Nanochemistry & Technology

 

NATURAL  CONCEPT 

Water is found in three different forms - liquid, solid or gas, depending on the temperature but it constantly changes from one form to another. Changes in temperature will determine which of these forms predominates in a particular area. Water is usually encountered in the liquid state, because this is its natural state when temperatures are between 0° C and 100° C. 'Fresh' or drinking water is found as groundwater in underground aquifers, and on the surface in ponds, lakes, and rivers. Seas and oceans account for 97% of all water on Earth; but their waters contain dissolved salts and are therefore unfit to drink. In regions of young volcanic activity, hot water emerges from the earth in hot springs (examples are Garampani in Assam and Badrinath in Uttaranchal). How does this phenomenon occur? Surface water percolates downward through the rocks below the Earth's surface to high-temperature regions surrounding a magma reservoir, either active, or recently solidified but still hot. There the water is heated, becomes less dense, and rises back to the surface through fissures and cracks.

Ice is the frozen form of water. It occurs when temperatures are below 0°C (32°F). For a given mass, ice occupies 9% more volume than water, which is why when water enters cracks in rocks and freezes it causes the rocks to crack and split. Being less dense than water, ice floats. This property of ice is vital to aquatic life in cold regions. As the temperature drops, ice forms a protective, insulating layer on the  of streams, pools and other water bodies, allowing water to remain liquid in the layers beneath and life to survive. Glaciers, icebergs, and ice caps are all frozen water.

STATE

A large amount of water is wasted in agriculture, industry, and urban areas. It has been estimated that with available technologies and better operational practices, agricultural water demand could be cut by about 50%, and that in urban areas by about 33% without affecting the quality or economics of life.

But most governments do not have adequate laws or regulations to protect their water systems. Due to the increase in population there has been a rise in the demand for food, space for housing, consumer products, etc., which has in turn resulted in increased industrialization, urbanization, and remands in agriculture thereby leading to both river and groundwater contamination.

Composting toilet is not allowed in urban dwelling. Water treatment involves science, engineering, business, and art. The treatment may include mechanical, physical, biological, and chemical methods. As with any technology, science is the foundation, and engineering makes sure that the technology works as designed. The appearance and application of water is an art.

REMOVAL

All water treatments involve the removal of solids, bacteria, algae, plants, inorganic compounds, and organic compounds. Removal of solids is usually done by filtration and sediment. Bacteria digestion is an important process to remove harmful pollutants. Converting used water into environmentally acceptable water or even drinking water is wastewater treatment.

Water in the Great Lakes Region is an organization dealing with the water resources. Ontario Clean Water Agency (OCWA) is a provincial Crown corporation in business to provide environmentally responsible and cost-efficient water and wastewater services. It currently operates more than 400 facilities for 200 municipalities.

NANO  ENVIRONMENT 

 

The oxygen can come from air or water. Iron oxide is actually more stable than pure iron or steel and therefore rusting is very common. Prevention of rust is actually a means of retaining a less stable or higher energy, state. Humans have most likely been trying to understand and control corrosion for as long as they have been using metal objects. The most important periods of prerecorded history are named for the metals that were used for tools and weapons. With a few exceptions, metals are unstable in ordinary aqueous environments. Metals are usually extracted from ores through the application of a considerable amount of energy.

Certain environments offer opportunities for these metals to combine chemically with elements to form compounds and return to their lower energy levels. Corrosion is the primary means by which metals deteriorate. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to gaseous materials like acid vapours, formaldehyde gas, ammonia gas, and sulfur containing gases. Corrosion specifically refers to any process involving the deterioration or degradation of metal components. The best known case is that of the rusting of steel. Corrosion processes are usually electrochemical in nature, having the essential features of a battery. When metal atoms are exposed to an environment containing water molecules they can give up electrons, becoming themselves positively charged ions, provided an electrical circuit can be completed. This effect can be concentrated locally to form a pit or, sometimes a crack, or it can extend across a wide area to produce general wastage. Localized corrosion that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive agents like seawater may lead to greatly enhanced growth of the fatigue crack. Pitting corrosion also occurs much faster in areas where microstructural changes have occurred due to welding operations. The corrosion process (anodic reaction) of the metal dissolving as ions generates some electrons, as shown in the simple model on the left, that are consumed by a secondary process (cathodic reaction). These two processes have to balance their charges.

REACTION

The anodic reaction may occur uniformly over a metal surface or may be localized to a specific area. If the dissolved metal ion can react with the solution to form an insoluble compound, then a buildup of corrosion products may accumulate at the anodic site. In the absence of any applied voltage, the electrons generated by the anodic reaction are consumed by the cathodic reaction. For most practical situations, the cathodic reaction is either the hydrogen-evolution reaction or the oxygen-reduction reaction. The hydrogen-evolution reaction can be summarized as reaction.

Reactions above represent the overall reactions which, in practice, may occur by a sequence of reaction steps. In addition, the reaction sequence may be dependent upon the metal surface, resulting in significantly different rates of the overall reaction. The cathodic reactions are important to corrosion processes since many methods of corrosion control depend on altering the cathodic process. Although the cathodic reactions may be related to corrosion processes which are usually unwanted, they are essential for many applications such as energy storage and generation.

