แสดงบทความที่มีป้ายกำกับ Should แสดงบทความทั้งหมด
แสดงบทความที่มีป้ายกำกับ Should แสดงบทความทั้งหมด

วันอังคารที่ 16 มีนาคม พ.ศ. 2553

What You Should Know about the Correct Paint Job

As we have mentioned in another article, one of the most common method to prevent corrosion in metals is to cover up the surfaces with a coating of paint. This prevents the flow of current whenever the metal is in contact with water.

To be effective, the first coating on the bare metal must have a very strong bond. This first paint coat is called the primer coat.

This primer paint coat is used to protect the base metal area as well as to provide good adhesion for the topcoat. Surface preparation is of utmost importance. It is useless to have a good primer when the surface itself is covered with loose dust. The primer coat will simply peel off later because the bond is weak.

For heavy-duty paint coatings, the surfaces need to be cleaned by sand-blasting. The idea of sand-blasting is to bombard the surface with abrasives under high pressure to scrape it clean to the bare metal. For less stringent coatings, wire-brushing is good enough.

Choosing the correct paint to suit the environment will ensure good protection for many years.

Choosing the Right Paint System

Epoxy paints provide very good protection from corrosion. This is because of the impervious nature of the epoxy coating layer. These are excellent for protection against corrosive chemicals, salt and fresh water abrasion and are ideal for steel piles, marine structures, ballast tanks and sewage treatment plants.

Polyurethane paints are used in body paint and auto paint of cars. These produce high gloss and are excellent body repair paints with good durability. These are special paints for special applications.

For architectural paint applications in the homes and buildings there are paints specially formulated for exteriors as well as interiors. Obviously those paints manufactured for exteriors will offer more protection from the effects of weather like rain, sunshine, fungus growth, rot, insect attack, and rusts.

Exterior applications includes masonry walls, brick walls, ceilings, wooden doors and windows, roof tiles, metal gates or other structures.

As before, the surface preparation and the first coat of paint is the most important. For masonry walls, paint manufacturers often have alkali resisting wall sealers or some other adhesion promoting wall sealers. A good surface preparation with a high pressure water jet will often be sufficient to loosen any weak bond.

For wood, there are wood primer sealers available as the first coat. Similarly, there are sealers and primers for roof tiles and metal.

Choose the top coat to suit the application. If your area is humid, probably you will need to have some fungus or mildew protection. For outdoor wood application, you may like to use transparent varnish coatings to bring out the beauty of wood. Just make sure that the varnish contains ultraviolet protection in the formulae. Walls that are often exposed to water may need some sort of silicone water repellent.

Interior applications are not so stringent. For long lasting paintwork, it is again advisable to have good surface preparation, sealers and primer coating as the first coat. The top coat should be formulated for good finishes and ease of maintenance. The surfaces should be easy to clean and the colors should last for a long time. Sometimes special effect patterns can be achieved with a little creativity.

Choosing the Right Color

Paints can determine the mood of our homes and offices. We can use one color, or we can use a combination of colors. Warm colors are the reds, oranges and yellows. Cool colors are blues, greens and violet. Let the colors of the rainbow guide you.

For those who want to try out combinations of colors, these are the main combinations:

Contrasting schemes - Colors that are opposite to each other, like having a blue wall with a yellow lining.
Blending schemes - Having different shades of one color together. For example you may have dark blue at one place and light blue in another.
Harmonious schemes - Having colors that complement each other by choosing shades that are not more than two colors apart. For example placing red and orange colors together.

By choosing bright or pastel hues, you can achieve further enhance the color combinations above to achieve freshness, vibrant, warmth, calmness.

The suggested steps to choose colors are as follows:

1. Select the preferred mood of each room

2. Select the preferred color hues for each respective room.

3. Finish by picking from the color swatches for the desired tone of the room.

Now that you have selected which paint to use, and what color scheme you want, it is time to start the painting job itself. Should you do it yourself or get a contractor to do it for you? Perhaps you should give it some thought before you start the job. It could be just as important as choosing the paint.

Air ForceCivil Engineer งานวิศวกรรมโยธา

วันศุกร์ที่ 12 มีนาคม พ.ศ. 2553

What You Should Know About Corrosion

Corrosion causes a lot of problems in maintenance. Machines get worn out, structures become weakened, appearance gets tarnished and metals like iron do not last as long as we wanted them to.

Yet not every wearing of machines can be attributed to corrosion. Erosion, abrasion and scuffing can also wear down machines. However the major reason for machine wear down is still corrosion. Why is that so?

We live in a world of chemicals and corrosion is a chemical reaction. In the process, metal is dissolved. Since iron and steel is used in almost every machine and structure, we particularly want to avoid corrosion in them.

Corrosion of iron and steel occurs only when 3 conditions occur:

1. A potential difference exists.

2. The surfaces are covered by an electrolyte.

3. Oxygen is present.

Potential Difference
Acids are the chief culprit in any corrosion. Sulphuric acid from acid rain causes a lot of corrosion damage. However, many people do not realize that it is the movement of electrons in the chemicals that causes the iron to get dissolved. The potential difference is the one that causes the electrons to flow in a closed electrical circuit, similarly to a battery.

