Top priority was anti corrosion protection Metal Structures and manufacturing equipment industry, are prone aggressive effects acid and alkaline environments in petrochemical, oil and gas, energy, and other industry.
Corrosion control methods
These methods may be divided into two groups. The first two methods are usually accomplished before metal products are operated in industrial environment (selection of structural materials and their combinations as early as at a stage, when a product is designed and manufactured, and protective coating application thereon).
The last two methods, on the contrary, may be used only, while the product is being operated (current passage to achieve a protective potential, and injecting special inhibitors into a process medium), and do not relate to any pretreatment before operation.
When the first two methods are used, a steel composition and the nature of protective coatings for the metal product involved, while it is operated in conditions, where medium corrosiveness varies, may not be changed. The second group of methods permits, if required, to create new protection modes ensuring that the product is exposed to corrosion as less as possible, when its operation conditions change. For example, depending on soil corrosiveness, various cathode current densities may be maintained in various pipeline sections or for various grades of oil pumped via pipes of particular composition, and various inhibitors may be used.
Widely used are the following main solutions for protecting metal structures from corrosion:
1. Protective Coatings
Metallic Coatings.
By protective effect the coatings may be anodic and cathodic.
In an aqueous electrolyte solution the electrochemical potential of anodic coatings is more negative than that of the metal protected, while that of the cathodic ones is more positive.
Due to a potential displacement the anodic coatings reduce or completely eliminate base metal corrosion in coating pores, i.e. provide electrochemical protection. Despite the fact that the cathodic coatings may intensify the base metal corrosion in the pores, they are used, as they improve such physical and mechanical properties of the metals, as wear resistance and hardness. But this requires considerably more coating thicknesses, and an additional protection is to be provided in a number of cases.
The metallic coatings are also divided by their application processes (electrolytic deposition, chemical deposition, hot-dip and cold application, thermodiffusion treatment, metal spraying, plating).
Nonmetallic Coatings
These coatings are obtained by superficial application of various nonmetallic materials: paintwork, rubber, plastic, ceramic and others.
Most widely used are paint coatings that may be divided into
By purpose:
– weatherproof,
– partially weatherproof,
– waterproof,
– special,
– oil/gasoline-proof,
– chemical-resistant,
– heat-resistant,
– insulating,
– preservative.
By film former compositionя:
– bituminous,
– epoxy,
– organosilicon,
– polyurethane,
– pentaphthalic and etc.
Coatings obtained by chemical and electrochemical surface treatment. These coatings represent the films of insoluble products generated as a result of chemical interaction between the metals and environment. As many of them are porous, they are used, mainly, as sublayers for lubricants and paint coatings to improve the protective capacity of metal coating and ensure reliable adhesion.
Application Methods:
– oxidation,
– phosphating,
– passivation,
– anodization.
2. Corrosion environment treatment to reduce corrosivity.
Such treatment examples may include corrosion environment neutralization and deoxygenation, and applying various anticorrosive agents that are introduced in small quantities into the aggressive medium and create an adsorbed film on a metal surface. Such film inhibits electrode processes and changes electrochemical metal parameters.
3. Electrochemical Protection.
By cathodic or anodic polarization from an external current source or connecting protectors to the structure protected a metal potential is displaced to values, at which the corrosion decelerates or completely stops.
4. New metal structural materials
New metal structural materials are developed and produced by eliminating impurities accelerating corrosion processes from a metal or alloy (iron from magnesium or aluminum alloys, sulfur from iron alloys and etc.), or introducing new components into the alloys to greatly improve their corrosion resistance (for example, chromium into iron, manganese into magnesium alloys, nickel into iron alloys, copper into nickel alloys and etc.).
5. Efficient designing and maintenance of metal structures and parts
A rational approach excluding an undesirable contact with a metal or its insulation:
– eliminating openings and gaps in structures,
– eliminating moisture stagnation zones, impact jet effects and sharp flow rate changes in structures and etc.
When electing design corrosion control solutions for building structures, an attack nature, structure maintenance conditions and moral service life of buildings, structures and equipment should be thoroughly studied. In which case, the recommendations of corrosion control material manufacturers having laboratories analyzing and treating protection systems made of materials manufactured by them are widely used.
Urgency of solving a corrosion control problem is driven by the necessity to preserve natural resources and environment.