Corrosion rates are usually expressed in terms of loss of thickness per unit time. General corrosion rates may vary from on the order of centimeters per year to micrometers per year. Relatively large corrosion rates may be tolerated for some large structures, whereas for other structures small amounts of corrosion may result in catastrophic failure. For example, with the advent of technology for making extremely small devices, future generations of integrated circuits will contain components that are on the order of nanometers (109 m) in size, and even small amounts of corrosion could cause a device failure. In some situations, corrosion may occur only at localized regions on a metal surface. This type of corrosion is characterized by regions of locally severe corrosion, although the general loss of thickness may be relatively small. Pitting corrosion is usually associated with passive metals, although this is not always the case. Pit initiation is usually related to the local breakdown of a passive film and can often be related to the presence of halide ions in solution.

Crevice corrosion occurs in restricted or occluded regions, such as at a bolted joint, and is often associated with solutions that contain halide ions. Crevice corrosion is initiated by a depletion of the dissolved oxygen in the restricted region. As the supply of oxygen within the crevice is depleted, because of cathodic oxygen reduction, the metal surface within the crevice becomes activated, and the anodic current is balanced by cathodic oxygen reduction from the region adjacent to the crevice. The ensuing reactions within the crevice are the same as those described for pitting corrosion: halide ions migrate to the crevice, where they are then hydrolyzed to form metal hydroxides and hydrochloric acid.

Corrosion can also be accelerated in situations where two dissimilar metals are in contact in the same solution. This form of corrosion is known as galvanic corrosion. The metal with the more negative potential becomes the anode, while the metal with the more positive potential sustains the cathodic reaction. In many cases the table of equilibrium potential can be used to predict which metal of galvanic couple will corrode. For example, aluminum-graphite composites generally exhibit poor corrosion resistance since graphite has a positive potential and aluminum exhibits a highly negative potential. As a result, in corrosive environments the aluminum will tend to corrode while the graphite remains unaffected.

Stress corrosion cracking and hydrogen embrittlement are corrosion- related phenomena associated with the presence of a tensile stress. Stress corrosion cracking results from a combination of stress and specific environmental conditions so that localized corrosion initiates cracks that propagate in the presence of stress. Mild steels are susceptible to stress corrosion cracking in environments containing hydroxyl ions (O; often called caustic cracking) or nitrate ions (NO3). Austenitic stainless steels are susceptible in the presence of chloride ions (CI") and hydroxyl ions (O=).

Other alloys that are susceptible under specific conditions include certain brasses, aluminum and titanium alloys. Hydrogen embrittlement is caused by the entry of hydrogen atoms into a metal or alloy, resulting in a loss of ductility or cracking if the stress level is sufficiently high. The source of the hydrogen is usually from corrosion (that is, cathodic hydrogen evolution) or from cathodic polarization. In these cases the presence of certain substances in the metal or electrolyte can enhance the amount of hydrogen entry into the alloy by poisoning the formation of molecular hydrogen. Metals and alloys that are susceptible to hydrogen embrittlement include certain carbon steels, high-strength steels, nickel-based alloys, titanium alloys, and some aluminum

alloys. A reduction in the rate of corrosion is usually achieved through consideration of the materials or the environment. Materials selection is usually determined by economic constraints. The corrosion resistance of a specific metal or alloy may be limited to a certain range of pH, potential, or anion concentration. As a result, replacement metal or alloy systems are usually selected on the basis of cost for an estimated service lifetime.

ELECTRON TRANSFER

Wet corrosion of metals occurs through electron transfer, involving two processes, oxidation and reduction. In oxidation, the metal atoms lose electrons. The surrounding environment then gains the electrons in reduction. The metal, where electrons are lost, is called the anode. The other metal, liquid or gas which gains the electrons is called the cathode.

The corrosion resistance of aluminium varies widely depending on alloy, environment, design and protective measures taken. However, it is possible to give some general guide-lines.

A clean aluminium surface is reactive and will react spontaneously with water or air and form aluminium oxide. This oxide is very stable and has in addition a very good adhesion to the metal surface and thus protects aluminium from corrosion or further oxidation. This means that aluminium has good corrosion resistance in environments where the oxide layer is stable. Aggressive ions will break down the oxide layer locally and start local corrosion attacks. Among the aggressive ions, chloride (Cl-), is the one with the most practical importance, because it is present in large amounts in both sea-water, road salts and some soils and in lower concentrations in other natural sources.

A so called general corrosion attack proceeds at about the same rate on the entire metal surface. Because of the stability of the oxide layer, general corrosion will rarely be a problem on aluminium, except in very alkaline or acidic environments. Aluminium may however experience local attacks in connection with formation of small anodic areas as a result of a local breakdown of the oxide layer. Some of the most typical local attacks on aluminium are pitting corrosion, crevice corrosion, intergranular corrosion and galvanic corrosion.

