Atmospheric corrosion is the deterioration and destruction of metallic materials, as well as their structure and properties, caused by interaction with the terrestrial atmosphere at its various temperature, moisture, chemistry, and climatic values. Atmospheric corrosion is distinguished from the dry or gaseous corrosion at high temperatures in the absence of moisture, the latter form of corrosion does not require atmospheric humidity to occur. Dry or gaseous corrosion is a chemical corrosion and is quite different from atmospheric corrosion.

Many industries such as the Construction and Oil/Gas industries suffer from corrosion risks. As metal structures and equipment experience terrestrial air conditions and therefore can suffer from atmospheric corrosion and in some severe cases, the metal can be completely destroyed which can be quite costly and detrimental to the continuity of the operations. Fortunately, with sufficient background information of the various exposure conditions and how these influence metal corrosion, most serious corrosion problems can be reduced. This form of corrosion occurs spontaneously, however, it may be slowed, prevented, and controlled but never completely stopped.

Most used metals are not in a pure state. These metals are usually in ores, chemical compounds that include O₂ (oxygen), H₂ (Hydrogen), and S₂ (Sulfur). These represent the mineral compounds which are the thermodynamic steady state of the metals. For the separation of the metals from their ores and for metallurgical and manufacturing processes, energy, in the form of heat, chemical, electrical, or mechanical, elevates the metal to higher energy levels and the metal product is not thermodynamically stable. This drives metals to convert into corrosive products. When metals come in contact with the atmosphere (oxygen) and water (moisture) in the presence of corrosive compounds such as chlorides or sulfur dioxide corrosion starts, and corrosion products such as oxides, hydroxides, or oxyhydroxides are formed.

Most used metals are not in a pure state. These metals are usually in ores, chemical compounds that include O₂ (oxygen), H₂ (Hydrogen), and S₂ (Sulfur). These represent the mineral compounds which are the thermodynamic steady state of the metals. For the separation of the metals from their ores and for metallurgical and manufacturing processes, energy, in the form of heat, chemical, electrical, or mechanical, elevates the metal to higher energy levels and the metal product is not thermodynamically stable. This drives metals to convert into corrosive products. When metals come in contact with the atmosphere (oxygen) and water (moisture) in the presence of corrosive compounds such as chlorides or sulfur dioxide corrosion starts, and corrosion products such as oxides, hydroxides, or oxyhydroxides are formed.

Atmospheric corrosion is an aqueous process, and its mechanism is electrochemical. Meaning that there's both transfer of mass during the chemical change also as an interchange of electrons and ions. The flow of electrical current (transfer of electrons) only happens because of the buildup of galvanic corrosion cell on the surface of the metal. For this to occur, anode and cathode sites, an electrolyte, and an oxidizing agent must be present

• - Anodes are the areas on the metal where several factors may be present such as inhomogeneous metal composition, grain boundary, multiple metallurgical phases, local metal defects, and nonuniform metal treatments - these form with a higher energy state.
• - Cathodes however, are the areas with lower energy state such as inert non-metallic inclusions and lower active-metal phases or structures. The cathodic reaction occurs on these sites and involves the reduction of an oxidizing agent, such as air, oxygen, or hydrogen ions.
• - An electrolyte, such as moisture, moisture contains dissolved atmospheric pollutants which works as an ionic conductor that will sustain electrochemical reactions.
• - An oxidizing agent, such as O₂ and H₂ ions, and is necessary for accepting the electrons emitted from the metal in the anode reaction.

Figure 1 below shows the basic corrosion mechanism of iron under a drop of water. Both iron dissolution and oxygen reduction reactions take place with slight separation on the surface, and their products (Fe ions and OH ions) react in the water drop to form red rust (corrosion product). The simple model of the corrosion reaction of Figure 1 explains many forms of corrosion and help understand measures to reduce corrosion. By preventing or slowing down one of the partial reactions, the overall corrosion rate can be reduced.

There are two basic forms of Atmospheric Corrosion, uniform (general) and non-uniform (localized). Uniform corrosion results at a similar corrosion rate over the metal surface and has the same appearance throughout. Uniform attack is typical for atmospheric corrosion of steel and copper. On the other hand, non-uniform corrosion usually occurs at relatively small and specific areas on the metal surface where the corrosion process is focused, resulting in local accelerated corrosion rate (i.e. Pitting).

Metal cracking is a very dangerous type of atmospheric corrosion which can occur when a metal structure is exposed to a corrosive environment and continuous or cyclic mechanical loading. These conditions can be widely seen in construction and oil/gas industries where continuous cyclic mechanical loading is the nature of the metal exposure. This combination leads to micro cracks, fissures, and big cracks that result in stress-corrosion cracking (under relatively constant loads) or fatigue corrosion (under cyclic deformation).

Corrosion may be expensive if not properly managed. Corrosion is a constant issue in construction and oil and gas industries due to poor material selection. The projected yearly direct cost of corrosion in the United States is $276 billion. When selecting building materials and accessories, corrosion concerns must be seriously evaluated. Figure 5 shows an overall distribution of yearly cost of corrosion in the United States.

It is evident that corrosion protection is often a necessary consideration in selecting materials for the construction and oil and gas industries and many others. Corrosion occurrence will reduce the load-carrying capacity of a component either by generally reducing its size (cross section) or by pitting, which not only reduces the effective cross section in the pitted region but also introduces stress raisers which can initiate cracks and reduce the lifetime of the structure. Many measures are present to reduce corrosion and extend the life of a component, increasing its overall reliability. The economics, environmental conditions, degree of protection needed for the projected life of the part, consequences of unexpected service failure, and importance of appearance are the main factors which determine not only whether a component needs to be protected against corrosion but also the most effective and economic method of achieving that protection.

There are two methods of minimizing the corrosion of steels. The first is to separate the reacting phases, and the second is to reduce the reactivity of the reacting phases. The separation of the reacting phases can be accomplished by metallic, inorganic or organic coatings, and film-forming inhibitors. Reactivity can be reduced by alloying, anodic or cathodic protection, and chemical treatment of the environment. Some methods of protection combine two or more forms. In most environments, the corrosion rate of carbon steel is typically around 20 micrometers per year in a rural outdoor atmosphere and rising to more than 100 micrometers per year in coastal environments. It is normally too high for a satisfactory application. The product design does not generally account for a base material loss. Hence, cost-efficient corrosion protection solutions are necessary.

Coating material is a tried and tested approach for extending the life of a product by reducing corrosion risks. At Engineering Edge, we offer a wide range of innovative anti-corrosion fasteners that are guaranteed to perform in various corrosive environments. Check out our CORROSHIELD® coating technology for more details.

At Engineering Edge, we guarantee that each fastener undergoes extensive research and testing. Each fastener passes the most detailed quality checks to exceed your expectations and perform for a minimum number of years before becoming structurally unreliable due to premature corrosion.

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