Types Of Steel Corrosion

Corrosion is the process of eroding of the top layer of metals as a result of an electrochemical reaction with the environment. When metals are exposed to moisture or wetness, an orangish discolouration known as rust forms on the surface of the metal. This causes deterioration and the gradual destruction of the metal, if not treated effectively. The study of prevention of corrosion and its treatment is known as Corrosion Engineering.

There are many forms of corrosion. The most common types are explained below.

General Attack Corrosion

This the most common type of corrosion which is also treatable. Also known as uniform attack corrosion, this phenomenon occurs when the entire surface of a metal is exposed to air or a wet surface which results in complete damage of the metal. General attack corrosion generally results in the most amount of destruction to the material however, it can be treated. Hence, it is also known to be the safest form of corrosion.

Galvanic Corrosion

Galvanic corrosion is a phenomenon that occurs when two distinct metals come in contact with each other through an electrolyte. One metal acts as a cathode while the other metal behaves like an anode. The rate of corrosion of the anode metal is amplified while the cathode metal takes longer to deteriorate. Galvanic corrosion can occur only when both the metals are placed in an electrolyte that is susceptible to corrosive action when electricity is passed through it.

Corrosion Engineering

Corrosion is the destructive attack of a material by reaction with its environment. The serious consequences of the corrosion process have become a problem of worldwide significance. In addition to our every day encounters with this form of degradation, corrosion causes plant shutdowns, waste of valuable resources, loss or contamination of product, reduction in efficiency, costly maintenance, and expensive over-design; it also jeopardizes safety and inhibits technological progress.

The multidisciplinary aspect of corrosion problems combined with the distributed responsibilities associated with such problems only increase the complexity of the subject. Corrosion control is achieved by recognizing and understanding corrosion mechanisms, by using corrosion-resistant materials and designs, and by using protective systems, devices, and treatments. Major  orporations, industries, and government agencies have established groups and committees to look after corrosion-related issues, but in many cases the responsibilities are spread between the manufacturers or producers of systems and their users. Such a situation can easily breed negligence and be quite costly in terms of dollars and human lives.

EFFECTS OF RAINFALL

The sudden dilution of a river by heavy rainfall can be a disruptive factor in a water treatment plant. The location of a river water intake should be carefully chosen with this problem in mind. In the operation of a treatment plant, it is common practice to adjust the chemical dosages according to effluent water quality. However, there are many water supplies so variable that it is necessary to base changes in chemical treatment on raw water characteristics, rather than on finished water quality. This imposes a hardship on the treatment plant operators requiring their constant attention to analysis and control.

Tides create another important influence on surface water quality in that they slow, or actually reverse, normal river flow. This is particularly pronounced during periods of low rainfall. The change in water quality between high and low tide sometimes justifies the installation of raw water supply reservoirs to receive water at low tide when the river flows unimpeded and quality is at its best. Plants so equipped stop pumping at high tide when saline bay waters move upstream into the upper channel.

Another characteristic of surface waters is seasonal temperature changes. This complicates treatment, particularly affecting the coagulation process in the winter. Low temperatures also create problems with air-binding of filters due to the increased solubility of gases and higher water viscosity. This binding causes pressure drop through the filter beds to increase, releasing gas and disrupting flow.

Another effect of temperature change occurs in water-cooled systems of industrial plants where heat exchange equipment is usually designed for the least favorable condition—the higher summer temperatures of surface waters. In winter, when the temperature is low, the flow must often be restricted to prevent over cooling. Lower water velocities may allow silting in heat transfer equipment, which can lead to corrosion and to pressure loss when higher cooling rates are needed.

Because rivers are warmer in the summer, designers take this into account in most water-dependent systems. But a complication arises in that many waste-waters contain heat from plant processes, and this added heat compounds the natural rise of the summer, sometimes producing an effluent warm enough to create an unhealthy condition for aquatic life. Pollution discharges not only add to the heat load of the river, but they usually also add to the oxygen demand and may have a pronounced influence on the oxygen content of the river water.

source: The NALCO Water Handbook

THE WATER MOLECULE

Three-quarters of the surface of the earth is covered with water. While this is an impressive statistic, it is pale beside the spectacular photographs that have come to us from outer space.

They reveal a beautiful blue planet bathed in water, partly hidden by a veil of vapor. Life came into being in this water. As living things became more complex and specialized, they left the sea  or the land, taking water with them as the major part of their bodies.

On the Planet Earth, water is life.