INTRODUCTION

The annual cost of corrosion and of protection against corrosion in the world is estimated at about 50 billion dollars. The tremendous cost is less surprising when we consider that corrosion occurs, with varying degrees of severity, wherever metals are used. For example, one plant spends 5 million pesos a year for repainting steel to prevent rusting. One large chemical company spent more than 3 million pesos per year for corrosion maintenance in its sulfuric acid plants, though the corrosion conditions were not considered to be parcticularly severe.

In fact the world's economy would be drastically changed if there were NO corrosion. For example, automobiles, ships, underground pipelines, and household appliances would not require coatings. The stainless steel industry would essentially dissapear and copper would be used for only electrical purposes. Most metallic plants, as well as consumer products, would be made of steel or cast iron. Corrosion touches all inside and outside the home, on the road, on the sea, in the plant, and in aerospace vehicles.

But while corrosion is inevitable, its cost can be considerably reduced. For example, an inexpensive magnesium anode could double the life of a domestic hot water tank. Washing a car to remove road deicing salts is helpful. Proper selection of materials and good design reduce cost of corrosion. A good maintenance painting program pays for itself many times over. Here is where the corrosion engineer enters the picture and is effective his primary function is to combat corrosion.

Aside from its direct costs, corrosion is a serious problem because it definitely contributes to the depletion of our natural resources. For example, steel is made from iron ore, and our domestic supply of high grade directly smeltable iron ore has dwindled. Another important factor concerns the world's supply of metal resources. The rapid industrialization of many countries indicates that the competition for and the price of metal resources will increase.

HOW TO PREVENT CORROSION

Various factors affect the occurence of corrosion and its rate of development. Corrosion preventive methods are applied to vary these factors for corrosion control. In the principle, there are four methods for corrosion prevention. The most common and familiar one is coating of the metal surface to intercept corrosive substances in the environment. Typical examples are painting and metal plating. The other corrosion prevention methods are the control of the corrosive environment and proper material selection.

CORROSION PREVENTION BY COATINGS

Corrosion prevention by paint coating has a long history of corrosion prevention, and is the method most frequently used today. Corrosion prevention of many objects, namely, land structures such as bridges, buildings, steel towers and various plants, marine structures such as sea walls and off shore structures, many kinds of trains, automobiles, electric appliances, and many others all depend on painting. Although painting looks simple, there are many technically important points to be considered in painting, and the durability (the life) of the paint coatings differ depending on the quality of the painting system.

Paints are a mixture of insoluble particles of pigment suspended in a film forming substance (vehicle). When paints are applied on metals after adjusting viscosity by adding a solvent, they form paint films by evaporation of the solvent through an application of heat. The most classic vehicle is the natural oil, example of which are linseed oil and tung oil. They are drying oils, and when exposed to air, they form paint films by oxidation, polymerization and curing. Such paints are called oil paints and it is among the standard specification of painting for steel structures until now.

There are some types of pigment which are added to the vehicle to produce paints. Pigments of one type are added to give color to paints; titanium oxide for white, carbon black for black, russian blue for blue and so on. Pigments of other types are used as fillers to adjust gloss, to improve strength of paints, and to increase the volume of paints. Example of fillers commonly used are barim sulfate, calcium carbonate, clay and diatomaceous earth, and gypsum.

Important pigments for corrosion protection are rust preventive ones. Typical examples are red lead (PB3O4) and Zinc Chromate (ZnCrO4). These are added to prime coats. When water penetrates the paint film and reaches the metal surface, the rust preventive pigments dissolve in water in just sufficient amounts to decrease corrosivity of water. They act as a corrosion inhibitor. If the solubility is too low, it will not be effective. If it is too high, the pigments will be leached out of the paint in just a short time.

ENVIRONMENT

  • Practically all environments are corrosive to some degree. Some examples are air in moisture; fresh, distilled, salt, and mine waters; rural, urban and industrial atmospheres; steam and other gases such as chlorine, ammonia, hydrogen sulfide, sulfur dioxide, and fuel gases; mineral acids such as hydrochloric, sulfuric, and nitric; organic acids such as naphthenic, acetic, and formic; alkalies; soils; solvents; vegetable and petroleum oils; and a variety of food products. In general, the "inorganic" materials are more corrosive than the "organics." For example, corrosion in the petroleum industry is due more to sodium chloride, sulfur, hydrochloric and sulfuric acids, and water, than to the oil, naptha, or gasoline.

  • The trend in the chemical process industries toward higher temperatures and pressures has made possible new processes or improvements in old processes, for example, better yields, greater speed, or lower cost of production. This also applies to power production, including nuclear power, missiles, and many other methods and processes. Higher temperatures and pressures usually involve more severe corrosion conditions. Many of the present day operations would not have been possible or economical without the use of corrosion resistant materials.

  • By far the most important form of corrosion is the rusting of iron and steel. Rusting is a process of oxidation in which iron combines with water and oxygen to form rust, the reddish-brown crust that forms on the surface of the iron. Because iron is so widely used, e.g., in building construction and in tools, its protection against rusting is important. Rusting can be prevented by excluding air and water from the iron surface, e.g., by painting, oiling, or greasing, or by plating the iron with a protective coating of another metal. Many alloys of iron are resistant to corrosion. Stainless steels are alloys of iron with such metals as chromium and nickel; they do not corrode because the added metals help form a hard, adherent oxide coating that resists further attack.

QUESTIONS & ANSWERS

 What is Corrosion?

Corrosion may be defined in several ways: (1) destruction or deterioration of a material because of reaction with its environment; (2) destruction of materials by means other than straight mechanical; and (3) extractive metallurgy in reverse. Extractive metallurgy is concerned primarily with the winning of the metal from the ore and refining or alloying the metal for use. Most iron ores contain oxides of iron, and rusting of steel by water and oxygen results in a hydrated iron oxide. Rusting is a term reserved for steel and iron corrosion, although many other metals form their oxides when corrosion occurs.

 What is the cheapest way to eliminate corrosion?

In our view, the most cheapest way to fight and eliminate corrosion is by proper surface preparation and application of special purpose coatings. This method ensures the surface to be corrosion free as long as there is proper surface preparation, design and painting system. This method may be expensive at first, but the effects of proper surface preparation and painting system will still be effective in years to come. Using this method singnificantly Minimizes expensive maintenance and replacement costs. Also, take into account the downtime and shutdown time of production, imagine how much profit will the plant be losing just for major maintenace operations. Therefore, it is better to invest on proper surface preparation and painting costs than to lose more money in the following years to come.

 How do we know when to call the experts?

The time to invite corrosion prevention experts is when you can see extensive corrosion on your establishment. Corrosion can be a disease, spreading through every nook and cranny of your plant or establishment. Not to mention it might come into contact to your products ruining your business. When the time comes, let the experts examine your location and make note of the recommendations sent by them. Because the proper surface preparation and painting system is unique in every location. So, invite the experts, Call Precision Sandblasting Incorporated for a free consultation and quote.