Galvanic Corrosion
In the galvanic series of metals, noble metals like gold, silver, and platinum are almost immune to the attack of acids. They are considered inactive. The most active are metals like potassium and sodium. Somewhere in between these extremes are metals like iron, copper, zinc, tin, etc.

In a normal galvanic corrosion, electrical current flows from a less active metal to a more active metal, i.e. from the cathode to the anode. The anode gets wasted away. When a steel structure is immersed in salt water together with a less active metal like copper, the steel gets corroded very quickly.

When dissimilar metals are placed together in an electrolyte, a potential difference is created between them. The more active metal is corroded when a current flows through the acidic water or electrolyte. The faster the flow, the faster is the rate of corrosion. In normal atmospheric corrosion, the concentration of the acid in the water electrolyte is very low, so the rate of corrosion is slow. With a higher concentration, the corrosion also quickens.

Presence of Oxygen
Rust is one of the most common effects of corrosion. It is the oxidation of iron to form ferrous oxide. Corrosion is therefore highest in water tanks at the surface of the water where it is wet and contains a lot of dissolved air from the atmosphere. In a closed vessel, corrosion stops when all the oxygen has been used up for rusting.

Corrosion Prevention Measures
Methods that are used to prevent corrosion are therefore aimed at preventing the presence of an electrical potential difference, using less active metals and removing the presence of moisture and oxygen.

Painting
The most common method is to cover up the surfaces of bare metal with a coating of paint. This prevents the flow of current whenever the metal is in contact with water. To be effective, the coating on the bare metal must have a very strong bond. This first paint coat is called the primer coat.

The purpose of the primer paint coat is to protect the base metal area and also have adhesion of the topcoat. Some examples of primers are zinc chromate primer, red oxide and red lead primer. For galvanized steel and light alloys you will need to use a polyvinyl butyral-based, acid-catalyzed etching primer.

Some alkyd-based paint coatings pigmented with micaceous iron oxide provides good protection for iron and steel surfaces and are suitable for storage tanks, pipes, steel structures and grills.

For heavy-duty paint coatings, the surfaces need to be cleaned by sand-blasting. The idea of sand-blasting is to bombard the surface with abrasives under high pressure to scrape it clean to the bare metal.

Epoxy paints provide very good protection from corrosion. This is because of the impervious nature of the epoxy coating layer. These are excellent for protection against corrosive chemicals, salt and fresh water abrasion and are ideal for steel piles, marine structures, ballast tanks and sewage treatment plants.

Polyurethane paints are used in body paint and auto paint of cars. These produce high gloss and are excellent body repair paints with good durability.

Rubber and Plastic linings
By using rubber and plastic linings and laminates to coat the bare metal, a chemical resistant elastomer layer can be strengthened by the strong steel reinforcement underneath. Their use is not so much to protect the steel but to use the steel for support.

Cathodic Protection
Another way to protect steel is to coat the metal with a more reactive metal. This can be found in galvanizing. This is the coating of iron with zinc. The iron can be dipped in molten zinc to coat it. Because the zinc is reactive, it forms a tough oxide layer that prevents further oxidation and so protects the iron underneath. Many outdoor structural parts are often protected in this way. Some examples are cooling towers, air conditioning ducting, sheet metal, pipes, street lamps, etc.

In some submerged steel installations, zinc is used as sacrificial anodes. When zinc is placed together with iron, the former is sacrificed in favor of the latter. An example of this usage is in tube and shell heat exchangers where the presence of copper tubes will cause an electrical potential to exist between the steel shell and the copper tube nest. Sacrificial anodes of zinc are often then placed at the shell end covers to protect the iron from wasting away.

Impressed Current System
By putting the flow of current to the opposite direction, a way has been found to counter the inherent electrical potential caused by dissimilar metals placed together. By using noble metals like titanium, platinized niobium or lead/silver alloy and injecting electrical current in the opposite direction, the parent steel metal can be protected from the effects of corrosion.

A typical impressed current system controls current so that the inert anodes are made positive relative to the steel. When the steel is immersed in seawater, the normal galvanic current is counteracted by the supplied current. Different potentials are also applied for newly painted steel and for clean bare steel. The controller unit with its step-down transformer, rectifier, and the reference electrode are essential components of this impressed current system. The amount of current, and its evenly distribution is very important in this system. Over potential can produce OH- ions that can attack the surrounding paint.

Alkaline Chemical Treatment
Cement wash or alkaline chemical treatment for the water in contact with the metal is used in tanks where no painting or coating is allowed. Chemical treatment of water is also used in steam boilers where any coating on the heat transfer surfaces will be disastrous.

Use of less active metals
It is difficult to replace steel in many applications where strength is important. In applications where strength and low cost is not so critical, alloys of cupro nickel, bronze and brass can be used to substitute steel parts. Stainless steels containing iron and chromium can also be used where higher cost is not a concern.

Painting to prevent corrosion does make our world a more colorful place to live. We can prevent corrosion in so many different ways. Corrosion need not be a dirty word. In most cases, just a coat of paint will do.

JUNIOR CIVIL ENGINEER Piles and Foundation โครงสร้าง ชนิดของโครงสร้าง