Corrosion is a naturally occurring effect, defined as destruction of substance, usually metals, or alteration of their properties due to the environment impact. As well as other natural events, such as natural disasters, corrosion may bring about dangerous and finance-consuming damage.
Corrosion is the process of metal destruction in the course of its physical-chemical or chemical interaction with the external environment. Corrosion is usually divided into:
In the course of operation tanks are subject to corrosion both from the outside and the inside.
The external side of the tank stains under the influence of atmospheric moisture and corrosive elements (particles) in the air.
The inside corrosion depends basically on the frequency of the tank’s loading with oil products, on chemical composition of the products, on the presence of water in the fuel. The tempo and the characteristics of the corrosion process are the most evident on the inside surface of the tanks at the points of separation of two structures: oil product – bottom water, oil-product – gas-vapour mixture.
Humidity and the external atmosphere temperature, as well as corrosion resistance of steel, — all influence the intensity of corrosion.
Corrosion preventive protection of tanks for oil and oil products should be worked out considering the requirements of building standards and regulations and in accordance with the corresponding standards, taking into consideration the construction peculiarities, terms of operation and the necessary operating period of the tank.
To reduce the danger of corrosion damage to the metal frameworks, that can bring the tank out of operation, it is essential to provide a certain system of measures, that includes applying protective coat and (or) increasing the thickness of the metal sheets (corrosion allowance), which is meant to consider the possible loss of the details’ thickness as a result of corrosion.
Moreover, you should provide periodic examination of the full surface of the tank not less than once in every 5 years for revealing corrosion damage and the areas of surface with damaged pain coating. If necessary, the protective coat is restored.
If the metal frameworks of tank roof and upper rings of the wall show extremely high tempo of corrosion process it is possible to use inert-gas atmosphere as additional corrosion preventive measure.
When choosing the protective coat and determining the corrosion allowances one should consider the level of aggressive impact of the environment on the metal details inside the tank and on its outside surface, exposed to the open air. The level of aggressive impact of environment on the inside framework elements is shown in the Table below.
The level of influence of external atmosphere on the open-air elements is determined by the temperature and humidity parameters of the air and loading of corrosion active gas in the air in accordance with the construction standards and regulations (Sanitary Standards and Regulations 2.03.11-85).
|Eements of the construction of tanks:||the extent of the aggressive impact of the products of the steel tanks:|
|crude oil||black oil, diesel fuel, kerosene||gasoline|
|the internal surface of the bottom and the lower belt on height of 1m from the bottom||aggressive medium||aggressive medium||weakly aggressive|
|the middle belt, the lower parts of the pontoons and floating roofs||weakly aggressive||weakly aggressive||weakly aggressive|
|the upper zone (zone of periodic moistening)||aggressive medium||weakly aggressive||aggressive medium|
|roof tank, top and side surfaces of pontoons and floating roofs||aggressive medium||aggressive medium||weakly aggressive|
Provided the level of hydrogen disulfide in the clean oil is higher than 10 g/l or it contains hydrogen disulfide and carbon dioxide in any proportion, the rate of aggressive impact on the internal surface of the bottom, the lower ring, roof, top and edges of the pontoons and floating roofs increases by one level.
Keeping the thickness of metal frameworks exposed to mildly aggressive influence is achieved only by means of corrosion allowances.
If the frameworks are subject to moderately aggressive and highly corrosive impact, the safe operation of the tank is ensured, apart from corrosion allowances, by applying protective coat. This method helps to raise the reliability of the metal frameworks in case of local destruction of protective coat before the planed examining of the corrosion condition of the tank.
The rate of corrosion allowance is set based on the tempo of corrosion damage process, which is determined by the type of environment aggressiveness:
Electrochemical protection of the frameworks’ elements of the tank is carried out by ground and cathodic protection units. The method is chosen based on the technical-economic parameters.
Surfaces of the metal frameworks, prepared for corrosion preventive procedures, should not have:
The welding joints should have smooth connection with the base metal without cutoffs and cold laps.
All internal elements of the tank, welded to the wall, bottom or roof, have to be scalded along the edges to avoid appearance of hollow spaces and spacing gaps.
Apart from this, all elements exposed to open air under the influence of moderately aggressive environment, should also be scalded along the edges to avoid appearance of hollow spaces and spacing gaps.
Before the protective coating is applied all surfaces must be degreased to the 2nddegree, cleaned from oxides to the 1st degree for applying sprayed metal and paint coating or to the 1st-2nd degree for applying paint coating. Dust should be alsoremoved from the surface.
The operating period of effectiveness for protective coating should be not less than 10 years.
When protecting the outside surface of the tank bottom you should meet the following requirements:
It is important to consider environmental protection regulations, requirements of accident prevention and safety instructions and other standards, controlling performing of this type of work.
According to the “Rules of installation of vertical cylindrical steel tanks for oil and oil products (PB 03-605-03)” the surface of the metal frameworks exposed to open air should be painted with lacquer paint materials. While choosing the colour the light reflection factor should be considered.
The state of corrosion preventive coating of the outside surface of the above ground tanks should be controlled periodically. Control is meant for checking defects in the external layer of the protective coating, continuity of the corrosion preventive coating throughout the external surface, the rate of adhesion of the coating to the metal surface of the tank.
