the range of 10 whereas in BM it was about to 9. We have seen from the tests that hydrogen evolution may shift the potential of aluminium but did not account as in case of ball milled aluminium. The following diagrams shows the results of pH of solutions.Polarization curves of aluminiumThe polarization curve is studied in the course of material selection which describes about the pH of different aluminium samples by comparing the strength of their E/V value. Hydrogen Evolution and PH of Al in water at 700C.This diagram shows the PH of different kind of samples of aluminium. This is studied in the corrosion control topic so we should acknowledge how to overcome the corrosion problems.
The reaction of aluminium water is exothermic which will automatically increase the rate of reaction. In such case the corrosion rate will be 10 times from 25-500C temperature. The subsequent reaction speed and the current increases up to 700C and a sudden change/decrease in the value at 850C due to AlOOH formed at this temperature as it was in the form of Al (OH)3 before this temperature. This shows a clear picture that corrosion increases by increase of temperature up to only 850C above than this temperature it will form more passivating film. This slowdown of Al corrosion with temperature can also be seen when measuring the amount of corrosion in loose powder in a beaker of water with time. The overall corrosion is between 55-700C because at 550C only Al (OH)3 is observed whereas at 1000C only AlOOH can be viewed. Percent of Aluminium corroded at 55, 70, 95 0CThis curve shows the reactivity of aluminium with atmosphere which shows the effect of temperature on aluminium corrosion. This curve explains that at high temperature there will be oxide layer which will protect the corrosion.
AreaAs the name suggest that like that of PH and temperature effect, there is a lot of chance for the properties to be measured by the surface area of the aluminium powder. As this is a difficult task to measure the surface area of three mixtures of aluminium and oxide. We can compare the corrosion rates by comparing the surface area and by this method we can calculate the corrosion rate accurately. This yields an aluminum powder that is 97% pure, with a surface area of over 13.5 m2/g. The oxide layer affects the corrosion rate in a positive way, which can view by the diagram.
To control corrosion, material selection should be adequate to avoid loss of material and avoid degradation of the process plant. The corrosion prevention is necessary for the plant life span. The material should be a metal or an alloy. There should be adequate knowledge of metal corrosion rate and other terminologies for the material selected. Protection mechanismCathodic protectionSteel is mostly used in plants for material to be consider for different process plants. So, the rate of corrosion for the steel in the presence of water/moisture is 0.5mm/yr. Such type of corrosion is not tolerable for many steel-oriented plants. That’s why such kind of structures are protected by cathode method to avoid corrosion to the minimum point.
Anode protection is applied where the cathodic coating is unnecessary because if the protection is lost then corrosion rate would be high and it would be impossible to treat such type of corrosion. This type of protection is applied for the storage of acid tanks.
There are many types of coatings and inhibitors present to avoid corrosion which we have discussed in the course. Whereas coatings comprise of organic compounds will protect the metal from the environment and their effect. Metallic coatings acts anode protection for various materials. Whereas inhibitors form a protection layer and avoid corrosion of metals. On the basis of study of coatings and its types each has its own advantages and disadvantages. We will choose according to our requirement and process type. And we have studied it in the course in the methods of corrosion control. Cathodic protection reduces the corrosion rate by cathodic polarization of a corroding metal. Pipelines are normally cathodically protected to reduce corrosion rates to negligible levels. Liquid resins and pigment are applied as a coating material. Coating provide physical barrier to the corrosive environment. Whereas inhibitors form protective coatings in situ corrosion inhibition is reversible such that a minimum concentration is required. Inhibitors are best used in re-circulating systems. Many inhibiting compounds are toxic. We have complete understood about them in course related to protection from corrosion
The corrosion affects the plant life and we have studied it about in course like its effects and different types like uniform corrosion, pitting corrosion, crevice corrosion and mainly with the Hight temperature corrosion. Because some metals corrode at high temperature due to the ability of less protective shield on them.
The corrosion of aluminium in water will raise the PH and Temperature which will greatly affect the corrosion rate. In the milling conditions the surface area of oxide particles remain the same. Whereas evolution of hydrogen remains proportional with the surface area of milled powder. When the reaction proceeds it suddenly increase the pH and temperature due to evolution of hydrogen from the reaction tank. Such type of increase of temperature and pH can be reduced by using excess of water. This will form the corrosion pits in the same way as the corrosion did. The reaction of aluminium water is exothermic which will automatically increase the rate of reaction.
Skrovan J., Alfantazi A, Troczynski T. (2009) Enhancing aluminum corrosion in water, J Appl Electrochem (2009) 39:1695″1702
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