When there is a relative movement between the corrosive liquid and the metal surface, the metal may be exposed to mechanical wear which removes the protective oxide layer and enhances the corrosion. This is called flow influenced corrosion and can be further divided into;

EROSION

Pure erosion is caused by a shear force from a flowing liquid which is higher than the adhesion of the oxide layer on the aluminium surface. The shear force from the liquid may be increased by turbulence or dissolved or suspended

solids causing abrasion. The shear force from the liquid is not necessarily caused by liquid flow, but can also be induced by a moving surface for example a propeller.

If the removal of the oxide layer occurs in a corrosive liquid, the corrosion will be enhanced because a bare aluminium surface will be exposed to the liquid for a while until the oxide layer is healed. This is called erosion-corrosion. The rapid self healing of the oxide layer on aluminium is a great advantage with regards to erosion-corrosion, but because aluminium is a fairly soft material, erosion-corrosion can be a serious problem.

The shear force from the liquid on the metal surface is the main design criteria with regards to erosion corrosion. In practice, this is often transferred to flow rate limits. For the same flow rate, the shear force in a small diameter tube will be higher than in a large diameter tube, but the differences will be small for normal flow rates. If the liquid contains no solids and in smooth pipes, with undisturbed flow, the tolerable flow rate will be fairly high, but normally some turbulence must be expected and the flow rate limitations must be decided, based on these critical areas.

Crevice corrosion occurs in narrow metal to metal or non-metal to metal gaps where the convection of water is hampered and a specific crevice chemistry different from the environment is allowed to develop. Aggressive ions like chlorides must be present in the electrolyte. The oxygen in the bottom of the crevice is consumed and an anodic area is developed adjacent to the oxygen depleted zone. Crevice corrosion develops quite similar to pitting corrosion after the initiation stage, with a gradual decrease of the pH and an increase of the chloride concentration within the crevice.

Crevice corrosion is normally not a serious problem on aluminium in the absence of aggressive ions, because of the very stable aluminium oxide. However in a crevice there will be a possibility for accumulation of moisture because of capillary forces and deposits with corrosive or hygroscopic species. In this way there will be a constant corrosive environment in the crevice which eventually can break down the oxide layer.

Crevice corrosion can occur during storage of aluminium such as water- staining which creates dark stains as a result of a surface etching caused by water trapped between the adjacent surfaces. It is most commonly seen on sheet products and is mainly an aesthetic problem as the mechanical integrity of the water stained aluminium is rarely impaired.

Stress Corrosion

Stress corrosion is another form of corrosion that is important to many fields including civil structures. Stress-corrosion occurs when a material exists in a relatively inert environment but corrodes due to an applied stress. The stress may be externally applied or residual. This form of corrosion is

particularly dangerous because it may not occur under a particular set of conditions until there is an applied stress. The corrosion is not clearly visible prior to fracture and can result in catastrophic failure. Many alloys can experience stress corrosion, and the applied stress may also be due to a residual stress in the material. An example of a residual stress could be a stress remaining in a material after forming, or a stress due to welding. Stress corrosion cracking will usually cause the material to fail in a brittle manner, which can have grave consequences as there is usually little or no warning before the failure occurs. Stress corrosion is a form of galvanic corrosion, where stressed areas of the material are anodic to the unstressed areas of the material. Practically the best way to control stress corrosion cracking is to limit or reduce the stresses a material is under while it is in a corrosive atmosphere.

ROOM TEMPERATURE

INTRODUCTION

At room temperature, most metals carry a thin oxide layer as a result of the reaction of metals with oxygen in the atmosphere. Increase of temperature may cause formation of a heavier layer, or the layer may detach. Zinc and zinc coatings carry a fairly protective zinc hydroxide or carbonate layer (zinc patina) which increases in thickness very slowly. Aluminium carries a thin, highly protective oxide layer. Some corrosion takes place even under completely dry conditions.

Recently, we invented novel nanocrystalline metal-base nanocomposites distributing nanoparticles of Cr or/and Al through co-electrode position of metal matrix with nanoparticles These nanocomposites containing appropriate contents of Cr or/and Al can quickly form protective films, when they are suffered the attack of corrosive species at ambient or high temperatures. Hence, they exhibit excellent corrosion resistance, as compared to conventionally coarse-grained (CG) alloys. These kinds of nanocomposites are proposed as surface coatings for metals against liquid corrosion hot corrosion and high temperature oxidation Moreover, the nanocomposites contain numerous grain boundaries (GBs); this causes highly enhanced diffusion kinetics of atoms in them with respect to in the CG alloy counterparts. For this reason, the nanocomposites may be applied to metals as precursor films for chemical heat treatment such as nitridation to further improve their surface properties for various proposes.

This improvement of surface properties would be more significantly, if the chemical treatment on the nanocomposites is conducted at temperatures greatly lower than normal temperatures adopted by similar treatment on conventional CG alloys. In this paper, the fabrication, wet and dry corrosion behaviours, and

plasma nitridation performance of the types of electro deposited nanocomposites (ENCs) are we briefly reviewed taking ENC Ni-Cr as an example.