All corrosion preventive works for above ground tanks are to be performed in accordance with RD 112-RSFSR-15-89 Basic requirements for corrosion preventive protection of the designed and reconstructed enterprises of the oil and oil products’ provision, GKNP RSFSR.
While preparing the external surface for corrosion preventive coating the worn out layer of protective coat, corrosion traces and dirt are removed either manually or by mechanical method.
Stain is removed either by cleaning pads № 25, 16 and 12 or wire brushes, or by chemical method, using cleaner, containing 35 % of orthophosphoric acid, 20 % of ethylic alcohol, 5 % of butyl alcohol, 1 % of hydroquinone and 39 % of water.
The cleaner is applied on the stained surface for 3-5 min, after which it is washed with hot water together with the corrosion products. After this the surface is wiped.
Mechanical cleaning and degreasing is done for the whole outside surface of the tank.
The prepared surface should be thoroughly wiped and dried.
A priming coat is applied on the prepared tank surface with a pneumatic spray gun. It is highly important to avoid paint drips. This process is meant for protecting metal from corrosion and ensuring adherence of the paint coating with the metal.
After the work is finished, the paint coating of light colour is applied on the outside surface. It gives heat-reflecting effect and corrosion preventive characteristics.
The finished painted surface should have equal layer thickness without paint dips and other defects.
The problem of tank bottom corrosion is very serious. For example, penetrating corrosion bottom destruction of the tanks, used for oil settling, and flow lines may lead to oil spills that pollute the environment. Moreover, there appears the need to replace the tank bottoms already after 5-6 years of operation, which is complicated, considering the diameter of the bottom equal to almost 50 m (for vertical steel tank 20 000).
The soil corrosion may be avoided by arranging water-proof layer and by using electro-chemical protection, when galvanic protectors are connected to the tank bottom. Apart from this, the bottom may be effectively protected by means of cathodic protection.
Electro-chemical protection of metals is based on the ability of metals to stop staining under the influence of flow of direct current.
The surface of any metal is inhomogeneous as regards galvanic characteristics, and this is to be seen as the main reason for metal to suffer corrosion in electrolytic conductors’ solutions, among which are sea water, local water and bottom water. In this case only areas with the most negative potential (anodes) are destroyed. While the areas of surface with the more positive potential (cathodes) do not show destruction.
The scheme of effect of the electrochemical protection implies that the whole surface of the metal framework is turned into one big indestructible cathode. Anodes are represented by electrodes of more electronegative material, connected to the bottom of the protected construction. They are called galvanic protectors.
The protective current appears as a result of operation of the galvanic pair: galvanic protector – protected construction. Protectors tend to gradually wear out, giving protection to the parent metal, that is why they are often called “sacrificial anode”.
Electro-chemical protection is the only effective method against the most local types of metal corrosion (pitting and gap corrosion, contact corrosion, intergranular corrosion and cracking). At the same time it helps to prevent from the further development of the on-going corrosion process, thus it is equally effective as for tanks and equipment under construction and for those, already in operation, too.
Galvanic protectors’ method is usually applied in combination with paint and lacquer coatings. Such combination of passive protection (painting) with active protection (electrochemical) enables to reduce the expenditure of galvanic protectors and prolong their operating life, to ensure more even distribution of electric protective current over the surface of protected constructions. It also helps to balance the coating defects, arising from its damage in the course of assembly, transporting and operation, including the results of natural ageing (swelling, buckling, cracking, flacking).
Protective current goes exactly to those metal areas, where the density of the coating is damaged. It reaches all shadowed areas, gaps, hollow spaces, preventing the bare metal from staining. It should be noted that in the course of cathodic polarization local and bottom water shows certain salt gel that contains insoluble Ca and Mg salts and plays the role of additional coating.
Along with this, electrochemical protection can provide full safety from corrosion without painting too. In this case the higher protective current density should be provided for unpainted steel surface, which will demand increasing the number of galvanic protectors and will enforce their expenditure. However, if we take into consideration the labour intensity of applying the protective coating, especially for tanks in operation, this method of corrosion preventive protection seems to be potentially productive.
As most metal frameworks are produced of steel, galvanic protectors may be made of metals with more negative electric potential, compared to steel. The basic are zinc, aluminum, magnesium.
It should be taken into consideration that if the potential shift to negative direction exceeds a certain limit, there is a threat of so-called “over-protection”, connected with release of hydrogen, changes of near-electrode layer’s composition, and other effects, that may lead to corrosion boosting.
Corrosion preventive works for internal surface are very labour-intensive. This is connected with complicated procedures of surface preparation and applying protective coating.
A priming coat is applied on the prepared inside tank surface with a pneumatic spray gun. It is highly important to avoid paint drips.
The inside surface of underground and above ground tanks are covered with paint and lacquer materials in 2-4 layers with the successive drying of each layer separately.
After the work is finished acceptance certificate is signed, which has to be accompanied by certificates for materials used for protection.
Table: «The level of aggressive impact of the external environment on the steel structures inside a vertical steel tank »:
Elements of a tank:
Level of product’s aggressive impact on the steel elements of a vertical tank:
Masut, diesel fuel, kerosene
internal surface of the bottom and the lower ring at 1 m’ height from the bottom
medium rings, lower parts of the pontoons and floating roofs
upper ring (splash zone)
tank roof, top and edges of the pontoons and floating roofs
Note: The rate of aggressive impact of masut is considered at the temperature level, not exceeding 90°C.