Corrosion: A dangerous phenomenon

The cost of corrosion involves an important part of the gross domestic product (GDP) and takes place in a wide range of examples, from large structures to implants placed in the human body.

From 1999 to 2001, the United States had a total annual direct cost due to corrosion issues of approximately $276 billion, or 3.1 percent of their GDP.

Similarly, in Peru, according to the Teknoquamica company, in the year 2000 losses by corrosion accounted for eight percent of GDP, ie, approximately $1,200 million.

Trinidad and Tobago hasn’t done any study to estimate costs that represent losses by corrosion.

Despite this, problems due to this phenomenon can be felt so it is clear then the need to implement the relevant measures.

There are different reasons why the phenomenon has not been controlled in an appropriate manner, ranging from climatic to economics. The situation, as you might guess, affects equally the majority of Latin American countries in which Governments and environmental conditions are similar. Governments practically do not pay attention to the problem of corrosion, not by lack of trained staff but by the lack of a unified policy on maintenance of structures.

Little by little private companies are becoming aware of the issue of corrosion and the damage the phenomenon causes when it is not addressed in a timely manner. But what is the corrosion? Corrosion can be defined in many ways.

Some definitions are very direct and focus on a specific type of corrosion, while others are very general and cover many forms of deterioration.

The word “corrode” is derived from the Latin word “corrodere” which means “gnawing parts”.

For most purposes, corrosion can be characterised as an electrochemical reaction between a material - usually a metal - and its environment which produces a deterioration of the material and its properties. The metals are rarely found in a pure state but rather are almost always combined with one or more non metallic chemical elements, and ores are usually an oxidised form of the metal. Therefore, a significant amount of energy should be applied to transform the ore into pure metal. This energy can be applied via metallurgy or chemical; additionally, extra energy is required in the form of cold work or by casting processes necessary to transform the pure metal into a usable piece.

Corrosion can be also defined as the tendency of a metal - produced and formed by a substantial application of energy - to return to its natural state of lower energy. From a thermodynamic perspective, the tendency to decrease the energy level is the main force that induces corrosion in metals.

The effects of corrosion in our daily life are classified into direct and indirect. Those affecting useful service of our real life are the direct. Indirect are those in which the producers and consumers of goods and services have influence on the corrosion costs. At home for instance, the phenomenon is observed directly in the automotive, steel burglar proof or windows, metal tools and others.

One of the more serious consequences of corrosion happens when it affects our safety or even life directly. When going about our daily business, we may see a number of problems due to corrosion. For example, on a bridge on the highway, corrosion may occur in the reinforcing steel rod inside the concrete, which can fracture it and, consequently, cause the failure of any section; similarly, the collapse of electric transmission towers may occur. These may damage buildings, structures, factories, etc, without mentioning the environmental impact and costly repairs that follow.

In December 1999, in the Bay of Biscay off the coast of France, the tanker MV Erika sank due to a hull rupture caused by corrosion. The result: approximately 20 thousand tons of crude oil spilled into the sea, which caused great damage to the marine ecosystem.

To control corrosion, it is necessary to know the process or mechanisms of corrosion, or in what manner it originates.

Corrosion takes place in different ways. Its classification is usually based on one of these three factors:

• Nature of the electrolyte: corrosion can be classified as “wet” or “dry”. It is necessary to have a liquid solution or mixture for wet corrosion to occur. Dry corrosion usually involves the reaction with gases at high temperature.

• Mechanism of corrosion: involves electrochemical reactions or direct reaction with a chemical.

• Appearance of corroded metal: corrosion can be uniform where metal corrodes at the same rate along the surface, or it may be localised, in which only small areas are affected.

The most common classification of wet corrosion based on the appearance of the metal is identified in the following forms: general or uniform corrosion; pitting corrosion; galvanic corrosion; corrosion-erosion, which includes cavitationerosion; intergranular corrosion, which includes sensitisation and exfoliation; de-alloying, including dezincification, and environmentally assisted cracking, which includes corrosion under stress, corrosion fatigue and damage by evolution of hydrogen.

In theory, the eight forms of corrosion are clearly distinct. In practice, however, there are cases of corrosion that includes more than one from; in other cases, it does not seem to conform to any of these forms.

Depending on the way in which corrosion occurs, an appropriate technique must be chosen to control it or prevent it.

There are five main primary corrosion control methods:

• Selection of materials.

Resistance to corrosion of a metal depends on the environment to which it is exposed. Once taking this into consideration you can carry out a good selection of materials for a specific use.

• Coatings. Coatings for protection against corrosion can be divided into two large groups: Metallic and non-metallic (organic and inorganic). With any type of coating that is selected the goal is the same: isolating the metallic surface from the corrosive environment. of the corrosive medium.

• Inhibitors. Some chemicals (salts, for example) cause corrosion, other chemicals inhibit it. Chromates and silicates and organic amines are common inhibitors. The mechanisms of inhibition can be a little complex.

Depending of the type, the inhibitor could be adsorbed on the specific sites to override the corrosion current. Others promote the formation of a protective film on the surface of the metal.

Inhibitors can be incorporated in a protective coating as well.

• Cathodic protection. The purpose of cathodic protection is to eliminate the current associated with the electrochemical process that occurs in wet corrosion.

For this an external artificially generated current is impressed over the structure. A system comprising a sacrificial material (anode) is used, such as zinc or magnesium, which are connected to the structure to provide protection. While the anode corrodes the adjacent structure is then protected.

• Design. The application of principles of design can eliminate many problems of corrosion and reduces the time and cost associated with maintenance and repair. Corrosion occurs frequently in small spaces or loopholes in which corrosive medium starts to be more aggressive. These areas can be eliminated or minimised in the design process. Where the corrosion under stress is possible, the components can be designed to operate at low levels of stress.

For all the above, it is very important that the engineers or specialist in corrosion, materials engineer, maintenance supervisors and reliability engineers to have sufficient knowledge to control this phenomenon. They must recognise it and know what is its origin and its severity; They must keep themselves updated on the tools and methods available today, as well as the inspection techniques, the effects of design variables, how to interpret and apply information on the corrosion and know where to get help.

For more information on corrosion you can contact our Metallurgy department at: Caribbean Industrial Research Institute (CARIRI) Trincity West Industrial Estate, Macoya.

Telephone: 285-5050 ext. 3110 E-mail: metallurgy@carir